Abstract
Water and oil do not mix. This essential statement of the hydrophobic effect explains why oil-in-water (O/W) emulsions are unstable and why energy must be supplied to form such emulsions. Breaking O/W emulsions is an exothermic event. Yet metastable O/W emulsions can be prepared with only water acting as the stabilizer by the adsorption of hydroxide ions formed from the enhanced autolysis of interfacial water. The heat of desorption of the hydroxide ions from the oil-water interface is not directly accessible but is obtained from the difference between the heat of reaction and the sum of the neutralization and interfacial heats when an emulsion is broken by the addition of acid. This experimental value of 28.4 k(B)T is in good agreement with the theoretical estimate of 16-20 k(B)T made from the fluctuation/correlation model of the hydrophobic force and the value of 14 k(B)T obtained recently from surface spectroscopy. Subsequent verification of the force driving ions to hydrophobic surfaces is shown for tetrabutylammonium bromide with a dielectric decrement value of 26 M(-1) compared to 20 M(-1) for NaOH. The positive cation preferentially adsorbs at the oil-water interface over hydroxide ions in agreement with the predicted model.