The Properties of Alkyl Polyglucosides

Similar to polyoxyethylene alkyl ethers, alkyl polyglycosides are usually technical surfactants. They are produced via different modes of Fischer synthesis and consist of a distribution of species with different degrees of glycosidation indicated by a mean n-value. This is defined as the ratio of the total molar amount of glucose to the molar amount of fatty alcohol in the alkyl polyglucoside, taking into account the average molecular weight when fatty alcohol blends are employed. As already mentioned most of the alkyl polyglucosides of importance to application have a mean n-value of 1.1-1.7. Hence, they contain alkyl monoglucosides and alkyl diglucosides as the main components, as well as smaller amounts of alkyl triglucosides, alkyl tetraglucosides, etc. up to alkyl octaglucosides besides the oligomers, minor amounts (typically 1-2%) of fatty alcohols used in the synthesis polyglucose, and salts, mainly due to catalysis (1.5-2.5%), are always present. The figures are calculated with respect to active matter. Whereas polyoxyethylene alkyl ethers or many other ethoxylates may be defined unambiguously by a distribution of molecular weights, an analogous description is by no means adequate for alkyl polyglucosides because different isomerism result in a much more complex range of products. The differences in the two surfactant classes result in rather different properties originating from the strong interaction of the headgroups with the water and in part with each other.

The ethoxylate group of the polyoxyethylene alkyl ether strongly interacts with water, forming hydrogen bonds between the ethylene oxygen and water molecules, hence building up micellar hydration shells where the structuring of water is greater (lower entropy and enthalpy) than in bulk water. The hydration structure is highly dynamic. Usually between two and three water molecules are associated with each EO group.

Considering glucosyl headgroups with three OH functions for a monoglucoside or seven for a diglucoside, alkyl glucoside behavior is expected to be very different from that of the polyoxyethylene alkyl ethers. Besides the strong interaction with water, there are also forces between the surfactant headgroups in the micelles as well as in other phases. Whereas comparable polyoxyethylene alkyl ethers alone are liquids or low melting solids, alkyl polyglucosides are higher melting solids because of intermolecular hydrogen bonding between neighboring glucosyl groups. They display distinct thermotropic liquid crystalline properties, as will be discussed below. Intermolecular hydrogen bonds between the headgroups are also responsible for their comparatively low solubility in water.

As for glucose itself, the interaction of the glucosyl group with the surrounding water molecules is due to extensive hydrogen bonding. For glucose, the concentration of tetrahedrally arranged water molecules is higher than in water alone. Hence, glucose, and probably also alkyl glucosides, can be classified as a “structure maker,” a behavior qualitatively similar to that of the ethoxylates.

In comparison to the behavior of the ethoxylate micelle, the effective interfacial dielectric constant of the alkyl glucoside is much higher and more similar to that of water than to that of the ethoxylate. Thus, the region around the headgroups at the alkyl glucoside micelle is aqueous-like.