Wormlike Micelles Based On “Pseudo-” Oligomeric Surfactants

Because of their unqiue molecular structure, oligomeric surfactants behave high surface activity, rich aggregate morphologies and excellent rheological properties. However, the investigation and application of some oligomeric surfactants are usually limited by their tedious synthetic process, low yield, and difficult purification. In recent years, super-amphiphilic molecules and oligomers based on simple and efficient way--non-covalent bond have attracted considerable attention. Here, we developed one kind of anionic and cationic “pseudo-”oligomeric surfactants based on non-covalent electrostatic interactions, respectively. Their self-assembly behaviors, viscoelasticity of the wormlike micelles solution, stimuli-responsive and the effects of hydrophobic volume, hydrophobic length, and hydrophilic group structure were systematically investigated through visual method, fluorescence spectroscopy, surface tension, rheology, kinematic viscosity, DLS, SAXS, FF-TEM, Cyro-TEM and 1H NMR. The main results are summarized as follows:1. Anionic “pseudo-”gemini surfactants:（1） The addition of Bola type quaternary ammonium salt can efficiently decrease the Krafft temperature of C18 ONa, in which Bola2 be with the benzyl group possesses a stronger ability than that of Bola2 et and Bola2 hy.（2） “Pseudo-”gemini surfactants were formed by simply mixing Bola quaternary ammonium salts and fatty acid sodium C_nONa in molar ratio of 1:2, and showed high surface activity and thickening ability similar as the conventional covalent gemini surfactants. With increasing the hydrophobic volume, or the hydrophobic chain length, or the bydrophobicity of Bola type quaternary ammonium salt, C_nONa-Bolanx’s cmc decreased accordingly, and the ability to form viscoelastic wormlike micelles gradually increased accompanied with a decrease in the critical overlap concentration（C*）.（3） 2C₁₈ONa-Bola2 be wormlike micellar solutions show stronger temperature-sensitive than the conventional covalent bond gemini surfactants viscoelastic fluid, and its flow activation energy is up to 399.76 kJ/mol.（4） Due to the pH-sensitive of C18 ONa, 2C₁₈ONa-Bola2 be viscoelastic fluid can respond to common pH stimuli or green CO₂ gas, and shows a transition between a gel-like wormlike micellar network and a water-like dispersion with precipitate. However, the CO₂-responsive behavior is irreversible.2. Cationic “pseudo-”oligomeric surfactants:（1） A series of “pseudo-”oligomeric surfactants were formed by simple neutralization reaction in situ without extra inorganic salts, and the degree of oligomerization can be flexibly changed in 1³ by controlling the molar ratio.（2） The cmc of cationic “pseudo-”oligomeric surfactants is greatly decreased with the increasing of molar ratio of C₁₆ PMA and HCA, at the same time gcmc increases. The cmc of “pseudo-”trimeric surfactants 3C₁₆PMA-HCA is as low as 0.030 mM which is even smaller than the traditional covalent trimeric surfactant.（3） With increasing the hydrophobic tails, or the hydrophobic volumn, or decreasing temperature, the C* of “pseudo-”trimeric 3C₁₆PMA-HCA decreases and viscoelasticity increases, because of the formation and growth of wormlike micelles.（4） The viscoelastic wormlike micelle solution based on 3C₁₆PMA-HCA exhibits a unique "bell-shaped" pH responsive behaviors: when pH increases monotonically, functional molecules in the solution change from traditional single head single tail structure, to "pseudo-" trimeric structure, and then to non-surface active, and consequently the viscosity of solution increases first and then decreases. This reflects the continuous transformation of microscopic aggregate structure among spherical micelles-wormlike micelles-no aggregates, and vice versa.