Langmuir 1991, 7, 2048-2053
2048
Phase Behavior of Branched Surfactants in Oil and Water Klaus R. Wormuth’lt and Stephen Zushma Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801 Received January 2, 1991. I n Final Form: April 19, 1991 Systematic investigation of the phase behavior of 16 surfactants with linear and branched hydrocarbon tails in oil and water mixtures reveals new relationships between surfactant Structure and microemulsion formation. The surfactants examined are classified as “linear” (single-tail),“methyl branched”, “double tail”, or “highly branched”. Measurements of the phase behavior in oil and water clearly establish the lipophilicranking: highly branched = double tail > methyl branched > linear. In almost all cases, branched surfactants mix equal amounts of oil and water less efficiently than linear surfactants and phase behavior yields the efficiency ranking: linear > double tail >> methyl branched = highly branched. Since branched ethoxylated alcohol/oil/water mixtures exhibit a larger three-phase region than linear ethoxylated alcohols, branched surfactant/oil/water mixtures are further from the tricritical point.
Introduction The chemical structure of the amphiphile prescribes the conditions under which oil and water are stably mixed.14 Only surfactants, amphiphiles which contain both strong hydrophilic and lipophilic groups, can form the microstructures required for efficient mixing of oil and water into a thermodynamically stable phase called a microemulsion. Despite extensive experimental results, no methodology or model exists which quantitatively predicts the structure of the surfactant required to blend oil and water under specified conditions of temperature, pressure, type of oil, and salinity. In this work, we explore the phase behavior of surfactants with branched hydrocarbon tails to clarify the link between the chemical structure of the surfactant and the conditions of microemulsion formation. Many of the surfactants used in industrial applications are prepared from petroleum feedstocks and have branched hydrocarbon tails. Although it is difficult to quantitatively define the “degree of branching” of a hydrocarbon, the branched surfactants examined in this report may be roughly classified as ”methyl branched”, “double tail”, or “highly branched”. The methyl-branched surfactants consist of a single hydrocarbon chain with one or more small pendant groups (methyl or ethyl groups) attached a t any position along the main chain. The double-tail surfactants consist of a main hydrocarbon chain with one pendant chain. The pendant chain is unbranched, contains three or more carbon atoms, and is attached a t or near the hydrophilic group (at the CY or 0 carbon). The remaining branched surfactant structures examined here fall into the highly branched category and contain some combination of methyl- and double-tail types of branching. A comparison of the phase behavior of surfactants from each of the above categories with that of surfactants with unbranched (linear) hydrocarbon tails was undertaken to ~~~
* To whom correspondence should be addressed.
+ Current address: 3 M Center, Bldg 236-IS-03,St. Paul, M N 55144-
1OOO. (1) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.;Jen, J.; Schomiker, R. Langmuir 1988,4,499. (2)Kahlweit, M.; Strey, R.; SchomHker, R.; Haase, D.Langmuir 1989,
5 , 305. (3)Shinoda, K.; Kunieda, H.; Arai,T.; Saijo, H. J. Phys. Chem. 1984, 88,5126. (4)Bellocq,A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G.; Lalanne, P.; Lemaire, B.; Lemanceau, B.; ROW, D. Adu. Colloid Interface Sci. 1984,20,167.
0743-746319112407-20481B02.50I, 0 I
,
clarify the role of branching upon the lipophilic strength of the surfactant and its ability to efficiently mix oil and water. Most of the branched surfactants previously examined in the literature fall into the double-tail category. By movement of the attachment point of the hydrophilic group incrementally along a linear hydrocarbon chain, a series of surfactant isomers of constant carbon number is created in which the surfactant evolves from single-tail (hydrophilic group a t the end of the hydrocarbon chain) to doubletails of unequal length (hydrophilic group between the end and the center of the hydrocarbon chain) to doubletails of equal length (hydrophilic group a t the center of the hydrocarbon chain). As the surfactant tails are made more equal in length, studies indicate that the Kraft point decreases: the critical micelle concentration increases! the surfactant becomes more effective a t reducing the airwater surface tension,7 and cyclohexane solubilization increases.8tg In microemulsion mixtures containing alcohol cosurfactants and salt, the amount of cosurfactant required to form a microemulsion is reduced and the surfactant preferentially partitions into the oil phase (a lipophilic shift) as the surfactant tails are made more equal in length.1° In microemulsion mixtures without added cosurfactant, surfactants with double-tails form microemulsions a t lower salinity (are more lipophilic) and more efficiently mix oil and water than single-tail surfactants.ll Only a few studies of methyl- and highly branched surfactants exist. Monolayers of linear surfactants a t the air-liquid interface become incompressible as the surface pressure is increased, but methyl-branched surfactants form compressible expanded monolayers.12 Measurements of interfacial tensions with the same series of surfactants examined in this paper indicate that the critical micelle concentration (cmc)increases upon branching (cmc (5)Valint, P. L.;Bock, J.; Kim, M-W.; Robbins, M. L.; Steyn, P.; Zushma, S. Colloids Surf. 1987,26, 191. (6)Graciaa, A.; Barakat, Y.;El-Emary, M.; Fortney, L.; Schechter,R. S.;Yiv, S.;Wade, W. H. J. Colloid Interface Sci. 1982,89,209. (7) Lascaux, M. P.; Dueart, 0.;Granet, R.; Pickarski, S. J.Chim. Phye. 1983,BO, 615. (8)Sagitani,H.; Suzuki,T.; Nagai,M.; Shinoda, K. J.Colloidhterface Sci. 1982,87,11. (9)Sagitani, H.;Shinoda, K. J. Phys. Chem. 1983,87,2018. (IO) Barakat, Y.;Fortney, L. N.; Schechter, R. S.; Wade, W. H.; Yiv, S. H.; Graciaa, A. J. Colloid Interface Sci. 1983,92,561. (11) Abe, M.; Schechter, D.; Schechter, R. S.; Wade, W. H.; Weerasooriya, U.;Yiv, S. J. Colloid Interface Sci. 1986,114, 342. (12) Matuo, H.; Cadenhead, D.A. Colloids Surf. 1989,41,287.
0 1991 American Chemical Society
Phase Behavior of Branched Surfactants
Langmuir, Vol. 7, No.10, 1991 2049
Table I. Surfactant Structures ranking: highly branched > doubletail > methyl branched > linear) 'and the surface tension at the cmc decreases ethoxylated starting alcohol =tegorY alcohol sulfate upon branching.ls-16 In this study, the phase diagrams of a series of branched linear dodecanol l-C& l-CiB isomers of dodecanol methyl branched 3b-C1& 3bCiB ethoxylated alcohol surfactants in oil/water mixtures and 2-butyl-1-octanol double tail (Guerbet) g-ClzEs p.ClB sodium sulfate surfactants in cosurfactant/oil/water/salt tridecanol linear l-cl&K 1-c1& mixtures are measured. Methyl-branched surfactants with 2-methyl-1-dodecanol methyl branched Ib-ClsS up to about three branch points are compared to doubleisomers of tridecanol methyl branched 4b-C& 4b-c& tail surfactants produced from Guerbet alcohols (branched 3-neopentyl-5,5highly branched hb-CisEs hb-ClsS dimethyl-1-hexanol at the second or ,&carbon) and highly branched surfaclinear l-cles tants synthesized from 3-neopentyl-5,5-dimethyl-l-hex- hexadecanol 2-hexyl-1-decanol double tail (Guerbet) p.Cl& anol or from methyl-branched Guerbet alcohol. All of the isomers of hexadechighly branched bg.CieS anol (methyl branched, branched surfactants are much more lipophilic than linear Guerbet) (single-tail) surfactants. The methyl- and highly branched mL of pyridine) at 0 "C. After being stirred for 24 h, the mixture surfactants mix equal amounts of oil and water less was poured into a 5% hydrochloric acid solution at 0 OC and efficiently than linear and Guerbet-branched surfactants. extracted with ether. The ether layer was washed with 10% The results are discussed within the framework of current sdium bicarbonate in water, washed with saturated sodium ideas on the patterns of phase behavior in amphiphile/ chloride solution, dried over magnesium sulfate, and evaporated oil/water mixtures. to yield 81 g of methanesulfonate ester. Step 2. Pentaethylene glycol (40 g) was added to 3.8 g of Experimental Section sodium metal in 300 mL of diglyme at 95 "C. After consumption of the sodium, 42 g of the methanesulfonate ester was added The oils and cosurfactants, octane, decane, dodecane, 2-budropwise, and the reaction mixture was heated to 120 OC and toxyethanol (CdEJ, and 1-butanol were 99% pure from Aldrich stirred for 24 h. After extraction with ether, the ether extract Chemical Co. Tetradecane (Sigma Chemical Co.) and toluene was washed with saturated sodium chloride and dried over (J.T. Baker Chemical Co.) were also 99% pure. The surfactants magnesium sulfate. The ether was evaporated, and the residue sodium dodecyl sulfate (I-ClB, Aldrich Chemical Co.) and sodium distilledat low pressureto yield 25gof ethoxylated alcohol (boiling hexadecyl sulfate (l-C& Lancaster Synthesis, La.) were repoint 188 "C at 0.02 mmHg). The structures of all of the ethoxcrystallized from ethanol. Pentaethylene glycol mono-n-dodeylated alcoholswere confirmed by lacand lH NMR, and a l l were cy1ether (l-C12Ea)was 98% pure from Nikko ChemicalCo. NaCl greater than 98% pure as determined by gas chromatography. was reagent grade from Fisher Scientific,and the water was deionThe sodium sulfates were prepared by the procedure outlined ized. below for the specific case of hb-C&. Fifteen grams of chloThe following starting alcohols were used as received for the rosulfonic acid (diluted with 50 mL of methylene chloride) was synthesis of linear and branched surfactants: tridecanol from added dropwise to 25 g of 3-neopentyl-5,5-dimethyl-l-hexanol Aldrich Chemical Co., Exxal-12 (isomers of dodecanol), Exxal(diluted with 250 mL of methylene chloride) at 0 "C. After 13(isomers of tridecanol), and Exxal-16 (isomersof hexadecanol, warming and stirring, the reaction mixture was neutralized to a methyl-branched Guerbet) from Exxon Chemical Co., 2-butylpH 8 by addition of sodium hydroxide in ethanol. After 1-octanol from Wiley Organics, and Eutanol G-16 (2-hexyl-ldecanol)from Henkel Corp. 3-Neopentyl-5,5-dimethyl-l-hexanol evaporation of the solvent,the surfactant (yield 22 g) was purified by recrystallization from ethanol/water (9/1). The other alkyl was distilled by P. Geissler,Exxon Chemical Co., to 98% isomeric sulfates were purified by various recrystallization or extraction purity, and 2-methyl-1-dodecanol was synthesized by E. Mozprocedures. The surfactant structures were confirmed by 19C eleski,Exxon Chemical Co. The chemical structure of the starting and lH NMR, and the purity of all sulfates was at least 98% and alcoholswas confirmed by NMR and mass spectrometry. Chlousually near 99.9% as determined by Hyamine titration.lB rosulfonic acid (distilled before use), methane sulfonyl chloride, sodium metal, and pentaethylene glycol were from Aldrich Results Chemical Co. The surfactants prepared from isoalcohols, Exxal-12, ExxalAt a fixed oil/water ratio, the phase diagram of ethox13,and Exxal-16,contain a distribution of branched isomers and ylated alcohol in oil and water mixtures as a function of a narrow distribution of carbon numbers. On average, the isomers temperature and concentration of surfactant resembles a of dodecanol contain 12 carbon atoms with about 3.4 branch fish (Figure l).17 The body of the fish outlines the region points and the isomers of tridecanol contain 13 carbon atoms of three-phase coexistence (3)between oil-rich,water-rich, with about 3.6 branch points, but each isoalcohol contains at and surfactant-rich (microemulsion) phases. A two-phase least five different types of methyl-branched isomers (80mol %) region surrounds the fish: below the fish a lower surfacand some ethyl-branched isomers (20 mol 5%) according to '*C NMR data. The isomers of hexadecanol is prepared by the Guertant- and water-rich phase (microemulsion) coexists with bet dimerization of methyl-branched octanols. The resulting an excess oil phase and above the fish an upper suralcohol is highly branched with a main branch at the &carbon factant- and oil-rich phase (microemulsion) coexists with and random methyl and ethyl branches along the two main tails. an excess water phase (2). The tail of the fish contains The structures and nomenclature of the pentaethoxylated one-phase microemulsions (l),and at higher surfactant alcohol and sodium sulfate surfactants are described in Table I. concentrations ordered lamellar liquid crystals (LC) are The prefix 1- denotes linear hydrocarbon tail surfactants, xbfound. An important feature of the phase diagram is the denotes methyl branched with a rounded-off average of x methyl point (X)where the body and tail of the fish meet.' The groups along the tail, g- denotes Guerbet-branched,hb- denotes temperature at point X (Tx)is a relative measure of the highly branched, and bg- denotes methyl-branched Guerbet. hydrophilic/lipophilic balance of the surfactant: a lower The pentaethoxylated alcohols were prepared by the procedure TX indicates a more lipophilic surfactant; a higher TX outlined below for the specific case of hb-ClsE5. Step 1. Methyl sulfonyl chloride (38 g) was added dropwise indicates a more hydrophilic surfactant if oil-type and to 60 g of 3-neopentyl-5,5-dimethyl-l-hexanol (diluted with 300 oil/water ratio are held constant. The concentration of surfactant at point X (Cx)is the minimum amount of (13) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. J. surfactant required to totally mix oil and water into a Phya. Chem. 1981,95,1671. one-phase microemulsion; CXis a measure of the efficiency (14) Varadaraj, R.; Bock, J.; Valint, P.; Brons, N. J. Colloid Interface
(z),
Sci. 1990, 140, 31.
(15) Varadaraj, R.; Schaffer, H.; Bock, J.; Valint, P. Langmuir 1990,
6, 1372.
(16) Reid, V. W.; Longman, G. F.; Heinerth, E. Tenaide 1967,4,292. (17) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986,90,671.
Wotmuth and Zushma
2050 Langmuir, Vol. 7, No. 10,1991 50
1
401 30
P
I 4 1 3E5 Tetradecane
Dodecane
-
4b-Cl3E5
Tx ("C)
20 hb-Cl3E5 10
(50150 OiliWater)
-
wt% S u r f a c t a n t
Figure 1. Schematic phase diagram of ethoxylated alcohol/ oil/water mixtures as a function of temperature and surfactant concentration at fixed oil/water ratio (50/50). TX and Cx are the coordinates of the point (X)at which the body and tail of the fish meet. I-Cl2E5
0
10
20
30
c x (Wt%)
Figure 3. Coordinates of point X (Tx, CX) as a function of oiltype and surfactant branching for C13 ethoxylated alcohols.
Teliadecane
0 0
3b-Cl2E5
Decane
hb
10
octane
-
0'4
3b
b
0
AT ("C)
.
g-Cl2E5
I
51
10 0
10
°
20
L 30
ol
- .
0
cx (Wt%)
10
20
30
c x (Wt%)
Figure 2. Coordinates of point X (Tx,CX)as a function of oiltype and surfactant branching for C12 ethoxylated alcohols (1-ClZEa data taken from Kahlweit et al., ref. 1).
Figure 4. Width of the three-phase region at l / 2 of Cx (AT)as a function of CXfor various linear and branched isomers of ClzEa ( 0 )and ClsEs (0) in tetradecane/water (50/50) (see Table I).
of the surfactant or its "amphiphilicity". For comparison of different surfactants, Kahlweit et al. show that measurement of the point X is necessary and often sufficient to characterize the phase diagram.' For linear and branched surfactants, Tx and CXincrease upon increasing the carbon number of the oil from Ce (octane) to Clr (tetradecane) (Figures 2 and 3). A strong lipophilic shift occurs as the tail of Cl3E5 or C12E5 is branched. The Tx values of 4b-Cl3E5 and hb-C13E5 are respectively 14 OC and 26 "C lower than the Tx values of l-C13E5 (Figure 3). In addition, the Tx values of 3b-C12E5 and g-CI2E5 are also about 12 "C and 35 OC lower than those of l-C12E5 (Figure 2). Most of the branched surfactants mix oil and water less efficiently than linear surfactants since the CX values of 4b-ClaEb and hb-ClsE5 are more than twice as high as those of l-ClaE5 (a similar trend is found upon comparison of 3b-C12E5 with l-C12E5). The Guerbet-branched surfactant is almost as efficient as the linear surfactant since the CX values of g-C12E5 are only about 1 wt 5% greater than those of l-Cl2E5. The phase diagrams were more closely examined by measuring the temperature range of the three-phase region a t '/2 of Cx (AT) for each of the surfactants in tetradecane and water (Figure 4). As the surfactants are branched,
the body of the fish grows on both the temperature and concentration axes, and ATscales linearly with Cx. Singleisomer surfactants exhibit a fish phase diagram roughly symmetric around Tx (Figure l).17 Although the surfactants 3b-C12Eb and 4b-C13E5 contain a distribution of branched isomers, the isomers appear to mix ideally since the phase diagrams are also symmetric around Tx. The sodium sulfate head group (S)is much more hydrophilic than the E5 head group. For C12S and C13S surfactants, short-chain alcohol cosurfactants and salt must be added to induce a large enough lipophilic shift to bring the fish pattern to room temperatures. Since adding lyotropic salt to sulfate surfactant mixtures has the same effect as increasing the temperature of ethoxylated alcohol mixtures, a fish (often skewed) may be found if the phase diagram is mapped as salinity as a function of surfactant and cosurfactant concentration (at constant temperature, surfactant/cosurfactant ratio and oil/water ratio).'* To simplify comparison of the branched and linear sulfates, the salinity range of the body of the fish (three-phase region) was measured a t a fixed concentration of surfactant plus cosurfactant (IO w t % CiS/butanol (112 by -
~~
(18)Kahlweit, M.; Strey, R.;Haase, D.J . Phys. Chem. 1985,89,163.
Langmuir, Vol. 7, No. 10, 1991 2061
Phase Behavior of Branched Surfactants 4 OI.Cl2S
3
I
40 OI.Cl3S
Tx ("C) 30 0 Ib-CigS
I
20
0 3b-Cl2S
i
I
0 0
-1
I
0
1
0
in
04b-Cl3S
Ohb-Cl3S
I
20 6
7
0
lo-
1
.e
.
c x (wt%)
5
0
-
0 0
Cm ( ~ 1 % )
Figure 5. Salinity at midpoint of three-phase region (Sm) as a function of the concentration of surfactant in the middle microemulsion phase (C,) for various linear and branched isomers [ l O w t % Cis butanol (1/2 by weight)] of Cl&3( 0 )and C& (0) in toluene/water (1/1 by weight) at = 25 O C .
4
weight) in toluene/water (1/1by weight) a t T = 25 OC). The salinity a t the midpoint of the three-phase region (Sm)is qualitatively analogous to Tx:a lower Smindicates a more lipophilic Surfactant. S m is the optimal salinity for enhanced oil recovery applications.1° The mixing efficiency (Cm) is estimated by assuming that all of the sulfate surfactant resides in the microemulsion phase, measuring the volume of the microemulsion phase, and calculating the concentration of sulfate surfactant a t Sm. The strong lipophilic shift found with branched ethoxylated alcohols in three-component mixtures also occurs with branched sulfate surfactants in five-component mixtures (Figure 5). Lipophilicity increases (Smdecreases) and the efficiencyof the surfactant decreases (Cm increases) for the series of surfactants I-CUS lb-Cl3S 4b-C13S hb-C&, the same trend as found above for the surfactants. A similar trend is also found upon comparison of 3b-C& with 1-ClZS. The Guerbet branched g-ClzS is somewhat less efficient but more lipophilic than l-ClzS, the same trend as found for the nonionic surfactants. Indeed, the phase behavior of branched sulfates and ethoxylates is correlated: Tx and S, as well as Cx and C m scale linearly (Figure 6). Upon addition of salt, the longer tail Guerbet surfactant (g-cl&) forms microemulsions between 0 and 100 "C with less than 3 wt 96 surfactant.19 At fixed salinity, the fish is inverted (2 is above the body of the fish and 2 is below the body-of the fish).2 Since the temperature dependence of sulfates is opposite that of ethoxylated alcohols, a lower TXindicates that the surfactant is more hydrophilic, and a higher TX indicates the surfactant is more lipophilic. TXand CX measured for g-c16s in decane and 2 wt 96 NaCl in water compare favorably with previous measurements (Figure 7L2 If the Guerbetbranched g-C& is further branched with methyl branches to give a highly branched surfactant (bg-C&), lipophilicity increases slightly (TXrises), but the mixing efficiency is significantly reduced (Cxis higher for bg-C1& than for g-clss, Figure 7). Since l-C16S is too hydrophilic to form microemulsions between 0 and 100 OC, direct comparison of Guerbetbranched sulfates with linear sulfates requires the addition
-
"
I
5
8
Cm (wt%)
Figure 6. Correlation of the phase behavior of linear and branched isomers of ClPES ( 0 )and ClaEa ( 0 )in octane/water C,) with phase behavior of linear and branched mixtures (Tx, isomers of C& ( 0 )and ClSS ( 0 )in butanol/toluene/water/ NaCl mixtures (S,, C,).
I
t
- -
(19)Shinoda, K.; Shibata, Y. Colloids Surf. 1986, 19, 185.
7
6
0
I-Cl6S
(wt% NaCI) 2 1 1
-
Og-ClGS
0 bg-Cl6S
0 6
7 0 Cm (wt%)
9
Figure 7. (Top) Coordinates of point X (Tx, CX)for Guerbet (g-Cl&) and methyl-branched Guerbet (bg-C+),surfactants in decane and 2 wt 5% NaCl in water. (Bottom)Salinity at midpoint of three-phase region (S,) as a function of the concentration of surfactant in the middle microemulsion phase (C,) for linear, Guerbet, and methyl-branched Guerbet surfactants [lo wt 7% Cl&/CIEI (1/2 by weight)] in octane/water (1/1by weight) at T = 25 O C . of cosurfactant and salt. The same technique described above for the ClzS and C13S surfactants was used to measure S m and C,, except C4E1 and octane were substituted for butanol and toluene to adjust the salinity range. S m drops sharply upon Guerbet branching, the same strong lipophilic shift found above with g-C12Es (Figure 7). However, the Guerbet-branched surfactant g-C& is more efficient than l-Cl&. The relationship between g- and bg-C& remains similar to that found without cosurfactant, and within error b g - c u s and g-ClSs are of similar lipophilicity (Figure 7).
2052 Langmuir, Vol. 7, No. 10, 1991
Discussion The phase behavior of surfactant/oil/water mixtures depends intimately upon the molecular structure of the hydrocarbon tail of the surfactant. Branched surfactants are more lipophilic than linear (single-tail) surfactants since microemulsions form a t lower temperatures and salinities. In almost all cases observed here, branched surfactants mix equal amounts of oil and water less efficiently than linear surfactants. The patterns of phase behavior fall within the phenomenological framework proposed by Kahlweit et a1.lI2 The phase behavior of ternary amphiphile/oil/water mixtures is linked to the phase behavior of the binary amphiphile/water, water/oil, and oil/amphiphile mixtures according to extensive experimental results.' In water, ethoxylated alcohols phase separate upon heating and the phase diagram exhibits an upper miscibility gap with a lower critical-point a t the temperature T,. In oil, ethoxylated alcohols and oil become more miscible upon heating and a lower miscibility gap with an upper critical point a t the temperature TOis found (TOis often below the freezing point). The chemical structures of the amphiphile and oil set T , and TOand drive the phase behavior in ternary ethoxylated alcohol/oil/water mixtures. Upon increase of the chain length of the hydrocarbon tail or decrease of the moles of ethylene oxide of unbranched ethoxylated alcohols, a lipophilic shift occurs and the decrease of T , and TOcorrelates with the decrease of T X in the ternary mixtures.20 When the tail of ethoxylated alcohols is branched, a similar lipophilic shift occurs and T , and T X fall proportionally. Relative to 1412E5, T , of 3b-ClzE5 falls32 "C, and T,of g-ClzE5is much below0 "C according to our measurements. In ternary mixtures relative to 1412E5, TXof 3b-ClnEa falls 12 OC, and Tx of g-ClzE5 falls 35 "C (Figure 2 ) . TOlies below 0 "C for all of the surfactants studied here. The temperature difference between T , and TO( T , TO)sets the size of the three-phase region. If T , - TOis relatively large, the critical points are connected through the ternary phase diagram by a continuous critical line and a three-phase region is not found. As T, - To is decreased, the critical line breaks a t a tricritical point and a three-phase region appears. Further reduction of T , TOincreases the size of the three-phase region (body of the fish) on the temperature scale, and the mixture evolves from the tricritical point according to the scaling law17921
-
AT [Tx - T,I3" (1) where AT is the maximum temperature range of the threephase region and T, is the tricritical point. A shift toward the tricritical point occurs upon increasing the chain length of the hydrocarbon tail or decreasing the moles of ethylene oxide on unbranched ethoxylated alcohols: the body of the fish shrinks as AT and C X d e c r e a ~ e . ' ~Branching -~~ the hydrocarbon tail has the opposite effect and the body of the fish grows (C,and AT increase, Figure 4), and so an "equivalent linear chain length" cannot be assigned to branched surfactants. All of the phase diagrams measured here are a t 50/50 oil/ water ratio. The lipophilic rank of the surfactants will not depend upon oil/water ratio, but the efficiency rank could conceivably change a t high or low oil/water ratio. No correlation is found between the number of branch points within the molecules and the phase behavior. The (20) Kahlweit, M.; Strey, R.; Haase, D.; Firman, P. Langmuir 1988,4, 785.
(21)Griffiths, R. B. J. Chem. Phys. 1974, 60, 195.
Wormuth and Zushma relationship between Tx, AT, and the mixing efficiency ( C x ) of g-ClzE5 is significantly different than that of the other branched surfactants (Figures 2 and 4), so the change in chemical potential and the path taken away from the tricritical point depend upon the type of branching. Since sodium sulfate surfactants are hydrotropic salts which increase the miscibility of alcohol and water, and NaCl is a lyotropic salt which has the opposite effect, sulfates and NaCl compete to produce the phase behavior of sulfate/alcohol/oil/water/NaCl mixtures.18*22*23 Branched sulfate surfactants are weaker hydrotropic salts than linear surfactants since less NaCl is required to balance the phase behavior (Figure 5). As expected, the lipophilic rank of the branched surfactants does not change upon substitution of the head group (Figure 6). Remarkably, the phase behavior of the five-component mixtures linearly correlates with that of the three-component mixtures even though five-component mixtures require more parameters for complete description.2 The complexity of five-component mixtures may partially explain the different results found with cl&surfactants. As expected from the trends with ClzS and C13S surfactants, the branched c1&are much more lipophilic than l-C&, and the methyl-branched analogue of g-cl& (bg-C&) is much less efficient than g-Cl6s (Figure 7). However, l-C& is less efficient than &Cl& or bg-&S. Increasing salinity does move the phase diagram away from the tricritical point and reduce efficiency (Cx increases)23and the partitioning of cosurfactants may also depend upon salinity and SO change C m . The trends in phase behavior are a macroscopic manifestation of the specific microscopic interactions between oil, water, and surfactant. One approach to describing these interactions focuses upon the energetics of the surfactant-rich monolayer which separates the oil and water microdomains within the microemulsion.24 In the model, the spontaneous curvature and the elastic curvature energy of the surfactant monolayer are concepts used to calculate a fishlike phase diagram.26 According to simple geometric packing arguments,%% branched surfactants should spontaneously curve the monolayer toward water and prefer water-in-oil droplet microstructures. Since Tx and E," are lower for branched surfactants than linear surfactants, branched surfactants do prefer water-in-oil microemulsions. Since branched surfactants generally mix equal amounts of oil and water less efficiently than linear surfactants, perhaps branched surfactants pack less efficiently within the monolayer and produce more flexible monolayers. Quantitative prediction of the patterns of phase behavior observed upon changing the molecular structure of the surfactant remains elusive, but the results obtained here can guide the process of choosing the surfactant most effective a t mixing oil and water into a microemulsion.
Conclusions Examination of the phase behavior of 16different linear and branched surfactants in oil and water reveals the (22) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir 1985,1,718. (23) Kahlweit, M.; Strey, R. J. Phys. Chem. 1986,90,5239. (24) Andelman, D.; Cates, M. E.; Roux, D.; Safran, S. A. J. Chem. Phys. 1987,87, 7229. (25) Cates, M. E.; Andelman, D.; Safran, S. A.; Roux, D. Langmuir 1988, 4, 802. (26) Tanford, C. The Hydrophobic Effect; J. Wiley and Sone: New York. 1973. (27) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (28) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601.
Langmuir, Vol. 7, No. 10, 1991 2053
Phase Behavior of Branched Surfactants important patterns of phase behavior that result upon branching the hydrocarbon tail of the surfactant. All of the branched surfactants are more lipophilic (form microemulsions at lower temperatures and salinities) than linear (single-tail) surfactants, but lipophilicity depends intimately upon the amount and type of branching. In most cases, branched Surfactantsare less efficient a t mixing equal amounts of oil and water, which indicates that
branched surfactant/oil/water mixtures are further from the tricritical point than linear surfactant/oil/water mixtures.
Acknowledgment. We thank R. V. Kastrup for the NMR studies and R.Varadaraj and J. Bock for comments and support.
