Barry W. Ninham

Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers

A simple theory is developed that accounts for many of the observed physical properties of micelles, both globular and rod-like, and of bilayer vesicles composed of ionic or zwitterionic amphiphiles. The main point of departure from previous theories lies in the recognition and elucidation of the role of geometric constraints in self-assembly. The linking together of thermodynamics, interaction free energies and geometry results in a general framework which permits extension to more complicated self-assembly problems.

Theory of self-assembly of lipid bilayers and vesicles

A simple theory is developed that explains the formation of bilayers and vesicles and accounts quantitatively for many of their physical properties: Properties including vesicle size distributions and bilayer elasticity emerge from a unified theory that links thermodynamics, interaction free energy, and molecular geometry. The theory may be applied to the analysis of more complicated membrane structures and mechanisms.

Electrostatic potential between surfaces bearing ionizable groups in ionic equilibrium with physiologic saline solution

Abstract In the past calculations of electrostatic properties of interacting cellular surfaces have been restricted by assumptions of fixed surface charge or surface potential. For the most part these calculations have been confined to a linear approximation and neglect the small but important complement of divalent cations in the cellular environment. In the present paper these limitations are removed. Solutions are obtained to the full non-linear Poisson-Boltzmann equation, treating the fraction of dissociated ionizable surface groups as a self-consistent functional of the electrostatic potential. The potential is expressed in terms of relations between Jacobian elliptic functions. A general and efficient method of computation is developed through the theory of Jacobian theta functions. The treatment given here offers wide flexibility in dealing with cell surfaces in the languages of the cell physiologist, biochemist and physical chemist.

Micelles, vesicles and microemulsions

A theory of self-assembly of surfactant molecules into micelles and bilayers is critically examined and extended to include vesicles and microemulsions. The notion of hydrophilic–lipophilic balance is quantified. The theory gives a unified account of type, size and shape of the aggregates which form under various conditions. Observed trends due to change in salt concentration, temperature and oil type, and due to the addition of cosurfactants, are correlated and emerge from a simple global framework.

Interactions between water—stable hydrophobic Langmuir—Blodgett monolayers on mica

Abstract A Langmuir-Blodgett technique was used for depositing monolayers of dimethyldioctadecylammonium ions on molecularly smooth muscovite mica surfaces. Direct measurements of the interaction between such surfaces were performed with the surface force apparatus of Israelachvili. A long—range attractive force considerably stronger than the expected van der Waals force was observed. Studies on the salt dependence of this force indicate that it does not have an electrostatic origin. Instead the present results support the view that perturbations of the dynamic association of water molecules outside a hydrophobic surface are propagated several nanometers.

A model of solvent structure around ions

The nature of solvent structure around ions is considered using a model of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles. The statistical mechanics of this model is treated in the mean spherical approximation which is a natural extension of the Debye–Huckel theory of electrolytes to include discrete charges and dipoles of finite size. Our results include (i) a modified expression for the Born energy which had been used empirically to fit solubility data, (ii) explicit forms for the polarization density about an ion from which we can deduce the orientation order of the dipolar solvents and the validity or otherwise of the concept of a ’’local’’ dielectric constant near charged bodies, and (iii) the form of the interaction free energy (potential of mean force) between ions at separations comparable to the solvent size. In presenting these results which are given in detail in Section IV only a familiarity with the general description of this model given in ...

Specific Ion Effects: Why the Properties of Lysozyme in Salt Solutions Follow a Hofmeister Series

Protein solubility in aqueous solutions depends in a complicated and not well understood way on pH, salt type, and salt concentration. Why for instance does the use of two different monovalent salts, potassium thiocyanate and potassium chloride, produce such different results? One important and previously neglected source of ion specificity is the ionic dispersion potential that acts between each ion and the protein. This attractive potential is found to be much stronger for SCN(-) than it is for Cl(-). We present model calculations, performed within a modified ion-specific double-layer theory, that demonstrate the large effect of including these ionic dispersion potentials. The results are consistent with experiments performed on hen egg-white lysozymes and on neutral black lipid membranes. The calculated surface pH and net lysozyme charge depend strongly on the choice of anion. We demonstrate that the lysozyme net charge is larger, and the corresponding Debye length shorter, in a thiocyanate salt solution than in a chloride salt solution. Recent experiments have suggested that pK(a) values of histidines depend on salt concentration and on ionic species. We finally demonstrate that once ionic dispersion potentials are included in the theory these results can quantitatively be reinterpreted in terms of a highly specific surface pH (and a salt-independent pK(a)).

The structure of electrolytes at charged surfaces: The primitive model

The effects of ion size on the structure of a primitive model electrical double layer at a charged surface is considered using the integral equation method based on a combined hypernetted chain and mean spherical approximation (HNC/MSA). The HNC/MSA is shown to be a nonlinear weak field approximation. The contact values of the surface ion distribution functions are shown to have the correct quadratic dependence on the surface charge density, An analytical comparison between the HNC/MSA and earlier work based on the modified Poisson–Boltzmann equation is given.

Device for measuring the force and separation between two surfaces down to molecular separations

We present an apparatus for measuring the force as a function of distance between surfaces at separations down to the order of molecular dimensions. The device is a simplified yet improved version of the surface force apparatus first developed by Israelachvili and Adams [J. Chem. Soc. Faraday Trans. 1, 74, 975 (1978)]. It gives the same measurement resolution and has the same possibilities of studying various phenomena in thin films such as friction, viscosity, adhesion, and phase transitions. It offers improved performance with regard to control of surface separation and increased versatility by virtue of variable chamber dimensions and geometry.

Critical micelle concentrations for alkyltrimethylammonium bromides in water from 25 to 160°C

Critical micelle concentrations were determined by conductance measurements for decyl-, dodecyl-, tetradecyl- and hexadecyltrimethylammonium bromide in water at 25, 60, 95, 130, and 160°C. The results are discussed in terms of the equilibrium model and the nonlinear Poisson-Boltzmann model for micelle formation. The free energies of transferring a methylene group from water to the oil-like interior of the micelle are found to be −781 at 25°C, −796 at 60°C, −819 at 95°C, −815 at 130°C, and −787 at 160°C cal-mol−1.