ISSN 1061-933X, Colloid Journal, 2006, Vol. 68, No. 5, pp. 558–562. © Pleiades Publishing, Inc., 2006.Original Russian Text © A.N. Zhukov, L.I. Zavarovskaya, Yu.M. Chernoberezhskii, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68, No. 5, pp. 612–616.

558

INTRODUCTION

Previously [1, 2], the method of flow ultramicros-copy was employed to investigate the kinetics of thecoagulation of dilute fused quartz and monodisperseamorphous silica suspensions prepared by the additionof aliquots of concentrated dispersions of the afore-mentioned particles in 96% ethanol stored for morethan 20 days to water–ethanol mixtures. It was dis-closed that suspensions thus prepared were stable withrespect to aggregation at an ethanol content of 80–96vol % in the mixtures. Upon a decrease in ethanol con-tent to 30 vol %, the coagulation proceeded in an accel-erated mode with a characteristic time (coagulationperiod) that was noticeably shorter than the times cor-responding to the Smoluchowski theory of fast coagu-lation. The rate of coagulation was the highest in sus-pensions containing 40 vol % of ethanol, and theiraging for more than 24 h slowed down the coagulation,which retained its “superfast” character at ethanol con-centrations of 40–48 vol %.

The effect of water content on the electrophoreticmobility of fused quartz particles and conductivity oftheir suspensions in water–ethanol solutions of electro-lytes was studied in [3]. The electrokinetic potential ofquartz was determined from these data with allowancefor the surface conductance; the values of electrokinetic

potential varied from –26 to –35 mV upon reduction ofethanol content from 96 to 60 vol %. The observationof superfast coagulation under these conditions with anincrease in the absolute values of the electrokineticpotential indicates that the electrostatic interaction doesnot markedly affect the aggregative stability of the sus-pensions studied.

The analysis of the data obtained carried out withaccount of the reported factors that provide for theaggregative stability or accelerate the coagulation ofdisperse systems [4–11] allowed us to conclude that themain causes of the stabilization or accelerated coagula-tion of the suspensions in question are the products ofsilica dissolution in water–ethanol media. It is known[12] that, at standard temperature, silica is very slightlysoluble in lower aliphatic alcohols; the equilibrium val-ues of its dissolution are achieved over 1–2 months andare on the order of a few ppm. The products of the dis-solution are orthoesters and esters of polysilicic acids.It is obvious that, upon the aging of concentrated silicadispersions in ethanol, the dissolution is preceded bythe formation of surface ester ethoxy groups. The pres-ence of the layers of such groups on the surface of silicaparticles provides for their steric stabilization, that is,the aggregative stability of suspensions observed inwater–ethanol mixtures with high alcohol content.

The Effect of Preparation Procedure and Compositionof Water–Ethanol Suspensions of Amorphous Silica

on Their Aggregative Stability and Kinetics of Coagulation

A. N. Zhukov

a

, L. I. Zavarovskaya

a

, and Yu. M. Chernoberezhskii

b

a

Department of Chemistry, St. Petersburg State University, Universitetskii pr. 26, Petrodvorets, St. Petersburg,198504 Russia

b

St. Petersburg State Technological University of Plant Polymers, ul. Ivana Chernykh 4, St. Petersburg, 198095 Russia

Received December 22, 2005

Abstract

—The method of flow ultramicroscopy is employed to study the effect of the composition and prepa-ration procedure of dilute water–ethanol suspensions of two samples of amorphous silica (fractionated fusedquartz and monodisperse amorphous silica) on the kinetics of their coagulation. It is revealed that all suspen-sions prepared by the addition of silica powders to water–ethanol mixtures with ethanol contents of 96 and40 vol % are stable with respect to aggregation, as the suspensions prepared by the addition of aliquots of con-centrated dispersions of the aforementioned silica samples in 96% ethanol aged for different time periods towater–ethanol mixtures containing 96 vol % ethanol. At a 40-vol % content of ethanol in the mixture, the coag-ulation whose character (including “superfast” coagulation) substantially depends on the time of aging of initialconcentrated silica dispersions occurs. Furthermore, kinetic studies are performed for the coagulation of dilutesilica suspensions prepared by the addition of silica powders to water–ethanol solutions containing 40 vol % ofethanol and traces (<1 ppm) of poly(ethoxysilane), poly(acrylic acid), and a supernatant prepared by the cen-trifugation of concentrated silica dispersion in 96% ethanol aged for more than 3 months. It is found that theaddition of aliquots of the aforementioned ethanol solutions to silica suspensions in 40% ethanol, which areinitially stable with respect to aggregation, causes their superfast coagulation.

DOI:

10.1134/S1061933X0605005X

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THE EFFECT OF PREPARATION PROCEDURE AND COMPOSITION 559

It is also known that an increase in water content inalcohol enhances the solubility of silica and changesthe composition of its dissolution products through thereactions of hydrolysis and polycondensation. Obvi-ously, an increasing water concentration in water–etha-nol silica suspensions prepared by the addition of ali-quots of aged concentrated silica dispersions in 96%ethanol results in a similar increase in the solubility andthe degree of polymerization of surface ethoxysilanes.It is known that the addition of sufficiently high-molec-ular-weight polymers to the dispersions, even in con-centrations less than 1 ppm, leads to a quite fast bridg-ing flocculation (coagulation) of particles.

In order to prove that the main regularities ofchanges in the aggregative stability found in [1, 2] aredue to the presence of the interaction products of silicaand alcohol on the silica particle surface, in this work,we studied the effect of the preparation procedure andcomposition of silica water–ethanol suspensions, aswell as the presence of traces of polymers, on the aggre-gative stability and the kinetics of coagulation of thesedisperse systems.

EXPERIMENTAL

Experiments were performed using fractionatedfused quartz powders with the mean effective particlesize of

0.5

±

0.1

µ

m and amorphous silica (Monospher250, Merck) with monodisperse spherical particles250 nm in diameter. The quartz powder was treatedwith a 30% nitric acid solution, washed with distilledwater to constant pH and conductivity values of a fil-trate, and subjected to sedimentation fractionation andcentrifugation. Monodisperse silica was used asreceived. Both samples were dried at

105–110°ë

andstored in a desiccator over concentrated sulfuric acid.Ethanol was purified by double fractional distillation atatmospheric pressure.

The kinetics of coagulation was studied using dilutesuspensions of the aforementioned silica samples pre-pared by two procedures. According to procedure I, sil-ica powders taken in an amount required to achieve aparticle concentration of

~10

13

m

–3

were subjected toultrasonic dispersion in a water–ethanol solution withan alcohol content of 96 or 40 vol %. In procedure II,the aliquots of concentrated dispersions containing10 mg of a powder in 100 ml of 96% ethanol aged for atime period from 3 days to 6 months were added to200 ml of water–ethanol solutions. All examined sus-pensions were subjected to an ultrasonic treatmentimmediately before the measurements which were car-ried out at

25°ë

.

The effect of polymer additives on the kinetics ofcoagulation of water–ethanol suspensions of fusedquartz was investigated using ethanol-solublepoly(ethoxysilane) (PEOS) and poly(acrylic acid)(PAA) with molecular masses of 500 and 450000,respectively. Dilute suspensions were prepared by pro-

cedure I, that is, by the addition of quartz powder(10 mg) to a water–ethanol solution of a preset compo-sition containing PEOS or PAA in concentrations of 0.3and 0.05 ppm, respectively.

Concentration

n

of particles and their aggregates insuspensions was determined as a function of time

t

byvisual counting in a preset volume with a VDK-4 flowultramicroscope at a linear flow velocity of a suspen-sion equals 0.01 cm/s, at which a possible coagulationcannot be orthokinetic [13, 14]. Degree

m

of particleaggregation corresponding to time

t

was expressed as

m

(

t

) =

n

0

/

n

, where

n

0

is the particle concentration at thebeginning of observation (

t

= 0) immediately after theultrasonic treatment of suspensions. The relative errorin measuring particle concentration was no more than5% in the cases of aggregation-stable systems and slowcoagulation and no more than 10% in the cases of fastand superfast coagulation. Characteristic time (period)of coagulation corresponding to the Smoluchowski the-ory of fast coagulation was calculated by the equation

τ

Sm

= 3

η

/4

kTn

0

,

where

η

is the solution viscosity,

k

is Boltzmann’s con-stant, and

T

is the absolute temperature. The values of

τ

Sm

were used to plot theoretical kinetic curves for fastcoagulation according to Smoluchowski (dotted linesin the plots):

m

(

t

) = 1 +

t

/

τ

Sm

.

RESULTS AND DISCUSSION

Time dependences of the degree of particle aggrega-tion plotted for suspensions of two aforementioned sil-ica samples freshly prepared in water–ethanol solutionscontaining 96 and 40 vol % of ethanol suggest that theprocedure of suspension preparation substantiallyaffects the kinetics of coagulation.

All the suspensions prepared by the addition of sil-ica powders to water–ethanol solutions (method I) arestable to aggregation, while coagulation takes place insuspensions prepared by procedure II. The kinetics ofcoagulation depends on the aging time of initial con-centrated silica dispersions in 96% ethanol used for thesampling of aliquots and on the ethanol concentrationin the dispersion medium (Figs. 1–4). After these dis-persions were aged for several days, the character of

m

(

t

)

dependences is nonmonotonic for suspensions pre-pared by procedure II in 96% ethanol (Figs. 1a, 2a).Similar suspensions prepared by the addition of ali-quots of concentrated dispersions after their aging for25–30 days become stable to aggregation (Figs. 1b, 2b).This time seems to be required for the formation of con-tinuous layers of silica–ethanol interaction productsthat more or less are uniformly distributed over the sur-face and play the role of stabilizers. Oscillations in the

m

(

t

)

dependences for dispersions aged for 5–8 daysmay be explained by particle disaggregation associatedwith the processes of dissolution and formation of sta-

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ZHUKOV et al.

1.0

0.5

m

(‡)

(b)

1.0

0.50 50 100 150 200 250

t

, min

1.5

1.5

Fig. 1.

Time dependences of the degree of aggregation indilute fused quartz suspensions in 96% ethanol prepared bythe addition of aliquots of concentrated quartz dispersionsin 96% ethanol aged for (a) 8 and (b) 25 days.

1.0

0.5

1.0

0.50 50 100 150 200 250

m

t

, min

(b)

(‡)

1.5 (a)

1.5

Fig. 2.

Time dependences of the degree of aggregation indilute monodisperse dispersions of amorphous Monospher250 silica in 96% ethanol prepared from the aliquots of con-centrated silica dispersions in 96% ethanol aged for (a) 5and (b) 30 days.

(‡)

(b)

(c)

6

5

4

3

2

1

0 20 40 60 80 100 120 140

m

7654321

0 20 40 60 80 100 120 140 160

3

2

1

0 50 100 150 200 250

t

, min

Fig. 4.

Time dependences of the degree of particle aggrega-tion in dilute dispersions of monodisperse amorphousMonospher 250 silica in 40% ethanol prepared from ali-quots of concentrated silica dispersions in 96% ethanolaged for (a) 12 (

τ

Sm

= 840 min) and (b) 20 days (

τ

Sm

=493 min) and (c) 6 months (

τ

Sm

= 562 min).

(a)

(b)

(c)

5.0

3.5

2.0

0.50 50 100 150 200 250

m

25

20

15

10

5

0

0 20 40 60 80 100 120

25

20

15

10

5

10 20 30 40 50 60

t

, min

Fig. 3.

Time dependences of the degree of particle aggrega-tion in dilute fused quartz suspensions in water–ethanolsolutions with alcohol content of 40% prepared from ali-quots of concentrated quartz dispersions in 96% ethanolaged for (a) 3 (b) 20, and (c) 30 days. Dotted lines refer tothe

m

(

t

)

dependences corresponding to the Smoluchowskitheory of fast coagulation with characteristic times

τ

Sm

668

= (a) 668, (b) 787, and (c) 884 min.

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THE EFFECT OF PREPARATION PROCEDURE AND COMPOSITION 561

bilizing layers (or other reactions) that rupture interpar-ticle contacts.

Time dependences of the degree of particle aggrega-tion in dilute silica suspensions prepared by procedureII in water–ethanol solution containing 40 vol % of thealcohol are somewhat different. In the case of a fusedsilica suspension prepared by the addition of an aliquotof its concentrated dispersion in 96% ethanol afteraging for 3 days, the

m

(

t

)

dependence is also nonmono-tonic and testifies a superfast coagulation (Fig. 3).When aliquots of concentrated quartz suspensions agedfor 20–30 days are used, the

m

(

t

)

dependences are vir-tually linear and suggest the coagulation at a rather highrate which increases with the aging time.

In the case of monodisperse silica suspensions in40% ethanol prepared by the addition of aliquots of itsconcentrated dispersion in 96% ethanol after its agingfor 12 and 20 days, the

m

(

t

)

dependences (Fig. 4) com-prise two linear parts, with the slope of the first part cor-responding to the kinetics of coagulation with a ratesomewhat higher than the rate of fast coagulation andthe slope of the second linear part corresponding to pro-nounced superfast coagulation. The aging of the con-centrated dispersion for 6 months causes an increase in

the slope of the first part and a decrease in the slope ofthe second part of the curve.

The data illustrated by Figs. 5 and 6 suggest that thepresence of 0.3 ppm of PEOS and 0.05 ppm of PAA inthe fused quartz suspensions in 40% ethanol also leadsto superfast coagulation, thus complying with commonideas of the bridging flocculation [6–11]. The additionof a supernatant of a concentrated fused quartz disper-sion in 96% ethanol aged for 3 months to the stable toaggregation monodisperse silica suspension in 40%ethanol prepared by procedure I has a similar effect(Fig. 7). Qualitatively similar characters of the

m

(

t

)

dependences illustrated by Figs. 5–7 indicate that thesuperfast coagulation of suspensions prepared by pro-cedure II is caused by the presence of polymeric prod-ucts of silica dissolution in ethanol.

CONCLUSIONS

Earlier proposed [1, 2] mechanisms of the aggrega-tive stability and superfast coagulation of dilute silicasuspensions in water–ethanol solutions with differentcontents of alcohol are supported by the data on theeffect of the preparation procedure for such suspen-sions and the addition of traces of polymers or a super-natant of a concentrated fused quartz dispersion in 96%ethanol aged for 3 months. The aggregative stability ofthe systems under examination results from the stericstabilization of the particles by the products of silicadissolution, while the superfast coagulation of suspen-sions containing from 30 to 80% of ethanol and pre-pared by the addition of aliquots of concentrated silicadispersions in 96% ethanol is associated with the bridg-ing flocculation of particles with the products of silicadissolution such as orthoesters or esters of polysilicicacids.

6.5

5.0

3.5

2.0

0.50 50 100 150

t

, min

m

Fig. 5.

Time dependence of the degree of particle aggrega-tion in a fused quarts suspension in a 0.3 ppm PEOS solu-tion in 40% ethanol (

τ

Sm

= 1063 min).

6.5

5.0

3.5

2.0

0.50 50 100 150 200 250

t

, min

m

Fig. 6.

Time dependence of the degree of particle aggrega-tion in a fused quartz suspension in a 0.05 ppm PAA solu-tion in 40% ethanol (

τ

Sm

= 1192 min).

2.5

2.0

1.5

1.0

0.50 50 100 150 200 250

t

, min

m2

1

Fig. 7.

Time dependences of the degree of aggregation in afreshly prepared dilute suspension of monodisperse amor-phous Monospher 250 silica in 40% ethanol prepared by thedispersion of a powder (10 mg) in a water–ethanol solution(1 l) (

1

) before and (

2

) after the addition of supernatant(

τ

Sm

= 788 min).

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ACKNOWLEDGMENTSThis work was supported by the Russian Foundation

for Basic Research, project no. 03-03-32476, and Lead-ing Scientific Schools grant of the President of RussianFederation, no. NSh-789.2003.3.

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