Biochimica et Biophysica Acta, 1024(1990) 227-232 227 Elsevier BBAMEM74842 Influence of stearic acid monolayers upon the procaine adsorption from underlying alkaline aqueous solutions Jfinos Zsak61, Maria Tomoaia-Coti§el 1, Emil Chifu 1, Aurora Mocanu 1 and Petre T. Frangopol 2 l Department of Physical Chemistry, University of Cluj-Napoca, Cluj-Napoca and 2 Institute of Physics and Nuclear Engineering, Mdgurele-Bucharest (Romania) (Received24 January1990) Keywords: Adsorption; Monolayer penetration; Procaine;Stearicacid; pH effect Adsorption of procaine at the air/water interface and its penetration into stearic acid monolayers from aqueous subphase of pH 8 are studied by measuring surface tension of aqueous procaine solutions and by recording surface pressure vs. mean molecular area curves for stearic acid monolayers spread onto procaine solutions of different concentrations. The amount of procaine in the interface is derived by means of Gibbs' equation. Results are compared to those obtained earlier at pH 2 and on unbuffered subphases. With increasing pH an increasing procaine adsorption and procaine penetration is observed. This phenomenon is interpreted in terms of protolytic equilibria in which participate both surfactants procaine and stearic acid. InWoduction Studies performed by using different experimental techniques showed some anesthetics to extend the surface area of monolayer films maintained at constant surface pressure [1,2] and to increase the surface pres- sure of lipid films maintained at constant area [3-5]. The molecular origin of the effects observed is pre- sumed to be the weakening of the packing of lipids due to the anesthetic molecules inserted [2]. The efficiency of anesthetics in increasing the surface pressure was observed to be directly proportional to their relative anesthetic efficiency [3]. In these experiments frequently lipid monolayer membranes were used [2,4,5]. The reason for using these model system is the correlation between the anesthetic efficiency and the oil/water partition coefficients of anesthetics [5]. Further, the monolayer represents the half of a lipid bilayer, which plays a very important r61e in biomembranes. Monolayers spread at the plane air/ water interface are sufficiently stable and suitable for experimental research, providing a convenient structural framework for the experimental study of physico-chem- ical interactions between the film forming molecules Correspondence: E. Chifu, Department of PhysicalChemistry,Uni- versityof Cluj-Napoca, 3400 Cluj-Napoca,Romania. and various subphase components (drugs, electrolytes, soluble proteins etc.). These monolayer studies allow us to obtain direct information, at the molecular level, concerning the conformation and packing of molecules having biological significance in natural biomembranes in conditions near the in vivo ones. In our previous paper [6] the stearic acid monolayer has been chosen as lipid membrane model, since stearic acid is a classical film forming substance, frequently used in monolayer studies, having one of the simplest structures. The influence of procaine, dissolved in the aqueous subphase, upon the properties of the stearic acid monolayer, spread at the air/water interface, has been studied. These investigations showed procaine to have an expanding effect upon the stearic acid mono- layer, suggesting the idea that in the monolayer pro- caine molecules are inserted between the stearic acid molecules, i.e., in the interface the adsorption of pro- caine molecules occurs. The adsorption (/~2) of the soluble surfactant 2 dis- solved in water (noted as component 1) at the air/water interface obeys Gibbs' equation I i ao~ where 02 and c2 stand for the surface tension of the surfactant solution and the molar concentration of surfactant 2, respectively, k and T are Boltzmann's constant and absolute temperature, respectively. 0005-2736/90/$03.50 © 1990 ElsevierSciencePublishersB.V.(Biomedical Division)