Measurement of the adsorption of drug ions at model membranes by scanning electrochemical microscopy

We present quantitative measurements of the adsorption of drug ions at Langmuir monolayers, which are considered as a model for biological membranes. The technique we have employed is based on scanning electrochemical microscopy using a capillary probe filled with organic electrolyte solution and operated in an inverted submarine arrangement in a Langmuir trough. In this experimental arrangement, we have made potential step chronoamperometric measurements, with the probe adjacent to the Langmuir monolayer deposited at the air/water interface, to extract information on the adsorption process. We have measured the adsorption of a cationic beta-blocker drug, propranolol, at a Langmuir monolayer of stearic acid. By developing a theoretical simulation that assumes that the equilibrium at the interface is controlled by the competition between the acid/base equilibrium of the stearic acid molecules at the surface and the electrostatically induced adsorption of the positively charged propranolol ion, we have been able to extract from our data a drug adsorption constant of (5 +/- 1) x 10(-5) mol dm(-3) at the monolayer surface pressure of 10 mN m(-1).