Experimental and theoretical studies of the system n-decyl-β-D-maltopyranoside+water

Abstract

A class of non-ionic surfactants that is useful for formulating microemulsions, and that is becoming increasingly important for industrial purposes, is the series of alkyl polyglucosides (C X G Y ). These surfactants have x carbons in the hydrophobic alkyl chain and y glucose units in the hydrophilic headgroup, with commercial products typically containing noninteger values of both x and y. Commercial C X G Y blends contain many other compounds besides alkyl-β-D-glucopyranosides, including n-alkyl-α-glucopyranosides, n-alkyl-β-D-maltopyranosides, and other isomers and materials that contain a larger number of glucose units. In this paper, we investigate the physical properties of the system n-decyl-β-D-maltopyranoside (C 10G 2)+water over a wide concentration range, using various experimental techniques (surface tension measurement, rotation rheometer, DSC, polarising microscopy) and a molecular aggregation formation model. The theory is based on calculating the size distribution of the aggregates, which in turn depends on the free energy of forming an aggregate. This free energy is modelled as the sum of several free-energy contributions and an ideal entropy of mixing. For each free-energy contribution, we have highlighted schematically only the relevant characteristics of the surfactant tails or the surfactant heads. The theoretical results are compared to those found in the literature for alkyl-β-D-glucopyranosides (C X G 1) aqueous solutions. In surfactant solutions, rheological behaviour is intimately linked to internal microstructure and micellar architecture. The diluted surfactant system demonstrates Newtonian behaviour and complex non-Newtonian behaviour within the high shear stress regime. In the middle concentration range, the surfactant solutions exhibit an unexpected rheological behaviour, where the viscosities are not dependent on temperature. At high surfactant concentration phase transition, especially liquid-crystalline to isotropic solution, could be followed using rheological experiments. In performing DSC experiments, emphasis is put on the melting behaviour for the dry surfactant and C 10G 2+water systems at high surfactant concentrations. The melting behaviour can be characterised by transitions from a crystalline phase to a liquid crystalline phase and finally to an isotropic solution. The identification of the liquid-crystalline phase was carried out from textural observation, using polarising microscopy. The lyotropic behaviour follows the classical pattern established for the surfactants. Applying polarising microscopy, textures of the hexagonal and lamellar phases could be observed for the system C 10G 2+water.