The equilibria of lipid-k(+) ions in monolayer at the air/water interface.

The effect of K(+) ion interaction with monolayers of phosphatidylcholine (lecithin, PC) or cholesterol (Ch) was investigated at the air/water interface. We present surface tension measurements of lipid monolayers obtained using a Langmuir method as a function of K(+) ion concentration. Measurements were carried out at 22°C using a Teflon trough and a Nima 9000 tensiometer. Interactions between lecithin and K(+) ions or Ch and K(+) ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of lipid-K(+) ion complex (LK(+)). The stability constants for lecithin-K(+) ion (PCK(+)) complex, [Formula: see text], and for cholesterol-K(+) ion (ChK(+)) complex, [Formula: see text], were calculated by inserting the experimental data. The value of area occupied by one PCK(+) complex is 60 Å(2 )molecule(-1), while the area occupied by one ChK(+) complex is 40.9 Å(2 )molecule(-1). The complex formation energy (Gibbs free energy) values for the PCK(+) and ChK(+) complexes are -14.18 ± 0.71 and -16.92 ± 0.85 kJ mol(-1), respectively.