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_XG_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_XG_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 (C₁₀G₂)+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_XG₁) 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 C₁₀G₂+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 C₁₀G₂+water.

Introduction

Amphiphilic sugar derivatives are of increasing interest because of their significance in areas of self–assembly and molecular recognition in biological systems. Currently, technological developments aiming at the production of chemicals derived from agricultural raw materials are attracting renewed attention. Alkylpolyglucosides (APG) are non-ionic surfactants synthesised from fatty alcohols and saccharides [1]. They display dermatological safety, very good biodegradability and interesting surface active properties [2], [3]. Therefore, these surfactants have become increasingly important as ingredients of detergents and cosmetic products.

The surface activity of pure alkylglucosides at the air/water interface was first studied systematically by Shinoda et al. [4]. In past years, significant progress in the understanding of the physical chemical properties of alkyl glycosides has been achieved. Remarkable results concern the aqueous solution properties [5], [6], [7], [8], [9], [10], phase behaviour, the oil/water interface, the formation of microemulsions [11], [12], [13], [14], [15], [16] and the adsorption of this surfactant type on solid surfaces [17], [18], [19]. The majority of work has been primarily concerned with the physicochemical characteristics of n-alkyl-β-D-glucopyranosides (C₈G₁, C₁₀G₁ and C₁₂G₁). The micelle formation theory, developed by Nagarajan et al. [20], [21], was applied to various n-alkyl-β-D-glucopyranoside surfactants, differing in surfactant tail length (C₈G₁, C₁₀G₁ and C₁₂G₁) [7]. The model predicts that the carbohydrate surfactant molecules self-assemble for energetic reasons into spherical bilayer vesicles. The predicted aggregation properties (critical micellar concentration, aggregation numbers) were close to the experimental findings [7]. It has been demonstrated that the theoretical concept of Nagarajan and Ruckenstein [20], [21] in combination with phase separation thermodynamics can be used successfully to describe the phase separation, which occurs for the system C₁₀G₁+H₂O and C₁₂G₁+H₂O at low surfactant concentrations [7].

The degree of glucosidation is an important parameter in controlling the production of hydrophilic/hydrophobic balanced surfactants. The glucose molecule has a highly hydrophilic headgroup, and when the degree of glucosidation increases the surface active properties will decrease [22]. Thermodynamic studies on aqueous saccharine solutions suggest that the hydrogen bonds that exist between sugars and water are stronger and linked more extensively than those between water molecules alone [23]. It is also well-established that the nature and extent of the hydration of a sugar depends upon the stereochemistry of the particular carbohydrate molecule. This unique interaction that certain sugars have with water is believed to be responsible for the operation of a number of biological processes [24]. The importance of the hydrogen-bonding properties of different sugars was also demonstrated in a recent study [25] on the effects of carbohydrates on membrane stability at low water activities.

Kutschmann et al. [26] measured the interfacial tension at the decane–water interface as a function of the n-decyl-β-D-maltopyranoside (C₁₀G₂) concentration. From the dependence of the interfacial tension on the surfactant concentration below the critical micellar concentration (cmc) the cross-sectional area of the molecules at the decane–water interface was estimated [26]. Aveyard et al. [27] investigated the effects of changes in temperature and electrolyte concentration on the distribution and aggregation of sugar surfactants (mainly C₁₀G₁ and C₁₀G₂) in hydrocarbon+water systems. These authors determined the cmc using surface tension measurements. Drummond et al. [28] reported on the surface pressure characteristics of aqueous solutions of n-dodecyl-β-D-maltopyranoside (C₁₂G₂) and discussed both the adsorption of the surfactant at the air–saturated monolayer interface and the micellization process. In addition [28], the nature of the interfacial microenvironment of C₁₂G₂ micelles was determined from the ionisation behaviour of two micelle-solubilised pH indicators. Böcker et al. [29] found that the cmc of C₁₂G₂ is about a factor of 2 higher than that of the C₁₀G₁. Using dynamic light-scattering measurements, circular dichroism spectra and ¹H–NMR, Focher et al. [30] figured out that the headgroup configuration controls its orientation to the apolar residue and, consequently, the packing of monomers in self-assemblies. The mean micellar diffusion coefficient of C₈G₁ in water shows a strong dependence on the surfactant concentration [30]. By contrast, the diffusion coefficient, and hence the hydrodynamic radius, for C₁₂G₂ in water is almost unaffected by surfactant concentration. Zhang et al. [31] investigated the effect of various salts on the surface tension and critical micelle concentration of the aqueous solution of C₁₂G₂. Interestingly, while the packing of C₁₂G₂ molecules at the air–water interface was not affected by the nature of salt added, cations and anions were found to have markedly different effects on the surface activity and critical micelle concentration of the surfactant [31].

Holland et al. [32] describe results obtained by atomic force microscopy on the solid surface adsorption for a series of non-ionic N-alkylmaltonamide surfactants. The latter consist of a constant amide-linked maltose disaccharide headgroup, but with increasing alkyl chain length, from octyl to octadecyl N-octylmaltonamide, with the shortest alkyl segment, adsorbed uniformly over the graphite surface without assembling into ordered structures. Both N-decylmaltonamide and N-dodecylmaltonamide assembled into ordered structures that are spread over several hundred nanometer regions of the graphite, almost without observable defects [32].

Increasing the effective length of the surfactant headgroup by adding C₁₀G₂ to water+alkyl ethylene glycol ether+C₁₀G₁ mixture moves the phase behaviour dramatically up in temperature when even small amounts of C₁₀G₂ are used [33].

Classical endothermic transitions [34] were also present: the melting point, where the hydrocarbon chains disengage from the crystal lattice; and, at higher temperature, the clearing point, where the hydrogen bonds between carbohydrate moieties melt to form isotropic liquid.

This paper aims to characterise the physical properties of n-decyl-β-D-maltopyranoside (C₁₀G₂) in water over a wide concentration range, in comparison with the same concentrations of C₈G₁ and C₁₀G₁ [6] in water, and to predict its special aggregation behaviour using a micelle formation model [7], [20], [21]. The present study, involving surface tension measurements; density measurements; rheological methods; differential scanning calorimetry; polarisation microscopy; and calculations based on a molecular aggregation model, has been performed to obtain more information on the physical properties of C₁₀G₂, and discuss the influence of the hydrophilic headgroup.