Thermodynamics of membrane lipid hydration*

Zwitterionic or polar lipids that form lamellar phases swell in the presence of water to take up between 10 and 30 water molecules per lipid. The degree of the water uptake depends both on the state of the alkyl chains, liquid or solid, and on the nature of the polar group. The swelling behavior has been extensively characterized on the free-energy level through measuring the relation between the chemical potential of the water and the degree of swelling. In spite of the extensive studies of this type, consensus is still lacking concerning the molecular mechanism causing the swelling. The two main ideas that explain the existence of the effectively repulsive force between two opposing bilayers are (i) a water structure effect and (ii) thermal excitations of the lipid molecules. The first is from a thermodynamic perspective caused by a negative partial molar enthalpy of the water, whereas for the second, the repulsion is caused by positive entropy. A further insight into the swelling behavior is obtained by simultaneously measuring the partial molar free energy and the partial molar enthalpy using a twin double calorimeter. Such measurements show for binary lipid–water and for ternary lipid–cholesterol–water systems that the first four water molecules enter the bilayer driven by a favorable enthalpic interaction, whereas for higher water contents, the partial free energy and the partial enthalpy have opposite signs. In spite of the fact that the partial free energy varies with water content in a similar way, the partial enthalpies depend strongly on the nature of the lipid sample. Thus, it is obvious that a molecular interpretation of the enthalpy values has to involve the lipid degrees of freedom. This is a strong indication that the interpretation of the free energies should also include the same degrees of freedom. This gives experimental evidence in favor of an interpretation of the molecular mechanism causing the swelling that involves changes in the thermal excitation of the lipid degrees of freedom.