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 water and surfactant micelles. In order to better understand the effect of length of the aliphatic chain and polarity of the hydrophilic head of the surfactant on the thermodynamic properties of alcohols in micelles, the volume and heat capacity of transfer of pentanol (PenOH) from water to aqueous solutions of surfactants were either taken from the literature or measured for various cationic, anionic, and nonionic surfactants of chain lengths between 6 and 12. The transfer quantities were analyzed through the principle of relative hydrophobicities (J. Colloid Interface Sci. 1988, 122, 418), which states that ''an extremum appears in the transition region of the transfer function only if the transfered 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;parameter between PenOH and surfactant monomers, and the volume and heat capacity of transfer of PenOH from water to the micelles all vary linearly with the chain length of: the surfactant in the direction of stronger hydrophobic interactions. This suggests that, with longer-chain surfactants, there is a stronger tendency for PenOH to dissolve in-the interior of the micelle. There appears to be notable dependence of the thermodynamic 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 transfer for PenOH with anionic surfactants compared to cationic surfactants are probably related to stronger interactions between the head groups in the palisade layer of the anionic micelle. It is also shown that transfer functions can be used to predict the thermodynamic properties of mixed micelles.