The effect of Hunitrazepam (FNTZ) on molecular packing of distearoylphosphatidylglycerol (DSPG), cholesterol (CHOL) monolayers and DSPG + CHOL mixed monolayers adsorbed at the water/1,2-dichloroethane interface was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The interfacial packing of the monolayer was estimated by the analysis of the voltammetric transfer of tetraethyl ammonium cation (TEA(+)) from the aqueous to the organic phase in the presence of different cations (Li+, Ca2+ and K+) in water. An important blocking of the transfer process was observed in the case of DSPG monolayer when the aqueous phase contained Li+ and Ca2+ cations indicating a highly ordered structure. EIS experiments evidenced that these cations adsorb at polar head group leading to a decrease in interfacial dielectric constant. FNTZ produced an enhancement of this blocking effect. When K+ was employed as aqueous electrolyte, no blocking effect of DSPG monolayer with or without FNTZ was observed. It was concluded that FNTZ induces lateral interaction with DSPG molecules in highly structured monolayers in which the negative charge of polar head groups is neutralized by cation adsorption. An interesting substitution effect of FNTZ was observed in monolayers containing CHOL. (c) 2006 Elsevier Ltd. All fights reserved.