The effect of structural modifications on the solution and interfacial properties of straight and branched aliphatic alcohols: the role of hydrophobic effects.

The effect of structural modifications, such as branching of the hydrocarbon chain on the solution and interfacial properties of short-chain aliphatic alcohols has been investigated. Surface tension measurements have been used to study the adsorption of the alcohols at the aqueous solution/air interface from water/alcohol mixtures, and to determine the aqueous solubilities of the alcohols. The related process of the partitioning behaviour of the alcohols between two immiscible phases has also been studied. Standard free energies of adsorption at the aqueous solution/air interface, standard free energies of transfer between water and hexane, and standard free energies of solution were obtained for the alcohols. A linear "Traube" relationship, an elegant demonstration of the hydrophobic effect, was found to exist between the various free energies for the normal alcohols and the number of carbon atoms in the alcohol molecule. The free energies showed that structurally modified alcohols have less negative free energy of adsorption, transfer or solution compared to the normal alcohol with the same number of carbon atoms. We assign effective numbers of carbon atoms for each branched alcohol for each transfer process. The position of the hydroxyl group relative to the branched part of the molecule was found to be a factor which influences the hydrophobic contribution to the free energy of each transfer process. An attempt has been made to ascertain whether there is a correlation between the molecular surface area, or the molecular volume, and the interfacial and solution thermodynamic properties of an alcohol in aqueous solution. The standard free energies of some of the branched alcohols have been found to be inconsistent with the values expected from the energetic of the adsorption and the water/hexane transfer processes. This is thought to reflect the different modes of association of the branched and normal alcohols in the liquid state as revealed by small and wide angle X-ray scattering which identifies correlation peaks attributable to intermolecular hydrogen bonding and interchain associations consistent with non-polar segmentation. Crown