Surfactants are amphiphilic molecules, meaning that one part is hydrophobic and the other hydrophilic. This leads to that they adsorb at interfaces and self-assembles in solution into various structures. These properties make surfactants important ingredients in several products as eg. detergents, emulsifiers and rheological modifiers. The extensive use of surfactants results in a significant environmental footprint, which can be reduced by transitioning into using surfactants that are readily biodegradable and synthesized from renewable resources. Alkylglycosides are sugar-based surfactants that fit into this description. We have previously shown that the alkylglycoside hexadecyl maltoside (C16G2) forms worm-like micelles (WLM) in solution, which can be used to tune the rheological properties. However, the solubility of saturated long-chain surfactants, like C16G2, is often very low at room temperature, making it difficult to use in technical applications. The introduction of a double bond in the carbon chain of the surfactant was hypothesized to increase the solubility of the surfactant, and in our recent work the first synthesis and characterization of an unsaturated alkylglycoside was presented.
Unsaturated C16G2 (palmitoleyl-β-D-maltoside) was synthesized at DEMAX, the ESS deuteration facility. This surfactant showed a drastic increase in solubility at low temperatures compared to the saturated version and remained in the micellar phase from 5 to 95 °C. The micellar structure was studied with several scattering techniques. The neutron experiments were performed on two different instruments (VSANS and SANS) making it possible to access a wide q-range and study several different length scales.
The SAXS and SANS results, which were simultaneously fitted with a flexible cylinder model, revealed the formation of WLM with a contour length of up to 1 µm and a persistence length of about 200 Å. The cross-sectional dimensions of the cylindrical micelles were analyzed with a core-shell cylinder model. This resulted in a micelle radius of 30 Å with a carbon chain core of 15 Å and a hydrated carbohydrate shell of 15 Å. At high concentrations in the semi-dilute regime the surfactant solutions yielded highly viscous solutions with shear-thinning and viscoelastic properties. This behaviour can be connected to the WLM micellar structure which, at high concentrations, entangle resulting in the intriguing rheological properties.
Figure 1. SANS-results (blue) and SAXS-results (red) for unsaturated C16G2 fitted with a flexible cylinder model and a core-shell cylinder model respectively. Insets show the molecular structure of unsaturated C16G2 and a schematic representation of the micellar structure.
The micelle formation of these alkylglycosides will be further investigated by neutron spin echo and dynamic light scattering. This will provide information on the dynamics of the elongated micelles and how it differs between the saturated and unsaturated C16G2. In combination with the structural and rheological characterization of the C16G2 systems, this will provide detailed knowledge on the behaviour of long chain alkylglycosides and facilitate the development of more environmentally benign products.
This work has been a collaboration between the Division of Physical Chemistry at Lund University, the Department of Food Technology at Lund University and the Deuteration and Macromolecular Crystallisation (DEMAX) platform at ESS. The neutron experiments were performed at the Jülich Centre for Neutron Science at Heinz Maier-Leibnitz Zentrum. The project was funded by the Swedish Research Council Formas and Vinnova - Swedish Governmental Agency for Innovation Systems within the NextBioForm Competence Centre.
Read the full article in the Journal of Colloid and Interface Science 585 (2021) 178–183