The characterization of acid-set silica hydrosols, hydrogels, and dried gel

The Characterization of Acid-Set Silica Hydrosols, Hydrogels, and Dried Gel E. G. A C K E R W. R. Grace & Co., Washington Research Center at Clarksville, Maryland 21029

Received March 17, 1969; accepted June 14, 1969 Acid-set silica hydrosol, hydrogel, and the dried desiccant silica gel have been characterized by various types of measurements and discriminating tests. Acidic silica hydrosols have a molecular weight of about 2,000 at formation and reach about 2,500,000 at the point of gelation. Molecular growth of silieie acid in this hydrosol was found to be linear. Cross-linking of the polysilieic acid chains occurs in the hydrogel state, and the number of cross-links between chains have been estimated. Silicic acid bonding in acidic hydrosols is believed to be by hydrogen bonding between the silanol groups through water. The siloxane bonding in hydrogels aged beyond syneresis was determined by a depolymerization rate study; a low degree of siloxane type bonding was found. To account for the remaining bonding in the hydrogel, a high degree of hydrogen bonding is suspected. The reasons for hydrogen bonding are given. Most of the silanol condensation must occur during the drying of this type silica gel. INTRODUCTION

formed at the gelation point. The mechanism of silica gelation generally accepted is the condensation of silicic acid silanol groups to form infinite polymers of

Silica gels made from acid-set hydrosols have high surface areas and fine pore structures. These gels are important because of their use as desiccants and selective adsorbents. I t is useful to have a thorough knowledge of the molecular structure at all stages of the preparation in order to best understand and regulate the surface structure and porosity of the final desiccant gel. We have defined two types of silica gel on the basis of the mode of preparation. One, called acid-set gel, is prepared by neutralizing a sodium silicate solution with excess strong mineral acid (pI-I 0.5-4). The other type, called alkaline-set gel, is prepared similarly but the acid-neutralized solution has a pH greater than 4 and less than ii. In addition, we have defined the terms hydrosol and hydrogel as follows: the hydrosol is the clear mixture of sodium silicate and acid prior to gelation (the gelation point is arbitrarily defined as the point when the hydrosol viscosity reaches 500 cps). The hydrogel is the undried material

r

l

[Si--O--Si--O]~ units (1). This description probably fits the alkaline-set gels. The mechanism for gelation mentioned by Plank (2) seems to best describe the acidic hydrosols. Plank suggested that hydrogen bonding of silanol groups through a water molecule occurred in these hydrosols. Because very little work has been published on the complete characterization of the stages of acid-set silica gelation, a complete analysis was necessary. The details include the range of molecular weights occurring in the hydrosol state, the extent of cross-linking in the hydrogel, an analysis of the bonding that occurs during drying, and the type of bonding occurring in each stage of the preparation.

Journal of Colloid and Interface Science, Vol. 32, No. 1, J a n u a r y 1970 41

42

ACKER I. HYDROSOL STUDIES

EXPERIMENTAL

Preparation Acid-set silica hydrosol preparations were made by mixing sodium silicate (1 Na20: 3.25 SiO2 weight ratios) solutions with H2SO4 water solution in a continuous high-speed mixer maintaining a uniform pH of 1.5 at 27°C. The hydrosol formula was 15% SiO2--120 % neutralized at p H 1.5 and had a set time of 110 minutes. The alkaline type hydrosol was made by adding a I-I2SO4 solution to a stirred solution of the silicate until a composition of 10 % S i O 2 - 36 % neutralized was obtained at a p H 10.6. This is used only for a comparison of hydrosols in molecular weight studies. The procedures used to characterize the hydrosol state are described below.

Molecular Weight Determination The molecular weights of the 15 % SiO2 - 120 % neutralized hydrosol were determined b y light scattering. The hydrosol samples were taken at various times and diluted to 1 gm SiO2/100 ml distilled water adjusted to p H 2. Samples were centrifuged prior to examination. A Brice Phoenix Universal Light Scattering Photometer Model #1000D and a Brice Phoenix Differential Refraetometer No. 1130 were used to make the scattering and refractive index measurements, respectively. A good approximation of molecular weight M as weight average was obtained by using the equation M = r/H~ with data obtained from the diluted hydrosol samples. This method is a one-point version of the original Zimm (3) treatment. Here "r" is the turbidity as measured at 546 m#, and "HI' is 6.18 X 10-Jn02(n -- no)2/c~; the "no" is the refractive index of water, and (n - - n o ) i s the difference in refractive indices between solution and solvent. In our studies, we found the difference in refractive indices to be 0.141 for a solution of c = 0.01 gm SiO2/ ml. This value of An is similar to the one used by Otouma and Ukihashi (4) in their light-scattering studies with acidified solutions of sodium silicate. Dissymmetry measurements were made and applied to the calculations for molecular weight.

Viscosity Measurements The viscosity measurements of the hydrosol at 25°C were made on a Brookfield Viscometer Model LV using a number 1 spindle. The reduced viscosity was calculated from the formula: 'red.

=

(7

-

7o)/7oe,

where 70 is the viscosity of water at 25°C, n is the viscosity of the hydrosol, and c is the concentration of silica in the hydrosol expressed as grams of silica per 100 milliliters.

The Staudinger Equation This equation (5) is determined by plotting the logarithm of specific viscosity (reduced viscosity extrapolated to zero concentration) as a function of the logarithm of the molecular weight. The two constants K and a can be determined from the plot, and the equation is constructed as nrea. = K M ~. The reduced viscosities of the 15% Si02 hydrosol were essentially the same as Io s

/,

Ior

~ ~oG <

=

~o5 / to 4

-

%

•

_

%

,-

-

. 1115

$ A L K A L I N E SET HYDROSOL I 0 % SiO 2 - 5 6 " / , NEUT, °- p H - 1 0 . 6 "~"PREDICTEO MOLECULAR

WT.

Io3 o .I .4 .3 .~ .; .~ .+ .~ .; ,Io RELATIVE GELATION T I M E (T/TG }

FIG. 1. Molecular weights of polysilicic acids in acidic silica hydrosol and in alkaline catalyzed silica hydrosol.

Journal of CollMd and Interface Science, Vol. 32, No. 1, J a n u a r y 1970

ACID-SET SILICA HYDROSOLS, HYDttOGELS, AND I)I{IED GEL for a 5 % SiO2 hydrosol through most of the range. For practical purposes, the reduced viscosity was substituted for a specific viscosity in the Staudinger plot.

TABLE I AN&LYSIS OF ACID-SET SILIC& HYDROSOL POLYMERIZATION MECHANISM Time (rain)

]~ESULTS

The results are expressed in a series of graphs and tables. The logarithms of the molecular weights of the acid-set 15 % SiO2 hydrosol are plotted as a function of the relative time of gelation in Fig. 1. For comparison, the molecular weight data for an alkaline-set hydrosol are shown in the same figure. Table I lists the molecular weights of the acid-set hydrosol at 10-minute intervals, the predicted weights at the same intervals, and also the rate of weight increase per 2 minutes. (The basis for predicting molecular weights is given in the interpretations.) The viscosity data for the acid-set hydrosol are given in Table II. A Staudinger type plot made by combining the molecular weight-viscosity data on a log-log basis is given in Fig. 2. The proposed structure of polysilicic acid in the acidic hydrosol is shown in Fig. 3. INTERPRETATION OF I~ESULTS

Molecular Growth by Linear Association

43

0 10 20 30 40 50 60 70 80 90 100 110

Measured molecular weight

6,000 8,000 10,000 14,000 20,000 28,000 40,000 65,000 110,000 230,000 600,000 3,000,000

Theoretical Increasein molecular molecular weight weight/2 rnin 2,000 3,400 5 500 9 000 15.000 25.000 40000 65.000 110 000 230.000 600,000 3,000,000

280 420 900 1,200 2,000 3,000 5,000 9,000 24,000 74,000 480,000

TABLE II VISCOSITY DATA FOg 15% SIO2--120% N HYDROSOL Time (rain) 0 10 20 30 40 50 60 70 80 90 100 104 108 110 110.2

Viscosity (cps) 5.4 5.8 7.0 7.4 8.0 9.2 11.0 13.0 17.0 24.0 48.0 78.0 150.0 250.0 500.0

Acid-set silica hydrosol shows molecular weights (weight average) ranging from 6,000 to 2,500,000. The large molecular weight species of polysilicic acid appear to bond with the lower molecular weight species over a 2-minute period until close to the end of gelation. The 2-minute interval was used because the additions of the low molecular weight species are consistent with those reported by Nauman and Debye (6) in this type of silicate prior to reaction On the basis of information obtained from with acid. At the end of gelation, molecules Part II, the bonding appears to be a hydroof about equal molecular weights are bond- gen type through one or more molecules of ing in a 2-minute period. H20 (as suggested by Plank (2)). Figure 3 The Staudinger equation plot gives us shows this type of bonding. values of K and a which are similar to those Several investigators have shown that found for organic polymer systems. The aged silicate solutions have a molecular "a = 0.65" is characteristic of polymers weight on the order of 2,000 (6, 7). Since which are exhibiting linear polymerization it is reasonable that our silicate has such a with a small degree of branching. weight, we have corrected for the inaccuraThe above analyses suggest that a bi- cies possible at low molecular weight values molecular reaction is occurring during gela- and drawn the curve of theoretical molection and that the polymerization is linear. ular weights shown in Fig. i. Journal of Colloid and Interface Science, Vol, 32, No. 1, January 1970

44

ACKEg I.O

0 )O.1

t-

O > •~

lnl = 0.8

I~ 0 -4M

°'65

(.} a .OI

M.W. ( W T . ) .OOI

i

I0 3

i

IO 4

i

I0 5

I0 6

Fie. 2. Staudinger equation plot for polysilicic acid in 15% SiO~--120% N hydrosol

H

H

H

H

H

O

O

O

O

O O i I H Si - - O-Si--OH--OI"I-,

I H H 0 - - SIi - - 0 - - Si - - 0 --SIi - OH -,--OH - - 0

-- SIi - - 0 - -

H

]

I

I

I

I

I

0

0

0

0

0

0

H

H

H

H

H

H

H

I HO

--

OH

H O

H O

O

i

I

I

I

S I - - O - - Si - - 0 I

--

H

Si-- O-Si

I

I

--

OH

i

0

0

0

0

H

H

H

H

A SEGMENT OF HYDROSOL STRUCTURE SHOWING AN OCCASIONAL BRANCHING OFF THE LINEAR POLYMER { T H E BRANCHING DOES NOT OCCUR ON EVERY SEGMENT). H H Si--OH.-.-=,-OH--.-="O--Si

SHOWS HYDROGEN BONDING

BETWEEN SILICIC ACID THROUGH H20 REPRESENTS

FREE WATER

FIG. 3 II. HYDROGEL EXPERIMENTA

STUDIES

L

Preparation Acid-set silica hydrogel preparations were made from hydrosols of the type described

in Part I. These hydrosols were allowed to set at a viscosity of 500 centipoises and then used for the hydrogel studies. The following formulations were used: 5 gm SiO2/100 ml p H 1.5 10 gm SiO2/100 ml p i t 1.5

Journal of Colloid and Interface Science, Vol. 32, No. 1, J a n u a r y 1970

ACID-SET SILICA HYDROSOLS, HYDgOGELS, AND DI~IED GEL 15 gm SiO2/100 ml pH 1.5 20 gm SiO~/100 ml pH 1.5 The methods used to evaluate the changing properties of acid-set hydrogels are described below:

High-Viscosity Measurements High-viscosity measurements of the hydrogel were made with a Brookfield Viscometer LV mounted on a Heliopath Stand and using a special spindle for moving through jelly-like materials. Elasticity Measurements Elasticity measurements were made on a modified Precision Penetrometer measuring compression to J/J0 ram. A balance pan for weights was mounted on the top, and a 5 cm diameter piston was mounted on the lower portion of the same rod. Hydrogels were obtained as cylinders 4.5 cm in diameter and zI cm in height. The test involved placing the molded hydrogel plug under the piston, and adding sufficient weights to make a measurable change in height. The calculations were made by using the forF/A mula E - Ad/d, where E is Young's modulus of elasticity; F is the weight of piston, rod, pan, and weights used for the depression; A is the area of the cylinder in square centimeters; A d is the change in height of the hydrogel cylinder in centimeters; and d is the diameter of the hydrogel cylinder in centimeters. The reproducibility of the measurements was about ~=10 %. The method and equipment are similar to those used by Munro, ~lcNab, and Ott (8). The calculations of molecular weights between cross-links of a linear polymer were made by using the equation from the statistical theory of rubberlike elasticity (9). This equation is

M~-

3RTC E '

where M~ is the molecular weight between crosslinks; R is the gas constant; T is the absolute temperature; C is the concentration in grams per milliliter; and E is Young's modulus.

45

Syneresis Profile The syneresis profile was obtained by placing a molded hydrogel cylinder previously described in a loosely- covered funnel above a 10-ml graduated cylinder and collecting the water solution exuded on a time basis, avoiding evaporation of the liquid. Rate of Depolymerization of Hydrogels The procedure involved the titration of equal weights of silica as hydrogel or dried gel in 900 ml of water with 1 N Na0H solutions. The sodium hydroxide was added at a rate sufficient to maintain a constant 11.5 pI-I at 25°C. The concentration as SiO2 was 0.i mole/liter. The hydrogel was sized through a 200 mesh screen or -77#.

The dried standard gel was a spray-dried desiccant gel screened to a - 7 7 ~ size and had a surface area of 800 m2/gm and a pore volume of 0.5 cc/gm. The rate of depolymerization was determined by plotting the amount of soluble silica as a function of time. The amount of soluble silica is equated with the amount of NaOH needed to maintain pH 11.5. For purposes of calculation, the trisilicate unit was chosen as the average soluble silica specie resulting from the depolymerization process. The structure of the trisilicate molecule in solution at pH 11.5 is pictured as follows: H

H

H

I

I

I

O

HO

O

O

Si--O--Si--O--Si--OH

J

I

+

2NaOH

I

o

o

o

H

H

H

The rate constant for the depolymerization process has the dimensions of moles/literminute.

Relating of Rate Constant to Breaking of Siloxane Bonds The structure of our spray-dried desiccant gel was determined from the surface area of 800 m2/gm, and the theoretical value of 8 "OH" groups per m~2 surface area (10). The formula for the dried gel is [(HO)2Si3Os]x. The solubilization of this polymer at pH 11.5 required the breaking

Journal of Colloid and Interface Science, Vol. 32, No. 1, January 1970

ACKER

46

of an average of 3 SiO bonds per unit of (HO)2Si30~. The/c for depolymerization of this standard dried gel was determined from Fig. 8, and it was found to be /c = 1.27 X 10-3 moles/liter-minute. On the assumption that the three bonds break at the same rate, a K per Si--O bond would be 3 times/c = 1.27 X 10-3 or Ksi-o = 3.81 X 10 -3.

our depolymerized silicate solutions contain low molecular weight species of silicate ion, and the trisilieate ion is a good choice for purposes of calculation. The comparison of the rate of depolymerization of the standard dried gel to t h a t of the hydrogel for analysis of siloxane 10 9

Application of Si--O Bond Analysis to Hydrogels The rate constants for the hydrogel depolymerization can be used to approximate the number of siloxane (Si--O) bonds between trisilicate units. This is done by diriding the Ksi-o bond by the rate constant for the hydrogel. The value obtained is the number of Si--O bonds (or fraction of) broken per trisilicate unit. The structure of the hydrogel in regard to siloxane bonding can be visualized.

107

w 106 o 10 5 o o z iO z Vck iO?

ARBITRARY SET T I M E ( 5 0 0 cps)

Evaluation of Rate of Depolymerization The rate of depolymerization has been used by Alexander (11) in his study on polymerization of monosilieic acid. I-Ie used molybdic acid to detect depolymerized species of silicic acid, and he was able to differentiate polymerized silica gels which were made up of different molecular weight polysilicic acids. He observed that not all depolymel~zed silica readily gave monomeric silicic, but in some cases a mixture of silicic acids, such as monosilicie acid and disilieie acid. In this study, the depolymerized silicie acid was considered as a trisilicic acid on the basis of the work of N a u m a n and Debye (6). A silicate with a 3.32/1 mole ratio of Si02/ Na20 has a molecular weight of 325. Such a silicate solution should contain a considerable amount of trisilieate ion, and this solution is similar to the silicate solution obtained in our depolymerization studies. A eolorimetrie test, for low molecular weight species of silieie acid with molybdic acid on our p H 11.5 depolymerized silica, commercial sodium silicate (3.32/1 mole ratio), and sodium meta silicate (Na2SiO3.9H20) showed all developed maximum color within 30 seconds. This is a good indication that

/

10 8

~DI0 2 ~9

f

> IOI

,2 l

o

.f

;

.6

,8

I~o I~z r,,o

,'2 i; 2'o ~4 is

;

TIME

31

(HRS,)

FIG. 4:. Viscosity of silica hydrosol-hydrogel 10 gm SiO=/100 ml at 25°C. ,6o

ACID-SET HYDROGEL COMPOSITION 0 5gm. S i 0 2 / l O O m l . 0 IOgm Si02/lOOml. e 15gm SiO2/lOOml. • 20 grn. S i 0 2 / I 0 0 mL

140 120 100

4O 2O

0

I

2

5

4

5

6

7

O

I. I

I/2

FRACTION OF TiME BEFORE S Y N E R E S I S (T/TB.s.)

Fzo. 5. Kinetics of hydrogel bonding lowed by square root of elasticities.

Journal of Colloid and Interface Science, V o l . 32, N o . 1, January 1970

as

fol-

ACID-SET SILICA HYDI~OSOLS, HYDROGELS, AND DRIED GEL TABLE I I I

55O 15 gm S i 0 2 / I O O m L HYDROGEL (SET T I M E - 4 HRS.)

5OO

/~

SUMMARY OF ELASTICITY DATA ON HYDROGELB

/

BASEONEQUATION

/

Hydrogel composition

I

M

~ 450

c

%/E/gin SiO2-

E

5 gm SiO2/100 ml 10 gm SiO2/100 nil 15 gm SiO2/100 ml 20 gm SiO2/100 ml 6.6% Si02 hydrogel pK 5.6--data from Munro, McNab, and Ott (8) Gelatin at 5°C--data from Sheppard and Sweet (12)

~ 550 3

mo 300 t~ ~. 250 200

150

I00 /

Mc

~RT

=

-_o..400

Jz

47

//THEORETICAL

/

<5 6 4.7 4.5 3.4

<4,000 2,000 2,000 1,500 10,000

2.8 ~

10,000

~/E/gm gel~Un.

CR OSS-

LINKINGREGION

u0

49~

~ 50 0

2

4

6

8

AGING T I M E

10

12

14

Tc.$.=~ 6 hrs,

16

(HRS.)

FIe. 6. The kinetics of cross-linking of silica hydrogel. bonding in the hydrogel assumes t h a t the rate of diffusion of N a O H into both structures is about the same. This was checked and found to be rapid in both types of gels and nearly equal. RESULTS

5

2-

435 ~

x Ld 0

L0

I

>o

Viscosity T h e viscosity of the silica hydrosolhydrogel system is shown in Fig. 4 for a 10 g m Si02/100 ml hydrosol-hydrogel. The time axis is in terms of hours and in terms of time relative to gelation time shown as T/Ta. This is a typical viscosity curve for the acid-set hydrogel system.

Elasticity T h e elasticity data are given for hydrogels of different silica concentrations. Figure 5 is a plot of the square root of elasticities (Young's modulus) as a function of time relative to the initiation of hydrogel syneresis. The time required for setting and for syneresis is given in the section on the syneresis of hydrogels. The calculated molecular weights between cross-links according to the theory of rubber-like elastic-

/ / j

0~/~

0

o ,o~.~,~,oo ~ ~o.o~

i

i

,

I

i

I

i

i

i

.I

.2

.3

.4

.5

,6

E

,8

.9

TiME R E L A T I V E

'

1.0

TO COMPLETE SYNERESIS

(TITcs,) Fio. 7. Rate of syneresis of silica hydrogels at different Si02 concentrations. ity were made for a 15 gm SiO~/100 ml hydrogel at various time intervals. These data are plotted in Fig. 6 as the number of silanol groups cross-linking per one million molecular weight as a function of time. On the assumption of a second-order reaction, a dotted line is drawn to give a better prediction of cross-linking. The values of ~v/E/gm SiO2/100 ml and the molecular weight between cross-links at syneresis for the four hydrogels are tabulated in Table

Journa~ of Colloid and Interface Science, Vol. 32~ No. 1, Junu~ry 1970

48

ACKEt~ TABLE IV SUMMARY OF SYNERESIS D A T A FOR I-IYDROGELS 10 gm SiO2/100 ml

Performance Data Hydrogel weight (gm) Hydrosol set time Syneresis started Syneresis duration Analysis of Data Grams SiO2 in hydrogel Vol. of syneresis H20 (cc.) Moles of Syneresis H~O Moles of SiO2 cc Rate constant [(day) (gm SiO2)]

43.5 18 hr 54 hr 7 days

15 gm SiO2/100 ml

50 4.5 hr 13 hr 28 hr

20 gm SiO2/100 ml

49 40 min 3 hr 36 hr

3.9 2.0

6.3 2.6

7.8 4.2

1.7

1.5

1.8

0.8

0.37

0.36

INTeRPRetATION OF R~SULTS

ation by the modified penetrometer method. The polysilicic acid is continuing to associate and bond throughout this aging period until the completion of syneresis. The elasticity measurements bring out two significant aspects of this hydrogel. First, the ~¢/E/gm SiO2 at syneresis is constant through the range studied. This was also previously observed by IV[unro, NIcNab, and Ott (8). Second, by applying the equation for determining the molecular weight between cross-links, the linear polysilicic acid molecules are shown to crosslink up to the extent of one cross-link per 2,000 molecular weight. It is reasonable to assume the cross-linking continues to the end of the syneresis period in the absence of an experimental method to detect this occurrence. The line of theoretical cross-linking is drawn from Fig. 6 in the belief that the elasticity measurement at the beginning of the aging period is caused by the entanglement of molecules and not cross-linking. Also, cross-linking should be a second-order reaction and should follow the straight-line plot extrapolated from the region of higher elasticity measurements to the time axis.

Highly Cross-Linked Structure of Hydrogel Silica hydrogel at the set point and beyond is a viscoelastic material which increases in viscosity and in rigidity up to the point of syneresis. The hydrogel from the syneresis points to the completion of the bonding is too rigid and too brittle for evalu-

Syneresis Relation to Silica Concentration Syneresis of a hydrogel is thought to be caused by the compression upon the free water by further intermolecular attraction. The data from Fig. 7 and Table IV show the course of syneresis is almost linear with time, and the ratio of moles of water exuded

III. The elasticity data from 3/[unro et al. (8) for silica hydrogel and from Sheppard and Sweet (12) on gelatin studies are included.

Syneresis The syneresis period of a hydrogel marks the final stage of formation of the hydrogel. The amount of liquid exuded as a function of time relative to the completion of syneresis is plotted in Fig. 7. The hydrogel with 5 gm SiO2/100 ml did not show any syneresis liquid over an extended period of time. The data are tabulated in Table IV. Rate of Depolymerization The hydrogel depolymerization rate data are plotted in Fig. 8. Hydrogels at various stages in the aging process were titrated and their rate constants are reported along with a dried desiccant gel. Hydrogel State Based on the hydrogel studies, a sketch of the molecular state of completely aged hydrogel is shown in Fig. 9.

Journal of Colloid and Intorface Science, ¥ol. 32, No. 1, January t970

ACID-SET SILICA HYDROSOLS, HYDROGELS, AND DRIED GEL

49

.040,-

~o03C

(}

_1

/HYDROGELS

do

Io ,,, ~ . 0 2 C _IF-

MOLES

I Ogm. S i O 2 / I O O ml.

•

0 m u.o O~

z~ ¢o.OIO ~w

AGED 5 HOURS

20.OxlO-3

(5 AGED 53 HOURS (~ ooAGED PAST SYNERESIS O='oAGED+ pH 3.0 WASH AND @ SPRAY DRIED DESICCANT GEL

13.5x10-3 9 . 0 0 x l O -3 6.27x10 -3 1.27x10-3

bl Q. x w ¥

0

5

IIo

I

20

115

I

25

30

TIME ( M IN.)

FIG. 8. Rate of depolymerization of various hydrogels and the standard gel

H" 0

H 0

S'i- O

S[i- O--Sli-

I

HO E

H 0

f

H 0

O-Sli-

H 0

O - S'i - O - S l i -

H 0 O-Si--OH--

I

I

I

I

I

~

O H

O H t

O H

0 H I

O H

O H

H O

H O

H O

H O

H O

H

H O H i O--Si

H 0

I

I

I

~

0 - Si, - O H I

O

O

O

O

O

O

H

H

H

H

H

H

-

O--Si

-

0

--

Si

-

H

O H - O H

H

2

I

-

o,

-

O-Si- ,

HH -

H --

~H

A SEGMENT OF HYDROGEL STRUCTURE H H Si- OH~OH~O--Si SHOWS HYDROGEN BONDING

OF SILICIC

REPRESENTS

ACID THROUGH H20

FREE WATER

Fro. 9 Journal of Colloid and Interface Science, V o l . 32, ~ o . 1, J a n u a r y 1970

50

ACKEt~

to moles of Si02 in the hydrogel is nearly constant. Hydrogel of 5 gm SiO2/100 ml concentration did not exhibit syneresis over the extended period of time observed. This lack of syneresis indicates the dilution of the polysilicic acid is too great for further attraction between these associated molecules.

into clusters by hydrogen bonding forces (14). The water adsorbed on the surface of the silanol groups of the silica gel is held there by hydrogen bonding forces (15). Cellulose gels in water show evidence of hydrogen bonding between the starch hydroxyls and the four coordinated water lattice surrounding the starch (16). Five A Low Degree of Siloxane Bonding and Most requirements for hydrogen bonding are Likely, a High Degree of Hydrogen given by J. D. Bernal (17). If we compare Bonding in Hydrogel the composition and structure of the acidic The rate of depolymerization technique silica hydrogel to similar examples of hydrohas useful application in analyzing the gen bonding systems and consider the resiloxane bonding occurring in silica hydro- quirements of hydrogen bonding, it seems gel. On the basis of a Ksi_o = 3.81 X reasonable that hydrogen bonding is re10-3 moles/liter-minute, aged hydrogel has sponsible in part for the rigidity of the a k = 6.27 X 10-8, and we can approximate silica hydrogel. The depolymerization of hydrogels and that 0.6 SiO bond is broken per two trisilicic acid units or about one Si--O bond gels to silicate solutions containing only trisilicate ions may be an oversimplification. per three trisilicic acid molecules. The hydrogel structure is pictured in The possibility of the soluble silicate's conFig, 9 with approximately one siloxane bond taining a percentage of higher silicate ions is per three trisilicic acid units. Some of the real; however, it would increase only fracremainder of these bonds are shown to be tionally the number of siloxane bonds in the hydrogen bonded through a water molecule. hydrogel. This would not alter the qualitaThe basis of hydrogen bonding is discussed tive description of the hydrogel state. The proposed structure for the hydrogel in the following two paragraphs. The remainder of the water in the hydrogel is state consists of nearly linear associated shown as free water. The relatively fast molecules of polysilieie acid with considerarate constant for fresh hydrogel (k = 20.0 X ble bonding and cross-linking by hydrogen 10-3 moles/liter-minute) is probably a bonding through a water molecule or cluster measure of the rate of depolymerization of of molecules. There is a low degree of condensation bonding (siloxane) between silieie these hydrogen bonds. acid units in the structure and a high On the basis of the elasticity treatment of degree of hydrogen bonding between these hydrogels at syneresis, the linear polymer of silicic acid was cross-linked about every units. 2,000 molecular weight segment. The hy- I I I . H Y D R O G E L D t ~ Y I N G A N D T H E D R I E D drogel became more rigid after syneresis, SILICA GEL and it is reasonable to project that this was The methods used to understand acid-set the result of further bonding. The above hydrogel drying and to characterize the siloxane bond analysis for hydrogel indicates dried gel are given below. a much lower bonding than is shown by the elasticity measurement. This discrepancy EXPERIMENTAL can be cleared up if we consider another Rate of Drying type of bonding is occurring between the The rate of drying of the hydrogei was desilanol groups of the silicic acid units. Hydrogen bonding is the most likely suspect to termined by drying in a force draft oven at 150°C with an air flow of 3 cu ft/minute. furnish the additional bonding. The reasons for hydrogen bonding are The hydrogel was sized in pieces approxinumerous. Trimethyl and triethyl silanol mately ~ inch. A recording balance folcompounds are known to associate through lowed the loss in weight over the drying hydrogen bonding (13). Water is associated period. Journal of Colloid and Tnterface Science, Vol. 32, No. 1, January 1970

ACID-SET SILICA HYDROSOLS, HYDROGELS, AND DRIED GEL

51

J t,d £9 o o1

Surface Areas of Gels The surface areas of gels referred to in this article were measured by a PerkinElmer-Shell Sorptometer. Pore volumes of gels were measured by a water titration method (18). Pore size distribution was done on a B.E.T. (19) Adsorption Apparatus using the Barrett-Joyner-Halenda (20) method.

I o o to

3

IDEALIZED DRYING CURVE FOR SMALL

o. ._c E

2

Total Volatiles of Hydrogel The total volatile content of a hydrogel was determined by heating a 10 gm sample at 1750°F for 1 hour and then weighing the residue. The results are expressed as follows.

o

,

i

i

20

30

40

6qO

5~0

710

80

u_ o < c~

% Total Volatiles =

IlO

% TOTAL

VOLATILES

OF H Y D R O G E L

FIG. 10. Silica hydrogel drying curve

Original-Final Weight X 100. Original Weight

This is a measure of the water content of the hydrogel and water derived from silanol group condensation. I~ESULTS The results are plotted in Figs. i0 and 1 i. The drying rate curve for a pH 3 washed hydrogel of 15 % SiQ - 120 % neutralized composition is shown in Fig. i0. The surface properties of this dried gel are: surface area, 800 m2/gm and pore volume, 0.4 cc/ gm. The pore size distribution is given in Fig. 11. The electron micrograph of this silica gel (Fig. 12) shows that the dried gel structure consists of closely bonded primary particles about 7 m~ in diameter. From the physical characterization of the dried silica gel, a two-dimensional sketch of gel structure is shown in Fig. 13.

70

PORE RANGE B E L O W 14A¢ 1 4 - 20

20-50

60

% S.AIN PORE RANGE 45 3g

15 5 I

50-40 40

ABOVE 5o E ac uJ ~uJ <

40

30

g 20

a: ~0

0

,

0

10

I

20

50

AV. DIAMETER

40

(A*)

FIG. 11. Pore size distribution of dried desiccant silica gel.

INTERPRETATION OF RESULTS

Condensation Reaction during Drying of Hydrogel Analysis of the drying curve for hydrogel shows a tendency for a constant rate and a falling rate portion. The constant rate represents the removal of free water from the hydrogel. The falling rate depicts condensation of silanol groups and the diffusion of this water vapor through the newly formed porous structure. This is the point in the

gel's preparation where most of the condensation reaction is occurring and where the final rigid gel structure is formed. The actual drying curve does not coincide exactly with the calculated curve because the drying of hydrogel chunks is not uniform. The outer surface of the hydrogel may be on the failing rate while the interior is still on the constant rate.

Journal of Colloid and Interface Science,

V o l . 32, N o . 1, J a n u a r y

1970

52

ACKER

FIG. 12. Electron micrograph of desiccant silica gel/63,000X

Physical Structure of Dried Desiccant Gel The physical structure of dried gel is shown on the electron micrograph and depicted in Fig. 13. Desiccant gel is composed of spherical primary particles about 7 m~ in diameter, and these are formed during the

drying step by a condensation and shrinking mechanism. The finer pores are probably located within these primary particles, and the larger ones are located betweea the tightly bonded primary particles. The surface of the pores is composed of the silanol

Journal of Colloid and Interface Science, Vol. 32, No. I, J a n u a r y 1970

ACID-SET SILICA HYDROSOLS, HYDROGELS, AND DRIED GEL

53

7 m}J

o_ -o_ I l ~l

ROi. ~0-9'i-

I lsi-

FINE

PORES

HO i.

. 0i

O-S,

0 I

- O-

Si-

0 I

0 I

o0

FINE

HO [ O-

;o-

-o-

Si-

OH 0 - Si-

0

0 I

-

0 -Si-O-

0 I

PORES

ON i

OH

0 I

0

J Si- 0 - Si-O

OH

,. - Sl-

0 I -si- o- Sli- o-si

sli-°-siol-°osi-o-sio-o-sisi-O-o°l o-siSl-io-si l°-SlOoli-loi°-Sl/°°li-°-O°l $i~,./t/0 l 0

0

0

0

I

I

I

l

Si-O-Si-O-Si- O - Si-OI I I I 0 0 0 0

Si~-O-Sil -IO-Si1-O-Sl Hi-O--~"~IOH H H 0H H0 H0 0H 0

I

l

I

I

Sli-O -Sli- 0 - Sli- 0 - Si-0o

o

/....3,

\

SEGMENT OF A 7m,u PRIMARY PARTICLE OF S I L I C A GEL APPROXIMATING THE STRUCTURE

Fio. 13. Structure of dried desiccant silica gel end groups of the condensed silicic acid molecules. The adsorptive performance is related to the presence of these silanol groups distributed throughout the porous structure. The usual hard granular structure of desiccant gel is achieved by the ability of the ultra fine sized primary particles to bond extensively. ACKNOWLEDGMENT Appreciation is given to W. 1~. Grace & Co. for encouraging the publication of this article. The writer acknowledges the advice and assistance of many of the personnel at the Washington Research Center. The assistance of Mrs. N. W. Collins and Mr. C. M. Clay was especially noteworthy. The advice by Mr. S. A. Mitchell of Jos. Crossfield & Sons, Ltd. to include a depolymerization study of silica gel was very helpful. REFEI%ENCES 1. ALEXANDER,A. E., ANDJOHNSON,P., "Colloid Science," p. 608. Clarendon Press, Oxford, 1945. 2. PLANK,C. J., or. Colloid Sci. 2,413-427 (1947).

3. ZIMM, B. H., J. Chem. Phys. 16, 1093 (1948). 4. O~ou~A, H., AND UKIHASHI, H., Bull. Chem. Soc. Japan 39, (No. 10) 1634-1637 (1965). 5. BILLMEYER, F. W., "Textbook of Polymer Chemistry," p. 131. Interscience Publishers, New York, 1957. 6. NAUMAN, 1~. V., AND DEBYE, P., J . Phys. Colloid Chem. 55, 1-9 (1951). 7. BRADY,A. P., BROWN,A. G., AND I'I'IUFF,H., J. Colloid Sci. 8, 252-276 (1953).

8. MUNRO,L. A., McNAs, J. G., ANDOTT, W. L., Can. $. Research 27, Sec. B, No. 10,781-789 (1949). 9. FERRY, 5. D., "Viscoelastic Properties of Polymers," p. 392. Wiley, New York, 1961. 10. ILER, I¢. X., "The Colloid Chemistry of Silica and Silicates," p. 242. Cornell University Press, Ithaca, New York, 1955. 11. ALEXANDER, G. B., J. Am. Chem. Soc. 76, 2094-2096 (1954). 12. SHEPPARD, S. ]~., AND SWEET, S. S., J. A m . Chem. Soc. 43,539-547 (1921). 13. WEST, R., ANn BANEY, IL H., J. Am. Chem. Soc. 81, 6145-6148. 14. PIMENTEL, G. C., AND McCLELLAN, A. L.,

Journal of Colloidand Interface Science, Voi. 32, No. 1, January 1970

54

ACKE~

"The Hydrogen Bond," pp. 193-205. W. H. Freeman and Company, San Francisco and London, 1960. 15. McDoNALD, IZ. S., d. Am. Chem. Soc. 79, 850-854 (1957). 16. COLLISON, R., and McDONALD, M. P., Nature 186,548-549 (1960). 17. BERNAL, J. D., "The Function of the Hydrogen Bond in Solids and Liquids" in D.

Hadzi and H. W. Thompson, eds., "Hydrogen Bonding," pp. 7-9. Symposium Publication Division of Pergamon Press, New YorkLondon-Paris-Los Angeles, 1959. 18. INNES, W. B., Anal. Chem. 28,332-334 (1956). 19. BRUNAUER, S., EMMETT, P. n . , ~-NDTELLER, E., J . Am. Chem. Soe. 60,309 (1938). 20. BARRETT,E. P., 5OYNER,L. G., A_NDHALENDA, P. P., J . A m . Chem. Soe. 73,373-380 (1951).

Journal of Colloid and Interface Science, ¥ol. 32, No. 1, January 1970