Environ. Sci. Technol. 1996, 30, 2143-2147
Water/Chlorocarbon Winsor I S III S II Microemulsion Phase Behavior with Alkyl Glucamide Surfactants JIMMIE R. BARAN, JR.,† GARY A. POPE,‡ W I L L I A M H . W A D E , * ,† A N D VINITHA WEERASOORIYA† Department of Chemistry and Department of Petroleum Engineering, The University of Texas, Austin, Texas 78712
The investigation of the microemulsification of chlorocarbons that contaminate aquifers is addressed in this study. The surfactants employed are alkyl glucamidessglucamine coupled with either n-heptanoic, n-nonanoic, or n-dodecanoic acids as well as the Guerbet version of the dodecanoic acid. The chlorinated hydrocarbons studied include tetrachloroethylene (PCE), carbon tetrachloride, 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), 1,2-dichlorobenzene (DCB), 1,2-dichloroethane (DCE), chloroform, methylene chloride, and 1,1,2,2-tetrachloroethane. It was possible to generate classical Winsor I S III S II phase behavior with nearly all of the above chlorocarbons.
Introduction Recently, our laboratory has applied the knowledge gained about surfactants through enhanced oil recovery (EOR) to surfactant-enhanced aquifer remediation (SEAR) (1-5). Chlorocarbons are major contaminants of aquifers. These chlorinated hydrocarbons or DNAPLs (dense non-aqueous phase liquids) have presented unique problems in applying surfactant technology to SEAR. Since DNAPLs are somewhat more polar than the hydrocarbons studied in EOR, the surfactants employed must be more hydrophilic than those used in EOR to the extent that there are few commercially available candidates. Also, systems must exhibit classical Winsor phase behavior at temperatures of 25 °C or lower, which requires careful structuring of the surfactant’s hydrophobe. Often all types of Winsor phase behavior can be obtained by changing the electrolyte concentration. This is known as a salinity scan (2). At low salinity, an aqueous phase Winsor Type I (o/w) microemulsion is formed in the presence of excess oil. As the salinity is increased, one enters a Winsor Type III system in which a middle phase * Corresponding author Fax: (512) 471-8696; e-mail address:
[email protected]. † Department of Chemistry. ‡ Department of Petroleum Engineering.
S0013-936X(95)00514-1 CCC: $12.00
1996 American Chemical Society
microemulsion containing both oil and water coexists with excess oil and aqueous phases. Further increases in salinity lead to an oil phase Winsor Type II (w/o) microemulsion being formed in the presence of excess water. As opposed to the previously studied anionic surfactants, the surfactants employed here will generate Winsor phase behavior without any added electrolyte. Surfactant mixtures are identified that will form the same Types I, II, and III mentioned above with no electrolyte. For mobilization, it is desirable for the system to be in the Winsor III region. Winsor III systems are the regimes where one finds low interfacial tensionssthe fundamental requirement for mobilization (6, 7). For solubilization of chlorocarbons without mobilization, a Type I system is required. If a Winsor II system is accidentally produced, the oil in effect becomes an infinite sink for the surfactant with obvious disasterous consequences. In the Winsor III regime, at a critical salinity, the amounts of oil and water solubilized in the middle phase are equal. This is defined as an optimum system. The salinity at which this optimum system occurs is known as the optimum salinity, S*, expressed in units of wt % NaCl. The solubilization parameter, σ, is defined as the volume (in mL) of water or chlorocarbon solubilized per gram of surfactant. The optimum solubilization parameter, σ*, is therefore half the middle phase volume (since by definition equal volumes of water and oil are solubilized) per unit weight of surfactant at optimum salinity. The salinity window, ∆S, is the width of the middle phase region, in units of wt % NaCl. We have succeeded in finding anionic surfactants that produced classical Winsor I S III S II phase behavior with tetrachloroethylene, carbon tetrachloride, trichloroethylene, and 1,1,1-trichloroethane (1-3). Unfortunately, we had identified other commonly used chlorocarbons that failed to produce Winsor III phase behavior with these surfactants and instead produced only Winsor I S II transitions (3, 5). With the inability of anionic surfactants to successfully produce Winsor III phase behavior with all of the chlorocarbons we were interested in, we shifted the focus of our research to nonionic surfactants. Our initial studies were conducted with the Tween series of surfactants (8). In general, we found it necessary to have Tween 21 (sorbitan monolaurate with 4 mol of ethylene oxide) as the major component of the surfactant system in order to observe Winsor-type phase behavior, and even so, the Tweens were only successful in forming microemulsions with PCE and CCl4. These systems generally produced small solubilization at rather high salinities. For example, when Tween 21 was used with PCE, S* ) 8.2 wt % NaCl, with a σ* ) 2.8 mL/g and ∆S ) 8.55 wt % NaCl. These characteristics are actually worse than those obtained with the anionic surfactant, MA-100, a dihexyl sodium sulfosuccinate (13). But once again, the major problem was the limited number of DNAPLs that produced classical phase behavior with these surfactants. Our attention was then drawn to the recent patent literature, which taught the production of glucamine based surfactants (9) whose structures are
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CH3 O CH2 H HO H H
N
C
CH3 O R
OH H OH OH CH2OH
linear alkyl glucamide (R = C6–C9 and C11)
CH2 H HO H H
N
OH
C
C4H9 CH C6H13
H OH OH CH2OH branched alkyl glucamide
Various combinations of these surfactants succeeded in producing classical phase behavior at room temperature with all the chlorinated hydrocarbons listed in the following section as well as producing this behavior at lowsor zerossalinities. Optimum salinities (S*), optimum solubilization parameters (σ*), and salinity windows (∆S) for a number of these systems are reported.
Experimental Section Chemicals. The aqueous phase as constituted contained NaCl in distilled H2O adjusted to a variety of salinities in weight percent (wt %) units. DNAPLs used in the studies were C2Cl4 (PCE), 1,2-C6H4Cl2 (DCB), and 1,1,1-C2H2Cl3 (TCA) (Aldrich); CH2Cl2, CHCl3, and CCl4 (EM Science); TCE (Fisher); 1,2- C2H4Cl2 (DCE) (MCB Manufacturing); and 1,1,2,2-C2H2Cl4 (Eastman) and were obtained as reagent grade and used without further purification. The alkyl glucamide surfactants syntheses have been established previously (10). The notation used to label these surfactants has the A referring to the surfactant being a glucamine followed by a number, which represents the number of carbons in the acid that is combined with the glucamine to prepare the glucamide surfactant. Therefore, the abbreviation A9 represents N-methyl-D-glucanonanamide, while A12 represents N-methyl-D-glucadodecanamide. A12G denotes the Guerbet species. Formulation of Equilibrium Microemulsion Systems. The surfactants and NaCl were usually predissolved in the aqueous phase, and all systems, as initially constituted, contained equal volumes of aqueous and DNAPL phases. When systems required salinities above 12.5% NaCl, the surfactants were predissolved in the oil phase on a weight/ volume ratio to give the desired final concentration. All systems were shaken multiple times, and sufficient time was allowed for the initially formed unstable macroemulsions to decay to thermodynamically stable microemulsion systems. Depending upon the system in question, these times varied from overnight to several days. Equilibration was done at 25 °C unless otherwise noted. Most surfactants employed were mixtures of two species, and the total surfactant concentration was constant at 2 wt % of the aqueous phase, unless otherwise noted. Reproducibility of solubilization parameters is limited by one’s ability to read phase volumes, (0.01 mL. This leads to uncertainties varying from 2 to 10% depending on the magnitude of σ or σ*.
Results and Discussion The alkyl glucamide surfactants produced a broad range of microemulsion types with the chlorocarbons depending on the surfactant system composition. In but one case for all systems studied, it was necessary to make mixtures of surfactants, since one must adjust the partitioning characteristics between water and each DNAPL (11, 12) of the
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TABLE 1
Surfactant Compositions for Winsor Type I, II, and III Microemulsions Formed from a Variety of Chlorocarbons chlorocarbon PCE EACN ) 2.90 (3) CCl4 EACN ) -0.06 (3) TCA EACN ) -2.49 (5) TCE EACN ) -3.81 (3) 1,2-DCB EACN ) -4.89 (3) 1,2-DCE EACN ) -12.10 (3) CHCl3 EACN ) -13.67 (3) CH2Cl2 EACN ) -13.79 (3) 1,1,2,2-C2H2Cl4 EACN ) -22.15 (3)
Type I, 0% NaCl A12G/A9 (60/40) A12G/A9 (42/58) A12G/A9 (34/66) A12G/A9 (25/75) A12G/A9 (27/73) A12G/A9 (20/80) A9/A7 (20/80) 100% A9
Type III, 0% NaCl A12G/A9 (70/30) A12G/A9 (50/50) A12G/A9 (40/60) A12G/A9 (30/70) A12G/A9 (40/60) A12G/A9 (50/50) A9/A7 (30/70)
Type II, 0% NaCl 100% A12G A12G/A9 (80/20) A12G/A9 (60/40) A12G/A9 (40/60) A12G/A9 (50/50) A12G/A9 (70/30) A12G/A9 (60/40)
A9/A7 A9/A7 A9/A7 (10/90) (8%) (20/80) (8%) (50/50) (8%)
surfactant systems so as to be compatible with the Winsor system desired. Table 1 lists the surfactant ratios necessary to obtain Types I, II, and III systems for each DNAPL. The compositions listed for the Type I system at 0% NaCl also produced a Type I system at 0.10% NaCl but a Type III system at 0.50% NaCl (10). Thus these ratios produce Type I systems near the I/III boundary. The formulation ratios in Table 1 for the Type III systems at 0% NaCl are well removed from the I/III phase boundary but contain more aqueous phase than oil phase and are compositions for mobilization applications. Finally, the Type II system compositions shown for each DNAPL produce systems that are close to the III/II phase boundary. In most of the above studies, surfactant mixtures were varied in 10% increments. To use PCE as an example, at surfactant ratios of A12G/A9 from 0/100 to 60/40, a Type I system exists at 0% NaCl. With ratios of 70/30 to 90/10, Type III systems exist at 0% NaCl. And finally, at a ratio of 100/0, a Type II system exists at 0% NaCl. For all the system compositions shown in Table 1, we subsequently gradually added electrolyte until we obtained optimum formulation (equal volumes of aqueous and DNAPL incorporated in the middle phase) for the Type I and Type III systems. As noted earlier, the surfactant compositions at 0.5% salinity were just inside the threephase regime. Therefore, optimum salinities for the Type III salinities are always greater than 0.5% NaCl. Obviously, the Type I systems uniformly have greater optimum salinities since they start out further removed from the optimum state. It also is impossible to determine optimum values for the phase behavior characteristics resulting from the Type II surfactant mixtures. In order to obtain optimum values, one would require “negative salinities”, and since this is not physically possible, we must settle for nonoptimal solubilization parameters. In looking at values of σ*, one notes values between 1.2 and 3.75 mL/g. Moreover, one notes salinity windows ranging from 12.5 to 24.5 wt.% NaCl. These values are comparable to those reported earlier using the anionic dihexylsulfosuccinates (1-5). The common thread between
these different surfactants is the shortness of the hydrophobe tails required for obtaining appropriate Winsor phase behavior. 1,2-DCB demonstrated nonclassical phase behavior with the glucamides in attempting to obtain data for Type I systems. Using the mixtures described above for the respective chlorocarbons, it was found that, at salinities less than optimum but in the Type III regime, liquid crystals formed in addition to what appeared to be an otherwise normal middle phase. Liquid crystal formation began above 9% NaCl. So although complex phase behavior was found in this system, it should not interfere with the ability to recover 1,2-DCB from aquifers, since the complex behavior becomes a problem at salinities much higher that those found in typical groundwater. DCE on the other hand presented a problem for finding surfactant ratios to satisfy both columns 1 and 2. The column 1 ratio in Table 1 exhibited the same type of phase behavior as DCB. The formation of liquid crystals began at 5% NaCl, so once again the recovery of 1,2-DCE from aquifers should not be a problem. But, in attempting to find a ratio to satisfy column 2, a very complex system was detected. Surfactant ratios of 30/70 and 40/60 (A12G/A9, respectively) produced a middle phase consisting entirely of liquid crystals. Ratios of 45/55 to 47/53 (A12G/A9) produced what appeared to be a third phase (no liquid crystal formation), but while the middle phase/oil phase interface remained fluid, the middle phase/water phase interface did not. Ratios of 48/52 and 49/51 (A12G/A9) exhibited this same characteristic, but also contained another middle phase that existed entirely in the oil phase. Dichloromethane also exhibited complex phase behavior. Using pure A9 as the surfactant, dichloromethane produced a thin layer of liquid crystals at the top of the aqueous phase microemulsion at 0% and 0.1% NaCl. Above these salinities, classical phase behavior was exhibited (no liquid crystal formation was observed). As for trying to find data to fit the other two columns in the tables, this proved impossible. Increasing the lipophilicity of the surfactant mixture, by adding a longer chained surfactant to the A9, only led to liquid crystals being prevalent throughout the middle phase region. The middle phase/chlorocarbon boundary remained fluid, while the middle phase/aqueous boundary appeared to gel. At this time, there is no apparent explanation for these observations. We have chosen to select a salinity and vary the surfactant ratio scan to produce classical Winsor-type phase behavior. It is the method for identifying the surfactant ratio necessary to give an optimum system at a given salinity. For demonstration purposes, we chose 1,1,2,2-C2H2Cl4 as the oil phase and chose to study the phase behavior at 0% NaCl. From the data in Table 1, A9 and A7 were the surfactants of choice. 1,1,2,2-C2H2Cl4 was chosen because it was one of the previously studied DNAPLs that did not exhibit classical phase behavior (3). In addition, the chosen surfactant system produced fairly rapid equilibration times. The 0% NaCl requirement was selected because it represents the ultimate successful application of these surfactants in terms of minimal perterbation of an aquifer. Figure 1 presents these data graphically. At A9 mole fraction values of approximately 0.15 and below a Winsor Type I aqueous phase, microemulsion is produced. In the mole fraction range of approximately 0.15-0.45 is a Winsor Type III middle phase microemulsion system. Above a mole fraction value of approximately 0.45, a Winsor Type II oil
FIGURE 1. Solubilization parameters for oil/water, oil/middle phase, water/middle phase, and water/oil microemulsions vs mole fraction of A9.
FIGURE 2. Solubilization parameters for oil/water, oil/middle phase, water/middle phase, and water/oil microemulsions vs salinity for CHCl3 with A9/A7 (20/80) at 25 °C.
phase microemulsion system is produced. The two curves intersect at a mole fraction value of approximately 0.25, which represents the ratio of the two surfactants needed to produce an optimum system at 0% NaCl and to give the optimum solubilization parameter (σ* ) 1.20 mL/g) for this system. For comparison, a more conventional plot of solubilization parameter versus salinity is shown in Figure 2. For demonstration purposes, we chose CHCl3 as the oil phase. A9 and A7 were the surfactants of choice, and the ratio of the two is given in Table 1. The salinity ranged studied was from 0% to 25% NaCl. There is an inherent difference between a surfactant ratio scan and a salinity scan. One will notice that the solubilization curve for the water/oil or water/middle phase values actually shows an increase after obtaining the optimum value for the surfactant ratio scan (see Figure 1). It has been demonstrated previously that increasing the length of the hydrophobe leads to an increase in the solubilization parameter (13-15). This is exactly what one sees in Figure 1. As the amount of A9 increases in the surfactant system, the average hydrophobe chain is becoming longer, thus leading to a larger solubilization parameter. On the other hand, the salinity scan in Figure 2 shows a steady decrease with increased salinity for this particular
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TABLE 2
Optimum Salinity, Optimum Solubilization Parameter, and Salinity Window for Surfactant Compositions Shown in Table 1a chlorocarbon
Type I
Type III
Type II
PCE CCl4 TCA TCE DCB DCE CHCl3 CH2Cl2 1,1,2,2-TCE
S* ) 4.8%, σ* ) 1.25 mL/g, ∆S ) 24.5+% S* ) 13.5%, σ* ) 3.0 mL/g, ∆S ) 24.5+% S* ) 15.3%, σ* ) 3.6 mL/g, ∆S ) 24.5+% S* ) 9.6%, σ* ) 3.75 mL/g, ∆S ) 24.5+% + + S* ) 1.5%, σ* ) 1.25 mL/g, ∆S ) 21.5% S* ) 1.0%, σ* ) 2.75 mL/g, ∆S ) 24.5+% S* )12.5%, σ* ) 1.675 mL/g, ∆S ) 21.5%
S* ) 0.75%, σ* ) 1.20 mL/g, ∆S ) 25.0+% S* ) 1.2%, σ* ) 3.25 mL/g, ∆S ) 18.5% S* ) 2.5%, σ* ) 2.75 mL/g, ∆S ) 25.0+% S* ) 0.7%, σ* ) 3.40 mL/g, ∆S ) 12.5% S* ) 2.30%, σ* ) 2.15 mL/g, ∆S ) 19.5% + S* ) 1.2%, σ* ) 2.25 mL/g, ∆S ) 18.5% + S* ) 7.1%, σ* ) 1.375 mL/g, ∆S ) 15.5%
σ ) 0.25 mL/g σ ) 1.00 mL/g σ ) 0.25 mL/g σ ) 1.50 mL/g σ ) 0.50 mL/g σ ) 0.25 mL/g σ ) 1.50 mL/g + σ ) 1.375 mL/g
a
+, see text.
curve after obtaining the optimum solubilization value. It has been previously shown for salinity scans that the solubilization parameter actually decreases as the salinity increases (14). Surfactant HLB Values. Systems employing nonionic surfactants use the HLB (hydrophilic-lipophilic balance) scale to predict phase behavior (11). This scale requires the HLB of a surfactant to match the HLB requirement of the oil phase. A surfactant’s HLB is generally determined by comparing its performance with other nonionic surfactants having a head group composed of ethylene oxide (EO) units for which the HLB is defined as (12)
HLB )
wt % EO 5
(1)
This allows surfactant behavior to be tabulated and used accordingly. The HLB of a mixture of surfactants is determined on a mole fraction basis (16, 17):
HLBmix )
∑X *HLB * i
i
(2)
The effect of salinity on the phase behavior of nonionic surfactants is much less than that with anionic surfactants, as demonstrated by the large Winsor III ranges found here as well as in earlier results (16). Thus, as a good approximation, the salinity term in eq 4 can be omitted, and eq 4 reduces to
HLBmix ) 11.0-0.115(ACN)
(5)
or for chlorocarbons
HLBmix ) 11.0-0.115(EACN)
(6)
with the previously determined constants. Only CH2Cl2 required a single surfactant to produce an optimum system (see column 1 of Tables 1 and 2). Using the previously determined EACN for CH2Cl2 (see Table 1), (3, 5), one finds that the HLB for A9 is 12.6. The HLBs of the mixes in column 2 of Table 1 were determined by eq 5. Once the HLBmix was known, eq 3 was used to determined the HLBs of A12G and A7. The values were averaged for each surfactant (five for A12G and 2 for A7). These are
i
or for a two-component system:
HLBmix ) X1HLB1 + X2HLB2
(3)
Previous EOR work resulted in an equation showing a linear relationship between HLBmix and each of the following variables: alkane carbon number of the oil phase (ACN) [If the oil phase is not an alkane, then one uses the equivalent alkane carbon number (EACN), which is often negative for chlorocarbons. The previously determined EACN values (3) are given in Table 1.], temperature (T) in Celsius, alcohol concentration [A], and salt concentration [S], both in wt %:
HLBmix ) a - k(ACN) + f[A] + b[S] + c(T - 28 °C) (4) where a, k, f, b, and c are constants whose values differ depending on whether one is interested in the I/III boundary, the optimal system, or the III/II boundary (16). Since there is no alcohol in our systems, the f[A] term is zero. Previous results with four different nonionic surfactants have shown a and k to be independent of surfactant type as long as one is interested in only optimal phase behavior (which we are) (18). c was determined to be quite small, and since all systems were prepared and studied at 25 °C, this term is essentially negligible and is eliminated.
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surfactant
HLB
A7 A9 A12G
13.2 ( 0.6 12.6 9.3 ( 0.4
Qualitatively this is what one would expect. The shorter the hydrophobe, the larger the HLB. Previous results in a series of straight-chained hydrophobes have shown that for every additional carbon added to the chain, one expects the HLB to decrease by approximately 0.475 unit (18). The value here is 0.3 ( 0.3 unit per carbon. If the A12G surfactant was a straight-chained species, one would expect it to have a HLB value of approximately 11.5 (based on 0.3 HLB unit per carbon). But, previous results have shown that going from a straight chain to branching at the fourth carbon, results in the HLB being lowered by another 2 units (19). Therefore, the A12G is expected to have an HLB of approximately 9.5, which is within the experimental error of the obtained value. Temperature Dependence. Since most aquifer temperatures are below 25 °C, it is important to determine the phase behavior of these surfactants at lower temperatures. The TCE system with the Type I surfactant composition given in Table 1 was subjected to a salinity scan at a temperature of 16 °C. The entire system equilibrated overnight and produced an optimum salinity of S* ) 9.75 wt % NaCl and an optimum solubilization parameter of σ*
) 3.7 mL/g as well as a salinity window of ∆S ) 24.5+ wt % NaCl. When compared to the values in Table 2, one sees that these surfactants are basically temperature insensitive over the temperature range of interest.
Acknowledgments The authors wish to thank the State of Texas’ Advanced Technology Program for Grant 379 and Condea, Inc. for chemical samples.
Literature Cited (1) Baran, J. R., Jr.; Pope, G. A.; Wade, W. H.; Weerasooriya, V. Phase Behavior of Water/Perchloroethylene/Anionic Surfactant Systems. Langmuir 1994, 10, 1146. (2) Baran, J. R., Jr.; Pope, G. A.; Wade, W. H.; Weerasooriya, V.; Yapa, A. Microemulsion Formation with Chlorinated Hydrocarbons of Differing Polarity. Environ. Sci. Technol. 1994, 28, 1361. (3) Baran, J. R., Jr.; Pope, G. A.; Wade, W. H.; Weerasooriya, V.; Yapa, A. Microemulsion Formation with Mixed Chlorinated Hydrocarbon Liquids. J. Colloid Interface Sci. 1994, 168, 67. (4) Baran, J. R., Jr.; Pope, G. A.; Schultz, C.; Wade, W. H.; Weerasooriya, V. Mixed Surfactant Systems for Microemulsion Formation with Chlorinated Hydrocarbons; American Academy of Environmental Engineers: Washington, DC, in press. (5) Baran, J. R., Jr.; Pope, G. A.; Schultz, C.; Wade, W. H.; Weerasooriya, V.; Yapa, A. Toxic Spill Remediation of Chlorinated Hydrocarbons via Microemulsion Formation. In Proceedings of the 10th International Symposium on Surfactants in Solution, Caracas, Venezuela, June 1994; Marcel Dekker, Inc.: New York, 1996 (in press). (6) Huh, C. Interfacial Tensions and Solubilizing Ability of a Microemulsion Phase that Coexists with Oil and Brine. J. Colloid Interface Sci. 1979, 71, 408. (7) Israelachvili, J. Physical Principles of Surfactant Self-Association Into Micelles, Bilayers, Vesicles and Microemulsion Droplets. In Surfactants in Solution, Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 4, p 3. (8) Baran, J. R., Jr.; Pope, G. A.; Wade, W. H.Unpublished data. (9) Connor, D. S.; Scheibel, J. J.; Kao, J.-N. U.S. Patent 9,106,985, 1991.
(10) Arenas, E.; Baran, J. R., Jr.; Pope, G. A.; Wade,W. H.; Weerasooriya, V. Aqueous Phase Microemulsions Employing Alkyl Glucamide Surfactants with Chlorinated Hydrocarbons. Langmuir 1996, 12, 588. (11) Griffin, W. C. Classification of Surface-Active Agents by “HLB”. J. Soc. Cosmet. Chem. 1949, 1 (5), 311. (12) Griffin, W. C. Calculation of HLB Values of Non-Ionic Surfactants. J. Soc. Cosmet. Chem. 1954, 5 (4), 249. (13) Barakat, Y.; Fortney, L. N.; Schechter, R. S.; Wade, W. H.; Yiv, S. H. Criteria for Structuring Surfactants to Maximize Solubilization of Oil and Water II: Alkyl Benzene Sodium Sulfonates. J. Colloid Interface Sci. 1983, 92, 561. (14) Abe, M.; Schechter, D.; Schechter, R. S.; Wade, W. H.; Weerasooriya, U.; Yiv, S. H. Microemulsion Formation with Branched Tail Polyoxyethylene Sulfonate Surfactants. J. Colloid Interface Sci. 1986, 114, 343. (15) Lalanne-Cassou, C.; Carmona, I.; Fortney, L.; Samii, A.; Schechter, R. S.; Wade, W. H.; Weerasooriya, U.; Weerasooriya, V.; Yiv, S. H. Minimizing Cosolvent Requirements for Microemulsion Formed with Binary Surfactant Mixtures. J. Dispersion Sci. Technol. 1987, 8, 137. (16) Graciaa, A.; Schechter, R. S.; Wade, W. H.; Yiv, S. H.; Barakat, Y. Emulsion Stability and Phase Behavior for Ethoxylated Nonyl Phenol Surfactants. J. Colloid Interface Sci. 1982, 89, 217. (17) Bourrel, M.; Salager, J. L.; Schechter, R. S.; Wade, W. H. A Correlation for Phase Behavior of Nonionic Surfactants. J. Colloid Interface Sci. 1980, 75, 451. (18) Graciaa, A.; Fortney, L.; Schechter, R. S.; Wade, W. H.; Yiv, S. H. Criteria for Structuring Surfactants to Maximize Solubilization of Oil and Water I: Commerical Nonionics. J. Soc. Petrol. Eng. 1982, Oct, 743. (19) Graciaa, A.; Barakat, Y.; El-Emary, M.; Fortney, L.; Schechter, R. S.; Yiv, S.; Wade, W. H. HLB CMC and Phase Behavior as Related to Hydrophobe Branching. J. Colloid Interface Sci. 1982, 89, 209.
Received for review July 12, 1995. Revised manuscript received January 19, 1996. Accepted February 21, 1996.X ES950514B X
Abstract published in Advance ACS Abstracts, April 15, 1996.
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