The latest research work provides us with basic knowledge about some selected properties of systems containing sugar surfactants. Now we are able to understand the behavior of aqueous solutions reasonably well and very recently progress was made with respect to the properties of microemulsions. However, little is still known about the adsorption properties. Apart from the need for more fundamental research, the future task will be to handle more complex systems. Here complex refers to multi-component mixtures used in technical applications, but also to biotechnological applications where the structure of the surfactant is more complex. Promising future developments in this field are outlined.

Sugar surfactants — aggregation, interfacial, and adsorption phenomena

A matrix of anomerically pure glucose-based surfactants have been synthesized and their thermotropic and lyotropic liquid crystalline phase behavior, and air-aqueous solution interfacial adsorption were investigated. The surfactants, which represent the major components of the Fischer synthesis products, were the n-octyl, n-decyl and n-dodecyl homologues of alkyl α-d- and β-d-glucoside and alkyl β-d-maltoside. The matrix allowed the investigation of the effects of alkyl chain length, headgroup polymerization, and anomeric configuration on the surfactants' physicochemical properties. Increasing the alkyl chain length increases the hydrophobicity and the dispersion interaction between surfactant molecules, as one would expect, resulting in greater thermal stability of thermotropic and lyotropic phases. Phase transition temperatures are influenced significantly by the anomeric configuration in the shorter octyl derivatives, but to a lesser extent in the longer alkyl chain derivatives. The effect of increasin...

How chain length, headgroup polymerization, and anomeric configuration govern the thermotropic and lyotropic liquid crystalline phase behavior and the air-water interfacial adsorption of glucose-based surfactants

This contribution deals with the physical-chemical properties of surfactants carrying hydroxyl groups in the polar part. Topics discussed include surface and colloid properties, micelles (both single and multi-component), general phase behaviour and microemulsions. A general conclusion is that our understanding of polyhydroxy surfactants is increasing, but that further work is needed to unravel certain aspects of this important class of surfactants. Examples of such aspects are the origin of the liquid-liquid phase separation and the adsorption to solid surfaces of polyhydroxy surfactants.

Polyhydroxyl-based surfactants and their physico-chemical properties and applications

The liquid crystalline state is a fundamental organization of matter, which combines order and mobility on a molecular, supramolecular and macroscopic level. In many cases the molecules can show both thermotropic and lyotropic liquid-crystalline (LC) phases, which is described as amphotropic behavior. Block-copolymers, polyhydroxy amphiphiles, disc-like, rod-like, polycatenar and banana-shaped LC molecules are discussed with respect to their amphotropic behavior with specific and non-specific solvents. The interactions of salts with polyether chains, leading to halotropic mesophases, and the interaction of aromatic electron acceptor molecules with electron-rich aromatic molecular parts are discussed in relation to lyotropic mesomorphism induced by classical solvent molecules. Polyphilic amphotropic materials showing more complex mesophase morphology and amphiphiles showing a hierarchical order of different levels of order are pointed out as future directions.

Amphotropic liquid crystals

The equilibrium adsorption behavior of two n-alkyl-beta-D-glucosides (octyl (C8G1) and decyl (C10G1)) and four n-alkyl-beta-D-maltosides (octyl (C8G2), decyl (C10G2), dodecyl (C12G2), and tetradecyl (C14G2)) from aqueous solution on a titania surface, as measured by ellipsometry, has been investigated. The main focus has been on the effect of changes in the alkyl chain length and headgroup polymerization, but a comparison with their adsorption on the silica/water and air/water interfaces is also presented. Some comparison with the corresponding adsorption of ethylene oxide surfactants, in particular C10E6 and C12E6, is given as well. For all alkyl polyglucosides, the maximum adsorbed amount on titania is reached slightly below the critical micelle concentration (cmc), where it levels off to a plateau and the amount adsorbed corresponds roughly to a bilayer. However, there is no evidence that this is the actual conformation of the surfactant assemblies on the surface, but the surfactants could also be arranged in a micellar network. On hydrophilic silica, the adsorbed amount is a magnitude lower than on titania, corresponding roughly to a layer of surfactants lying flat on the surface. A change in the alkyl chain length does not result in any change in the plateau molar adsorbed amount at equilibrium; however, the isotherm slope for the alkyl maltosides increases with increasing chain length. Headgroup polymerization on the other hand affects the adsorbed amount. The alkyl glucosides start adsorbing at lower bulk concentrations than the maltosides and equilibrate at higher adsorbed amounts above the cmc. When compared with the ethylene oxide (EO) surfactants, it is confirmed that the EO surfactants hardly adsorb on titania, since the measured changes in the ellipsometric angles are within the noise level. They do, however, adsorb strongly on silica.

Adsorption of alkyl polyglucosides on the solid/water interface: Equilibrium effects of alkyl chain length and head group polymerization

The formation of lyotropic mesophases (liquid crystals) in four binary systems n-alkyl glycosides/water was examined in dependence on surfactant concentration, temperature and the chain lengths (alkyl = heptyl, octyl, nonyl, decyl). The binary phase diagrams were established and the enthalpies of phase transitions were measured. The following phase transitions were detected by texture observation and calorimetry: hexagonal phase H, lamellar phase L, cubic phase Q, gel phase G and crystalline phase C. The positions of the corresponding regions of these phases in the phase diagram were determined. Sequence of phases and the localization of the phase regions were depended on the chain length of the alkyl group. So in the binary system n-decyl-β-D-glucoside/water the H-phase was not observed.

A. Göpfert

Papers

Lyotropic liquid crystals in binary systems n-alkyl glycosides/water*