EFFECT OF THE INTRAMOLECULAR CHARGE SEPARATION DISTANCE ON THE SOLUTION PROPERTIES OF BETAINES AND SULFOBETAINES

The solution properties of zwitterionic surfactants are shown to be strong functions of the type of negatively charged center (carboxylate vs sulfonate) and the number of methylene groups separating the charged sites. For similar structures, differences such as the higher solubility and critical micelle concentration of the betaine relative to the sulfobetaine can be explained as a direct result of the carboxylate headgroup being more hydrophilic than the sulfonate. No evidence is seen of intramolecular ion-pair formation, indicating that in a polar medium such as water, the distance between the charges sites increases monotonically with increasing number of methylene units in the tether. The increasing headgroup area with tether length leads to poor foaming and aqueous solution thickening properties. The monomeric and micellar compositions of betaine surfactants as a function of pH and concentration can be determined directly from titration curves.