Association Phenomena in Aqueous Solutions of Surfactants and Hydrophobically Modified Polymers

Abstract: Interactions between ionic surfactants (sodium dodecyl sulfate and dodecyltrimethylammonium chloride) and nonionic polymers ((hydroxyethyl)cellulose, ethyl(hydroxyethyl)cellulose and their hydrophobically modified analogous) have been studied by means of rheology, pulsed field gradient nuclear magnetic resonance (PFG NMR), time-resolved fluorescence quenching (TRFQ), dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM). In all experiments surfactants have been added in various amounts to polymer solutions at fixed concentrations. The concentration of mixed micelles formed by surfactant molecules and hydrophobic tails of the polymer, binding isotherms, aggregation numbers and polymer self-diffusion coefficients have been obtained and correlated to viscosity. The viscosity increases where the number of mixed micelles is nearly unchanged but their aggregation number increases due to added surfactant, resulting in slower rheological and dynamic relaxation processes. The viscosity decrease occurs when the mixed micelles are dominated by surfactant molecules. Here, there is a strong increase in the number of mixed micelles and the number of hydrophobic tails per mixed micelle decreases. Under these circumstances when there are few polymer hydrophobic tails per mixed micelle, a high viscosity can be regained if the aggregation number increases, resulting in more polymer hydrophobic tails per mixed micelle. The aggregation number can be increased, by adding an oppositely charged surfactant or a screening electrolyte. The polymer-surfactant network undergo structural rearrangements from a heterogeneous structure at low surfactant concentration to a homogeneous structure at high levels of surfactant addition.