Determination of the Critical Micelle Concentration of Sodium Dodecyl Sulphate in Aqueous Solution in the Presence of an Organic Additive of Acetonitrile by Conductometry and an Inorganic Additive of Phosphate by Fluorometry

Abstract

The critical micelle concentration (CMC) is the single most useful quantity for characterizing surfactants and the most significant parameter when working with ionic micellar phases, such as micellar electrokinetic chromatography (MEKC). Sodium dodecyl sulphate (SDS) was the first and is the most popular surfactant used in MEKC. Acetonitrile is often used as an organic additive for selectivity adjustment of solutes and solubility enhancement of hydrophobic compounds and phosphate as an inorganic additive for obtaining desired pH and buffer capacity in MEKC. The effect of acetonitrile on the CMC of SDS was studied by conductometry and the effect of phosphate buffer on the CMC of SDS in acetonitrile-water binary solvent was studied by fluorometry with the use of pyrene as a probe. It was found that the CMC of SDS first decreased up to 3 % (v/v) and then increased with increasing of the volume ratio of acetonitrile to water up to 5 % (v/v). The minimum of the CMC of SDS in acetonitrile-water binary solvent was found at the volume ratio of 3 % (v/v) acetonitrile to water and was determined to be 7.7 mM. The CMC value of SDS in acetonitrile-water (3 %, v/v) binary solvent decreased with increasing of phosphate in the concentration range of 10-50 mM. The information provided in this paper is expected to be useful for optimization of separation and interpretation of migration behaviour of solutes in MEKC.