Effect of Hydrophilic Polymers on Physical Stability of Liposome Dispersions

Interactions between various hydrophilic polymers and liposomes composed of hydrogenated soybean lecithin were investigated by means of isothermal titration calorimetry (ITC), FT-IR, and dynamic light scattering (DLS) measurements. Their effect on the dispersity of liposomes was investigated by turbidity measurement. Sodium alginate was found to bind to lipid bilayers via electrostatic interaction and to stabilize dispersity over a wide concentration range despite its high hydrophilicity. This was elucidated in terms of the irreversible adsorption of sodium alginate onto the liposome surfaces. Carboxymethyl cellulose sodium salt and poly(vinyl alcohol) gave similar results, but the stabilization effect was reduced. Other polymers, which were adsorbed onto the liposome surfaces very weakly or not adsorbed at all, destabilized the dispersity significantly, just below the overlap concentration of polymers. This was interpreted in terms of depletion flocculation. The adsorption of the polymers onto the liposome surfaces was detected by ITC and DLS measurements, although the adsorbed fraction seemed to be very small. Notably, ITC enabled us to quantitatively estimate the heat flow accompanied by the adsorption. The sign of the heat flow due to the adsorption enabled us to detect the perturbation of the lipid membrane, which was also proved by FT-IR measurements. The relationship between the interaction strength and the stabilization effect is discussed.