Microstructures in the aqueous solutions of a hybrid anionic fluorocarbon/hydrocarbon surfactant

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Abstract

The aqueous solutions of the anionic hybrid fluorocarbon/hydrocarbon surfactant sodium 1-oxo-1[4-(tridecafluorohexyl)phenyl]-2-hexanesulfate (FC6HC4) shows peculiar rheological behavior. At 25 °C the viscosity vs concentration curve goes successively through a maximum and a minimum, while the viscosity vs temperature curve of the 10 wt% aqueous FC6HC4 solution goes through a marked maximum at 36 °C [Tobita et al., Langmuir 13 (1997) 5054]. In an attempt to explain these properties the microstructure of aqueous solutions of FC6HC4 has been investigated by means of digital light microscopy, transmission electron microscopy at cryogenic temperature (cryo-TEM), rheology, and self-diffusion NMR. At 20 °C, the increase of the FC6HC4 concentration was found to result in a progressive change of structure of the surfactant assemblies from mainly spherical micelles at 0.5 wt% to mainly cylindrical micelles at 10 wt%. At intermediate concentrations small disklike micelles and small complete and incomplete vesicles coexisting with cylindrical micelles were visualized. The occurrence of stretched cylindrical micelles is responsible for the effect of the surfactant concentration on the solution viscosity. Cryo-TEM, rheology, and self-diffusion NMR all suggest that an increase of the temperature brings about a growth of the assemblies present in the 10 wt% solution of FC6HC4. The structure of the assemblies present at the temperature where the viscosity is a maximum could not be elucidated by cryo-TEM because of the probable occurrence of an on-the-grid phase transformation, the result of blotting during specimen preparation. Nevertheless, the results show that the observed large assemblies break up at higher temperature to give rise to a more labile bicontinuous structure that consists of multiconnected disordered lamellae, with many folds and creases, and that may well be the L3 phase.

Introduction

Recently, hybrid fluorocarbon/hydrocarbon anionic surfactants of the general chemical structure CmF2m+1C6H4COCH(SO3Na)CnH2n+1 (referred to as FCmHCn, see Scheme 1; note that the carbon atom bearing the sodium sulfonate group is asymmetric) have been synthesized [1]. These surfactants are much more resistant to hydrolysis than other hybrid fluorocarbon/hydrocarbon surfactants [2]. Their aqueous solutions show very low critical micelle concentration (cmc) and are characterized by low surface tension (20 mN/m above the cmc). Besides, these surfactants are capable of emulsifying water/hydrocarbon/perfluoroethyl–oil ternary mixtures [1]. In view of those properties, the behavior of these surfactants both in aqueous solution and at interfaces has been extensively investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These studies revealed peculiar rheological behavior of the aqueous solutions of the FC6HC4 homologue. Thus, at 25 °C the viscosity of FC6HC4 solutions was found to go through a maximum at a surfactant concentration of about 9 wt% and then through a minimum at about 16 wt%. The effect of temperature on the viscosity of the 10 wt% FC6HC4 solution was even more striking. The viscosity (about 1 Pa s at 15 °C) increased with temperature up to a maximum value of about 102 Pa s at 36 °C and then decreased rapidly at higher temperature, down to a value only 10 times larger than the viscosity of water at 60 °C. The system was found to be viscoelastic in the temperature range around the viscosity maximum. This behavior was referred to as thermoresponsive viscoelasticity [7]. The peculiar concentration dependence of the viscosity of FC6HC4 solutions had been previously observed with other surfactants, generally of the cationic type [11], [12], [13] and attributed to two antagonistic effects. The first effect is the transformation of the nearly spherical micelles occurring at low concentration into threadlike micelles at high concentration and the entanglements of the threadlike micelles leading to increased viscosity [11], [12], [13]. The second effect may involve the dynamics of the micelles [11], [12] or reflect a true decrease of micelle length [13] or an increased branching of the threadlike micelles [14], which would result in a decrease of viscosity at high concentration. The presence of a minimum in the viscosity vs concentration curve, and the subsequent increase of viscosity at still higher surfactant concentration, have received no satisfactory explanation. Complex effects involving the relative values of the structural relaxation time of the system and the lifetime of the threadlike micelles appear to be responsible for the observed behavior [11], [12]. In the case of FC6HC4 solutions this behavior is apparently associated with the formation of a lamellar liquid-crystalline phase in the system [10].

The temperature dependence of the viscosity of the 10 wt% FC6HC4 solution is unusual. The literature shows one additional example of such variation, again for a solution of cationic surfactant [15]. In this case the presence of a maximum of viscosity upon increasing temperature was attributed to the transformation of the vesicles formed at low temperature into threadlike micelles at high temperature.

The thermoresponsive rheological behavior noted for FC6HC4 solutions is of interest for applications for which fluids that become more viscous at high temperature are required. For instance, the oil industry requires fluids of very high viscosity at high temperature for cracking oil-containing rock. Injecting such fluids into the oil well is difficult because of their high viscosity at ambient temperature. A fluid of low viscosity at low temperature that becomes highly viscous at the elevated temperature of the oil-containing rock would greatly facilitate the process. Anionic surfactants having rheological behavior similar to that of FC6HC4 solutions but at still higher temperature represent a possible solution. Research to explain this behavior may help direct the synthesis of similar and better performing compounds.

The microstructure of the 10 wt% FC6HC4 solution at 20 °C was studied by freeze-fracture transmission electron microscopy [8]. The micrographs showed large aggregates, about 500 nm in size, coexisting with smaller aggregates, about 80 nm in size [8]. In view of these results the maximum of viscosity was explained in terms of the effect of temperature on the friction arising from collisions between small and large aggregates as compared to the friction arising from collisions between small aggregates or between large aggregates [8]. However, freeze-fracture electron microscopy involves freezing that may affect the aggregates present in the investigated system, particularly if it is close to a phase change. This problem and others are largely avoided by examining vitrified specimen of the investigated systems by means of transmission electron microscopy at cryogenic temperature (cryo-TEM) [16], [17]. Indeed, the ultrafast vitrification of a thin film of the investigated solution ensures minimal, if any, structural changes of the aggregates present in the solution. Cryo-TEM has been instrumental in clarifying microstructures in surfactant solutions and thus providing explanation for their rheological behavior [16], [17].

In this paper we report the results of a microstructural study of FC6HC4 solutions as a function of concentration, between 2 and 10 wt%, and of temperature between 15 and 60 °C. We used cryo-TEM, digital light microscopy, measurements of self-diffusion coefficients by pulsed field gradient NMR spectroscopy, and measurements of rheological properties of the 10 wt% solution of FC6HC4 at temperatures from 20 to 60 °C. The sequence of structures occurring upon increasing concentration at 20 °C has been elucidated and permitted us to explain the effect of concentration on the solution viscosity. The results do not provide an unambiguous explanation for the thermoresponsive behavior of FC6HC4 solutions. There are indications that an on-the-grid phase transformation may be taking place during the preparation of some of the cryo-TEM specimens. That transformation prevents the observation of the original microstructure, particularly around the temperature corresponding to the maximum of viscosity. Nevertheless, it appears that the thermoresponsive behavior of FC6HC4 solution may reflect a transformation of the cylindrical micelles present at low temperature into a highly viscous structure at around 40 °C made up of large assemblies. These assemblies break up at higher temperature to give rise to a more labile bicontinuous structure that appears to consist of multiconnected disordered lamellae with many folds and creases, possibly an L3 phase. Although this explanation is still somewhat conjectural, we believe these results may provide a first explanation for the thermoresponsive rheological behavior of FC6HC4 solutions.

Section snippets

Materials

The surfactant FC6HC4 was synthesized and purified at the Tokyo University of Science, as previously described [1].

Methods

The preparation of vitrified specimens for cryo-TEM was performed in a controlled environment vitrification system (CEVS) [16], [17], [18]. All solutions were quenched from the selected temperature and 100% relative humidity. Specimens were examined in a Philips CM120 microscope, operated at 120 kV, using an Oxford CT-3500 cryo-holder system. All specimens were observed in the

Digital light microscopy

All solutions of FC6HC4 were clear and transparent at all temperatures (20–60 °C) and concentrations (up to 10 wt%) investigated. Digital light microscopy revealed no structure in the investigated solutions. Thus, these solutions contained no surfactant assemblies (vesicles or bilayer fragments) of length scale that can be resolved by this technique, that is, larger than say 1 μm. Digital light microscopy can also detect the presence of surfactant assemblies of size below 1 μm, down to about 100

Summary and conclusions

The behavior of the hybrid surfactant FC6HC4 has been investigated by digital light microscopy, cryo-TEM, rheology, and NMR self-diffusion, at concentrations between 2 and 10 wt% and temperatures up to 60 °C. The visual aspect of the solutions as well as their examination by digital light microscopy indicated the absence of aggregates with sizes larger than 1 μm under these conditions. Cryo-TEM clearly showed the presence of small micelles at low concentration. At 20 °C, these micelles grew and

Acknowledgements

This work was supported in part by a “Center of Excellence” grant from the Israel Science Foundation of the Israel Academy of Sciences and Humanities, and by a Technion VPR Fund, R. and M. Rochlin Fund. The cryo-TEM work was performed at the “Cryo-TEM Hannah and George Krumholz Laboratory for Advanced Microscopy” at the Technion, part of the “Technion Project on Complex Fluids.” We thank Judith Schmidt and Berta Shdemati for excellent technical assistance. D.W. is grateful for the Levi Eshkol

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    Present address: Department of Food Engineering and Biotechnology, Technion-Israel Institute of Technology, Haifa 32000, Israel.

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