Marija Bujan

Phase Behavior of Bis(Quaternary Ammonium Bromide)/Sodium Cholate/H2O System

Interactions in an oppositely charged surfactant mixture composed of a gemini surfactant (bis(quaternary ammonium bromide)) and a bile salt (sodium cholate) in water were studied at 30°C. A combination of techniques was used including surface tension, conductometry, light scattering, light microscopy, and microelectrophoretic measurements. A strong dependence of the phase behavior on the molar ratio and actual concentration of surfactants was found. The interplay between electrostatic effects, geometry of molecules, and dissimilar separation of the hydrophobic and hydrophilic moieties in the surfactants dictate the interaction mode and the microstructures formed. Instead of precipitation, in the equivalent mixtures formation of complexes, mixed micelles, vesicles, coacervates, and solid crystalline phases have been observed. The extent of interacting forces in mixed micelles formed in equivalent mixtures was evaluated by regular solution theory. A relatively high negative value of interaction parameter indicated a strong attractive interaction between surfactants. The compositions of both mixed micelles and mixed monolayer are found to be almost equimolar.

Interactions in aqueous mixtures of alkylammonium chlorides and sodium cholate

The interactions of alkylammonium chlorides (the number of carbon atom per chain was either 12, 14, or 16) with sodium cholate have been investigated by a combination of techniques including light and electron microscopy, surface tension, conductivity, light scattering, and microelectrophoretic measurements. The phase behavior has strongly depended on the molar ratio and actual concentration of oppositely charged surfactants. The change in the composition of the aggregates leads to a shape transformation from globular to elongated micelles to open and/or closed bilayers (vesicles) and precipitation. The length of micelles has been found to decrease dramatically with the concentration shift to the micellar regions of either surfactant. Upon a moderate excess of one surfactant, the mean hydrodynamic diameter of aggregates increases and wormlike micelles and/or open and closed bilayers are formed. Microscopic observations of alkylammonium cholates (novel catanionic surfactants precipitated in and/or close to equimolar region) have shown the presence of a variety of morphologies including twisted ribbons, tubules and bundles of tubules.

ADSORPTION AND ASSOCIATION IN BINARY MIXTURES OF CATIONIC SURFACTANTS

The adsorption and solution properties of cationic surfactants dodecylammonium chloride, tetradecylammonium chloride, and hexadecylammonium chloride, as well as hexadecyltrimtehylammonium bromide in pure and mixed states, were studied by surface tension and conductance measurements. The surfactants mixed non-ideally in both mixed monolayer and in the mixed micelles. The regular solution theory was used for evaluating the non-ideal interactions between molecules in adsorbed and micellar states. Similar values of the mixed monolayer and mixed micelle molecular interaction parameters in a mixture imply the molecules at the interface and in the mixed micelle have similar interactions. No synergism in surface tension effectiveness was observed. The adsorbed film at the air/water interface and mixed micelles were richer in more surface-active component.

Lamellar to hexagonal columnar liquid crystalline phase transition in a catanionic surfactant mixture: dodecylammonium chloride–sodium bis(2-ethylhexyl) sulfosuccinate

Phase transitions from a dispersed lamellar to hexagonal liquid crystalline phase have been investigated in a catanionic surfactant mixture formed by mixing a single tailed cationic surfactant, dodecylammonium chloride, with a double tailed anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate. Depending on the bulk composition and total surfactant concentration, mixed micelles, vesicles, lamellar and hexagonal columnar liquid crystalline phases have been identified. Differences in the geometry of the two hydrophobic chains stabilize vesicles of different shapes (spherical, tubular and pearled) relative to the liquid crystalline phase even in stoichiometric mixtures. At higher surfactant concentrations the phase transition from a dispersed lamellar to hexagonal columnar liquid crystalline phase proceeds continuously, with both phases coexisting over a range of concentrations. The transition proceeds through processes of vesicle aggregation, reorganization into multilayer sheets rolled-up into tubules, and formation of a hexagonal columnar liquid crystalline phase.

Influence of Dodecylammonium Chloride on the Properties of Carrageenan Gels

Rheological measurements, confocal laser scanning microscopy, and x-ray diffraction were used to study the influence of dodecylammonium chloride (DDACl) on the gelation of κ- and ι-carrageenans. The increase of DDACl concentration led to the changes of gel structure from nano to macro-scale. At the macroscopic level, two phase regions have been differentiated; one below the surfactant micellar region containing turbid gel and the other in the micellar region characterized by gel collapsing. The effect of monomer DDACl concentration on gel structure at the nano level was detected by rheological measurements. All gel samples exhibited shear thinning behavior, well fitted by power law equation, indicating that they maintain mainly carrageenan rheological properties. The change of flow behavior parameters with increasing DDACl concentration might be ascribed to the formation of surfactant domains inside gel networks. Progressive increase of surfactant concentration caused gradual break down of the gel structure and enhanced ordering of surfactant molecules. In the vicinity of the critical micelle concentration, the appearance of microphase separated domains, visualized by confocal laser scanning microscopy indicated pronounced structural changes of both, κ- and ι-carrageenan gels. X-ray diffraction of collapsed gels revealed the formation of a bilayered surfactant structures in conjunction with the carrageenan chains. The increase of DDACl concentration enhanced ordering and growth of mesostructural domains consisting of stacked bilayers. Both, local ordering of bilayers and regular stacking of lamellae revealed stronger electrostatic interactions between surfactant and carrageenan with higher charge density on chains.

Nano- and microcomplexes of biopolymer carrageenans and dodecylammonium chloride

Abstract Polymer and surfactant complexation was investigated in systems containing anionic biopolymers and cationic surfactants by various classical and modern methods. Differently charged carrageenans (one, two or three sulfate groups per monomeric unit) and dodecylammonium chloride (DDACl) were used as model systems. Formation of various soluble and insoluble complexes (from nano- to microdimensions) and gelation strongly depends on carrageenan and DDACl concentrations, their molar ratio and linear charge density on carrageenan chains. The main factors governing complexation include electrostatic and hydrophobic interactions as well as conformation of carrageenan chains. With increasing carrageenan concentration, the intramacromolecular complexes change to intermacromolecular, which subsequently reorganize into better ordered structures, giant vesicles, and precipitated stoichiometric compounds, dodecylammonium carrageenates. Structural analysis of the new compounds revealed the formation of a lamellar structure with the polar sublayer containing carrageenan chains and the non-polar sublayer consisting of disordered dodecylammonium chains electrostatically attached to the carrageenan backbone. At gelling carrageenan concentration, progressive addition of DDACl caused gradual transitions from the structure of carrageenan gel alone to lamellar ordering of collapsed gel balanced by intermolecular forces within the gel network, i.e., by hydrogen bonding, electrostatic, hydrophobic and van der Waals forces.