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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-Hexanol

R. 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, England

Received 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 50°C. 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 at- tracted considerable attention in view of their use in many industrial products and their po- tential application in enhanced tertiary oil re- covery. Various investigations have been car- ded 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

Journal of Cofloid and Interface Science. Vol. 100, No. 2, August 1984

(6, 7) are now available for the description of a number of microemulsion systems and the methods used for their study. The latter in- dude phase diagrams, scattering techniques, conductivity and dielectric studies, viscosity measurements, NMR investigations, and in- terfacial tension measurements. Moreover a number of theories have been put forward in recent years to explain the origin of ther- modynamic 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-dif- fusion studies by Lindman et al. (8) have

0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All fights of reproduction in any form reserved.

312 BAKER ET AL.

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 hy- drophobic 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 hy- drolysis 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 50°C. 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 co- surfactant (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

EXPERIMENTAL

Materials

Water. All water was twice distilled from an all glass apparatus and its surface tension was 72 ___ 0.2 mN m -~ at 25 + 0.1°C.

Xylene. Analar grade material (ex Koch Light) was used as received. The NMR 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 <9 9 10 11 12 13 14

Wt% 1 16 21 20 18 17 7

This distribution of alkyl chain length gives the surfactant a Krafft point well below room temperature and therefore the surfactant aqueous solution remained clear in all investigations.

The commercial material which was supplied as a 30% aqueous solution, contained a maximum of 0.8% unsulfonated material and 0.5,0.8% sodium sulfate. The surfactant was extracted from the aqueous solution using toluene. The water was distilled off from the mixture together with some of the toluene as an azeotropic mixture. The surfactant was then isolated from the toluene solution by vacuum distillation.

The sodium sulfate was removed by dis- solving the surfactant in chloroform, followed by removal of any solubilized water by addition of oven dried magnesium sulfate. Both sodium sulfate and magnesium sulfate are fil- tered offand the chloroform was removed under vacuum at 50°C. The surfactant was purified by repeated recrystallization from pro- pan-2-ol. The purified surfactant was dried and stored in a vacuum oven. The material

PHASE DIAGRAMS OF A TERNARY AND QUATERNARY SYSTEM 313

was a white crystalline solid and was char- acterized by its NMR spectrum.

Preparation of Samples for Phase Diagrams

Stock solutions of appropriate concentrations of surfactant in both water n-hexanol and xylene were made up to maximum concentrations of 30, 60, and 50 wt%, respectively. These solutions were then used to make samples containing a wide concentration range of the ingredients. For the ternary system, when samples contained more than 40% surfactant, solid surfactant was used in addition to the appropriate stock solution. Above 70 wt% surfactant, solid surfactant alone was used. For the quaternary system, a range of concentrations of water, n-hexanol, and xylene was used, while keeping the concentration of NaDBS fixed.

All samples were prepared by weight in glass screw capped vials and in all cases, the initial mixing was carried out using a laboratory "Whirlimixer." However, samples with a high viscosity and those containing solid surfactant were further mixed using mechanical rollers to ensure thorough mixing. When water was the major component (for the quaternary system), the order of addition was aqueous surfactant solution, water, xylene, and n-hexanol. On the other hand, when xylene was the major component the order of addition was surfactant in xylene, xylene, and n-hexanol. In the case of n-hexanol being the major component, the order of addition was surfactant solution in hexanol, n-hexanol, xylene, and water. Fi- nally all samples were sealed and placed in a temperature cabinet for 1 month at 25°C to reach equilibrium. The samples were then studied as below, resealed, mixed by "Whir- limixer," and transferred to a temperature- controlled cabinet at 37 + 1 °C. After a further months storage, the samples were studied and then the procedure repeated at 50°C.

Assessment of Samples

The different phases were assessed visually and by optical and polarizing microscopy.

Each phase, unless transparent, was observed under phase contrast and between crossed nicols using either a Zeiss or a Leitz microscope fitted with a 10× eyepiece, 16X objective lens, and a Reichart Kam-ES camera. The appearance of each phase under the microscope was noted and in some cases a photograph was taken.

To determine whether the phases were oil or water continuous, a dye test was used. A drop of the phase under investigation was placed on a clean microscope slide and a small amount of two dyes, Waxoline red (n-hexanol soluble) and Lissamine Scarlet (water soluble), was placed at two separate edges of the droplet. Continuity of the fluid phases was readily determined by observing which dye dissolved in the droplet. However, in the case of highly viscous phases, mixing had to be aided by a small spatula, thus making assessment difficult. For investigations at 37 and 50°C, a hot stage was fitted to the microscope.

Construction of the Phase Diagrams

The various phase regions were represented on a two-dimensional triangular phase diagram. These were constructed by making a large number (>200) of samples, to extend over a wide concentration range of each in- gredient. 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 Investigations

For 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 Linde- mann tubes and exposures were normally of approximately 16 hr. Photographs were taken using a Warhus low angle camera with a spec- imen to film distance of 295 mm. The dif-

Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984

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PHASE DIAGRAMS OF A TERNARY AND QUATERNARY SYSTEM

TABLE I

Designation and Nomenclature of the Observed Phases

315

Designation Basic structure Alphabetical notation

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

Normal micelles Lj Reversed micelles Lamellar, double layers D Two-dimensional hexagonal E Two-dimensional hexagonal F Face centered cubic FCC Primitive cubic 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 X- ray beam and n is an integer.

RESULTS

Ternary System

The phase diagrams for the ternary system studied are shown in Figs. l a-c at the three temperatures investigated, namely 25, 37, and 50°C. These diagrams show the typical phases previously described by many authors (11- 14). It should be mentioned, however, that dotted boundaries shown on the phase diagrams are more uncertain than those shown with a continuous line. A qualitative description of the phases observed and the nomenclature used (11) is given in Table I. Below a brief description of the various regions is given.

Isotropic Micellar Solution, L1 Phase

This limited region is found at the water- rich part of the phase diagram; its boundary along the water/NaDBS axis is determined by the solubility of NaDBS in water ( ~ 3 2 wt% at 25°C) and its boundary along the water-

hexanol axis is determined by the solubility of hexanol in water (<2 wt%) (15). With increase of temperature, the extent of the L~ region along the water-NaDBS axis increases as a result of the increase of solubility of NaDBS with increase of temperature ( ~ 40% at 37°C and ~43% at 50°C), whereas the extent along the water-hexanol axis does not change significantly since the amount of hexanol that is solubilized in the L~ region does not increase with increase of temperature.

Isotropic Inverse Micellar Solution, Le Phase

This region extends over a wide area of the phase diagram; the boundaries for the binary compositions are determined by the solubility of water in hexanol ( ~ 8 wt% at 25°C) (15) and NaDBS in n-hexanol ( ~ 6 0 wt% at 25°C). In the L2 region a wide range of water concentrations (8 to 57 wt%) can be solubilized depending on the surfactant concentration. The amount of solubilization changes slightly with increase in temperature as illustrated in Fig. ld; the most significant change occurs above 50 wt% NaDBS where the extent of the L2 region increases with increase of temperature (Fig. ld).

L1 + L2 Region

This region appears at low NaDBS concentrations (<2 wt%) when the amount of

FIG. 1. Phase diagrams of the ternary system water-sodium alkyi benzene sulfonate (NaDBS)-hexanol at three different temperatures.


316 BAKER ET AL.

hexanol in the L region exceeds its solubilization limit (>2%). It should be mentioned that there is some uncertainty in the hexanol- rich end of this region since the part with a dotted boundary cannot have a tie line between L2 and L1 without it crossing the L2 + LC region. This is clearly forbidden by the phase rule. Therefore such boundaries drawn by the dotted line are only to be considered approximate.

However, since both NaDBS and hexanol are hygroscopic, traces of water must have been present and, these are used in the formation of such liquid crystalline phases. As the temperature is increased, the extent of the F-phase decreases and the amount of water that can be solubilized is reduced to approximately 15 wt% at 37 and 50°C. Similar observations were made by Ekwall and Mandell (19) for the ternary system water-sodium caprylate-decanol.

L1 + L2, (L1 + L2) + Liquid Crystalline (LC) Phases

At an NaDBS concentration > 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) and middle phase (E).

Middle Phase, Normal (E)

This phase extends along the NaDBS-water axis between approximately 55 and 75 wt% NaDBS. Throughout its extent it is capable of solubilizing a maximum of approximately 4 wt% n-hexanol and this determines its upper boundary. This middle phase shows angular or fan-like textures of the E and F phases or a mixture of the two.

Middle Phase, Reversed (F)

This phase extends from the binary NaDBS-hexanol axis and is capable of solubilizing a maximum of 20 wt% water. Throughout its existence, the F phase is a slightly turbid, viscous gel and it shows the very striking fan-like angular textures characteristic of this phase. Furthermore, mixtures of fan-like and angular textures are observed as well as fan like units alone and mixed with a fan-like texture. Finally, in some cases a Very fine angular texture is also observed. It should be mentioned that the F phase appears to extend to the axis where no water is present.


Neat Phase (D)

A large area of D phase is found in the centre of the liquid crystalline region at all temperatures studied (Figs. 1 a-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 wt% n-hexanol is required to form the D phase. The location and extent of the D phase in this system is very similar to that observed in other systems (11, 14, 17). Throughout its existence, the D phase varies from a turbid to almost transparent, mucous-like gel and it displays some of the textures characteristic of this phase (11, 18-20).

D + L2, D + F , D + E , D + PC (Primitive Cubic)

The D phase exists in equilibrium with several other phases as shown in the phase diagrams (Figs. la-c); the extent and position of these two phase regions depends on both the NaDBS and water concentration. For example, between 5 and 10 wt% NaDBS, a woven texture characteristic of the D phase is observed (4, 19). At higher NaDBS concentrations (20-25 wt%) striated textures are observed. The X-ray data on samples from this part of the phase diagram (see below) suggest that a face centered cubic (FCC) and lamellar phase are in equilibrium. The appearance of a viscous transparent gel in some of the samples confirms the presence of the FCC phase.

PHASE DIAGRAMS OF A T E R NAR Y AND Q U A T E R N A R Y SYSTEM

TABLE II

Observed and Theoretical Ratios of d-spacings for Three Samples of Liquid Crystalline Phases

3 1 7

Composition (wt%) Ratio of d-spacings Sample No. NaDBS hexanol Water (expt) Structure Theoretical ratio

1 5 5 90 1:0.48:0.33 Lamellar 1:1/2:1/3:1/4 2 5 12,5 82.5 1:0.607:0.523 Face centered cubic 1:0.866:0.612:0.522 3 30 10 60 1:0.583:0.503 Hexagonal 1:0.577:0.500:0.378

X-Ray Results

L o w a n g l e X - r a y d i f f r a c t i o n o f f e r s t h e b e s t

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f u l l a n a l y s i s o f a l l t h e r e g i o n s w o u l d r e q u i r e

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o b t a i n e d i n t h e p r e s e n t i n v e s t i g a t i o n a r e b y

n o m e a n s e x h a u s t i v e a n d a r e o n l y u s e d t o

TABLE IIl

X-Ray Diffraction Resul ts - -Line Spacings are in ,~

(a) Face Centered Cubic and Lamellar

Sample composition (wt%) Face centered cubic

Sample No. NaDBS hexanol Water ( I 11 ) (200) (220) 3111

Lamellar

t/ a (A) n = 1 n = 2 n = 3 (A)

1 5 5 90 52.8 - - 32.5 - - 91 81.2 39.1 27.1 80.5 2 5 12.5 82.5 51.8 - - 31.4 27.1 90 - - 37.5 24.6 74.3 3 15 10 75 51.8 - - 32.8 27.1 90 78.5 40.8 27.1 80.5 4 20 15 65 - - - - 32.0 27.1 90 75.4 39.1 - - 77.0

(b) Hexagonal and Lamellar

Sample composition (wt%) Hexagonal Lamellar

a a

Sample No. NaDBS hexanol Water (1010) (1120) (2020) (2240) (,~,) n = I n ~ 2 n = 3 (A)

5 30 10 60 50.3 29.4 25.3 - - 58.4 - - 3 9 . 1 - - 78 6 40 20 40 . . . . . . . 27.1 81 7 50 10 40 . . . . . . . 27.1 81 8 a 50 30 20 - - 23.0 - - - - 45.9 . . . . 9 60 0 40 - - 29.3 - - 14.7 58.6 . . . .

10 60 20 20 . . . . . . . 26.7 80 11 70 0 30 - - 29.3 - - - - 58.6 . . . .

(c) Primitive Cubic and Hexagonal

Hexagonal Sample composition (wt%) primitive cubic

a a

Sample No. NaDBS hexanol Water (110) (111) (220) (222) (A) (1010) (I 120) (1240) (A)

12 70 10 20 33.0 26.7 16.3 13.2 46 - - 29.8 14.5 56 13 80 10 10 32.5 26.0 - - - - 46 . . . .

a Reversed hexagonal (F).


318 BAKER ET AL.

confirm some of the observations made by microscopy.

The structure of the samples was determined by comparing the ratio of the calculated d spacings to those expected from theory. As an illustration, some examples obtained with three samples are given in Table II. The results obtained with the various other samples are given in Table III, along with the unit cell dimensions which were calculated for each structure from the following equation,

a dhkl -- [2]

Vh 2 + k 2 + l 2

where h, k, l are the Miller indices and a gives the distance between the centers of adjacent units (11, 21).

The points corresponding to the compositions in Table III, along with the identified structures are shown in a temary phase diagram in Fig. 2. In some cases the X-ray results confirm the observations made by microscopy but there are a number of discrepancies. For example, at a sample composition of 80 wt% NaDBS, 20 wt% water 10 wt% n-hexanol (sample 13), the X-ray data suggest that a primitive cubic structure exists. If this was so,

the sample should be transparent, highly viscous, and isotropic. In fact the sample was turbid and showed strong birefringence between crossed nicols. The reason for this dis- crepancy is unclear, but it may be that other phases are dispersed in this primitive cubic structure.

The unit cell dimensions a given in Table III give the distance between the centers of adjacent units, i.e., the sum of the radii of two surfactant molecules plus any material (water or n-hexanol) contained inside or between the units. The results in Table III show an increase in the unit cell dimensions in the order primitive cubic ~ reversed hexagonal < lamellar < face centred cubic. Since the units in the primitive cubic and reversed hexagonal are essentially close-packed the unit cell dimension in this case should correspond to approximately twice the length of a surfactant molecule. The unit cell dimension obtained in these cases was "--46 A corresponding to a surfactant chain length of ~ 2 3 A. This value is in close agreement with the value of ~21 A calculated from the bond lengths.

From the unit cell dimensions of the other liquid crystalline phases, the thickness of the

Hexanol

X-Ray Data. / ~

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water layer was calculated using a value of 21 A for the surfactant chain length. Thus, the thickness of the water layer between cylinders in the hexagonal (E) phase is 16.6 A, between the layers in the lamellar (D) phase is 38/~, and between spheres in the FCC phase is 8 A. These values are in close agreement with those reported by previous authors (1 l, 21, 22). Since the unit cell dimension, and hence the thickness of the water layer, does not increase with an increase in the water content (e.g., samples l, 7, and 10 in Table II) it seems likely, from the limited amount of data ob-

tained by X-ray diffraction, that the present system forms a non-expanding lamellar (D) phase (11, 23). This result is rather surprising for an ionic surfactant/alcohol mixture, although this may be caused by the aromatic head group.

Extent of Water Solubilization in the L2 Region

Since one of the main objectives of the present study is to identify and characterize the thermodynamically stable w/o regions formed by the system under investigation, the

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Journal of Colloid and Interface Science. Vol. 100, No. 2, August 1984

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PHASE DIAGRAMS OF A TERNARY AND Q U A T E R N A R Y SYSTEM 321

L2 region is considered here in more detail, by considering the amount of water solubilized to the total surfactant (i.e., NaDBS + n-hexanol) concentration and to the ratio of NaDBS to n-hexanol. Figure 3 shows the number of moles of water per mole of total surfactant solubilized in the L 2 region at 25 and 50°C, as a function of the total surfactant concentration (the data for 37°C are not shown to avoid overcrowding of the graph, since the values lie between those at 25 and 50°C). The results show that solubilization does not change appreciably with temperature and the maximum water solubilization (approximately 11 mole of water per mole of total surfactant) occurs at a critical molar concentration of NaDBS + n-hexanol of ~0.3. Above the latter value, the amount of water solubilized decreases rapidly.

When the amount of water solubilized per mole of total surfactant is plotted versus mole ratio of n-hexanol to NaDBS (Fig. 4), a sharp maximum is found at a ratio of n-hexanol to NaDBS of 4.5:1. This shows that the molar ratio of the two surfactants is critical for maximum solubilization of water. Furthermore, the values of this optimum ratio do not change significantly with temperature.

The results obtained in this study also show that the maximum amount of water solubilized exceeds that required for the hydration of NaDBS and n-hexanol ( ~ 5 and 3 molecules, respectively) (11, 12) indicating that at least in this part of the L2 region water ag- gregates must be present. The most likely structure is that of an inverse micelle made up of n-hexanol and NaDBS molecules with a molar ratio of 4.5:1 and swollen with water (11, 12).

Quaternary System

The phase diagrams for the four component systems, containing fixed concentrations of 5, 10, 15, and 20 wt% NaDBS, are shown in

Figs. 5-8(a-c) at three different temperatures, namely 25, 37, and 50°C. The influence of temperature on the extent of the transparent region (i.e., L2 + M) is shown in Figs. 5d to 8d for the four systems investigated. Each phase diagram is plotted in a two-dimensional triangle in the same manner as those described by Friberg and co-workers (24, 25). Since the NaDBS concentration is fixed for each set of phase diagrams, the comers of the triangles represent the other three components, i.e., n- hexanol, water, and xylene plus x wt% NaDBS, where x is the NaDBS concentration for that particular set of diagrams. In other words, the points in the phase diagrams are obtained by multiplying each particular composition by 100/(100 - x). This was done in order to en- large the variable components to fill the whole of the equilateral triangles. However, in the discussion of the phase diagrams all concentrations of ingredients mentioned refer to ac- tual composition, i.e., corrected values.

At each concentration ofNaDBS, a number of different phases were identified and labeled using the same nomenclature as with the ternary phase diagram (Table I). It should be mentioned that the phase boundaries are only approximate; accuracy of the location varies between 1 and 5%. Moreover, in the present study, the regions containing liquid crystalline phases were not fully investigated and the phases in the regions were grouped together under the general headings of gel or E/LC (emulsion containing liquid crystalline phase). Below a brief description of the various regions is given.

Isotropic L1 Phase

This extends over a very limited area in the high water content part of the phase diagrams and is, in general, capable of solubilizing only a small amount of xylene or n-hexanol. How- ever, the amount of xylene solubilized increases with increase in NaDBS concentration

F1G. 5. Phase diagrams for the quaternary system at three different temperatures. Total concentration of NaDBS is 5 wt%.

Journal of ColloM and Interface Science, Vol. 100, No. 2, August 1984

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C

L1

G~

10

,

Wet

er-P

lOw

t °/o

NaD

B$

O

100

He

xan

ol~

lOw

t~o

Na

OB

S

Xyl

ene~

_lO

wtO

lo N

aDB

S

XY

len

e~

lOw

t °/

o N

aDB

S

O

100

O

100

C

E L2

~-E

ILC

d

50

°C

--

2

5°C

/ ~

\

....

. ~

o~

~.S

--,~

\

__. =oo

/

' ,\

1~.~

\

L1-P

E

L1

10

~,

0 10

00

. 0

0 "

"

100

mm

~Bm

- W

ater

~-l

owt

°1o

NaD

BS

H

exa

n o

l~-l

Ow

t °1o

NaD

BS

W

ater

-]-l

Ow

t°/o

NaD

B S

H

exa

no

H 1

Ow

t%

NaD

BS

t~

~0

> t-n

rn

>

PHASE DIAGRAMS OF A TERNARY AND Q U A T E R N A R Y SYSTEM 323

and temperature, reaching a maximum of 15 wt% at 15 wt% NaDBS and 50°C. On further increase of the NaDBS concentration to 20 wt% the solubilization of xylene is reduced to approximately 5 wt%. The solubilization of n-hexanol does not show an increase with increase of NaDBS concentration or temperature and remains constant at approximately 1-2 wt%.

Isotropic Le Phase and Microemulsion (M) Region

The L2 phase extends over a much wider area than the L~ phase. The L2 phase extends over the whole range of xylene concentrations but the amount of water solubilized in the inverse micelles changes with both NaDBS concentration and temperature. For example, at 5 wt% NaDBS (Fig. 5) the maximum amount of solubilized water at 25°C is approximately 10 wt% throughout most of the L2 region. However, within a narrow range of n-hexanol concentration (5-20 wt%) an extension of the L2 region occurs and the amount of water incorporated increases to a maximum of approximately 30 wt%. This extension of the L2 region remains transparent, but is char- acterized by the appearance of a blue tinge and is usually referred to as the microemulsion (M) region (24, 26). Although the transition from the L2 to the M region is a gradual one, a dotted line is placed on the phase diagram (Fig. 5) to indicate the approximate transition point. A similar behavior is observed at 10 and 15 wt% NaDBS but the extent of both the L 2 and M regions increases with an increase in the NaDBS concentration (Figs. 6 and 7). The maximum amount of water that can be incorporated in the M region reaches a value of 55 wt% at both NaDBS concentrations. Moreover, the intensity of the blue tinge in the M region decreases with an increase in the NaDBS concentration, until at 15 wt%

NaDBS, it becomes very difficult to detect. This vague distinction between the Lz and M regions disappears completely at 20 wt% NaDBS which shows (Fig. 8) a large region of colorless, isotropic solution.

The extent of the L2 region at 5 and 10 wt% remains essentially the same as the temperature is increased (Figs. 5 and 6), but the extent of the microemulsion region is reduced. The range of n-hexanol concentrations over which the microemulsion region remains stable is also reduced with increase of temperature (Figs. 5d and 6d). In contrast, at 15 wt% NaDBS, both the extent of the L2 and M regions increase slightly with an increase of temperature (Fig. 7d). Finally, at 20 wt% NaDBS there does not seem to be any significant change in the extent of the transparent region with an increase of temperature (Fig. 8d).

LI + Le Region

This region appears in the 5 and 10 wt% NaDBS phase diagrams (Figs. 5 and 6), at all temperatures, when the water content increases beyond the solubilization limit in the L2 region. In both cases, the extent of the L~ + Le region increases with increase of temperature. In contrast, this region only appears at 37 and 50°C in the 15 wt% NaDBS system and no such region exists at 20 wt% NaDBS.

L1 + E (Emulsion) and L1 + E/LC (Liquid Crystalline) Regions

When the solubilization limit of xylene is exceeded in the LI region, a two phase region appears which, at 5 and 10 wt% NaDBS (Figs. 5 and 6) consists of L1 and oil in water emulsion (L1 + E). Except at high water content, the emulsions in this two phase region appear viscous and are highly stable. Microscopic investigation showed a decrease of droplet size as the xylene concentration is increased. This

FIG. 6. Phase diagrams for the quaternary system at three different temperatures. Total concentration o f NaDBS is 10 wt%.


z p >

X y

]en

e +

15w

t ~ N

aDBS

~

/~

O

1OO

a ~

~ 2

5°C

////

',,\

E

ILC

O

1OO

Wat

er+1

5wt ~

NaD

BS

im~

Hex

anol

+15w

t~ N

aDBS

X yl

ene+

15 w

t ~0 N

aDBS

0

100

c

LI+

E~L

C

Gel

10

0 0

i~

lO0

Wat

e~+1

5wt ~

NaD

BS

Hex

anol

+1

5wi ~

NaD

BS

Xyl

ene+

15w

t ~ N

aDBS

0

100

b ~

37O

c

EIL

C

LI+E

ILC

L 1

10

0 10

0 W

ater

+15w

t~N

aDBS

i~

H

exan

ot+1

5wt~

NaD

BS

X yle

ne+1

5wt~

oNaD

BS

0 10

0

d ~

2 5°

C

/ /

~ ..

...

37°C

,/

\,___

,0%

100

O

t~

> ~n

po

m

>


is difficult to explain since all samples were made by mixing the two phases with simple agitation.

Increasing the n-hexanol concentration beyond the solubilization limit in the L1 r eg ion

also results in the formation of two phase regions. For example at 5 wt% NaDBS the L1 + E region is formed whereas at 10 wt% NaDBS the Lx + E/LC region is formed (Figs. 5 and 6). From the observed textures in the latter region, the liquid crystalline phase appears to be a neat of D phase. The L1 + E/ LC region also appears at 15 and 20 wt% NaDBS when either the n-hexanol or xylene is increased beyond its solubilization limit. These results demonstrate that a minimum amount of surfactant is required to form a liquid crystalline phase in an emulsion, as previously found by Friberg and Mandell (27).

L 2 q- E (Inverse Emulsion), L 2 q- E/LC, Le + Gel, L2 + E + Gd Regions

By gradually increasing the xylene concentration, while maintaining that of n-hexanol at a low level, inversion of the system into a w/o emulsion takes place above a critical concentration of xylene which depends on surfactant concentration and temperature.

As the xylene concentration is further increased at 5 wt% NaDBS the inverse emulsion is found in equilibrium with the L2 phase. At 10, 15, and 20 wt% NaDBS the inverse emulsion remains in equilibrium with the L2 phase at higher xylene concentrations but contains a liquid crystalline phase.

The 1-2 phase also appears in equilibrium with gel and emulsion/LC phases (see Figs. 5-8). The L2-gel regions appear when the water content is increased beyond the L~ + L2 phase. Throughout most of these regions, the samples are viscous and either very weakly or non- birefringent. In contrast the E/LC phase in the [.2 + E/LC region shows stronger bire-

fringence and the observed textures at 10 wt% NaDBS indicate that these consist of predom- inantly D phase.

M + E, M + E/LC

Binary phases of microemulsion in equilibrium with either E or E/LC appear in the vicinity of the M region. In all cases, the amount of emulsion in the M + E region k, quite small and this region changes to M + Lt with an increase of temperature. The textures observed in the M + E/LC region (mosaic and oily streaks) indicate that in all cases, this region consists of the D phase. The latter also constitutes most of the gel region in equilibrium with the M phase at 15% NaDBS (Fig. 7).

Gel Regions

Various other gel regions (apart from those described above) can be distinguished in the phase diagrams. For example, in the 5 wt% NaDBS there is a large E + gel region (Fig. 5) whereas at 10 wt% NaDBS this region, E/ LC + gel (Fig. 6), consists of a mixture of emulsions and gels of high viscosity; the emulsions are located mainly at low water content and the gels in the high water content part of the region. In the 15 and 20 wt% NaDBS systems (Figs. 7 and 8) the gel region appears transparent at a high water content and more turbid in the lower water content part of the region. At each concentration of NaDBS, the textures are very similar in these gel regions as proved using phase contrast and polarizing microscopy.

Comparison of the Transparent Regions ( M q- L2) in the Four Systems Studied

The extent of the oil continuous transparent region, i.e., M + L2 for each concentration of NaDBS, can be represented in a tetrahedron

FIG. 7. Phase diagrams for the quaternary system at three different temperatures. Total concentration of NaDBS is 15 wt%.


z p >

Xyl

ene

+ 20

wt%

N

aDB

S

O

100

a 25

°c

LI+

EIL

C

O

0 m

~

100

Wat

er +

20w

t%

NaD

BS

H

exa

nol+

2Ow

t %

NaD

BS

LI~

EIL

C

Gel

R

egio

n

O

Wa

ter÷

2O

wt

°lo

NaD

BS

Xyl

ene~

- 20w

t°lo

NaD

BS

O

1O

O

Gel

O

100

Hex

anol

+2O

wt°

lo N

aDBS

Xyl

ene

• 20

wt °

IoN

aOBS

0

100

b ~

37°C

LI~

E/L

C

L 1

10

0 O

"

" m

l~

" "

100

Wat

er +

20w

t%

N

aDB

S

He

xan

o ~

+ 20

wt°

lo N

BDBS

10

0~

O

W

ar e

r-l-2

Ow

t %

Na D

BS

Xyl

ene~

20 w

t°lo

NaD

BS

0 lO

0

__

25°C

L2

~x

. .M

~ "

" '°°

° H

e x an

o I -

t- 20

w tO

Io N

a DB

S

L~

t~

Cr~

>

PHASE DIAGRAMS OF A T E R N A R Y AND Q U A T E R N A R Y SYSTEM 327

lOOwt ~ NaDBS

Xylene

Water ,q~ Hexanol

FIG. 9. Transparent regions at 25°C for various NaDBS concentrations.

as illustrated in Fig. 9 for the results obtained at 25°C. A similar representation can be made for the other temperatures. The front of the tetrahedron represents the ternary water-n- hexanol-xylene systems and the corners of the triangle represent the pure components. The fourth corner of the tetrahedron represents pure NaDBS. An expanded scale is used for the NaDBS composition to allow a clear representation of the transparent regions. This diagram clearly shows the increase of the extent of transparent regions as the surfactant concentration is increased.

In Fig. 10 a distorted tetrahedron is used to show the effect of the xylene concentration on the extent of the transparent region. The right-hand side triangle represents the ternary water-NaDBS-n-hexanol system and the corners of the triangle the pure components. In this case the fourth corner of the tetrahedron represents pure xylene. The data for this diagram were interpolated from the phase diagrams shown in Figs. 5-8 by noting the composition at the extremities of the transparent region for each NaDBS concentration. The same procedure described for Figs. 5-8 was used for expanding the composition of the variable components to fill the whole triangle at each xylene composition.

Amount of Water Solubilized in the L2 Region

This was calculated in the manner described before for the three component system. Figures 11 and 12 show the number of moles of water solubilized per mole of total surfactant (n- hexanol + NaDBS) plotted versus the total surfactant concentration at 25 and 50°C for various xylene concentrations. Figure 13 shows the corresponding plots as a function of the molar ratio of n-hexanol to NaDBS. For comparison the values obtained in absence of xylene (i.e., for the ternary system) are shown on the same plots. In both cases, the area under the graphs represent the transparent regions. In all cases a maximum is found at a given total surfactant concentration and molar ratio. However, the maximum amount of water incorporated per mole of total surfactant (22 mole of water per mole surfactant) at 25 and 50 wt% xylene is twice as high as compared to the value (11 mole water per mole surfactant) in the absence of xylene, i.e., for the ternary water-NaDBS-n-hexanol system. Further increase in the xylene content results in a reduction of the maximum amount of water incorporated (17 mole of water per mole of surfactant). Similar results were ob-

FIG. 8. Phase diagrams for the quaternary system at three different temperatures. Total concentration of NaDBS is 20 wt%.


328 BAKER ET AL.

Xylene

1 0 ( ~ . ~ 75

Water "~ NaDBS

FIG. 10. Transparent regions at 25°C for various xylene concentrations.

served by Cooke and Schulman (28) and this led these workers to draw a clear distinction between inverse micelles and microemulsions. The total surfactant concentration at which the maximum solubilizafion of water occurs is shifted to lower values as the xylene content is increased. With an increase of the temperature to 50°C (Fig. 12) a small reduction in the maximum amount of water incorporated occurs at 25°C and 75 wt% xylene, although the value at 50 wt% xylene remains (essentially) constant.

Figure 13 shows that the maximum solubilization of water occurs at an opt imum ratio of n-hexanol to NaDBS which depends on the xylene content. The ratios observed were 4.0, 3.0, and 2.0:1 at 0, 25, and 50 wt% xylene,

respectively. A similar dependence on the concentration of oil phase was observed by Sj6blom and Friberg (25). When the temperature is increased to 50°C, the ratios change slightly and the corresponding values are approximately 5.0, 2.5, and 2.0:1.

DISCUSSION

A comparison of the phases described in the quaternary system can be made with those for the three component system. For example, the same type of liquid crystalline phases, two phase regions and a large extent of the I-2 region are observed in both the three and four component systems. This shows that the four component system is an extension of the three component one. This can be clearly seen if

2c

~= ~- Owt%xyte.e . - 2 5

. - s o

. - 75 ,,

I o . 1'.o

Mo les NaDBS÷ Hexanol

FIG. 11. Solubilization of water as a function of total surfactant (NaDBS + he×ano|) concentration at various xylene concentrations (25°C).


PHASE DIAGRAMS OF A T E R NAR Y AND Q U A T E R N A R Y SYSTEM 329

20

"~ 0 - 0 wt ~ Xylene e=

• ~25 +

• - 5 0

z • - 7 5

. . J

~ j

0 ' 0"5 1~0

Moles N a D B S + Hexanol

FIG. 12. Solubilization of water as a function of total surfactant (NaDBS + hexanol) concentration at various xylene concentrations (50°C).

one follows the sequence of the changes that occur with an increase in n-hexanol concentration at a fixed level of NaDBS. For example, at 10 wt% NaDBS (Fig. 6) the sequence of the phase transition observed at vanishingly small xylene concentrations is Lx ---* Lt + E/LC ---* E / L C + g e l ~ L 2 + g e l ~ L 2 + L l ~ L 2 a s the concentration of n-hexanol is increased. The corresponding sequence with a zero concentration of xylene, i.e., in the ternary system (Fig. 1) is L1 --~ L1 + LC ---, LC ~ L2

+ LC ~ L2 + LI ~ L2 with increasing n- hexanol concentration.

The phase diagrams can be used to describe the phase changes which may appear during the preparation of microemulsions by the titration technique. For example, Schul- man and co-workers (26, 28-30) observed that the titration of an o/w emulsion with an alcohol resulted in the formation of a highly viscous phase, later thought to be lamellar liquid crystalline phase (26), before finally con-

2 0 -

lO

n0 z

:E 20~

~ lO

Owt ~ Xylene

10 20 30

10 20 3 0

25wt ~ Xylene

l b ' 2b

' ~ b ' 2'0 Molar Ratio Hexanol :NaDBS

5Owt ~ Xylene

: O ' 1£) ' 2 '0 ' 3~3

2 5 ° C

10 2 0 30

FIG. 13. Solubilization of water as a function of molar ratio of hexane NaDBS at various xylene concentrations (25 and 50°C),


330 BAKER ET AL.

verting to a w/o microemulsion. The phase diagrams in Figs. 5-8 dearly show that small additions of n-hexanol produce a gel or highly viscous emulsion which, above 5 wt% NaDBS is liquid crystalline in nature. At 10, 15, and 20 wt% NaDBS, this liquid crystalline phase appears to be lamellar throughout most of its extent, but above 40-50% water, the observed textures suggest that the middle (E) phase is present. This seems rather surprising since, in the three component system, a m i n i m u m of about 25-30 wt% NaDBS is required to form the E phase. However, the observed textures are rather difficult to identify and were not confirmed by any other technique. Further addition of alcohol results in the formation of a w/o microemulsion, providing that the water concentration is not too high.

The phase diagrams given in Fig. 5 also show why a sharp end point is observed when using the titration technique, since the amount of n-hexanol required to form a microemulsion is very critical.

Figure I0 clearly illustrates that the microemulsion region is an extension of the L2 region. This result is in agreement with observations made by Friberg and co-workers (24, 25, 31) for other systems. It was the type of phase behavior shown in Fig. 10 that led Friberg and co-workers (24, 26, 32) to consider microemulsions to be swollen inverse micellar systems. The results shown in Figs. 11-13 clearly show the role played by xylene in the solubilization of water in the microemulsion region. First, a lower n-hexanol:NaDBS ratio is required for opt imum solubilization of water in the presence of xylene, i.e., less n-hexanol is required. Moreover, the systems containing 25 and 50 wt% xylene are capable of incor- porating twice as many moles of water per mole of total surfactant when compared to that obtained without xylene (i.e., ternary system). These results indicate that, although there is a gradual transition from the L2 to the microemulsion region, the structures may be different, especially at the extremes of the two regions.

It should be mentioned that using the present surfactant system, water in xylene microemulsions with a high water content, reaching ~ 5 5%, could be prepared using rea- sonably low NaDBS levels, namely, 10 and 15 wt%. Such high water volume fractions cannot be reached with the potassium oleate system, in which, max imum of only ~ 3 0 wt% water can be incorporated (25).

An important point which emerges from the present investigation is the effect of temperature. It is generally accepted that microemulsions ionic surfactants are not very sen- sitive to temperature variation. However, the present results clearly show that this is not the case, at least when relatively low concentrations of NaDBS (< 15 wt%) are used (see Figs. 5d and 6d) when the extent of the microemulsion region is significantly reduced by increase of temperature.

ACKNOWLEDGMENTS

The authors are indebted to Mr. C. A. Hart for his help with the microscopic investigations. They are also grateful for the ICI management for their support for Dr. R. C. Baker for carrying out this work at Jealott's Hill for his thesis.

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