A plot showing surface tension as function of pH showed quite similar relative changes in pH 5-9. Johlin measured using sessile bubble method with different timing (hours) to expect equilibrium.

However, the pattern isn’t the same as Sugden’ result that was imposed over the same plot.

abstract here

A regularity has been found: a larger hydrophilic head gives rise to a lower interfacial tension. A relationship was found between the size of the hydrophilic head of lipid and the isoelectric point pH value. With larger hydrophilic heads, the isoelectric point appears at lower pH.

abstract:
Vol7_suppl_05_39.pdf

obtained by the sessile bubble method of measuring surface tensions

J. Biol. Chem.-1930-Johlin-319-25.pdf

A theoretical equation is derived to describe the dependence of the interfacial tension of a lipid bilayer on the pH of the aqueous solution. Interfacial tension measurements of an egg phosphatidylcholine bilayer were carried out. The experimental results agreed with those derived from the theoretical equation obtained close to the isoelectric point within a range of three pH units. A maximum corresponding to the isoelectric point appears both in the theoretical equation and in the experiment.

abstract here