Alkylglucosides - physical chemical properties
Abstract: The aim of this study is to characterise the physical-chemical properties of alkylglucosides. Alkylglucosides are non-ionic surfactants synthesized from glucose and a fatty alcohol. The glucose molecule constitutes the hydrophilic head group and the fatty alcohol constitutes the hydrophobic tail. The two parts are linked together with a glucoside bond. In nature, the building and breakage of a glucoside bond are controlled by different enzymes called glucosidases. This makes alkylglucosides highly biodegradable. Alkylglucosides also have low toxicity towards biological life. Therefore, alkylglucosides are very interesting in applications like laundry, car wash, shampoo, personal care products and cosmetics. There are two possible ways of attaching a glucose molecule to the fatty alcohol, viz. a- or b-bond. The two ways give different physical-chemical properties to the alkylglucoside. One such property is different packing in a crystal lattice. In this study, the physical-chemical properties of some short-chain alkylglucosides have been determined, viz. C8-, C9-, and C10-alkylglucosides. Alkylglucosides with a- and b-bonding, as well as straight and branched tails have been studied. The study includes both alkylmonoglucosides, which are anomerically pure, and commercially available alkylpolyglucosides. The study is based on the determination of binary surfactant/water phase diagrams (temperature vs. concentration) and ternary surfactant/ surfactant/water phase diagrams. The different phases in the phase diagram have been investigated to obtain information concerning aggregates size/structure and aggregate-aggregate interactions. Techniques like nuclear magnetic resonance (NMR) self-diffusion, microscope, small-angel X-ray scattering (SAXS), differential scanning calorimetry (DSC) and fluorescence have been used to determine the physical-chemical properties. A general feature of alkylglucosides is a broad solution phase extending from neat water to approximately 60-70 wt% surfactant. At higher surfactant concentrations, liquid crystalline phases are formed. Alkylglucosides with straight tails can form hexagonal, cubic and lamellar liquid crystalline phases, whereas alkylglucosides with branched tails only form lamellar liquid crystalline phases. A conclusion from the study is that alkylglucosides do not form spherical micelles but elongated aggregates. The aggregates size increase significantly when the number of carbon atoms in the tail increase. When there are ten or more carbon atoms in the tail a phase separation occurs at dilute concentration. A mechanism behind the phase separation has been proposed based on the dilution of a surfactant network. Repetition distances and head group areas in the liquid crystalline phases have been calculated. A general observation is the surfactant head group area are fairly unchanged when the concentration increases. An explanation for this behaviour is the glucose molecule requires a certain amount of hydration water. When this condition is met there is no gain in free energy by changing the head group area at the hydrophobic/hydrophilic interface.
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