Abstract
Abstract Surface potentials, ψg, of mixtures of synthetic phosphatides dispersed in uni-univalent electrolyte have been calculated from the limiting molecular areas through use of the Gouy equation. Electrophoretic mobilities of the dispersed particles were measured and used to calculate ζ potentials from the Helmholtz-Smoluchowski equation. In analogy with several structurally related systems, the ratio ζ/ψg was found to be unity at low surface charge density (σ l 20,000 e.s.u. cm-2), decreasing progressively at higher surface charge density to a minimum of 0.58. The ζ potential at low surface charge density is appreciably affected by the composition of the fatty acyl substituents to the extent that this determines the cross-sectional area per net negative charge in the bilayer. Charge reversal concentrations have been measured for various metal ions on several phosphatide surfaces differing in charge density. Inserting the conditions for charge reversal into the Stern equation, the standard free energy of adsorption for each cation was calculated and used to obtain a value for the logarithm of the apparent association constant, K'. At an initial surface charge density below 20,000 e.s.u. cm-2, the sequence of values of log K' was Ag l Ba l Sr l Ni l Mg l Ca l Co l Zn l Cu l Mn l Pb l Cd l La l Ce l Th l UO2. Comparison of log K' for magnesium, calcium, strontium, and barium as a function of surface charge density suggests that the larger cations are able to form ion triplets at high surface charge density while the smaller cations are unable to do so. The high affinity of UO22+ for phosphatide surfaces is also considered. The results obtained are discussed in terms of their potential applicability to several problems of current biochemical interest.
Highlights
Surface potentials, tic, of mixtures of synthetic phosphatides dispersed in uni-univalent electrolyte have been calculated from the limiting molecular areas through use of the
Charge reversal concentrations have been measured for various metal ions on several phosphatide surfaces differing in charge density
The surface potential, #G, can be calculated from the Gouy equation, assuming 1 net negative charge for each acidic species at pH 4.8 and using information on molecular areas of the phosphatides obtained from force-area curves [15, 16]
Summary
Tic, of mixtures of synthetic phosphatides dispersed in uni-univalent electrolyte have been calculated from the limiting molecular areas through use of the. Electrophoretic mobilities of the dispersed particles were measured and used to calculate { potentials from the Hehnholtz-Smoluchowski equation. The surface potential, #G, can be calculated from the Gouy equation, assuming 1 net negative charge for each acidic species at pH 4.8 and using information on molecular areas of the phosphatides obtained from force-area curves [15, 16]. Adsorption of metal ions onto a negatively charged surface can cause changes in the magnitude and sign of .C. it is recognized that the transport and equilibrium effects of cations are not identical, it is considered that cation binding can be most evaluated by means of charge reversal measurements. Charge reversal data have been obtained for a variety of cations on synthetic phosphatides with varying surface charge density, and have been used to determine apparent association constants. Some of the factors which influence the cation sequences can be deduced
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