Langmuir 1988,4 , 44-49
44
In principle, small chromium hydrous oxide particles could precipitate first and then adhere to hematite to produce a close-packed surface layer. Again, the coating particles and the cores would both be positively charged. However, if the potential on two kinds of solids is of the same sign but differs in magnitude, heterocoagulation is quite possible.44 The smooth deposited layers would indicate the last proposed mechanism to be less likely. The coated particles have the same isoelectric point as the solids obtained from the blank. This behavior is as expected since the measured mobilities depend essentially on the properties of the surface layer. The formation of the coating critically depends on the heating rate. When relatively large solution volumes are processed, the heating time to reach the required temperature is also long. In addition, the temperature distribution within the solution volume is expected to be less uniform with increasing vessel size; the zones adjacent to (42)MatijeviE, E.;Bell, A. In Particle Growth in Suspensions;Smith, A. L.. Ed.: Academic: New York. 1973: D 179. (43) MatijeviE, E.; Sapieszko, R. S.; M&lIe, J. B. J. Colloid Interface Sci. 1975,50, 567. (44)Barouch, E.; MatjeviE, E. J . Chem. SOC.,Faraday Trans. 1. 1985, 81,1797.
the glass wall would have a higher temperature than those in the interior of the solution. Both these factors could be responsible for less uniform coatings in experiments using large solution volumes. This problem could be circumvented by preheating the dispersions of core particles prior to addition of coating solutions. the aging times required to produce coatings by method I1 are much shorter than the times required by method I. This effect is attributed to the addition of base, which promotes the complex formation of chromium species in solution more efficiently than storing the same solution a t room temperature. The described coating procedure is a modification of a previously developed method for the preparation of spherical particles of chromium hydrous oxide of narrow size distribution. It has thus been possible to extend the knowledge of generation of “monodispersed” particles of one substance to its coating on particles of another substance. The modification introduced in this work has actually shown that uniform colloidal chromium hydrous oxide dispersion particles can be prepared in the absence of cores much faster than previously reported. Registry No. Fe203,1309-37-1; chromium hydrous oxide,
50814-20-5.
Toroidal Microstructures from Water-in-Oil Emulsions Joel M. Williams Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received July 10, 1987. I n Final Form: September 1, 1987 Polymerization of emulsions made from styrene-divinylbenzene-dodecane-water mixtures provides a useful route to open-microcellular rigid foams. The structures of the resulting foams demonstrate that the emulsions possess considerable segregation and alignment of each molecular species at the time of polymerization. Simple pictorial models are sufficient to explain the emulsion structures and the resulting foams. Toroidal microstructures are predictable and observed.
Introduction Over the last decade, rigid open-microcellularpolymeric foams have been needed for high-energy physics experiments, especially those for inertially confined fusion. The required foams needed densities less than 0.1 g/cm3 and cell sizes of 30 pm or less. Loa Alamos National Laboratory has investigated a variety of routes for producing these special foams. Successful routes have generally involved a polymer phase that separated from a hot solution and then removal of the solvent in a manner such that the polymer did not m0ve.l Along with workers at Lawrence Livermore National Laboratory,2 the Atomic Weapons Research Establishment (England),3 and Case Western Reserve U n i ~ e r s i t ywe , ~ are now exploring the usefulness (1)Young, A. T. J. Vac. Sci. Technol. A 1986,A4(3),1128. (2)Letts, S.A.;Lucht, L. M.; Morgan, R. J.; Cook,R. C.;Tillotson, T. M.; Mercer, M. B.; Miller, D.E. Progress in Development of Low Density Polymer Foams for the ZCF Program; Lawrence Livermore National Laboratory Report UCID-20537,1985. (3)Gunn, T. Atomic Weapons Research Establishment, Aldermaston, England, 1985,private communication. (4)Litt, M. L.; Hsieh, B. R.; Krieger, I. M.; Chen, T. T.; Lu, H. L. J. Colloid Interface Sci. 1987,115(2),312 (contains photomicrographs).
of emulsion technology in producing rigid microcellular foams. Unilever5 has described the production of a porous, homogeneous, cross-linkedpolymeric block material having a dry density of less than 0.1 g/cm3 by a process in which monomer (styrene or substituted styrene plus comonomers) are polymerized after they have formed an emulsion having a high (at least 90%) internal phase comprised of water. The emulsion polymerization of styrene is not new, of course, and has been the subject of numerous patents because of the technical importance of polystyrene.6 Monodispersed polystyrene latexes, for example, have been prepared by seeded-emulsionpolymerizations and 3-5-pm particles were successfully prepared on a US space shuttle in 1982.’ Styrene is readily polymerized in an emulsion (5)Barby, D.;Haq. Z.,low density porous cross-linked polymeric materials and their preparation and use as carriers for included liquids, European Patent 0 060 138 (Unilever, applicant), September 1982. (6) Sumner, C. G. Clayton’s The Theory of Emulsions and Their Technical Treatment; Blakiston Co.: New York, 1954. (7)Vanderhoff, J. W.; El-Aasser, M. S.; Micale, F. J.; Sudol, E. D.; Tseng, C. M.; Silwanowicz, A.; Kornfeld, D.M.; Vicente, F.A. J. Dispersion sci. Technol. 1984,5(3/4),231.
0743-7463/88/2404-0044$01.50/0 0 1988 American Chemical Society
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Langmuir, Vol. 4, No. 1. 1988 45
Figure 1. SEM of sample A from polymerization of an aqueous emulsion containing 4.7% styrene/divinylbenzene (50/50)and 2.1% sorbitan monooleate. [Each SEM pair is a stereo couple. Stereo viewing can be accomplished by holding the images about 55 em (22 in.) away from your eyes and gently crossing (converging) your eyes to produce three images, viewing the middle one. As a crutch, try looking a t the two images normally (i.e., see two separate images), and then place a pencil (or your finger) hetween you and the images so that the pencil appears in the center of each image. Now look a t the pencil and think about the middle image that appears. Remove the pencil when the middle image sharpens. The white rectangles should overlap exactly when stereo viewing is done properly. A second stereo pair Gust below the first) is provided for people with stereo viewers (see the Experimental Section) or for those who find that diverging their eyes is easier than converging them. To use the diverging eye method, look off into space (out the window to infinity; a blank wall is too close) for 5 s or so and slowly raise the SEM images into view ahout 55 cm from your eyes. Think about the images, but do not try to foeus on them immediately; just continue to look into space. With practice the middle image will sharpen for serious viewing. A crutch for this method is a cardboard blinder. Cut two 1.6-cm (s/8-in.) round holes in a piece of cardboard. The distance between the holes should he the same as the distance between the pupils of your eyes. Hold the cardboard to your nose. Hold the stereo imaging pair next to the cardboard (centered hetween the holds) and very slowly move it away from you. Stare straight ahead; do not let your eyes move from side to side. A single 3-D image will come into view before you get the imaging pair to your normal reading distance. After viewing the 3-D image a t your normal reading distance for awhile (l(t15s), flip the cardboard blinder out of view while concentrating on the 3-D image. The 3-D image will persist if your brain is properly conditioned. It takes practice to keep your eyes fixed in a nonfocusing position, so do not despair. More than two-thirds of the population should be able to achieve 3-D imaging by one of these techniques.]
system using persulfate i n the aqueous phase as t h e initiator. Sorbitan fatty esters, used in t h e Unilever patent, have been described as emulsifiers for the emulsion polymerization of styrene and other water-insoluble monomers? During our initial investigations into making foams b y polymerizing water-in-oil (w/o) emulsions containing styrene/divinylbenzene as t h e oil phase, it became clear to us that t h e foam structures could provide significant information about the structure of emulsions. In particular these foam structures seemed appropriate for addressing t h e observed difference t h a t aliphatic and aromatic hydrocarbons have on emulsion stability. For example, (8) Danfmth, J. D.US.Patent 2468212 (to Universal Oil Products), 1949.
Daviess reported that coalescenceno longer occurred when an aliphatic hydrocarbon was replaced b y an aromatic one. T h e phase inversion temperature (PIT) was also found to b e sensitive to t h e hydrocarbon type.'O For various hydrocarbons added to a water/stufactant/n-heptanesystem, paraffin destabilized t h e system, while n-hexane a n d benzene (even more) stabilized it. For the n-C6-CIe hydrocarbons, t h e lower members of the series were the most For micellar solutions, t h e r e is stable to coalescence." evidence that aromatic molecules are located in t h e polar interface layer i n preference to t h e more hydrophobic re(9) Davin, J. T.Recent Pros. Surf. Sei. 1964.2. 129. (10) Friberg. S. E.;Solans, C. Lonsmuir I986 2(2), 121. (11) Vincent, B. 'Emulaions and Foams"; In Surfwtonts; Tadms, Th. F., Ed.; Academic: London. 1984.
Williams
46 Langmuir, Vol. 4, No. 1, 1988 TOUCHING BUBBLES WITH THIN LAYER INTERFACE (Water- in - Oil Emulsion) ' i '-\
strengths were measured on cubes approximately 1.6 cm on a side by using an Instron. sample A 0.065 density (g/cm3) compression modulus (psi) 1200 8.3 compression modulus (MPa)
ORGANIC CONTENT
Figure 2. Effect of changing the organic content on droplet interaction and size.
gions of the aliphatic hydrocarbon chains of the surfactant.12 To explore how these parameters might affect our foams and what information the foam structures might give toward understanding the structure of emulsions, we modified the oil composition of our emulsions to include a nonpolymerizable, aliphatic hydrocarbon that would allow the aromatic monomer to occupy its preferred positions and polymerize in place. The effects of this modification were dramatic and enlightening and are described in this paper.
Experimental Section Emulsion Preparation. Sorbitan monoleate surfactant (Lonzest SMO, HLB = 4.3) and organic components were mixed in a 300-mL Fleaker beaker. With magnetic bar stirring, water with potassium persulfate was added dropwise for the first 50 mL; then eyedropper aliquots were added. The total addition time was 3/4 h. Afterwards, the mixture was vigorously stirred for l/z h at 1728 rpm with a splashless paint stirrer; stirring of sample A was stopped after 1/4 h as stirring could not be effected uniformly throughout the volume. Treating a mixture similar to sample A with an ultrasonic disperser caused the mixture to "cream". Quantities used are given (in grams) below. sample component styrene divinylbenzene SMO dodecane H*0 K2S208
B
A 3.78 3.78 3.43 0 150.0 0.20 very stiff,
3.79 3.75 3.28 5.05 145.1 0.21
C 3.75 3.78 2.65 30.0 110.3 0.20
fluid, like smooth, like like paste thick pudding milk deep ripples gentle ripples smooth surface texture no air bubbles in mix Yes yes consistency
Polymerization. Each beaker was covered with an airtight cover and placed overnight in a 60 "C oven. Each sample produced a stiff white solid. Sample C had 20 mL of a colorless liquid above the solid. Foam Production. Fragments were cut from each polymerized solid and placed on a watch glasa in a 65 "C oven overnight. The samples were next placed in a 85 "C vacuum oven to remove residual volatiles. Densities were determined from weight and volume measurements and are given below. Compression (12) Boyde, A. Scanning Electron Microscopy; Wells, 0. C., Ed.; McCraw-Hill: New York, 1974.
B
C
0.051 340 2.3
0.042 210 1.4
Microscopy. The leached foam was mounted on a metal stalk with DUCO cement. After the cement dried, a smooth surface was cut with a vibrating razor blade. The device used was a Vibratome Model lo00 sold by Ted Pella, Inc., Tustin, CA. The sides of the foam and the mount were then coated with silver paint for conductivity. After the silver paint dried, the foam was coated with a few angstroms of gold. The prepared foam was then analyzed with a Hitachi Model S-520LB scanning electron microscope. Stereo views were taken by tilting the stage a t 10" for one of the shots. Boyde13J4has described stereo viewing of SEM images. For unaided viewing, he prefers the diverging eye method. I find it much easier to use the converging eye method, because it can accommodate a wider range of image sizes. The diverging eye method is much easier on the eyes, however. A cheap ( ~ $ 5 ) stereo viewer is the "Stereoscope" made by Hubbard Scientific Co., Northbrook, IL, and also available from Ernest F. Fullam, Inc., Latham, NY.
Results and Discussion The rigid foam produced when a water-in-oil (w/o) emulsion containing 5% styrene/divinylbenzene (50150) and 2.5% sorbitan monooleate surfactant polymerizes has a well-connected, open-microcellular structure (Figure 1). The cells are not dodecahedral, but generally spherical. (For comparison and contrast, see the closed-cell structures observed by Lissant and Mayhan.15) In emulsions with as much as 95-99 vol % internal phase, the droplets of the dispersed phase should interact strongly.16 According to the literature," the droplets should behave as undeformable spheres in w/o emulsions; in contrast, those of o/w systems deform in response to the applied rate of shear. Assuming that the cell sizes of the foam represent the size of the aqueous droplets in the emulsion, the emulsion possessed droplets ranging in diameter from 5 pm to around 0.5 pm. Particle (or droplet) size has been related to the mode of preparation by Jurgen-Lohmann'* and Kiyama, Kinoshita, and Suzuki.lg Becher has given a schematic of packed nonuniform droplets.20 A simple equation for the droplet size distribution in emulsions has been developed by Schwarz and BezemereZ1They found that the equation fits the data in the literature very satisfactorily when the emulsions are prepared mechanically in a mixer, by shaking, or in a turbulent field. Our results are supportive of that conclusion. A striking feature of the cells is their immense open interconnectivity. Indeed, every cell has an open window to each of its nearest neighbors. The polymerized organic material is confined (13) Boyde, A. Scanning Electron Microscopy; SEM, Inc.: AMF 0'Hare,IL, 1979; Vol. 11. (14) Ward, A. J. I.; Rananavare, S. B.; Friberg, S. E. Langmuir 1986, 2, 373.
(15) Lisaant, K. J.; Mayhan, K. G.J. Colloid Interface Sci. 1973,42(1), 201. (16) Smith, D. H. J. Colloid Interface Sci. 1985, 108(2), 471. (17) Sherman, P. Proc. Int. Congr. Surf.Act., 3rd 1960,2,596 (cited in ref 18, p 80). (18) Jurgen-Lohman, L. Kolloid-2.1951,124,41(cited in ref 9, p 52). (19) Kiyama, R.; Kinoshita, H.; Suzuki, K. Reu. Phys. Chem. Jpn. 1951. 21. 82. (20) Becher, P. Emulsions: Theory and Practice, 2nd ed.; Reinhold: New York, 1957. (21) Schwarz, N.; Bezemer, C. Kolloid-2.1966, 146, 139.
Toroidal Microstructures from wlo Emulsions
Langmuir, Vol. 4, No.1, 1988 41
Figurs 3. SEM of sample C from polymerization of an aqueous emulsion containing 5% styreneJdivinylbenzene (50/50), 20% dodeeane, and 1.8% sorbitan monooleate. (See comments for Figure 1.) WATER - IN - OIL EMULSION INTERFACE FAVORITE POSITION OF AROMATIC MONOMERS
OF AROMATIC OMERS OR ADDED HATlC MOLECULES
Figure 4. segregation of aromatic and aliphatic molecules in the interstices of touching surfactant-coated water droplets.
to the interstices formed by three or more water droplets. The adjoining space between two droplets is devoid of any polymer. When the organic content of the emulsion is increased, the water droplets respond by moving apart (Figure 2) and growing in size. When the diluent is a nonpolymerizing material, like dcdecane, the foam structure produced is a weak one in which the water droplets are thinly coated with polymer (Figure 3). Polymer concentrating a t the surface of the water droplet is not too surprising since the aqueous phase contains the polymerization catalyst. What is surprising is the uniformity of the thin coat throughout the sample and the absence of significant excursions by the polymer into the dcdecane region (the exospherical void of the foam). This result suggests that significant molecular segregation has occurred and that the aromatic
Figure 5. Toroid formed by aromatic monomers around the surfactant window between two water droplets.
molecules are indeed concentrated a t the surfactant-organic interface, as suggested earlier,'* rather than being dispersed in the organic phase. The occurrence of little windows and their accompanying hyperboloids (Figure 3), even when a large excess of dodecane is used, suggests that aromatic monomers also congregate where surfactant surfaces merge. A zonal molecular preference is envisioned (Figure 4). Part of the preference may he due to the double bond network supplied by the tail of the oleate surfactant used in these
48 Langmuir, Vol. 4, No.I , 1988
Williams
Figure 6. SEM ofsample B from polymerization of an aqueous emulsion containing 4.7% atyrene/divinylbemne (50/50), 3.1% dad-e, and 2.1% sorbitan monooleate. (See comments far Figure 1.) n
0.1
I
1
WINDOW DIAMETER ( II rn)
Figure 7. Relationship of toroid wall width to toroid window size. The solid line is an "eyeball" evaluation,while the dashed line is a least-squaresfit using a second degree polynomial. experiments. The preference of the aromatic monomers for the area where the surfactant surfaces merge may be related to the pore-limiting exclusion mechanism so important in gel permeation chromatography (GPC). Aromatic molecules, like styrene and divinylbenzene, appear to be very small molecules when analyzed by GPC, whereas alkanes, hexane and up, appear significantly larger (J.M.W. unpublished data). If no aromatic monomer were present, the aliphatic hydrocarbon molecules would he unable to fill the pore-limiting region. The system would adjust to this void by eliminating it entirely through droplet coalescence. Such an emulsion instability, obtained when aromatic hydrocarbon is replaced by aliphatic hydro-
carbon, was reported by Daviess and recently discussed at length by Friherg and Solans.'O Proper choice of the aliphatic hydrocarbon level mlead to a situation where aromatic monomer units will concentrate at their energetically most favored spot, i.e., at the merging surfactant surfaces (Figure 4). In this case the aromatic monomers would form toroidal bodies around the surfactant surfaces that form the planes (windows) between two water droplets (Figure 5). By further choosing the hydrocarbon level, each toroidal structure could be made to completely join each of its neighbors or just occasionallytouch them. The latter case is beautifully exemplified by the SEM of sample B (see Figure 6). The windows of the toroids formed in sample B are generally proportional to the sue (diameter) of the smaller of the two adjoining water droplets. The window/droplet diameter ratio seems to vary from about to with the former more prevalent. Princen" has shown that the wall-flattened parts of the droplets are a function of the total wall area and can be related by an equation for a discontinuous phase up to 97.5% (o/w). With his formula, the wall-flattened parta (windows here) of the droplets of a 95% internal phase emulsion have been calculated to be 38% of the total wall area. The experimental results agree well with this value. The width of the toroidal walls (wedges), on the other hand, is not so sensitive (Figure 7). Mason et al." demonstrated an interrelationship of wedges (22) hincen.
H.M.J . Colloid Interfnce Sei.
1988, 91(1), 150.
Langmuir 1988,4, 49-51
49
contained in the same region. They find that the radius a t the end of each wedge will be the same. The insensitivity of the wedge width requires a new look at the physics of the situation. Some areas in sample A, which has only aromatic monomers in the organic phase, look like they are membrane-covered windows (Figure 1). This appears to result from the juncture of five (or more) droplets wherein three of them prevent the fourth from contacting the fifth. Such a case is clearly visible in the two large cells a t the upper left of Figure 1. Where droplets touch there are only open windows. The open centers of the toroids of sample B (Figure 6) should dispel1 any thoughts that the openings between cells in these foams could have been produced by retraction of polymeric membranes present in the emulsion during the removal of water to produce the foam. In summary, a straightforward pictorial representation of concentrated water-in-oil emulsions is possible. At the time of polymerization, each major component type (water, surfactant, aromatic monomers, and aliphatic diluent) occupies a specific physical location. Aromatic components interact more strongly with the surfactant than do aliphatic ones, although the oleate surfactant tail used in this work may help in this regard. Small-size aromatic components also pervade nooks and crannies not available to aliphatic molecules and thus provide a stabilizing influence by eliminating void regions. Thus, it is not sufficient to know what building materials are present and at what temper-
atures and concentrations things change, but it is also necessary to know how these materials can be assembled to produce a structure. PIT and phase diagrams center on the former. Studying the foam structures prepared from various emulsions centers on the latter and can provide a clearer picture of molecular alignments than might be speculated from other observations and measurements. Rigid foam structures point out, for example, that filling the narrowing (capillary) space created when two internal phase droplets come into contact with one another is important in stabilizing the emulsion. The common use of cosurfactants may be a means of having the surfactants perform this task themselves or relieving the restraints on other molecules so that they may perform it. The foam structures also point out why aromatic and aliphatic hydrocarbons stabilize emulsions differently. The problem now is to find some way to include the physical alignment of the various components into the systematic (mathematical) prediction of emulsion stability.
(23) Mason, G.; Nguyen, M. D.; Morrow, N. R. J. Colloid Interface Sci. 1983, 95(2), 494.
Registry No. Styrene, 100-42-5; divinylbenzene, 1321-74-0; dodecane, 112-40-3.
Acknowledgment. I greatly appreciate the suggestions, comments, and counsel provided by Ainslie T. Young. Ted Gunn, Steve Letts, and Mort Litt are thanked for their open sharing. Dave Duchane, Rai Liepins, Betty Jorgensen, and Steve Newfield are thanked for manuscript comments. The scanning electron microscopy by A. James Gray is especially appreciated. Thanks to Debbie Vigil for typing the manuscript and to Cynthia Archuleta and Cindy Boone for the drawings. The generous supply of Lonzest SMO from Lonza is gratefully acknowledged.
Fate of Interfacial Adsorbates in Solvent Extraction Systems V. G. Chamupathi and H. Freiser* Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received December 6, 1986. I n Final Form: February 17, 1987 Adsorbed extractants are quantitatively stripped from a liquid-liquid interface when the organic phase is removed from a rapidly stirring two-phase mixture through a microporous Teflon phase separator. A method is described which enables the determination of interfacial excess from more dilute solutions than was possible heretofore.
Introduction The recent introduction of the microporous Teflon phase separator (MTPS) in the study of solvent extraction processed+ has had a positive significant impact on increased understanding of such separations. With the MTPS, it is possible to conduct extractions under conditions of high, (1) Watarai, H.; Cunningham, L.; Freiser, H. Anal. Chern. 1980,54, 2390. (2) Watarai, H.; Freiser, H. J. Am. Chern. SOC.1983, 105, 191-194. (3) Watarai, H. J. Phys. Chem. 1985, 89, 384-387. (4) Watarai, H. Talanta 1985, 32,817-820. (5) Watarai, H.; Sasabuchi, K. Solvent Eztr. Zon Ezch. 1985, 3(6), 881-893. (6)Aprahamian,E.,Jr.; Cantwell, F. F.; Freiser, H. Langrnuir 1985, 1 , 79.
known interfacial areas6 and, hence, to properly assess the relative importance of the interface in relation to either bulk phase. Quantitative estimation of the amount of extractant6 and/or chelate4,' that is adsorbed in the interfacial region generated under conditions of controlled high-speed stirring is now fairly routine. The MTPS will selectively pass bulk organic phase even when the aqueous-organic phase pair is subject to high-speed stirring. At any time, about 1% of the total organic phase is withdrawn and circulated through the flow-through cell of the spectrophotometer (Figure l ) , where its clarity (freedom from turbidity is a sign of the completeness of (7) Dietz, M., unpublished observations.
0743-746318812404-0049$01.50/0 0 1988 American Chemical Society
