title: pH-Induced Re-entrant Microstructural Transitions in Cationic Surfactant-Hydrotrope Mixtures.
abstract: The structural transitions occurring with change in pH for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal) were investigated at various temperatures using dynamic light scattering and small-angle neutron scattering. Direct structural studies show a transition from rigid cylindrical micelles at neutral pH to spherical micelles at ∼ pH 2 upon protonation of salicylate molecules; however, an unanticipated reversion to flexible cylindrical micelles with further decrease in pH was observed. We also observed these microstructure transitions from cylinders at high pH to spherical micelles at intermediate pH to flexible cylindrical micelles at low pH were highly sensitive to temperature. Our results suggest that, in addition to the well-described electrostatic and hydrophobic interactions in cationic surfactant-hydrotrope mixtures, the pH-induced microstructural changes are potentially governed by complementary cation-π and hydrogen bonding interactions.
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of Houston , Houston, Texas 77204-4004, United States.
authors.forename: Chinedu D
authors.initials: CD
authors.lastname: Umeasiegbu
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of Houston , Houston, Texas 77204-4004, United States.
authors.forename: Vemuri
authors.initials: V
authors.lastname: Balakotaiah
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of Houston , Houston, Texas 77204-4004, United States.
authors.forename: Ramanan
authors.initials: R
authors.lastname: Krishnamoorti