Interfacial acid–base equilibrium and electrostatic potentials of model Langmuir–Blodgett membranes in contact with phosphate buffer

The effect of a cationic Langmuir–Blodgett (LB) multilayer of varying thickness on pKi of the weak acid 4-heptadecyl-7-hydroxy coumarin (HHC) located at the multilayer-phosphate buffer interface is investigated. The amphiphilic fluorescent dye is embedded in a neutral methyl arachidate (MA) monolayer that has been deposited atop of the LB-film during dipping. Such a system is considered as an asymmetric model membrane built up of docosyl ammonium cations and phosphate counter-ions (HPO42−). Its electrostatic properties are compared with those of the previously studied LB-membrane of Cd-arachidate [1] consisting of arachidate anions and Cd2+ counter-ions. The fluorimetric titration of HHC in the neutral matrix at the multilayer-phosphate buffer interface shows that pKm/w at the membrane–water interface increases with increasing membrane thickness, reaching saturation at about 250 Å. The plateau of pKm/w=6.8 found for this system significantly differs from the plateau value of 10.6, observed for the Cd-arachidate multilayer-phosphate buffer boundary [1], and from pKa/w=8.2 obtained at the air–phosphate buffer interface [2]. The shifts, ΔpKi=pKa/w−pKm/w, imply positive membrane–water Gouy–Chapman potentials for all studied thicknesses of docosyl ammonium phosphate (DCAP) multilayer, and negative ψm/w potentials for all Cd-arachidate membranes. The plateau values, obtained for membranes more then 250 Å thick, are ψm/w∞=+81 and −195 mV, respectively. Thus, the membrane–water interface appears charged, in spite of the presence of a neutral MA monolayer at the LB-film-phosphate buffer boundary. The ψm/w/d dependencies can be well fitted with the equationψmw=ψsm∞+ψmw∞ coshκdsinhκddescribing the variation of the double layer potentials of asymmetric thin films with film thickness at constant surface charge densities [21a]. The fit of this equation to the experimental data for ψm/w/d yields the Gouy–Chapman potential at the glass-LB membrane interface, ψs/m∞ and the Debye length, κ−1, of the diffuse double layers at the s/m and m/w interfaces. The values obtained show that the cationic membrane has two positively charged surfaces with different (small) charge densities, while the anionic membrane has a positive s/m and a negative m/w interface, both having large charge densities. The values of κ−1 enable estimation of the concentration of the free divalent charges in the membrane, which appear to be of the order of (4–5)×1022 m−3. The dipole potential at the interface cationic membrane–phosphate buffer is positive and linearly increases with increasing membrane thickness. It results from the different density of DCAP dipoles deposited on the solid substrate at dipping and withdrawal. For anionic membrane the density of Cd-arachidate dipoles in the adjacent monolayers is the same, and only the last MA monolayer contributes a small positive dipole potential that is independent on membrane thickness.