Binding of a Cationic Surfactant by Polyacrylate, Poly(vinyl sulfate), and Montmorillonite. Tensammetric Measurements in Very Dilute Aqueous Solution

Abstract

Out-of-phase alternating current polarography, a tensammetric method, was used to directly measure the concentration of hexadecylbenzyldimethylammonium (HBDA), a cationic surfactant, in equilibrium with very dilute solutions of anionic polymers such as poly(vinyl sulfate) (PVS) and polyacrylate (PA), as well as montmorillonite, a negatively charged clay mineral. The sensitive response of the time-controlled mercury drop electrode allows binding isotherms to be calculated in a μM range of HBDA. In the case of montmorillonite, a double-step binding isotherm is observed which is equal to twice the cation exchange capacity (CEC)ofthe clay mineral. A cation exchange reaction for the first step and hydrophobic interactions between adsorbed surfactant molecules are commonly considered in the adsorption process. Electrophoretic measurements along the binding isotherm allow the formation of a surfactant molecule bilayer to be observed. Up to a 1.5-1.6 CEC equivalent of adsorbed HBDA, the formation of montmorillonite plate aggregates can also be proposed by considering a demixing of Na and HBDA cations between external and interplate sites. In the case of PA and PVS, a quantitative binding of HBDA to the carboxylate and sulfate functional groups respectively is found. Two domains of cooperativity for the binding process are discriminated along the isotherms. Considering a specific charge neutralization with HBDA, it can be proposed, in a first step, that the Na counterion condensation controls the surfactant fixation at both vinyl polyelectrolyte, PVS and PA, surfaces at an ionic strength of 10 -2 M. This can be satisfactorily modeled by a one-dimensional nearest neighbor lattice model. In a further step, a higher cooperative character for the binding of surfactant aggregates is observed, which can be related to the breakdown of the Na counterion condensation. At high HBDA solution concentrations, the building of micelles in solution competes with this binding mechanism.