N e w Class of Crystalline Soluble Silicates Helmut H. Weldes Philadelphia Quartz Go. Research Laboratories, Primos, Pa. Crystalline,
water-soluble
double
silicates
containing alkali
19018
metal and quaternary
ammonium ions have been prepared and characterized. These silicates can contain sodium, potassium, or lithium ions as well as mixtures of these alkali metal cations in
combination
with
a
number
of
quaternary
ammonium
ions,
such
as
tetra-
methylammonium, tetra(2-hydroxyethyl)ammonium, N,N-di(2-hydroxyethyl)morpholinium, and N,N,N~,N’-tetra(2-hydroxyethyI)piperazinium.
Tm
CRYSTALLINE, water-soluble alkali metal-quaternary ammonium double silicates described are an entirely new family of compounds (Weldes, 1966). Some quaternary ammonium silicates containing no inorganic alkali have been described in the scientific literature. They were all prepared by dissolving various forms of silica in quaternary ammonium hydroxide solutions. Schwarz (1916) reported the formation of a crystalline salt with the formula [N(C2H5)4]2Si03. Glixelli and Krokowski (1937) claim to have produced a crystalline salt with the formula (CH3)4NHSi03 .8H20. Merrill and Spencer (1951) described the preparation of crystalline, hydrated quaternary ammonium silicates by dissolving silica gel in solutions of tetramethylammonium hydroxide, tetraethylammonium hydroxide, benzyl trimethylammonium hydroxide, and tetra(2-hydroxyethy1)ammonium hydroxide. I t has now been found that crystalline double silicates containing alkali metal cations and quaternary ammonium cations can be prepared very easily from solutions of quaternary ammonium hydroxide in alkali metal silicate solutions.
Experimental
Materials. The alkali metal silicates used were commercial products in the case of sodium and potassium silicates and experimental products in the case of lithium silicates, all produced by the Philadelphia Quartz Co. These materials had the composition shown in Table I . Ethylene oxide was obtained from Matheson Co. in 99.5% purity. Mono-, di-, and triethanolamine were obtained as pure liquids from the Union Carbide Chemicals Co. Tetra(2-hydroxyethy1)ammonium hydroxide was used from two sources: one, supplied by the Carbide and Carbon Chemicals Co., consisted of an aqueous solution containing 50.36% [ N ( C r H 4 0 H ) 4 ] 0 H ,12.16% NasO, and
0.29% Cog; the other was supplied by the R. S. A. Corp. as a 40% solution in methanol. Other bases included 28 to 30% aqueous solution of ammonium hydroxide, reagent grade (Allied Chemical Corp.), 10% solution in water of tetramethylammonium hydroxide (Eastman Kodak Co.) , methylamine 40% in water (Eastman Kodak Co.), morpholine (Eastman Kodak Co.), N,N’-bis(2-hydroxyethyl) piperazine (Union Carbide Chemicals Corp.), anhydrous piperazine (Eastman Kodak Co.), N- (2-aminoethyl)piperazine (Dow Chemical Co.), 50% methanolic choline base (Rohm & Haas Co.), 96% active tetra-N,N,N‘,N’2-hydroxyethylethylenediamine(Visco Products Co.) , and N-ethyl morpholine (Union Carbide Chemicals Co.). Preparation of Crystalline Double Silicates. GENERAL PROCEDURES. The alkali metal quaternary ammonium silicate solutions from which the double silicates are crystallized can be produced in two different ways. Method A. A suitable quaternary ammonium hydroxide or its solution is mixed with an alkali metal silicate solution, and the mixture is concentrated in vacuo at a temperature not exceeding 40” C. Method B. The quaternary ion intended to form the double silicate can be formed in situ. I n this case, a suitable amine is mixed with the alkali metal silicate solution and ethylene oxide is introduced into this mixture and allowed to react with the amine to form the quaternary ion. The reaction is permitted to go to completion and the solution is then concentrated as in Method A. The concentrated solution obtained by either method is cooled to facilitate crystallization. Several general rules are important for preparation of crystalline alkali metal-quaternary ammonium silicate: The concentration of alkali metal silicate in the reaction mixture should not be too high; above a certain limit, depending on the type of silicate, gel formation might Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970 243
occur and crystallization is slow. The isolation of the crystals becomes difficult because of the high viscosity of the mother liquor. The amount of water distilled off from the final reaction mixture is critical. Optimum performance is usually realized if enough water is removed to give a solution containing 97% of the amount present in the particular silicate used as starting material. The reaction temperature should be as low as possible-room temperature or possibly lower. However, below room temperature, reaction time becomes unreasonably long. The final solutions supersaturate very easily and need seeding and mechanical aid for faster crystallization. These two methods are described in detail in the example of sodium tetra(2-hydroxyethy1)ammonium silicate. Other compounds made by these methods are given in Table 11, indicating the method used and molar composition of the crystalline products obtained. Sodium Tetra(2-hydroxyethyl)ammonium Silicate. METHOD A. T o 1067 grams of E sodium silicate (weight ratio 3.22 S i 0 2 / N a z 0 ,27.7% SiOz) was added 1767 grams of a 50% aqueous tetra(2-hydroxyethyl)ammonium hydroxide solution. No precipitation nor coacervation formed on mixing and a clear solution was obtained. This was cooled and held in the refrigerator a t approximately 2" C. Crystals began to form in about 1 day and continued to grow in number and size for about 6 days. The crystals were then filtered off, washed with alcohol and ether, and dried in vacuo a t 30°C. The yield was 777 grams of crystals. The mother liquor was placed in the refrigerator and during 2 weeks 152 grams more of crystals formed. The mother liquor was then mixed with ethanol, which caused precipitation of an additional amount of crystalline product. The combined yield of crystalline material was 1105 grams. This was recrystallized from water to which
ethanol was added a t a ratio of 150 ml of water t o 20 ml of ethanol. The crystals were filtered on a Buchner funnel, washed twice with ethanol and twice with ether, and dried in vacuo a t 40°C. The pseudocubic crystals have a melting point of 57-59°C and soften a t 53°C. Analysis. Ignition loss 63.79'%, quaternary ion Nf(C2H40H)4 33.09'%, water 30.707, SiO? 28.317, NazO 7.71% with a mole ratio of 1 N a 2 0 to 1.4 N*(CzHaOH)4 to 3.8 Si02 to 13.7 HzO. The same product was obtained from other sodium silicates ranging in weight ratios of SiOn/NanO from 2.00 to 3.75, when equal weights of the 50% tetra(2hydroxyethy1)ammonium hydroxide solution were mixed with the undiluted silicate and the mixtures refrigerated. Attempts to obtain the product from metasilicate (ratio 1 NssO to 1 SiOz) were unsuccessful. METHODB. A solution of 120 grams of E sodium silicate in 300 grams of water was placed in a three-necked flask equipped with a stirrer, an inside thermometer, a gas inlet tube, and a low temperature reflux condenser filled with dry ice and acetone. T o this solution was added 24 grams of 29% aqueous ammonia. The vessel was immersed in a cooling bath and 74 grams of ethylene oxide was gassed into the reaction mixture through the gas inlet tube while stirring vigorously. The exothermic reaction, which started immediately, made it necessary to cool the reaction vessel to hold the temperature of the contents between 25" and 30°C. Otherwise, the internal temperature rises to about 80°C, which reduces the final yield. After about % of the ethylene oxide was added, the solution became turbid and a coacervate formed which became heavier as the final portions of the ethylene oxide were mixed in. During this period of about 45 minutes,
Table I . Composition of Alkali Metal Silicates Weight/Ratio SiO?/M20"
Type
Sodium silicates s 35
N E
K
RU D Metso Granular Potassium silicate Kasil 1 Lithium silicate
Mz0,
O h
SiO2,
Yo
HzO, %
Sp. Gr. at 20" C.
20" C., Poises
2.2 1.8 1.0 9.6 21.0 3.5
3.75 3.22 3.22 2.90 2.40 2.00
6.75 8.90 8.60 11.00 13.80 14.70
25.3 28.7 27.7 31.9 33.2 29.4
67.9 62.4 63.6 57.0 53.0 55.8
1.318 1.394 1.381 1.480 1.559 1.534
0.985:1:4.85b
29.5
28.7
41.7
...
20.8 19.8
70.5 76.9
1.259
2.50 6.00
8.30 3.30
...
Viscosity at
0.4
...
M = Na, K , or Li. *Mole ratio Na20:Si02:H?0. Table II. Composition of Alkali Metal-Quaternary Ammonium Silicates Compound
Lithium tetra(2-hydroxyethy1)ammonium silicate Sodium tetra(2-hydroxyethyljammonium silicate Sodium tetramethylammonium silicate Sodium methyl triethanolammonium silicate Sodium N,N-di(2-hydroxyethy1)morpholiniumsilicate Sodium N-di(2-hydroxyethyl)-N'[ (tris-2-hydroxyethyl)ethylammonium]piperaziniumsilicate Sodium N,N,N',N'-tetra(2-hydroxyethyl)piperaziniumsilicate Potassium tetra(2-hydroxyethy1)ammoniumsilicate Potassium trimethyl-2-hydroxyethylammoniumsilicate a
M 2 0 = Li20; NaZO; KzO. ' (NR,)>O= quaternary oxide.
244
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
Prepn. Method
B AIB A
B B B B B A
M20":(NR~)2Ob::Si02:HzO
SiOz/MzO+(NR,)yO
1:0.6:3.4:10.0 1:0.7:3.8:13.7 1:2.3:6.6:32.0 1:0.9:3.7: 7.3 1:0.5:3.2:12.7
2.12 2.23 2.00 1.95 2.13
1:1.3:4.8:23.5 1:1.7:5.0:20.4 1:0.7:3.9:13.3 1:3.4:9.8:28.4
2.09 1.85 2.29 2.04
the ethylene oxide refluxed in the low temperature reflux condenser. This stopped as soon as all the ethylene oxide had been incorporated. Agitation was continued a t 25" to 30°C, until all of the coacervate had redissolved and the clear solution had reformed. This took about 2 hours; then the vessel was closed and held a t room temperature overnight. Following this period, 290 grams of water was distilled off under a water aspirator vacuum a t 35" to 40°C. This left a clear, rather viscous solution which was placed in the refrigerator a t a temperature of 2 ° C . Crystals started to form very quickly and were filtered off after 12 hours. The crystals were filtered and washed as described under Method A. They had the same composition as those obtained in Method A. Again, the same product is obtained when sodium silicates of other mole ratios than 3.22 are used, as long as the ratio is 2.0 Si02/Na20 or greater. Mono-, di-, and triethanolamine may also be used in place of the ammonia, if the differences in ethylene oxide requirements for conversion of these materials to the quaternary ion are taken into account. Analytical Procedures. In analyzing the alkali metalquaternary ammonium double silicates, special procedures are necessary. If these procedures are not followed exactly as described, it is most likely that incorrect data will result. IGNITIONLOSS. The ignition loss is determined on a 1-gram sample. I t is extremely important to heat the sample in a closed platinum crucible very slowly. If the ignition is carried out too fast, silicon carbide forms, which is almost impossible to burn. Therefore, the crucible is heated very slowly on one side until all the organic matter has charred completely; this should take a t least 1 hour. Then the heat is increased slowly to full blast of the Tirrill burner and continued until the sample has turned completely to a water-clear melt; this takes about 2 hours. When the sample is clear, it is transferred to a Fisher burner and heated full blast for one-half hour. QUATERNARY AMMONIUM BASE. The nitrogen content is determined using the Kjeldahl procedure with special modifications for this type of compound. About 0.75 gram of sample is weighed into a 500-ml round-bottomed, twonecked flask, and 10 grams of dehydrated KrSO, and 2 grams of dehydrated CuS04 are added. After admixing 12 ml of concentrated H2SO4,a reflux glass tube is set on top of the flask and the mixture heated over a wire gauze in the hood slowly and cautiously, close to the boiling point of the sulfuric acid. Heating is continued until the originally dark solution becomes clear and no dark specks remain. This digestion period takes between 3 and 24 hours, depending on the composition of the sample. After the contents are cooled to room temperature, 100 ml of distilled water is added carefully through the reflux tube while swirling the contents. Then a few Alundum boiling stones are added and a magnetic stirrer. The reflux glass tube is removed and the flask is connected to the distillation equipment. The end of the condenser dips into a receiver filled with sufficient 0.2N HC1 to reduce the pH of the solution well below neutral. This acid is added with 100 ml of distilled water. Then about 130 ml of 6N sodium hydroxide is added through a dropping funnel while stirring with a magnetic stirrer. When all the sodium hydroxide is added, the reaction mixture is heated for 1 hour to vigorous boiling. The ammonia formed during the digestion is driven over to the 0.2N
HCl and after completion the free hydrochloric acid is back-titrated with 0.2N NaOH. ALKALIMETALDETERMINATION. The alkali metals in the double silicates are easily determined by standard flame photometric methods. For best results it is advisable, however, to remove the silica by precipitation before the alkali metal is determined. Potassium may also be determined by the usual gravimetric procedure in the residue of the ignition loss after removing the Si02 by treatment with H F . Sodium cannot be determined by the usual gravimetric procedure with magnesium uranyl acetate, because the quaternary bases also form precipitates with uranyl acetate. Sodium can be determined with adequate accuracy, however, as the difference between total titratable alkali of the alkali metal quaternary ammonium silicate and its quaternary content determined by the Kjeldah1 method. SILICADETERMINATION. Silica may be determined in solution using the usual volumetric or gravimetric procedures. POTENTIOMETRIC TITRATION. A potentiometric backtitration method for the determination of weak bases besides strong bases in alkali metal-quaternary ammonium double silicates has been developed. Silica interferes in the titrations, so that the following method has to be used: The total titratable alkali of the OAS material is determined and from this a sample weight is calculated which requires about 200 ml of 0.2N HCl for neutralization. The proper sample weight is weighed into a 50-ml beaker and diluted with water to about a 20 to 25% SiOr concentration. T o this solution is added exactly 2.5 ml of 2.ON HCl from a microburet. The gelled mixture is stirred by hand with a stirring rod, diluted with about 20 ml of water, and filtered by decantation on a Buchner funnel using Whatman No. 4 filter paper. The gel is washed free of chloride and all the filtrates are collected and combined in a suitably sized beaker containing a magnetic stirrer and pH electrodes standardized a t pH 7. The agitated solution is titrated with 0.2N NaOH and pH readings are taken a t intervals of 1 ml except a t the inflection points, where readings are taken at 0.1ml intervals. The results are plotted on regular chart paper as pH us. milliliters expressed as HC1. The number of milliliters to the inflection point a t the pH of 9 to 10 is calculated as the quaternary ion. The number of milliliters from the inflection point a t a pH of 9 to 10 to the second inflection point a t a pH of 4.5 to 5.5 is calculated as tertiary amine. Since in many cases the base strength of the* quaternary ammonium hydroxide is very similar to that of the sodium hydroxide, they cannot be distinguished. In such cases one has to determine the sodium ion gravimetrically or flame photometrically and then subtract this value from the amount of acid corresponding to the inflection point at 9 to 10 pH. The difference can then be calculated as the quaternary ion. Results
Synthesis. Alkali metal-quaternary ammonium silicates crystallize very well from solutions of quaternary ammonium hydroxides in alkali metal silicate solutions. Two methods of incorporating the quaternary ammonium hydroxide into the silicate solution are possible. I n one, a suitable preformed quaternary ammonium hydroxide or its solution is mixed with the alkali metal silicate solution at concentrations which avoid coacervation. The douInd. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970 245
Inflection point for tsrtiory omine
3
5
10 15 mi. 0.2 N HCI Solution.
20
Figure 1. Potentiometric titration curve for sodium tetra(2-hydroxyethy1)ammonium silicate solution before crystallization Showing inflection point for quaternary ion at about pH 9 and tertiary amine at about pH 4.5
ble silicates crystallize easily on concentrating the mixture to a point where no phase separation is observed on cooling, followed by refrigeration of the solution. Quaternary ammonium salts, such as tetraethanol-ammonium chloride, cannot be used in the formation of these crystalline double silicates. These salts are compatible with alkali metal silicate solutions only a t concentrations much too low for crystal formation; a t higher concentrations (where crystals might form) the compatibility is poor and coacervation or phase separation occurs, rather than crystal formation. I n a second method, the quaternary ammonium hydroxide intended to form the double silicate is synthesized in the alkali metal silicate solution. Any water-soluble amine that forms stable solutions in alkali metal silicate solutions can be used. The amine-alkali metal silicate solution is treated with ethylene oxide to convert the amine to the quaternary ammonium ion. The conversion of the amine to the quaternary ion by this reaction with ethylene oxide in the presence of alkali metal silicate solution is about 60% complete if stoichiometric amounts of ethylene oxide and amine are used. This is illustrated in Figure 1, which shows a potentiometric titration curve for a reaction solution of ammonia, ethylene oxide, and sodium silicate. Quantitative analysis of this curve indicates a per cent ratio of 1.4 tetra(2-hydroxyethy1)ammonium ion to 1 triethanolamine. The conversion can be increased even further, if a large excess of ethylene oxide is applied. The special potentiometric titration method used with these products is described in the experimental section.
The high rate of conversion of ammonia or amines with ethylene oxide to the quaternary ammonium ion in the presence of alkali metal silicate contrasts strongly with the conversion in the absence of silicate. The formation of ethanolamine from ammonia and ethylene oxide is a case of simultaneous, consecutive, competitive, secondorder reactions. This reaction in water, in the absence of silicate, does not go considerably beyond the tertiary amine formation, although Potter and McLaughlin (1947) mention in their kinetic study of ethanolamine formation in water from ammonia and ethylene oxide that they find some indication for a very limited and slow formation of the tetra(2-hydroxyethy1)ammonium ion. Mixed alkali metal-quaternary ammonium silicates are obtained when mixtures of different alkali metal silicates are used as starting materials. The different alkali metals are built into the crystalline product in about the proportions in which they are present in the original reaction mixture. There seems t o be no particular preference for one alkali metal cation over another. Generally, however, the rate of crystallization is faster for crystals containing potassium ions than for those containing the other alkali metal ions. Composition. The composition of a number of crystalline alkali metal-quaternary ammonium silicates is given in Tables I1 and 111. In describing these silicates in chemical terms the traditional silicate terminology was chosen by expressing the relative amounts of inorganic alkali present as the oxide ( M 2 0 , where M can be Li, Na, or K) and the organic quaternary ammonium ion in the same way-
Table Ill. Composition of Sodium-Potassium Tetra( 2-hydroxyethyl)ammonium Silicates Inorganic Alkali in Crystalline Product
Sodium Sodium, potassium Sodium, potassium Sodium, potassium Potassium
Si02 in Reaction Mixture Contributed from Sodium and/or Potassium Silicate
100% 7570~25% 50%50% 25%:75% 10070
"(NR4)nO = [ N ( C ~ H ~ O H ) ~ ]'MzO Z O . = Kz0
246
Mole Ratio
K?O:Na20:(NR4)nOn:SiOr
M20b:(NR4)20:Si02
R20C:Si02
:1 :0.7: 3.8 1:8.3:5.3:35.4 1:3.2:2.8:14.7 1:1.8:2.0:10.1 1:.. .:0.7: 3.9
1:0.7:3.8 1:0.6:3.8 1:0.7:3.5 1:0.7:3.6 1:0.7:3.9
1:2.2 1:2.4 1:Z.l 1:Z.l 1:2.3
+ NazO. 'RzO = KzO + Na20 + (NRJzO.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 197-0
Inflection point quaternary ion
for
3t
/I
\ i
I t I
5
I
I
I
15
10
ml. 0 . 2 N HCI
20
Solution
Figure 2. Potentiometric titration curve for solution of sodium tetra(2hydroxyethy1)ammonium silicate crystals Only one inflection point for strong alkali present; inflection point for tertiary amine has disappeared
Le., (NR4)zO both set in proportion to the SiO, content. This permits easy comparison of these compounds with regular alkali metal silicates. The last column in Tables I1 and I11 gives the mole ratio of SiO? to total alkali oxide in the mixed silicates. Most of these mole ratios are close t o that of a disilicate. Potentiometric titrations of crystallized double silicates redissolved in water indicate that only the quaternary ammonium ion is built into the crystals (Figure 2 ) . Only one inflection point for strong alkali is observed as compared to Figure 1, which shows an inflection point for lower amines of weaker basicity for an original reaction solution before crystallization. Properties. The physical properties of a few different crystalline alkali metal-quaternary ammonium silicates are summarized in Table IV. Most of these compounds are very soluble in water but insoluble in organic solvents. The solubility in water of sodium tetra(2-hydroxyethy1)ammonium silicate is 180
grams in 100 ml of water at 20°C and 35.5 grams in 100 ml at 1”C. The only exception is the sodium N,N,N’,N’-tetra(2hydroxyethy1)piperazinium silicate. This is insoluble in water, although the inorganic alkali can be leached out by successive washing with water. The ion exchange properties of this compound are being studied. Aqueous solutions of alkali metal-quaternary ammonium silicates are as incompatible with water-miscible organic solvents as regular sodium silicate solutions. Viscosity and pH of aqueous sodium tetra(2-hydroxyethyl) ammonium silicate solutions were determined a t 20” C as a function of concentration (Table V). The gelling properties of sodium tetra(2-hydroxyethy1)ammonium silicate solutions acidified with sulfuric acid were determined. The gel curves expressed as gel time us. pH a t different silica concentrations are shown in Figure 3. The curve for 12% SiO? concentration could be obtained only in the acid pH region; in the alkaline
Table IV. Properties of Alkali Metal-Quaternary Ammonium Silicates Compound
Melting Point, ’C
64-68 M“ 163-164 Dh Sodium tetra(2-hydroxyethy1)ammonium silicate 53 s’ 57-59 M 145 S Sodium tetramethyl-ammonium silicate 158-159 M Sodium methyl triethanolammonium silicate 70 S 75-77 M Sodium N,N-di(2-hydroxyethy1)morpholinium silicate 63 S 64-66 M Sodium N-di(Z-hydroxyethyl)-N’-[ (tris-2-hydroxy61 S ethyl) -ethylammonium]piperazinium silicate 68-69 M Sodium N,N,N’,N’-tetrakis(2-hvdroxvethvl)uiuerazinium 103 Bd - - _ _ silicate 130-135 D Potassium tetra(2-hydroxyethyl)ammonium silicate 52 S 56-58 M Potassium trimethyl-2-hydroxyethylammoniumsilicate 93 s 94-95 M Lithium tetra(2-hydroxyethyl)ammonium silicate
Refractive Indices
Density at 20”C
a
P
Y
Crystal Habit
...
1.500
...
...
Thin plates
1.604
1.498
1.506
1.528
Cube-like
...
,450
1.460
1.466
Irregular
...
...
...
...
Cube-like
1.623
.490
1.489
1.510
Cube-like
1.514
.494
1.500
1.506
Irregular
1.513
,490
1.494
1.498
1.624
1.490
1.496
1.502
Elongated plates Cube-like
...
...
...
...
Rhombic
M = melting. D = decomposing. S = softening. B = brown.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
247
Table V. Viscosity and pH of Sodium Tetra( 2-hydroxyethyl)ammonium Silicate Solutions a t 2OoC
Table VI. X-Ray Analyses of Sodium Tetra( 2-hydroxyethyl) ammonium Silicate Hydrated Sodium Tetra(2-hydroxyethy1)ommonium Silicate
Solids Content, Yo
Viscosity a t 20" C., Centipoiser
pH a t 20" C.
d,A
38.8 41.0 43.2 46.1
14 20 29 48
10.68 10.68 10.69 10.70
4.12 3.97 3.85 3.72 3.45 3.31 3.16 2.96 2.84
1/10
d,A
80 100 65 10 35 45 45 70 30
4.12 3.97 3.78 3.65
...
40 30 10 10
...
3.33 3.19 3.03 3.83
100 80 40 50
Anhydrous Sodium Tetra(2-hydroxyethy1)ammonium Silicate
3.07 2.65 2.45 2.37 2.26 1.78 1.75
Sodium Metosilicote Pentahydrate"
80 100 55 60 60 30 35
1/10
Anhydrous Sodium Metosilicate
3.04 2.57 2.40
100 48 64
...
... ... ...
1.75
40
... ...
Data from ASTM diffraction data cards.
PH
Figure 3. Gel curves (pH vs. gel time) for sodium tetra(2hydroxyethy1)ammonium silicate solutions at 12, 3, 2, and 1 Yo Si02 concentration Gelling agent sulfuric acid
pH region the difference between immediate gel formation and complete stability was too small to be measurable. Minimum gel time for all concentrations is at pH 7 to 8. These curves are very similar to those for regular sodium silicate solutions (Merrill and Spencer, 1950). The gel curves for other gelling agents, such as sodium bisulfate, boric acid, oxalic acid, etc., are very similar to those with sulfuric acid. X-ray powder diffraction data were obtained for the hydrated and anhydrous sodium tetra(2-hydroxyethy1)ammonium silicate (Table VI). The data were measured with a GE X R D 3 diffractometer using copper K radiation and a simulation counter. The lattice spacings are similar to those for hydrated and anhydrous sodium metasilicate. I t is most likely that the quaternary ammonium ion replaces sodium ions in the metasilicate structure, thus somewhat expanding the crystal lattice. Only the 3.45-A line in the hydrated crystals is not present in the sodium metasilicate pentahydrate but is present in tridymite. I t is very unlikely that tridymite is present,
248
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
and this line probably belongs to the lattice (7f the double salt. Upon heating (dehydrating), the lattice undergoes drastic change and then corresponds to that of anhydrous metasilicate. I t is, however, a somewhat expanded metasilicate lattice to accommodate the organic cation. Thus, these data indicate that, in the hydrated sample, lattice positions normally occupied by sodium ions have been taken by the quaternary ammonium ion and that the tetra(2-hydroxyethy1)ammonium ion is of about the same size as the hydrated sodium ion. When water is lost on dehydrating, a tighter lattice is formed containing nonhydrated sodium ions and tetra(2-hydroxyethyl) ammonium ions; the latter are oversized and expand the anhydrous metasilicate structure. Three distinctive lines a t 1.78, 2.26, and 2.37 are shown by the quaternary ammonium silicate which are not present in the anhydrous sodium metasilicate. Acknowledgrnen t
The author thanks M. R . Derolf and J. S. S. Bobb for their help with the experimental and analytical work. literature Cited
Glixelli, S., Krokowski, K., Rozniki Chem. 19, 309 (1937). Merrill, R. C., Spencer, R. W., J . Phys. Colloid Chem. 54, 806 (1950). Merrill, R. C., Spencer, R. W., J . Phys. Colloid Chem. 55, 187 (1951). Potter, C., McLaughlin, R. R., Can. J . Res. 25, Sec. B, 405 (1947). Schwarz, R., Ber. 49, 2358 (1916). Weldes, H. H., U.S. Patent 3,239,549 (March 8, 1966). RECEIVED for review September 4, 1969 ACCEPTED February 24, 1970
