Nuclear magnetic resonance spectroscopy of frozen phosphatidylcholine–D2O suspensions: a new technique for measuring hydration forces
At temperature below freezing, lamellar phases of lipids in water separate into regions of ice in equilibrium with regions of lamellar phase with reduced water content. The repulsive force per unit area between lamellae is the negative of the pressure in the unfrozen, interlamellar water, which is determined by the chemical potential and thus by the temperature. When deuteriated water is used, the fraction remaining unfrozen is determined from its contribution to the nuclear magnetic resonance signal. When the molecular area in the plane of the lamellae and the density of water are known or may be inferred, the interlamellar separation may be calculated. We report the use of this technique to measure force–hydration and force–distance curves at freezing temperatures for egg yolk lecithin.