Langmuir 1991, 7, 1370-1377
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High Internal Phase Water-in-Oil Emulsions: Influence of Surfactants and Cosurfactants on Emulsion Stability and Foam Quality Joel M. Williams Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received July 9,1990. I n Final Form: January 11,1991 The relationship between the hydrophilic-lipophilic balance (HLB) number of a cosurfactant and the stability of a styrene-based emulsion is studied for 8 surfactants and 22 cosurfactants. The amount of cosurfactant that can be incorporated into a water-in-oil emulsion is stronglyand inversely coupled to the HLB number of the cosurfactant with a correlationof 96%. The higher the HLB number the smallerthe amount of cosurfactant required to form large, coalesced droplets in the emulsion and, hence, in the ultimate polymeric foam. Methanol is an obvious deviant from the typical HLB number-related, cosurfactant disintegration of the sorbitan monooleate containing emulsions. Some modifications to the literature assignments of HLB numbers are recommended. A mechanism is presented for the dual droplet distribution observed for nonionic cosurfactants. The mechanism for the disintegration of emulsions by anionic and nonionic surfactants amears to be different, but the ultimate dependence on HLB number appears to be similar.
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Introduction Although foams prepared by the polymerization of the high internal phase emulsions (HIPEs) containing styrenetype monomers satisfy some of the needs of the high energy physics community, physicists prefer materials with cell sizes less than 1pm in diameter.*I2 Cells of HIPE-derived foams, unfortunately, are generally in the 1-10 pm size range.3-6 As part of our continuing effort to reduce the cell size, the effects of 8 surfactants and 22 cosurfactants have been explored. During our analysis of the data, we found it necessary to make some modifications to the literature assignments for hydrophilic-lipophilic balance (HLB) values.6 The modifications correct several assignments that were very disturbing to this physical organic chemist; e.g., an ester group should not be more hydrophilic than an alcohol group or a carboxylic acid group. The relationship between the HLB number of a cosurfactant and the stability of the standard styrene-based HIPE is studied herein.'*E Experimental Section Standard Emulsion Preparation. The emulsification technique used to prepare the foam samples has been (1)Haendler, B. L. Low-Density Polymer Foams for Fusion-Fuel Capsules. In Energy and Technology Review. Lawrence Livermore National Laboratory Report UCRL-52000-89-10; Lawrence Livermore National Laboratory: Livermore, CA, October 1989. (2) Williams, J. M.; Wrobleski, D. A. J. Mater. Sci. 1989,24, 4062. (3) Williams, J. M. Langmuir 1988, 4, 44. (4) Williams, J. M.; Wrobleski, D. A. Langmuir 1988, 4, 656. (5) Haendler, B. L.; Buckley, S. R.; Chen, C.; Cook, A. R.; Cook, R. C.; Hair, L. M.; Kong, F. M.; Kramer, H. D.; Letts, S. A.; Overturf, G. E., III; Schumann, D. L.; Upadhye, R. 5.Lawrence Livermore National Laboratory Report UCm-21080-87;LawrenceLivermoreNational Laboratory: Livermore, CA, 1988. (6)Williams,J. M.;Gray, A. J.; and Wilkerson, M. H. Langmuir 1990, 6, 437. (7) This paper discusses emulsion stability in terms of the weighted HLB number of the cosurfactants and not the HLB of the emulsion itself. A surfactant can be likened unto a seeeaw with the hydrophilic end on one side of the pivot and the lypophilic end on the other. Unfortantely, the HLB number of a surfactant only tells how out of balance the seesaw is and not how long the seeeawarms are. Knowledgeof the phase inversion temperature (PIT) of the systems containing different cosurfactant is needed to adequately address that issue. The reader is referred to Shinoda and Friberg! (8) Shinoda, K.; Friberg, S. Emulsions andsolubilitation;Wiley: New York, 1986.
described p r e v i o ~ l y .Each ~ emulsion was prep-red as a batch consisting of 100 cm3. The styrene and divinylbenzene were obtained from Polysciencesand used without removing the inhibitors. The amounts of styrene and divinylbenzene used in each batch were 9 and 1g, respectively. When present, the amount of sorbitan monooleate (SMO) surfactant was 2 g per batch as this was previously found to be optimum? Commercially available surfactants and cosurfactants were used without further purification and were added to the oil component of the emulsion before emulsification. Deionized water containing 1.5 g/L of potassium persulfate was added to give the final volume of 100 cm3. Emulsion Polymerization, Foam Production, and Characterization. Polymerizations were conducted in a forced air oven at 60 "C. Removal of the polymerized masses from the jars and drying have been previously described? Evaluations of emulsion and foam quality were made visually. Scanning electron micrographs were made on razor-finished surfaces as previously de~cribed.~
Results Emulsions and Foams from Single Surfactants. The radius of curvature at the oil/water interface is determined in part by the head and tail components of the surfactant. Unilever, in their patent: claims that styrene/divinylbenzene water-in-oil emulsions can be made with surfactants having HLB numbers from 2 to 6. Several common surfactants (Table I) with HLB numbers near this range were substituted for sorbitan monooleate (SMO)to see if they could produce smaller cell sizes. None was better than SMO. Surprisingly, although sorbitan monostearate (SMS) had a HLB number similar to that of SMO, it did not produce a viable emulsion in our laboratory. The combination of higher surface tension and a melting point above room temperature seems problematical. Indeed, SMS appeared to be too insoluble to remain dissolved in the oil when present at 20 wt % of the monomers. Interestingly,sorbitan Surfactants [monopalmitate (SMP) and monolaurate (SML)] with HLB numbers above 6 were able to form emulsions and (9) Barby, D.; Haq, 2.European Patent 60138assigned to Unilever (3 September 1982). HLB 2-6 (preferably 4).
0743-7463 9112407-137Q$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 7,1991 1371
High Internal Phase Water-in-Oil Emulsions
Table I. Properties of Some Nonionic Surfactants surfactant sorbitan trioleate [STO,Span 851 sorbitan monooleate [SMO,Span 801 nonylphenylpolyoxyethylene(1.5) alcohol [NPE0(1.5)] Igepal CO-210 [NPE0(2)] sorbitan monostearate [SMS,Span 601 sorbitan monopalmitate [SMP, Span 401 sorbitan monolaurate [SML,Span 201 Igepal CO-520 [NPE0(5)1
state= liq liq liq mp 53 "C mp 48 "C liq liq
HLBb 1.8 4.3 4.6 4.7 6.7 8.6 10.5
surface tension: dynlcm 32 30
stable emulsione
46 36 28
0 Benson, F. R. Poly01 Surfactants. In Nonionic Surfactants;Schick, M.J., Ed.;Dekker: New York, 1967; p 277. Reference 10. Stable is defined here as a uniform single component at room temperature.
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15 pm
A
B
Figure 1. Scanning electron micrographs of foams prepared from emulsions containing sorbitan monolaurate (A) and sorbitan monopalmitate (B).
subsequently foams as illustrated by Figure 1. The foam from the higher HLB-numbered surfactant, SML, was better, but neither foam was as good as that from SMO. SML has a lower melting point and lower surface tension value than SMP. A t 20% of the monomers, the surfactant level is about 4 times that needed to form a monolayer.4 It is not clear at this point whether the differences in the surfactant properties manifest themselves in the oil-water interface, the oil, or both; probably both. A class of surfactants based on substituted phenyl polyethylene oxides has been studied extensively by Shinoda and his co-workers. The HLB numbers of some are reported to be in the 2-6 range.1° For example, Igepal CO-210 (4-nonylphenylpoly(oxyethylene) alcohol) is a liquid with a HLB number that should be near 5 (see Table I). Neither it nor Igepal CO-520 showed any tendency to form water-in-oil emulsions with styrene/ divinylbenzene when present at 20% of the monomers. HLB numbers, if they are correct for these two surfactants, are not sufficient criteria for forming water-in-oil emulsions that can be used for making foams. Phase inversion temperature (PIT) data and length and type of hydrophilic and lypophilic components are needed to properly evaluate the system? Emulsions and Foams from SMO and Cosurfactants. "It is a well-known fact that pure surfactants are (10) Griffin, W. C. Emulsions. In Kirk-Othmer Encyclopedia of Chemical Technology;Wiley: New York, 1979; p 913. Reproduced by Shinoda, K.; Kunieda, H. Phase Properties of Emulsions: PIT and HLB. In Encyclopedia of Emulsion Technology;Becher, P., Ed.;Dekker: New York, 1983; Vol. 1, p 356.
not useful for application as stabilizers for foams, emulsions,or dispersions".ll Commercialformulationsseldom contain a single surfactant. Friberg and Venable have discussed the use of cosurfactants extensivelywith regard to microemulsions.12 The addition of a cosurfactant to the best singlesurfactant emulsion systemseemed a logical extension of our research. Table I1 contains data for 22 cosurfactants. All but three (nos. 2, 16, and 22) had terminal alcohol groups. The molecular weights are from the suppliers' catalogs. HLB numbers are discussedbelow. Under "mol(co)/mol(smo)"of Table 11,the "stable" column indicates the amount of cosurfactant that can be added to the SMO/oil component and still obtain a foam, while the *unstable"column indicates the amount of cosurfactant that definitely will not yield a foam. In effect, these two columns bracket the instability of the emulsion at the polymerization temperature (60 O C ) . No attempt was made to refine the gap because the data already contained over a hundred samples. Under the "data at stable condition", the "rating" column corresponds to a visual inspectionof each foam produced. The subjective ratings were from 0 to 10 and were performed in one sitting by the author without reference to sample identification. These ratings were used to rank the cosurfactants that fell in the same "mol(co)/mol(smo)" group. The other two columns convertthe "mol(co)/mol(smo)"data togram and volume equivalent data. For a point of reference, adding 4 mol of SMO to the standard emulsion also yields an unstable emulsion.6 HLB Numbers Reassigned. The amount of cosurfactant that can be incorporated in the SMO/monomer phase appears (Table 11)to be roughly inversely related to the HLB number of the cosurfactant. The exact relationship requires a knowledge of the HLB number for each cosurfactant. The methodology proposed by Davies and Rideal seemed straightforward, but the literature values13 assigned to some functional groups were very disturbing (Table 111). Thus, not only was an ester group very hydrophilic (AHLB = 2.4), it became extremely hydrophilic (AHLB = 6.8) when attached to a sorbitan ring! Having trouble with these assignments, this author has taken a number of surfactants from the literaturelo(Table IV) and performed a multivariatelinear regressionon their HLB numbers. The "sorbitan ester" designation used previously has been dropped. Three new functional groups, or substituents, have been added (1)a sorbitan ring without OH groups; (2) an imbedded, and very likely interfacial in the surfactants used here, benzene ring; (3) "extra tails". The "extra tails" designation accounts for the fact that dividing a long chain tail into shorter ones (11) Friberg, S. E. Langmuir 1988,4,487; during a review of a book. (12) Friberg, S. E. and Venable,R. L. Microemulsions. InEncycbpedh of Emulsion Technology;Becher, P., Ed.; Dekker: New York, 1983;Vol. 1, p 287.
(13) Myers,D. Surfactant Science and Technology;VCH Publishers: New York, 1988.
1372 Langmuir, Vol. 7,No.7, 1991
William
Table 11. Cosurfactants and Their Influence on Emulsion Stability* no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
cosurfactant methanol stearic acid 1-octadecanol (stearyl) oleyl alcohol 1-hexadecanol (cetyl) decanol 9-decen o1 2-methyl-2-hexanol 1-pentanol cyclohexanol Igepal CO 210 (9-Ph0-2)
3-phenoxy-lJ2-propanedio1 3-phenylpropanol Brij 30 (lauryl-4) Igepal CO 520 (9-PhO-5) sodium dioctylsulfoeuccinate Tween 80 (SMO-20) Brij 78 (stearyl-20) Brij 99 (oleyl-20) Brij 58 (cetyl-20) Brij 35 (lauryl-20) potassium oleate
physical state liq mp 67 OC mp 60 "C liq mp 54 "C liq liq liq liq liq liq liq liq liq liq solid liq mp 44 OC mp 25 "C mp 38 "C mp 38 "C solid
mol wt
32 284 270 268 242 158 156 116
88 100 308 168 136 363 440 445 1310 1150 1150 1120 1200 321
HLB 1it.b tale 9.3 1.6 4.0 3.4 4.6 6.5 5.6 7.4 8.1 7.4 4.6 7.7 10.8 8.1 9.5 8.2 10.5 9.4 15.0 15.3 15.3 15.7 20.0
16.3 15.4 14.8 16.0 17.3 20.1
mol(co)/mol(smo) stable unstable 50 55 6 12 6 12 6 12 3 6 3 6 3 6 1 6 1 6 1 3 1 3 1 3 1 3 1 3 0.3 1 0.1 1 0.1 0.3 0.1 0.3 0.1 0.3 0.1 0.3 0.05 0.1 0.02 0.03
data at stable condition gm/gvol % oil ratingd 9.9 3.74 20.5 9.6 3.98 21.0 20.8 7.1 3.79 6.5 3.76 20.5 9.5 1.70 16.4 7.3 1.11 15.2 15.2 3.6 1.09 13.5 7.7 0.27 13.4 7.5 0.21 13.4 9.8 0.23 13.4 7.9 0.72 13.6 7.4 0.39 13.8 3.3 0.32 14.7 3.1 0.85 13.7 3.5 0.31 2.5 0.10 13.2 13.6 0.6 0.31 0.4 0.27 13.5 0.3 0.27 13.5 13.6 0.2 0.26 0.3 0.14 13.3 13.1 6.4 0.015
a The emulaions were styrene/divinylbenzene/SMO/coeurfactant and water. b Shinoda, K.; Kunieda, H. In Encyclopedia of Emulsion Technoloav: Volume 1,Becher.. P... Ed.:. Dekker: New York, 1983; Vol. 1,p 356. From equation developed herein. d Blind (1-10) rating baaed on physi
