The electrokinetic properties of cationic surfactants adsorbed on a hydrophobic substrate: effect of chain length and concentration

Abstract

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 cetyl trimethylammonium bromide (CTAB), trimethylammonium bromide (TTAB), dodecyl trimethylammonium bromide (DTAB) in PMMA microcapillaries. The EO mobility curve as a function of concentration shows three regimes. First, at very low concentrations below 0.002 mM, the mobility is constant and approximately equal to the value obtained with the surfactant-free electrolyte (1 mM KCl). Next, the EOF reverses and mobility increases linearly with surfactant concentration. Finally, the mobility reaches a plateau at a concentration well below surfactant CMC (0.2-mM CTAB, 0.5-mM TTAB and 2-mM 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 alkyl trimethylammonium surfactants adsorbed on the hydrophobic surface of PMMA, this energy was found to be 0.114 kT smaller than the reported value for ionic surfactants adsorbed on a hydrophilic surface.