Bernard A. Pailthorpe

Lipid packing and transbilayer asymmetries of mixed lipid vesicles

Predictions, based on a previously developed theory, of the radii and asymmetric lipid distribution of mixed phosphatidylcholine/lysophosphatidylcholine and phosphatidylcholine/cholesterol vesicles of variable composition are presented. The results compare well with available experimental data, except for cis-unsaturated phosphatidylcholine/cholesterol vesicles at high concentrations of cholesterol. It is concluded that specific lipid-lipid interactions need not be invoked for saturated and trans-unsaturated phosphatidylcholine mixed with lysophosphatidylcholine or cholesterol. A discussion of the effect of packing stresses on induced flip-flop and non-spherical vesicles is also presented.

Short-range interactions mediated by a solvent with surface adhesion

At short range, interaction between two surfaces mediated by a solvent with surface adhesion is calculated in the hypernetted-chain approximation. A smooth monotonic attraction is found when the solvent-surface attraction is weak. There is also significant depletion of fluid densities near such surfaces. Depending on the solvent parameters, the interaction between a weakly attractive and strongly attractive surface can be repulsive or attractive. Extension of these calculations to include a Lennard-Jones solvent also indicates monotonic attractions.

Ion–solvent interactions and the activity coefficients of real electrolyte solutions

A thermodynamic argument is developed which gives the activity coefficient of a real electrolyte solution in terms of a formula involving integrals over the radial distribution functions of the several ionic species, together with additional terms which take account of ion–solvent interactions. The ion–solvent interaction contributions can be calculated from a knowledge of the molal volume of the solution. Both required quantities are observable and easily accessible. This permits an evaluation of the validity of the restricted primitive model and of Debye–Huckel theory for real electrolyte solutions.

On the theory of dipolar fluids and ion–dipole mixtures

The Stillinger–Lovett condition for the second moment of ion–ion distribution functions is obtained from first principles for an ion–dipole mixture. The Debye–Huckel form for the asymptotic behavior of the potential of mean force between ions is shown to emerge for arbitrary density of the dipolar solvent in the limit of low ionic concentration.

Solvent structure in particle interactions. Part 2.—Forces at short range

At small separations, less than 5–10 molecular diameters, standard theories of dispersion interactions between particles immersed in a liquid break down. The Lifshitz theory is valid only asymptotically and must be replaced by a more general theory which takes into account liquid structure near interfaces. The nature of structural forces is explored for a model system comprising two attractive half-spaces separated by a thin liquid film [Lennard-Jones fluid]. An exponentially decaying attractive potential is found, in agreement with earlier predictions based on mean field theories.

Competitive adsorption from binary mixtures: Adhesive hard sphere model

Abstract A model study is given of the phenomenon of competitive adsorption using the theories of liquid state physics. The adsorbate is modeled as a mixture of hard spheres with a short-range attraction. The composition dependence of the specific surface excess is studied as a function of molecular size ratio and intermolecular potentials.

Solvent structure in particle interactions. Part 1.—The asymptotic regime

The structural contribution to dispersion energies is obtained for the interaction of two solute molecules immersed in a liquid, for a solvent molecule with a wall, and for two half-spaces separated by a liquid. The interaction potentials are derived from the Ornstein–Zernike equations using the known asymptotic forms of the direct correlation functions. While the expressions obtained derive from a specific molecular interaction model they have wider validity. Significant changes emerge in the usual Hamaker or Lifshitz expressions based on a continuum solvent picture even at distances of the order of 50 A for systems with small Hamaker constants.

Dispersion interactions across binary liquid mixtures. A proper account of structural effects

Abstract We have extended earlier work on the effects of solvent structure upon dispersion interactions to the case involving binary mixtures at arbitrary densities. Specifically, asymptotic expressions for the potential of mean force between a planar substrate and a molecule in a binary solution and also the interaction energy of two planar half spaces across a binary mixture are obtained. Both of these results include effects due to inhomogeneities in the intervening medium under the action of surface forces. An anomaly in a result of Deryaguin and Churaev has been traced to an invalid assumption about the additivity of molecule-substrate interaction potentials.

Solvent Structure and Hydrophobic Solutions

In a paper on aqueous solutions of nonpolar solutes published in 1973 F. H. Stillinger(827) acknowledged his debt to H. S. Frank in these words: The author has enjoyed a protracted and constructive dialogue with Professor H. S. Frank on the nature of water and aqueous solutions. This interaction has helped the former to maintain in his work a respectable balance between the sterile intricacy of formal theory and the seductive simplicity of poetic “explanation.”

Solvent mediated interactions—solute size effects and predictions of mean field theory

Abstract We investigate the effect of solute/solvent size ratio on the solvent mediated potential of mean force between solutes at infinite dilution. Predictions of Landau—Ginzburg mean field theory are compared with a Percus—Yevick—hypernetted chain theory of a mixture of infinitely dilute hard sphere solutes in a solvent of hard spheres with surface adhesion (sticky spheres).