The influence of alcohols on microemulsion composition

The Influence of Alcohols on Microemulsion Composition MARC BAVIERE, 1 ROBERT SCHECHTER, AND WILLIAM WADE The University of Texas, Austin, Texas 78712 Received July 28, 1980; accepted October 6, 1980 The composition of a microemulsion in equilibrium with one or more excess phases depends on the relative amounts of the hydrocarbon and the aqueous phases as well as on many other compositional variables such as surfactant, alcohol, or electrolyte concentrations. It is shown here that the microemulsion composition changes in a predictable direction as the water-to-oil ratio is changed. This shift in concentration is primarily due to alcohol partitioning between the coexisting phases. The change in microemulsion composition resulting from a change in the water-to-oil ratio can sometimes be compensated by altering the electrolyte concentration. The change in electrolyte concentration has been termed a shift in optimum salinity and this shift is related to the alcohol partitioning and to the alcohol structure. A number of observations not yet fully understood are also reported. I. INTRODUCTION

A microemulsion is most generally defined as a transparent or translucent stable mixture of oil, water, electrolyte, and surfactant or combination of surfactants (1, 2). In many applications alcohols are used as an additional ingredient of the surfactant formulation and are often referred to as cosolvents (1, 3) or cosurfactants (4, 5) playing an important role in achieving the desired stability. As stable mixtures of oil and water, there are a number of practical applications for microemulsions including enhanced oil recovery (4, 7, 8). For example, Bowcott and Schulman (6) investigated microemulsions by titrating dispersions of hydrocarbon, water, and surfactant to transparency with alcohol. The mechanism by which alcohols act to stabilize microemulsions is not yet fully understood. The issue is complicated by the fact that several different phenomena all appear to be acting in parallel. It is, therefore, desirable to better understand the role i Permanent address: Institut Fran~ais du Petrole, Paris, France.

of alcohols in the formulation of stable microemulsions. The first step in accomplishing this goal is to develop a graphic representation of microemulsion phase behavior. If the aqueous electrolyte solution can be considered to act as a single component (always partition in such a way that the proportions of water and electrolyte are constant)--this assumption has been shown to be reasonable for the cases considered here (9, 10), then the composition of a mixture of electrolyte, surfactant, hydrocarbon, and cosurfactant (alcohol) can be represented as a point within the quaternary diagram shown in Fig. la. For systems of interest, a mixture having the overall composition defined by point O may lie in a one-, two-, or three-phase region. If it is in a three-phase region, the composition of each of the phases can be represented by the points 1 (lower phase), M (microemulsion), and u (upper phase). As shown by Fig. la, the microemulsion phase (or middle phase, as it is sometimes referred to) contains the bulk of the surfactant so that the points 1 and u (excess phases) almost coincide with the base of the quadralateral. This is generally

266

0021-9797/81/050266-14502.00/0 Copyright © 1981 by Academic Press, Inc. All fights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

267

I N F L U E N C E OF A L C O H O L S ON M I C R O E M U L S I O N S

the case over most of the range of alcohol concentrations for which three phases are observed, however, there may be compositions for which excess phases contain significant quantities of surfactant. In such cases there would be two microemulsion phases (by the definition used here) in equilibrium with an excess phase, either 1 or u. However, for the systems here studied, the bulk of the surfactant is found in but one of the phases when either two or three phases are present. If the overall mixture denoted by point O coexists in three phases, 1, M, and u, then the point O necessarily lies in the plane of the triangle defined by the points 1, M, and u. Furthermore, any other overall composition lying in the plane of this tie triangle will divide into the same three phases. Any other mixture in the three-phase region but not on this tie triangle will exhibit equilibrium phases having different compositions. Thus, for example, if a sequence of mixtures is prepared with each element containing a successively greater proportion of alcohol, a progression of microemulsion compositions will result. It is of interest to further investigate this progression of microemulsion compositions. To make visualization of the trends in microemulsion composition easier, the threedimensional phase diagram can be reduced to a two-dimensional figure following a procedure used by Salter (11). A projected point M' (see Fig. la) can be associated with point M by obtaining the intersection of the normal to the base of the quadralateral which passes through point M. Thus O', M', I', u' shown in Fig. lb are the projections of O, M, 1, u. Using this projection construction, Fig. 2 shows a series of mixtures, containing an increasing proportion of alcohol, but equal quantities of hydrocarbon and electrolyte. The corresponding projected microemulsion compositions are also shown. It is easily seen that by adding alcohol I (Fig. 2b), the microemulsion becomes richer in both alcohol and electrolyte. Thus a progressively smaller

SURFACTANT

ALCOHOL~

HYDROCARBON

ELECTROLYTE SOLUTION ALCOHOL

HYDROCARBON bELECTROLYTE FIG.1.(a)Quaternarydiagramshowingcomposition of microemulsion phase in equilibrium with an excess aqueous phase (1) and an excess oil phase (u); (b) composition points projected to the base of the quadralateral. Primes denote projected points.

amount of hydrocarbon is solubilized. The addition of a different alcohol can result in a different trend as shown in Fig. 2b. We shall find that other, more complex behavior is possible. The projected microemulsion compositions shown in Fig. 2a contain more hydrocarbon than electrolyte when the alcohol concentration is small. The ratio of electrolyte solution to hydrocarbon is seen to increase as the alcohol concentration increases. The open circles denote the overall composition and it is seen that the ratio of hydrocarbon to electrolyte is unity. If the proportions are expressed on a volume basis, then the electrolyte concentration for such partitioning has been called the optimal salinity by Healy, Reed et al. (1, 2). This state is an important one because many other properties of the microemulsion system have been shown to be correlated to the optimal salinity (2, 12, 13). In addition, Graciaa et al. (14) have recently related the concept of optimal Journal of Colloid and Interface Science,

Vol. 81, No. 1, May 1981

268

BAVIERE, SCHECHTER,AND WADE ALCOHOLI

ELECTROLYTE a

HYDROCARBON ALCOHOL"IT ~'~ • Projec'ledMiddle / ~ Compositic~

/

ELECTROLYTE

....A~ O Projected.Overall

HYDROCARBON

FIG. 2. Shown in each triangular diagram are a sequence of overall compositions (projected)each containing equal proportionsof electrolyteand hydrocarbon but increasing amounts of alcohol. Fig. 2a shows the locus of correspondingmiddle phase compositions which increase in electrolyteand decrease in oil as the alcohol concentration is increased. Figure 2b depicts same except that this alcohol gives increasing oil compositions.

salinity which deals with microemulsions to macroemulsion stability (HLB theory). Because of these factors it is interesting to determine the relationship between the alcohol structure and the locus of middle phase microemulsion compositions. Furthermore, this locus may depend to some extent on a number of other factors including the electrolyte concentration, the hydrocarbon molecular weight, and the hydrocarbon-toelectrolyte ratio in the overall mixture (called the water-oil ratio, WOR, for want of a better abbreviation). Partial answers to our questions have appeared in the literature. Jones and Dreher (15) have distinguished between water soluble alcohols such as isopropanol and heavier or oil soluble alcohols such as isopentanol and noted that the oil soluble ones can sometimes be used to obtain stable microemulsions if the electrolyte concentration is too Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

low, the oil molecular weight too high, or the surfactant molecular weight too small. Salter (16) has provided further insight by correlating alcohol partitioning between oil and electrolyte to observed shifts in optimal salinity. The alcohol partition coefficients were measured for fixed proportions of oil, water, and electrolyte. This study extends the work of Salter in that complete electrolyte-hydrocarbon-alcoholphase diagrams are constructed and shown to be useful in interpreting the results. Partition coefficients are reported as a function of alcohol concentration, electrolyte concentration, water-oil ratio (WOR) and hydrocarbon (alkane) molecular weight. These results have provided new insights regarding the locus of middle phase compositions. Finally, Hsieh and Shah (17) have reported a reduction in the amount of oil and electrolyte solubilized in the microemulsion for a given amount of surfactant. The surfactant concentration is represented as a parameter associated with each projected point in Fig. 2. Thus this concentration would be shown to be increasing in the progression 1 to 4. In systematically optimizing the surfactant formulation, Vinateri and Fleming (18), Bourrel et al. (19), and Salter (16) have concluded that the amount of alcohol should be minimized. These conclusions will also be considered further here. II. EXPERIMENTAL The systems discussed here contain ACS reagent grade NaCI, Analytical reagent grade alcohols, Phillips Petroleum Company fine grade (99 mol%) hydrocarbons, and sodium dodecyl orthoxylene sulfonate supplied by Exxon (1, 2). All chemicals except the surfactant were used as received. The surfactant was deoiled using a silica gel column in which the oil was first eluted with chloroform and the surfactant was then recovered by a methanol elution. Varying quantities of electrolyte (NaCI in distilled water), hydrocarbon, surfactant, and alcohol were mixed and allowed to equili-

269

INFLUENCE OF ALCOHOLS ON MICROEMULSIONS ETHANOL

ELECTROLYTE

((I)

OCTANE TERT- BUTANOL

ISOPROPANOL

ELECTROLYTE ( b )

OCTANE

ELECTROLYTE

(C)

OCTANE

SEC - BUTANOL

ELECTROLYTE

(d )

OC TAN E

FIG. 3. Ternary phase diagrams for systems consisting of electrolyte, octane, and a variety of alcohols.

brate at 29°C until constant phase volumes and clear phases were obtained. The volumes were measured and each phase analyzed. It was found that the error in assuming the additivity of phase volumes was less than 0.5%. An aliquot of each phase was analyzed for water, hydrocarbon, and alcohol using gas chromatography. The NaCI was assumed to remain in the aqueous phase and phase diagrams are constructed assuming that the ratio of salt to water remains constant (pseudo component). There is experimental evidence to indicate that this may be in error for some microemulsions (9). In every case the surfactant concentration was 1 wt% based on the overall weight. The

alcohol concentrations cited are in weight percent of the overall mixture compositions and likewise for a particular phase unless otherwise noted. The surfactant concentrations in the excess phases were measured for a few cases using UV spectrophotometry. These concentrations were found to be quite small, at least l0 bless than that of the microemulsion phase in agreement with the results reported by Vasquez et al. (20) for sulfonated surfactants. Thus for the results reported here, only a small error is incurred by assuming that all of the surfactant is in the microemulsion phase in equilibrium with one or more excess phases. Journal of Colloid and Interface Science,

Vol. 81, No. 1, May 1981

270

BAVIERE, SCHECHTER, AND WADE TABLE I

The Alcohol Concentrations for Which the Concentration in Octane Equals That in W a t e r

Alcohols

Alcohol content in aqueous phase at Pc = 1a (g)

Alcohol solubility a (g)

TBA 2-Butanol Isobutanol n-Butanol TAA 2-Pentanol Isopentanol n-Pentanol n-Hexanol

27.87 12.11 -5.66 4.78 -0.61 ~0.06 ~-0.008

oo 21.84 8.20 7.45 12.35 2.79 2.61 2.25 0.48

a In 100 g water.

III. A L C O H O L P A R T I T I O N I N G

A. Triangular Diagrams Several different types of surfactant-free alcohol phase behavior are depicted in Fig. 3. Ethanol, which is not miscible with octane, exhibits no plait point and partitions preferentially into the aqueous phase. The partition coefficient, Pc, defined as the ratio of the alcohol concentration in the hydrocarbon phase divided by the aqueous phase concentration is less than unity. The slope of the tie lines in such cases is negative. It is significant to note that as the alcohol concentration is increased from, for example, point (a) to point (b) of Fig. 3a, the slope of the tie line decreases, which normally (but not necessarily) corresponds to a decrease in the alcohol partition coefficient. The important point is that Pc depends on a number of factors including the alcohol concentration, tie line slope~ position of the plait point, and the shape of the binodal curve. Increasing the amount of water relative to that of hydrocarbon, but keeping the alcohol constant such that, for example, the overall composition changes from point (a) of Fig. 3a to point (c) results in the alcohol concentration being reduced in both the hydrocarbon and the electrolyte solution as can be seen by observing the intersections of the Journal of Colloid and Interface Science, Vol. 81, N o . 1, M a y 1981

respective tie lines passing through the two points. This is a necessary consequence of Pc being less than unity. Since the slope of the tie line passing through point (c) is greater than that through point (a), then the Pc increases with increasing water-oil ratio

(WOR). Isopropanol and tertiary butanol are miscible with both octane and water and exhibit similar phase diagrams (see Figs. 3b and 3c). The slopes of the tie lines are negative for low concentrations of tertiary butanol (Pc < 1), but become positive at higher concentrations (Pc > 1). This phenomenon of tie line inversion was observed for all the higher molecular weight alcohols distributing between octane and water but at decreasing aqueous phase concentrations, making such observations increasingly difficult. Table I shows the alcohol concentration at which the tie line inversion is observed.

B. The Influence of Electrolyte Concentration and Hydrocarbon Molecular Weight Figure 4 shows the tie lines resulting when 4 vol% secondary butanol is mixed with equal volumes of oil and electrolyte (WOR

RATIO WATER OCTANE = I ?.-BUTANOL 4 % Vol.

tu c., I.O E

h 8

z

O.I

0

i IO0

i 200

ELECTROLYTE CONCENTRATION (g/I NaCI)

FIG. 4. T h e influence of salt concentration on the partitioning o f 2-butanol b e t w e e n an a q u e o u s p h a s e and octane.

INFLUENCE OF ALCOHOLS ON MICROEMULSIONS

RATIO WATER/HYDRocARBON = I

ta. t)

5I--

271

secondary butanol and isopentanol. The presence of the isopentanol increases the fraction of the secondary butanol which partitions into the oil phase and vice versa. Thus by mixing secondary butanol and isopentanol, the concentration of both in the oil phase is increased.

cr

Z

b.J

,,z. • ISOPENTANOL 4.0% Vol. • ISOPE~ITANOE 1.0% Vol. • ISOPENTANOL O. 1% Vol. 0.1

8

12

f 16

ALKANE CARBON NUMBER

FIG. 5. Partition coefficients as a function ofisopentanol concentration and alkane carbon number.

~l.O n,, .I OPENTANOL

d z

//

u ISO~NTANOL + SEC- BUTANOL 4 % Vol.

,,=,

/

[] ISOPENTANOL + ISOPROPANOL 4 % Vol. o ISOPENTANOL + ETHANOL

= 1). As expected, Pc increases as the con0 centration of NaC1 is increased. However, 0 the variation is small provided changes in electrolyte concentrations are kept moderate. a This point will be relevant in the discussion 1.0 to follow. Similarly, it is anticipated that the affinity of a given alcohol for the hydrocarbon phase relative to that of the aqueous phase should la.I decrease as the molecular weight of the alkane is increased. This trend is observed in I.iJ o.~ g Fig. 5.

I

2

5

4 % VoI.

4

ISOPENTANOL(%Vol)

5 I-

C. Alcohol Mixtures

Microemulsion formulations utilizing alcohol mixtures have been reported (21). The partitioning of an alcohol is influenced by the presence of a second alcohol. Figures 6a and 6b show this effect for secondary butanol, isopropanol, and ethanol, each mixed with varying amounts of isopentanol. The Pc of ethanol, for example, is seen to increase as the concentration of isopentanol increases. On the other hand, the presence of ethanol decreases the Pc of the isopentanol. A different trend is seen for mixtures of

" ~ ETHANOL 4 % Vol.

d ~0.01

.001 0

b

'1

l 2

i 3

u 4

ISOPENTANOL (%Vo I.)

Fro. 6. Partition coefficients with mixed alcohol systems. Journal of Colloid and Interface Science, Vol. 81, No. 1, M a y 1981

272

BAVIERE, SCHECHTER, AND WADE T A B L E II Salinity Scan wit h S B A 3 W T % Phase composition(vol %)

Salinity (g/liter NaCI)

Phase volume(vol %)

LP

LP

MP

UP

Total

Brine

SBA

C8

Brine

SBA

Sulfonate

Ca

SBA

Cs

WOR in micellar phase

16 17 18 19 20 21 22 23 24 24*

-40.5 46.0 47.0 48.0 50.0 49.0 49.5 5.15 --

58.8 18.2 14.4 13.1 13.1 13.0 13.3 49.5 48.5 48.5

40.5 40.5 40.5 39.0 38.0 37.0 36.0 --.

99.3 99.2 100.9 99.1 99.1 100.0 98.3 99.0 100.0 . .

-94.76 95.28 95.37 95.48 95.43 95.48 95.24 95.55 .

-4.73 4.68 4.60 4.53 4.56 4.51 4.75 4.45 .

-0.51 0.05 0.03 0.004 0.01 0.01 0.01 0.004

84.87 64.98 53.06 46.60 38.97 32.63 27.13 14.69 7.99 7.46

5.96 5.93 5.70 5.28 5.23 4.83 4.37 3.01 2.13 2.12

1.90 5.58 7.11 7.55 7.71 7.69 7.38 2.79 2.05 1.99

7.27 23.51 34.13 40.56 48.09 54.86 61.12 79.50 87.83 88.43

1.26 1.37 1.39 1.36 1.59 1.41 1.82 ----

98.74 98.63 98,61 98.64 98.41 98.59 98.18 ----

11.7 2.8 1.6 1.2 0.8 0.6 0.4 0.2 0.1 0.1

Micellarphase

UP

N o t e . Octan e W O R = 1, C12OXSOaNa 1 wt%, T = 29°C.

IV. M I C R O E M U L S I O N C O M P O S I T I O N

A. Optimal Salinities

One definition of optimal salinity has been cited. Others which yield approximately (perhaps identically) the same result have also been stated (2, 13). Clearly, the results presented in Table II may be used to estimate that electrolyte concentration for which the volumes of octane and electrolyte solution contained in the middle microemulsion phase are equal. Since the overall WOR = 1, this concentration is the previously defined optimal salinity. For the system described in Table II, the optimal salinity is seen to be 19.5 g/liter. Salager et al. (13) have defined an optimal salinity as the average of the two electrolyte concentrations defining the range for which middle phase microemulsions are observed. For the case given in Table II, these limits are seen to be 17 and 22 g/liter, respectively, and the average is, therefore, 19.5 g/liter. Thus the two different definitions give almost the same values, although not generally true. It has also been found t h a t a t 19.5 g/liter the interfacial tensions between M and 1 equals that between M and u. This has also been proposed as an equivalent definition of Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

the optimal salinity (2). It is remarkable that so many seemingly independent phenomena can be correlated to the existence of middle phase microemulsions. An empirical study defining the factors which determine the optimal salinity has been reported (13, 22). One of the important parameters was shown to be the alcohol concentration. Increasing the alcohol concentration can either increase or decrease the optimal salinity depending on the nature of the alcohol. It has been found that the optimal salinity decreases with increasing alcohol Concentration for all monoalcohols having four or more carbons regardless of the molecular arrangement of the alcohol, independent of the surfactant structure, for all electrolyte concentrations tested, for all alkanes which are liquid at room temperature, and at all alcohol concentrations studied (13, 16, 21). Thus dS*

- -

dA

< 0

[1]

for all alcohols having four or more carbons, where S* is the optimal salinity and A is the alcohol concentration. All of the definitions of the optimal salinity depend on the overall WOR and this variable

INFLUENCE

OF ALCOHOLS TABLE

273

ON MICROEMULSIONS III

Salinity Scan with SBA 3 WT% Phase composition (vol %) Salinity (g/liter NaCI)

Phase volume (vol %)

LP

LP

MP

UP

Total

16.56 17.34 18.13 18.75 19.69 21.25 22.50

--70.0 73.0 75.5 77.5 78.5

89.0 88.5 20.0 17.0 15.5 15.5 19.5

10.0 9.5 8.5 8.0 8.0 6.0 1.0

99.0 98.0 98.5 98.0 99.0 99.0 99.0

28.81 23.59

79.8 80.0

20.2 19.5

---

100.0 99.5

Brine

Micellar phase

UP

SBA

C8

Brine

SBA

Sulfonate

C8

95.75 96.05 96.37 96.44 96.32

--3.63 3.59 3.52 3.55 3.67

--0.62 0.36 0.11 0.01 0.01

89.50 88.49 64.07 54.26 47.62 36.28 25.85

4.09 3.94 3.79 3.72 3.59 3.21 2.81

1.22 1.20 5.10 5.98 6.46 6.40 5.77

5.18 6.38 27.05 36.04 42.33 54.12 66.17

96.27 96.41

3.72 3.59

0.01 0.004

22.58 20.02

2.57 2.51

5.01 5.15

69.84 72.32

-

-

-

-

SBA

C8

0.82 99.18 0.81 99.19 0.82 99.18 0.78 99.22 0 . 7 9 99.21 0.86 99,14 too small to be analyzed -----

WOR in micellar phase 17.3 13.8 2.4 1.5 1.1 0.7 0.4 0.3 0.3

N o t e . O c t a n e W O R = 4, C 1 2 O X S O s N a 1 w t % , T = 29°C.

has not received much attention. However, it is now possible to understand the changes in microemulsion composition which take place as the water-to-oil ratio is varied. Tables III and IV show the results of experiments carried out at WOR = 0.25 and 4.0 under the same conditions as those reported in Table II. The projected middle phase microemulsion compositions and the corresponding projected overall compositions are shown in Fig. 7 for the case of 18 g/liter NaCI. Also shown is one tie line TABLE

(the dashed line) for the secondary butanolelectrolyte-octane surfactant-free system. As the overall mixture composition is changed from O~ to Of to O~ (all containing 1 wt% surfactant and 3 wt% alcohol), the secondary butanol concentration decreases in both the octane and the aqueous phases sincePc < 1. This trend can be seen by comparing the alcohol concentrations at a given salinity but at differing WOR in Tables II, II, and IV. Since the effect of decreased secondary butanol concentration is to cause IV

Salinity Scan with SBA 3 WT% Phase composition (vol %) Salinity (g/liter NaCI)

Phase volume (vol %)

LP

Micellar phase

UP

LP

MP

UP

Total

Brine

SBA

C8

Brine

SBA

Sulfonate

Cs

SBA

Cs

WOR in micellar phase

15 16 17 18 19 20 21 22 23

-11.0 15.0 15.8 18.0 18.8 17.5 19.0 19.5

26.5 15.0 12.5 11.2 11.5 11.2 10.5 81.0 80.5

72.5 73.0 71.5 72.0 70.5 69.5 70.0 ---

99.0 99.0 99.0 99.0 100.0 97.5 98,0 100.0 100.0

-93.66 93.20 93.53 93.43 95.11 93.53 94.07 94.46

6.34 6.80 6.47 6.57 4.89 6.47 5.33 5.54

---------

76.46 63.72 57.16 44.88 37.26 32.52 26.46 4.01 6.07

7.18 7.44 7.53 8.00 8.24 7.21 6.23 3.09 3.39

3.86 6.61 8.02 8.78 8.66 8.83 9.34 1.26 1.34

12.50 22.24 32.30 38.34 45.84 51.44 57.98 91.64 89.09

2.43 2.24 2.39 2.47 2.46 2.65 2.52 ---

97.57 97.76 97.61 97.53 97.54 97.35 97.48 ---

6.1 2.9 1.6 1.2 0.8 0.6 0.5 0.04 0.07

N o t e . O c t a n e W O R = .25, C I ~ O X S O z N a 1 w t % . Journal of Colloid and Interface Science, Vol. 81, No. I, May 1981

274

BAVIERE, SCHECHTER, AND WADE

l SECONDARY

BUTANOL

o.,/ i

<

•

%

oz

ELECTROLYTE

ALCO.OL\0' TIE LINE

o,

..... -~ OCTANE

18 g/l NoCI

Fro. 7. Projected microemulsion composition corresponding to systems for which the overall water to hydrocarbon ratio is increased.

the salinity (optimal) at which the microemulsion concentration contains equal volumes of hydrocarbon and water to be increased (dS*/dA < 0) then decreasing the secondary butanol concentration without making a corresponding and compensating reduction in the electrolyte concentration will necessarily leave the microemulsion poorer in hydrocarbon. Thus, increasing the concentration of an alcohol for which dS/dA < 0 will result in a microemulsion which has increased proportions of oil in the microemulsion. The sequence of compositions M~---> M~---> M~ is therefore predictable since each succeeding microemulsion contains a smaller portion of octane. The fact

that alcohol concentration in the microemulsion decreases from M[---> M~--> M~ is a consequence of the tie line slope. These considerations are important because it has been convenient to represent microemulsion behavior on a triangular diagram. Such representations are termed pseudoternary diagrams because systems consisting of at least five components cannot accurately be so depicted. Fleming and Vinatieri (23, 24) discuss this difficulty and recommend a systematic procedure for selecting pseudo components. Their procedure may very well be accurate enough for many applications, but because of the variability of the Pc it will not apply over a wide range of alcohol concentrations. The locus of middle phase microemulsion concentrations as a function of WOR is then dependent on two factors, the alcohol partition coefficient (tie line slope) and the relationship of the optimal salinity to alcohol concentration. The possible combinations are enumerated in Table V. Shown also are the expected microemulsion transitions. Thus if dS*/dA < 0 and Pc < 1 (Case A, Table V) we expect first that the alcohol concentration will decrease as WOR increases, and this will result in a corresponding increase in the optimal salinity. Since the electrolyte concentration will then be less than optimum, the surfactant tends to exhibit a larger affinity for the aqueous phase

TABLE V The Possible Trends Resulting from a Change in Water/Oil Ratio Alcohol properties

Result of increasing WOR

Effect on optimal salinity

Partition coefficient

Optimal salinity

Microemulsion composition

<0

1

increases decreases

II ~ III --> I I --~ III ~ II

secondary butanol isopentanol

A B

=1

constant

no change

tertiary amyl alcohol

C

< 1 > I

decreases increases

I ~ I I I ~ II II --~ III ~ I

isopropanol ?

D D

<0 or >0 >0

Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

Examples

Case

INFLUENCE

OF ALCOHOLS

O Projected Overall Composition • Projected Microemulsion Composition A

E

A

A = ALCOHOL

H

E

CASE A A

E

A

H CASE C

H CASE B

E

H CASE P

FIG. 8. Locus of projected middle phase microemulsion compositions depending on the Water-Oil Ratio.

and the microemulsion will tend to become richer in water. Thus the tendency exists for the microemulsion to undergo the transformations II ~ III ~ I. Similar considerations will reveal the validity of the microemulsion compositional changes indicated for the other cases cited in Table V. The qualitative trends shown schematically in Table V can be given quantitative representation using triangular diagrams. Figure 8 depicts the four cases corresponding to those presented in Table V. For example, consider Case A shown by Figure 8. As the overall water-oil ratio is increased, O1 --~ 02 ~ 03, a sequence of microemulsions M, ~ M2 ~ M3 is obtained. These microemulsions contain decreased amounts of alcohol and increasing amounts of aqueous phase (see Fig. 8, Case A). Microemulsion M3 is sometimes termed an underoptimum system, since the electrolyte concentration would have to be increased to restore a microemulsion system containing equal proportions of aqueous and hydrocarbon phases.

ON MICROEMULSIONS

275

Figure 7 shows the data for 18 g/liter NaC1 tabulated in Tables II, III, and IV plotted on a triangular diagram. Secondary butanol exhibits properties corresponding to Case A and clearly, the experimental results are in agreement with the predicted trends shown in Fig. 8. Alcohol systems corresponding to other cases have also been studied. Agreement with predicted trends has been found in every case. The results for the isopropanol system are shown in Fig. 9. It is seen that increasing the electrolyte concentration decreases WOR in the microemulsion phase as expected (I ~ III ~ II transition). At a given electrolyte concentration, it is seen that increasing the overall WOR decreases the proportion of aqueous phase solubilized in the microemulsion phase (I ~ III ~ II transition). This trend is in full agreement with the predictions given in Table V. Trends for these two and the secondary butanol-isopentanol system are shown in Table VI. This table has been prepared to emphasize an interesting aspect of the trends shown in Table V, namely, increasing WOR in the presence of a light alcohol (Case D) and a heavy alcohol (Case B) yields precisely the same trend in microemulsion composition. Table VI shows that the optimal salinity of both the isopropanol and the isopentanol secondary butanol systems decrease with increasing WOR, whereas the optimal salinity for the secondary butanol system increases are expected.

B. The Alcohol Content of Microemulsions For every case studied there was more alcohol in the microemulsion than would be calculated by assuming that the aqueous and hydrocarbon phases each contained the same proportions of alcohol as those found in the equilibrium excess phases. This is interpreted to mean that a portion of the alcohol in the microemulsion is incorporated into the interfacial region separating the hyJournal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

276

BAVIERE, SCHECHTER, AND WADE I0 OCTANE CI20XS03 No I wt % ISOPROPANOL 3wl %

1.0

g

.

_

• 5.25

0J

~

25

,

50

i

,

i

35

~

40

i

,

45

i

50

i

55

r

60

ELECTROLYTE CONCENTRATION(g/I NoCr)

FIG. 9. The relation of WOR of the overall system to WOR of the microemulsion phase. drocarbon and aqueous phases. One function of the alcohol then appears to be truly that of a cosurfactant. This is especially true of the higher molecular weight alcohols since the excess concentration (and thus the proportion of the alcohol associated with the interfacial region) appears to increase with molecular weight. For isopropanol, the excess is quite small. A second factor is the effect of the alcohol on the solvent properties of a particular phase with respect to the surfactant. Thus isopropanol is known to partition preferentially into the aqueous phase. It might be expected that the presence of the isopropanol in the aqueous phase should tend to increase the solubility Of the surfactants. Thus, with the addition of a l o w molecular weight alcohol, a microemulsion phase is expected to enrich the proportion of aqueous phase in the microemulsion. Table VII shows the phase volumes resulting from the addition of methanol, isopropanol, and secondary butanol to a microemulsion system. Increasing the methanol concentration systematically increases the proportion of aqueous phase in the microemulsion. At low isopropanol concentrations (less than 0.48%) the observed trend shown i n Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

Table VII differs from that expected. The excess oil phase decreases and the excess a q u e o u s phase increases. The expected behavior is, however, observed as the isopropanol concentration increases. This would indicate that the role played by alcohol is a complex one and is not wholly predictable based on simple partitioning arguments. Another example which cannot be explained based on simple partitioning considerations is shown in Figure 10. It has been shown that adding secondary butanol to a TABLE VI The Optimal Salinity and Middle Phase Microemulsion Volume Are Shown for Different Ratios of Water to Hydrocarbon Overall WOR

0.25

1.00

4.00

Isopropanol 3 wt%

OS 41.5 MPVol 7.0

31,5 14.0

29.0 21.0

2-Butanol 3 wt%

OS MPVol

18.5 11.5

19.3 13.0

20.0 15.5

2-Butanol 3 wt% Isopentanol 3 wt%

OS MPVol

10.65 15.5

9.35 19.0

7.75 23.0

N ot e . OS =Optimum Salinity in g/liter NaC1; MP Vol = Middle Phase Volume in vol %.

277

I N F L U E N C E OF A L C O H O L S ON M I C R O E M U L S I O N S T A B L E VII Effect of Methanol, Isopropanol, and Secondary Butanol on the Phase Behavior of a Type III System P h a s e v o l u m e s (%) Alcohol (vol %)

Methanol 0 0.13 0.51 3.70 6.05

Isopropanol 0 0.13 0.51 3.68 5.98 7.09

sec.-Butanol 0 0.13 0.51 3.68 5.98

LP

MP

UP

28.0 27.0 22.0 18.0 no excess water

38.0 35.5 38.0 41.0 58.4

34.0 37.5 40.0 41.0 41.6

28.0 30.3 35.3 43.9 45.7 no excess water

38.0 37.4 35.9 16.1 12.0 57.8

38.0 EMULSIONS 41.0 59.0

34.0 32.3 28.8 40.0 42.3 42.2

28.0

34.0

48.5 49.5

no excess oil ---

54.5 50.5

Note. 27.5 g/liter NaC1, octane (WOR = 1) and 1 wt% C12OXSO3Na, T = 29°C.

two-phase system containing isopentanol increases the partition coefficient for both the secondary butanol and the isopenta- nol. If the alcohol concentration in the hydrocarbon phase is increased, one would expect a corresponding increase in surfactant preference for the hydrocarbon resulting in an increase in the proportions of oil present in the microemulsion. Figure 10 shows precisely the opposite trend. Adding secondary butanol unexpectedly increases the water content of the microemulsion. Clearly, there are a number of factors to be considered in defining the role that alcohols play in stabilizing microemulsion. Even though a mixed cosurfactant system consisting of the isopentanol and secondary butanol produced unexpected changes in

the microemulsion composition, the effect of WOR for this mixed system is quite predictable. Increasing WOR reduces the alcohol concentration in bothphases which, for these alcohols, should increase the proportion of water to oil in the microemulsion. This trend is observed experimentally as shown in Figure 11 for a fixed overall concentration of alcohol.

C. The Triangles Three-phase microemulsion phase equilibria are represented by tie triangles such as the one shown in Fig. 1. It has been found that the base of this triangle closely corresponds to the tie lines observed in the absence of surfactant. For example, for WOR = 1, Pc = 0.28 at 17 g/liter for 4 vol% secondary butanol (see Fig. 4). Table IV shows the distribution of secondary butanol among the three equilibrium phases. The base of the tie triangle corresponds to a line connecting the composition of the excess phases. The partition coefficient for alcohol between these excess phases is seen to be rather insensitive to salinity and for a salinity

SALIr,~/TYIOg/INaCI / ~ 1.5 OCTANEWOR=I ~ CmOXSO3 Na I wt%

/ / .1"

I'~ 1.0

z

(3.5

I

0

0

i

i

I

2

i

i

i

3 4 5 SEC-BUTANOLWT.%

i

6

FIG. 10. Phase behavior in mixed alcohol systems. Journal of Colloid and Interface Science, Vol. 81, N o . 1, M a y 1981

278

BAVIERE, SCHECHTER, AND WADE OCTANE CBzOXSO3 Na I wt % ISOPENTANOL I wt %

=, 8

=E k._z 1.0

,.=, o • 0.27 I.!

•

• 0.1

5.23 i

7

i

|

8

i

i

9

i

i

i

I0

i

i

II

SALINITY (g/I NoCI)

FIG. 11. The relation of WOR of the overall system to WOR of the microemulsion phase. of 17 g/liter, Pe = 0.35. This corresponds closely to that observed between octane and electrolyte in the absence of surfactant. Similar agreement is found under other conditions. Thus to a very good approximation the base of the tie triangle corresponds to a tie line in the surfactant-free system. Since the microemulsion is generally relatively richer in alcohol, the vertex of the triangle is tilted toward the alcohol vertex. Thus the projected concentrations should indicate an alcohol content above any point on a line connecting the two excess phases (essentially a tie line) as shown in Fig. la. Because the mechanism by which alcohol stabilizes the microemulsion is not fully understood, it is not yet possible to give rules for the location of point M. v. CONCLUSIONS The partitioning of alcohol between the aqueous and hydrocarbon phases determines in part the variations in microemulsion composition observed when the proportions of hydrocarbon and water used to prepare the microemulsion are varied. Given Journal of Colloid and Interface Science, Vol. 81, No. 1, May 1981

further information defining the variation of the optimal salinity with increasing alcohol composition, the trends in microemulsion compositional changes are predictable. There are a number of definitions of the optimal salinity. These appear to yield approximately the same values. The experiments shown here indicate that the water to oil ratio in the microemulsion is unity at the optimal salinity defined by Salager et al. (13), independent of the overall WOR. This implies that at least one of the definitions offered by Reed and Healy (2) will correspond exactly to that presented by Salger et al. if the overall WOR is the same in both cases. The results reaffirm that the optimal salinity does depend strongly on the overall WOR, thereby limiting to some extent the generality of this concept and indicating the complexity of these systems. Finally, a number of surprising results which are not yet understood have been reported. This indicates that continued study of the stabilizing influence of alcohols is warranted. ACKNOWLEDGMENTS The authors wish to express their appreciation to the Department of Energy, the Robert A. Welch Foundation, Delegation Generale a la Recherche Scientifique et Technique, and the followingoil and chemical companies: Amoco, Ashland, Atlantic Richfield, British Petroleum, Chevron, Continental, Elf-Aquitaine, Exxon, Gulf, Marathon, Mobil, Shell, Stepan, Suntech, Tenneco, Texaco, Union, and Witco. REFERENCES 1. Healy, R. N., Reed, R. L., and Stenmark, D. G., Soc. Petrol. Eng. J. 16, 147 (1976). 2. Reed, R. L., and Healy, R. N., Some PhysicoChemical Aspects of Microemulsion Flooding: A Review, in "Improved Oil Recovery by Surfactant and Polymer Flooding" (D. O. Shah and R. S. Schechter, Eds.), p. 383. Academic Press, New York, 1977. 3. Puerto, M. C., and Gale, W. M., Soc. Petrol. Eng. J. 17, 193 (1977). 4. Gogarty, W. B., and Tosch, W. C., Petrol. Technol. 1047 (December 1968). 5. Dominguez, J. G., Willhite, G. P., and Green,

I N F L U E N C E OF ALCOHOLS ON MICROEMULSIONS

6. 7.

8. 9. 10.

11.

12.

13.

14. 15. 16.

17.

D. W., "Solution Chemistry of Surfactants" (Mittal, Ed.), Vol. 2. Plenum, New York, 1979. Bowcott, J. E., and Schulman, J. H., Z. Elektrochem. 59, 282 (1955). Holm, L. M., in "Improved Oil Recovery by Surfactant and Polymer Flooding" (D. O. Shah and R. S. Schechter, Eds.), p. 453. Academic Press, New York, 1977. Healy, R. N., Reed, R. L., and Carpenter, C. W., Soc. Petrol. Eng. J. 15, 87 (1975). Tosch, W. C., Jones, S. C., and Adamson, A. W., J. Colloid Interface Sci. 31, 297 (1969). Dominguez, J. G., "PhaseBehaviorofMicroemulsion Systems with Emphasis on Effects of Paraffinic Hydrocarbons and Alcohols" Ph.D. Dissertation, University of Kansas, 1977. Salter, S. J., "Selection of Pseudo-Components in Surfactant-Oil-Brine-Alcohol Systems," Preprint SPE 7056, Proceedings of Fifth Symposium on Improved Methods of Oil Recovery of the Society of Petroleum Engineers of the AIME, 1978. Wade, W. H., Morgan, J. C., Schechter, R. S., Jacobson, J. K., and Salager, J. L., Soc. Petrol. Eng. J. 18, 242 (1978). Salager, J. L., Vasquez, E., Morgan, J. C., Wade, W. H., and Schechter, R. S., Soc. Petrol. Eng. J. 19, 107 (1979). Graciaa, A., Bourrel, M., Schechter, R. S., and Wade, W. H., submitted for publication. Jones, S. C., and Dreher, K. D., Soc. Petrol. Eng. J. 16, 161 (1976). Salter, S. J., "The Influence of Type and Amount of Alcohol on Surfactant-Oil-Brine Phase Behavior and Properties," Preprint No. SPE 6843, presented at the 52nd Annual Fall Technical Meeting of the Society of Petroleum Engineers, 1977. Hsieh, W. C. and Shah, D. O., "The Effect of Chain Length of Oil and Alcohol as Well as

18.

19.

20.

21.

22.

23. 24.

279

Surfactant to Alcohol Ratio on Solubilization, Phase Behavior and Interfacial Tension of Oil/ Brine/Surfactant/Alcohol Systems," Preprint No. SPE 6594, presented at the International Symposium on Oilfield and Geothermal Chemistry, LaJolla, 1977. Fleming, P. D. and Vinatieri, J. E., "Multivariate Optimization of Surfactant Systems for Tertiary Oil Recovery," Preprint No. SPE 7582, presented at the Fall Technical Meeting of the Society of Petroleum Engineers, 1978. Bourrel, M., Salager, J. L., Lipow, A., Wade, W. H., and Schechter, R. S., "Properties of Amphiphile/Oil/Water Systems at an Optimum Formulation for Phase Behavior," Preprint No. SPE 7450, presented at the Fall Technical Meeting of the Society of Petroleum Engineers, 1978. Vasquez, E., Salager, J. L., E1-Emary, M., Koukounis, C., Wade, W. H., and Schechter, R. S., in "Solution Chemistry of Surfactant" (Mittal, Ed.), Vol. 2, p. 801. Academic Press, New York, 1979. Baviere, M., Wade, W. H., and Schechter, R. S., "The Effect of Salt, Alcohol and Surfactant on Optimum Middle Phase Composition," presented at the Enhanced Oil Recovery Symposium, Stockholm, August, 1979. Salager, J. L., "Physico-Chemical Properties of Surfactant-Water-Oil Mixtures: Phase Behavior, Microemulsion Formation and Interfacial Tension," Ph.D. Dissertation, The University of Texas at Austin, 1977. Fleming, P. D. and Vinatieri, J. E.,J. Chem. Phys. 66, 3147 (1977). Vinatieri, J. E., and Fleming, P. D., "Use of PseudoComponents in the Representation of Phase Behavior of Suffactant Systems," Preprint No. SPE 7057 in the Fifth Symposium on Improved Methods for Oil Recovery of the Society of Petroleum Engineers of AIME, 1978.

Journal of Colloid and Interface Science, Vol. 8l, No. 1, May 1981