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Abstract: The effect of the presence of gramicidin D in a lecithin membrane on its interfacial tension has been studied. The studies have been carried out at various forming solution compositions and at various potassium ion concentrations in the electrolyte solution. Potassium chloride was used as the electrolyte. The complex was formed between the gramicidin molecule and K+ ion. The following parameters describing the complex were determined: the surface area occupied by GK+ complex (A GK+ ), the interfacial tension of the GK+ membrane complex (γ GK+ ), and the stability constant of the gramicidin–K+ complex (K). These values are 156 Å2, 1.89 mN m−1 and 0.033 m3 mol−1, respectively. [Copyright &y& Elsevier]