1061-933X/04/6604- © 2004

MAIK “Nauka

/Interperiodica”0439

Colloid Journal, Vol. 66, No. 4, 2004, pp. 439–443. Translated from Kolloidnyi Zhurnal, Vol. 66, No. 4, 2004, pp. 491–496.Original Russian Text Copyright © 2004 by Zhukov, Kim, Chernoberezhskii.

INTRODUCTION

The aggregation stability of disperse systems whichcan be determined as their capacity to retain the degreeof dispersity with time, as well as its loss (coagulation)are among the fundamental problems of colloid sciencethat is corroborated by the yearly growing number ofpublications in this field. The main results and progressin solving this problem are treated in detail in mono-graphs, textbooks, and reviews [1–6]; as a rule, they arerelated to further development and experimental verifi-cation of the Derjaguin–Landau–Verwey–Overbeektheory of the aggregation stability of lyophobic colloids(the DLVO theory). According to this theory, the stabil-ity in a kinetic sense is ensured by the predominance ofsurface forces of electrostatic repulsion of interactingparticles over the forces of molecular attraction. In thiscase the dependence of potential energy of particlesinteraction on the interparticle distance is characterizedby the presence of a high energy barrier

U

max

. The low-ering of this barrier height for one reason or other leadsto slow coagulation, and its elimination results in fastcoagulation.

The quantitative measure of aggregation stability isthe coagulation rate which is determined by decreasingnumber concentration

n

of dispersed particles in solu-tion with time due to aggregation, compared to theirinitial concentration

n

0

. According to the classicalSmoluchowski theory of fast coagulation, the kineticsof this process at

U

max

= 0 is determined solely by theintensity of the Brownian motion of particles (diffu-sion) and can de described by the following equation:

n

0

/

n

= 1 +

t

/

τ

1/2

, (1)

where

τ

1/2

is the time of the half-coagulation (a twofolddecrease in the initial concentration of particles) whichis called the coagulation period and is given by

τ

1/2

= 3

η

/4

kTn

0

, (2)

where

η

is the solution viscosity,

T

is the absolute tem-perature, and

k

is Boltzmann’s constant. To describe thekinetics of slow coagulation which takes place at

U

max

> 0,i.e., when not all particle collisions end with the aggre-gation, Derjaguin proposed to use factor

W

~exp(

U

max

/

kT

) > 1

, called the stability ratio [2]. Theintroduction of this factor into the Smoluchowski equa-tions results in an increase in time of half-coagulationby a factor of

W

.In works on the kinetics of fast coagulation (earlier

works are reviewed in [1]; recent ones, in [3, 6]), it isshown that, in some cases, the rate of this processexceeds the values calculated by the Smoluchowskitheory. It is found that such an ultrafast or acceleratedcoagulation takes place in suspensions and sols of con-siderable polydispersity (Wiegner’s effect), in the casesof anisometry of dispersed particles (Müller’s effect)and their directional motion upon the sedimentation orstirring. In the latter case, the coagulation is calledorthokinetic, in contrast to perikinetic coagulation,which is due only to Brownian motion of particles. Thetheoretical works performed with allowance made forthe effect of these factors on the kinetics of fast coagu-lation are reviewed in [1, 3, 5, 6].

The special case is the ultrafast coagulation, whichoccurs upon the addition of water-soluble polymers toaqueous disperse systems, called flocculation [7–9]. Asa rule, the flocculation is observed in solution either in

The Effect of Composition of Water–Ethanol Solutions on the Kinetics of Coagulation of Fused Quartz Suspensions

A. N. Zhukov*, L. I. Kim*, and Yu. M. Chernoberezhskii**

* Department of Chemistry, St. Petersburg State University (Petrodvorets Branch), Universitetskii pr. 26, St. Petersburg, 198504 Russia

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

Received December 25, 2003

Abstract

—The effect of composition of water–ethanol solutions containing 0–96 vol % alcohol on the coagu-lation kinetics of dilute fused quartz suspensions is studied using flow ultramicroscopy technique. It is estab-lished that, in both water and ethanol containing 4 vol % water, freshly prepared quartz suspensions are stablewith respect to aggregation; at ethanol concentrations from 10 to 90 vol %, an ultrafast coagulation takes place,with characteristic time (coagulation period) being essentially smaller compared to a value corresponding tothe Smoluchowski theory of fast coagulation kinetics. It was shown that the aging of the aforementioned sus-pensions for more than 24 h results in slow coagulation at high and low alcohol content, but its ultrafast char-acter is retained in the ethanol concentrations range of 40–48 vol %. The probable reasons for the ultrafast coag-ulation in the disperse systems specified are discussed.

440

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et al

.

the presence of small amounts (<1 ppm) of high-molec-ular-weight (

~10

6

) linear macromolecules with polargroups on both chain ends or in the presence of poly-electrolytes with lower molecular mass (but with higherconcentration) and high charge density, the charge signbeing opposite to that of particles. In the first case, thisprocess is called bridging flocculation, because itoccurs due to the formation of a bridge between dis-persed particles as a result of adsorption of the two endsof a macromolecule on particle surfaces [4, 5, 9]. In thesecond case, the flocculation is related to a nonuniform(mosaic) distribution of electric charge on the particlesurface due to the presence of similarly charged patcheson the initial surface and oppositely charged patchesformed by polyions adsorbed in a planar conformation[5, 10]. This charge patchiness leads to an additionalelectrostatic attraction of particles and a significantincrease in the flocculation rate [8].

There is further circumstance, which should betaken in consideration in the experiments on the coagu-lation kinetics. Although the coagulation itself is a non-equilibrium process, in all the above cases, the equilib-rium between the dispersed particles and the dispersionmedium is implied. However, the time of the establish-ment of such equilibrium can be comparable with thecoagulation period or even longer, and the establish-ment of the equilibrium itself can be rather complexprocess because of the presence of various diffusionflows. For example, moderate or even very low solubil-ity of dispersed particles in the dispersion medium(depending on physicochemical nature of contactingphases and particle size) results in the appearance ofdissolution products and gradients of their concentra-tions in a liquid phase. As a result, this gives rise to theadditional kinetic effects (duffusiophoresis, capillaryosmosis) and corresponding nonequilibrium surfaceforces which can affect the coagulation kinetics [2].

The majority of experimental works on aggregationstability and coagulation was performed for aqueousdispersions. Since water is not a typical solvent becauseof some specific features of its physicochemical prop-erties, similar studies of nonaqueous disperse systemsseem to be of great theoretical and practical interest.The reviews of such works [11, 12] show that, in spiteof versatile characteristics of different organic solvents(apolar, aprotic, and amphiprotic), the basic concepts ofthe DLVO theory are also applicable to the dispersionsof solid particles in such media. Thus, an ionic–electro-static component of the particle interaction energy isalso very important for these systems, although its rolediminishes significantly in apolar media because ofextremely low ionic strength. Therefore, the most uni-versal for ensuring the aggregation stability of bothaqueous and nonaqueous disperse systems is theadsorption or steric component of interparticle interac-tion energy, which is realized upon the adsorption ofsurfactants or polymers from solutions on the particlesurface.

The mechanisms of steric stabilization by polymersand the thermodynamics of the interaction of thus sta-bilized particles are discussed in detail in a monograph[13]. The necessary condition of steric stabilization isthe mutual repulsion of extended into solution macro-molecular chains of adsorption layers on the surface oftwo approaching particles. For the repulsion be real-ized, it is necessary that the interaction between poly-mer chains and the solvent was stronger than the inter-action between these chains, otherwise the flocculationtakes place. To determine the conditions of the transi-tion from the aggregation stability to flocculation, anotion of

θ

-point taken from the theory of polymersolutions and similar to the Boyle point in the theory ofreal gases is used. In this point, the intensities of mac-romolecules interaction with the solvent and betweenthemselves are equal, and its measure can be both thetemperature (critical flocculation temperature) and thecomposition of dispersion medium which is expressedby the concentration of an electrolyte or another lesspolar solvent added to solvent corresponding to theflocculation point (critical flocculation volume).

The aggregation stability of quartz suspensions inaqueous electrolyte solutions is well-studied [14]. It isestablished that an essential role in the stability of suchsystems is played not only by an ion–electrostatic, but astructural component of the particle interaction energyattributed to the presence of water boundary layers on thequartz surface. Similar conclusions were drawn from theresults of determining the stability of quartz suspensionsin butanol solutions of electrolytes [15].

The aim of this work is to study the aggregation sta-bility and the specific features of coagulation of fusedquartz suspensions in binary water–ethanol mixturescontaining 0–96 vol % alcohol.

MATERIALS AND METHODS

We used a fractionated powder of fused quartz withan average effective particle size of 0.5

±

0.1

µ

m. Theinitial powder was treated with 30% nitric acid fol-lowed by washing with distilled water to constant val-ues of pH and conductivity of the filtrate. Then the pow-der was treated using sedimentation fractionation andcentrifugation, dried at 105–110

°

C and stored in andesiccator over concentrated sulfuric acid.

Ethanol was purified by double fractionation atatmospheric pressure.

For measuring aggregation stability, we used dilutedquartz suspensions with a particle concentration ofabout

10

13

m

–3

prepared from a concentrated suspen-sion (10 mg quartz in 100 ml 96% ethanol) which wassubjected to ultrasonic treatment for 2 min and stored ina glass flask. The studied samples were prepared byadding an aliquot of concentrated suspension to water–ethanol mixtures (200 ml, 0–96 vol % ethanol) storedfor 24 h. The mixtures were preliminarily (more than24 h prior to experiment) prepared by the dilution of

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THE EFFECT OF COMPOSITION OF WATER–ETHANOL SOLUTIONS 441

ethanol with distilled water or by the addition of etha-nol to water. For measurements, both freshly preparedand aged (for 24 h and longer time) suspensions wereused. All samples were subjected to ultrasonic treat-ment immediately before measurements which wereperformed at

20°ë

.The aggregation stability and coagulation kinetics

of the above suspensions were studied with a VDK-4flow ultramicroscope which allows one to determinetime dependences of the particle concentration in sus-pension,

n

, and the aggregation degree

n

0

/

n

of the par-ticles by their visual counting.

RESULTS AND DISCUSSION

Figures 1–3 show the time dependences of aggrega-tion degree of freshly prepared quartz suspensions inwater–ethanol solutions with 0–96 vol % ethanol. It isobvious that freshly prepared suspensions are stablewith respect to aggregation in water and 96% ethanol(see Fig. 1); at the ethanol content from 10 to 93%, thecoagulation occurs: the process is slow at 93% ethanolconcentration and ultrafast, at 10–90% ethanol. More-over, at an ethanol content of 30–50 vol % (Fig. 2), theobserved coagulation rate exceeds significantly the val-ues corresponding to the Smoluchowski theory (seetable).

The aging of suspensions leads to a marked decreasein the coagulation rate. The data represented in Figs. 4and 5 and in the table show that, after 24 h of aging, thecoagulation of suspensions containing 80–90 and 10–20% ethanol changes its character from ultrafast toslow coagulation; after longer aging (exceeding 48 h),the range of alcohol concentrations, where the ultrafastcoagulation is observed, becomes even narrower (40–48 vol %). Figure 6 demonstrates the effect of the sys-

tem aging on the aggregation degree of particles corre-sponding to the observation time equal to 120 min.

Obviously, such a strong dependence of the coagu-lation kinetics and the observed degree of particleaggregation on the aging time for the systems studied isrelated to the long establishment of equilibriumbetween the surface of fused quartz and water–ethanolsolutions. As is known, the dissolution of silica (partic-ularly, amorphous silica) is a limiting stage of the pro-cess. As was shown by Iler [16], at 25

°

C, the increaseof methanol concentration in water from 0 to 90 wt %is accompanied by a decrease in the equilibrium solu-bility of amorphous silica from 140 to 5 ppm, withreaching the latter value for two months. Just as the sil-ica dissolution in water results in the formation of pol-ysilicic acid, the products of its dissolution in alcoholsare esters of polysilicic acid; their presence in the con-centration of the order of 1 ppm can initiate the bridg-ing flocculation.

However, freshly prepared quartz suspensions in96% ethanol obtained by adding an aliquot of the con-centrated quartz suspension in the same ethanol storedfor a period exceeding 1 month display the aggregationstability (Fig. 1). Hence, the products of silica dissolutionact as stabilizers, if their concentration exceeds 1 ppm.Moreover, the dissolution of silica is preceded by theesterification of the quartz particles surface, i.e., by for-mation of surface ethoxy groups in the presence of eth-anol which can also provide for the steric stabilizationof such suspensions [16, 17].

The coagulation and, particularly, ultrafast coagula-tion of freshly prepared suspensions obtained by add-ing the mentioned aliquots to solutions with the ethanolconcentration lower than 96 vol % can be due to boththe nonequilibrium state of the quartz surface in the ini-tial solutions; i. e., the presence of diffusion flows initi-ating additional kinetic effects (diffusiophoresis, capil-lary osmosis) and corresponding nonequilibrium sur-

0 40

1.0

1.2

80 120 160 200 240 280

t

, min

1.4

1.6

1.8

2.0

n

0

/

n

2

1

,

8

6

,

7543

Fig. 1

. Time dependences of the aggregation degree offreshly prepared suspensions of quartz in water–ethanolsolutions with various alcohol content: (

1

) 96, (

2

) 93, (

3

) 90,(

4

) 87, (

5

) 80, (

6

) 70, (

7

) 60, and (

8

) 0 vol %.

0 400

80 120 160 200 240 280

n

0

/

n

2

31

t

, min

25

20

15

10

5

Fig. 2.

Time dependences of the aggregation degree of freshlyprepared suspensions of quartz in water–ethanol solutions withvarious alcohol content: (

1

) 48, (

2

) 40, and (

3

) 30 vol %.

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.

face forces, and to the influence of the solventcomposition on the interaction of macromoleculesadsorbed on the particle surface between themselvesand the liquid (the concept of

θ

-point).

After the prolonged aging of the studied suspen-sions (for more than 24 h), the coagulation becomesslow at high and low alcohol concentrations. This isindicative of a significant importance of nonequilib-rium processes in the ultrafast coagulation. However,after so long-term aging, the coagulation process

retains its ultrafast character in the range of ethanolconcentrations of 40–48 vol %. For water–ethanol mix-tures of this composition, it was found (beginning withMendeleev’s work [18]) that some physicochemicalproperties (compressibility, heat capacity, viscosity,and enthalpy of electrolyte dissolution) display theextremal dependences [19]. This resulted from themaximal stabilization of the structure of a water–alco-hol mixture related to the fact that, at the water : ethanolmolar ratio of 3 : 1, all cavities of an icelike skeletonformed by water molecules are filled with alcohol mol-

0 40

1.0

1.4

80 120 160 200 240

t

, min

1.6

1.8

2.0

2.2

n

0

/

n

2

1

1.2

Fig. 3.

Time dependences of the aggregation degree of freshlyprepared suspensions of fused quartz in water–ethanol solu-tions with the alcohol content of (

1

) 20 and (

2

) 10 vol %.

0 40

1.0

1.2

80 120 160 200 240 280

t

, min

1.4

1.6

1.8

2.0

n

0

/

n

1

4

2

,

63

5

Fig. 4.

Time dependences of the aggregation degree of agedfor 24 h suspensions of fused quartz in water–ethanol solu-tions with various alcohol content: (

1

) 90, (

2

) 70, (

3

) 48, (

4

) 40,(

5

) 30, and (

6

) 20 vol %.

Degree of particle aggregation of

freshly

prepared and aged fused quartz suspensions in water–ethanol solutions with variousalcohol concentrations

Alcohol concen-tration, vol %

η ×

10

3

, Pa s

n

0

×

10

12

, m

–3

, min

n

0

/

n

**

A B C D

96 1.40 7.7 563 1.43 1.00 – –

93 1.50 7.6 610 1.39 1.24 – –

90 1.61 7.8 639 1.38 1.55 1.10 –

87 1.80 7.9 707 1.34 1.52 – –

80 2.01 7.6 812 1.30 1.80 – –

70 2.37 8.1 905 1.27 1.88 1.31 –

60 2.67 7.9 1046 1.23 1.90 – 1.02

48 2.87 6.8 1432 1.17 5.00 1.42 1.19

40 2.90 6.9 1300 1.18 >25.5 1.92 1.28

30 2.70 5.5 1567 1.15 18.0 1.59 1.03

20 2.18 9.7 695 1.35 2.20 1.32 –

10 1.54 9.5 502 1.48 1.71 – –

* Characteristic time of the fast coagulation according to Smoluchowski (Eq. (2)).** Degree of particle aggregation corresponding to an observation time of 240 min: (A) values calculated by the Smoluchowski equation (1);(B–D) experimental values for freshly prepared suspensions and suspensions aged for 24 and more than 48 h, respectively.

τ1/2*

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THE EFFECT OF COMPOSITION OF WATER–ETHANOL SOLUTIONS 443

ecules strongly bound to water. Obviously, the presenceof such structure should influence also the interactionbetween the mixed solvent and the quartz surface; con-sequently, this is exhibited in the kinetics of dissolutionand coagulation.

To reveal the mechanism of ultrafast coagulation offused quartz suspensions in water–ethanol solutions,the kinetics of quartz dissolution and the resultant prod-ucts should be investigated as a function of the alcoholcontent in the system.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project no. 03-03-32476, and bythe program “Leading Scientific Schools of RussianFederation,” project no. NSh-789.2003.3.

REFERENCES

1. Kruyt, H.R., Colloid Science, Amsterdam: Elsevier,1952.

2. Derjaguin, B.V., Churaev, N.V., and Muller, V.M., Sur-face Forces, New York: Consultants Bureau, 1987.

3. Sonntag, H. and Strenge, K., Coagulation Kinetics andStructure Formation, London: Plenum, 1987.

4. Fridrikhsberg, D.A., Kurs kolloidnoi khimii (Textbookof Colloid Chemistry), Leningrad: Khimiya, 1984.

5. Hunter, R.J., Foundations of Colloid Science, New York:Oxford Univ. Press, 2001, 2nd ed.

6. Huang, C. and Pan, J.R., Encyclopedia of Surface andColloid Science, Hubbard, A.T., Ed., New York: MarcelDekker, 2002, p. 1049.

7. Adachi, Y. and Wada, T., J. Colloid Interface Sci., 2000,vol. 229, p. 148.

8. Bouyer, W.L.Y.F. and Borkovec, M., J. Colloid InterfaceSci., 2001, vol. 241, p. 392.

9. La Mer, V.K., Discuss. Faraday Soc., 1966, vol. 42,p. 248.

10. Gregory, J., J. Colloid Interface Sci., 1973, vol. 42,p. 448.

11. Lyklema, J., Adv. Colloid Interface Sci., 1968, vol. 2,p. 65.

12. Parfitt, G.D. and Peacock, J., Surface and Colloid Sci-ence, Matijevic, E., Ed., London: Plenum, 1978, vol. 10,p. 163.

13. Napper, D.H., Polymeric Stabilization of Colloidal Dis-persions, London: Academic, 1983.

14. Golikova, E.V., Chernoberezhskii, Yu.M., Ioganson, O.N.,et al., Kolloidn. Zh., 2003, vol. 65, p. 460.

15. Varzhel’, V.I., Golikova, E.V., and Zhukov, A.N., Vestn.Leningr. Gos. Univ., Ser. 4: Fiz., Khim., 1990, no. 2,p. 92.

16. Iler, R.K., The Chemistry of Silica: Solubility, Polymer-ization, Colloid and Surface Properties, and Biochemis-try of Silica, New York: Wiley, 1979.

17. Kruglitskii, N.N. and Kruglitskaya, V.Ya., Dispersnyestruktury v organicheskikh i kremniiorganicheskikh sre-dakh (Dispersed Structures in Organic and Organosili-con Media), Kiev: Naukova Dumka, 1981.

18. Mendeleev, D.I., Sochineniya (Collected Works), Lenin-grad: ONTI Khimteor., 1937, vol. 4, p. 1.

19. Mishchenko, K.P. and Poltoratskii, G.M., Voprosy ter-modinamiki i stroeniya vodnykh i nevodnykh rastvorovelektrolitov (Problems of Thermodynamics and Struc-ture of Aqueous and Nonaqueous Electrolyte Solutions),Leningrad: Khimiya, 1968.

0 400.9

1.0

80 120 160 200 240 280t, min

1.1

1.2

1.3

n0/n

2

1, 4

3

13

24

Fig. 5. Time dependences of the aggregation degree of agedfor more than 48 h suspensions of fused quartz in water–ethanol solutions with various alcohol content: (1) 30, (2) 40,(3) 48, and (4) 60 vol %.

0 20

1.4

1.6

1.2

1.0

n0/n

25.4

25.6

25.8

40 60 80 100C, wt %

1

23

Fig. 6. Aggregation degree of fused quartz suspensions vs.alcohol concentration C in water–ethanol solutions withaging time: (1) 0, (2) 24, and (3) longer than 48 h. Theobservation time is 120 min.