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 thick ness on pK(i) of the weak acid 4-heptadecyl-7-hydroxy coumarin (HHC) locate d at the multilayer-phosphate buffer interface is investigated. The amphiph ilic fluorescent dye is embedded in a neutral methyl arachidate (MA) monola yer that has been deposited atop of the LB-film during dipping. Such a syst em is considered as an asymmetric model membrane built up of docosyl ammoni um cations and phosphate counter-ions (HPO42-). Its electrostatic propertie s are compared with those of the previously studied LB-membrane of Cd-arach idate [1] consisting of arachidate anions and Cd2+ counter-ions. The fluori metric titration of HHC in the neutral matrix at the multilayer-phosphate b uffer interface shows that pK(m/w) at the membrane-water interface increase s with increasing membrane thickness, reaching saturation at about 250 Angs trom. The plateau of pK(m/w) = 6.8 found for this system significantly diff ers from the plateau value of 10.6, observed for the Cd-arachidate multilay er-phosphate buffer boundary [1], and from pK(a/w) = 8.2 obtained at the ai r-phosphate buffer interface [2]. The shifts, Delta pK(i) = pK(a/w) - pK(m/ w) imply positive membrane-water Gouy-Chapman potentials for all studied th icknesses of docosyl ammonium phosphate (DCAP) multilayer, and negative psi (m/w) potentials for all Cd-arachidate membranes. The plateau values, obtai ned for membranes more then 250 Angstrom thick, are psi(m/w)(infinity) = 81 and - 195 mV, respectively. Thus, the membrane-water interface appears c harged, in spite of the presence of a neutral MA monolayer at the LB-film-p hosphate buffer boundary. The psi(m/w)/d dependencies can be well fitted wi th the equation psi(m/w) = psi(s/m)(infinity) + psi(m/w)(infinity) cosh(kappa d) / sinh(kap pa d) describing the variation of the double layer potentials of asymmetric thin films with film thickness at constant surface charge densities [21a]. The f it of this equation to the experimental data for psi(m/w)/d yields the Gouy -Chapman potential at the glass-LB membrane interface, psi(s/m)(infinity) a nd the Debye length, kappa(-1), of the diffuse double layers at the sim and m/w interfaces. The values obtained show that the cationic :membrane has t wo positively charged surfaces with different (small) charge densities, whi le the anionic membrane has a positive s/m and a negative m/w interface, bo th having large charge densities. The values of kappa(-1) enable estimation of the concentration of the free divalent charges in the membrane, which a ppear to be of the order of (4-5) x 10(22) m(-3). The dipole potential at t he interface cationic membrane-phosphate buffer is positive and linearly in creases with increasing membrane thickness. It results from the different d ensity of DCAP dipoles deposited on the solid substrate at dipping and with drawal. For anionic membrane the density of Cd-arachidate dipoles in the ad jacent monolayers is the same, and only the last MA monolayer contributes a small positive dipole potential that is independent on membrane thickness. (C) 2000 Elsevier Science B.V. All rights reserved.