I
CHARLES E. GRABIEL, LEO R. MORRIS, PAUL J. SIENKNECHT, and RHODES N. FARRIS The Dow Chemical Co., Midland, Mich.
Recrystallization of Sodium Vinylbenzenesulfonate With this method, purified monomer can be recovered in 80% yield from reaction mixtures containing sodium halide
Sodium vinylbenzenesulfonate, more commonly known as sodium styrenesulfonate, is an ionic water-soluble vinyl monomer. As a comonomer in a vinyl copolymerization, it imparts to the finished product properties such as dyeability, water sensitivity, stability, resistance to static build-up, ion exchange and polyelectrolyte properties. The degree to which these properties are exhibited, of course, depends on the proportion of sulfonate monomer present in the copolymer. Inorganic salts, which are formed when preparing sodium vinylbenzenesulfonate, are often undesirable in the final copolymer formulation. However, the monomer can be purified efficiently by the recrystallization method described.
V.
IUYLBENZENESULFONIC ACID derivatives have been used for preparing watersoluble polyelectrolytes and water-insoluble ion exchange resins. Various methods for preparing vinylbenzenesulfonic acid derivatives result in the formation of esters, amides, or salts of sulfonic acid, depending on the conditions of the reaction. Basically, these processes involve the dehydrohalogenation of haloalkylbenzenesulfonic acid derivatives according to the following equation :
X C 1 H (O - S 0 2 Y
+ MOH
the water-soluble halide presents no difficulty. However, when the derivatives are water-soluble, the problem of separating the halide from the sulfonate arises. Because solubility of sodium arylsulfonates is decreased by the presence of sodium chloride ( I ) , fractional crystallization of the sulfonate is one method of preparing purified sodium vinylbenzenesulfonate. Therefore, an investigation was undertaken, of the phase equilibria in the threecomponent system: sodium vinylbenzenesulfonate-sodium halide-water.
After equilibrium had been attained (at least 12 hours), samples were centrifuged in a clinical centrifuge, and then filtered through a medium porosity fritted glass funnel, using a suction flask with a small vial in it to recover the filtrate. T h e filtrate was analyzed for sodium vinylbenzenesulfonate and sodium halide to determine its composition. Sodium vinylbenzenesulfonate was determined by the standard bromatebromide titration technique used for determining carbon-carbon double bonds. T h e sodium halides were determined by potentiometric titration with silver nitrate, using a calomel electrode as the reference and a silver electrode as the indicator. The results were calculated on a weight per cent basis.
Results Sodium VinylbenzenesulfonateSodium Bromide-Water System. Bccause of either analytical error or undetermined impurities, analyzed constituents of the sodium vinylbenzenesulfonate used in this system total only
Experimental ___ h
A'
=
MX
+ HzO
Br, C1; Y = OH, CI; and Z =
OM, OR, NHz
Where 2 is an amino group, the sulfonyl halide is treated with ammonia or an amine prior to dehydrohalogenation. With a n alcohol as solvent, the haloethylbenzenesulfonyl halide gives a vinylbenzenesulfonic ester but in aqueous medium, the metal salt of sulfonic acid is formed. Similarly in aqueous solution, haloethylbenzenesulfonic acid yields a metal salt of vinylbenzenesulfonic acid. In the foregoing preparative methods, a metal halide is always present in the reaction mixture. Where vinylbenzenesulfonic acid derivatives are oil-solublee.g, esters and amides- the presence of
Phase diagrams for the sodium vinylbenzenesulfonate-sodium halide-water systems at 25" C. were constructed. From these the proper conditions, if any, can be determined for purifying sodium vinylbenzenesulfonate by recrystallization. Data for constructing the phase diagrams were obtained by the method of Purdon and Slater (2). Complexes of various composition were synthesized from purified sodium vinylbenzenesulfonate, sodium halide, and water. The system was allowed to come to equilibrium with gentle agitation on a sample roller at room temperature. No special precautions were taken to control the temperature which remained about 25' C.
Analysis
of
Sodium Vinylbenzenesulfonate 70
Sodium vinylbenzenesulfonate (by unsaturation) Sodium bromide (silver nitrate titration) Water (Karl Fisher titration) Polymer (organic sulfurmonomer) Sodium sulfate Total
A-
B'
72.4
92.1
12.4
1.6
1.4
0.9
4.6 2.4
3.0 nil 97.6
93.2
For the system containing sodium bromide. For the system containing sodium chloride. a
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ON SOLUBILITY CURVE
0
POINT
-X-
COMPLEX POINT
":'
A
Figure 1. Cornpsilion of the solid hase in equilibrium with fhe solution is located by the intersection of the tie line which connects a point o n the solubility curzle and its corresponding complex point with the side of the triangle. Temperature, 25" C . 4
NaBr
NoSS
93.270 (see column A of the tabulation). Therefore the complexes were made so that the polymer and other impurities were calculated as water. In Figure 1, concentration of each component is plotted along an altitude of the isosceles triangle with the vertex representing 100% of the component. The circles indicate the solubility curvei.e., composition of the filtrate in equilibrium with the solid phase. The X's indicate the complex composition which is the composition of the total system. The intersection of the tie line connecting a point on the solubility curve
0
*
and its corresponding complex point with the side of the triangle locates the composition of the solid phase in equilibrium with the solution. In the sodium vinylbenzenesulfonatesodium bromide-water system the tie lines converge on the sodium vinylbenzenesulfonate-water side close to the sodium vinylbenzenesulfonate vertex. The undetermined impurities present in the monomer, as well as the proximity of the solubility and complex points, in a few cases are responsible for the tie-line scatter about the monomer apex. This scatter precludes a positive
FILTRATE COMPLEX
statement about the exact composition of the solid phase other than that no sodium bromide is precipitated with the vinylbenzenesulfonate when the complcx has a composition which lies above the line from the invariant point 10 the monomer vertex. The invariant point in the sodium vinylbenzenesulfonate-sodium bromidewater system is so close to the sodium bromide-water edge of the triangle that determination of its exact position is beyond the accuracy of the measurements. The intersection of the tit: lines from the points grouped about the sodium bromide-water point with the sodium vinylbenzenesulfonate-sodium bromide side indicates that these points lie close to the invariant point. Therefore, in this system, sodium vinylbenzencsulfonate is the only component which can be separated in a pure state by crystallization. Sodium VinylbenzenesulfonateSodium Chloridewater. The monomer purity here was higher than that in the preceding system (see column B of the tabulation). Some small amount of residual sodium bromide was present in the monomer. To conserve a threeComponent system, this sodium bromide was calculated as sodium chloride on a molar basis. Thus the monomer sample was considered to have 0.88YGsodium chloride by weight rather than l.6yG sodium bromide. This assumption has no significant effect on the results a t high sodium chloride concentrations, and has little or no effect at low salt concentration. A s in the sodium bromide system, all components except monomer and halide were calculated as water. In Figure 2, the monomer and the sodium chloride have about the same solubility, 26 and 26.5%, respectively.
4
Figure 2. Convergence of the lie lines o n the sodium vinylbenzenesulfonate vertex indicates that the solid phase is pure monomer. Temperature, 25" C.
846
INDUSTRIAL AND ENGINEERING CHEMISTRY
SODIUM VINYLBENZENESULFONATE
Figure 3. Effect of water concentralion o n the crystallizing temperature of monomer-halide solutions. The sodium bromide system contains from 2 to 4% more solids at a given temperature than does the sodium chloride system. Temperature. 24" to 88" C. b
In this graph convergence of the tie lines on the sodium vinylbenzenesulfonate vertex indicates that the solid phase is pure monomer. As in the sodium bromide system, because the invariant point lies close to the sodium chloride-water side of the triangle, only sodium vinylbenzenesulfonate can be recovered in a pure state from the mixture by recrystallization. Effect of Temperature on Solubility. The entire temperature-composition surface was not investigated. Rather the temperature curve a t one fixed ratio of monomer to sodium halide was studied. Equimolar amounts of sodium halide and monomer were dissolved in water. The saturation temperature was determined as the temperature a t which sodium vinylbenzenesulfonate first started to precipitate. The system was agitated to prevent supercooling. I n Figure 3, saturation temperature is plotted us. the per cent of water in the complex. The sodium bromide system contains from 2 to 4y0 more solids at a given temperature than does the sodium chloride system.
11% of sodium vinylbenzenesulfonate, but a 5% sodium bromide solution will dissolve 16Y0 of monomer. Reduction of the monomer concentration of a given solution to 1% requires a sodium chloride concentration of l8y0, and a sodium bromide concentration of 33%. However, when the salting-out effect of the two sodium halides are compared on a molar basis, the picture changes somewhat. In Figure 5 , the solubility curves for the two systems are plotted on the same graph with an expanded scale. The points for the two systems lie on the same curve. Therefore,
when compared on a molar basis, sodium bromide and sodium chloride are equally effective in salting-out sodium vinylbenzenesulfonate. Thus, a concentration of 5 mole yo of either sodium halide in a solution of sodium vinylbenzenesulfonate will leave only 0.2 mole yG monomer in solution. These results validate the assumption that the small quantity of sodium bromide present in the monomer used in the sodium chloride system can be calculated as sodium chloride on a molar basis. They also indicate that the same saltingout effect can be expected from mixed
Discussion
Comparison of the Two Systems Both solubility curves in Figure 4 have the same shape with the invariant point so close to the side of the triangle that determination of the exact location was beyond the limits of accuracy of the experiments. The straight lines, A and A', are the loci of complex points which are equimolar in the salts, sodium chloride and sodium bromide, respectively, and sodium vinylbenzenesulfonate. Sodium chloride gives more efficient salting-out on a weight basis than does sodium bromide. A 5y0 sodium chloride solution will dissolve only
0 SODIUM CHLORIDE 0 SODIUM BROMIDE
b
Figure 4. On a weight basis, sodium chloride gives more eflcient saltingout than does sodium bromide. Temperature, 25" C . NaCl NaBr
NaSS
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847
0 0
X=CI X=Br
Figure 5. Comparisons on a molar basis and weight basis di$er. When compared o n a molar basis, sodium chloride is no more effectioe than sodium bromide in salting-out. Temperature, 25’ C., concentration units, mole % 4
NaX (20%)
N a S S (209bl
halides such as are found in the dehydrohalogenation of bromoethylbenzenesulfonyl chloride. The molar equivalence of the two sodium halides for salting-out sodium vinylbenzenesulfonate does not hold at the sodium halide end of the solubilitv curve, because of the differences in solubility of the two halides. The effect of this difference is s e m in a comparison of the maximum concentrations of equimolar mixtures of salt and monomer which will result in the precipitation of pure sodium vinylbenzenesulfonate. The line connecting the invariant point of the solubility curve with the composition of the solid phase (in this case pure sodium vinylbenzenesulfonate) is the boundary line for the precipitation of a pure component. A complex whose composition lies above this line will give pure monomer in the solid phases,
but one whose composition lies below will give a solid phase composed of a mixture of salt and monomer. Because the invariant points in the solubilitv curves in the sodium halide-sodium vinylbenzenesulfonate-water systems lie at the pure salt point, the phase boundaries for the two systems will be different. In the sodium chloride system, the maximum solids concentration of an equimolar mixture of salt and monomer which will yield pure monomer is 18 mole yo. I n the sodium bromide system, the comparable concentratior is 26 mole yo. Efficiency of Monomer Recovery. ’The efficiency for recovering pure sodium vinylbenzenesulfonate from a complex containing a given molar ratio of salt to monomer can be calculated from the temperature-composition curve (Figure 3) and the phase diagram (Figure 4). For
an equimolar mixture of salt and sodium: vinylbenzenesulfonate? the results ar? presented in Figure 6. The theoretical recovery of pure monomer is plotted us. the saturation temperature for a onestage process. The composition of the complex is obtained from the plot of saturation temperature us. composition (Figure 3). IYithin experimental error, the recovery of sodium vinylbenzenesulfonate is the same for both salts on a molar basjs. .l‘he maximum recovery of sodium vinylbenzenesulfonate from an equimolar mixture of salt and monomer in a one-stage process is in the range of 80 %. Isolation of P u r e Monomer. Pure sodium vinylbenzenesulfonate was prepared by repeated recrystallization of crude reaction product. The purity of the recrystallized product was determined by elemental analysis and chcmical analysis. Elemental analysis indicates a purity greater than 98.5%. Calcd. for C8H,SaO3S: C. 4 6 . 6 ; H, 3.4; S:15.5. Found: C, 47.2; H, 3.5; S, 15.5. Analysis of the product for monomer’ by bromare-bromide titration, salt by silver nitrate titration, and water by Karl Fisher titration gave the following values : Monomer, 96% : sodium bromide, nil; water, 1.1%. ‘The ultraviolet spectrum of the sodium vinylbenzenesulfonate has an absorption band at 256 rnp with a molar extinction coefficient of 20,200. Literature Cited (1) Cook?, W. T., J. Chem. SOC. Ind 40,
56T (1921).
(2) Purdon, F. F.; Slater, V. W., “Aqueous
351b 2b
3’0’
,
Solutions and the Phase Diagram,” Edward A4rnoldand Go., London, 1946.
1
40 510 $0 70 SATURATION TEMPERATURE, O
810
90
Id0
C
Figure 6. For both sodium chloride and bromide, recovery of sodium vinylbenzenesulfonate is about the same. Saturated solution of dehydrohalogenation mixture 848
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review February 2 3 , 1960 ACCEPTED July 5, 1960 Division of Physical and Inorganic Chrinistry, 131st Meeting, ACS, Miami, Fla., April 1957.
