On understanding hydrophobicity : Chapter 7

• In phenomena such as micellization, solubilization and detergency the hydrophobicity is the driving force. This force causes nonpolar parts of surfactant molecules to minimize their contact with water. It is shown that the adsorption of surfactants on hydrophobic surfaces, such as latexes, is totally dominated by the hydrophobic effect. Thus, the nature of the latex particle surface only plays a minor role. The adsorption of surfactants on hydrophobic surfaces is therefore akin to micellization. Thermodynamic analysis of these phenomena reveal that the hydrophobic effect consists of two parts. The first part is due to the structuring of water molecules around a hydrophobic group resulting in a decrease in entropy, enthalpy as well as Gibbs free energy. It is thus favourable and counteracts the hydrophobic effect. The second contribution is due to the large amount of energy that is required to form a cavity in the water in order to accommodate a hydrophobe This contribution is attributed to the large cohesive energy density in water, which in turn is due to the small size of water molecules, meaning that many hydrogen bonds have to be broken in order to form a cavity for the hydrophobe. This second contribution dominates over the first one and is the cause for the hydrophobic effect. On the other hand the temperature dependence of the hydrophobic effect is totally dominated by the first contribution i.e. the water structuring.

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