Search

We present results from an experimental dynamic light-scattering study of poly(ethylene oxide) (PEO) in both a pure solvent (water) and a mixed solvent (tert-butanol + water). The concentration dependence of the diffusive relaxation of the PEO molecules is found to be typical of polymers in a good solvent. However, the mesoscopic diffusive behavior of PEO in the mixed solvent is very different, indicating an initial collapse and subsequent reswelling of PEO caused by co-nonsolvency. Furthermore, in the solutions of PEO with very large molecular weights, we found additional hydrodynamic modes indicating the presence of PEO clusters and aggregates similar to those found by some other investigators. [ABSTRACT FROM AUTHOR]

We have resolved a long-standing issue in the discussion on the origin of the mesoscale inhomogeneities observed in aqueous solutions of tertiary butyl alcohol (TBA). We have shown that the formation of stable mesoscale particles (of about 100 nm in size) can be triggered by the addition of trace amounts of propylene oxide (an impurity expected to be present in all commercial samples of TBA) to a solution, which was previously filtered at a low temperature to remove these inhomogeneities. We hypothesize that these particles are aggregates of mixed clathrate-hydrates that are formed through the stabilization of fluctuations of the intrinsic structure in TBA aqueous solutions by the clathrate-forming ability of propylene oxide. [ABSTRACT FROM AUTHOR]

Nonionic hydrotropes (low-molecular-weight amphiphiles) demonstrate striking dual actions in bulk solutions and interfaces, exhibiting both surfactant-like and co-solvent properties. We report on peculiar, strongly affected by this duality, liquid-liquid and air-liquid-liquid interfacial behavior in aqueous ternary systems, containing hydrotropes and hydrocarbons, in a broad range of compositions and at various temperatures. Phase diagrams of the studied systems, containing tertiary butanol (TBA), as a hydrotrope, are of Type 1: the hydrotrope, at the experimental conditions, is completely miscible with water and with all investigated hydrocarbons [cyclohexane (CHX), toluene (TOL), and n -decane (DEC)], whereas the ternary mixtures exhibit liquid-liquid phase separation terminated at corresponding critical points. The shape and location of the phase separation boundary are only weakly dependent on temperature and the hydrocarbon's nature; however, the critical point in the water-TBA-DEC system is significantly shifted toward a higher TBA concentration. For the experimentally studied systems and for available data reported in the literature, we confirmed an apparently generic (for nonionic hydrotropes) phenomenon of a dual action at water-oil interfaces (earlier found in water-TBA-CHX [ J. Phys. Chem. C 2017 , 16423]): at low concentrations, hydrotropes saturate the water-oil interface like a surfactant, whereas at higher concentrations they act as co-solvents, resulting in vanishing interfacial tension at the liquid-liquid critical point. We suggest a universal crossover function that accurately interpolates the two theoretically based limits of interfacial behavior. This crossover function also accounts for earlier deviations from Langmuir-von Szyszkowski limiting behavior in the water-TBA-DEC system, caused by lower solubility (relative to other studied hydrocarbons) of DEC in water. An intriguing correlation between the dual action of hydrotropes at the water-oil interface and the behavior of the liquid-air interfaces is also discussed.121 , 16423]): at low concentrations, hydrotropes saturate the water-oil interface like a surfactant, whereas at higher concentrations they act as co-solvents, resulting in vanishing interfacial tension at the liquid-liquid critical point. We suggest a universal crossover function that accurately interpolates the two theoretically based limits of interfacial behavior. This crossover function also accounts for earlier deviations from Langmuir-von Szyszkowski limiting behavior in the water-TBA-DEC system, caused by lower solubility (relative to other studied hydrocarbons) of DEC in water. An intriguing correlation between the dual action of hydrotropes at the water-oil interface and the behavior of the liquid-air interfaces is also discussed.

We present results from an experimental dynamic light-scattering study of poly(ethylene oxide) (PEO) in both a pure solvent (water) and a mixed solvent (tert-butanol + water). The concentration dependence of the diffusive relaxation of the PEO molecules is found to be typical of polymers in a good solvent. However, the mesoscopic diffusive behavior of PEO in the mixed solvent is very different, indicating an initial collapse and subsequent reswelling of PEO caused by co-nonsolvency. Furthermore, in the solutions of PEO with very large molecular weights, we found additional hydrodynamic modes indicating the presence of PEO clusters and aggregates similar to those found by some other investigators.

The phase behavior and mesoscopic inhomogeneities in the ternary system of tertiary butyl alcohol (TBA), water, and propylene oxide (PO) have been studied by static and dynamic light scattering, gas chromatography, mass spectrometry, and molecular dynamics simulations. Mesoscale inhomogeneities are observed in this system in a broad range of PO concentrations, from 0.02 to about 65 mass %, and at certain TBA/water mass ratios varying from about 3/97 to about 30/70. This TBA/water composition domain corresponds to a region where short-lived micelle-like molecular clustering and thermodynamic anomalies are observed in the TBA-water binary system. At dilute PO concentrations (0.02 to about 1 mass %) the mesoscale inhomogeneities are Brownian diffusive droplets, with a size of the order of a hundred nanometers. We hypothesize that these droplets have a hydrophobic core enriched by oily impurities and oligomerized PO molecules. A hydrogen-bonded layer of TBA, water, and PO molecules surrounds this hydrophobic core. At high PO concentrations (beyond 50 mass %), the interfacial curvature of the mesoscopic inhomogeneities changes its sign and the exteriors of these inhomogeneities become hydrophobic. The inversion of the internal curvature may result in the formation of a spongelike bicontinuous mesoscale structure at intermediate PO concentrations. This mesostructure appears to be at a nonequilibrium state, although extremely long-lived.

Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10 degrees C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.