(403b) The Electrokinetic Properties of Cationic Surfactants Adsorbed On a Hydrophobic Substrate: Effect of Chain Length and Concentration

Electrokinetic(EK) properties such as the electro-osmotic flow(EOF) are influenced by surfactant adsorption at the solid-liquid interface. With the growing popularity of poly(methyl methacrylate) (PMMA) as the substrate for polymeric-based microfluidics, it is important to understand the effect of surfactants on EOF in these devices. Here, we investigate the effect of surfactant chain length and concentration on the electro-osmotic (EO) mobility induced by three cationic surfactants cetyltrimethylammonium bromide (CTAB), tertradecyltrimethylammonium bromide (TTAB), and dodecyltrimethylammonium bromide (DTAB) in PMMA microcapillaries. The presented EO measurements provide a simple and rapid method for characterization of surface hydrophobicity through adsorption of surfactants. The EO mobility as a function of concentration shows three regimes. First, at very low concentrations below 0.002mM the mobility is constant and approximately equal to the value obtained with the surfactant-free electrolyte (1mM KCl). Next, the EOF reverses and mobility increases linearly with surfactant concentration. Finally, the mobility reaches a plateau at a concentration well below critical micelle concentration (CMC), 0.2mM CTAB, 0.5mM TTAB and 2mM DTAB and decreases at the vicinity of CMC. Our results show that the rate of change in mobility with respect to concentration is a linear function of chain length and increases with longer chain surfactants.  In addition, we deduce the magnitude of Van der Waals or cohesive energy between the adsorbed alkyl chains from the EO mobility values. For the alkyltrimethyl ammonium surfactants adsorbed on the hydrophobic surface of PMMA, this energy was found to be 0.114KT, smaller than the reported value for ionic surfactants adsorbed on a hydrophilic surface.