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Investigations into the Formation and Characterization of Microemulsions I Phase Diagrams of the Ternary System Water-Sodium Alkyl Benzene Sulfonate-Hexanol and the Quaternary System Water-Xylene-Sodium Alkyl Benzene Sulfonate-HexanolR. C. BAKER, *'1 A. T. FLORENCE,t TH. F. TADROS, *'2 AND R. M. WOOD~*ICI Plant ProteCtion Division, Jealott's Hill Research Station, Bracknell, Berkshire RG12 6EY, England, t Department of Pharmacy, University of Strathclyde, Royal College, 204 George Street, Glasgow G1 1XW, Scotland, and ~Department of Applied Physics, Sheffield City Polytechnic, Sheffield $1 1 WB, EnglandReceived June 28, 1983; accepted January 25, 1984 The phase diagrams of the ternary system water-sodium alkylbenzene sulfonate (NaDBS)-hexanol and the quaternary system water-xylene-NADBS-hexanol have been established at three different temperatures, namely 25, 37, and 50C. The different phases formed have been qualitatively examined using optical (phase contrast and polarizing) microscopy. The textures of the various liquid crystalline phases in the ternary system have been identified, by comparison with previous studies in the literature. Some of the liquid crystalline phases have been quantitatively assessed using low angle X-ray diffraction. The latter measurements were also used to determine the unit cell dimensions in the various phases studied. With the quaternary system, particular attention was paid to the transparent region which consisted of an L2 (inverse micellar) phase extending into another transparent region which has a blue "tinge" in some cases, namely the microemulsion (M) region. The amount of water solubilized in the L2 (reverse micelle) or M + L2 phase was calculated from the phase diagrams. With the ternary system the results showed a maximum in moles of water solubilized per mole total surfactant (NaDBS + hexanol) at a concentration of 0.3 mole surfactant, at an optimum molar ratio of n-hexanol to NaDBS of 4.5: I. This maximum was about twice with the quaternary system, when compared with that of the ternary system, indicating the importance of the role of xylene in solubilization of water by the surfactants, The present investigation has also shown that the extent of the microemulsion region is significantly reduced by increases of temperature when the NaDBS is lower than 15 wt%. INTRODUCTION

Since their introduction in 1943 by Hoar and Schulman (1), microemulsions have attracted considerable attention in view of their use in many industrial products and their potential application in enhanced tertiary oil recovery. Various investigations have been carded out to study the formation and stability of such systems, and various techniques have been employed for their characterization. Several review articles (2-5) and monographs~Present address: The Wellcome Foundation Ltd., Berkhamsted, Herts HP4 2D4, U.K. 2 Author to whom correspondence should be addressed. 31

(6, 7) are now available for the description of a number of microemulsion systems and the methods used for their study. The latter indude phase diagrams, scattering techniques, conductivity and dielectric studies, viscosity measurements, NMR investigations, and interfacial tension measurements. Moreover a number of theories have been put forward in recent years to explain the origin of thermodynamic stability of such systems and these have been recently reviewed (5). However, in spite of this amount of research, which expanded rapidly in the last decade or so, there is still some controversy about the nature of such systems. The most recent NMR self-diffusion studies by Lindman et al. (8) have0021-9797/84 $3.00 Copyright 1984by AcademicPress,Inc. All fightsof reproductionin any form reserved.

JournalofCofloidandInterfaceScience.Vol. 100,No. 2, August 1984

312

BAKER ET AL. EXPERIMENTAL

dearly shown that such systems can vary from the so called bicontinuous (with flexible and highly disorganized internal interfaces and no definite separation into hydrophilic and hydrophobic domains) to thermodynamically stable "dispersions" with definite droplet cores. In the first case, there is no justification for the use of the term "microemulsion" to describe such systems and these are better referred to as "thermodynamically stable isotropic solutions" (9). On the other hand, in the latter case, the term "microemulsion" or "swollen micellar system" may still be appropriate. In the present two papers, we will describe some systematic investigations on microemulsions using a quaternary system of water, xylene, sodium alkyl benzene sulfonate (to be referred to as NaDBS), and hexanol. NaDBS was selected as the surfactant since it can be easily purified and it is stable to hydrolysis in solution. For a systematic study of microemulsion composition, it is essential to establish the phase diagrams of the system under investigation. From these one can identify the extent of the microemulsion region and its relation to other phases. One of the objectives of the present investigations was to establish the effect of temperature and therefore studies were made at 25, 37, and 50C. Since the ternary phase diagrams of the system water-NaDBS-heXanol have not been established previously, we found it necessary first to establish such phase diagrams. The various phases formed were identified qualitatively using phase contrast and polarizing microscopy. Quantitative analysis of some of the phases formed was attempted using low angle X-ray diffraction. The amount of water solubilized in the inverse mieelle region was determined as a function of total surfactant concentration and the r a t i o o f surfactant to cosurfactant (hexanol). This is then followed by an investigation of the quaternary system and in particular the extent of the microemulsion in each system. Characterization of the w/o microemulsions was carried out using light scattering, conductivity, and viscosity (Part II) (10) measurements.Jqurnal of Colloid aod Interface Science, Vol. 100, No. 2, August 1984

Materials Water. All water was twice distilled from an all glass apparatus and its surface tension was 72 ___0.2 m N m -~ at 25 + 0.1C. Xylene. Analar grade material (ex Koch Light) was used as received. The N M R spectrum showed the material to be a mixture of ortho, meta, and para isomers and containing a few percent ethyl benzene. Surfactant. The surfactant was a commercial product, Nansa 1106, supplied by Albright and Wilson (Whitehaven). Although this material is usually referred to as sodium dodecyl benzene sulfonate, the linear hydrocarbon chain consists of a distribution from C9 to C 1 4 as shown below.Carbon chain length Wt% 200) of samples, to extend over a wide concentration range of each ingredient. Once the phase in a sample had been identified, the point on the phase diagram representing that sample was coded according to the phases found. When this procedure had been completed for all the samples, the extent of each phase region and the phase boundaries were determined by drawing a line through the points with the same code.

X-Ray Diffraction InvestigationsFor such investigations, cobalt radiation was used with an iron filter to give a radiation wavelength of 0.17902 nm. The specimens were sealed inside 0.5-mm-diameter Lindemann tubes and exposures were normally of approximately 16 hr. Photographs were taken using a Warhus low angle camera with a specimen to film distance of 295 mm. The difJournal of Colloid and Interface Science, Vol. 100, No. 2, August 1984

Assessment of SamplesThe different phases were assessed visually and by optical and polarizing microscopy.

a

Hexano| 0 100 Hexanol

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Water 0

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Hexanot 0 100

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PHASE DIAGRAMS OF A TERNARY AND QUATERNARY SYSTEM TABLE I Designation and Nomenclatureof the Observed PhasesDesignation Basic structure Alphabetical notation

315

Water continuous isotropic liquid Oil continuous isotropic liquid Neat phase Middle phase, normal Middle phase, reversed Optically isotropic, normal Optically isotropic, normal

Normal micelles Reversed micelles Lamellar, double layers Two-dimensional hexagonal Two-dimensional hexagonal Face centered cubic Primitive cubic

Lj D E F FCC PC

fraction line positions were measured using a Hilger and Watts measuring device with a precision of 0.05 mm. The values of the scattering angle 0 were calculated from the diffraction patterns and the d-spacings followed from the Bragg equation, i.e., nX = 2d sin 0 [1]

where )~ is the wavelength of the incident Xray beam and n is an integer. RESULTS

hexanol axis is determined by the solubility of hexanol in water ( 20 wt% and when the amount of hexanol exceeds its solubilization limit (>2 wt%) various liquid crystalline phases appear in equilibrium with L~, L2, or (L~ + L2) phases. Various textures of liquid crystalline phases were identified, namely, mosaic (neat phase D) a n d middle phase (E).

Neat Phase (D)A large area of D phase is found in the centre of the liquid crystalline region at all temperatures studied (Figs. 1a-c). This region extends over a wide range of water concentrations (15-80 wt%) but exists only over a relatively limited range of n-hexanol concentrations (6-27 wt%). In the region of high surfactant concentrations ( ~ 70 wt%) a minimum of approximately 15 wt% water and 10