THE INTRINSIC PK(A) VALUES FOR PHOSPHATIDIC-ACID IN MONOLAYERS DEPOSITED ON MERCURY-ELECTRODES

The intrinsic K-a values of the phosphate group of phosphatidic acid ( PA) in self-organized monolayers deposited on a hanging mercury drop e lectrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In line w ith the Gouy-Chapman theory, plots of 1/C at constant bulk pH and vari able KCl concentration against the reciprocal of the calculated diffus e-layer capacity C-d,C-0 at zero charge exibit slopes that decrease fr om an almost unit value to zero as the absolute value of the charge de nsity on the lipid increases from zero to approximate to 2 mu C cm(-2) . The values so determined are K-1 = 10(8) M(-1) and K-2 = 10(4) M(-1) . The plots of 1/C against 1/C-d,C-0 for PA exhibit slopes that pass f rom zero to a maximum value and then again to zero as pH is varied fro m 7.5 to 1.5, indicating that the charge density of the lipid film pas ses from slightly negative to slightly positive values over this pH ra nge. An explanation for this anomalous behavior is provided.