Robert Aveyard

Interfacial tensions at alkane-aqueous electrolyte interfaces

Surface tensions of aqueous electrolytes, and their interfacial tensions against n-dodecane, have been determined at 20°C. The salts studied were LiCl, NaCl, KCl, KBr, NaBr, KI and Na2SO4 at concentrations up to about 1 mol kg–1. For the alkali metal chlorides and Na2SO4 the surface and interfacial tension increments are similar for a given electrolyte. The corresponding increments for KBr, NaBr and KI however are found to differ considerably. The results are discussed in terms of both electrostatic theory and dispersion force theory of interfaces. With respect to the latter, it is found that if due allowance is made for the presence of an ion-free layer of water at the interface, an approximate approach in which only a single dominant interaction frequency is considered, gives results in reasonable accord with experiment.

Salting-out of alkanols by inorganic electrolytes

The free energies, entropies and enthalpies associated with the salting-out of alkanols (mainly butanol) by eight inorganic electrolytes have been determined at 20°C. The results are discussed in terms of the scaled particle theory (in the case of 1 : 1 electrolytes) and the thermodynamic theory of McDevit and Long. It is concluded that alternative approaches to the treatment of salting-out would be desirable. A simple, semi-empirical method for the calculation of salting coefficients is proposed. It is based on the effect which the electrolyte has on the surface tension of the solvent (water) and is found to be relatively successful for many systems.

Activity coefficients and association of n-alkanols in n-octane

The infra-red absorption at 1.405 µm, due to O—H stretching, has been determined for solutions of n-dodecanol and of n-octanol up to concentrations of 0.13 mol dm–3 in n-octane. On the assumptions that the systems behave as ideal associated solutions, and that the absorbance at 1.405 µm is due solely to OH groups in unassociated alcohol, solute activity coefficients have been calculated. The values for dodecanol agree well with those determined previously for the same system, using vapour pressure osmometry, justifying the assignment of the absorption at 1.405 µm to monomeric alcohol alone.The activity coefficients have been used to estimate the nature and extent of association. It is concluded that for concentrations up to 0.13 mol dm–3 more than one associated species coexist.

Desorption of electrolytes at liquid–vapour and liquid–liquid interfaces

The desorption of various simple inorganic salts at the decanol–water interface has been studied, and analysed together with previously obtained results for the air–electrolyte and dodecane–electrolyte interfaces. The desorption of the salts is much more dependent on the nature of the anion, at all three interface types, than it is on the cation and the results are discussed in terms of the possible interaction between anions and hydration layers at the interfaces. The dependence of free energies of adsorption of alkanols, both from alkane and from electrolyte, on the alkane–electrolyte interface has also been investigated and it is concluded that the chemical potential of the alkanol at the surface is not much affected by the salts, but the influence is greater the less the desorption of the salt.

Salting-in of alcohols by symmetrical tetra-alkylammonium bromides

The changes in free energy, enthalpy and entropy associated with the transfer of several alkanols at high dilution from water to aqueous solutions of some symmetrical tetra-alkylammonium bromides (R4NBr) have been determined at 20°C. The enthalpies were determined calorimetrically for several salt concentrations.The treatment of salt effects based on the scaled particle theory has been applied to the salting-in of butanol by solutions of R4NBr. For this purpose it was necessary to obtain the cation diameter indirectly by fitting experimental salting-in data for alkanes to the theory. It is concluded that the theory is as successful for butanol as it is for the alkanes.The thermodynamic parameters of transfer are consistent with the assumption that the salting-in of alkanols (and alkanes) by R4N+ ions is caused by the formation of short-lived cation-solute “pairs” and the resultant disruption of water structure surrounding both the cation and non-electrolyte.

Adhesion at the alkane/water and ester/water interfaces

The surface tension of 1-decanol and of several methyl esters of n-alkanoic acids, and the interfacial tension of these materials against water, have been determined in the temperature range 20–32°C. The interfacial tensions of n-nonane and n-undecane against water in the range 20–30°C are also reported. A simple model is proposed to account for the variation with chain length of the thermodynamic parameters of adhesion of the liquids with water. The results are consistent with n-alkanes assuming an orientation parallel to the water surface and the esters being essentially normal to the surface.

Work and entropy of adhesion at liquid–liquid interfaces and the relationship to salt desorption

Works of adhesion of cyclohexane, toluene, chloro-octane and nonan-2-one, with water, have been determined as a function of temperature. From these results, and others in the literature, it is shown that a rectilinear relationship exists between the work and entropy of adhesion. Results for systems where water is replaced by ethan-1,2-diol do not conform to this relationship, and it is concluded that the linearity observed for the aqueous systems is a result of the promotion, by the organic liquids, of interfacial water structure. It is also found that the desorption of NaCl at the aqueous electrolyte–organic liquid interface is less the greater the work of adhesion between water and the organic liquid.