Surface Properties of Zwitterionic Surfactants

The effect of pH and electrolyte on the surface properties of a betaine surfactant, C 12 H 25 N + (CH 2 C 6 H 5 )(CH 3 )CH 2 COO -, were studied. The ΔG° ad of both the zwitterionic form of the betaine and of the cationic protonated betaine were calculated. Surface activity decreases slightly with decrease in pH. As expected, the cationic form has somewhat lower surface activity than the zwitterionic form. Although the pK a of the protonated betaine is 2.8, the properties of both the zwitterionic betaine and the cationic protonated betaine determine the surface properties, even in distilled water (pH = 5.85). At pH = 5.85, the betaine interacts more strongly with Na 2 SO 4 than with NaCl or CaCl 2; anionic surfactants also show stronger interaction with the betaine than do cationic surfactants. These effects cannot be attributed to the zwitterion, but to the presence of the protonated cationic form in equilibrium with the zwitterion.