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Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed...
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Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt

A special issue of Liquids (ISSN 2673-8015).

Deadline for manuscript submissions: closed (30 November 2023) | Viewed by 28122

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Dr. William E. Acree, Jr.

E-Mail Website
Guest Editor
Department of Chemistry, University of North Texas, Denton, TX 76203, USA
Interests: chemical and solution thermodynamics; solute transfer properties; phase equilibria
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Franco Cataldo

E-Mail Website
Guest Editor
Actinium Chemical Research, Via Casilina 1626A, 00133 Rome, Italy
Interests: solvents properties and effects; plasticizers; solvatochromism; charge-transfer interactions

Special Issue Information

Dear Colleagues,

For more than a century, researchers have tried to understand and quantify the effects that solvent properties have on the solubility, chemical reactivity and spectral properties of dissolved solute molecules. Solvents play a vital role in commercial manufacturing processes and in our everyday lives. Solvents are routinely used (a) as a reaction medium in the preparation of new chemicals, medicinal compounds and chemical materials; (b) as an extraction and pre-concentration solvent for the removal of trace analytes from unknown chemical samples prior to identification and quantification via standard analytical methods; (c) as a mobile and/or stationary phase in chromatographic separations; (d) as solubilizing agents in the development of personal health-care and household cleaning products, paints and varnishes, and pharmaceutical formations; and (e) on the industrial scale: liquid–liquid solvent extraction is currently applied, for instance, in certain petrochemical fractionation steps of oil, while solid–liquid extraction is applied in vegetable oil extraction from a number of different seeds. Each of the aforementioned applications are largely governed by molecular interactions between dissolved chemicals and their surrounding solvent medium. The progress made in recent years in discovering how solvent polarity and hydrogen-bonding character influence reaction kinetics and solution behavior led to several of the modern conveniences that we enjoy today. These discoveries resulted from the outstanding contributions of prominent scientists, who are considered to be the leading experts in their respective research fields.

Prof. Dr. Christian Reichardt is a pioneer in solvatochromic probe studies—most notably for the development of the ET(30) and normalized ETN solvent polarity scales based on the UV-visible absorption spectrum of the zwitterionic 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinium)phenolate dye molecule (commonly referred to as Reichardt’s dye). To date, numerical ET(30) and ETN values have been determined for more than 400 different organic mono-solvents and ionic liquids and for many common binary-aqueous solvent mixtures. ET(30) values have been used to examine preferential solvation in binary solvent mixtures, to measure trace water concentrations in organic mono-solvents, to probe surface polarities of chemically functionalized silica particles, to examine interfacial regions of aqueous micelles formed by cationic surfactants, and as one of several empirical input parameters in the development of multiparameter quantitative structure–property relationships for the estimation of a specific solvent-dependent physicochemical property of a given solute molecule in a series of organic mono-solvents.

Liquids is pleased to be publishing an honorary Special Issue in honor of Prof. Dr. Christian Reichardt for his many outstanding contributions regarding solvatochromic probe molecules and their applications which furthered our understanding of molecular interactions. This Special Issue welcomes the submission of unpublished manuscripts of both original research and review articles involving spectroscopic probes used in (although not limited to) critical micelle and trace water determinations and in solvent polarity, preferential solvation, hydrogen bonding, microviscosity and solvent classification (e.g., the affinity and basicity scale or others) studies. Papers reporting quantum chemical calculations of donor–acceptor interactions, solvent classification and solute–solvent interactions are also welcomed.

Prof. Dr. William E. Acree, Jr.
Prof. Dr. Franco Cataldo
Guest Editors

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Keywords

  • solvents’ effects on the spectra of organic/inorganic compounds
  • solvatochromism
  • application of Reichardt's solvatochromic dye
  • parameters of solvents’ polarity
  • solvents’ classification (e.g., the affinity and basicity scale or others)
  • solute–solvent interactions
  • solvent effects in homogeneous chemical equilibria and reactions
  • deep eutectic solvents and ionic liquids
  • hydrogen-bonding effects
  • quantum chemical calculations of donor–acceptor interactions
  • quantum chemical calculations in solvents’ classification and solute–solvent interactions

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Published Papers (15 papers)

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Research

Jump to: Review, Other

17 pages, 1629 KiB  
Article
Conventional and Green Rubber Plasticizers Classified through Nile Red [E(NR)] and Reichardt’s Polarity Scale [ET(30)]
by Franco Cataldo
Liquids 2024, 4(2), 305-321; https://doi.org/10.3390/liquids4020015 - 31 Mar 2024
Viewed by 1092
Abstract
After a survey on polymer plasticization theories and conventional criteria to evaluate polymer–plasticizer compatibility through the solubility parameter, an attempt to create a polymer–plasticizer polarity scale through solvatochromic dyes has been made. Since Reichardt’s ET(30) dye is insoluble in rubber hydrocarbon [...] Read more.
After a survey on polymer plasticization theories and conventional criteria to evaluate polymer–plasticizer compatibility through the solubility parameter, an attempt to create a polymer–plasticizer polarity scale through solvatochromic dyes has been made. Since Reichardt’s ET(30) dye is insoluble in rubber hydrocarbon polymers like polyisoprene, polybutadiene and styrene–butadiene copolymers and is not useful for the evaluation of the hydrocarbons and ester plasticizers, the Nile Red solvatochromic dye was instead used extensively and successfully for this class of compounds. A total of 53 different compounds were evaluated with the Nile Red dye and wherever possible also with Reichardt’s ET(33) dye. A very good correlation was then found between the Nile Red scale E(NR) and Reichardt’s ET(30) scale for this class of compounds focusing on diene rubbers and their typical hydrocarbons and new ester plasticizers. Furthermore, the E(NR) scale also shows a reasonable correlation with the total solubility parameter calculated according to the Van Krevelen method. Based on the above results, some conclusion was made about the compatibility between the diene rubbers and the conventional plasticizers, as well as a new and green plasticizer proposed for the rubber compounds. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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17 pages, 7522 KiB  
Article
Exploring Solvation Properties of Protic Ionic Liquids by Employing Solvatochromic Dyes and Molecular Dynamics Simulation Analysis
by Stuart J. Brown, Andrew J. Christofferson, Calum J. Drummond, Qi Han and Tamar L. Greaves
Liquids 2024, 4(1), 288-304; https://doi.org/10.3390/liquids4010014 - 20 Mar 2024
Cited by 1 | Viewed by 1201
Abstract
Solvation properties are key for understanding the interactions between solvents and solutes, making them critical for optimizing chemical synthesis and biochemical applications. Designable solvents for targeted optimization of these end-uses could, therefore, play a big role in the future of the relevant industries. [...] Read more.
Solvation properties are key for understanding the interactions between solvents and solutes, making them critical for optimizing chemical synthesis and biochemical applications. Designable solvents for targeted optimization of these end-uses could, therefore, play a big role in the future of the relevant industries. The tailorable nature of protic ionic liquids (PILs) as designable solvents makes them ideal candidates. By alteration of their constituent structural groups, their solvation properties can be tuned as required. The solvation properties are determined by the polar and non-polar interactions of the PIL, but they remain relatively unknown for PILs as compared to aprotic ILs and their characterization is non-trivial. Here, we use solvatochromic dyes as probe molecules to investigate the solvation properties of nine previously uncharacterized alkyl- and dialkylammonium PILs. These properties include the Kamlet–Aboud–Taft (KAT) parameters: π* (dipolarity/polarizability), α (H-bond acidity) and β (H-bond basicity), along with the ET(30) scale (electrophilicity/polarizability). We then used molecular dynamics simulations to calculate the radial distribution functions (RDF) of 21 PILs, which were correlated to their solvation properties and liquid nanostructure. It was identified that the hydroxyl groups on the PIL cation increase α, π* and ET(30), and correspondingly increase the cation–anion distance in their RDF plots. The hydroxyl group, therefore, reduces the strength of the ionic interaction but increases the polarizability of the ions. An increase in the alkyl chain length on the cation led to a decrease in the distances between cations, while also increasing the β value. The effect of the anion on the PIL solvation properties was found to be variable, with the nitrate anion greatly increasing π*, α and anion–anion distances. The research presented herein advances the understanding of PIL structure–property relationships while also showcasing the complimentary use of molecular dynamics simulations and solvatochromic analysis together. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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10 pages, 4548 KiB  
Communication
The Photophysics of Diphenyl Polyenes Analyzed by Their Solvatochromism
by Javier Catalán and Henning Hopf
Liquids 2024, 4(1), 278-287; https://doi.org/10.3390/liquids4010013 - 13 Mar 2024
Viewed by 903
Abstract
The solvent-dependent intensity changes in the first UV/Vis absorption band of the three polyenes DPH, DPHb, and ttbP3 dissolved in a hydrocarbon solvent with temperature allow for the conclusion that, at temperatures above 233 K, the two phenyl groups of DPH are rotated [...] Read more.
The solvent-dependent intensity changes in the first UV/Vis absorption band of the three polyenes DPH, DPHb, and ttbP3 dissolved in a hydrocarbon solvent with temperature allow for the conclusion that, at temperatures above 233 K, the two phenyl groups of DPH are rotated out-of-plane to yield a non-coplanar molecular structure. This leads to the conclusion that DPH becomes increasingly less coplanar as the temperature rises above 233 K. When the phenyl groups rotate out-of-plane, the polarizability decreases, and the energy of the first electronic transition increases by an extra value. Therefore, below 233 K, the correlation lines between the absorption energy of the 0–0 component of the UV/Vis absorption band and the solvent polarizability, as measured by the SP values, show bilinear behavior. The unexpected behavior shown by DPH dissolved in tetrachloro- and dichloromethane is discussed. We dedicate this research as a tribute to the very important contribution to the solvent effect made by Prof. Christian Reichardt and also to his generous and altruistic scientific help that he has always shown. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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20 pages, 5794 KiB  
Article
Solvatochromic and Computational Study of Some Cycloimmonium Ylids
by Daniela Babusca, Andrei Vleoanga and Dana Ortansa Dorohoi
Liquids 2024, 4(1), 171-190; https://doi.org/10.3390/liquids4010009 - 12 Feb 2024
Viewed by 1523
Abstract
This article contains a comparative spectral analysis corroborated with the quantum mechanical computations of four cycloimmonium ylids. The spectral shift of the visible electronic absorption band of the studied molecules in 20 solvents with different empirical parameters is expressed by linear multi-parametric dependences [...] Read more.
This article contains a comparative spectral analysis corroborated with the quantum mechanical computations of four cycloimmonium ylids. The spectral shift of the visible electronic absorption band of the studied molecules in 20 solvents with different empirical parameters is expressed by linear multi-parametric dependences that emphasize the intramolecular charge transfer (ICT) process. The nature of molecular interactions and their contribution to the spectral shift of the visible ICT band of solutes are also established in this manuscript. The results of the statistical analysis are used to estimate the cycloimmonium ylids’ excited dipole moment by the variational method, using the hypothesis of McRae. The importance of the structure of both the heterocycle and carbanion substituents to the stability and reactivity of the studied cycloimmonium ylids is underlined by the quantum mechanical computations of the molecular descriptors. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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8 pages, 632 KiB  
Communication
Solvent Polarity/Polarizability Parameters: A Study of Catalan’s SPPN, Using Computationally Derived Molecular Properties, and Comparison with π* and ET(30)
by W. Earle Waghorne
Liquids 2024, 4(1), 163-170; https://doi.org/10.3390/liquids4010008 - 8 Feb 2024
Cited by 1 | Viewed by 1147
Abstract
Catalan’s SPPN, a measure of solvent polarity/polarizability has been analysed in terms of molecular properties derived from computational chemistry. The results show that SPPN correlates positively with the molecular dipole moment and quadrupolar amplitude and negatively with the molecular polarizability. [...] Read more.
Catalan’s SPPN, a measure of solvent polarity/polarizability has been analysed in terms of molecular properties derived from computational chemistry. The results show that SPPN correlates positively with the molecular dipole moment and quadrupolar amplitude and negatively with the molecular polarizability. These correlations are shared with Kamet and Taft’s π* and Reichardt and Dimroth’s ET(30). Thus, one can associate the solvent polarity with non-specific interactions involving the permanent charges on solvent molecules. It is also noted that the opposite correlations, all three parameters increasing with increasing solvent polarity but decreasing with increasing solvent polarizability, creates an ambiguity in their use, for example, in linear free energy relationships. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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15 pages, 13494 KiB  
Article
Use of DFT Calculations as a Tool for Designing New Solvatochromic Probes for Biological Applications
by Cynthia M. Dupureur
Liquids 2024, 4(1), 148-162; https://doi.org/10.3390/liquids4010007 - 4 Feb 2024
Viewed by 1416
Abstract
The intramolecular charge transfer behavior of push–pull dyes is the origin of their sensitivity to environment. Such compounds are of interest as probes for bioimaging and as biosensors to monitor cellular dynamics and molecular interactions. Those that are solvatochromic are of particular interest [...] Read more.
The intramolecular charge transfer behavior of push–pull dyes is the origin of their sensitivity to environment. Such compounds are of interest as probes for bioimaging and as biosensors to monitor cellular dynamics and molecular interactions. Those that are solvatochromic are of particular interest in studies of lipid dynamics and heterogeneity. The development of new solvatochromic probes has been driven largely by the need to tune desirable properties such as solubility, emission wavelength, or the targeting of a particular cellular structure. DFT calculations are often used to characterize these dyes. However, if a correlation between computed (dipole moment) and experimentally measured solvatochromic behavior can be established, they can also be used as a design tool that is accessible to students. Here, we examine this correlation and include case studies of the effects of probe modifications and conformation on dipole moments within families of solvatochromic probes. Indeed, the ground state dipole moment, an easily computed parameter, is correlated with experimental solvatochromic behavior and can be used in the design of new environment-sensitive probes before committing resources to synthesis. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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10 pages, 279 KiB  
Communication
Prediction of Paracetamol Solubility in Binary Solvents Using Reichardt’s Polarity Parameter Combined Model
by Elaheh Rahimpour and Abolghasem Jouyban
Liquids 2023, 3(4), 512-521; https://doi.org/10.3390/liquids3040032 - 14 Dec 2023
Viewed by 2236
Abstract
The objective of this research is to propose a general model utilizing the solvatochromic polarity of electronic transition energy (ET) of the Reichardt indicator to predict paracetamol solubility in the solvent mixtures. In order to model validation, the available ET (30) values of [...] Read more.
The objective of this research is to propose a general model utilizing the solvatochromic polarity of electronic transition energy (ET) of the Reichardt indicator to predict paracetamol solubility in the solvent mixtures. In order to model validation, the available ET (30) values of nine aqueous mixtures obtained from existing literature sources were utilized. The trained model yielded a relatively accurate estimation of paracetamol solubility in the investigated systems. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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9 pages, 1581 KiB  
Communication
Effective Recognition of Lithium Salt in (Choline Chloride: Glycerol) Deep Eutectic Solvent by Reichardt’s Betaine Dye 33
by Manish Kumar, Abhishek Kumar and Siddharth Pandey
Liquids 2023, 3(4), 393-401; https://doi.org/10.3390/liquids3040024 - 28 Sep 2023
Viewed by 1313
Abstract
Deep eutectic solvents (DESs) have emerged as novel alternatives to common solvents and VOCs. Their employment as electrolytes in batteries has been an area of intense research. In this context, understanding changes in the physicochemical properties of DESs in the presence of Li [...] Read more.
Deep eutectic solvents (DESs) have emerged as novel alternatives to common solvents and VOCs. Their employment as electrolytes in batteries has been an area of intense research. In this context, understanding changes in the physicochemical properties of DESs in the presence of Li salts becomes of utmost importance. Solvatochromic probes have the potential to gauge such changes. It is reported herein that one such UV–vis molecular absorbance probe, Reichardt’s betaine dye 33, effectively manifests changes taking place in a DES Glyceline composed of H-bond accepting salt choline chloride and H-bond donor glycerol in a 1:2 molar ratio, as salt LiCl is added. The lowest energy intramolecular charge–transfer absorbance band of this dye exhibits a 17 nm hypsochromic shift as up to 3.0 molal LiCl is added to Glyceline. The estimated ETN parameter shows a linear increase with the LiCl mole fraction. Spectroscopic responses of betaine dye 33, N,N-diethyl-4-nitroaniline and 4-nitroaniline are used to assess empirical Kamlet–Taft parameters of dipolarity/polarizability (π*), H-bond-donating acidity (α) and H-bond-accepting basicity (β) as a function of LiCl concentration in Glyceline. LiCl addition to Glyceline results in an increase in α and no change in π* and β. It is proposed that the added lithium interacts with the oxygen of the –OH functionalities on the glycerol rendering of the solvent with increased H-bond-donating acidity. It is observed that pyrene, a popular fluorescence probe of solvent polarity, does respond to the addition of LiCl to Glyceline, however, the change in pyrene response starts to become noticeable only at higher LiCl concentrations (mLiCl ≥ 1.5 m). Reichardt’s betaine dye is found to be highly sensitive and versatile in gauging the physicochemical properties of DESs in the presence of LiCl. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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11 pages, 1356 KiB  
Article
Reichardt’s Dye-Based Solvent Polarity and Abraham Solvent Parameters: Examining Correlations and Predictive Modeling
by William E. Acree, Jr. and Andrew S. I. D. Lang
Liquids 2023, 3(3), 303-313; https://doi.org/10.3390/liquids3030020 - 2 Aug 2023
Cited by 4 | Viewed by 2286
Abstract
The concept of “solvent polarity” is widely used to explain the effects of using different solvents in various scientific applications. However, a consensus regarding its definition and quantitative measure is still lacking, hindering progress in solvent-based research. This study hopes to add to [...] Read more.
The concept of “solvent polarity” is widely used to explain the effects of using different solvents in various scientific applications. However, a consensus regarding its definition and quantitative measure is still lacking, hindering progress in solvent-based research. This study hopes to add to the conversation by presenting the development of two linear regression models for solvent polarity, based on Reichardt’s ET(30) solvent polarity scale, using Abraham solvent parameters and a transformer-based model for predicting solvent polarity directly from molecular structure. The first linear model incorporates the standard Abraham solvent descriptors s, a, b, and the extended model ionic descriptors j+ and j−, achieving impressive test-set statistics of R2 = 0.940 (coefficient of determination), MAE = 0.037 (mean absolute error), and RMSE = 0.050 (Root-Mean-Square Error). The second model, covering a more extensive chemical space but only using the descriptors s, a, and b, achieves test-set statistics of R2 = 0.842, MAE = 0.085, and RMSE = 0.104. The transformer-based model, applicable to any solvent with an associated SMILES string, achieves test-set statistics of R2 = 0.824, MAE = 0.066, and RMSE = 0.095. Our findings highlight the significance of Abraham solvent parameters, especially the dipolarity/polarizability, hydrogen-bond acidity/basicity, and ionic descriptors, in predicting solvent polarity. These models offer valuable insights for researchers interested in Reichardt’s ET(30) solvent polarity parameter and solvent polarity in general. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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Review

Jump to: Research, Other

30 pages, 11132 KiB  
Review
Solvent Replacement Strategies for Processing Pharmaceuticals and Bio-Related Compounds—A Review
by Jia Lin Lee, Gun Hean Chong, Masaki Ota, Haixin Guo and Richard Lee Smith, Jr.
Liquids 2024, 4(2), 352-381; https://doi.org/10.3390/liquids4020018 - 9 Apr 2024
Cited by 1 | Viewed by 2912
Abstract
An overview of solvent replacement strategies shows that there is great progress in green chemistry for replacing hazardous di-polar aprotic solvents, such as N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), and 1,4-dioxane (DI), used in processing active industrial ingredients (APIs). In synthetic chemistry, alcohols, carbonates, ethers, [...] Read more.
An overview of solvent replacement strategies shows that there is great progress in green chemistry for replacing hazardous di-polar aprotic solvents, such as N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), and 1,4-dioxane (DI), used in processing active industrial ingredients (APIs). In synthetic chemistry, alcohols, carbonates, ethers, eucalyptol, glycols, furans, ketones, cycloalkanones, lactones, pyrrolidinone or solvent mixtures, 2-methyl tetrahydrofuran in methanol, HCl in cyclopentyl methyl ether, or trifluoroacetic acid in propylene carbonate or surfactant water (no organic solvents) are suggested replacement solvents. For the replacement of dichloromethane (DCM) used in chromatography, ethyl acetate ethanol or 2-propanol in heptanes, with or without acetic acid or ammonium hydroxide additives, are suggested, along with methanol acetic acid in ethyl acetate or methyl tert-butyl ether, ethyl acetate in ethanol in cyclohexane, CO2-ethyl acetate, CO2-methanol, CO2-acetone, and CO2-isopropanol. Supercritical CO2 (scCO2) can be used to replace many organic solvents used in processing materials from natural sources. Vegetable, drupe, legume, and seed oils used as co-extractants (mixed with substrate before extraction) can be used to replace the typical organic co-solvents (ethanol, acetone) used in scCO2 extraction. Mixed solvents consisting of a hydrogen bond donor (HBD) solvent and a hydrogen bond acceptor (HBA) are not addressed in GSK or CHEM21 solvent replacement guides. Published data for 100 water-soluble and water-insoluble APIs in mono-solvents show polarity ranges appropriate for the processing of APIs with mixed solvents. When water is used, possible HBA candidate solvents are acetone, acetic acid, acetonitrile, ethanol, methanol, 2-methyl tetrahydrofuran, 2,2,5,5-tetramethyloxolane, dimethylisosorbide, Cyrene, Cygnet 0.0, or diformylxylose. When alcohol is used, possible HBA candidates are cyclopentanone, esters, lactone, eucalytol, MeSesamol, or diformylxylose. HBA—HBA mixed solvents, such as Cyrene—Cygnet 0.0, could provide interesting new combinations. Solubility parameters, Reichardt polarity, Kamlet—Taft parameters, and linear solvation energy relationships provide practical ways for identifying mixed solvents applicable to API systems. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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40 pages, 2522 KiB  
Review
Polarity of Organic Solvent/Water Mixtures Measured with Reichardt’s B30 and Related Solvatochromic Probes—A Critical Review
by Stefan Spange
Liquids 2024, 4(1), 191-230; https://doi.org/10.3390/liquids4010010 - 17 Feb 2024
Cited by 1 | Viewed by 2094
Abstract
The UV/Vis absorption energies (νmax) of different solvatochromic probes measured in co-solvent/water mixtures are re-analyzed as a function of the average molar concentration (Nav) of the solvent composition compared to the use of the mole fraction. The [...] Read more.
The UV/Vis absorption energies (νmax) of different solvatochromic probes measured in co-solvent/water mixtures are re-analyzed as a function of the average molar concentration (Nav) of the solvent composition compared to the use of the mole fraction. The empirical ET(30) parameter of Reichardt’s dye B30 is the focus of the analysis. The Marcus classification of aqueous solvent mixtures is a useful guide for co-solvent selection. Methanol, ethanol, 1,2-ethanediol, 2-propanol, 2-methyl-2-propanol, 2-butoxyethanol, formamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF), N-formylmorpholine (NFM), 1,4-dioxane and DMSO were considered as co-solvents. The ET(30) values of the binary solvent mixtures are discussed in relation to the physical properties of the co-solvent/water mixtures in terms of quantitative composition, refractive index, thermodynamics of the mixture and the non-uniformity of the mixture. Significant linear dependencies of ET(30) as a function of Nav can be demonstrated for formamide/water, 1,2-ethanediol/water, NMF/water and DMSO/water mixtures over the entire compositional range. These mixtures belong to the group of solvents that do not enhance the water structure according to the Marcus classification. The influence of the solvent microstructure on the non-linearity ET(30) as a function of Nav is particularly clear for alcohol/water mixtures with an enhanced water structure. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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22 pages, 8945 KiB  
Review
Solvatochromism in Solvent Mixtures: A Practical Solution for a Complex Problem
by Omar A. El Seoud, Shirley Possidonio and Naved I. Malek
Liquids 2024, 4(1), 73-94; https://doi.org/10.3390/liquids4010003 - 3 Jan 2024
Cited by 2 | Viewed by 1915
Abstract
Many reactions are carried out in solvent mixtures, mainly because of practical reasons. For example, E2 eliminations are favored over SN2 substitutions in aqueous organic solvents because the bases are desolvated. This example raises the question: how do we chose binary [...] Read more.
Many reactions are carried out in solvent mixtures, mainly because of practical reasons. For example, E2 eliminations are favored over SN2 substitutions in aqueous organic solvents because the bases are desolvated. This example raises the question: how do we chose binary solvents to favor reaction outcomes? This important question is deceptively simple because it requires that we understand the details of all interactions within the system. Solvatochromism (solvent-dependent color change of a substance) has contributed a great deal to answer this difficult question, because it gives information on the interactions between solvents, solute-solvent, and presumably transition state-solvent. This wealth of information is achieved by simple spectroscopic measurements of selected (solvatochromic) substances, or probes. An important outcome of solvatochromism is that the probe solvation layer composition is almost always different from that of bulk mixed solvent. In principle, this difference can be exploited to “tune” the composition of solvent mixture to favor the reaction outcome. This minireview addresses the use of solvatochromic probes to quantify solute-solvent interactions, leading to a better understanding of the complex effects of solvent mixtures on chemical phenomena. Because of their extensive use in chemistry, we focus on binary mixtures containing protic-, and protic-dipolar aprotic solvents. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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31 pages, 7153 KiB  
Review
How the Concept of Solvent Polarity Investigated with Solvatochromic Probes Helps Studying Intermolecular Interactions
by Heinz Langhals
Liquids 2023, 3(4), 481-511; https://doi.org/10.3390/liquids3040031 - 6 Dec 2023
Cited by 3 | Viewed by 2872
Abstract
Intermolecular interactions form the basis of the properties of solvents, such as their polarity, and are of central importance for chemistry; such interactions are widely discussed. Solvent effects were reported on the basis of various polarity probes with the ET(30) polarity [...] Read more.
Intermolecular interactions form the basis of the properties of solvents, such as their polarity, and are of central importance for chemistry; such interactions are widely discussed. Solvent effects were reported on the basis of various polarity probes with the ET(30) polarity scale of Dimroth and Reichardt being of special interest because of its sensitivity, precise measurability and other advantages, and has been used for the investigation of solvent interactions. A two-parameter equation for the concentration dependence of medium effects has been developed, providing insights into structural changes in liquid phases. Moving from condensed gases to binary solvent mixtures, where the property of one solvent can be continuously transformed to the other, it was shown how the polarity of a solvent can be composed from the effect of polar functional groups and other structural elements that form the matrix. Thermochromism was discussed as well as the effect of very long-range interactions. Practical applications were demonstrated. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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57 pages, 15000 KiB  
Review
Solvatochromic and Acid–Base Molecular Probes in Surfactant Micelles: Comparison of Molecular Dynamics Simulation with the Experiment
by Nikolay O. Mchedlov-Petrossyan, Vladimir S. Farafonov and Alexander V. Lebed
Liquids 2023, 3(3), 314-370; https://doi.org/10.3390/liquids3030021 - 16 Aug 2023
Cited by 1 | Viewed by 1832
Abstract
This article summarizes a series of seventeen publications by the authors devoted to molecular dynamics modeling of various indicator dyes (molecular probes) enclosed in surfactant micelles. These dyes serve as generally recognized tools for studying various types of organized solutions, among which surfactant [...] Read more.
This article summarizes a series of seventeen publications by the authors devoted to molecular dynamics modeling of various indicator dyes (molecular probes) enclosed in surfactant micelles. These dyes serve as generally recognized tools for studying various types of organized solutions, among which surfactant micelles in water are the simplest and most explored. The modeling procedure involves altogether 50 to 95 surfactant molecules, 16 to 28 thousand water molecules, and a single dye molecule. The presentation of the simulation results was preceded by a brief review of the state of experimental studies. This article consists of three parts. First, despite numerous literature data devoted to modeling the micelles itself, we decided to revisit this issue. The structure and hydration of the surface of micelles of surfactants, first of all of sodium n-dodecylsulfate, SDS, and cetyltrimethylammonium bromide, CTAB, were studied. The values of the electrical potential, Ψ, were estimated as functions of the ionic strength and distance from the surface. The decrease in the Ψ value with distance is gradual. Attempts to consider both DS− and CTA+ micelles in water without counterions result in a decay into two smaller aggregates. Obviously, the hydrophobic interaction (association) of the hydrocarbon tails balances the repulsion of the charged headgroups of these small “bare” micelles. The second part is devoted to the study of seven pyridinium N-phenolates, known as Reichardt’s dyes, in ionic micelles. These most powerful solvatochromic indicators are now used for examining various colloidal systems. The localization and orientation of both zwitterionic and (colorless) cationic forms are generally consistent with intuitive ideas about the hydrophobicity of substituents. Hydration has been quantitatively described for both the dye molecule as a whole and the oxygen atom. A number of markers, including the visible absorption spectra of Reichardt’s dyes, enable assuming a better hydration of the micellar surface of SDS than that of CTAB. However, our data show that it is more correct to speak about the more pronounced hydrogen-bonding ability of water molecules in anionic micelles than about better hydration of the SDS micelles as compared to CTAB ones. Finally, a set of acid–base indicators firmly fixed in the micellar pseudophase were studied by molecular dynamics. They are instruments for estimating electrostatic potentials of micelles and related aggregates as Ψ= 2.303RTF−1 (pKai − pKaapp), where pKai and pKaapp are indices of so-called intrinsic and apparent dissociation constants. In this case, in addition to the location, orientation, and hydration, the differences between values of pKaapp and indices of the dissociation constants in water were estimated. Only a semi-quantitative agreement with the experimental data was obtained. However, the differences between pKaapp of a given indicator in two micellar solutions do much better agree with the experimental data. Accordingly, the experimental Ψ values of ionic micelles, as determined using the pKaapp in nonionic micelles as pKai, are reproduced with reasonable accuracy for the corresponding indicator. However, following the experimental data, a scatter of the Ψ values obtained with different indicators for given micelles is observed. This problem may be the subject of further research. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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Jump to: Research, Review

10 pages, 698 KiB  
Perspective
Polarity of Aqueous Solutions
by Pedro P. Madeira, Luisa A. Ferreira, Vladimir N. Uversky and Boris Y. Zaslavsky
Liquids 2024, 4(1), 107-116; https://doi.org/10.3390/liquids4010005 - 12 Jan 2024
Cited by 1 | Viewed by 1635
Abstract
This short review describes the expansion of the solvatochromic approach utilizing water-soluble solvatochromic dyes to the analysis of solvent features of aqueous media in solutions of various compounds. These solvent features (polarity/dipolarity, hydrogen bond donor ability (HBD acidity), and hydrogen bond acceptor ability [...] Read more.
This short review describes the expansion of the solvatochromic approach utilizing water-soluble solvatochromic dyes to the analysis of solvent features of aqueous media in solutions of various compounds. These solvent features (polarity/dipolarity, hydrogen bond donor ability (HBD acidity), and hydrogen bond acceptor ability (HBA basicity)) vary depending on the nature and concentration of a solute. Furthermore, the solvent features of water (the solvent dipolarity/polarizability and hydrogen bond donor ability) in solutions of various compounds describe multiple physicochemical properties of these solutions (such as the solubility of various compounds in aqueous solutions, salting-out and salting-in constants for polar organic compounds in the presence of different inorganic salts, as well as water activity, osmotic coefficients, surface tension, viscosity, and the relative permittivity of aqueous solutions of different individual compounds) and are likely related to changes in the arrangement of hydrogen bonds of water in these solutions. Full article
(This article belongs to the Special Issue Solvatochromic Probes and Their Applications in Molecular Interaction Studies—a Themed Issue to Honor Professor Dr. Christian Reichardt)
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