Sorption of Sodium Silicates and Silica Sols by Cellulose Fibers KEYSOLD C. JIERRILL AND ROBERT W. SPESCER Philadelphia Quartz Company, Philadelphia 6 , Pa. Juday, and Xeloche ( 6 ) . The solution to be analyzed mas first boiled in a gold dish with 1.0 ml. of 1.0 AT sodium hydroxide for 5 minutes to convert all of the colloidal silica t o simple silicate ions. After cooling and transferring to a 100-ml. volumetric glass flask, 2.0 ml. of 10% ammonium molybdate solution were added, followed immediately by 5.0 ml. of 1.0 S sulfuric acid. The solution was then diluted to 100.0 ml., mixed, and allowed to stand for 8 to 10 minutes to complete development of the yellow color. The intensity of the color of the solution in a cell with a depth of 20 mm. was measured in a Klett-Summerson photoelectric colorimeter using the blue (4200 mG) filter. Strict adherence t o this method was necessary to obtain accurate results. Reagents were stored in hard rubber bottles. Relatively binall corrections were made for absorption of light by the reagents and mateiial leached out of the paper. These were determined daily. The sodium hydroxide was Baker's analyzed grade which eontained 9i.375 anhydrous base. The silicates were AIetso Crgstals (Philadelphia Quartz Company), sodium metasilicate which contained 29.24q10 sodium oxide, and 28.20% silica, and Nsilicate which analyzed 8.84% sodium oxide and 28.86% silica corresponding to a silica to alkali (NazO) molecular ratio of 3.367. The ammonium sulfate, sulfuric acid, and other chemicals used were reagent grade products. Analyses of the typical comniercial bleached paper pulps are summarized in Table I.
Sorption of both alkali and silica from sodium metasilicate, Naz0.3.37SiOn, N-Sol A, and a sulfuric acid-treated silica sol wvas measured at 25" C. for bleached sulfite, soda, kraft, and groundwood cellulose pulps. Similar data on sodium hydroxide are given for comparison. Both alkali and silica are sorbed by the cellulose fibers, but the amount of the former sorbed is usually much greater than that of the latter. The mechanisms of the sorption probably involve van der Waals forces, hydrogen bonding, and chemical reactions with metal oxide impurities. Practical applications of the data are discussed.
T
HE interaction of sodium silicates with cellulose paper pulp
is of great interest and practical importance in connection with their use as adhesives (9, l6),sizings, and coatings for paper and in bleaching and deinking ( I O ) . Similarly, applications of activated silica sols in sizings, and to increase retention of filler and fiber on the x ire of the paper machine and in savealls make important a knonledge of their interaction with cellulose fibers. Such information can be used also in understanding the action of silicates as detergents and soap and synthetic detergent builders on cellulose textile fibers. although much work has been reported on the sorption of sodium hydrovide by cellulose (f-5, i f , 12, fa) apparently no similar data are available for various silicates and activated silica sols. This paper reports quantitative data on the sorption a t 25.0" C. of both alkali or sodium ions, and silica, silicate ions, or siliceous silica micelles by cellulose fibers from solutions of sodium hydroxide, sodium metasilicate, a sodium silicate with a silica to alkali (NalO) molecular ratio of 3.37, and activated silica sols produced by neutralization with sulfuric acid and ammonium sulfate. Concentrations from 0.0005 .V t o 0.1 Jf for the sodium hydroxide and silicate solutions and 0.01 X or 0.05 -If for the activated silica sols were studied using bleached sulfite, kraft, soda, and spruce groundwood cellulose pulps
TABLE
Constituent
1.
A$S.iI,YSEs O F P.4PER PITLPS
Sulfite 6.3.5 0.011 0.015 0.159 0.006 0.036 0.22
Kraft R 29 0 020 0 031 0 120 0.007 0.010 0.18
Soda 5.76 0.049
Groundwood 8.36 0,123
0,048 0.349 0,001 0.001
0.173 0.010 0,007
0.44
0.045
0.36
Preparation of activated .silica sols involves dilution of the commercial sodium silicates, reaction with an acid or ammonium salt, aging to permit micelle growth, and subsequent dilut,ion to prevent gelation ( 8 ) . The aging period for both the sulfuric acid sol (Baglis sol) and the ammonium sulfate sol (N-Sol 9) used in this work was 15% of their gel life. Baylis sol was prepared by neut'ralizing 82% of the alkalinity of N-silicate and aging a t 2.62% silica for exactly 4 minutes and 12 seconds before dilution to 1.0% silica to stabilize the sol. N-Sol A was made by aging equal molal quantities of diluted N-silicate and ammonium sulfate a t a concentration of 2.50% silica for 5 minutes before dilut,ion t o 1.0% silica.
EXPERIMENTAL PROCEDURE
Eight grams of air-diy pulp 13 ere thoroughly mived with 200 ml. of solution in a Waring Blendor in order to simulate the mechanical treatment given in paper mills by the beater and Jordans. The pulp suspension was then placed in hard rubber bottles to avoid attack of the alkali on glass and allowed to stand in a water bath a t 25.0" C. for 24 hours. Sorption of both alkali and silica was relatively rapid; moat of the sorption occurred in less than an hour. Equilibrium was essentially attained in less than 24 hours. The pulp was then placed on a bare Buchner funnel and the solution filtered off by slight suction. The filtrate was refiltered through the mat formed by the pulp, and then analyzed. The change in concentration of the solution per gram of pulp represents the amount sorbed. No correction was made for sorbed water. The sodium oxide (Sa20) content was determined by titrating a suitable aliquot to the methyl orange end point. Silica was determined colorimetrically. Some of the silica analyses on the more concentrated solutions were checked by the standard gravimetric procedure with fairly good agreement. The colorimetric procedure was a modification of that used by Knudson,
SORPTION DATA
The sorption of alkali by four representative paper pulps from solutions of sodium hydroxide, metasilicate, and >-azo.3.37Si0, as determined by titration with acid is shown in Figures 1, 2, and 3. When apparent sorption of sodium hydroxide per gram of groundwood is plotted as a function of the alkali concent,rat,ion on a logarithmic scale two straight lines result. The data up to 0.025 -If sodium hydroxide represent sorption ; solutions above this concentration are brown owing
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April 1950
C, X NoOH
02
0B
04
25
C(M0LES NaOHILITER)
Figure 1.
Sorption of Alkali from Sodium Hydroxide Solutions by Cellulose Paper Pulps
0=
Bleached spruce groundwood
0 = Bleached soda
0 = Bleached sulfite = Bleached kraft
to solution of lignins. Sorption data for the other pulps and the silicates give a single straight line when plotted on logarithmic scales. At equal molal concentrations of sodium ion the sorption of alkali from sodium hydroxide and metasilicate solutions is about the same for the bleached soda and kraft pulps; whereas, the bleached sulfite sorbs slightly more alkali from the hydroxide. Over most of the concentration range studied groundwood appears to sorb much more alkali from sodium hydroxide than from metasilicate. More than twice as many moles of alkali are sorbed from solutions of the metasilicate than from Naz0.3.37SiOz solutions of equivalent concentration (Figures 2 and 3). Sorption of alkali from the two silicates increases in the order: krafts eris absorbed by filter paper from a 3.0 ratio silicate. The nature of the sorption, whether physical or chemical, cannot be determined from these data. Both sorption by van der Waals forces and hydrogen bonds, and chemical reaction with a multiplicity of groups on the long cellulose molecule which do not interact with each other give the same type of apparent x / m versus C curves observed here. >fore elaborate treatments of this type of data than the classical sorption isotherm have been developed; some of these take into consideration electrostatic effects ( 1 4 ) . Corrections for differences in sorbed water could also be made ( 2 ) . This possible correction appears to have been overlooked in recent' work on interaction of small molecules or ions Tvith colloids in solution. However, the impurities contained in these cellulose samples and lack of knowledge regarding their exact structure and composition make the value of more elaborate treatments of these data dubious. A probable mechanism of t,he sorption of alkali is hydrogen bonding of the hydroxyl ion formed by hydrolysis with the corresponding hydroxyl or carboxyl groups on the cellulose. B e cause of electrostatic attraction, sorption of hydroxyl ions would
DISCUS SIOK
The authors' data show clearly that both the alkali and silica in sodium silicate solutions are sorbed by cellulose fibers but that they interact with the pulp separately. In almost all cases
TABLE 111. SORPTIOX OF SII~ICA FROM N-SOL A SOLUTIOXS BY PAPER PULPS Sulfite P u-__lp C1"
C2b
0.263 0.622 2.04 4.02 8.38 43.2
1.50 2.98 8.09 15.2 28.3 150
K r a f t Pulp 2c -
Ci
m
Cn
0.15 0.455 0.38 0.882 0.40 2.36 0.75 4.36 2.00 8.62 12.5 41.0
Soda Pulp z -
Cz
Ci
m
1.48 0.20 2.95 0.45 7.99 0.65 15.0 1.25 2R.3 2.00 151 8.50
0,443 0.735 2.10 4.02 8.27 41.8
1.44 2.86 7.85 14.8 28.0
151.0
1
I
I
I
I
,001
,005
.02
.05
2 m
0.30 0.68 1.00
1.75
2.75 8.50
a CI = final concentration of sodium silicate determined by titration, millimoles per liter. b C2 = final silica concentration in millimoles of silica per liter. C 5 moles of silica sorbed per gram of air-dry pulp. na
TABLEIv. CLAsSICAL SORPTION ISOTHER4f C O N S T l N T S FOR SILICATES AND SILICA SOLS o s CELLULOSEFIBERS Sulfite Adsorbate NanSiOa Saz0.3.37Si02
N-Sol A
n
k
1 . 1 0.00039 1 . 3 0.000046 0 96 0.0079
Kraft
n
Soda
k
1 . 0 0.00015 0 . 9 3 0.0042 0.77 0.076
n
Groundwood
k
0.41 0.10 0.40 4.3 0 . 6 7 0.017
n
k
0.32 2 . 2 0 . 6 9 0.016
..
.,.
.0002 C.
IMOLES No SILICATE /LITER1
Figure 6. Log-Log Plots of Sorption of Alkali by Cellulose Paper Pulps
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
April 1950
be accompanied by sorption of sodium ions. The silicate thereby would be hydrolyzed further and the silica to alkali (NanO) ratio in the solution increased as is observed. A possible mechanism for the sorption of silicate ions or micelles might be condensation of their SiOH groups with the alcoholic hydroxyl groups of the cellulose. Silica cannot be removed wholly from natural celluloses by the strongest reagents and apparently has a definite influence on certain of their properties (13). This has been attributed to the existence of covalent bonds between silicic acids and alcoholic hydroxyl groups in cellulose. It does not seem likely, however, that this formation of covalent bonds occurred under the conditions of the present experiments.
747
X Nap0-3.37 SIOo
I
Y Figure 8.
05 I
.
I
025
LO
1.5
2.0
I
I
I
I
2.5
I
050 075 C, (MOLES Nap 0 - 3 3 7 SiOZ/LITER)
Sorption of Silica from Naz0.3.37SiOs Solutions by Cellulose Paper Pulps Symbols same as Figure 1
I ,050
I
,025 C,
Figure 7.
(MOLES No, SiO,
I
,075 /LITER )
Sorption of Silica from Sodium Metasilicate by Cellulose Paper Pulps Symbols same as Figure 1
A mechanism for sorption of both alkali and silicate particles involves chemical reactions with impurities such as aluminum, calcium, and magnesium oxides t o form insoluble complexes or, particularly in the case of impure cellulose pulps such as groundwood, with acidic impurities such as lignin and decomposition products. However, there appears to be no relationship between amounts of alkali or silica sorbed by the four pulps and their content of these oxides. Differences in the order of effectiveness of the celluloses in removing alkali and silica indicate that different sorption mechanisms are effective. This is particularly apparent in the inperted order of effectiveness in sodium metasilicate solutions when comparing alkali and silica removal. Probably both van der Waals forces, hydrogen bonding, and chemical reactions are involved in the sorption of sodium silicates by cellulose fibers. There are several practical applications of these data. One is in minimizing the alkali staining of paper by silicate adhesives. The authors’ data show that use of a siliceous silicate not only makes it possible to obtain adhesion with less alkali present but t h a t i t also greatly reduces sorption of the available alkali. Even at the same sodium oxide concentration, sorption of alkali from the siliceous silicate Naz0.3.37SiOz is only about half that
from sodium metasilicate. The data also indicate that addition of ammonium sulfate to minimize alkali staining actually increases sorption of the alkali, although it still may be effective by neutralizing sorbed alkali. The data also explain the practical observations that N-Sol A may be more effective in retaining cellulose fibers on the wire of the paper machine and in coagulating them in the saveall, than the sulfuric acid silica sol. None of the four pulps sorbed silica from the sulfuric acid sol. The soda, sulfite, and kraft pulps readily sorbed silica from N-Sol A. The failure of groundwood t o sorb silica from either the sulfuric acid sol or N-Sol A is probably a t least partially responsible for difficulties in coagulating this type of pulp, although satisfactory coagulation of groundwood pulps by N-Sol A is observed under proper conditions. LITERATURE CITED
(1) Bancroft, W. D., J . Phys. Chem., 40, 44 (1936). (2) Bancroft, W. D., and Calkin, J. B., Textile Research 4, 119, 1;9, 371 (1934); J . Phys. Chem., 39, 1 (1935). (3) Bialkowsky, H. W., Paper Trade J., 97, No. 13, 29 (1933). (4) Calkin, J. B., in Colloid Symposium Monograph, 12 (1936); J . Phys. Chem., 40, 33 (1936). (5) Downs. M. L.. Tech. Assoc. Pamrs. 21. 367 (1938). (6j Knudson, H. W., Juday, C., and‘Meloche: V. W., IND. ENG. CHEM.,ANAL.ED., 12, 270 (1940). (7) McBain, J. W., and Hopkins, D. G., Dept. Sci. Ind. Research, 2nd Rept. of Adhesives Research Commission, 1932, appendix 111, p. 72. (8) Merrill, R. C., IND.E N G .CHEM.,40, 1355 (1948). (9) Ibid., 41,337 (1949). (10) Merrill, R. C., Tech. Assoc. Papers, 32, 520-9 (1949). (11) Rowland, B. W., J . Phys. Chem., 41, 997 (1937). (12) Sait6, Gi-iti, J . SOC.Chem. I n d . J a p a n , 43, 133-6B (1940); Cellulosechemie, 18, 106 (1940). (13) Samec, M., in “Colloid Chemistry,” ed. by J. Alexander, Vol. 4, p. 7, New York, Chemical Catalog Co., 1932. (14) Scatchard, G., N . Y . h a d . Sci., 51, 660 (1949). (15) Wills, J. H., and Sams, R. H., IND.ENQ.CHEW.,41, 81 (1949). RECEIVED September 29, 1949.
