A humidity titration calorimetry technique to study the thermodynamics of hydration

Isothermal titration calorimetry has been applied in combination with a moisture generator and, independently, gravimetry to study the adsorption of water onto 50 mu g of the lipid dioleoylphosphatidylcholine. During the experiment, the relative humidity is varied stepwise, e.g. from 2% to 92% in increments of 2% every 15 min. The difference between the partial molar enthalpy of the water molecules which interact directly with the lipid headgroups and the molar enthalpy of bulk water is endothermic (10-15 kT/mol). We conclude that the hydration of lipids with phosphatidylcholine headgroups is entropy-driven at room temperature. (C) 1999 Elsevier Science B.V. All rights reserved.