The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.

We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months.

The titration of poly(methacrylic acid) gel with sodium methoxide was studied in water, methanol, dioxane, and their mixtures. There are three main regimes of the swelling behavior of the gels depending on the dielectric constant ε of the medium. In polar media with ε larger than a critical value ε1, the gel swells monotonically with the increase of the degree of ionization, α, due to the osmotic pressure of counterions. In media with ε smaller than a critical value ε2, the gel always shrinks with increasing α due to ion-pair formation with their aggregation to multiplets. In media with dielectric constants ranging from ε1 to ε2, the gel swells at α up to ca. 0.1 and then collapses. The collapse induced by ionization is in agreement with recent theoretical findings.

Microheterogeneities in positively charged gels of diallyldimethylammonium bromide copolymerized with acrylamide and negatively charged gels of sodium and cesium methacrylate copolymerized with acrylamide appear as a result of the collapse of these gels in poor solvents (waterethanol mixtures). Three possible reasons for microheterogeneities are analyzed: the polyelectrolyte effect (i.e., competition between the attraction of the uncharged parts of the chains in poor solvent and electrostatic repulsion coupled with osmotic pressure of counterions), the ionomer effect (i.e., the formation of ionomer multiplet structure), and vitrification (i.e., partial formation of glassy kinetically frozen polyacrylamide-rich regions). Both macroscopic observations and SAXS experiments were made. The dry and water-swollen gels immersed in the water-ethanol mixtures show different final states at high ethanol contents, which proves the existence of kinetically frozen structures for these cases. For highly charged cationic gels we observed an increase of the scattering exponent which correlates with the volume phase transition while for smaller charge density this increase occurs at higher ethanol contents with the appearance of kinetically frozen structures. These results suggest that in the former case the microdomain structure appears mainly as a result of the ionomer effect while in the latter case the main factor is partial vitrification. In contrast to recent results in purely aqueous systems, no SAXS scattering maxima were observed, apparently due to the high irregularity of the microstructures.

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