Nov., 1957
EFFECT OF PARTICLE SIZEON SOLUBILITY OF AMORPHOUS SILICA IN WATER
1563
THE EFFECT OF PARTICLE SIZE ON THE SOLUBILITY OF AMORPHOUS SILICA IN WATER BY G. B. ALEXANDER Contribution from the Grasselli Chemicals Department, E . I. du Pond de Nemours & Co., Inc., Wilmington, Delaware Received June 4, 1967
The solubility of amorphous silica is related to the ?article size by the equation log S = 4.80 X 10-4A - 2.043, where 8 is the solubility in per cent. by weight in water a t 25 , and A is the specific surface area in square meters per gram. This solubility is affected by the amount of silanol (SiOH) groups in the internal structure of the silica polymer and by metal impurities. The surface energy of amorphous silica particles is estimated to be 1.1 X cal./cm.z
Introduction The solubility of amorphous silica in water is reported by Alexander, Heston and Iler’ to be about 0.0157&in the pH range from 2 to 8. Holt and KinG2 state that quartz particles behave as if soluble silica were being leached from their surfaces and that the amount of leachable silica which can be dissolved from the particles is greater with small sized quartz than with those of larger size. Greenberg and Sinclair3 conclude that the polymerization of monosilicic acid proceeds through the formation of siloxane bonds with the elimination of water, and that this reaction is a reversible phenomenon. White, Bannock and Murata4 conclude that nearly all silica of hot springs is in the monomeric form. They further conclude that solubility equilibrium exists between dissolved (monomeric) and amorphous silica and that in supersaturated solutions at low temperature, monosilicic acid polymerizes slowly to colloidal silica. Greenbergs discusses the thermodynamic functions of silica in water and concludes that, although theoretically solubility is a function of particle size, in the case of published data on silica, particle size seemed to have no influence on solubility. Ilers calculates the expected solubility for quartz as a function of particle size, but states that the surface energy for amorphous silica is not known, hence such a calculation for amorphous silica cannot be made. The purpose of the present paper is to expand the data given previouslyi and show that solubility of amorphous silica is a function of particle size.
Experimental The silica sols used in this study were prepared from sodium silicate solutions by ion-exchange techniques. Thus, C.P. sodium metasilicate, Na&iOs.9HzO, was dissolved in water and diluted to 2% SiO,. This sodium silicate solution was converted to a silicic acid solution by passing it through a column of cation-exchange resin of ”Nalcite” HCR in the hydrogen form. The resulting silicic acid solution was then alkalized by adding one mole of sodium metasilicate as a 2y0 SiO, solution for each 90 moles of silicic acid. Thereafter silica sols containing particles of different sizes were prepared from this solution by heating the solution for (1) G. B. Alexander, W. M. Heston, Jr., and R. K. Iler, THIS JOURNAL, 68, 453 (1954). (2) P. F. Holt and D. T. King, Nature, 1’71,514 (1955). (3) 9. A. Greenberg and D. Sinolair, THISJOURNAL, 69, 435 (1955). (4) D.E. White, W. W. Bannock and K. J. Murata, “Silioa in Hot Spring Waters,” U. 9. Geological Survey, Washington, D. C., January 6, 1956, (5) 9. A. Greenberg, THIS JOURNAL, 61, 190 (1957). (6) R. K. Iler, “The Colloid Chemiatry of Silica and Silicates,” Cornell University Press, Ithaca, N. Y.,1955,Chapter I.
different lengths of time at 80’. The resulting sols were then cooled, and deionized, by contacting the sol with further ion-exchange resin in the hydrogen form. Finally fractions were prepared by adjusting the pH to 2.0,4.0, 6.0 and 8.0. The resulting samples were stored in polythene a t room temperature for 3 months, and the per cent. soluble silica was then determined by the method given by Alexander, Heston and Iler.1 The particle surface area of the silica particles in the sols was determined by a titration technique, involving the determination of the amount of alkali required to react with the silica particles over a given pH range.’ Particle size can be calculated from surface area from the relationship D = K / A where D is the particle diameter, A the specific surface area, and K is a constant, of about 3000. It was observed that for a given particle size, solubility did not vary with pH, over the range pH 2 to 8. However, sols of pH 2 and 8 come to equilibrium more rapidly than sols of pH 4 and 6. Results of these studies are shown in Fig. 1, and a t 25O, may be summarized by the reIation log S
0.000480A
- 2.043
Although the effect of temperature has not yet been checked, i t is expected that a generalized formula for the solubility of amorphous silica i s given by 0.143 A logS/So == T
-
where 8 is solubility in % Si02 of a particle having a surface area of A , m.2/g. and SOis the solubility of a massive piece of amorphous silica a t TOK. From the relationship above cited, one can calculate the solubility of massive forms of amorphous silica at 25”, as 0.0091 %. Solubllity of Silica Particles Prepared from Commercial Silicates.-A series of dilute silica sols was prepared in a way similar to that described in paragra hs 1 and 2 of example 1 of U. S. Patent 2,574,902.s f n this case, “F” grade sodium silicate (Si0z:NazO = 3.25; SiOn = 28.4%) was used. Heating time was varied in order t o control particle size. The solubility of these amorphous colloidal silica particles in water at pH 8 was quite similar to the solubility of silica prepared from C.P. sodium silicate. However, at lower pH the solubility was lower and did not vary in any reguIar fashion. It is believed that this variable solubility may be due to differences in cationic content of the various sols, since it has been shown that cations such as aluminum, iron and magnesium affect the solubility of silica, and it is known that these impurities are present to varying extents in different lots of commercial sodium silicate. Solubility as a Function of Internal Structure.-It has been shown that colloidal silica particles which are polymerized in the temperature range of 80-loo”,consist almost entirely in their internal structure of amorphous SiOa. However, silica particles which are polymerized at a lower temperature appear to contain appreciable amounts of silanol groups, -$i-OH in the internal structure of the amorphous
I
particle. It has been found that a silica product which was prepared by polymerizing silica in a dilute solution a t room temperature for two years, until the particle size of the silica ___(7) G. W. Sears, Anal. Chem., 28, 1981 (1956). (8) M. F. Bechtold and 0. E. Snyder, U. S. Patent 2,574,902 (du Pont).
JEANA. SABATKA AND P. W. SELWOOD
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cn
100
200
300 400 500 600 PARTICLE SURFACE AREA ( d / g . ) ,
700
Fig. 1.-Solubility of silica sol particles a t pH 2.0. polymers corresponded to a surface mea of 307 m.2/g., had a solubility of 0.0163% at 25'. The solubility of particles of this size polymerized a t 80-100°, is 0.0129%, or 24% less than the solubility of the product polymerized at room temperature. It is postulated that this increase in solubility is due to the more open internal structure of the silica particles polymerized at room temperature. Reaction of Silica Particles with Molybdic Acid.-It has been observed that silica polymers can be depolymerized in the presence of molybdic acid reagent which is used to determine the content of soluble silica in silica sols. A similar result was observed by Goto and Ookura.9 Goto suggested that the rate of this reaction is a way of determining particle size.10 However, i t would appear that such a proposal would be complicated because the rate of this reaction is affected by ( a ) the internal silica structure, specifically the amount of internal silanol content and (b) presence of metal silicate in the structure of the silica particles. (9) K. Goto and T. Ookura, Japan Analyst, 4, 175 (1955). (10) Katsumi Goto, J . Chem. SOC.Japan, Pure Chem. Sec., 7 6 , 929 (1955).
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It has been observed that the reaction of silica polymers with molybdate usually could be ignored, if the particle size is greater than 20 mp. However, in sols containing smaller particles, the colored silicomolybdic acid complex forms a t a slow, constant rate for a long period of time, due to the depolymerization of silica and reaction of the monomer thus formed with molybdic acid. I n order to determine the original concentration of monomer, a correction was made for the color which develops due to this depolymerization. This was done by extrapolating the color-time curves to zero time and subtracting from the total color, the color due to depolymerization. That depolymerization is the cause of this color formation was shown by diluting a given silica sol to '/a of its original silica concentration. After standing for three weeks, this sol approached the same value for solubility of monosilicic acid as the original, but showed only l / 8 of the color formation due to depolymerization.
Discussion The surface energy of amorphous silica particles can be calculated, assuming the relationship R - -T=l n- S m SO
2E dr
where R = T = m = S = SO= d = E =
gas constant, cal./deg./mole O Absolute molecular weight of monomer solubility of partide radius, r, cm. solubility of massive particle density of solid, g. per cc. surface energy, cal./cm.2
whence E
=
15.7~log S/So
For amorphous silica particles prepared from C.P. sodium metasilicate, and in aqueous solution in the pH range 2-8, E = 1.1 X 10-6 cal./cm.2.
THE STRUCTURE OF NICKEL-COPPER-SILICA CATALYSTS BY JEANA. SABATKA AND P. W. SELWOOD Contribution from the Chemical Laboratory of Northwestern University, Evanslon, Ill. Received June 6 , 1967
Specific magnetizations are given for several nickel-copper-silica catalyst samples, and for one unsupported nickelcopper Sam le. Measurements were made from -196" to 600°, after reduction, and both before and after sintering. The degree to wtich true solid solution of nickel and copper occurs in these systems is found to depend on the over-all nickel concentration. Samples containing high concentrations of nickel tend to yield more nearly homogeneous solutions. Lower concentrations of nickel result in the nickel and copper being separately aggregated, as in a mechanical mixture. This difference is related to the relative accessibility of large and small nickel particles.
Introduction The chief question to be answered in this conCopper sometimes is added to nickel, or to nickel- nection is: when copper is added to a nickel-silica silica catalysts. The purpose of this addition is catalyst in the preparative stage do the metals form twofold: first, to alter the d-band electron concen- a true, homogeneous, solid solution, or are they septration in fundamental studies relating to the mech- arately aggregated as a mechanical mixture? Measanism of heterogeneous catalysis; and second, be- urements of specific magnetization may, in princicause of a belief that the activity, or useful life, of ple, be used to answer this question. This is true because the magnetization and Curie point of a the catalyst may be improved in this way. The structural relationship of copper to nickel is nickel-copper solid solution are quite different from easily determined for systems containing these two those of a mechanical mixture of the two. In a metals only, without a support. One example of typical nickel-silica catalyst, however, the problem such a system is described below. But for supported is complicated because the nickel particles are so nickel-copper-silica systems the problem is more small that they exhibit super-paramagnetism' difkult because the components in these systems (1) C. P. Bean [ J . Applied Phus., 26, 1881 (195511 has used the are generally so nearly amorphous as to give only term "super-paramagnetio" for syatema so small that, while the diffuse X-ray diffraction bands. atomic magnetio moments within each particle possess long range ferro-
