Wormlike Micellar Systems Formed By Quaternary Ammonium Gemini Surfactants: Effect Of Intermolecular Hydrogen Bonding

Hydrogen bonding as a directive force plays an important role in designing andsynthesizing super-molecules. The process of the self-assembly of surfactant inaqueous solution was attributed in large part to ‘‘hydrophobic effects” which is anentropically favored release of interfacial water. This is a kind of nondirectivedriving forces and generally results in the aggregates with symmetric structure suchas spherical micelles. Wormlike micelle has a quite long contour length and isdissymmetric in structure, which is the consequence of micellar growth alongone-dimensional direction. If a driving force could be served as a supplementarycontribution to the micellar system, this will undoubtedly accelerate the growth ofmicelles. However, this special effect was not emphasized and effectively played inthe past. Minimization of the free energy in end-caps was still a main factor to growwormlike micelles. In this dissertation, we represent a systematic study on thewormlike micelle systems formed by quaternary ammonium gemini surfactants witha hydroxyl-substituted spacer, by which we expected to reveal the role ofintermolecular hydrogen bonds in constructing wormlike micelles. The main pointsof the results are summarized as follows:1. The effects of the intermolecular hydrogen bonding between thehydroxyl-substituted spacers of12-3（OH）-12or12-4（OH）_n-12（n=1,2） on theiradsorption and aggregation were studied by equilibrium and dynamic surface tension,conductivity, fluorescence probing, dynamic light scattering and IR measurements.The results strongly suggested the dimolecular moieties formed in dilute systemsprior to their adsorption and aggregation due to the intermolecular hydrogen bonding.Of course, this interaction was also existed in the adsorption layer and the aggregatesand resulted in strongly influence for the self-assembly, e.g. promoting the growth of micelles and stimulating the dissociation of more counterions on the aggregatesurfaces which made the molecules arranged tighter in aggregates.2. The viscoelastic property of the wormlike micelles formed by12-3（OH）-12and12-3-12was studied. The results indicated that under the reinforcement ofintermolecular hydrogen bonding, the12-3（OH）-12solution produced far higherzero-shear viscosity than the12-3-12one and the required concentrations of theformer were also considerably lower. The dynamic frequency-sweep rheology alsoindicated that12-3（OH）-12formed longer wormlike micelles than12-3-12. Theseresults confirmed that the micellar growth can be effectively promoted by theintermolecular hydrogen bonding.3. By systematic study and comparison of the viscoelastic properties for thesystems of m-3（OH）-m with different lengths of alkyl tail, it was found that withslight increase in the alkyl tail length, increased hydrophobic interaction between thealkyl tails led to evidently strengthening of the viscoelasticity of solutions and evenformed gel-like solid. The frequency-sweep rheology measurements also providedthe information that the increase in alkyl tail yielded longer wormlike micelles. Thepresent results provided a simple and efficient method to strength the hydrogenbonding interaction and thus follow the finer viscoelasticity of solution.4. The effect of NaSal on the properties of wormlike micelles in the aqueoussolution of12-3（OH）-12has been examined. The surface of12-3（OH）-12aggregateshad larger degree of counterion disassociation due to the effect of the intermolecularhydrogen bonding. This made that12-3（OH）-12aggregates could much stronglyassociate with Sal. The screening of salt to the ionic headgroups resulted in tightlypacking of12-3（OH）-12molecules in aggregate, which further promoting theintermolecular hydrogen bonding and thus rapid micellar growth. Therefore, additionof salt was another method to promote the hydrogen bonding interaction.5. The viscoelastic property of wormlike micellar solutions of mixedcationic/anionic gemini surfactants C₁₂φC₁₂/12-3（OH）-12have also been investigated. With addition of a small amount of C₁₂φC₁₂into the solution of12-3（OH）-12, the solution viscosity was pronouncedly enhanced. Dynamic rheologymeasurements showed that12-3（OH）-12/C₁₂φC₁₂formed long wormlike micelles.This becomes third method to promote the hydrogen bonding interaction, whichcould also effectively improve the solution’s viscoelasticity.