A humidity titration calorimetry technique to study the thermodynamics of hydration

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