Amphiphile bilayer films from DPPC: bilayer lipid membranes and Newton black films

Abstract

Amphiphile bilayer films are obtained from 1,2 dipalmitoyl-glycero-3-phosphocholine (DPPC): bilayer lipid membranes (BLM) and Newton black films (NBF), through thinning of the respective thin liquid films, thus allowing for a very precise determination of the moment of their formation. Stability (or rupture) and formation of BLM and NBF are considered from a unified point of view with the microscopic theory of Kashchiev–Exerowa [J. Colloid Interface Sci., 77 (1980) 501–511], based on the formation of nanoscopic holes in them. BLM and NBF are obtained and studied with the microinterferometric method of Scheludko–Exerowa in its contemporary version. The equivalent thickness of both BLM (in benzene solution between two water phases with 0.1 M NaCl) and NBF in aqueous DPPC solution (in the presence of 0.1 M NaCl) is determined as being h w = 7.0 nm for BLM and h w = 7.8 nm for NBF. By means of the dependences: BLM lifetime versus DPPC concentration and probability for BLM formation versus DPPC concentration, it is established that there exist metastable BLM and stable NBF. The good fit between the experimental results of τ( C) dependence and theory in the case of BLM allow to determine the three constants: pre-exponential factor A = 1.5 × 10 −3 s, related to the process kinetics; constant B = 20.2 ± 0.2, related to the specific hole energy γ = 1.7 × 10 −11 J/m and the equilibrium concentration C e = 6 × 10 −4 ± 7.2 × 10 −6 m/l. The specific hole linear energy γ = 1.7 × 10 −11 J/m determined as well as the binding energy Q between first neighbor molecules in the bilayers Q = 1.48 × 10 −19 J (36 kT) are lower than the ones determined for DPPC foam bilayer in gel state γ = 9.1 × 10 −11 J/m and Q = 55 kT. This means that interaction is weaker in the case of BLM. The critical concentration C c at which bilayer formation starts is: for BLM C c = 30 μg/ml and for NBF C c = 70 μg/ml. This concentration characterizes quantitatively the formation of the amphiphile bilayer and is a very useful parameter that can be used for various purposes.