The Equilibria of Lipid–K+ Ions in Monolayer at the Air/Water Interface

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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, K_{{{text{PCK}}^{ + } }} = { 3}. 2 6times 10^{ 2} {text{dm}}^{ 3} ,{text{mol}}^{ - 1} , and for cholesterol–K+ ion (ChK+) complex, K_{{{text{ChK}}^{ + } }} = { 1}.00 times 10^{ 3} {text{dm}}^{ 3} ,{text{mol}}^{ - 1} , were calculated by inserting the experimental data. The value of area occupied by one PCK+ complex is 60 Å2 molecule−1, while the area occupied by one ChK+ complex is 40.9 Å2 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.