Liquids, Volume 6, Issue 2 (June 2026) – 9 articles

Balancing ionic transport, thermal robustness, and electrochemical stability remains an important challenge in the design of ionic liquid (IL) electrolytes for lithium-based energy storage. Here, quantitative structure–transport relationships were established through a systematic comparison of six bis(trifluoromethanesulfonyl)imide ([Tf₂N]⁻)-based ILs spanning imidazolium, pyrrolidinium, and quaternary ammonium cation families, each examined in both conventional alkyl and ether-functionalized forms. Density, viscosity, and ionic conductivity were measured over broad temperature ranges, while Raman spectroscopy and electrochemical stability measurements were used to probe ion association and voltage stability under selected conditions for both neat ILs and LiTf₂N-containing electrolytes. Ether functionalization consistently lowered viscosity and enhanced conductivity in the neat ILs, whereas LiTf₂N addition markedly increased viscosity and reduced conductivity in all systems. The magnitude of this lithium-induced transport penalty depended on cation architecture, being smallest for imidazolium systems and largest for ammonium analogues. Raman spectra indicate that these trends are associated with competition between Li⁺–anion coordination and ether-mediated solvation, which modifies ion association and local coordination environments. Walden analysis showed subionic behavior for all systems, with larger deviations after lithium incorporation, suggesting increased ion correlation. Electrochemical measurements revealed a complementary trade-off between transport and stability: the ether-functionalized imidazolium electrolyte containing 0.65 mmol g⁻¹ LiTf₂N exhibited the highest ionic conductivity among the lithium-containing systems, reaching 1.6 and 12.6 mS cm⁻¹ at 25 and 80 °C, respectively, but the corresponding imidazolium IL had the narrowest electrochemical stability window, about 4.3 V. In contrast, the ether-functionalized pyrrolidinium and ammonium ILs exhibited wider electrochemical stability windows of about 5.5 V, with improved cathodic stability and somewhat higher anodic stability than the imidazolium analogue. Full article

Conventional liquid springs enable storage of energy in the form of interfacial tension at forcibly wetted lyophobic surfaces. The pressure–volume work performed to compress the liquid into a poorly wettable porous medium is recovered during spontaneous expulsion when pressure falls below the capillary pressure characteristic of a given system. Our study explores generalizations to easily wettable materials where liquid infiltration is opposed solely by steric hindrance exerted on liquid molecules in micro-sized pores. The concept is exemplified in molecular simulations of prototypical model systems with methanol intruding narrow slits between hydrocarbon or graphene surfaces. While these materials show significant wetting propensities at macroscopic interfaces with liquid methanol, substantial compression is required to wet molecular-sized pores barely accommodating a monolayer of liquid molecules. The observed O(10³) bar intrusion pressures secure stored energy densities competitive with supercapacitors and amenable to improvement. Wall–liquid attraction and small pore diameters lead to intrusion–expulsion pathways along cooperative-adsorption isotherms. The process avoids abrupt liquid/vapor transitions and associated nucleation barriers, responsible for cycle hysteresis in experiments with water in hydrophobic capillaries. Using open ensemble (Grand Canonical) Monte Carlo sampling, we identify the range of porosities supporting reversible energy storage/recovery operation in lyophilic media; the results can assist with the design of molecular spring devices with competitive storage and power capacities in pragmatic contexts. Full article

Poly(l-lactic acid) or poly(l-lactide) (PLLA) is an optically active polymer derived from renewable sources and fully biodegradable. It is known that PLLA assumes a left-handed helix in the solid state and also in solution it still keeps a certain degree of helical structure. Here, we examine the Optical Rotatory Dispersion (ORD) behavior of two grades of PLLA (medium molecular weight and hexadecyl-terminated or high molecular weight for 3D printing) in 13 different solvents and analyze the experimental ORD data through the Moffitt–Yang equation. Furthermore, the ORD data of PLLA in additional 6 solvents were taken from the literature and analyzed with the Moffitt-Yang approach. The results suggest that, also in solution, PLLA maintains the left-handed helix, and the most structurizing and helicogenic solvents for PLLA are ethyl acetate, acetonitrile, and certain chlorinated solvents. The equilibrium association constant (K) and other thermodynamic parameters (ΔG°, ΔH° and ΔS°) between PLLA and polyphenylacetylene (PPA, another helical polymer in the solid state and in solution) were determined in trichloromethane, dichloromethane, and tetrahydrofuran. The K values found suggest a strong helix-helix interaction between the two polymers. The ORD analysis of the PLLA-PPA solutions shows evidence of the extrinsic Cotton effect and confirms the chiral helicity induction between the two polymers with 1:1 complex formation. Full article

Bubble bursting at liquid surfaces was investigated experimentally using high-speed imaging at 25,000 fps and micro-particle image velocimetry (µ-PIV) at up to 4000 flow fields per second. Three fluids with distinct rheological properties were studied: a viscous Newtonian fluid (Emkarox, η₀ = 0.072 Pa·s) and two non-Newtonian fluids (highly viscous Carboxymethyl Cellulose, HV CMC, η₀ = 0.53 Pa·s, and viscoelastic Polyacrylamide, PAAm, η₀ = 57.17 Pa·s). Bubble radii ranged from 1.2 to 4.0 mm, with corresponding lifetimes spanning from O(10⁻²) to O(10¹) s depending on fluid properties. The relationship between bubble size and lifetime at the air–liquid interface was quantified for the non-Newtonian fluids, using the Newtonian fluid as a reference. µ-PIV measurements further captured the rapid dynamics of bubble bursting beneath the interface in the liquids. These findings provide new insight into the complex interfacial mechanisms governing bubble rupture and fluid motion. Full article

Based on the Lie group method (symmetry transformation groups), an analysis of an unsteady (start-up) flow over a flat surface was performed. This approach enabled reducing the number of independent arguments, which significantly simplifies the process of numerical modeling. An unsteady solution was obtained for the velocity profile in the boundary layer. This enabled estimating the dynamics of the velocity profile transformation and its transition to a steady-state mode. It was shown that in the limit of infinite time of the process, the velocity profile tends to the classical steady-state Blasius profile in the boundary layer. The dynamics of the friction coefficient variation over time were elucidated too. Full article

Metal oxide nanoparticles dispersed in water are difficult to recover because of their small size and colloidal stability. In this study, the interfacial adsorption behavior of Fe₂O₃, CoO, and CuO nanoparticles at hydrophobic ionic liquid (IL)–water interfaces was investigated and compared with that at molecular solvent–water interfaces. When CuO nanoparticle dispersions were shaken with hydrophobic ILs, bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻) salts of 1-butyl-3-methylimidazolium ([BMIm]⁺) and 1-octyl-3-methylimidazolium ([OMIm]⁺), the nanoparticles were removed from the aqueous phase and accumulated at the IL–water interface, while negligible Cu was detected in the bulk IL phase. The removal efficiency decreased with increasing ionic strength below 0.05 mol/dm³ and increased with pH, indicating that electrostatic interactions between charged nanoparticles and the IL–water interface contribute to adsorption. Adsorption isotherms were empirically fitted with the Langmuir equation to estimate the maximum adsorption capacity. For negatively charged Fe₂O₃ and CuO nanoparticles, the maximum adsorption capacities at IL–water interfaces exceeded those at molecular solvent–water interfaces and the theoretical monolayer capacity estimated from nanoparticle size, suggesting multilayer adsorption or aggregation at the interfaces. These results demonstrate the potential of hydrophobic IL–water interfaces for the separation and recovery of metal oxide nanoparticles from aqueous media. Full article

This paper provides a comprehensive overview of research findings concerning the acid catalytic effect (ACE) of hot compressed water and water–alcohol mixtures, along with the applications of these solvents. The ACE observed during reactions can be categorized into three types: inherent, associated, and interfering. These ACE types originate from the solvent, solutes, and reactor, respectively. Distinguishing and evaluating these ACEs is crucial for elucidating reaction mechanisms and developing reaction models. Water exhibits inherent ACE in both its dissociated and undissociated forms under hot compressed conditions. Hot compressed water–alcohol mixtures possess the capability to tune the characteristics of solvents, including ACE, through their composition. The application of hot compressed water and water–alcohol is prevalent in a variety of fields, including the conversion of biomass and biomass-derived materials, extraction, biodiesel production, organic synthesis reactions, recycling via the decomposition of polymers, and inorganic material synthesis. In these applications, the utilization of water–alcohol mixtures resulted in a higher yield of target products and/or superior properties of products compared to the use of pure solvents, such as water alone or alcohol alone. The observed results can be attributed to the optimization of the roles of water and alcohol in the reaction through mixing them. Full article

In this work, the volumetric properties of nine binary systems composed of ethyl propanoate and n-alkylcyclohexanes (from cyclohexane to decylcyclohexane) were investigated. Densities were measured at atmospheric pressure (101 kPa) over the entire composition range and at temperatures from 288.15 K to 328.15 K. A total of 525 density data points were obtained. Excess molar volumes were derived from the experimental densities and correlated using a Redlich–Kister equation, while mixture densities were modeled with the Jouyban–Acree model. All systems exhibit positive excess molar volumes over the studied temperature and composition ranges, indicating volume expansion upon mixing due to dominant repulsive interactions. The magnitude of the excess molar volume increases with increasing alkyl chain length of the branched naphthenic compound: for an equimolar mixture, V^E is about 0.65 cm³·mol⁻¹ for the methylcylohexane + ethyl propanoate mixture and reaches 0.83 cm³·mol⁻¹ for the heptylcylohexane + ethyl propanoate binary system, although a plateau tendency is observed for longer alkyl chains. Excess molar volumes increase linearly with temperature, with a more pronounced temperature effect for shorter-chain alkylcyclohexanes. The Jouyban–Acree model provides an excellent correlation of the density data, yielding average relative deviations between 0.02% and 0.04%, and allows reliable predictions within the investigated temperature range. Full article

We report an optical method to estimate local microviscosity in thermotropic liquid crystals using viscosity-responsive aggregation-induced emission luminogens. Pendant-type luminogens were designed by covalently attaching 4-cyano-4′-n-octyloxybiphenyl mesogens (n = 8, 10) to a bis(N,N-dialkylamino)anthracene emissive core. When introduced at 1.0 wt% into 8OCB and 10OCB, thermal and optical analyses showed that the intrinsic liquid crystal properties were essentially unchanged, indicating good structural compatibility. Temperature-dependent fluorescence and polarization measurements revealed that emission changes are governed mainly by microviscosity rather than macroscopic phase disruption. Effective microviscosity was evaluated from absolute fluorescence quantum yields using the Förster–Hoffmann relation. On this basis, the microviscosity in the nematic phase is 21 mPa·s for 8OCB upon cooling, which correlates with the enhancement in fluorescence. In the smectic phase, although the director distribution parameter remains nearly constant, the effective microviscosity is ca. 21 mPa·s for 10OCB and ca. 54 mPa·s for 8OCB, and the fluorescence varies smoothly with temperature, reflecting changes in local segmental mobility within the layered structure. These values are broadly consistent with reported viscosity ranges/trends for cyanobiphenyl-type liquid crystals. Full article