Critical fluctuations of membranes: Spectroscopy meets thermodynamics

Abstract

Broadband ultrasonic spectra (100 kHz to 2 GHz) of aqueous solutions of liposomes from phospholipids without and with cholesterol or with a peptide added are analyzed near the main phase-transition temperatures of the bilayer systems. All spectra reveal a critical term due to domain structure fluctuations and up to three relaxation terms with discrete relaxation times. By analogy with critically demixing binary fluids, the former is evaluated within the framework of the Bhattacharjee-Ferrell dynamic scaling theory. The relaxation rates of fluctuations and the critical amplitudes resulting from this analysis are discussed briefly. Consistency with and differences to critically demixing systems are demonstrated. The terms with discrete relaxation times are related to short-scale liposome deformation, rotational diffusion of phospholipid molecules, and conformational isomerization of alkyl chains within the bilayers, respectively. © 2012 The Author(s).