A synchrotron X-ray study of competing undulation and electrostatic interlayer interactions in fluid multimembrane lyotropic phases

Recent X-ray work [1] in a quaternary lyotropic lamellar phase demonstrated that entropically induced undulation forces dominated the free energy of interaction between fluid membranes swollen by dodecane. Here we report on a comprehensive X-ray study of competing electrostatic and undulation forces in two multimembrane systems in the lamellar Lα phase as a function of the intermembrane distance. In each case, the negatively charged membrane is composed of a mixture of Sodium Dodecyl Sulfate (SDS) and pentanol, while the solvent separating the membranes is either pure water or brine (~ 0.5 mole. 1-1 of NaCl). A specialized high resolution X-ray spectrometer enables us to measure, as a function of the intermembrane distance (d ), the exponent η (d ) which characterizes the algebraic decay of layer correlations. In turn, η (d ) is directly related to the intermembrane interactions. The results clearly show that when diluting with pure water the interactions are dominated by long range electrostatic forces. In the brine dilution system, the addition of free ions (NaCl) to the solvent yields a small Debye length (λD/d<<; 1). In this case, with short range electrostatic interactions, undulation forces are restored with η(d)=1.33(1-δ/d)2 (δ is the membrane thickness) approaching a constant value for large d, as predicted by Helfrich [4].