VOLUME AND HEAT-CAPACITY OF N-PENTANOL IN AQUEOUS SURFACTANTS - EFFECT OF CHAIN-LENGTH AND POLARITY OF THE HEAD GROUP

Medium chain length alcohols tend to distribute themselves between wat er and surfactant micelles. In order to better understand the effect o f length of the aliphatic chain and polarity of the hydrophilic head o f the surfactant on the thermodynamic properties of alcohols in micell es, the volume and heat capacity of transfer of pentanol (PenOH) from water to aqueous solutions of surfactants were either taken from the l iterature or measured for various cationic, anionic, and nonionic surf actants of chain lengths between 6 and 12. The transfer quantities wer e analyzed through the principle of relative hydrophobicities (J. Coll oid Interface Sci. 1988, 122, 418), which states that ''an extremum ap pears in the transition region of the transfer function only if the tr ansfered solute is more hydrophobic than the main solute'', and with a chemical distribution model (J. Solution Chem. 1984, 13, 1; 1987, 16, 529). The distribution constant K-D, the volumetric interaction;param eter between PenOH and surfactant monomers, and the volume and heat ca pacity of transfer of PenOH from water to the micelles all vary linear ly with the chain length of: the surfactant in the direction of strong er hydrophobic interactions. This suggests that, with longer-chain sur factants, there is a stronger tendency for PenOH to dissolve in-the in terior of the micelle. There appears to be notable dependence of the t hermodynamic parameters on the nature of the head group in the case of K-D and volumes of transfer. The larger K-D and smaller volume of tra nsfer for PenOH with anionic surfactants compared to cationic surfacta nts are probably related to stronger interactions between the head gro ups in the palisade layer of the anionic micelle. It is also shown tha t transfer functions can be used to predict the thermodynamic properti es of mixed micelles.