CMC determination in the presence of surfactant-adsorbing inorganic particulates

An efficient method for determining the critical micelle concentration (CMC) of surfactants in the presence of micron-sized surfactant adsorbing particulates via the utilization ultrafiltration membranes is presented. The capabilities and limitations of this method to measure free surfactant concentration both above and below the CMC are explored and discussed. This method was used to determine the CMC of sodium dodecyl sulfate (SDS) in the presence of aluminum oxide and titanium dioxide. For both substrates, the CMC corresponded to the turning point of the adsorption isotherm, far below the CMC measured without the solid substrate. This reduction in CMC was attributed to excess sodium ions in sloution, indicating that metal cations at the solid surface were able to neutralize the dodecyl sulfate anion. This agreement between the CMC and the turning point of the isotherm may indicate that adsorption halts because micelles form; however, the surface might still be saturated with surfactant if surface saturation coincides with the CMC.