Volume constraint effect on solute partitioning to Triton X-100 micelles in water

On the basis of the simple two-phase model the partition constants (K_d) of various organic solutes and metal chelates between aqueous and Triton X-100 micellar phases are derived spectrophotometrically. The magnitude and the order of K_d are discussed in terms of molecular size, hydrophilicity and ion-association character of the solutes. The K_d values of neutral species increase almost linearly with their van der Waals molar volumes until the critical value near 110 ml mol^-1 is reached. Above this critical volume, the constraint effect of the micelles gives rise to severe levelling off the partition constants at ca. 10^3.0 for bulky solutes. The K_d values of 2-(thiazol-2-ylazo)-4-tert-octylphenol, 2-(thiazol-2-ylazo)-4-methylphenol (HL) and the metal chelates of [Fe^IIL₂] and [Ni^IIL₂] are in a very narrow range of 10^2.83, 10^2.93, 10^3.06 and 10^3.16, respectively. The effects of micellar size are also examined using Brij-35 and Brij-58 surfactants.