The thermodynamics of the interaction between a hydrophobically modified cationic polyelectrolyte and an anionic surfactant (sodium dodecyl sulfate, SDS) has been investigated by microcalorimetry, conductivity, and UV−vis spectrophotometry. The polyelectrolyte employed was a newly synthesized polymer (D40OCT30) based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with a degree of substitution (DS) of 28.1%. The interaction between D40OCT30 and SDS was found to be very strong because of the introduction of ionic and hydrophobic moieties on the backbone of the dextran polymer. The aggregation concentration of polyelectrolyte−SDS complex (CACcomplex) was derived from the curves of variation of the observed enthalpy, solution conductivity, and optical dispersion with SDS concentrations. The results show that these values obtained from different methods are coincident and increase with D40OCT30 concentration. A mechanism of interaction is proposed and discussed in detail in the text. The total interaction enthalpies were derived from the observed enthalpy curves. The results indicate that the total interaction process is entropy-driven. From the calorimetric and turbidity measurements, the partial phase diagram that describes the dependence of the phase boundary on polymer alkyl side chain concentration is also deduced.

Bile acids are biological compounds in the body that have interesting properties and have been used to make special chemical structures in molecular recognition. Various polymers have been synthesized from bile acids. The materials should preserve some of the properties of bile acids, such as biocompatibility, high stability of the steroid nucleus, reactivity of the side groups, optical activity and self-assembling capacity. The synthesis and applications proposed for such polymers are discussed.

The intra- and/or intermolecular aggregation and the structure of the aggregates formed in water by hydrophobically modified dextrans, prepared by covalent attachment of cholic or deoxycholic acid to dextran of M v = 30 000, were investigated by various fluorescence and light scattering techniques. From the variation of the fluorescence quantum yield and fluorescence emission maximum of a hydrophobic fluorescent probe, N-phenyl-1-naphthylamine, with polymer concentration, we conclude about the value of the critical aggregation concentration, cac, and the existence of intermolecular aggregation below cac. The values of cac are found to depend on the nature of the hydrophobic moiety and the degree of substitution. The hydrodynamic radii of the aggregates are determined using dynamic light scattering, and the apparent weight-average molecular weights, radii of gyration, and second virial coefficients are evaluated by static light scattering over a very large concentration range. The results show that below a given critical concentration the hydrophobically modified dextrans form big and loose aggregates and small and compact ones at concentrations higher than 0.2 g %. A transition between the two types of aggregates begins at 0.02 g %, the concentration that we keep calling cac despite the fact that it is the concentration at which the formation of compact aggregates begins rather than that for which intermolecular aggregation occurs. The results are compared with those for unmodified dextran for which no aggregation was observed.

The self-aggregation of polyelectrolytes having N-alkyl-N, N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendant groups (alkyl ) octyl, dodecyl, or cetyl) randomly distributed along a polysaccharide backbone (dextran) was studied by steady-state fluorescence techniques using several free fluorescent probes or pyrenelabeled polymers and by viscometry. The onset, offset, and highest values of the fluorescence response of N-phenylnaphthylamine (NPN), pyrene (Py), and 1,6-diphenyl-3,5,6-hexatriene (DPH) were corroborated with NPN and DPH anisotropy and quenching experiments to describe the dynamic of hydrophobic microdomain formation and microdomain characteristics. The start of the aggregation process (critical aggregation concentration, cac) and the microdomain characteristics such as polarity, microviscosity, size, and number strongly depend on the alkyl chain length and the degree of substitution with cationic pendant groups. Fluorescence experiments with pyrene-labeled polymers and viscosity data suggest that microdomains are mainly formed by intramolecular hydrophobic associations, except for the polymers carrying octyl groups, where some intermolecular associations were revealed.

We have used a precision isothermal titration microcalorimeter (ITC) to measure the enthalpy curves for the interaction of a hydrophobically modified polyelectrolyte (D40OCT30) with oppositely charged surfactants (SC(n)S) in aqueous solution. D40OCT30 is a newly synthesized polymer based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with degree of substitution of 28.1%. The employed anionic surfactants are sodium octyl sulfate (SC(8)S) and sodium tetradecyl sulfate (SC(14)S). Microcalorimetric results along with turbidity and kinematic viscosity measurements demonstrate systematically the thermodynamic characterization of the interaction of D40OCT30/SC(n)S. A three-dimensional diagram with the derived phase boundaries is drawn to describe the effect of the alkyl chain length of surfactant and of the ratio between surfactant and pendant groups on the interaction. A more complete picture of the interaction mechanism for D40OCT30/SC(n)S systems is proposed here.

Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant.

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