Adsorption of dipolar (zwitterionic) surfactants to dipolar surfaces

The adsorption of the neutral but dipolar (zwitterionic) surfactants, dodecyldimethyl(3-sulfopropyl)ammonium hydroxide and diheptanoic-sn-glycero-3-phosphocholine to the silicon nitride/water interface has been studied via contact angle measurements. Adsorption is highest in the pH range where the Si3N4 surface is also electrically neutral, but dipolar. The adsorption decreases sharply above the isoelectric point of Si3N4, when the surface is negatively charged, and is more complex below the isoelectric point, when the surface charge is positive. Qualitatively similar results were obtained with alumina (Al2O3) surfaces. The results indicate the importance of discrete or individual charge-charge interactions, rather than the net or smeared-out surface charge interaction in driving the binding of zwitterionic surfactants to amphoteric surfaces that carry both positive and negative charges.