Microemulsion formation with branched tail polyoxyethylene sulfonate surfactants

Microemulsion Formation with Branched Tail Polyoxyethylene Sulfonate Surfactants MASAHIKO ABE,* DAVID SCHECHTER,~f R. S. SCHECHTER,$ W. H. W A D E , t UPALI WEERASOORIYA,§ AND SEANG YIV tt *Department of Industrial Engineering Chemistry, Science University of Tokyo, Noda, Chiba, 278 Japan; tDepartment of Chemistry and ~.Department of Chemical Engineering, The University of Texas, Austin, Texas 78712; §Celanese Company, Box 9077, Corpus ChristL Texas 78649; and IIAdamantech, Inc., P.O. Box 1195, Marcus Hook, Pennsylvania 19061-0895

Received September30, 1985;acceptedFebruary 4, 1986 Pure monoisomericsurfactantshavebeen synthesizedwiththe objectivebeingto formulatecosolventfreemiddlephase microemulsionsat ambienttemperatureand at any requiredelectrolyteconcentration. The objectivewas attained when carefulattention was paid to appropriate hydrophobebranching used in concert with the correct ethoxylationlevel. © 1986AcademiePress,Inc. middle phase microemulsion contains equal volumes of oil and water, called the optimum Microemulsions can often be recognized by formulation (2, 3), is often considered to proa number of features which distinguish them vide the reference state for comparing propfrom other families of dispersed systems. Mi- erties among different systems or surfactants. croemulsions are generally found as transparIn most surfactant formulations involving ent or translucent thermodynamically stable microemulsions, a cosurfactant (or cosolvent) mixtures of oil, water, and surfactant. They is generally used in combination with the prican usually be made to contain any phase vol- mary surfactant. The issue concerning the role ume ratio of two immiscible liquids, generally of this added component and the understandhydrocarbon and electrolyte. Microemulsions ing of the necessity for adding them has parform spontaneously upon contact between alleled the research and development of micomponents concomitant with the occurrence croemulsion technology since they were first of low interfacial tensions. Most microemul- introduced by Schulman and Bowcott (4). In sions are of low viscosity and can be of the oil- Schulman's formulations, alcohol was an in-water or water-in-oil type, with dispersed added ingredient, necessary for converting the phase characteristic lengths in the range of macroemulsion of hydrocarbon, water, and 102 ~. potassium oleate to a stable transparent miThose skilled in their preparation know a croemulsion. More recent studies for envariety of procedures to produce microemul- hanced oil recovery processes have unraveled sions but one established technique utilizes several different phenomena associated with Winsor (1) type phase behavior in combina- the presence of alcohol in microemulsion systion with Healy and Reed optimum concepts tems. The most fundamental role of alcohol (2). The systems of most interest are generally is probably its ability to destroy liquid cryslocated in the neighborhood of a phase region talline and/or gel structures which obviate the where a microemulsion phase exists in equi- formation of microemulsion (5, 6). In fact, librium with excess oil and electrolyte (Winsor Winsor type phase behavior in most oil/water/ type III system). The precise condition where surfactant systems cannot be obtained at low INTRODUCTION

342 0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.

JournalofColloidandInterfaceScience,Vol.

114, No. 2, December 1986

343

MICROEMULSION FORMATION \

~L4 ~

55

50 45

~ ~

.

CI8 ~-OLEFIN SULFONATE n-DECANE 2 g p d l ISOPENTANOL

x

MIC ROEMULSION

REGIME

L,ou,o R.S. .

\

REGIME 30

20 0

J I

I ~'

I 3

I 4.

[sec-BUTANOL] l (gpdl)

FIG. 1. Interplay of cosolvent concentration and temperature for determination of the liquid crystal/microemulsion phase boundary for the system as noted.

sumably, the effects of alcohol on solvent properties of oil and water, and the ability of cosurfactant molecules to penetrate the surfactant interfacial monolayer were among those responsible for the cosurfactant functions described above (10). The importance of molecular kinetic conditions of various components, including alcohol, during microemusification are emphasized in another work on microemulsions (11). An example was given where the occurrence of a favorable kinetic regime were necessary for microemulsion formation. The same line of thought with regards to the dynamic role of cosurfactant was suggested in the work on micellar solution containing alcohols (12) and on interfacial transport phenomena in microemulsion systems (13). One of the most fascinating aspects of microemulsion formulation probably concerns the role of cosurfactant vis-a-vis surfactant molecular structure. A number of surfactants are known to form cosurfactant-free microemulsions: double tailed species such as Aerosol OT, p-dihexylbenzene sulfonate (14), or didodecyldimethylammonium hydroxide (15). At the other end of the spectrum, hydrophilic straight chain surfactants such as a-olefin sulfonates (Fig. 1), ethoxylated oleyl sulfonates (16), or end-attachment alkane sulfonates (6, 17) require large alcohol concentrations to form microemulsion.

temperature in the absence of alcohols. Exactly how the cosurfactant functions can best be illustrated in Fig. 1 for data taken from an earlier publication (5). It shows the liquid crystal regime, the microemulsion regime, and the boundary separating the two regimes. All variables in the system are held constant except the temperature, [sec-butanol], and [NaCI] which are required to be just inside the microemulsion regime but at an optimum. Such reciprocal relationships between transition temperature and required alcohol concentration are a universal observation in this laboratory. Under different circumstances, alcohols CIB ALKANE SULFONATES may be employed not so much as a required 7.6 n- DECANE, 4 0 " C ingredient for microemulsion formation but, instead, to change surfactant partitioning B MICROEMULStON REGIME characteristics or to modify microemulsion properties to meet a specific application such as a desire for clarity of solution. For example, in a detailed phase behavior study, Salager (3, "~f0.54 7) found that alcohol is only one common " ~ ' 2 REGIME \ control variable capable of bringing the sur0 * factant formulation to its optimum state. Also, Salter showed that additions of alcohol depress I 2 5 4 5 6 7 8 B the solubilization in microemulsion (8) while ISOMER NUMBER our laboratory found that it decreases the senFIG. 2. Minimal cosolvent requirements for an isomeric sitivity to composition fluctuations (9). Pre- series of alkane sulfonates. I

I

1

i

I

I

I

I

I

Journal of Colloid and Interface Science, Vol. I I4, No. 2, December1986

344

A B E E T AL.

Recent reports show direct correlation between tail branching and the alcohol requirement on both commercial products (18) and pure monoisomeric species (6, 17). Figure 2 presents earlier published data (6) which shows the change in alcohol requirement for microemulsion formation and in surfactant hydrophilicity (as indicated by the optimum salinity by each data point) with change in sulfonate head group attachment position in n-octadecane. For the isomeric series depicted in Fig. 2, the end-attached hydrophilic species requires 10 g per deciliter (gpdl) of sec-butanol and 11.4 gpdl NaCI to produce optimum middle phase microemulsion whereas no cosurfactant is needed when the more hydrophobic species with sulfonate attached to the sixth and seventh carbon atoms are used. This information can be synthesized into a general conclusion that hydrophobic surfactants, at least those of the ionic type with optimal tail branching, may need no alcohol to microemulsify mixtures of oil and water at least at low ionic strength. The purpose of this study is to expand the realm of cosurfactant-free microemulsions into more hostile environments, e.g., high salinity and low temperature. Published results on existing commercial or pure surfactants invariably indicate that significant quantities of cosurfactant are required in these systems. To the best of our knowledge, no one has pub-

ROH

lished results on alcohol-free middle phase microemulsions above 5% NaC1 and under 50°C utilizing monoisomeric surfactant species. Assuming that the optimum tail branching with respect to the alcohol requirement remains unchanged regardless of the hydrophilic structure, our research efforts have been directed toward synthesizing and performing phase behavior studies as model surfactants using optimum hydrophobe branching coupled with head groups of increasing hydrophilicity to impart appropriate partitioning characteristics to the surfactant. We will report the results on synthesis, phase behavior, and solubilization parameters for a series of branched alkane sulfonates with precise numbers of ethylene oxide moieties with the general structure:

I / C\ - R ( 0 ~ / ) x SO~ Na+ R All molecules will be designated mC.(EO)x w h e r e r n = R + 1 = 5, 6, 7, n = R + R + 1 = 12, 14, 16, 18, 20, and x = 0, 2, 3. EXPERIMENTAL

1. Synthesis Previously we reported (20) the synthesis of linear monoisomeric polyoxyethylene sulfonate surfactants.

Na H ~ H 2 ~,+ RO e Nae B,,r'X/S05N~aNaBr + R O ~

SO 5 Na

[11

TH F, A

R = alkyl, alkyl oxyethyl, alkyl oxyethoxyethyl. Synthesis of branched chain ethylene oxide sulfonates with one ethoxy unit can be achieved in the same manner.

R~

OH Nail R ~ O

R, ~

THF, Z~~ R, /

e Na~Br/X~SO3Na R ~ _ _ --Na Br

R'

0/v SO3 Na [21

R = CH3; R' = C16H33 , C14H29. However, attempts to synthesize the currently needed ethoxylated alcohols which are precursors for branched chain ethylene oxide sulfonates with more Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

MICROEMULSION

345

FORMATION

than one ethoxy unit as was done previously were not successful because of a competitive elimination process.

R

MsCI

OH pyridine

O°C

R R--~. - OMs Na HO'VOH. 9 C03=" RR-~

0 "v OH+ R ' ' ~

"-' 180°C,4h

Therefore, it became necessary to develop a method for homologating a secondary alcohol to give branched chain alkoxy alkanols and their subsequent conversion via sulfoethylation to the corresponding ethoxylated sulfonates. This objective was realized by allowing the alkoxide from the secondary alcohol to react with sodium chloracetate, followed

0%

Rj

[3]

[00%

by acidification and subsequent reduction. The alcohol thus formed can be either sulfoetholated to yield a branched chain ethoxylated sulfonate with 2 ethylene oxide units or subjected to a repetition of the homologation technique to yield an alcohol which can serve as the precursor for the surfactant with 3 ethylene oxide units as follows.

R ~ Nail R ~ ~ ~CI-CHgCOgNaR-~ A R'/'~-OHTH~.~ R f~'-O~Na~ -Na G/ " R ' / - - O / \CO2Na -H21 ^ / R ;~.LIP,I1"14 R/

N H R THIR/

e

~Br/v --,,(aun

S O~Na+ R R, /

[41

R,/--o~x~vCO2N~ z UA,H4 ¢)--0%0,/'OH

I. NaH(-H2t)

R R I ' ~ O ' / N ¢ , O - , ~ O~ / S

O3 Na

2. B r / ~ S O 3 N a (-NaBr)

A typical experimental procedure is as follows. In a three-necked !-liter round-bottom flask equipped with a reflux condenser, a nitrogen inlet, and a magnetic stirring bar, was dispersed sodium hydride (14.4 g, 0.6 mole) in 1000 ml oftetrahydrofuran (THF) at room temperature and under an atmosphere of dry nitrogen. A solution of 6-hexadecanol (48.4 g, 0.2 mole) in 50 ml of T H F was added slowly over a period of 3 min, and the resulting mixture was heated at reflux by means of an oil bath and stirred overnight. Subsequently, after the reaction flask was cooled down to room

temperature, chloracetic acid (18.8 g, 0.20 mole) in 100 ml of T H F was added dropwise over 1 h. Heating was resumed and the reaction mixture was stirred for another 16 h. Next, after the excess sodium hydride was destroyed with 2-propanol, the volatiles were removed. The residue was then dissolved in hot water and acidified with C.P. H2SO 4 (pH < 1). After this solution was cooled down, ether was added to extract the alkoxyacetic acid. This ether extraction was repeated once more and the ethereal fractions were combined and washed five Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

346

ABE ET AL.

times with water. The organic fraction was next dried with anhydrous magnesium sulfate and filtered. After removing the solvent first by rotary evaporation and then by high vacuum evaporation, the crude acid was obtained. This acid without further purification was subjected to reduction. Thus, lithium aluminum hydride, LAH (11.4 g, 0.30 mole), was dispersed in 300 ml of dry ether, at room temperature, inside a three-necked, l-liter roundbottom flask equipped with a reflux condenser, a nitrogen inlet, and a magnetic stirring bar. The carboxylic acid prepared earlier was dissolved in 200 ml of ether and was added dropwise so as to maintain a gentle reflux. After the addition was complete, the resulting mixture was stirred at room temperature overnight ( ~ 16 h). Next, the reaction flask was cooled with ice and the excess LAH was decomposed by adding cold water dropwise, followed by the slow addition of 450 ml of 10% (v/v) sulfuric acid. The ethoxylated alcohol thus liberated was extracted three times with ether. The combined ethereal fractions were washed twice with water, twice with saturated solutions of sodium bicarbonate, and once more with water. After the ether solution was dried, it was volatilized to yield the ethoxylated alcohol. This was isolated in pure form by vacuum distillation (yield 38 g, 70%), bp 135°C at 0.25 mm of Hg pressure.

Spectral data of 6-hexadecyloxyethanol. 1HNNR (CDC13): 6 0.85(2t, 6H), 1.25(m, 26H) 2.40(s, 1H), 3.25(m, 1H), 3.55(m, 4H). 13CNMR (CDC13): 6 14.07(2c), 22.71(2c), 25.13, 25.47, 29.39-29.92(5c), 31.98, 32.12, 33.99(2c), 62.25, 59.82, 80.16. This same experimental procedure was repeated for the synthesis of 6-hexadecyloxyethoxyethanol from 6-hexadecyloxyethanol (28.0 g, 0.98 mole) (yield 22 g, 63%) b.p. 150°C at 0.15 mm of Hg pressure.

Spectral data of 6-hexadecyloxyethoxy ethanol. IHNMR (CDCI3): 6 0.85(2t, 6H), 1.25(m, 26H), 2.95(s, 1H) 3.20(m, 1H), 3.55 (m, 8H). 1 3 C N M R (CDC13): ~ 13.96(2c) 22.60(2c) 25.01, 25.36, 29.29-29.80(5c), Journal of Colloid and Interface Science, Vol. 114,No. 2, December1986

31.87, 32.01, 33.86(2c), 61.76, 68.17, 70.84, 72.58, 80.25. The detailed experimental procedure for sulfoethylation has been published previously (21) and for the two alcohols above we find:

Spectral data of sodium 2-{6-hexadecyloxyethoxy}ethanesulfonate. 1HNMR (CDC13): 6 0.85(2t, 6H), 1.25(m, 26H), 3.20(2m, 3H) 3.55(m, 4H), 3.85(m, 2H). 13CNMR (CDC13): 14.12(2c), 22.72(2c), 25.18, 25.56, 29.4529.78(5c), 32.01(2c), 33.67(2c), 50.90, 66.95, 67.78, 70.48, 80.66.

Spectral data of sodium 2-{ (6-hexadecyloxyethoxy }ethoxy} ethanesulfonate. IHNMR (CDC13): 6 0.85(2t, 6H), 1.25(m, 26H), 3.25(2m, 3H), 3.60(m, 8H), 3.90(m, 2H). 13CNMR (CDC13): 6 14.09(2c), 22.70(2c), 25.07, 25.46, 29.39-29.96(5c), 31.96, 32.11, 33.80(2c), 50.83, 66.94, 68.12, 70.10, 70.16, 70.61, 80.18. The following sulfonate surfactants were also prepared by using the same methodology: sodium 2- {6-dodecyloxyethoxy }ethan e sulfonate, sodium 2-{ {6-dodecyloxyethoxy}ethoxy }ethanesulfonate, sodium 2-{ 5-tetradecyl oxyethoxy)ethanesulfonate, sodium 2-{6-tetradecyloxyethoxy}ethanesulfonate, sodium 2- {7-tetradecyloxyethoxy}ethanesulfonate sodium 2-6-octadecyl oxyethane sulfonate, sodium 2- { {6-octadecyloxyethoxy }ethoxy }ethanesulfonate, sodium 2- { {6-eicosanoyloxyethoxy }ethoxy }ethanesulfonate.

2. CMC Determination Various values of CMCs for branched tail ethoxylated (as well as unethoxylated species for comparison purposes) sulfonates were measured by the technique of electrical conductivity. The instrument used was a Model RC-I 8A conductivity bridge made by Beckman Instruments. The bridge utilizes a pre-

347

MICROEMULSION FORMATION

cision ac Wheatstone Bridge to measure solution conductivity or resistivity and a cathode-ray oscilloscope detector. The temperature of all solutions was maintained at a constant 25.6°C by a temperature-controlled water jacket surrounding the conductivity cell. The conductivity cell and bridge were cleaned first with acetone and then with tripledistilled water until the measured resistance was greater than 106 ohms. An initial volume of water was placed in the cell along with a magnetic stirring bar. A fixed volume increment of surfactant solution was injected into the cell; the size and concentration were dependent on previous estimates of the CMC in order to obtain numerous points above and below the CMC. After each increment the system was allowed 2 or 3 min to stabilize and equilibrate and the resulting inverse of resistance was plotted against concentration of surfactant (moles/liter). After the breakpoint was reached, another 10 to 15 points were obtained for a meaningful least-squares analysis of the lines both above and below the breakpoint. By algebraic manipulation, the intersection of the two lines was solved to obtain the CMC.

3. Phase Behavior Studies The surfactant concentration used in this study, unless otherwise indicated, was 0.57

15,0

~.: 6 C I 4 (EO) I

256"C

~,°.°~ o

5.0

0

l

0.5

1.0

!/

Critical Micelle Concentrations Surfactants branched at sixth carbon

CMC

6C12 SO3Na 6~bCt2 SO3Na 6CI2(EO)2SO3Na 6CI2(EO)3SOaNa

8.96 2.21 3.57 5.25

X 10-4 >( 10-3 × 10-3 × 10-3

6~bC14 SO3Na 6CI4(EO)2SO3Na

7.60 X 10-4 1.82 × 10 -4

6C16 SO3Na 6q~Ct6 SO3Na 6CIr(EO)2SO3Na 6C16(EO)3SO3Na

1.18 2.90 9.28 1.35

6C18 SO3Na 6CIs(EO)ISO3Na 6CIs(EO)3SO3Na

4.22 × 10-5 2.04 X 10-4 6.58 X 10-4

X X X ×

10 -4 10-4 10-4 10-3

gpdl. All systems were equilibrated by mixing appropriate volumes of surfactant solution, salt solution, and hydrocarbon in sealed 5- or 10-cm 3 graduated tubes. All tubes are kept in thermostated baths and shaken several times daily to assure proper mixing and until phase volumes reach constancy. All the solubilization parameters reported here are for middle phase microemulsion at optimum. The solubilization parameter at the optimum (a*) is defined as the volume of oil or water (they are equal) in the optimum microemulsion per unit volume of neat surfactant. All solubilization parameters were calculated with the assumption that all the surfactant inventory was contained in the microemulsion phase. RESULTS A N D DISCUSSION

1. C M C Determination

CMC = 1.10 X I0 - 3 t o o l / I

,

TABLE I

I

,

15

2.O

CONC. (x 1 0 " 3 ) ( m o l e / I )

FIG. 3. Conductivity/concentration plot for C M C determination.

CMCs were determined for a variety ofsurfactants with the hydrophile attached at the sixth carbon. A typical conductivity plot is shown in Fig. 3 with the crossing of the leastsquares lines taken as the CMC. All the CMCs so determined are presented in Table I and are plotted in Fig. 4. Data are presented for alkane sulfonates (6Cn), alkylbenzene sulfoJournal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

348

ABE ET AL. TABLE II ISOMER # 6

SULFONATES, 2 5 . 6 ° C

Data Fit to log CMC = A - B n for the Four Branched Suffactant Series

10 - 3 6Cn(EO) 3

o

Species

A

B

6Cn 6q~Cn 6Cn(EO)2 6Cn(EO)3

-0.29 +0.24 -0.79 -0.63

+0.23 +0.24 +0.14 +0.14

2. Phase Behavior Studies 10- 4

i

i

i

i

i

12

14

16

18

20

n • T A I L CARBON NUMBER

FIG. 4. CMC data for four homologous series of surfactants with hydrophobes branched at the sixth carbon.

nates (6~Cn), and ethoxylated alkane sulfonates 6Cn(EO)2 and 6Cn(EO)3. The trends are quite regular. Increasing hydrophobe size at constant branching leads to a logarithmic decrease in CMC for all four series. The slope for the alkylbenzene sulfonates closely parallels that for the alkane sulfonates with the CMCs of the former being approximately a factor of 2 higher than those of the latter. Added ethylene oxide leads to a further increase in CMC and a reduction in slope. Grtte (21) and Barry and Wilson (22) studying straight tailed dodecyl ethoxysulfates, formed the opposite trend with added ethylene oxide--a decrease in CMC. When the data were fitted to the relationship log CMC = A - B n it was possible to determine the values of A and B (see Table II). The values of B are less than those reported by Barry and Wilson (22) and Rosen (23), based once again on studies of straight tailed species. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

A. Branched tailed alkane sulfonates. Typical phase behavior plotted in salinity]hydrocarbon alkane carbon number (ACN) space is shown for 7C18 at four different temperatures in Fig. 5. The locations of phase boundaries and optimum lines were obtained by varying electrolyte concentration for each hydrocarbon. Winsor type III phase behavior exists within the area between the two solid lines. To the upper left and lower right sides of the three phase region are two-phase o/w and w/o systems, respectively. The optimum salinity (S*), depicted by dashed lines, is shown to be linearly related to

/

O. 57gpdl 7Ci8 14

NO ALCOHOL

8

5 14

10 8 0

i

i

i

I

2

I

[NoC~

i

/ (gpdl)

FIG. 5. Phase maps at four temperatures in ACN/[NaC1] space for 7C~8.

MICROEMULSION FORMATION

349

the oil phase ACN at all temperatures in the NaC1 concentration range studied with an average slope of

0.579pd I

7Ci8

NO ALCOHOL 8 6O ~x ~ 9 I 0 / A C N

AACN --=5.5. A[NaC1]

o

The three-phase region, most narrow at low salinities and low temperatures, gradually broadens as the salt concentrations and temperatures are increased. These variations are consistent with previous reports on phase study (9). The study of the same system below 40°C indicated that liquid crystals appeared in an increasing n u m b e r of systems at low temperatures so that microemulsions could be obtained only in a limited range of hydrocarbon and salt concentrations. The data in Fig. 5 also indicate a m i n o r increase in surfactant hydrophilicity at higher temperatures as evidenced by a slight shift to the right of the three-phase region. A n u m b e r of alkane sulfonates similar in tail branching to 7C18 also produce cosurfacrant-free microemulsion. Molecules with chain length in the range ofC14 to C18 listed in Table III are capable of forming microemulsions TABLE III Optimum Parameters for Branched Tailed Alkane Sulfonates T (°C)

9C!8 6Ci8

5C18 6C~8

80 25 35 45 55 70 25

7C14

30 40 50 25

6C12

ACN

10.0 7.1 10.0 11.4 13.5 7.8 Methylcyclohexane (4.0) 6.4 7.3 10.0 Methylcyclohexane (4.0)

S* (gpdl)

a* (cm3/cm 3)

0.37 1.30 2.40 3.00 4.00 4.00 1.30

42.0 37.0 22.0 18.0 11.0 25.0 37.0

2.50 3.00 4.00 1.70

19.0 15.0 9.5 17.0

No microemulsion was found

50

"' i"

40

b

50

20 I0

•

40*C

•

50* C

•

60* C

•

70"C

i

I

0

0.5

1.0

1.5

I

2.0

[NaC,]I (gpdl,

FIG. 6. Solubilization parameters for 7C~8as a function of [NaC1], temperature, and oil phase ACN.

without cosurfactant. Values of o p t i m u m salinities and solubilization parameters at optim u m for these surfactants indicated that salt tolerance can be improved by lowering the surfactant molecular weight. Unfortunately, a* rapidly decreases when surfactant chain length is shortened, as revealed in a previous work (19). Furthermore the 6C14 alkane sulfonate m a y be the smallest near midchain branched molecule capable of forming microemulsions since we were not able to find any microemulsions with 6C12. Solubilization parameters at o p t i m u m versus [NaC1] at different temperatures are plotted in Fig. 6. O p t i m u m alkane carbon numbers are also noted in the graph. One sees that 7C18 produces microemulsions with good solubilization in the entire range of ACN, salinity, and temperatures studied. Solubilizations are the highest at low salinity, low ACN, and low temperatures (5, 9, 19). Electrolyte has the most effective influence on a* whereas the effect of temperature on a* is more significant at low, rather than high, ACN. The dashed lines for constant A C N have a negative slope consistent with surfactant hydrophilicity as it increases with temperature. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

350

ABE ET AL.

These studies on alkane sulfonates which identify cosurfactant-free microemulsion regimes give a clear message that for a given head group such as sulfonate, cosurfactant-free microemulsion can be attained but only at relatively low ionic strengths. In other words, when attached to an optimal tall, the sulfonate head group appears too weak in polarity or hydrophilicity to function efficiently at higher salt concentrations. Higher salinity systems can be functional but only at the expense of reduced solubilization parameters. It is therefore necessary to modify the hydrophilic portion of the surfactant molecule if the goal is to find alcohol-free microemulsions at higher salt concentrations. We chose to retain the sulfonate head group since other known polar head groups, e.g., carboxylates, are even less hydrophilic than sulfonates. On the other hand, we assumed that jumping to significantly more water soluble hydrophiles, such as disulfonates, would be too dramatic a change in polarity. We chose the option of gradually augmenting the sulfonate by adding an increasing number of ethylene oxide units

16

0.57 gpdl 6CI6(E0)2 NO ALCOHOL, W0R=I

14 12 I0

16

~

14

12

I0

~

~

6

40°C

7

8

C

I

I

I

6

7

8

No CI]/(gpdl) FIG. 7. Phase maps at four temperatures in AGN/[NaC]]

space for 6C16(EO)2. Journal of Colloid and Interface Science, Vol. 114, No. 2, D e c e m b e r 1986

0.57gpdl 6Cl6 (.EO)2 NO ALCOHOL,WOR • I

6O

5O Aca ¢J 40 o

b

30 20 10 I

i

i

I

5

6

7

8

[N,cl]/~g,,,) FIG. 8. Solubilizationparameters for 6C16(EO)2 as a function of [NaC1],temperature, and oil phase ACN.

until the partitioning became appropriate at the desired ionic strength.

B. Ethoxylated branched alkane sulfonates. Data in Fig. 7 show that 6C16 with two ethylene oxide units added between the hydrophobe tail and sulfonate head forms alcohol-free microemulsion in the 5-8% NaC1 concentration range. Comparison with the results on 6C16 in Table III indicates that addition of two ethylene oxide units shifts the optimum salinity upward nearly 1.4 gpdl while retaining the ability to generate microemulsion without alcohol. It is interesting to note that the width of the three-phase region is nearly the same regardless of the salinity, ACN, or temperature--quite different from the general trend previously observed (10). The corresponding solubilization parameters at the optimum are displayed in Fig. 8 showing that this molecule produces very good solubilization even at elevated temperatures. Once again, however, the highest solubilization parameters are produced at low temperatures, ACN, and salinities. The electrolyte appears to depress ~* to a lesser extent as evidenced by a comparison with Fig. 6. Compared to 6C16, the ethoxylated species gives rise to much higher solubilization which im-

MICROEMULSION FORMATION 16

'0 579pdl 6CI4(E0)2 NO ALCOHOL WOR=I

i2 i0 C

C 8

6 I

t

I

I

I

1

i

6

8

I0

16

14

10 °C 8

6

6

i

I

8

I0

12

No CI]/(gpdl)

FIG. 9. Phase maps at four temperatures in ACN/[NaC1] space for 6C14(EO)2.

plies that ethylene oxide in addition to increasing hydrophilicity promotes higher solubilization efficiency. Another interesting feature found in comparing Figs. 6 and 7 concerns the effect of temperature on surfactant partitioning. While ionic surfactants tend to become somewhat more hydrophilic at elevated temperatures, results in Figs. 7 and 8 and for other ethoxylated species which follow indicate that addition of ethylene oxide reverses this effect. Figures 7 and 8 show that for a given hydrocarbon, optimum salinity gradually decreased with increasing temperature. This sign reversal in the temperature coefficient is characteristic of EO type nonionics. A temperature-indifferent EO sulfonate will obviously need to contain less than 2 EOs per sulfonate. It is reasonable to assume that the effect of temperature on the hydration state of the ethylene oxide moieties is accountable for the decreased

351

hydrophilicity at elevated temperatures in EO sulfonates as in the pure nonionic species. The next species synthesized was found to function at even higher salinity, also without added alcohol. This molecule is similar to the previous one except that the tail length is two carbons shorter, i.e., 6C14(EO)2. It forms microemulsions with various hydrocarbons with an aqueous phase containing 6 to 12% NaC1, as shown in Figs. 9 and 10. One sees that at a given temperature, the width of the threephase region is nearly independent of the ACN and salinity. An increase in the temperature, however, broadens this width and once again causes the amphiphile to become more hydrophobic. Note that the three-phase region clearly curves toward the ACN axis which means that higher ionic strength is more effective in changing surfactant partitioning between water and oil. This ACN/electrolyte behavior is also unusual when compared to previously observed results which generally show that S* increases linearly with oil phase ACN (5) or with the opposite curvature (3, 5). Figure 10 shows the dependence of a* on

6CI4(E0) 2 0.57 gpdl 7

t

40

8

30

NO ALCOHOL, WOR =1

ACN

\ D, C

!

20

i

6

i

8

i

i

I0

i

i

12

IN.Cl]l(9pdl) FIG. 10. Solubilization parameters for 6C14(EO)2 as a function of [NaC1], temperature, and oil phase ACN.

Journal of Colloid and Interface Science, Vol. 114, No. 2, D e c e m b e r 1986

352

ABE ET AL.

optimum salinity, oil phase ACN, and temperature. One notes that the variation of tr* with salinity is rather flat compared to the results in Figs. 5 and 7, especially at elevated temperature. Values of a* are lower than those seen in Fig. 8 due to the decrease in surfactant molecular weight (9, 19) but they are still good for the entire range of experimental conditions studied (generally, a value for a* of more than 10 is considered acceptable for most applications). Figures 11 and 12 show the effect of calcium on phase behavior and solubilization parameters at optimum. In these experiments, all variables including hydrocarbon are held constant while an increasing amount of NaC1 is removed to compensate for the CaC12 addition. Data in Fig. 11 show that 0.7% CaC12 replaces approximately 3.2% NaCl which means that CaC12 is f o u r t o five times more effective than NaC1 in forming these optimum systems. Figure 12 shows that for a given hydrocarbon, a* decrease slightly when part of the NaC1 in the system is replaced by CaC12 to maintain the formulation at an optimum. Figure 13 shows the effect of tail branching on the optimum phase behavior of three ethoxylated C~4sulfonates. Note that optimum salinity is very sensitive to the tail branching: S* increases twofold when the attachment point moves from the seventh to the fifth carbon atom. Figures 14 and 15 show that the values of tr* for 5C14(EO)2 and 7C14(EO)2 are approximately in the same range as compared

0.57 gpdl 6CI4 (EO) 2 NO ALCOHOL OIL PHASE- DECANE

30 u "~ ou ~b

/ 8.9

NoCI /(gpdl)

70

8.1

20

500c

v

~

70 ° C

I0

i

I

i

i

I

0

0.2

0.4

0.6

0.8

[Co CI2] / ( g p d l )

FIG. 12. Solubilization parameters for 6C~JEO)2 with CaC12 exchanged for NaC1 as electrolyte.

to the result for the 6 isomer in Fig. 10. Data in Fig. 15 indicate that alcohol-free microemulsions can be obtained with 5C14(EO)2 only at 50°C and higher temperatures. Gels are formed at 40°C except with octane which confirms that decreased tail branching requires increased temperature or cosurfactant to melt extended phases. Figures 16 and 17 show that the salinity at which the optimal microemulsion forms can be increased further by lowering the molecular weight of the hydrophobe tail to yield 6C12(EO)2. This species produces alcohol-free

NO ALCOHOL 50°C

6Ct4 (EO)2

14

O)2 7CI4(E012

0 57gpOI 6CI6(EO) 2

NO ALCOHOL OIL PHASE-DECANE

io

Io 4 8 I

O2

O4

O6

0.2

04

0.6

02

04

06

[c.c,2],~,,, FIG. 11. Phase behavior for 6C14(EO)2 CaC12exchanged for NaCl as electrolyte.

Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

4

I

6

I

L 8

I

I

I0

i

I

12

L

14

[ NoCI] /(g•dl

FIG. 13. Optimum phase behavior lines in ACN/[NaC1] space for 7CtJEO)2, 6C~4(EO)2 and 5CI4(EO)2.

MICROEMULSION FORMATION

16

40°C

50*C'~ \ ~

0~

CIO ClZ / / ,Cl4

/ : ; I , ~ , ' c,~

c8

0.57gpdl 7C14(EO)2SO3Na÷

~

40

353

40"C ~

8

ACN

//,~-.C

,' j.,,.w/

,,

16

12 30

,o.c'\ u o

20

/

t

60 c-~:_~.~,

8

b

:\/;

~ o . c ~

8

16

I

I

: /

,

D

16

:

12

X,'5 /

'~

o. /

70.0~0.57.~pdl

I0

6Cl2 (EO)2 SO,No"

8 NO

0

I 2.0

i 4.0

i

[No

i 60

i

i

i 8.0

i

i

i

I0

ALCOHOL,

i

WOR

J

12

• I

i

14

i

J

115

I

[No CI] /(gpdl)

CI] /(opd,)

FIG. 14. Solubilization parameters for 7C14(EO)2 as a function of [NaC1], temperature, and oil phase ACN.

FIG. 16. Phase maps for 6C12(EOh in ACN/[NaC1]space at four temperatures.

C8 0 57 gpdi 6CI2(EO)2SO3No* 8

/

0 57gpdt 5C~ (EO)2S03No*

12

/W

30

u u

8

NO ALCOHOL WOR : 1

//

4O

/ /

400C

,,/

50"C

, CI0

/

/ , CI2

I0

~a "tO'C

"b ~o 8

6

I0

0

i 8

i

J I0

J

i 12

i

i 14

[No C'] /(,pdl)

FIG. 15. Solubilization parameters for 5Ct4(EO)2 as a function of [NaC1], temperature, and oil phase ACN.

I

i

IO

l

i

12

i

14

~

i

16

[NoCll/(~pdl) FIG. 17. S o l u b i l i z a t i o n p a r a m e t e r s for 6C12(EO)2 as a f u n c t i o n o f [NaC1], oil p h a s e A C N , a n d t e m p e r a t u r e .

Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

354

ABE ET AL.

microemulsions between 10 and 15 gpdl NaC1. One notes that the effect of temperature on o p t i m u m salinity becomes more pronounced for any hydrocarbon used. The width of the three-phase region become relatively broad and the solubilization parameters at o p t i m u m are low, especially at high temperatures and with high ACN. These results are to be expected even with the relatively low molecular weight of this molecule. In fact, as we mentioned earlier, 6C12 sulfonate with no ethylene oxide added is not even capable of producing a middle phase microemulsion. Although we have reached the lower limit in hydrophobe molecular weight it is still possible to increase surfactant molecular weight and at the same time its hydrophilicity by adding more ethylene oxide to the 6C]2 sulfonate. When this is done, the surfactant, 6C12(EO)3, is seen (in Figs. 18 and 19) as able to form alcohol-free microemulsions not only at very high salinity (around 20 gpdl NaC1) but also at temperatures as low as 25°C with acceptable solubilization parameters. All the results are recapitulated in Fig. 20,

li~ 14 12 I0

6CI2(E0) 3 0.57 gpdl NO ALCOHOL

25 /

~. ao b

8

ACN

/~'~40"C

15

~

IC

I

//

I

/-

I

|

16

14

1 (3

I

I

18

I

20

I

22

[NoCl]/(gpd,)

FIG. 19. Solubilization parameter for 6C12(EO)3 as a function of [NaC1], oil phase ACN, and temperature.

to show that alcohol-flee microemulsions can be obtained at any electrolyte concentration between 0.5 and 20% NaC1 provided that the optimal structures are identified. One should speculate in part on the fundamental reason for the surfactant structures required but from all evidence in hand it would appear that there are three unifying but often conflicting concepts which must be simultaneously evoked in designing cosolventfree high quality microemulsions: (1) The system must be designed so that there is near equal partitioning of surfactant between the

8 6Cl2 (EO)3 0.57gpdl NO ALCOHOL

16

/

~o

12

0

~<~ 4o

8 8

~7C,8

30

I0

40"C;NO ALCOHOL

I0

6O

14

057gpdl SURFACTANT

ACN

6Cl6 ( E°12

\\1-~

_

i ,2

\ ~o ,. ,o\ \.o,.,.% \,2

20 8

12

14

16

h

i

i

i

i

i

i

18

20

22

14

16

18

20

[No e l ] / ( g p d l )

i0

0

81~. . 6 Ci~I 2 ( EO)$ 8 6CI2 (EO) 2 ~'~12

8CI6(E0)2 I

I

i

J

i

2

4

6

8

I0

16

L 12

14

16

IB

20

[Noc,],(g,d,) FIG. 18. Phasemaps for 6C12(EO)3in ACN/[NaC1]space at four temperatures. Journal of Colloid and Interface Science, Vol. 114, No. 2, D e c e m b e r 1986

FIG.20. Summary of behavior for a variety ofsurfactants.

MICROEMULSION FORMATION

355

motion of good solubilization, and (4) correct balance of the surfactant hydrophile and hydrophobe structures to a functional state at any salt concentration between 0.5 and 20 gpdl NaC1 without compromising any other microemulsion quality. These are some of the features regarding optimal surfactant structure for cosurfactantfree microemulsion: (1) for linear chain hydrophobes such as the molecules studied here, optimum tail branchings are those with the polar head group attached to the fifth, sixth, or seventh carbon atoms. It is probable that when multiple branched tails such as propylene oligomer are used, the optimum attachment point will shift to a more nearly endattached position in the carbon backbone. It is reasonable to assume that in the case of alkyl aryl species, the attachment point corresponding to minimal alcohol will also shift somewhat toward the end of the chain because benzene, toluene, or xylene can be considered as a branching part of the hydrophobe. (2) The molecular weights of the surfactant studied here vary only from 350 to 410 for the entire SUMMARY range of salinities studied. The relatively small We have identified a spectrum of ethoxy- variation of molecular weights is due to the lated double branched tail alkane sulfonates decrease in hydrophobe chain length being which inherently have the ability to micro- compensated by the increase in number of emulsify hydrocarbon and electrolyte con- ethylene oxide units. Apparently, this range of taining a broad range of salt concentration molecular weights give rise to good solubiliwithout added cosurfactant at low tempera- zation parameters regardless of the salt contures. These molecules spontaneously form a centration. The surfactant molecular sizes in microemulsion upon contact with oil, surfac- terms of total length (major tail + ethylene tant, and electrolyte solution at 40°C and oxide + sulfonate) also vary little from the sometimes at room temperature, with at least most hydrophobic to the most hydrophilic end acceptable solubilization parameters. of the spectrum. When alkyl aryl compounds These optimal structures were obtained are used, the molecular weight factor should through parallel optimization of (1) hydro- be taken into account. Since the aromatic phobe branching, (2) surfactant molecular group contributes as 3-4 methylene groups in weight including both the hydrophobe and terms of a molecular weight effect on surfachydrophile, and (3) surfactant head group. tant hydrophobicity and solubilization, the tail This multidimensional optimization has led length should be lowered by 3-4 methylene to (1) complete removal of the cosurfactant groups when alkyl aryl species are used. In the from the microemulsion formulation, (2) a re- case of sulfonates, addition of 3 or at most 4 duction of the lower limit of temperature at units of ethylene oxide is sufficient to satisfy which microemulsion can be formed, (3) pro- all the criteria for microemulsion quality inliquid phases. This involves simultaneously optimizing temperature, cosolvent type and concentration, surfactant head group polarity and hydrophobe size and structure, oil phase composition, and electrolyte type and composition. (2) High solubilization or low interfacial tensions require maximal linear extension of the surfactant molecules and this can be accomplished on either the hydrophobic or hydrophylic end of the molecule or both. Obviously surfactant activity must be sufficiently strong to be above the critical micelle concentration. (3) Net surfactant lateral interactions must be sufficiently weak so that the system is above the melting point of all extended structures, eg liquid crystals, gels, etc., so that the more disordered microemulsion state is thermodynamically stable. Lateral interactions are decreased by adding hydrophobe branching, adding ethylene oxide, increasing temperature, decreasing hydrophobe length, adding cosolvent, and decreasing electrolyte concentration.

Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

356

ABE ET AL.

cluding high salt tolerance, alcohol-free form u l a t i o n , a n d good solubilization. This implies that m o r e complicated species such as m i x e d E O / P O molecules should n o t be necessary. W e have conclusively resolved the issue regarding the alcohol r e q u i r e m e n t i n microe m u l s i o n formation. W e have shown that simple molecules such as ethoxylated double c h a i n alkane sulfonates c o n t a i n all the elem e n t s a n d characteristics which enable t h e m to m i c r o e m u l s i f y oil a n d electrolyte a n d stabilize m i c r o e m u l s i o n s even in extremely hostile e n v i r o n m e n t s . Apparently, these molecules are sufficiently short i n length to provide a fast m o l e c u l a r diffusion for s p o n t a n e o u s l y emulsification a n d sufficiently large i n size to provide sufficient interfacial m o l e c u l a r interaction for good solubilization a n d stabilization. ACKNOWLEDGMENTS The authors thank the Department of Energy,the Robert A. Welch Foundation, and the followingcompanies: Amoco, Arco, British Petroleum, Chevron, Elf-Aquitane, Exxon, Gulf, Norsk-Hydro, Shell, Sohio, Sun, Tenneco, Texaco, Union, and Witco for their support. REFERENCES 1. Winsor, P. A., "Solvent Properties of Amphiphilic Compounds." Butterworths London, 1954. 2. Healy, R. N., and Reed, R. L., Trans. A I M E 257, 491 (1974). 3. Salager,J. L., Vasquez, E., Morgan, J. C., Schechter, R. S., and Wade, W. H., Soc. Pet. Eng. J. 19, 107 (1979). 4. Bowcott,J. E., and Schulman, J. H., Z. Elektrochem. 59, 283 (1955). 5. Barakat, Y., Fortney, L. N., Schechter, R. S., Wade, W. H., and Yiv, H. S., "Alpha Olelin Sulfonates for Enhanced Oil Recovery," ARTEP 2nd Euro-

JournalofColloidandInterfaceScience,Vol.114,No.2, December1986

pean Symposium on Enhanced Oil Recovery, Paris, p. 11. Editions Technip, Paris, 1982. 6. Barakat, Y., Fortney, L. N., Lalanne-Cassou, C., Schechter, R. S., Wade, W. H., Weerasooriya, U., and Yiv, S. H., Soe. Pet. Eng. J. 913 (1983). 7. Bourrel,M., Salager,J. L, Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 75, 451 (1980). 8. Salter, S. J., "The Influence of Type and Amount of Alcohol on Surfactant-Oil-Brine Phase Behavior and Properties," SPE Preprint 6843. 9. Graciaa, A., Fortney, L. N., Schechter, R. S., Wade, W. H., Yiv, S., Soc. Pet. Eng. J. 743 (1982). 10. Bourrel, M., and Chambu, C., "The Rules for Achieving High Solubilization of Brine or Oil by Amphiphillic Molecules," SPE Reprint 10676. 11. Gerbacia, W., and Rosano, H. L., J. Colloid Interface Sci. 44, 242 (1973). 12. Yiv, S. H., Zana, R., Ulbricht, W., and Hoffmann, H., J. Colloid Interface Sci. 80, 224 (1981). 13. Hsieh, W. C., and Shah, D. O., "The Effect of Chain Length of Oil and Alcohol as Well as Surfactant to AlcoholRatio on Solubilization,Phase Behavior and Interfacial Tension of Oil/Brine/Surfactant/ Alcohol Systems," SPE Preprint 6594. 14. Fortney, L. N., and Wade, W. H., unpublishedresults. 15. Ninhan, B. W., Chen, S. J., and Evans, D. F. J. Phys. Chem. 88, 24 (1984); Blum, F. D., Pickup, S., Ninham, B. W., Chen, S. J., and Evans, D. F., J. Phys. Chem. 89, 711 (1985). 16. Carmona, I., Schechter, R. S., Wade, W. H., and Weerasooriya, U., Soc. Pet. Eng. J. 351 (1985). 17. Schechter, R. S., Wade, W. H., Weerasooriya, U., Weerasooriya, V., and Yiv, S. H., J. Dispersion Sci. Technol. 6, No. 2, (1985). 18. Puerto, M. C., and Reed, R. L., "A Three-Parameter Representation of Surfactant-Oil-Brine Interaction," SPE Reprint 10678. 19. Barakat, Y., Fortney, L. N., Schechter, R. S., Wade, W. H., and Yiv, S., J. Colloid Interface Sci. 92, 561 (1983). 20. Carmona, I., Schechter, R. S., Wade, W. H., Weerasooriya, U., and Weerasooriya, V., J. Dispersion Sci. Technol. 4, 361 (1983). 21. G6tte, E., Proc. Int. Congr. Surf. Act. 3rd Cologne 45, 1 (1960). 22. Barry, B. W., and Wilson, R., Colloid Polym. Sci. 256, 251 (1978). 23. Rosen, M., "Surfactants and InterfacialPhenomena," Chap. 3. Wiley-Interscience, New York, 1978.