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Behavior of a pyrogenic silica in simple electrolytes
Behavior of a Pyrogenic Silica in Simple Electrolytes R.P.
ABENDROTH
Fundamental Research, Owens-Illinois, Inc., Toledo, Ohio 48601 Received January 9, 1970; accepted March 3, 1970 Adsorption of hydrogen and hydroxyl ions on a nonporous pyrogenic silica was determined by potentiometrie titration in LiC1, I4C1, and CsCI electrolytes over the pH range 1.8-9.0. No evidence of hydrogen ion adsorption was found. The sequence of counterion adsorption was Cs + > K + > Li +. An attempt was made to increase the extent of surface hydration of this silica by treatment in aqueous solution. This product had the same properties as untreated silica, except for slightly lower charge densities above pH 6. This difference was attributed to adsorption of impurities during treatment rather than to any significant effect of hydration state. Charge densities were found to be somewhat higher than those reported for precipitated siliea, but much lower than those reported for mieroporous silica. Differential capacitance curves indicate the eounterions for precipitated silica to be further away from the surface. It is suggested that Cs+, classed as a structure breaker in aqueous solution, is better able than Li+ to penetrate any structured water phase existing at the silica surface, in accordance with observed adsorbabilities. INTRODUCTION The surface properties of high surface area amorphous silicas can v a r y according to the method of preparation. These variations have been interpreted as indicating differences in the n u m b e r and arrangement of surface silanol groups (1). Pyrogenic silicas, e.g., CAB-O-SIL (2), are considered to have a high proportion of free or isolated silanol groups, whereas precipitated silicas have a higher proportion of silanol groups hydrogen bonded to each other (3,4) as determined from gas phase studies. Although these various silicas have been well characterized with respect to water v a p o r adsorption, their interaction with hydrogen and hydroxyl ions in aqueous solutions is not well understood. Tadros and L y k l e m a (5) determined adsorption isotherms for a microporous precipitated silica and observed charge densities much larger t h a n those observed b y Bolt (6) for a nonporous precipitated silica. The intent of this work was to characterize in aqueous solutions the adsorptive properties of a nonporous, high-purity silica of
pyrogenic origin, and compare results with those reported for nonporous and microporous precipitated silicas. MATERIALS AND METHODS The pyrogenic silica used was CAB-O~ S I L M-7, with an Ar B E T surface area of 170 m~/gm. I t was used as received after drying at 115°C. A complete Ar adsorptiondesorption isotherm showed negligible hysteresis. I n an effort to increase the extent of surface hydration, M-7 was heated for 24 hours in 6N HC1 at 90°-95°C. This product, designated silica 153, was then filtered and resuspended in distilled deionized water a number of times until the p H of the filtrate approached that of the water. After drying at 115°C, the Ar B E T surface area was 166 m2/gm. Semiquantitative spectrographic analyses of these silicas are given in Table I. The titrants, HC1 and appropriate bases, and electrolytes were prepared with distilled deionized water and were standardized prior to use. Potentiometric titration of the suspension and of an equal volume of blank electro-
Journal of Cvlloid and Interface Science, VoL 34, No. 4, D e c e m b e r 1970
591
592
ABENDROTH
lyre was used to determine the extent of hydrogen and hydroxyl ion adsorption as a function of pH. The suspension and blank were contained in borosilicate vessels with tightly fitting covers and were maintained at 20°C in a water bath. Wet prepurified N.o was bubbled through both vessels to prevent effects due to C02. Titrants were dispensed from microburets read to 4-0.05 ml. The ionic strength of the suspension and blank was maintained constant during titration. The usual suspension concentration was 5.0 gm silica per 400 ml electrolyte. Titration of a suspension containing 1.0 gm per 400 ml showed that results were independent of suspension concentration. The silica was introduced into the electrolyte approximately i8 hours before titration commenced. The t:trations were started at pH 4 to 5, carried Out to pH 9, then down to about pH 1.8. :In some cases, HC1 additions were made down to pH 1.5. However, titration errors and uncertainty in pH readings become larger in this region, leading to excessive scatter in the experimental points. It is felt that pH 1.8 to 1.9 probably represents the lower limit in the acid region for this technique. Qualitative pI-I drift experiments were conducted according to the method of B~rub6 and de Bruyn (7) in the region pH 1.4 to 1.6. In these experiments, additions of dry silica were made to the electrolyte, and any change in pH was noted. Each addition comprised about 600 m ~ of surface area. Instrumentation included an Orion 801 pH meter and Beckman glass and reference electrodes. TABLE I SPECTROGI~API-1IC ANALYSES OF THE SILICAS USED Element concentration range (ppm) Not detected Less than 5
Less than
i0
L e s s t h a n 25 3-30 2-20
h~[-7
153
B
B
Cu, Fe, Mg, Mn, Mo Ni, Sn, Pb, Ti Cr, A1
Cu, Mg, Mn, Mo Ni, Sn, Pb, Ti Cr
Ca, Zn, Zr
Ca, Zn, Zr Al
-
-
--
Fe
RESULTS
Charge densities are based on the Ar BET surface areas and are calculated from = F(r.+ - to,-),
[1]
where (r~+ - ro~-) is the adsorption density of hydrogen over hydroxyl ions, per square centimeter. Charge densities as a function of pH for M-7 in KC1 at ionic strengths of 10°, 10-1, 10-2 M are shown in Fig. 1, and in 10-1 M CsC1, KC1, and LiC1 in Fig. 2. It was observed in the solutions above PI.I 5 that after an addition of base, rapid adsorption occurred, followed by extremely slow adsorption. These observations are similar to those made previously (5, 7, 8). Below about pi.i 5, any changes in pH upon addition of titrant were complete within 15 to 20 rain and the pH was invariant thereafter. The fast adsorptions were complete within 30 rain, and comprise the present results. Extension of the equilibration time to several hours showed that the additional change in pH did not exceed the indicated uncertainties in the plotted results. Some hysteresis was observed upon titrating from high pH to lower pH's but is not shown in the plotted results. A comparison of the results for 1V[-7 and silica 153 at 10-1 M ionic strength in CsC1, KCI, and LiCl is shown in Fig. 3. An unequivocal point of zero charge (pzc) can in principle be established by the intersection of isotherms determined at varying ionic strengths. It was not possible to do this in this study, since the isotherms in Fig. 1 superimpose below pi.i 3-3.5. The problem of where to place the isotherms with respect to absolute charge density was resolved by noting that the pH region where the isotherms are horizontal is also the region where identical additions of HC1 to suspension and blank give identical changes in pH. It is possible that the silicas could be carrying a charge constant with respect to pH in this region, but this possibility was disposed of by considering the results of the pH drift experiments. Addition of dry silica to electrolyte in the horizontal region of the isotherms gave no change in pit down to pit 1.4. This is interpreted as no net interaction
Journal of Colloid and Interface Soience, Vo]. 34, No. 4, December 1970
BEHAVIOR OF PYROGENIC SILICA IN ELECTROLYTES
593
10 o
-22
/
-20 -18 -16
/J(//
~ g -'" %
-12 -I
-6-4-2-
0I
I
l
I
2
3
4
5
pH
I
I
l
I
g
7
8
9
FIG. 1. Charge densities for CAB-O-SIL M-7 in KC1 as a function of ionic strength. -22.
?csc,
-181
CI
-16:
LiCl
-J4:
~l? -,2: - Jo:
-8: -6:
pH
FIG. 2. Charge densities for M-7 in 10-1 M CsC1, KC1, LiC1. occurring between silica and hydrogen or hydroxyl ions in this region, indicating an essentially neutral surface. Differential capacities were derived for M-7 and silica 153 b y graphical differentiation of the e h a r g e - - p H curves. The relation used was given b y B6rub6 and de B r u y n (7), as corrected b y Ball (9). c =
F 2 d(r~+ -- r o ~ - ) 2.3 R T d p H
[2]
These results are plotted in Fig. 4 at 10-~ M ionic strength. Also plotted are values derived from the results of Bolt (6) for a nonporous precipitated silica in NaC1. DISCUSSION CAB-O-SIL M-7 and silica 153 do not show any quantitative evidence of hydrogen ion adsorption at ionic strengths 10- I and 10-2 M down to p H 1.8, or in 10 ° M solu-
Journal of Colloid and Interface Science, Vol. 34, No. 4, D e c e m b e r 1970
594
ABENDROTH
tions down to pH 1.5. No qualitative evidence of hydrogen ion adsorption was detected down to pH 1.4 in the pH drift experiments. These data are in contrast to the M-7
-so:
/ t ~ 3
-t6: -12 .41 o'
4
5
6
7
s
9
-IE
-4: O'
~ 4
Kcl
~~-------------------"~" - ' 56 - 7
~
9"
-201
M-7
-161
,
153
-12:
-41 o
4
5
6
pH
?
8
9
F i e . 3. Charge densities for M-7 ~nd silica 153 n 10-1 M CsC1, KC1, LiC1. LiCl
KCI
CsCl M-7
22O-
I I I I ]
2O0180160~
120I00-
eo~ so20~ 0
extensive positive charge development at pH 2 observed for microporous silica (5). Precipitated silica (6) shows several points at pH 2 that indicate hydrogen ion adsorption; however, these points are not unequivocally determined in view of the titration errors associated with this pH region and the low indicated charge densities. It has been suggested (10) that the silicon cation sufficiently polarizes the OH group so that a protonated water molecule is unable to bond to it. The present results support this view. The charge densities for AJl-7 are compared to the results of Tadros and Lyklema (5) in 10-1 M KC1, and to Bolt's data (6) in 10-1 M NaCI in Fig. 5. The much higher charge densities exhibited for microporous silica have been attributed to the presence of internal hydroxyl or hydrogen ions, as opposed to groups interacting on the surface alone. It is apparent that M-7 exhibits slightly higher charge densities than silica 153, and both of these are greater than for precipitated silica. The treatment of M-7, resulting in silica 153, was done in an effort to make :V[-7 behave similarly to precipitated silica on the assumption that the essential difference is due to extent of surface hydration. Inspection of Fig. 4 indicates this was not achieved.
~
Cs(;I KCl LiCI
I I
t I
NaCl
I 3
I M-7
'// I
I
t
I pH
FIG. 4. Differentia] capacities of M-7 and silica 153 in 10-I M CsC], KC1, LiC]. Journal of Colloid and Interface Science, Vo]. 34, No. 4, December 1970
BEHAVIOR
OF PYROGENIC
SILICA IN ELECTROLYTES
595
I00
-
-80
~LyklO adr°s nd erna
~l~ - 60 -
40
M-7
-20 0 +20 ~40
2
5
4
5
6
7
8
9
pH F r o . 5. C o m p a r i s o n of M - 7 w i t h t h e r e s u l t s of T a d r o s a n d L y k l e m a in 10-~ M KC1, a n d w i t h B o l t ' s r e s u l t s in 10- t M NaC1.
It is apparent, however, that some difference Cs + > K + > Li+, confirming previously in behavior was achieved by this treatment. reported results (5, 13). The precipitated To determine whether silica 153 was more silica in NaCI shows similar differential cafully hydrated than M-7, the ignition method pacitance at pH 6, but lower values at of de Boer and Vleeskens (11) was employed. higher pH's. Generally speaking, the pyroSince M-7 may have hydrated to an unknown genie silica and precipitated silica have extent during immersion in the electrolyte roughly similar properties but it is perhaps before titration began, these determinations surprising that this pyrogenic silica exhibits were carried out for M-7 as received and higher charge densities, since it is thougl~t after immersion in distilled deionized water, that pyrogenie silicas in general have fewer and for silica 153. Since internal as well as reactive sites. Heston, Iler, and Sears (14) surface hydroxyls can be removed by this were able to fully ionize the surface of a method (12), the percentage weight loss on precipitated silica without dissolution at pH ignition is reported rather than surface hy- 12 and found Noi~ = 3.5. Application of this droxyl concentration. These weight losses technique to M-7 and silica 153 indicated are: M-7 before immersion, 1.67 %; ~.I-7 after that full ionization, if achieved, was also immersion, 1.64%; silica 153, 2.32%. Thus accompanied by dissolution, and a deterit is possible that the surface of silica 153 is mination of NcH was not possible. The difmore fully hydrated than M-7, but the ferential capacitance values indicate the extent is unknown outing to possible intro- eounterions are further away from the duction of bulk hydroxyls. In view of this, charged surface of the precipitated silica it is not possible to conclude unequivocally than for M-7, possibly indicating a more that the slight differences in the adsorption tightly packed structured water phase (15, isotherms and differential capacitance curves 16) adjacent to the surface. Thus, there for M-7 and silica 153 are due to any change appear to be significant differences in the in the degree of surface hydration. The structure of the electrical double layer formed spectrographic analysis for silica 153 shows on those two types of silicas. that it contains increased amounts of aluA model of the electrical double layer minum and iron. If these were confined to formed on oxides that exhibit extensive inthe surface, they could possibly have the teraction with water has been advanced by effect of reducing hydroxyl ion adsorption up B6rub6 and de Bruyn (7) to explain the to the pH range 8-9, and hence might better sequence of eounterion adsorbability on ruaccount for the slight difference in adsorp- tile. Their argument is that the observed tion behavior of silica 153 as compared to sequence Li+ > Na + > Cs+ exists because M-7. Li+ would tend less to disrupt the structured Examination of Fig. 4 shows that the water layer next to the futile surface than adsorbability of the eounterions for both Cs +, since Li+ is more highly hydrated and M-7 and silica 153 increases in the order is classed as a structure promoter in aqueous Journal of Colloid and Interface Science, VoL 34, No. 4, December 1970
596
ABENDROTH
solutions. Thus Li + would be better able to penetrate the structured layer t h a n Cs +, resulting in the higher observed differential capacitances. This does not appear to be the case for silica. I f Cs + is to be classed as a structure breaker, then it might better be able to penetrate this layer, resulting in the sequence observed in this study. REFERENCES 1. HOCKEY, J. A., Chem. Ind. (London) 1965,
57. 2. Product of the Cabot Corp., Boston, Mass. 3. SNYDER, L. R., AND WARD, J. W., J. Phys. Chem. 70, 3941 (1966). 4. McDoNALD, R. S., ]. Phys. Chem. 62, 1168 (1958). 5. TAD~OS, Tm F., AND L:~KLEMA,J., J. Electroanal. Chem. 17, 268 (1968). 6. BOLT, G. H., J. Phys. Chem. 61, 1166 (1957).
7. B~RUB]~, Y. G., AND DE BRUYN, P. L., J. Colloid and Interface Sci. 28, 92 (1968). 8. PARKS, G. A., AND D~ BRVYN, P. L., J. Phys. Chem. 66, 967 (1962). 9. BALL, B., J. Colloid and Interface Sci. 30, 424 (1969). 10. L I , I-I. C., AND DE BRUYN, P. L., Surface Sci.
5, 203 (1966). 11. DE BOER, J. H., AND VLEESKENS, J. M.,
Koninkl. Ned. Akad. Wetensch. Proc. Set. B
61, 2 (1958). 12. DAVYDOV, V. YA., KISELEV, A. V., AND ZIiURAVLEV, L. T., Trans. Faraday Soc. 60, 2254 (1964). 13. TIEN, H. TI., J. Phys. Chem. 69, 350 (1965). 14. HESTON, W. M. JR., ILER, R. K., AND SEARS, G. W. JR., J. Phys. Chem 64, 147 (1960). 15. DERJAGCIN, B, V., Discussions Faraday Soe. 42, 109 (1966). 16. LIDSTROM, L., Acta Polytech. Scand., Chem. Met. Ser. 7~ (1968).
Journal of Colloid and Interface Science, Vol. 34, No. 4, December 1970
ABENDROTH
Fundamental Research, Owens-Illinois, Inc., Toledo, Ohio 48601 Received January 9, 1970; accepted March 3, 1970 Adsorption of hydrogen and hydroxyl ions on a nonporous pyrogenic silica was determined by potentiometrie titration in LiC1, I4C1, and CsCI electrolytes over the pH range 1.8-9.0. No evidence of hydrogen ion adsorption was found. The sequence of counterion adsorption was Cs + > K + > Li +. An attempt was made to increase the extent of surface hydration of this silica by treatment in aqueous solution. This product had the same properties as untreated silica, except for slightly lower charge densities above pH 6. This difference was attributed to adsorption of impurities during treatment rather than to any significant effect of hydration state. Charge densities were found to be somewhat higher than those reported for precipitated siliea, but much lower than those reported for mieroporous silica. Differential capacitance curves indicate the eounterions for precipitated silica to be further away from the surface. It is suggested that Cs+, classed as a structure breaker in aqueous solution, is better able than Li+ to penetrate any structured water phase existing at the silica surface, in accordance with observed adsorbabilities. INTRODUCTION The surface properties of high surface area amorphous silicas can v a r y according to the method of preparation. These variations have been interpreted as indicating differences in the n u m b e r and arrangement of surface silanol groups (1). Pyrogenic silicas, e.g., CAB-O-SIL (2), are considered to have a high proportion of free or isolated silanol groups, whereas precipitated silicas have a higher proportion of silanol groups hydrogen bonded to each other (3,4) as determined from gas phase studies. Although these various silicas have been well characterized with respect to water v a p o r adsorption, their interaction with hydrogen and hydroxyl ions in aqueous solutions is not well understood. Tadros and L y k l e m a (5) determined adsorption isotherms for a microporous precipitated silica and observed charge densities much larger t h a n those observed b y Bolt (6) for a nonporous precipitated silica. The intent of this work was to characterize in aqueous solutions the adsorptive properties of a nonporous, high-purity silica of
pyrogenic origin, and compare results with those reported for nonporous and microporous precipitated silicas. MATERIALS AND METHODS The pyrogenic silica used was CAB-O~ S I L M-7, with an Ar B E T surface area of 170 m~/gm. I t was used as received after drying at 115°C. A complete Ar adsorptiondesorption isotherm showed negligible hysteresis. I n an effort to increase the extent of surface hydration, M-7 was heated for 24 hours in 6N HC1 at 90°-95°C. This product, designated silica 153, was then filtered and resuspended in distilled deionized water a number of times until the p H of the filtrate approached that of the water. After drying at 115°C, the Ar B E T surface area was 166 m2/gm. Semiquantitative spectrographic analyses of these silicas are given in Table I. The titrants, HC1 and appropriate bases, and electrolytes were prepared with distilled deionized water and were standardized prior to use. Potentiometric titration of the suspension and of an equal volume of blank electro-
Journal of Cvlloid and Interface Science, VoL 34, No. 4, D e c e m b e r 1970
591
592
ABENDROTH
lyre was used to determine the extent of hydrogen and hydroxyl ion adsorption as a function of pH. The suspension and blank were contained in borosilicate vessels with tightly fitting covers and were maintained at 20°C in a water bath. Wet prepurified N.o was bubbled through both vessels to prevent effects due to C02. Titrants were dispensed from microburets read to 4-0.05 ml. The ionic strength of the suspension and blank was maintained constant during titration. The usual suspension concentration was 5.0 gm silica per 400 ml electrolyte. Titration of a suspension containing 1.0 gm per 400 ml showed that results were independent of suspension concentration. The silica was introduced into the electrolyte approximately i8 hours before titration commenced. The t:trations were started at pH 4 to 5, carried Out to pH 9, then down to about pH 1.8. :In some cases, HC1 additions were made down to pH 1.5. However, titration errors and uncertainty in pH readings become larger in this region, leading to excessive scatter in the experimental points. It is felt that pH 1.8 to 1.9 probably represents the lower limit in the acid region for this technique. Qualitative pI-I drift experiments were conducted according to the method of B~rub6 and de Bruyn (7) in the region pH 1.4 to 1.6. In these experiments, additions of dry silica were made to the electrolyte, and any change in pH was noted. Each addition comprised about 600 m ~ of surface area. Instrumentation included an Orion 801 pH meter and Beckman glass and reference electrodes. TABLE I SPECTROGI~API-1IC ANALYSES OF THE SILICAS USED Element concentration range (ppm) Not detected Less than 5
Less than
i0
L e s s t h a n 25 3-30 2-20
h~[-7
153
B
B
Cu, Fe, Mg, Mn, Mo Ni, Sn, Pb, Ti Cr, A1
Cu, Mg, Mn, Mo Ni, Sn, Pb, Ti Cr
Ca, Zn, Zr
Ca, Zn, Zr Al
-
-
--
Fe
RESULTS
Charge densities are based on the Ar BET surface areas and are calculated from = F(r.+ - to,-),
[1]
where (r~+ - ro~-) is the adsorption density of hydrogen over hydroxyl ions, per square centimeter. Charge densities as a function of pH for M-7 in KC1 at ionic strengths of 10°, 10-1, 10-2 M are shown in Fig. 1, and in 10-1 M CsC1, KC1, and LiC1 in Fig. 2. It was observed in the solutions above PI.I 5 that after an addition of base, rapid adsorption occurred, followed by extremely slow adsorption. These observations are similar to those made previously (5, 7, 8). Below about pi.i 5, any changes in pH upon addition of titrant were complete within 15 to 20 rain and the pH was invariant thereafter. The fast adsorptions were complete within 30 rain, and comprise the present results. Extension of the equilibration time to several hours showed that the additional change in pH did not exceed the indicated uncertainties in the plotted results. Some hysteresis was observed upon titrating from high pH to lower pH's but is not shown in the plotted results. A comparison of the results for 1V[-7 and silica 153 at 10-1 M ionic strength in CsC1, KCI, and LiCl is shown in Fig. 3. An unequivocal point of zero charge (pzc) can in principle be established by the intersection of isotherms determined at varying ionic strengths. It was not possible to do this in this study, since the isotherms in Fig. 1 superimpose below pi.i 3-3.5. The problem of where to place the isotherms with respect to absolute charge density was resolved by noting that the pH region where the isotherms are horizontal is also the region where identical additions of HC1 to suspension and blank give identical changes in pH. It is possible that the silicas could be carrying a charge constant with respect to pH in this region, but this possibility was disposed of by considering the results of the pH drift experiments. Addition of dry silica to electrolyte in the horizontal region of the isotherms gave no change in pit down to pit 1.4. This is interpreted as no net interaction
Journal of Colloid and Interface Soience, Vo]. 34, No. 4, December 1970
BEHAVIOR OF PYROGENIC SILICA IN ELECTROLYTES
593
10 o
-22
/
-20 -18 -16
/J(//
~ g -'" %
-12 -I
-6-4-2-
0I
I
l
I
2
3
4
5
pH
I
I
l
I
g
7
8
9
FIG. 1. Charge densities for CAB-O-SIL M-7 in KC1 as a function of ionic strength. -22.
?csc,
-181
CI
-16:
LiCl
-J4:
~l? -,2: - Jo:
-8: -6:
pH
FIG. 2. Charge densities for M-7 in 10-1 M CsC1, KC1, LiC1. occurring between silica and hydrogen or hydroxyl ions in this region, indicating an essentially neutral surface. Differential capacities were derived for M-7 and silica 153 b y graphical differentiation of the e h a r g e - - p H curves. The relation used was given b y B6rub6 and de B r u y n (7), as corrected b y Ball (9). c =
F 2 d(r~+ -- r o ~ - ) 2.3 R T d p H
[2]
These results are plotted in Fig. 4 at 10-~ M ionic strength. Also plotted are values derived from the results of Bolt (6) for a nonporous precipitated silica in NaC1. DISCUSSION CAB-O-SIL M-7 and silica 153 do not show any quantitative evidence of hydrogen ion adsorption at ionic strengths 10- I and 10-2 M down to p H 1.8, or in 10 ° M solu-
Journal of Colloid and Interface Science, Vol. 34, No. 4, D e c e m b e r 1970
594
ABENDROTH
tions down to pH 1.5. No qualitative evidence of hydrogen ion adsorption was detected down to pH 1.4 in the pH drift experiments. These data are in contrast to the M-7
-so:
/ t ~ 3
-t6: -12 .41 o'
4
5
6
7
s
9
-IE
-4: O'
~ 4
Kcl
~~-------------------"~" - ' 56 - 7
~
9"
-201
M-7
-161
,
153
-12:
-41 o
4
5
6
pH
?
8
9
F i e . 3. Charge densities for M-7 ~nd silica 153 n 10-1 M CsC1, KC1, LiC1. LiCl
KCI
CsCl M-7
22O-
I I I I ]
2O0180160~
120I00-
eo~ so20~ 0
extensive positive charge development at pH 2 observed for microporous silica (5). Precipitated silica (6) shows several points at pH 2 that indicate hydrogen ion adsorption; however, these points are not unequivocally determined in view of the titration errors associated with this pH region and the low indicated charge densities. It has been suggested (10) that the silicon cation sufficiently polarizes the OH group so that a protonated water molecule is unable to bond to it. The present results support this view. The charge densities for AJl-7 are compared to the results of Tadros and Lyklema (5) in 10-1 M KC1, and to Bolt's data (6) in 10-1 M NaCI in Fig. 5. The much higher charge densities exhibited for microporous silica have been attributed to the presence of internal hydroxyl or hydrogen ions, as opposed to groups interacting on the surface alone. It is apparent that M-7 exhibits slightly higher charge densities than silica 153, and both of these are greater than for precipitated silica. The treatment of M-7, resulting in silica 153, was done in an effort to make :V[-7 behave similarly to precipitated silica on the assumption that the essential difference is due to extent of surface hydration. Inspection of Fig. 4 indicates this was not achieved.
~
Cs(;I KCl LiCI
I I
t I
NaCl
I 3
I M-7
'// I
I
t
I pH
FIG. 4. Differentia] capacities of M-7 and silica 153 in 10-I M CsC], KC1, LiC]. Journal of Colloid and Interface Science, Vo]. 34, No. 4, December 1970
BEHAVIOR
OF PYROGENIC
SILICA IN ELECTROLYTES
595
I00
-
-80
~LyklO adr°s nd erna
~l~ - 60 -
40
M-7
-20 0 +20 ~40
2
5
4
5
6
7
8
9
pH F r o . 5. C o m p a r i s o n of M - 7 w i t h t h e r e s u l t s of T a d r o s a n d L y k l e m a in 10-~ M KC1, a n d w i t h B o l t ' s r e s u l t s in 10- t M NaC1.
It is apparent, however, that some difference Cs + > K + > Li+, confirming previously in behavior was achieved by this treatment. reported results (5, 13). The precipitated To determine whether silica 153 was more silica in NaCI shows similar differential cafully hydrated than M-7, the ignition method pacitance at pH 6, but lower values at of de Boer and Vleeskens (11) was employed. higher pH's. Generally speaking, the pyroSince M-7 may have hydrated to an unknown genie silica and precipitated silica have extent during immersion in the electrolyte roughly similar properties but it is perhaps before titration began, these determinations surprising that this pyrogenic silica exhibits were carried out for M-7 as received and higher charge densities, since it is thougl~t after immersion in distilled deionized water, that pyrogenie silicas in general have fewer and for silica 153. Since internal as well as reactive sites. Heston, Iler, and Sears (14) surface hydroxyls can be removed by this were able to fully ionize the surface of a method (12), the percentage weight loss on precipitated silica without dissolution at pH ignition is reported rather than surface hy- 12 and found Noi~ = 3.5. Application of this droxyl concentration. These weight losses technique to M-7 and silica 153 indicated are: M-7 before immersion, 1.67 %; ~.I-7 after that full ionization, if achieved, was also immersion, 1.64%; silica 153, 2.32%. Thus accompanied by dissolution, and a deterit is possible that the surface of silica 153 is mination of NcH was not possible. The difmore fully hydrated than M-7, but the ferential capacitance values indicate the extent is unknown outing to possible intro- eounterions are further away from the duction of bulk hydroxyls. In view of this, charged surface of the precipitated silica it is not possible to conclude unequivocally than for M-7, possibly indicating a more that the slight differences in the adsorption tightly packed structured water phase (15, isotherms and differential capacitance curves 16) adjacent to the surface. Thus, there for M-7 and silica 153 are due to any change appear to be significant differences in the in the degree of surface hydration. The structure of the electrical double layer formed spectrographic analysis for silica 153 shows on those two types of silicas. that it contains increased amounts of aluA model of the electrical double layer minum and iron. If these were confined to formed on oxides that exhibit extensive inthe surface, they could possibly have the teraction with water has been advanced by effect of reducing hydroxyl ion adsorption up B6rub6 and de Bruyn (7) to explain the to the pH range 8-9, and hence might better sequence of eounterion adsorbability on ruaccount for the slight difference in adsorp- tile. Their argument is that the observed tion behavior of silica 153 as compared to sequence Li+ > Na + > Cs+ exists because M-7. Li+ would tend less to disrupt the structured Examination of Fig. 4 shows that the water layer next to the futile surface than adsorbability of the eounterions for both Cs +, since Li+ is more highly hydrated and M-7 and silica 153 increases in the order is classed as a structure promoter in aqueous Journal of Colloid and Interface Science, VoL 34, No. 4, December 1970
596
ABENDROTH
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Journal of Colloid and Interface Science, Vol. 34, No. 4, December 1970