A Simple Interpretation of Hydrophobic Interactions and Critical Concentrations in Micellar Solutions

Abstract

Surfactant molecules possess a hydrophobic tail and a hydrophilic head group. Because of this dual property, they form in water (or other polar liquids) a large number of relatively large aggregates, as soon as their concentration becomes greater than the critical micelle concentration[1,2]. In the aggregates, the hydrocarbon tails are shielded from water by the polar head groups. Two factors are mainly responsible for the formation of micelles. One of them, the hydrophobic bonding, is primarily due to the incompatibility of the hydrocarbon tails of the surfactant with the polar solvent. Indeed, the tails interfere with the strong polar interactions between the water molecules and, therefore, the free energy of the system is decreased by diminishing their contact with water. Should this factor act alone, the surfactant molecules will form “infinite size” aggregates, thus leading to a separate surfactant phase. The repulsion that arises between the head groups competes with the hydrophobic bonding and ensures the formation of a large number of finite (but relatively large) size aggregates. The present paper is concerned with the mechanistic interpretation of the hydrophobic bonding, as well as with the critical micelle concentration in both polar and nonpolar solvents. The latter quantity provides information about the conditions under which a large number of aggregates can form.