The equilibrium surface tension of anionic surfactant n-decyl sulfate (DS-) for various monovalent (alkali) counterions was investigated. It was found that surface activity of surface chemically pure DS- significantly increases with decreasing hydrated size of the counterion. We describe our experimental results in terms of the previously developed adsorption model, which assumes that the counterions may penetrate the Stern layer where the surfactant headgroups are adsorbed. The headgroups and counterions have a finite size that leads to the surface exclusion effects in the adsorption isotherm. The model is improved by explicitly taking into account the electric interactions between adsorbed ions in the adsorbed layer. We obtain a good correlation between the relative counterion size in the Stern layer, the measure of the area excluded by the adsorbed counterions, and the effective radius of the hydrated counterion in the solution. The limiting areas per molecule at the critical micelle concentration for the adsorbed decyl sulfate for various counterions are in good agreement with those measured by neutron scattering.

No abstract

Thermodynamics of micelle formation of anionic surfactants was investigated by using isothermal titration calorimetry (ITC). Highly purified decyl and dodecyl sulfates have been used to analyze the effect of counterions (Li+, Na+, K+, and Cs+) on critical micelle concentration (cmc) and enthalpy of micellization (ΔH mic) determined between 10 and 60 °C. The enthalpy of micellization decreases strongly with increasing temperature and passes trough zero (endothermic to exothermic processes), while the cmc versus temperature exhibits a minimum. At a given temperature and for a fixed chain length, the decrease of cmc and ΔH mic in the order Li+ > Na+ > K+ > Cs+ is related to the increase of the binding of counterions to micelles. The electrostatic repulsions between ionic headgroups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The heat of dilution of micelles is markedly dependent on temperature and is correlated with the temperature-dependent shape of micelles. The cmc concept has an exact meaning within the so-called phase separation model of micelle formation. Therefore, free energy and entropy were deduced from the cmc and enthalpy of micellation using this model by taking into account the counterion binding. The temperature changes of ΔS mic indicate that the process of micellization is entropically driven. ΔG mic is always negative (thermodynamically favored process) and slightly temperature and counterion dependent.

Defined surfactants of general formula [(CH3)3SiO]2CH3Si(CH2)3(OCH2CH2)3–9OCH3, have been synthesized from the corresponding oligoethylene glycol monoallyl monomethyl ethers via hydrosilylation. The concentration‐dependent spreading performance on hydrophobized silicon wafers has been investigated and compared with that of Silwet L77. For the hexaethylene glycol derivative the highest initial spreading velocities and largest spreading areas were found. Since Silwet L77 spreads faster than all the other derivatives under investigation, a synergistic effect of different compounds is unlikely. Minor differences were found for handshaken and sonicated solutions. Copyright © 1999 John Wiley & Sons, Ltd.