Measurement of the Solubilities of Vinylic Monomers in Water

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Measurement of the Solubilities of Vinylic Monomers in Water† X.-S. Chai,‡ F. J. Schork,*,§ Anthony DeCinque,§ and Karl Wilson§ Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, Atlanta, Georgia 30332, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100

In aqueous emulsion and miniemulsion polymerization, the product may be dependent upon the ability of the monomer(s) to diffuse across the continuous aqueous phase. This, in turn, is dependent upon the solubility of the monomer in the aqueous phase. Reference solubilities are often difficult to find or measure using different methods and temperatures. This paper presents the solubilities of a number of low-solubility monomers at 60 °C, using the multiple headspace extraction gas chromatography technique described by Chai et al. (J. Appl. Polym. Sci. 2004, in press). The data were collected under uniform conditions and provide a base for judging relative solubilities. These data will be useful not only as absolute measurements of water solubility but also as a tool to judge the relative solubilities of various monomers. Introduction Aqueous dispersed-phase polymerization (suspension, microsuspension, emulsion, and miniemulsion) is widely used in the polymerization of vinylic monomers because of the low viscosity and good heat transfer imparted by the continuous aqueous phase and because of the environmentally benign nature of water as compared with the organic solvents used in solution polymerization. (Suspension polymerization refers to macroscopic monomer droplets dispersed in water and then polymerized via an oil-soluble free-radical initiator. Microsuspension polymerization uses small amounts of surfactant and mild shear to reduce the droplet size to approximately 50 µm. Polymer particles are of the same size. Emulsion polymerization produces submicron polymer particles through the use of high concentrations of surfactant and water-soluble free-radical initiators via nucleation of micelles. Miniemulsion polymerization produces submicron polymer particles from submicron monomer droplets formed by high shear and a combination of surfactant and cosurfactant.) In many aqueous dispersed-phase polymerization systems, the molecular structure of the polymer product, as well as the morphology and particle size distribution of the product, depends on the solubility of the monomer in the aqueous phase. In emulsion polymerization, poor water solubility of a comonomer may result in poor incorporation of the monomer into the polymer product because monomer transport through the continuous aqueous phase may be restricted for a highly water-insoluble monomer. In suspension polymerization, high water solubility may result in the production of “fines”, or very small particles formed by homogeneous nucleation in the aqueous phase. This encompasses the concepts of reactionlimited versus transport-limited reactions, so pervasive in the field of reaction engineering and in the work of Dr. Dudukovic. Because monomers are generally very † This paper is dedicated to Prof. Mike Dudukovic. * To whom correspondence should be addressed. Tel.: (404) 894-3274. Fax: (404) 894-2866. E-mail: joseph.schork@ chbe.gatech.edu. ‡ Institute of Paper Science and Technology. § School of Chemical and Biomolecular Engineering.

insoluble in water, references often list monomers as “insoluble” or give unreliable data. The data are unreliable because different methods are often used for different monomers and may be reported at different temperatures. This paper reports the water solubility of a number of monomers, all measured by the same method, in the same laboratory, and at the same temperature. These data will be useful not only as absolute measurements of water solubility but also as a tool to judge the relative solubilities of various monomers. The technique developed by Chai et al.1 was applied to a wide variety of monomers in order to construct a base of monomer solubilities under uniform conditions. The basic concept of the method is to allow a known monomer-water mixture to achieve vapor-liquid equilibrium in a closed vial. The headspace of the vial is then sampled and fed to a gas chromatograph (GC) where the output peak size gives a measure of the monomer concentration in the vapor phase. The vapor removed is replaced by inert gas to maintain the pressure. Between each headspace sample, an equilibrium time is provided to allow vapor-liquid equilibrium to reestablish. The trends in the GC peak size over time allow extrapolation back to the calibrated monomer solubility. One now has a peak area that corresponds to a known solution of monomer. Now a saturated solution of monomer is prepared in the same way. As the experiment progresses, the GC peaks will maintain a constant value until all of the excess (insoluble) monomer is consumed. This constant saturated peak size and the known unsaturated solution and peak size can be used to determine the unknown solution saturation point by proportion.

calculated area saturated area ) known solution unknown saturation point During experimentation, the limits of this method were tested and some method modifications were incorporated. These modifications are explained, and the final solubility data are presented. Experimental Section Monomers. Monomers that range from slightly soluble to almost completely insoluble were selected. All

10.1021/ie0492303 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/12/2005

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5257 Table 1. Solubilities in Water at 60 °C monomer

MW

CAS no.

solubility (wt %)

solubility (mol %)

molarity

acrylonitrile methyl acrylate vinyl acetate methyl methacrylate glycidyl methacrylate ethyl acrylate norbornylene 4-methylstyrene 3,3-dimethylacrylic acid 1-octene butyl acrylate styrene R-methylstyrene stearyl methacrylate divinylbenzene 2-ethylhexyl acrylate 4-vinylbenzyl chloride VEOVA 10 isooctyl acrylate isodecyl acrylate tert-dodecanethiola isobornyl acrylate hexadecanea lauryl methacrylate stearic acida acrylic acid stearyl ester dioctyl maleate 1-dodecanethiola

63.1 86.09 86.09 100.12 142.15 100.11 94.16 118.2 100.1 112.22 128.17 104.151 118.2 338.6 130.2 184.278 152.62 198 184.2 212 202.4 208.3 226.45 254.4 284.5 324.5 340.56 202.4

107-13-1 96-33-3 108-05-4 80-62-6 106-91-2 140-88-5 498-66-8 622-97-9 541-47-9 111-66-0 141-32-2 100-42-5 98-83-9 32360-05-7 1321-74-0 103-11-7 1592-20-7 26544-09-2 29590-42-9 1330-61-6 25103-58-6 5888-33-5 544-76-3 142-90-5 57-11-4 4813-57-4 142-16-5 112-55-0

10.32 5.53 4.27 2.25 2.67 2.13 0.655 0.649 0.523 0.437 0.338 0.237 0.089 0.0473 0.0173 0.0164 0.00712 0.00361 0.00297 0.00258 0.000669 0.000312 too insoluble too insoluble too insoluble too insoluble too insoluble too insoluble



a

Not monomers but often used in emulsion/miniemulsion formulations.

solubilities were measured at 60 °C because it is in about the midrange of reaction temperatures commonly used for dispersed-phase polymerization. In addition, the water solubilities of other compounds used in aqueous dispersed-phase polymerization formulations were measured. The compounds measured are listed in Table 1. Sample Preparation. Two samples of each monomer in water were required, one saturated and one below saturation. Both of these samples were prepared in 2 mL of water except for the most insoluble monomers, which were prepared in 4-10 mL. The 2 mL of water also contained 15 ppm of surfactant to help accelerate dissolution and monomer droplet stabilization. This very low level of surfactant should not affect monomer solubility measurements. The amount of monomer required, of course, depended upon each monomer, but values were mostly in the microliter region. To prepare nonsaturated solutions of the most insoluble monomers, a dilute solution was prepared first. The samples were heated at 60 °C for at least 15 min before being run in the multiple headspace extraction gas chromatograph (HS-GC). Experimental Apparatus. All measurements were carried out using an HP-7694 automatic headspace sampler and a model HP-6890 capillary gas chromatograph (Agilent Technologies, Palo Alto, CA). GC operating conditions are listed in Table 2. Results and Discussion Modifications to the Method. The role played by the GC in this method was not the standard role of separation and detection but only of detection. The separation is not necessary; indeed, there should be only one compound present. Instead, the GC was used only for detection. A long GC column adds to the retention time and reduces the sharpness of the sample peak. Because many of these monomers are of low volatility

Table 2. Operating Conditions for HS-GC column type column size column temp carrier gas carrier gas flow rate detector type hydrogen flow rate air flow rate equilibrium timea vial pressurization time sample loop fill time loop equilibration time extraction cycle timea

HP-5 capillary 0.5m × 0.35 mm × 0.25 µm 200 °C helium 3.8 mL/min flame ionization detector 35 mL/min 400 mL/min 6-10 min 0.2 min 1.0 min 0.05 min 8-12 min

a Times depended upon the sample volume. Larger volumes required longer times.

and are present in extremely low concentrations, long columns resulted in significant peak tailing in the chromatogram. To rectify this, the GC column was replaced with a short 0.5-m column. Another choice made in these experiments was to use a minimal amount of water. As shown by Chai et al.,1 the equilibration time increases exponentially with the sample size. However, using less water meant that there was also less monomer to detect. Larger volumes were required to improve the detection sensitivity for the most insoluble monomers. The equilibrium heating time was increased for these samples to 25 min. The modification that proved to be most efficient was a change in the method. The technique originally described by running the saturated sample as the GC peak areas approached a constant value. This value denoted the saturation point of the monomer. The extractions would then continue until a “transition point” was reached and the peak area began to decrease. This denoted the removal of all excess monomer until only the aqueous phase remained. The peak area at the transition point was used as the peak area corresponding to the saturation point of the monomer itself. However, because the peak areas became constant for

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several samples before the transition point was reached, it was not necessary to wait for the transition point to appear. As soon as the saturated peak areas reached a constant value for several samples in a row, the experiment was stopped and the constant value was considered as the transition point peak area. Limits of the Method. The method works best for the range of solubilities between 10-6 and 10-2 wt %. One of the assumptions in the method is that Henry’s law applies. (This is in part because a linear change is assumed between each sampling.) If the monomer is too soluble, then a significant volume of the solution will be made up of monomer and Henry’s law cannot be applied. Acrylonitrile is an example of a monomer that was too soluble to be tested by this method. A monomer can also be too insoluble in water to measure. If only a small amount of monomer can be added without saturating the solution, then the liquid concentration will be very small. The vapor concentration, depending on Henry’s constant, will be proportional. If the vapor concentration drops below the detection limit of the GC, the method fails. As mentioned before, a shorter GC column was used to help mitigate this. Larger volumes of water can also be used, but one must wait accordingly longer between samples. Lauryl methacrylate is an example of a monomer that was unable to be measured because it was too insoluble. Other compounds for which the method failed are listed in Table 1. As the monomer solubility decreased, measurements became less accurate. In the parts per million range, traces of solution on the walls of the vial, the measurement accuracy, and the surfactant added to speed up equilibrium all began to be affected. The most insoluble monomers began to deviate from the exponential decay shown by other solutions. Instead, the decay approached a sharper 1/(peak area) decline. After efforts were made to avoid this, it was decided that the trend could still be used for extrapolation. When the extraction number versus 1/(peak area) was plotted, a linear trend line could still be fitted. In no case was the correlation value allowed to fall below R2 ) 0.96. For the most insoluble monomers, the readings could be very sensitive. These monomers were also the ones that required a dilution step to prepare the solution. It was found that running a sample using the wastediluted solution could help “clear” the GC column of any monomer remaining from an earlier sample. For much the same reason, it was also easier to run the saturated solution first. The saturated solution was expected to need a few samplings before settling to a constant saturated value, so there is more tolerance by running the saturated sample first. Solubility Data. Monomer solubilities in water at 60 °C are listed in Table 1, as weight percent, mole percent, and molarity. A few observations can be made. Hexadecane, generally considered to be the most effective miniemulsion costabilizer because of its extreme water insolubility, is indeed so water insoluble that it is immeasurable by this method. Lauryl methacrylate, used as a miniemulsion costabilizer and comonomer by Chern and Chen,2,3 is also too insoluble to be measured by this method. Interestingly, Chern and Chen found stearyl methacrylate to be a more effective costabilizer, whereas our data indicate that it is more water soluble than lauryl methacrylate. There are, of course, other

Figure 1. Relationship of the water solubility (as weight percent at 60 °C) of alkyl acrylate monomers as a function of molecular weight.

factors influencing the effectiveness as a costabilizer. Dodecanethiols, used by the rubber industry for their known low water solubility, are shown to be extremely insoluble by this method. It has been said4 that styrene is the least water-soluble monomer for which conventional macroemulsion polymerization is entirely reaction-limited. By this criterion, a number of common monomers might be suspected of exhibiting diffusion limitations in macroemulsion polymerization; however, most of the sparingly soluble monomers are used as comonomers in conjunction with more soluble monomers. In this case, the more soluble comonomer in the aqueous phase will serve to solubilize the less soluble comonomer. Figure 1 shows that, for a given class of monomers (alkyl acrylates, the only class for which we have enough data points to make a meaningful correlation), it may be possible to correlate water solubility to molecular weight. It is hoped that these data, by virtue of the fact that all data were taken at the same temperature, by the same method, and in the same laboratory, will give a useful index as to the relative water solubility of vinylic monomers and will spark additional discussions of the issues of monomer transport in aqueous dispersed-phase polymerization. Note Added after ASAP Publication. This paper was released ASAP on January 12, 2005, with incorrect values in row 1 of Table 1. The correct version was posted on February 8, 2005. Literature Cited (1) Chai, X. S.; Hou, Q. X.; Schork, F. J. Determination of Solubility of Monomer in Water by Multiple Headspace Extraction Gas Chromatography. J. Appl. Polym. Sci. 2004, in press. (2) Chern, C. S.; Chen, T. J. Stability of Polymerizable Surfactant-Stabilized Latex Particles during Semibatch Emulsion Polymerization. Colloid Polym. Sci. 1997, 275 (11), 1060. (3) Chern, C. S.; Chen, T. J. Effect of Ostwald Ripening on Styrene Miniemulsion Stabilized by Reactive Cosurfactants. Colloids Surf. A 1998, 138 (1), 65. (4) Vanderhoff, J. W., private communication.

Received for review August 23, 2004 Revised manuscript received October 21, 2004 Accepted October 25, 2004 IE0492303

Measurement of the Solubilities of Vinylic Monomers in Water

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