Modification of anomalous swelling in multilamellar vesicles induced by alkali halide salts

By use of small-angle X-ray scattering it is shown that addition of alkali halide salts in small amounts (0-200 mM) shifts the repeat spacing in multi lamellar DC13PC vesicles and alters the anomalous swelling behaviour close to the main transition. Both effects follow the Hofmeister series of the io ns. We suggest that the shift of repeat spacing can be explained by ion eff ects on the van der Waals attractive forces between the membranes and on th e decay length of the repulsive hydration force. The anomalous swelling is explained in terms of a critical unbinding of the membranes. The proximity of the critical temperature of the unbinding to the main transition tempera ture can be tuned by varying the concentration and type of salt in the samp le.