Interactions of Some Hofmeister Cations with Sodium Dodecyl Sulfate in Aqueous Solution

In this work, we have investigated the influence of some alkali metal ions on the Krafft temperature (T-K) and critical micelle concentration (CMC) of a classical ionic surfactant, sodium dodecyl sulfate (SDS), over a wide range of temperature. The alkali metal cations such as Li+, Na+, Cs+, and K+ are found to affect the solubility and hence the T-K of the surfactant. It was observed that kosmotropic Li+ lowers the T-K of the surfactant. Due to the common ion effect, the solubility of SDS decreases in the presence of Na+, resulting in an increase in the T-K. On the other hand, chaotropic K+ and Cs+, capable of forming contact ion pairs with the chaotropic dodecyl sulfate ion, lower the solubility and hence elevate the T-K. In terms of decreasing the T-K, the ions follow the trend: Li+ > Na+ > Cs+ > K+ except for 0.0025 M CsCl. The added cations screen the charge of the micelle surface and facilitate closer packing of the surfactant with a consequent decrease in the CMC. In terms of the effectiveness in lowering the CMC, the ions follow the order: Cs+ > K+ > Na+ > Li+. In the presence of added electrolytes, the (CMC) values are found to be lower than the corresponding values in pure water. The thermodynamic parameters (Gibbs free energy, enthalpy, and entropy changes) of micellization were calculated to gain insights into the mechanism of the process.