Micellization and adsorption characteristics of CHAPS

The adsorption of CHAPS on hydrophobic latex particles was studied at 22 an d 36 degrees C by determining the adsorbed amount and the enthalpy of adsor ption. The adsorption process was compared to the micellization of the surf actant. Therefore, the critical micelle concentration (cmc) and the heat of micellization were also determined at both temperatures. From these two qu antities the Gibbs energy and the entropy of the process were calculated. T he cmc and the heat of micellization are temperature dependent; the cmc inc reases as the temperature rises, and the heat of micellization goes from po sitive at 22 degrees C to negative at 36 degrees C. There is an entropy-ent halpy compensation in the micellization process, characteristic of hydropho bic interactions. The maximum adsorbed amount is independent of the tempera ture, while the initial slope of the isotherms is slightly steeper at 22 de grees C. Although the adsorption process is exothermic at both temperatures , the enthalpy of adsorption is more negative at 36 degrees C. Since the ad sorption process is more favorable at 22 degrees C, there is a substantial entropy contribution to the overall process, suggesting that hydrophobic in teractions are also dominating the adsorption of CHAPS on latex particles. The orientation of the hydroxyl groups in the steroid nucleus of the surfac tant is mainly responsible for the aggregation and adsorption behavior of C HAPS. Indeed, the mechanisms of the micellization and the adsorption proces ses are strongly related: both are driven by hydrophobic interactions betwe en the apolar faces of the CHAPS molecules (micellization) or between the h ydrophobic parts of the molecules and the hydrophobic latex particle surfac e (adsorption).