Physico-chemical studies on mixed oppositely charged surfactants: Their uses in the preparation of surfactant ion selective membrane and monolayer behavior at the air water interface

Surfactant ion selective membranes, prepared using different sets of catanionic species, have been found superior to the membranes prepared earlier in terms of sensitivity and durability. The aggregates (or coacervates) of cetyltrimethylammonium (CTA+) ion with dodecylsulfate (DS−), dodecylbenzenesulfonate (DBS−), dioctylsulfosuccinate (AOT−), deoxycholate (DC−), and taurochenodeoxycholate (TCDC−) ions in 1:1 molar ratio were used as the carrier complexes in the membrane for estimating anionic surfactants whereas carrier complexes of DBS− with CTA+, tetradecyltrimethylammonium (TTA+) and dodecyltrimethylammonium (DTA+) cations (1:1 molar ratio) were employed for estimating the cationic surfactants. Electrochemical behavior of membrane electrodes was, although, not always Nernstian, the cell emf versus log[surfactant] plots were linear with sharp breaks corresponding to the critical micelle concentration (CMC). Film behavior of complex aggregates (coacervates) was studied at the air–water interface by surface pressure (π)–area (A) measurements. Limiting area per molecule, maximum attainable surface pressure, film compressibility, etc., were evaluated from the experimental data. Formation of probe-excluded regions of compressed monolayers, doped with a fluorescent probe (NBD-PC) was observed by epifluorescence microscopy.