Novel Gemini Surfactants Based on Natural Products Aggregation Behaviour in Aqueous Solutions
Doctoral thesis, 2002
There is currently much interest in using natural products to obtain novel types of surfactants. One of the reasons for increasing the proportion of surfactants based on natural products and replacing products based on petroleum raw materials is the growing concern about the environment.
Gemini surfactants are novel surfactants that are presently attracting considerable interest in the academic and industrial communities working on surfactants. These surfactants are generally composed of two hydrophobic tails connected by a short linker unit at, or in close vicinity to, the surfactant head group.
In this work, a series of gemini surfactants referred to as heterogeminis (HG's) with non-identical, directly linked head groups, has been investigated. This distinguishing feature makes the HG's resemble a mixed system of two different surfactants built into the same molecule. The following aspects of these surfactants are successively reported in this thesis: the synthesis route, the aggregation behaviour in aqueous solutions, the adsorption at the air-water and solid-water interfaces, the micelle microstructure, micelle dynamics, micellar solubilization, interaction with cyclodextrin or with other surfactants.
The results demonstrate that a HG molecule with its unusual structure forms normal micelles in a way similar to that of standard surfactants. The HG's were shown to exhibit improved performance over the mixed system both in terms of micellization and packing. Nonionic HG's form spherical micelles that grow with concentration, whereas the ionic HG's form slightly ellipsoidal micelles. Swollen or elongated micelles were observed by adding either toluene or decanol to nonionic HG's. Micelles were found to form in mixtures of nonionic HG surfactant and cyclodextrin. This gives HG surfactants an advantage in mixed solubilizing systems compared to single head and tail surfactants, which generally lose their ability to micellize at much lower additions of cyclodextrin. However, on large additions of cyclodextrin rod-shaped mixed complexes form. The zwitterionic surfactants that are counterion-free were reported to self-assemble at very low concentrations. Thread-like micelles were found to form resulting in a very high viscosity. By adding ionic surfactants these micelles dissolve and mixed isolated micelles with a spheroidal shape form.