Fourier transform NMR pulsed-gradient spin—echo (FT-PGSE) self-diffusion measurements of solubilization equilibria in SDS solutions

Solubilization equilibria for homologous series of alcohols, methyl ketones, aromatics, and crown ethers with SDS micelles in D 2 O solution at 25°C were determined. The measurements are based on multicomponent self-diffusion studies through an improved version of a Fourier transform NMR pulsed-gradient spin-echo technique. As expected, the degree of solubilization increases strongly with the number of carbon atoms in the solubilizate alkyl chains, corresponding to an increment in the standard free energy for transfer of solubilizate molecules from the aqueous to the micellar phase (ΔG p o ) of ≈ −2.6 kJ mole −1 for each −CH 2− group. Hydrocarbon chain branching in the solubilizate molecules decreases the degree of solubilization, as compared to conditions for n -alkyl compounds. Crown ether solubilization is found to increase strongly with ring size. Comparative studies with lauryltrimethylammonium bromide micelles do not show this trend, suggesting that the effect can be ascribed to the sodium binding capability of the larger crown ethers, and that crown ethers are “solubilized” at the SDS micellar surface.