Water Systems at Different Concentrations and Temperatures

CHEM. RES. CHINESE UNIVERSITIES 2011, 27(6), 1065—1071 Dielectric Spectra of Aerosol OT/Water Systems at Different Concentrations and Temperatures ZH...
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CHEM. RES. CHINESE UNIVERSITIES 2011, 27(6), 1065—1071

Dielectric Spectra of Aerosol OT/Water Systems at Different Concentrations and Temperatures ZHAO Kong-shuang1*, JIA Zhan-jie1, YANG Li-kun1 and XIAO Jin-xin2 1. College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China; 2. Beijing FLUOBON Surfactant Institute, Beijing 100080, P. R. China Abstract The dielectric spectra of aerosol OT[AOT, sodium(bis-2-ethylhexyl)sulfosuccinate]/water systems at different concentrations and temperatures were investigated by the dielectric relaxation spectroscopy(DRS). Through the dielectric spectra of different concentrations, two dielectric relaxations were observed over a frequency range from 40 Hz to 110 MHz and the mechanisms of the relaxations were also interpreted based on the Grosse’s model. The low-frequency relaxation(around 105 Hz) was attributed to the radial diffusion of counterions along the long-half axis of the rod-like micelle, and the high-frequency relaxation(around 106 Hz) was ascribed to the radial diffusion of counterions along the short-half axis. Furthermore, specific emphasis was placed on studying the effects of temperature on system’s conductivity. It was observed that the low-frequency limit of conductivity(κl) decreased and then increased with the increment of temperature under the measured concentration. On the other hand, the conductivity(κm and κh) in meso- and high-frequency ranges always increased as temperature increased. Both the tendencies of alteration which mentioned above should be related to the phase transition of AOT/water system. Keywords Aerosol OT; Rod-like micelle; Dielectric relaxation spectroscopy Article ID 1005-9040(2011)-06-1065-07

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Introduction

Aerosol OT(AOT), sodium bis-2-ethylhexyl-sulfosuccinate, is a typical anion surfactant with a small branched double tail[1], which can easily form reverse micelle and microemulsions with other solvent in the absence of cosurfactant. In addition, the AOT-water-oil system is a particularly useful model system for measuring the characteristics of simple microemulsions since it can form one of the few three-component microemulsions in the vicinity of room temperature[2]. Recently, the structures and properties of AOT reverse micelle and the phase behavior of AOT-water-oil system have been studied by means of various techniques, such as the fluorescence probe technique[3], rheometry and small angle X-ray scattering (SAXS) experiments under static and shear conditions[4,5], 1H and 13C NMR spectroscopy[6], circular dichroism(CD)[7], UV-spectrometry and conductivity meter[8] and dynamic light scattering(DLS)[9]. In Fig.1, the AOT/water system exhibits a phase behavior at 25 °C. The AOT/water system forms isotropic liquid solution (L) which consists of normal spherical or rod-like micelle when wt(wt is the surfactant mass fraction) is below 1.4%. When the value of wt ranges from 1.4% to 24%, the mixture will correspond to L-La equilibrium. And the system forms a lamellar liquid crystalline phase(La) if wt extends from 24% to 77%[10,11]. If wt is higher than 77%, anisotropic liquid crystal phase forms, such as viscous isotropic cubic phase(V1), and reversed hexagonal phase(H2)[12,13]. An important issue to address,

however, is that a macroscopic phase separation spontaneously takes place after a few days for the mixtures with wt below 8.0%, while those with wt above 8.0%, systems are much stable during a long period. As mentioned above, the AOT/water system’s phase behavior is rather complicated. Ficheux et al.[13] reported two hydrophilic neutral polymers[polyethylene glycol (PEG) and polyacrylamide(PAM)] affected the stability of lamellar phase made of AOT/water mixtures. Callaghan et al.[12] studied the diffusion of water in randomly oriented lamellae of AOT/water solutions using the pulsed field gradient nuclear magnetic resonance(PFG NMR). The value of wt in the both studies, however, is higher than 24%. When wt is below 24%, the AOT/water mixtures form a lamellar liquid crystalline phase(La). To the best of our knowledge, few researches on AOT/water system(wt

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