A microcalorimetric study of the interaction of phospholipid liposomes with colloidal titanium dioxide and silica: an example of enthalpy–entropy compensation

The interaction of extruded cationic and anionic liposomes (VETs), prepared from dipalmitoylphosphatidylcholine (DPPC)–cholesterol–dimethyldioctadecylammonium bromide (DDAB) and phosphatidylinositol (PI) mixtures, respectively, with colloidal titanium dioxide and silica suspensions in phosphate buffered saline (pH 7.4) has been investigated by isothermal titration calorimetry (ITC). Measurements were made at 25 °C as a function of liposomal lipid concentration and charged lipid content (PI and DDAB), in a range 4–19 mol%. The interactions were found to be exothermic and the cumulative enthalpies as a function of the amount of lipid bound were interpreted using Langmuir adsorption isotherms from which the Gibbs energies of adsorption were derived. The Gibbs energies were in a narrow range −29 to −32 kJ mol−1 for both titanium dioxide and silica and showed not systematic variation with charged lipid content of the liposomes. When the enthalpies of intention (ΔH) were plotted against the entropy term (TΔS) linear plots were obtained with a slope of unity indicating clear enthalpy–entropy compensation with a compensation temperature of approximately 300 K. Similar effects were found for liposome–zinc citrate interactions previously reported by Scott and Jones (Colloids Surfaces A, 182 (2001) 247–256). The interaction of liposomes with the solid surfaces of colloidal particles appears to be a new example of enthalpy–entropy compensation.