Two alkyl chains of sulfoxides in water; Intramolecular association

Abstract

The CMC or solubility of a series of sulfoxides in water was measured by the surface tension method. The thermodynamic parameters of micellization or solution were calculated from these values and discussed in terms of the intramolecular association between two alkyl chains in the monomeric state. The two alkyl chains of some sulfoxides are too long or too similar to form micelles. Sulfoxides, alkylsulfinylalkanols, and sodium alkyl sulfates (different in the position of the sulfate group), which all have two hydrocarbon chains on a molecule, were classified into the three types: IIL, IIS, and IIE. These types, respectively, correspond to molecules in which a variable chain is longer, shorter, and equal compared with the other chain. The logarithm of CMC or solubility showed a linear relationship with the total number of carbon atoms in the hydrocarbon chains and the slope of this straight line was in the order, single chain ≑ IIL > IIE > IIS. This order was explicable on the basis of an increase in the water contact area accompanying the addition of one methylene group, taking into account the intramolecular association. The increment per methylene in the enthalpy of micellization for type IIS was positive, whereas that for other types was negative.