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Colloids Interfaces, Volume 2, Issue 4 (December 2018)

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Open AccessArticle Facile Fabrication of Multifunctional ZnO Urchins on Surfaces
Colloids Interfaces 2018, 2(4), 74; https://doi.org/10.3390/colloids2040074
Received: 19 November 2018 / Revised: 10 December 2018 / Accepted: 10 December 2018 / Published: 14 December 2018
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Abstract
Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed, μm-long, tapered nanoneedles radiating from the [...] Read more.
Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed, μ m-long, tapered nanoneedles radiating from the urchin center. The ZnO urchin structures were successfully formed on several different substrates with high surface density and coverage, including silicon (Si), glass, polydimethylsiloxane (PDMS), and copper (Cu) sheets, as well as Si seeded with ZnO nanocrystals. Time-resolved SEM revealed growth kinetics of the ZnO nanostructures on Si, capturing the emergence of “infant” urchins at the early growth stage and subsequent progressive increases in the urchin nanoneedle length and density, whilst the spiky nanoneedle morphology was retained throughout the growth. ε -Zn(OH)2 orthorhombic crystals were also observed alongside the urchins. The crystal structures of the nanostructures at different growth times were confirmed by synchrotron X-ray diffraction measurements. On seeded Si substrates, a two-stage growth mechanism was identified, with a primary growth step of vertically aligned ZnO nanoneedle arrays preceding the secondary growth of the urchins atop the nanoneedle array. The antibacterial, anti-reflective, and wetting functionality of the ZnO urchins—with spiky nanoneedles and at high surface density—on Si substrates was demonstrated. First, bacteria colonization was found to be suppressed on the surface after 24 h incubation in gram-negative Escherichia coli (E. coli) culture, in contrast to control substrates (bare Si and Si sputtered with a 20 nm ZnO thin film). Secondly, the ZnO urchin surface, exhibiting superhydrophilic property with a water contact angle ~   0 ° , could be rendered superhydrophobic with a simple silanization step, characterized by an apparent water contact angle θ of 159° ± 1.4° and contact angle hysteresis Δ θ < 7°. The dynamic superhydrophobicity of the surface was demonstrated by the bouncing-off of a falling 10 μ L water droplet, with a contact time of 15.3 milliseconds (ms), captured using a high-speed camera. Thirdly, it was shown that the presence of dense spiky ZnO nanoneedles and urchins on the seeded Si substrate exhibited a reflectance R < 1% over the wavelength range λ = 200–800 nm. The ZnO urchins with a unique morphology fabricated via a simple route at room temperature, and readily implementable on different substrates, may be further exploited for multifunctional surfaces and product formulations. Full article
(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)
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Open AccessArticle Systematic Characterization of DMPC/DHPC Self-Assemblies and Their Phase Behaviors in Aqueous Solution
Colloids Interfaces 2018, 2(4), 73; https://doi.org/10.3390/colloids2040073
Received: 22 November 2018 / Revised: 10 December 2018 / Accepted: 11 December 2018 / Published: 14 December 2018
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Abstract
Self-assemblies composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) form several kinds of structures, such as vesicle, micelle, and bicelle. Their morphological properties have been studied widely, but their interfacial membrane properties have not been adequately investigated. Herein, we report [...] Read more.
Self-assemblies composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) form several kinds of structures, such as vesicle, micelle, and bicelle. Their morphological properties have been studied widely, but their interfacial membrane properties have not been adequately investigated. Herein, we report a systematic characterization of DMPC/DHPC assemblies at 20 °C. To investigate the phase behavior, optical density OD500, size (by dynamic light scattering), membrane fluidity 1/PDPH (using 1,6-diphenyl-1,3,5-hexatriene), and membrane polarity GP340 (using 6-dodecanoyl-N,N-dimethyl-2-naphthylamine) were measured as a function of molar ratio of DHPC (XDHPC). Based on structural properties (OD500 and size), large and small assemblies were categorized into Region (i) (XDHPC < 0.4) and Region (ii) (XDHPC ≥ 0.4), respectively. The DMPC/DHPC assemblies with 0.33 ≤ XDHPC ≤ 0.67 (Region (ii-1)) showed gel-phase-like interfacial membrane properties, whereas DHPC-rich assemblies (XDHPC ≥ 0.77) showed disordered membrane properties (Region (ii-2)). Considering the structural and interfacial membrane properties, the DMPC/DHPC assemblies in Regions (i), (ii-1), and (ii-2) can be determined to be vesicle, bicelle, and micelle, respectively. Full article
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Open AccessArticle Physicochemical Parameters for Brea Gum Exudate from Cercidium praecox Tree
Colloids Interfaces 2018, 2(4), 72; https://doi.org/10.3390/colloids2040072
Received: 9 November 2018 / Revised: 10 December 2018 / Accepted: 11 December 2018 / Published: 12 December 2018
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Abstract
Brea gum (BG) is a hydrocolloid obtained as an exudate from the Cercidium praecox tree. The physicochemical properties of brea gum are similar to those of the arabic gum and, in many cases, the former can replace the latter. The brea gum was [...] Read more.
Brea gum (BG) is a hydrocolloid obtained as an exudate from the Cercidium praecox tree. The physicochemical properties of brea gum are similar to those of the arabic gum and, in many cases, the former can replace the latter. The brea gum was incorporated in 2013 into the Argentine Food Code because of its ancestral background and its current food uses. Brea gum could be also used as additive or excipient for pharmacological formulations. This work reports intrinsic viscosity, coil overlap, and Mark–Houwink–Kuhn–Sakurada (MHKS) parameters of BG solutions. Partially hydrolyzed BG solution was analyzed using intrinsic viscosity measurements, dynamic light scattering and size-exclusion chromatography (SEC). The MHKS parameters, a and k, were determined for BG at 25 °C, with values of 0.4133 and 0.1347 cm3 g−1, respectively. The viscometric molecular weight of BG was 1890 kg mol−1. The hydrodynamic parameters of BG were indicative of a hyperbranched structure and spherical conformation. The knowledge obtained on the physicochemical properties of brea gum favors its use in food and pharmaceutical applications. Full article
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Open AccessCommunication Self-Alignment Sequence of Colloidal Cellulose Nanofibers Induced by Evaporation from Aqueous Suspensions
Colloids Interfaces 2018, 2(4), 71; https://doi.org/10.3390/colloids2040071
Received: 29 November 2018 / Revised: 10 December 2018 / Accepted: 11 December 2018 / Published: 12 December 2018
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Abstract
Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and [...] Read more.
Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration. Full article
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Open AccessArticle Morphological Features and Sorption Performance of Materials Based on Birnessite Exposed to Various Reductive Conditions
Colloids Interfaces 2018, 2(4), 70; https://doi.org/10.3390/colloids2040070
Received: 9 November 2018 / Revised: 4 December 2018 / Accepted: 6 December 2018 / Published: 9 December 2018
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Abstract
The article is devoted to the evolution of structural, morphological, and sorption characteristics of layered manganese oxide (birnessite) under various conditions close to the real operating regime of the sorbents for radioactive waste processing. To identify the phase composition in the birnessites, we [...] Read more.
The article is devoted to the evolution of structural, morphological, and sorption characteristics of layered manganese oxide (birnessite) under various conditions close to the real operating regime of the sorbents for radioactive waste processing. To identify the phase composition in the birnessites, we implemented XRD analysis, while SEM and temperature-programmed reduction (TPR) were used to study morphological and redox features of the materials, respectively. Structural changes after various kinds of treatment of birnessites were tracked using low temperature nitrogen sorption. Sorption characteristics were assessed under static and in dynamic conditions on the efficiency of Sr2+ removal from simulated seawater. TPR combined with kinetic analysis revealed the decrease of particle sizes in the birnessites after repeated use in sorption-regeneration cycle and reduction with hydrazine. Despite the fact that the porous structure of the materials remains preserved, the surface morphology of birnessite changes drastically depending on the reducing agent. Hydrazine treatment increases the sorption performance of the birnessite followed by degradation of mechanical properties, thus, preventing such sorbent from repeated use. Kinetic analysis of TPR allows quantifying differences in morphology and porous structure of manganese oxide materials. The specific surface area, amorphous surface structure, and accessibility of Mn+3 sites are the most important factors for birnessite sorption performance. Full article
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Open AccessArticle Dynamic Measurements with the Bicone Interfacial Shear Rheometer: Numerical Bench-Marking of Flow Field-Based Data Processing
Colloids Interfaces 2018, 2(4), 69; https://doi.org/10.3390/colloids2040069
Received: 7 October 2018 / Revised: 19 November 2018 / Accepted: 3 December 2018 / Published: 7 December 2018
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Abstract
Flow field-based methods are becoming increasingly popular for the analysis of interfacial shear rheology data. Such methods take properly into account the subphase drag by solving the Navier–Stokes equations for the bulk phase flows, together with the Boussinesq–Scriven boundary condition at the fluid–fluid [...] Read more.
Flow field-based methods are becoming increasingly popular for the analysis of interfacial shear rheology data. Such methods take properly into account the subphase drag by solving the Navier–Stokes equations for the bulk phase flows, together with the Boussinesq–Scriven boundary condition at the fluid–fluid interface and the probe equation of motion. Such methods have been successfully implemented on the double wall-ring (DWR), the magnetic rod (MR), and the bicone interfacial shear rheometers. However, a study of the errors introduced directly by the numerical processing is still lacking. Here, we report on a study of the errors introduced exclusively by the numerical procedure corresponding to the bicone geometry at an air–water interface. In our study, we set an input value of the complex interfacial viscosity, and we numerically obtained the corresponding flow field and the complex amplitude ratio for the probe motion. Then, we used the standard iterative procedure to obtain the calculated complex viscosity value. A detailed comparison of the set and calculated complex viscosity values was made in wide ranges of the three parameters herein used, namely the real and imaginary parts of the complex interfacial viscosity and the frequency. The observed discrepancies yield a detailed landscape of the numerically-introduced errors. Full article
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Open AccessArticle Influence of Magnetic Field on Evaporation Rate and Surface Tension of Water
Colloids Interfaces 2018, 2(4), 68; https://doi.org/10.3390/colloids2040068
Received: 2 October 2018 / Revised: 28 November 2018 / Accepted: 28 November 2018 / Published: 5 December 2018
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Abstract
Using neodymium ring magnets (0.5–0.65 T), the experiments on the magnetic field (MF) effects on water evaporation rate and surface tension were performed at room temperature (22–24 °C). In accordance with the literature data, the enhanced evaporation rates were observed in the experiments [...] Read more.
Using neodymium ring magnets (0.5–0.65 T), the experiments on the magnetic field (MF) effects on water evaporation rate and surface tension were performed at room temperature (22–24 °C). In accordance with the literature data, the enhanced evaporation rates were observed in the experiments conducted in a period of several days or weeks. However, the evaporated amounts of water (up to 440 mg over 150 min) in particular experiments differed. The evaporated amounts depended partially on which pole of the ring magnet was directed up. The relatively strong MF (0.65 T) caused a slight decrease in surface tension (−2.11 mN/m) which lasted longer than 60 min and the memory effect vanished slowly. The surface tension data reduced by the MF action are reported in the literature, although contrary results can be also found. The observed effects can be explained based on literature data of molecular simulations and the suggestion that MF affects the hydrogen bonds of intra- and inter-clusters of water molecules, possibly even causing breakage some of them. The Lorentz force influence is also considered. These mechanisms are discussed in the paper. Full article
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Open AccessArticle Synthesis of Submicrocontainers with “Green” Biocide and Study of Their Antimicrobial Activity
Colloids Interfaces 2018, 2(4), 67; https://doi.org/10.3390/colloids2040067
Received: 27 September 2018 / Revised: 27 November 2018 / Accepted: 28 November 2018 / Published: 3 December 2018
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Abstract
The synthesis and properties of submicrocontainers with a shell of nanoparticles of silicon dioxide and a core of polymerized 3-(Trimethoxysilyl) propyl methacrylate loaded with 5-Dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) are considered. The resulting containers were characterized by scanning electron microscopy SEM, laser correlation spectroscopy and thermogravimetric [...] Read more.
The synthesis and properties of submicrocontainers with a shell of nanoparticles of silicon dioxide and a core of polymerized 3-(Trimethoxysilyl) propyl methacrylate loaded with 5-Dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) are considered. The resulting containers were characterized by scanning electron microscopy SEM, laser correlation spectroscopy and thermogravimetric analysis. The obtained submicrocontainers show low polydispersity with a small increase in size in comparison with the initial droplet size of the Pickering emulsion. The Zeta potential of the final containers was sufficiently negative at pH7 to be stable. The maximum release of encapsulated biocide was observed over approximately 24–27 h with a lease of about 78% of the encapsulated biocide during 3.5 h. The effectiveness of the encapsulated biocide by the Pickering emulsion technique was studied by tests on the growth rate of a microfungi colony (Aspergillus niger, Aspergillus awamori) and the growth rate of the bacteria Bacillus cereus. The test shows that the submicrocontainers of DCOIT facilitate a growth inhibition of 70% against 52% for the free biocide after 5 days; this is due to the fact that free biocide loses its activity promptly, while the encapsulated biocide is released gradually, and thus retains its effectivity for a longer time. Full article
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Open AccessArticle Elemental Mercury Adsorption by Cupric Chloride-Modified Mesoporous Carbon Aerogel
Colloids Interfaces 2018, 2(4), 66; https://doi.org/10.3390/colloids2040066
Received: 28 October 2018 / Revised: 22 November 2018 / Accepted: 30 November 2018 / Published: 2 December 2018
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Abstract
Mesoporous carbon aerogels (MCA) synthesized via aqueous polymerization of resorcinol and formaldehyde were modified by copper chloride and applied for adsorption removal of elemental mercury (Hg0) at a low temperature. The sorbents were characterized by field emission scanning electron microscopy (FESEM), [...] Read more.
Mesoporous carbon aerogels (MCA) synthesized via aqueous polymerization of resorcinol and formaldehyde were modified by copper chloride and applied for adsorption removal of elemental mercury (Hg0) at a low temperature. The sorbents were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, and X-ray photoelectron spectroscopy (XPS) techniques. The results show that CuCl2-modified MCA exhibited good ability for Hg0 adsorption in the 40–160 °C temperature range, with Hg0 removal efficiency all above 95.0%. The Hg0 removal efficiency first increased and then decreased with the elevation of reaction temperature. It performed optimally at 80 °C with the highest Hg0 removal efficiency of 98.7%. XPS results indicate that covalent chlorines (C-Cl groups) play an important role in elemental mercury adsorption process. Hg0 is firstly captured in the form of oxidized mercury (Hg2+) and then reacts with C-Cl groups to form HgCl2. Full article
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Open AccessArticle Measuring Inner Layer Capacitance with the Colloidal Probe Technique
Colloids Interfaces 2018, 2(4), 65; https://doi.org/10.3390/colloids2040065
Received: 26 October 2018 / Revised: 21 November 2018 / Accepted: 22 November 2018 / Published: 27 November 2018
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Abstract
The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were [...] Read more.
The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica. Full article
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Open AccessArticle Rheology of Water-in-Crude Oil Emulsions: Influence of Concentration and Temperature
Colloids Interfaces 2018, 2(4), 64; https://doi.org/10.3390/colloids2040064
Received: 29 September 2018 / Revised: 22 November 2018 / Accepted: 22 November 2018 / Published: 26 November 2018
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Abstract
The effect of aqueous phase content and temperature North Sea crude oil emulsion viscoelastic behavior has been studied. Heavy crude oil from the North Sea is of high viscosity and is capable of forming stable water-in-crude oil (w/o) emulsions without introducing any synthetic [...] Read more.
The effect of aqueous phase content and temperature North Sea crude oil emulsion viscoelastic behavior has been studied. Heavy crude oil from the North Sea is of high viscosity and is capable of forming stable water-in-crude oil (w/o) emulsions without introducing any synthetic surfactants. The aqueous phase volume content was varied from 1 to 40%, and the temperature was varied from 0 to 30 °C. The w/o emulsion viscosity increased sharply when the aqueous phase content exceeds 20%, being more pronounced at the lower temperatures. The viscosity flow curves for emulsions containing more than 20% aqueous phase demonstrate non-Newtonian behavior, in contrast to crude oil, which is Newtonian. The coefficients in the master curve describing the viscosity-temperature dependence were determined. Oscillatory rheological tests showed that the loss modulus substantially exceeds the storage modulus which indicates the liquid-like state of the emulsions. Full article
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Open AccessArticle Production of a Biosurfactant by Cunninghamella echinulata Using Renewable Substrates and Its Applications in Enhanced Oil Spill Recovery
Colloids Interfaces 2018, 2(4), 63; https://doi.org/10.3390/colloids2040063
Received: 1 September 2018 / Revised: 5 November 2018 / Accepted: 14 November 2018 / Published: 24 November 2018
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Abstract
This study aimed to evaluate the production of a surfactant by Cunninghamella echinulata, using agro-industrial residues, corn steep liquor (CSL), and soybean oil waste (SOW). The study had a factorial design, using as a variable response to the reduction of surface tension. [...] Read more.
This study aimed to evaluate the production of a surfactant by Cunninghamella echinulata, using agro-industrial residues, corn steep liquor (CSL), and soybean oil waste (SOW). The study had a factorial design, using as a variable response to the reduction of surface tension. C. echinulata was able to produce biosurfactant in assay, CSL (8.82%) and SOW (2%). The results showed that the biosurfactant was successfully produced by C. echinulata and had attractive properties, such as a low surface tension (31.7 mN/m), a yield of 5.18 g/L at 120 h of cultivation, and an anionic profile. It also achieved a reduction in surface tension stability in a wide range of pH values, temperatures, and salinity values. The biosurfactant produced by C. echinulata showed an absence of toxicity to Artemia salina. The influence of the biosurfactant on the viscosity of engine oil, burnt engine oil, diesel, soybean oil post-frying, canola oil, and water was investigated. The results reveal a mechanism for the decrease of the viscosity using hydrophobic substrates and the new biosurfactant solution at 1.5% of the (CMC). This enables the formulation of a low-cost culture medium alternative, based on corn steep liquor and the reuse of soybean oil after frying to produce a biosurfactant. Additionally, performance of the biosurfactant isolated from C. echinulata showed an excellent ability to remove spilled oil, such as diesel (98.7%) and kerosene (92.3%) from marine sand. Full article
(This article belongs to the Special Issue Biosurfactants: Trends and Applications)
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Open AccessArticle Low Pressure Hysteresis in Materials with Narrow Slit Pores
Colloids Interfaces 2018, 2(4), 62; https://doi.org/10.3390/colloids2040062
Received: 26 September 2018 / Revised: 11 November 2018 / Accepted: 12 November 2018 / Published: 16 November 2018
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Abstract
Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy [...] Read more.
Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy jumps by a finite value when touching slit walls are separated from each other. We consider a more realistic adhesion model by introducing a smoothly-varying adhesion force, which depends on the distance between the slit walls. The range of the attraction between the slit walls is found to have a pronounced influence on the shape of hysteresis loops at low vapor pressure. A large interaction range avoids an extraordinarily small relative vapor pressure necessary for pore closing, which is a precondition for low pressure hysteresis. Our extended model allows us to describe a discontinuity, which can appear in the desorption branch of swelling/shrinkage diagrams for hardened cement paste. Full article
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Open AccessFeature PaperArticle Water Vapor Adsorption by Some Manganese Oxide Forms
Colloids Interfaces 2018, 2(4), 61; https://doi.org/10.3390/colloids2040061
Received: 19 October 2018 / Revised: 12 November 2018 / Accepted: 14 November 2018 / Published: 15 November 2018
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Abstract
Manganese oxide forms prepared by different methods differ by their compositions, phase ratios in polyphase samples, and crystallite sizes (XRD and TEM characterization). Among the phases, tunnel-structured β-MnO2 (pyrolusite), α-MnO2 (cryptomelane), ε-MnO2 (akhtenskite), and β-Mn2O3 (bixbyite) have [...] Read more.
Manganese oxide forms prepared by different methods differ by their compositions, phase ratios in polyphase samples, and crystallite sizes (XRD and TEM characterization). Among the phases, tunnel-structured β-MnO2 (pyrolusite), α-MnO2 (cryptomelane), ε-MnO2 (akhtenskite), and β-Mn2O3 (bixbyite) have been identified. Water vapor sorption isotherms showed substantial differences in the affinities of water molecules to oxide surfaces of the manganese oxide forms under study. The parameters of the BET equation and pore size distribution curves have been calculated. The manganese oxide forms have mesoporous structures characterized by uniform and non-uniform pore sizes as well as by moderate hydrophilic behavior. Full article
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Open AccessArticle Anomalous Interfacial Structuring of a Non-Halogenated Ionic Liquid: Effect of Substrate and Temperature
Colloids Interfaces 2018, 2(4), 60; https://doi.org/10.3390/colloids2040060
Received: 19 October 2018 / Revised: 6 November 2018 / Accepted: 13 November 2018 / Published: 14 November 2018
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Abstract
We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, [...] Read more.
We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact. Full article
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Open AccessReview Synthesis and Properties of SBA-15 Modified with Non-Noble Metals
Colloids Interfaces 2018, 2(4), 59; https://doi.org/10.3390/colloids2040059
Received: 2 October 2018 / Revised: 5 November 2018 / Accepted: 12 November 2018 / Published: 14 November 2018
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Abstract
Modification of SBA-15 with non-noble metal leads to functional materials, which can be applied as gas sensors, adsorbents, and catalysts of various reactions. The new materials contain up to four various metals, which are deposited consecutively or simultaneously at various concentrations ranging from [...] Read more.
Modification of SBA-15 with non-noble metal leads to functional materials, which can be applied as gas sensors, adsorbents, and catalysts of various reactions. The new materials contain up to four various metals, which are deposited consecutively or simultaneously at various concentrations ranging from a fraction of 1% to an amount that is comparable with the mass of silica-support. These materials contain metals at various oxidation levels, usually as oxides, which occur in crystalline form (a typical crystallite size of about 10 nm matches the width of the SBA-15 channels), but in a few other materials, crystalline metal compounds have not been detected. Many researchers have provided detailed physico- chemical characteristics of SBA-15 modified with non-noble metals by the means of various microscopic and spectroscopic techniques. Full article
Open AccessArticle An Efficient Bioemulsifier-Producing Bacillus subtilis UCP 0146 Isolated from Mangrove Sediments
Colloids Interfaces 2018, 2(4), 58; https://doi.org/10.3390/colloids2040058
Received: 1 September 2018 / Revised: 24 October 2018 / Accepted: 6 November 2018 / Published: 13 November 2018
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Abstract
In this work, we investigated the potential of Bacillus subtilis UCP 0146 in the bioconversion of a medium containing 100% cassava flour wastewater to obtain a bioemulsifier. The evaluation of the production was carried out by the emulsification index (IE24) and [...] Read more.
In this work, we investigated the potential of Bacillus subtilis UCP 0146 in the bioconversion of a medium containing 100% cassava flour wastewater to obtain a bioemulsifier. The evaluation of the production was carried out by the emulsification index (IE24) and the surface tension (ST). The ionic charge, stability (temperature, salinity, and pH measured by IE24 and viscosity), and ability to remove and disperse oil and textile dye were investigated. B. subtilis produced an anionic bioemulsifier in the medium containing 100% cassava wastewater under Condition 4 of the factorial design (inoculum 9% at a temperature of 35 °C and shaken at 100 rpm), and showed a surface tension of 39 mN/m, an IE24 of 95.2%, and a yield of 2.69 g·L−1. The bioemulsifier showed stability at different pH (2–8), temperatures (0–120 °C), and NaCl concentrations, a dispersion oil displacement area (ODA) test of 55.83 cm2, and a reduction of the viscosity of the burned engine oil (90.5 Cp). The bioemulsifier was able to remove petroleum (94.4%) and methylene blue azo dye (62.2%). The bioemulsifier and its synthesis from bacteria also emphasizes the role of surfactants in oil remediation. Full article
(This article belongs to the Special Issue Biosurfactants: Trends and Applications)
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Open AccessFeature PaperArticle Effect of Amplitude on the Surface Dilational Visco-Elasticity of Protein Solutions
Colloids Interfaces 2018, 2(4), 57; https://doi.org/10.3390/colloids2040057
Received: 17 October 2018 / Revised: 6 November 2018 / Accepted: 7 November 2018 / Published: 10 November 2018
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Abstract
Harmonic drop surface area oscillations are performed at a fixed frequency (0.1 Hz) to measure the dilational visco-elasticity for three proteins: β-casein (BCS), β-lactoglobulin (BLG), and human serum albumin (HSA). The surface area oscillations were performed with different amplitudes in order to find [...] Read more.
Harmonic drop surface area oscillations are performed at a fixed frequency (0.1 Hz) to measure the dilational visco-elasticity for three proteins: β-casein (BCS), β-lactoglobulin (BLG), and human serum albumin (HSA). The surface area oscillations were performed with different amplitudes in order to find the origin of non-linearity effects. The analysis of data shows that the non-linearity in the equation of state—i.e., the relation between surface pressure and surface concentration of adsorbed protein molecules—is the main source of the amplitude effects on the apparent visco-elasticity, while perturbations due to non-uniform expansions and compressions of the surface layer, inertia effects leading to deviations of the drop profile from the Laplacian shape, or convective transport in the drop bulk are of less importance. While for the globular proteins, HSA and BLG the amplitude effects on the apparent visco-elasticity are rather large, for the non-globular protein BCS this effect is negligible in the studied range of up to 10% area deformation. Full article
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Open AccessReview Sessile Droplets on Deformable Substrates
Colloids Interfaces 2018, 2(4), 56; https://doi.org/10.3390/colloids2040056
Received: 20 September 2018 / Revised: 19 October 2018 / Accepted: 2 November 2018 / Published: 6 November 2018
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Abstract
Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet [...] Read more.
Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet and an elastic substrate. Researchers have developed several theories to explain the interaction between the two phases and describe the process of wetting of deformable/soft substrates. A summary of the most recent advances on static wetting of deformable substrates is given in this review. It is demonstrated that action of surface forces (disjoining/conjoining pressure) near the apparent three-phase contact line should be considered. Any consideration of equilibrium droplets on deformable (as well as on non-deformable) substrates should be based on consideration of the excess free energy of the system. The equilibrium shapes of both droplet and deformable substrate should correspond to the minimum of the excess free energy of the system. It has never been considered in the literature that the obtained equilibrium profiles must satisfy sufficient Jacobi’s condition. If Jacobi’s condition is not satisfied, it is impossible to claim that the obtained solution really corresponds to equilibrium. In recently published studies, equilibrium of droplets on deformable substrates: (1) provided a solution that corresponds to the minimum of the excess free energy; and (2) the obtained solution satisfies the Jacobi’s condition. Based on consideration of disjoining/conjoining pressure acting in the vicinity of the apparent three-phase contact line, the hysteresis of contact angle of sessile droplets on deformable substrates is considered. It is shown that both advancing and receding contact angles decrease as the elasticity of the substrate is increased and the effect of disjoining/conjoining pressure is discussed. Fluid inside the droplet partially wets the deformable substrate. It is shown that just these forces coupled with the surface elasticity determine the deformation of the deformable substrates. Full article
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Open AccessArticle Rubber Surface Change and Static Charging under Periodic Stress
Colloids Interfaces 2018, 2(4), 55; https://doi.org/10.3390/colloids2040055
Received: 28 September 2018 / Revised: 27 October 2018 / Accepted: 2 November 2018 / Published: 6 November 2018
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Abstract
Rubber materials play an important role in robotics, due to their sensing and actuating abilities, that are exploited in soft smart materials endowed with shape-adaptive and electroadhesive properties. The application of an electric field produces non-linear deformation that has been extensively modelled, but [...] Read more.
Rubber materials play an important role in robotics, due to their sensing and actuating abilities, that are exploited in soft smart materials endowed with shape-adaptive and electroadhesive properties. The application of an electric field produces non-linear deformation that has been extensively modelled, but is not understood at the molecular level. The symmetric effect (the production of an electric field due to rubber deformation) was recently discovered and explained as follows: rubber surface chemical composition and adsorptive properties change during rubber deformation, allowing the surface to exchange charge with the atmosphere. The present work describes the complex surface morphology and microchemistry of tubing made from vulcanized natural rubber, showing that it is rough and made from two domain types: stiffer elevations containing Br or Al (depending on the sample used) and O, that rise above an elastic base that is exempt of elements other than C and H. The surface area fraction occupied by the elastic base is higher in the strained rubber than when it is relaxed. Electrostatic potential on rubber surfaces was measured as a function of the stretching frequency, using Kelvin electrodes and showing frequency-dependent potential variation. This is explained considering charge exchange between the atmosphere and rubber surface, mediated by water vapor adsorbed in the stretched rubber and trapped when it relaxes. Full article
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Open AccessAddendum Addendum: Taylor, S.E., et al. Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface. Colloids Interfaces 2018, 2, 40.
Colloids Interfaces 2018, 2(4), 54; https://doi.org/10.3390/colloids2040054
Received: 25 October 2018 / Accepted: 2 November 2018 / Published: 6 November 2018
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Abstract
In the article recently published in Colloids and Interfaces [...] Full article
(This article belongs to the Special Issue Colloids and Interfaces in Oil Recovery)
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Open AccessFeature PaperArticle Dynamic Properties of Mixed Cationic/Nonionic Adsorbed Layers at the N-Hexane/Water Interface: Capillary Pressure Experiments Under Low Gravity Conditions
Colloids Interfaces 2018, 2(4), 53; https://doi.org/10.3390/colloids2040053
Received: 4 October 2018 / Revised: 26 October 2018 / Accepted: 29 October 2018 / Published: 2 November 2018
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Abstract
Capillary pressure experiments are performed in microgravity conditions on board the International Space Station to quantify the dynamic interfacial behavior of mixed adsorption layers of TTAB and C13DMPO at the water/hexane interface. While the non-ionic surfactant C13DMPO is soluble [...] Read more.
Capillary pressure experiments are performed in microgravity conditions on board the International Space Station to quantify the dynamic interfacial behavior of mixed adsorption layers of TTAB and C13DMPO at the water/hexane interface. While the non-ionic surfactant C13DMPO is soluble in both bulk phases, water and hexane, the cationic surfactant TTAB is only soluble in the aqueous phase. The interfacial layer is thus formed by TTAB molecules adsorbing from the aqueous phase while the C13DMPO molecules adsorb from the aqueous phase, and transfer partially into the hexane phase until both the equilibrium of adsorption and the distribution between the two adjacent liquid phases is established. The experimental constrains as well as all possible influencing parameters, such as interfacial and bulk phase compressibility, interfacial curvature, calibration of pressure and absolute geometry size, are discussed in detail. The experimental results in terms of the dilational interfacial viscoelasticity of the mixed adsorption layers in a wide range of oscillation frequencies show that the existing theoretical background had to be extended in order to consider the effect of transfer of the non-ionic surfactant across the interface, and the curvature of the water/hexane interface. A good qualitative agreement between theory and experiment was obtained, however, for a quantitative comparison, additional accurate information on the adsorption isotherms and diffusion coefficients of the two studied surfactants in water and hexane, alone and in a mixed system, are required. Full article
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Open AccessArticle Photoluminescence Property of Eu3+ doped CaSiO3 Nano-phosphor with Controlled Grain Size
Colloids Interfaces 2018, 2(4), 52; https://doi.org/10.3390/colloids2040052
Received: 18 September 2018 / Revised: 26 October 2018 / Accepted: 26 October 2018 / Published: 28 October 2018
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Abstract
A series of Eu3+ doped CaSiO3/SiO2 nano-phosphor powder of controlled grain size, crystalline structure, and chemical composition were synthesized using the microemulsion technique. The morphology, size, and shape of the synthesized nanophosphorous powder were investigated using transmission electron microscopy [...] Read more.
A series of Eu3+ doped CaSiO3/SiO2 nano-phosphor powder of controlled grain size, crystalline structure, and chemical composition were synthesized using the microemulsion technique. The morphology, size, and shape of the synthesized nanophosphorous powder were investigated using transmission electron microscopy and X-ray diffraction (XRD) analysis. XRD profiles of samples sintered over 600 °C, suggested phase shift from amorphous powder grain to more ordered polycrystalline powder of triclinic type wollastonite, CaSiO3, with preferred crystal phase orientation of (112) and tetragonal type cristobalites of SiO2. The grain size, crystallinity, and chemical composition of the host matrix, activator and sensitizer strongly affected both the absorption and emission bands of these samples. The amplitude of both the orange and red emission bands significantly increased with sintering temperature. The emission band is red-shifted with decreasing grain sizes. These bands displayed good sensitivity to ionic concentration of the Si4+, Ca2+, and Eu3+. With increasing Ca2+ ion concentration both the intensity of the red photoluminescence (PL) band increased and a concentration quenching observed. Increase in Si4+ ion concentration led to quenching in PL intensity of both the orange and red bands, whereas the amplitude of the blue-band slightly increased. With increasing Eu3+ ion concentration the red-band initially increased whereas it started decreasing at higher sample concentration. In the presence of Ca2+ ion as a sensitizer, the sample showed a remarkable PL property-including–about 100% photon conversion efficiency and a two-fold increase in excitation and emission photons. Full article
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Open AccessArticle Failure of Debye-Hückel Screening in Low-Charge Colloidal Suspensions
Colloids Interfaces 2018, 2(4), 51; https://doi.org/10.3390/colloids2040051
Received: 30 September 2018 / Revised: 17 October 2018 / Accepted: 18 October 2018 / Published: 22 October 2018
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Abstract
Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone of colloid stability. Electrostatic interactions dominate van der Waals attractions at large colloid-colloid separations h, unless strongly screened. Under these conditions, the potential U(h) between charged colloids is expected to be exponentially screened, [...] Read more.
Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone of colloid stability. Electrostatic interactions dominate van der Waals attractions at large colloid-colloid separations h, unless strongly screened. Under these conditions, the potential U ( h ) between charged colloids is expected to be exponentially screened, U ( h ) exp ( κ h ) / h , with κ 1 = λ D where λ D is the classical Debye-Hückel screening length. By measuring the force between individual charged particles at dilute electrolyte concentrations (<mM) using optical tweezers, we tested experimentally the prediction κ 1 = λ D in a nonpolar solvent. At low salt concentrations, we found close agreement between the directly-measured decay length κ 1 and Debye-Hückel predictions. However, above a critical electrolyte concentration (≈450 μ M), we obtained significant discrepancies between measured and predicted screening lengths, with κ 1 λ D . In marked contrast to expectations, we found that the measured screening length κ 1 appears to grow as the ionic strength of the solution is increased. The origin of this discrepancy is discussed and the importance of considering the surface is highlighted. Full article
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Open AccessArticle Nanoparticle-Lipid Interaction: Job Scattering Plots to Differentiate Vesicle Aggregation from Supported Lipid Bilayer Formation
Colloids Interfaces 2018, 2(4), 50; https://doi.org/10.3390/colloids2040050
Received: 20 September 2018 / Revised: 13 October 2018 / Accepted: 17 October 2018 / Published: 22 October 2018
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Abstract
The impact of nanomaterials on lung fluids, or on the plasma membrane of living cells, has prompted researchers to examine the interactions between nanoparticles and lipid vesicles. Recent studies have shown that nanoparticle-lipid interaction leads to a broad range of structures including supported [...] Read more.
The impact of nanomaterials on lung fluids, or on the plasma membrane of living cells, has prompted researchers to examine the interactions between nanoparticles and lipid vesicles. Recent studies have shown that nanoparticle-lipid interaction leads to a broad range of structures including supported lipid bilayers (SLB), particles adsorbed at the surface or internalized inside vesicles, and mixed aggregates. Currently, there is a need to have simple protocols that can readily evaluate the structures made from particles and vesicles. Here we apply the method of continuous variation for measuring Job scattering plots and provide analytical expressions for the scattering intensity in various scenarios. The result that emerges from the comparison between experiments and modeling is that electrostatics play a key role in the association, but it is not sufficient to induce the formation of supported lipid bilayers. Full article
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Open AccessArticle Culture Medium Optimization for Production of Rhamnolipids by Burkholderia glumae
Colloids Interfaces 2018, 2(4), 49; https://doi.org/10.3390/colloids2040049
Received: 6 September 2018 / Revised: 10 October 2018 / Accepted: 15 October 2018 / Published: 18 October 2018
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Abstract
Burkholderia glumae is a biosafety level 1 bacterium capable of producing rhamnolipid biosurfactant with longer 3-hydroxy fatty acid chains moieties than those produced by the prototypal producer, the opportunistic pathogen Pseudomonas aeruginosa. Although the capacity of production of rhamnolipid, and the parameters [...] Read more.
Burkholderia glumae is a biosafety level 1 bacterium capable of producing rhamnolipid biosurfactant with longer 3-hydroxy fatty acid chains moieties than those produced by the prototypal producer, the opportunistic pathogen Pseudomonas aeruginosa. Although the capacity of production of rhamnolipid, and the parameters affecting this production, are well established for P. aeruginosa, little is known about the factors that may affect their production in B. glumae. Hence, to evaluate and enhance the production of rhamnolipids in B. glumae, following the selection of best carbon and nitrogen sources, a two-level fractional factorial design experiment was performed to identify the limiting factors significantly affecting the production of rhamnolipids in this bacterial species. Effects of six inorganic nutrients and two physical parameters were studied, and mannitol, urea, CaCl2, and potassium phosphate buffer were selected for further optimization by applying a response surface methodology (RSM). Under the identified optimized conditions, a rhamnolipid production of 1.66 g/L was obtained, about five times higher than that of the initial non-optimized conditions. This represents a key step in the development of large-scale production processes. Full article
(This article belongs to the Special Issue Biosurfactants: Trends and Applications)
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Open AccessArticle Rheological Properties of Alginate–Essential Oil Nanodispersions
Colloids Interfaces 2018, 2(4), 48; https://doi.org/10.3390/colloids2040048
Received: 27 September 2018 / Revised: 13 October 2018 / Accepted: 15 October 2018 / Published: 17 October 2018
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Abstract
Due to its favorable structural properties and biocompatibility, alginate is recognized as a suitable versatile biopolymer for use in a broad range of applications ranging from drug delivery, wound healing, tissue engineering, and food formulations such as nanodispersions. Rheological analysis plays a crucial [...] Read more.
Due to its favorable structural properties and biocompatibility, alginate is recognized as a suitable versatile biopolymer for use in a broad range of applications ranging from drug delivery, wound healing, tissue engineering, and food formulations such as nanodispersions. Rheological analysis plays a crucial role in the design of suitable nanoemulsion based coatings. Different essential oil and alginate nanodispersion compositions stabilized by Tween 80 were analyzed for rheological and conductometric properties. The results confirmed that the nanoformulations shared a pseudoplastic non-Newtonian behavior that was more evident with higher alginate concentrations (2%). Nanodispersions made of alginate and essential oil exhibited a slight thixotropic behavior, demonstrating the aptitude to instantaneously recover from the applied stress or strain. Oscillatory frequency sweep tests showed a similar fluid-like behavior for 1% and 2% alginate nanodispersions. Finally, it was demonstrated that advantages coming with the use of the essential oil are added to the positive aspects of alginate with no dramatic modification on the flow behavior. Full article
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Open AccessFeature PaperArticle Imparting Photo-responsive Function to Thermo-responsive Iridescent Emulsions
Colloids Interfaces 2018, 2(4), 47; https://doi.org/10.3390/colloids2040047
Received: 29 September 2018 / Revised: 11 October 2018 / Accepted: 11 October 2018 / Published: 13 October 2018
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Abstract
In our previous paper, we reported that thermo-responsive emulsions can be prepared based on a long-chain amidoamine derivative (C18AA) and tetraoctylammonium bromide (TOAB), and that the C18AA + TOAB emulsions developed a characteristic interference color in a narrow temperature range. However, the coloration [...] Read more.
In our previous paper, we reported that thermo-responsive emulsions can be prepared based on a long-chain amidoamine derivative (C18AA) and tetraoctylammonium bromide (TOAB), and that the C18AA + TOAB emulsions developed a characteristic interference color in a narrow temperature range. However, the coloration of the original C18AA + TOAB at room temperature exhibited poor brightness. In the present study, we show that the addition of NaOH is effective in both lowering the coloration temperature and improving the brightness of C18AA + TOAB emulsion considerably. Furthermore, we demonstrate that photo-response function can be imparted to C18AA + TOAB iridescent emulsions by introducing a photochromic naphthopyran derivative (Pyran) that reversibly changes from white to yellow upon UV irradiation. The C18AA + TOAB emulsions containing Pyran shows a dual stimuli-responsive iridescent property, and the emulsion color is controllable and reversible through both UV irradiation and temperature. Full article
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Open AccessArticle Tailoring Acrylated Soybean Oil-Containing Terpolymers through Emulsion Polymerization
Colloids Interfaces 2018, 2(4), 46; https://doi.org/10.3390/colloids2040046
Received: 9 September 2018 / Revised: 2 October 2018 / Accepted: 10 October 2018 / Published: 12 October 2018
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Abstract
This work focuses on the synthesis of terpolymers using methyl methacrylate (MMA) and vinyl pivalate (VPi), along with the incorporation of methacrylate acid (MA) and acrylated fatty acids (AFA) derived from commercial soybean oil. Emulsion polymerizations were carried out using different monomeric species, [...] Read more.
This work focuses on the synthesis of terpolymers using methyl methacrylate (MMA) and vinyl pivalate (VPi), along with the incorporation of methacrylate acid (MA) and acrylated fatty acids (AFA) derived from commercial soybean oil. Emulsion polymerizations were carried out using different monomeric species, along with different initiator concentrations ranging from 0.5 g∙L−1 to 2.2 g∙L−1. The thermal properties of the terpolymers were improved when acrylated fatty acid was incorporated into the polymer chains, expressing glass transition temperatures (Tg) ranging from 70 °C to 90 °C and degradation temperatures in the interval between 350 °C and 450 °C for acrylated fatty acid concentrations ranging from 5 wt% to 10 wt%. Furthermore, a change was noted in the molar mass distributions as a result of acrylated fatty oil present in the polymers. The materials with 5 and 10 wt% of acrylated fatty oil presented mass-average molar masses of 225 kg∙mol−1 and 181 kg∙mol−1, respectively. As the results in this work suggest, the molar masses of the formed polymers are significantly altered by the presence of modified fatty acids. Full article
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Open AccessArticle Hydrodynamic Parameters of Strelitzia Gum
Colloids Interfaces 2018, 2(4), 45; https://doi.org/10.3390/colloids2040045
Received: 29 August 2018 / Revised: 20 September 2018 / Accepted: 21 September 2018 / Published: 10 October 2018
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Abstract
The flower of Strelitzia reginae generates abundant and viscous mucilage as exudate, which is purified in periods of heating–cooling, and finally precipitated with ethanol, obtaining strelitzia gum (StrG). By means of intrinsic viscosity measurement, the viscometric molecular weight (MWv) is [...] Read more.
The flower of Strelitzia reginae generates abundant and viscous mucilage as exudate, which is purified in periods of heating–cooling, and finally precipitated with ethanol, obtaining strelitzia gum (StrG). By means of intrinsic viscosity measurement, the viscometric molecular weight (MWv) is determined, with a value of 200,000 g/mol, as well as a hydrodynamic radius of 20 ± 1 nm and a hydration value of 445 ± 34 g/g. The size of StrG was compared against dynamic light scattering data with a value of 16 ± 2 nm and a MWDLS of 230,000 g/mol. StrG is a biopolyelectrolyte with an “a” value of 0.85, which corresponds to a flexible behavior with a great effect of volume exclusion. This statement is based on the difficulty of gum dissolution, that should be performed at 80 °C. This macromolecule is very promising and can potentially be used in several industrial applications, such as in film forming, and as a gel, thickener, and coemulsifier. Full article
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