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 mg 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 Ž . bulk water is endothermic 10–15 kJrmol . We conclude that the hydration of lipids with phosphatidylcholine headgroups is entropy-driven at room temperature. q 1999 Elsevier Science B.V. All rights reserved.