Langmuir 1991, 7, 854-867
a54
Effect of the Intramolecular Charge Separation Distance on the Solution Properties of Betaines and Sulfobetaines Jeffry G. Weers,. James F. Rathman, Frank U. Axe, Charles A. Crichlow, Lafayette D. Foland, David R. Scheuing, Richard J. Wiersema, and Alfred G. Zielske Clorox Technical Center, 7200 Johnson Drive, Pleasanton, California 94588-8004 Received May 25,1990.In Final Form: August 31, 1990 The solution properties of zwitterionic Surfactants are shown to be strong functions of the type of negatively charged center (carboxylatevs 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-pairformation, indicatingthat 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. and the dielectric properties of the medi~m.'$~J However, the distance between the charged centers is not necessarily Although zwitterionic surfactants are formally unproportional to the number of methylene units separating charged, they do contain both positively and negatively them since the tether group is flexible and may adopt a charged hydrophilic groups in the same molecule. These number of conformations. opposing charges lead to large dipole moments for zwitThe question arises as to how the charged groups are terionic surfactant molecules with hydrophilicites interoriented with respect to one another and whether this mediate between the ionic and conventional nonionic structure is different for surfactant in monomeric and miclasses of surfactants.' cellar form. For a monomer in solution, electrostatic considerations suggest that the opposite charges would As such, zwitterionicSurfactants have solution properties preferentially be as close as possible, resulting in a cyclic similar in some respects to both ionic and nonionic surstructure, providing that the tether is sufficiently long so factants. For example, many sulfobetaines exhibit Krafft that the strain on the methylene chain is not too severe. points, a phenomenon usually associated with ionic surThis might indeed be the case for molecules in a gaseous factants; however, unlike ionics, many zwitterionic amstate, but in aqueous solutions, solvation forces counter phiphiles are very soluble in aqueous solutions over wide ranges of concentration, pH, hardness, and ~ a l i n i t y . ~ - ~ the effect of electrostatic attraction, since the hydrophilicity of the headgroup is maximized when the anionic and Their insensitivity to electrolyte is typical of polyethoxycationic moieties are as far apart as possible. The headlated nonionic surfactants; however most zwitterionic surgroup hydrophilicity also depends on the type of anionic factant solutions do not undergo a phase separation (cloud group (SO4- < SOS- > PKa, only the zwitterionic form is present and the cmc equals cmQ; a t pH
