Electrochemical analysis of a phospholipid phase transition

Phospholipid monolayers in the presence of electric fields show three phase transitions characterised by capacitance peaks at potentials ∼−0.94 V, ∼−1.0 V and ∼−1.25 V versus Ag/AgCl. This paper focuses on studying the phase transition characterised by the capacitance peak at potential ∼−1.0 V versus Ag/AgCl. After the application of potential steps in the negative potential direction, the current transients are characteristic of a nucleation and growth process. After the application of potential steps in the positive potential direction the transition is faster and not characteristic of a nucleation and growth process. When the change in charge associated with the phase transition was calculated from potential pulses in either direction it increases through the same potential window of ∼0.015 V. Impedance data plotted in the complex capacitance plane shows a significant extra capacitative element within the potential window characterising the phase transition. For applied ΔE of 0.002 V, the extra element fits a Debye relaxation process but for higher ΔE of 0.005 V the fit is not so good and the relaxation time constant is shorter. The data is consistent with a model whereby a structured electrolyte-phospholipid emulsion breaks up into two phases: electrolyte and phospholipid at more negative potentials. The two phases remain in equilibrium with each other throughout the potential window characterising the transition. An explanation for the Debye relaxation in the impedance data is that it represents the diffusion of phospholipids and increased adsorption of phospholipid head groups coincident with the increasing radius of phospholipid domains.