Surfactant adsorption at solid-aqueous interfaces containing fixed charges: Experiments revealing the role of surface charge density and surface charge regulation

The charge on the solid-liquid interface affects both the amount of ionic surfactant and the organization of the surfactant adsorbed at the solid-liquid interface. For most solids, the surface charge is not fixed; it is regulated by the adsorption or desorption of ions. The aim of this work is to study surfactant adsorption to surfaces that have a controlled and fixed density of covalently bound surface charges. The desired surface charge density is achieved by the use of gold-thiol self-assembled monolayers (SAMs) of different omega-groups (-OH and -N+(CH3)(3)). The mole fraction of -N+(CH3)(3) on the mixed SAM dictates the density of covalently bound charges. The charge on -N+(CH3)(3) is fixed and does not self-regulate, but the total surface charge can be changed through adsorption of other ions. We have studied the adsorption of sodium dodecyl sulfate (SDS) to the interface between these model surfaces and aqueous solutions of SDS. Atomic force microscopy (AFM) of the adsorbed surfactant reveals no surface micelles above the critical micelle concentration, cmc, over a wide variety of -N+(CH3)(3) densities. This leads us to hypothesize that lateral mobility of ions other than surfactant at the interface is important for the formation of surface micelles of ionic surfactants. Adsorption isotherms of SDS (with no added salt) measured by surface plasmon resonance (SPR) show a plateau region that ends at about cmc/25. In this plateau region, the surface excess of SDS is equal to the known fixed surface charge. There is a second plateau region above the cmc. In this concentration range, the surface excess is approximately twice the fixed surface charge. Desorption experiments starting above the cmc show rapid desorption of SDS into water until the surface excess is equal to the fixed surface charge. The rapid desorption is followed by a much slower desorption.