S. A. GREENBERG
960
TABLE I11 THEHEATOF THE REACTION 2MgO(s) 2B(s)
+
Temp.
("IC.)
Hole area (cm.2 X 102)
1337 1.107 1422 1.107 1436 1.107 1357" 2.47 1359" 2.47 1375" 0.502 0 For the last three
Pressure
BaOz
(atm. X 106)
Pressure M g (atm. --(F X 108)
;H3*)2~g(B) -
('
=
Vol. 61 2Mg(g)
-THzBs)BzOn(g)
+ BaOa(g)AT 298°K.
-(F
-TH2es)2B(a)
-(F -Tos)2MgO(s)
2.87 1.93 78.20 72.27 8.80 28.16 6.21 4.19 78.68 73.11 9.32 29.24 10.27 6.94 78.76 73.24 9,40 29.44 1.98 1.33 78.32 72.47 8.94 28.42 2.15 1.45 78.28 72.49 8.96 28.46 9.72 6.56 78.42 72.66 9.06 28.66 measurements, the boron-magnesium oxide mixtures were compacted a t 16,000 p.s.i.
lium oxide, the monomer should be present in a higher relative concentration from reaction between those materials than from reaction between boron and magnesium oxide or boric oxide. Consequently, Soulen and Margrave used the beryllium oxide-boron reaction data of Seamy4 to calculate a limit of L +5.3 kcal. per mole for formation of BO from the elements a t 0°K. This value corresponds to a dissociation energy a t 0°K. 58.1 e.v. Spectroscopic data are consistent with a dissociation energy between 7.2 and 9.1 e.v.l0
AH2;B
253 264 263 262 262 252
For a chemical determination of the heat of formation of BO, reaction conditions are required that yield only very low concentrations of,dimer. Under such conditions the favorable entropy of dissociation of the dimer will increase the relative concentrations of the monomer. The monomer would probably be the major oxide species yield,ed by a reaction of a hafnium or zirconium boride with hafnium or zirconium oxide. A mass spectroscopic study of such a reaction might be the best means of fixing the stability of BO.
THE DEPOLYMERIZATION OF SILICA IN SODIUM HYDROXIDE SOLUTIONS BY S. A. GREEN BERG^ Laboratory for Physical and Inorganic Chemistry, Leiden, Holland Received March 1 1 , 1067
The rates of depolymerization of quartz and several amorphous silicas in sodium hydroxide solutions were followed by electrical conductance and light scattering measurements. From the results it was concluded that the activation energy for depolymerization is 21.5 kcal./mole. Also the rate is directly proportional to the silica concentration. Above pH 11 the hydroxyl ion concentration does not markedly affect the rate if the solutions are stirred. No influence on rate was found on adding sodium sulfate to the reaction mixtures. The rate is a function of the surface area and structure of the amorphous silicas. Quartz shows an extremely slow rate of reaction compared to the amor hous silicas. A tentative mechanism is proposed. The degree of polymerization of the silica in solutions of depolymeriie$silica was estimated from light scattering measurements.
It is well known that silica gels will dissolve in concentrated sodium hydroxide s o l ~ t i o n s . ~ JHowever, very little quantitative research on the process has been reported in the literature. Since it is generally agreed that silica is a polycondensation polymer of silicic acid Si(OH)r, then the depolymerization reaction involves hydrolysis Si-0-Si
+ HgO = 2SiOH
(1)
Although this reaction occurs in water,4it is catalyzed by hydroxyl ions and is found to go to completion only in solutions which are sufficiently alkaline. I n this study the rates of depolymerization of amorphous silicas and quartz in sodium hydroxide solutions with pH values from 7.1 to 13.0 were followed by electrical conductivity and light ( 1 ) Chemistry Department, Seton Hall University, South Orange,
N. J. (2) For discussion see R. K. Iler, "Colloidal Chemistry of Silica and the Silicates," Cornell University Press, Ithaca, N. Y., 1955. (3) G. B. Hurd and S. H. Laning, THISJOURNAL, 68,914 (1954). (4) G. B. Alexander, W. H. Hcston and R. K. Iler, ibid., 58, 453 (1954).
scattering measurements. I n addition, the degrees of polymerization of silicic acid a t equilibrium in this pH range were examined. To obtain some understanding of the mechanism of the reaction, the influences on the rate of depolymerization of concentration of reactants, pH, stirring, electrolyte concentration and temperature were measured.
Experimental Materials.~-Mallinckrodt special bulky (85.6% Si02 414.4% H,O)? standard luminescent (81.2% Si02 18.8% HzO) and silica aerogel (93.5% Si02 Nags04 HzO) were used. Also a relatively pure (99.9% SiOg) sample of gulverized a-quartz was examined. The other materials employed in the study were chemically-pure. Apparatus.-The conductivity apparatus consisted of a Philips (Eindhoven) Bridge GM 4249 and dip conductivity cells with constants of approximately one cm.-l. All the conductivity measurements were performed in a constant temperature bath (&O.Ol"). The light scattering equipment was manufactured by Netheler and Hinz (Hamburg) after the design of Cantow. The details of a similar apparatus were described by Cantow.6 To maintain the tempera-
+
(5) 8. A. Greenberg, ibid.,60, 325 (1956). (6) H. J. Cantow, Z . Nakrforsch., 78, 485 (1952),
++
July, 1957
96 1
DEPOLYMERIZATION OF SILICA IN SODIUMHYDROXIDE
ture of the solution in the cell, water was passed through a hollow jacket surrounding the cell. By means of this arrangement the temperature of the solution was held constant to 3=0.05°. A Beckman Model H glass electrode pH meter was used for the measurement of hydrogen ion activities at 25'. At the higher pH values, corrections were made €or the sodium-ion effect. Procedures.-For the conductivity experiments 800 ml. of sodium hydroxide solutions was placed in a three-necked flask, into which a stirrer, thermometer and dip cell were inserted. After the solutions reached the desired temperatures the silica samples were added. I n each case stirring was continued throughout the entire experiment exce t in a series of runs which were made to investigate the eiect of stirring speed on reaction rate. A blank run was conducted for 72 hr. with a sodium hydroxide solution to establish that no appreciable changes in conductance occur because of carbonation, evaporation, or soluble impurities in the glass. The light scattering apparatus was calibrated with silica sol mixtures of known turbidities. These turbidities were determined with a Unicam spectrophotometer by transmission measurements on water and solutions.? The solutions examined contained only silica sol and sodium hydroxide. Kinetics of Depolymerization.-The depolymerization of silica in alkaline solutions can be expressed by the over-all eq. 1. The acidic SiOH groups will react with OH- ions to form SiO- groups. A reduction in the conductance of the solution will consequently occur as the reaction proceeds* because the conductance of SiO- ions is less than that of OHions. Only SiO- and SiOH groups will appear below pH 11.8 on molecules of different molecular weights.B Doubly charged >Si('-
will form above this pH value. When one 0OH- ion reacts an SiO- ion forms which may be on a monomer or polymer of silicic acid. Also it will be assumed that the conductivity of the SiO- ions is not strongly dependent upon molecular weight. I n order to apply the rate equations to the conductivity measurements the concentration of unreacted silica was assumed to be proportionnl to Lt L , where Lt and L, are the specific conductances of the solutions at time t and infinity, refipectively. The quantity LO Lt where Lo is the specific conductance at time zero, is then proportional to the concentration of depolymerized silica. Therefore LO- L , is proportional to the total concentration of silica and ( L t - L,)/(Lo -. L,) is the fraction of unreacted silica x remaining at any time 1 , and 1 - x is the fraction of silica which has been depolymerized in this time. Similarly the readings r made with light-scattering equipment will be handled in the same manner. The readings r made with this apparatus are proportional to the weight average molecular weight of the silicate particles. The concentration of unreacted silica is set proportional to r L T, and the concentration of product is then proportional t80 T O - r l . The fraction of unreacted silica x remaining in solution a t time t is (rt r,)/(ro T,) and the fraction of product formed in time t is 1 - z.
-
-
Results Conductivity Experiments. A. Rate Dependence on Silica Concentration.-In Fig. 1 the rate of depolymerization a t 40" as a function of silica concentration is shown in LO- Lt os. time plots. For these experiments the quantities of special bulky silica necessary to make up NazO:SiOzmole ratios 1:2, 1: 1.5, 1 : 1 and 1 :O.GG were added to 800 nil. of 0.025 N sodium hydroxide solutions. If the fraction of silica depolymerized 1 - x is plotted as a function of time then all the points fall on the same curve. From this it may be concluded that the rate of reaction is directly proportional to the silica concentration since the total change in conductance Lo - L , will depend on the amount of silica present. (7) P. Dotyand R. F. Steinw. J . C h e m Phus , 18, 1211 (1030). (8) S. A . Greenberg and J . J. Herlnans, tn press.
1
I
I
I
I
I
10
20 30 40 50 Time, minutes. Fig. 1.-Rate dependence on silica concentration; NhO: SiOz mole ratios are shown on the curves.
The usual functions of Lt - L,, which is proportional to the concentration of unreacted silica, were plotted as a function of time, but no linear relationships were obtained. It must, therefore, be assumed that the over-all mechanism is complex and probably changes order during the course of the reactions. This is perhaps shown by the fact that a TABLE I SUMMARY OF DATAO N DEPOLYMERIZED SILICA SOLUTIONS r X
Expt, no.
1
2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17 18 19 20 21 22 23
106,
NazO : Si01
SiOz, %
1:0.5 l:O.G6 1:l 1:1.5 1:2 1:0.33 1.0.5 1:l 1 :2 1:4 1:0+ 1:l 1.2 1 :3 Orig. soln. silica sol. 1:20 1:6.6 1:4 1:3 1:2 1:0.70
0.075 ,050 ,075 .117 ,150 0.075 ,075 ,075 ,075 ,075 1.5 3 .O 6.0 9.0
0.025 ,025 ,025 ,025 ,025 0.075 ,050 ,025 .0125 ,0062 1 .o 1.0 1 .o
0.1 .1
1700 negl. 7.1 1500 0.0016 9.7 1000 ,0048 9.9 670 ,0080 10.43 .0104 10.45 480 ,0160 10.7, 220 32 ,0480 12.1 24 ,0800 12.7 57 ,8000 12.8
1:0.40 1:O.O-l
.I .1
.I .I .1
.1 .I
INa+l
1 .o
PH
12.0 12.0 11.9 11.6 11.0 12.5 12.4 11.9 10.3 10.0 13.0 12.8 12.3 11.3
an1.-1
-0 -0 4 21
.. -0 N O
4 184 650
..
I13
240 416
902
S. A. GREENBERG
Vol. 61
ln(Lt - Lm) us. time plot becomes linear as the reaction proceeds to completion. At equilibrium the solutions exhibit p H values above 11 (Table I, expts. 2-5). B. Rate Dependence on pH.-In Fig. 2 the rate dependence a t 40" on concentration of sodium hydroxide is shown graphically in Lo - L1 us. time plots. In each case, to 800 ml. of solution with the normalities indicated in the figure, 0.7 g. of special bulky silica (0.6 g. SiOz) was added. It may be seen that in 0.025, 0.05 and 0.075 N solutions the curves exhibit essentially the same shape! However, the rates in the 0.0125 and 0.0062 N solutions are much slower. When plots of 1 - x are made as a function of time all the curves exhibit the I same shape. This suggests that the fraction of 10 20 30 40 50 silica depolymerized in unit time is independent of Time, minutes. sodium hydroxide concentrations for solutions a t Fig. 2.-Rate dependence on concentration of sodium equilibrium with pH values above 10 (Table I, hydroxide (0.8 1. solution + 0.6 g. SiOz). Symbols for normalit of sodium hydroxide solution: @, 0.0062 N : 0, expts. 6-10). It is interesting to note that the conductance changes Lo - Lm are approximately the 0.0125 Q, 0.025 N ; 8 , 0.050 N ; 0 , 0.075 N . same for pH values above 11.9, but at pH 10.3 and 10 the conductance changes are appreciably 100 smaller, which shows clearly that depolymerization is incomplete in the solutions with the lower p H values. C. Rate Dependence on Temperature.-In Fig. 3 80 are shown the rates a t which silica is depolymerized at temperatures between 30 and 60" in 1 - 2 us. time plots. Each solution contained the same mixture: 800 ml. of 0.025 N sodium hydroxide and 60 Fi 0.7 g. of special bulky silica (0.6 g. SiO,). If it is I assumed that the initial portions of the curves are 3 linear then d ( l - z)/dt values may be estimated. 40 Plots of loglo d ( l - x)/dt us. 1/T where T is the absolute temperature are given in Fig. 4. From this curve the activation energy was found to be 21.5 kcal./mole. 20 D. Effect of Stirring on Rate.-When the mixture of 800 ml. of 0.025 N sodium hydroxide and 0.7 g. of special bulky silica (0.6 g. SiOz) is allowed to react a t 40" with varying degrees of stirring, I I then rate curves of the type shown in Fig. 5 are 10 20 30 obtained. When no stirring is applied the rate Time, minutes. Fig. 3.-Effect of temperature on rate of silica depolymeriza- is markedly slower, but if stirring is applied motion. mentarily in this experiment after 100 and 135 minutes of reaction, then the conductance rapidly approaches the value obtained with continuous stir2.5 ring. I n the cases where varying amounts of stirring were applied: (1) stirring only before conductance readings were taken and (2) speed 1 and speed 2 (approximately double speed 1) stirring, only small differences in rate may be observed. H Apparently only a small amount of stirring is necessary to increase the rate markedly. E. Effect of Sulfate Ions on Rate.-Because of the influence of sulfate ions in cement chemistry sa ,o 1.5 and to examine the effect of electrolyte on the rate, sodium sulfate was added to the reaction mixtures used in the experiments described in the previous section. The electrolyte concentration was doubled by this addition. In Fig. 5 it may be noted that the course of the reaction did not change sub3 10 320 330 stantially because of the sodium sulfate addition. 1/T x 106. F. Rate Dependence on Structure of Silica.Fig. 4-Arrhenius plot to obtain activation energy for depolymerization of silica. In Fig. 6 the variation of reaction rate at 40" with I
1
I
-
h
3
,
I
I
I
I
silica structure is shown in 1 - 2 vs. time plots. The reaction mixtures contained 800 ml. of 0.025 N sodium hydroxide and 0.6 g. of SiOz. The nitrogen adsorption surface areas of the silicas are listed in Table 11. TABLE I1 THEEFFECTOF SURFACE AREA O N RATE Silica
Std. Lum. Sp. B Aerogel Quartz
963
DEPOLYMERIZATION OF SILICA IN SODIUM HYDROXIDE
July, 1957
d(1
Rate
- s)/dt 15 3.0 1.3 a0
Surface area,
sq. m./g. Si02
750 380 250
