Colloids and Surfaces A: Physicochemical and Engineering Aspects
Rheological behaviours of the hexagonal and lamellar phases of glucopone (APG) surfactant
Introduction
Considering the wide range of applications for surfactant-containing systems, detailed rheological characterizations becomes increasingly important, for example in the design of pumping, agitation and mixing operational units. Moreover, it also helps one to in establish theoretical understanding of the relationships between the viscoelastic properties and the microstructures. It has been known that such systems exhibit complex flow properties especially at high surfactant concentrations [1], [2].
The growing interest concerning sugar ester surfactants over petrochemically derived standard products is mainly due to their advantages with regard to performance and environmental compatibility [3], [4]. Moreover, sugar ester surfactants are mostly obtained from renewable raw materials such as fatty acids and sucrose [5], [6]. They are readily biodegradable in aqueous environments and in most situations found to be non-toxic and non-allergenic [3]. Hence, sugar surfactants are widely used in the fields of cosmetics [7], [8] and food additives [9] for a variety of functions that include emulsifying and foaming in various products such as bread, ice cream, margarine, and fat substitutes [10], [11], [12]. Studies on lyotropic behavior of amphiphilic sugar derivatives are still relatively limited [13], [14]. Interestingly, depending on the type of product being studied, an entire range of lyotropic mesophases ranging from hexagonal to reverse hexagonal via cubic fluid lamellar can be obtained. The lamellar phase is the most commonly prepared liquid crystal mesophase in concentrated surfactant systems. Considerable amount of work has been focused towards understanding the changes in the microstructure of the lamellar phase under shear. In particular, the effects of surfactant concentration on the rheology and microstructure of the lamellar phase have been recently reported [15]. The rheological response during processing is directly related to microstructural changes, which contributes to significant functional benefits. In contrast, the rheological behaviour of the hexagonal liquid crystals is less studied [16], [17]. They are known to be more viscous than the lamellar liquid crystals, with the viscosity decreasing as the shear rate is increased. They are also known to exhibit elastic properties [16].
In this work, the hexagonal and lamellar phases of the glucopone–water–hydrocarbon systems were considered. The phase behaviours of glucopone–water–hydrocarbone systems were investigated by Siddig et al. [18]. Optical microscopy and small-angle X-ray scattering (SAXS) were used for phase identification and structure characterization. SAXS experiments are useful in conveying information on the area per surfactant head group in the liquid crystalline phases. Rheological measurements were performed in order to study the viscoelastic properties of the liquid crystalline phases. The effects of different types of hydrocarbon and temperature on the rheological behaviours of hexagonal and lamellar phases were also investigated.
Section snippets
Material and samples preparation
Technical grade alkyl polyglucosides, APG (commercially known as glucopone 215 CSUP), octane and dodecane were purchased from Fluka. The APG represents a mixture of α- and β-glucosides [18] and is supplied as a 65 wt.% solution in water. The surfactant was used without any further purification. Heptane was purchased from Ajax chemicals, while tetradecane was purchased from Tokyo Kasei Kogyo Co. (TCI). The APG solution was prepared by weighing the surfactant into a glass tube and diluting it to
Small-angle X-ray scattering
The birefringence patterns of lyotropic mesophases of hexagonal and lamellar structures were observed first using an optical polarizing microscope. This was followed by SAXS measurement in order to confirm the formation of these phases. The characterization of liquid crystalline phases by X-ray diffraction is based on the long-range order in the liquid crystalline state which gives rise to Bragg reflections. The positions of peaks are characteristic of the different liquid crystalline phases.
Conclusions
The rheological behaviours of the hexagonal and lamellar liquid crystalline phases of glucopone–water–hydrocarbon systems have been investigated. The liquid crystalline phases were formed at high surfactant concentrations and were identified by means of microscopy and SAXS. The SAXS measurements showed that the area per head groups of hexagonal phases are almost unaffected, while the area per surfactant head groups for the lamellar phases were found to increase as the alkane chain length of
Acknowledgment
This work was supported by the Malaysian Ministry of Science, Technology and Environment, under research grant IRPA 09-02-02-0032-SR0004/04-04.
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