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Biomedical subjects

B W Ninham

Publications and source records attributed to B W Ninham.

At least 19 recordsLinked to original sources
PubMed2001

Specific ion effects: why DLVO theory fails for biology and colloid systems.

The classical Derjaguin-Landau-Verwey-Overbeek theory that underpins colloid and surface science is shown to be flawed, especially at biological salt concentrations. This is in part because the dispersion forces acting on the ions are ignored. When these are included properly very different results are obtained. These results have substantial implications for biological and for ordinary colloid systems at moderate salt concentrations.

Algorithms↗
PubMed2001

Supra-aggregates.

Mixtures of oil (isooctane), water, and a double-chained ionic surfactant, Cu(AOT)2 [copper(II) bis(2-ethylhexyl)sulfosuccinate] form equilibrium phases consisting of spontaneous emulsions thermodynamically stable systems. The experimental conditions are such that the head polar group of the surfactant is totally hydrated. The emulsion droplets are comprised of supra aggregates, lamellar spherulites in which interior and exterior are bicontinuous microemulsions. The microstructures and component ratios can be predicted from elementary considerations that require only the notion of local and global packing constraints. The phenomenon of supra aggregation, microphases within any topologically closed container, in equilibrium with an external phase, appears quite general. The results throw some light on the meaning of 'phase' in mesostructured fluids.

Journal Article↗
PubMed1999

Mechanism of low-threshold hypersonic cavitation stimulated by broadband laser pump.

A low threshold acoustic cavitation regime was observed for the excitation of hypersonic waves due to a stimulated Brillouin scattering (SBS) mechanism, when the optical pump lies within the uv frequency range. Cavitation occurs if the optical pump bandwidth Delta(+)>>Omega(0), where Omega(0) is the Stokes frequency shift (the hypersonic frequency). In the opposite case (Delta(+)<<Omega(0)), cavitation does not occur despite the fact that the hypersonic wave intensity is much higher. The effect is associated with the stimulation of a broad frequency spectrum of hypersonic pressure in a field provided by the broadband optical pump. In contrast, for a monochromatic optical pump, the hypersonic wave is of single-frequency character. Induction of cavitation at the low intensities of acoustic pressure is attributed to nanobubbles of fixed size that occur in the liquid. The resonant frequency of the nanobubbles coincides with the frequency of some spectral component of hypersound present in the broadband SBS process. That conclusion is reinforced by the further observation that at the same intensity of broadband pumping the cavitation vanishes after degassing the liquid. In parallel experiments on four-photon polarization Rayleigh wing spectroscopy, it was also demonstrated that spectral lines exist in ordinary (not degassed) water, which can be ascribed to resonances of radial vibrations of nanobubbles. Those lines are absent in the degassed water spectrum.

Journal Article↗
PubMed1996

Dressed polyions, counterion condensation, and adsorption excess in polyelectrolyte solutions.

The phenomenon of Manning-Oosawa counterion condensation is given an explicit statistical mechanical and qualitative basis via a dressed polyelectrolyte formalism in connection with the topology of the electrostatic free-energy surface and is derived explicitly in terms of the adsorption excess of ions about the polyion via the nonlinear Poisson-Boltzmann equation. The approach is closely analogous to the theory of ion binding in micelles. Our results not only elucidate a Poisson-Boltzmann analysis, which shows that a fraction of the counterions lie within a finite volume around the polyion even if the volume of the system tends towards infinity, but also provide a direct link between Manning's theta-the number of condensed counterions for each polyion site-and a statistical thermodynamic quantity, namely, the adsorption excess per monomer.

Adsorption↗
PubMed1989

Conformation of surface bound polyelectrolytes: I. Implications for cell electrophoresis.

We present a self-consistent calculation for a layer of short-chain polyelectrolytes grafted to a planar interface. Individual self-avoiding chain configurations are generated on a tetrahedral lattice. By evaluating the complete ensemble of configurations in the presence of a self-consistent field we determine averages for the equilibrium layer height, segment and charge density profiles. These describe the response of the layer to changes in the surrounding medium, in particular to electrolyte concentration. This self-consistent model of a biological cell, which is known to have an extended layer of charge exterior to the membrane bilayer surface, is applied to the problem of cell electrophoretic mobility. We compare the electrophoretic mobility deduced from the application of this model with that for a constant profile of charge and segments and with the case of a constant segment profile with a plane of charge, usually assumed in the literature. We find that, within the region of concentrations studied here and with the chain lengths used, there is general agreement between the different descriptions.

Animals↗
PubMed1988

The attractive forces between polar lipid bilayers.

Long-range attractive forces between lipid bilayers are not well described by the Lifshitz theory of Van der Waals forces between macroscopic media. It is shown that when correlations between polar headgroups are taken into account, the predicted forces take a qualitatively different form consistent with the measured data.

Biophysical Phenomena↗
PubMed1985

Attractive forces between uncharged hydrophobic surfaces: direct measurements in aqueous solution.

Long, double-chained alkylammonium acetate surfactants are soluble in water and, under suitable conditions, adsorb onto sheets of muscovite mica, forming an electrically neutral, hydrophobic surface. Attractive forces measured between such surfaces are 10 to 100 times stronger than expected from van der Waals theory over distances D up to about 10 nanometers. The forces decay exponentially [with a force proportional to exp(-D/1.4)] instead of following the power-law behavior of continuum theory. The results of these and earlier experiments indicate that the strength of these attractive forces depends critically on the degree of hydrophobicity of the surface and is due to the long-range influence of the surface on the structure of water. In addition, for very hydrophobic surfaces, the cavitation effects on pulling the surfaces apart are described.

Adsorption↗
PubMed1985

Immunosuppressive effects of cationic vesicles.

Double chained cationic amphiphiles closely related to those in widespread household and commercial use have been shown to inhibit the generation of cytoxic T lymphocytes in mixed lymphocyte reactions in vitro, and to inhibit inflammatory responses caused by the injection of alloreactive cytotoxic lymphocytes into the footpads of mice. The immunosuppressive effects are not due to non-specific toxicity, but are probably due to inactivation or deletion of cytotoxic T cell precursors in the in vitro experiments, and modification of the inflammation induced by activated T cells in vivo. Apart from the cautionary aspects in relation to public health, the results offer a new approach to immunology and cell biology in terms of modification of cell membranes, and a rationale for the systematic design of a class of immunosuppressive drugs.

Animals↗
PubMed1977

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.

Liposomes↗
PubMed1977

Phase transition in charged lipid membranes.

Experimental results on the effect of electrostatics on bilayer phase transitions are compared with corresponding data for monolayers and the predictions of electrical double layer theory. The two substantial conclusions which emerge are that: (i) double layer theory based on a continuous surface charge distribution cannot explain all the relevant data, a situation which may be improved by taking into account the discrete nature of the surface charge distribution; (ii) the crystal - liquid crystal phase transition of charged bilayer membranes is always a continuous one which takes place through an intermediate state consisting of both fluid and frozen domains.

Electricity↗
PubMed1973

Van der Waals forces between cylinders. II. Retarded interaction between thin isotropic rods.

We derive the retarded van der Waals interaction between two long thin parallel dielectric cylinders immersed in a solvent. The result shows that the ultraviolet interactions which may be responsible for the specificity of macromolecular interactions do not operate strongly over distances R >/= 50 A. For greater distances the contribution of these frequencies xi is damped by a factor infinity e(-xiR/c).

Macromolecular Substances↗
PubMed1970

Van der Waals forces. Special characteristics in lipid-water systems and a general method of calculation based on the Lifshitz theory.

A practical method for examining and calculating van der Waals forces is derived from Lifshitz' theory. Rather than treat the total van der Waals energy as a sum of pairwise interactions between atoms, the Lifshitz theory treats component materials as continua in which there are electromagnetic fluctuations at all frequencies over the entire body. It is necessary in principle to use total macroscopic dielectric data from component substances to analyze the permitted fluctuations; in practice it is possible to use only partial information to perform satisfactory calculations. The biologically interesting case of lipid-water systems is considered in detail for illustration. The method gives good agreement with measured van der Waals energy of interaction across a lipid film. It appears that fluctuations at infrared frequencies and microwave frequencies are very important although these are usually ignored in preference to UV contributions. "Retardation effects" are such as to damp out high frequency fluctuation contributions; if interaction specificity is due to UV spectra, this will be revealed only at interactions across <200 angstrom (A). Dependence of van der Waals forces on material electric properties is discussed in terms of illustrative numerical calculations.

Electrochemistry↗