Shrinking Kinetics of Polyacrylate Gels in Surfactant Solution

Abstract

Volume transition of covalently cross-linked sodium polyacrylate gels (micrometer-sized) due to the absorption of dodecyltrimethylammonium bromide from bulk aqueous solution is studied by means of light and fluorescence microscopy. The volume transition occurs in a narrow range of surfactant concentrations. In the presence of 0.01 M sodium bromide in the solution, the equilibrium volume is reduced typically 50 times for surfactant concentrations larger that 4 × 10-4 M. During the transition, a collapsed, surfactant-rich, surface phase is formed, enclosing the swollen gel core. Evidence is found for equilibrium between the core and surface phase in the collapsed state of the gel. The relation to surfactant self-assembly in solutions of linear polyion and in centimeter-sized gels is discussed. The kinetics of diffusion-controlled shrinking is analyzed theoretically. The model considers the transport of surfactant from the bulk, through the “stagnant” liquid layer and the surface phase, to the gel core. During shrinking, the osmotic swelling of the gel is assumed to be in quasi-equilibrium with the bulk and calculated using a recent model for equilibrium swelling of phase-separated gels (Hansson et al. J. Phys. Chem. B 2002, 106, 9777). The kinetic model is used to analyze time-resolved experimental shrinking curves. The result suggests that the shrinking is controlled by stagnant layer diffusion of surfactant, and that the relative swelling at intermediate stages during the collapse is the same as for slab gels of the same composition. The lag time measured before shrinking starts is longer than the time expected for the surfactant concentration to exceed the critical aggregation concentration in the gels, suggesting that the gels are arrested for some time in a metastable state before the surface phase starts to form.