Micellization and adsorption characteristics of CHAPS

The adsorption of CHAPS on hydrophobic latex particles was studied at 22 and 36 degrees C by determining the adsorbed amount and the enthalpy of adsorption. The adsorption process was compared to the micellization of the surfactant. Therefore, the critical micelle concentration (cmc) and the heat of micellization were also determined at both temperatures. From these two quantities the Gibbs energy and the entropy of the process were calculated. The cmc and the heat of micellization are temperature dependent; the cmc increases as the temperature rises, and the heat of micellization goes from positive at 22 degrees C to negative at 36 degrees C. There is an entropy-enthalpy compensation in the micellization process, characteristic of hydrophobic interactions. The maximum adsorbed amount is independent of the temperature, while the initial slope of the isotherms is slightly steeper at 22 degrees C. Although the adsorption process is exothermic at both temperatures, the enthalpy of adsorption is more negative at 36 degrees C. Since the adsorption process is more favorable at 22 degrees C, there is a substantial entropy contribution to the overall process, suggesting that hydrophobic interactions are also dominating the adsorption of CHAPS on latex particles. The orientation of the hydroxyl groups in the steroid nucleus of the surfactant is mainly responsible for the aggregation and adsorption behavior of CHAPS. Indeed, the mechanisms of the micellization and the adsorption processes are strongly related: both are driven by hydrophobic interactions between the apolar faces of the CHAPS molecules (micellization) or between the hydrophobic parts of the molecules and the hydrophobic latex particle surface (adsorption).