Relationship between the adsorption behavior and bulk complexation in oppositely charged polyelectrolyte-surfactant systems: Effect of polyelectrolyte concentration, molecular weight, charge localization and backbone rigidity/hydrophobicity
Due to a wide range of applications, much emphasis has been placed on understanding the physicochemical behavior of polyelectrolyte/surfactant complexes, both at air-water interface and in the bulk. However, the correlation between the adsorption behavior and complexation in the bulk is less explored. In this research, this correlation is investigated and its dependence on polyelectrolyte concentration, molecular weight, charge localization and backbone rigidity and hydrophobicity is studied. The polyelectrolyte concentration is normalized with respect to it critical overlap concentration in order to compare the polymer in same concentration regime. Different polyelectrolyte systems were used to analyze the polyelectrolyte structural effect: * The molecular weight effect was studied between low molecular weight cationic hydroxyethylcellulose (JR400)/SDS and high molecular weight hydroxyethylcellulose (JR30M)/SDS system * The charge localization effect was studied between the linearly charged poly(methacrylamidopropyltrimethylammonium chloride) (MAPTAC) and iii locally charged poly(methacrylamide propyl (methoxy-carbonyl-methyl) dimethyl ammonium chloride) (AMT) * The effect of rigidity and hydrophilicity/hydrophobicity of the polyelectrolyte backbone was studied by comparing and contrasting flexible/hydrophobic MAPTAC and semi rigid/hydrophilic JR30M All of these polyelectrolytes were interacted with sodium dodecyl sulfate (SDS) surfactant. The concentration of these polyelectrolytes was varied over a sufficient range to analyze the polyelectrolyte-surfactant interaction in different concentration regimes. The adsorption behavior was analyzed by surface tension measurements, while, the complexation in the bulk was examined by rheological measurements. Fluorescence measurement techniques were additionally used to analyze the effect of charge localization on the structure of the polyelectrolyte/surfactant complexes. In the molecular weight study, as a function of surfactant concentration, the surface tension at the interface varied in the one phase region above the surfactant's critical aggregation concentration (CAC). The surface tension increased with increase in the SDS concentration and this became more pronounced with increase in polyelectrolyte molecular weight. This is counterintuitive. This increase can be explained by intermolecular association taking place in the bulk between the polymer and surfactant through hydrophobic association of the bound surfactants. This bulk intermolecular association was favored by the interfacial complex over the surface adsorption, thereby increasing desorption of the polyelectrolyte-surfactant complex from the surface. Increase in molecular weight increased this intermolecular association and the surface tension increased with increase in the molecular weight. Charge localization on the polyelectrolyte chain increased the surface tension in the two phase region. From fluorescence measurements it was observed that the charge localization enhances SDS aggregation. The charge localization of AMT causes an observed increase in the surface tension above the surfactant CAC and this can be interpreted as desorption of the surface complex as the bulk complex phase-separates. On the other hand, in the linearly charged MAPTAC/SDS complex the surface tension stays constant as SDS concentration is increased, signifying that the surface adsorbed complex cannot be desorbed by the formation of the MAPTAC/SDS complex in the bulk. Unlike the polyelectrolytes which have flexible hydrophobic backbones, the stiff, hydrophilic polymers were observed to increase the surface tension even while they remained in the one phase region. The association between the rigid and hydrophilic JR30M polymer and surfactant is higher than MAPTAC. For JR30M, pronounced desorption of the surface active species is observed in the one phase region above the CAC. On the other hand, in the case of flexible and hydrophobic MAPTAC, the association with SDS at the surfactant CAC causes the viscosity of the MAPTAC/SDS system reduces by addition of SDS. We deduce from this that the MAPTAC collapses with addition on surfactant in the one phase region. Thus, the surface tension stays constant. In all the studied polymer/surfactant systems, the adsorption behavior of the complex at the interface was linked to the changes taking place in the bulk complexation.