Effect of surfactant concentration on the stability of aqueous titanium dioxide suspensions

Effect of Surfactant Concentration on the Stability of Aqueous Titanium Dioxide Suspensions TATSUO SATO AND SHIGERU KOHNOSU Monsanto Agricultural Research Station, Kawachi, Ibaraki, Japan

Received June 5, 1990; accepted November 1, 1990 The effect of a surfactant (polyoxyethylene diethylenetriamine dialkylamide) on the stability of aqueous titanium dioxide (TiO2) dispersion was studied by measuring adsorption and ~ potentials. It was found that as the surfactant concentration increases, stability decreases initially and then increases, showing a minimum. As the concentration increases further, stability decreases, showing a maximum. The f potential changes from negative to positive as the adsorption increases and reaches a maximum when the adsorption reaches a plateau value. The dispersion flocculated when the absolute values of the ~"potential dropped below 10 mV, indicating that the destabilization by the surfactant in the low surfactant concentration is due to reduction of the electrical repulsion. Destabilization at the higher surfactant concentration is likely caused by the depletion effect since there is no change in adsorption and ~"potential in the concentration range, and the surfactant molecules are not sufficiently large to form bridges between particles. © 1991AcademicPress,Inc. INTRODUCTION

Surfactants are widely used as dispersants or flocculants in many industries. Surfactants (or polymeric surfactants) affect the stability of a dispersion by changing the electrical repulsive energy (DLVO theory) or steric repulsive energy (steric stabilization theory) between particles by adsorption. The theories on the effect of polymer adsorption on the dispersion stability have been reviewed by several investigators ( 1-8 ). Recently, however, it was found that free nonadsorbing polymers (or polymeric surfactants) can destabilize (or stabilize) dispersions, so-called depletion flocculation (or depletion stabilization). Many theories have been proposed to account for the mechanism of the depletion effect in the past decade and these theories were reviewed by Napper (6) and Scheutijens and Fleer (8). One of the authors (9) found depletion flocculation in nonaqueous dispersions in 1971 and proposed two theories ( 10, 11 ) on the flocculation mechanism. In some cases, the stability of a dispersion changes with surfactant (or polymer) concen-

tration in a complex manner. Nabzar et al. (12) found recently, in studying the stability of polyacrylamide-clay suspensions, that the polymer works as a stabilizer and a flocculant depending on concentration. As the polymer concentration increases, the stability increases initially and then decreases, showing a maximum. As the concentration increases further the stability increases again. They attributed the stabilization to the steric stabilization and the destabilization to bridging flocculation. Similar complex behavior was observed in the present work on the effect of a surfactant on the aqueous titanium dioxide dispersion. The surfactant destabilizes the dispersion in low concentration and in high concentration and stabilizes the dispersion in a limited medium surfactant concentration range. The aim of this work was to investigate the mechanism of the effect of the surfactant on the dispersion in aqueous solutions in a wide range of surfactant concentration, since the destabilization by the surfactant was observed in both low concentration and high concentration and the destabilization mechanisms seem to be different. The aqueous suspension

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Journalof Colloidand lnte(aee Science,Vol. 143,No. 2, May 1991

S U R F A C T A N T EFFECT ON DISPERSION

of titanium dioxide was used as a model since this system is widely used in many industrial products such as paints, cosmetics, and paper. EXPERIMENTS

Materials Surfactant. The surfactant used in this work was polyoxyethylene (6 moles)-diethylenetriamine dialkylamide ( P O E - D E T A D A A ) of the following structure, supplied by Takemoto Oil and Fat Company, Aichi, Japan: RCNH--

CH2

--

CH2NH - - CH2 - -

I[ O CH2 - - N H C R (C2H40)6

I[

O R = C12 Average molecular weight = 744

Titanium dioxide. Titanium dioxide used was rutile-type Tipaque R-680 supplied by Ishihara Industry Company, Tokyo. The surface area determined by the BET method using nitrogen at - 195 °C was 8.0 mZ/g. The average particle size calculated from electron microscopic observation was a 0.21-gin diameter. The specific gravity was 4.2. Procedures Determination of adsorption of POE-DETADAA. The amount of adsorption of the surfaetant onto titanium dioxide was calculated from measurements of the difference in concentrations of the surfaetant solutions before and after preparation of the suspensions. Carefully stoppered glass bottles containing known amounts of titanium dioxide and surfactant solution were agitated gently at room temperature (25°C) for 24 h. The surfactant solution was then centrifuged and analyzed for surfactant concentration. The surfactant concentration was determined by measuring the intensity of the adsorption band at 210

435

nm for 0.1-0.4% solution and at 225 nm for 0.4-3.0% solution by a Shimadzu Spectrophotometer UV- 160. Determination of ~potential. The ~potential was determined with an Acoustophoretic Titrator System 7000 (Pen Kem Inc, New York). The samples were introduced into a Teflon measuring unit while being stirred and the ~" potential was measured at 20°C. This apparatus was used because the ~ potential of concentrated suspension (20%) could be measured without dilution. Determination of stability of dispersion. The stability of the dispersion was determined by a sedimentation method. Titanium dioxide pigments were dispersed by a paint shaker with glass beads in the surfactant solution in glass bottles for 20 min. The solution was then poured into a sedimentation test tube. Stability was determined by measuring the settling rate and the sedimentation volume. As the stability increases, the settling rate decreases and the sedimentation volume formed in long-term storage (about 1 month) decreases. The stability was graded 0 (very unstable) to 5 (very stable) by an overall evaluation. RESULTS A N D DISCUSSION

Effect of Adsorption and ~ Potential on Dispersion Stability in Low-Su(actantConcentration Solutions The change in the stability of aqueous suspension of titanium dioxide (20%) with surfactant concentration is shown in Fig. 1. The stability is initially very good in low surfactant concentration up to 0.05%. As the surfactant concentration increases from 0.05%, the stability decreases initially and reaches a minim u m (very poor) at about 0.2%. As the concentration increases further, the stability increases rapidly and then decreases, showing a maximum at about 1.0%. The change in ~"potential with surfactant concentration is also shown in Fig. 1 to show the correlation between stability and ~ potenJournal of Colloid and Interface Science, VoI. 143, No. 2, May 1991

436

SATO AND KOHNOSU 20

2,

at the concentration where the adsorption reaches a plateau.

10

Computation of Total Potential Energy 0

3

-10

-3

-2

-1

0

The electrical repulsive potential energy due to the electrical charge of the particles in aqueous solution can be approximated by E ~ -- --5-ln{1 ~r~'2 + e x p ( - KH)}

-20

[1]

Log C (log % wtlwt)

FIG. h Change in dispersion stability of TiO2 and ~"potential with surfactant concentration at TiO2 20% (w/w). tial. The ~ potential of titanium dioxide in the aqueous solution is initially negative. As the surfactant concentration increases, the potential changes from negative to positive, passing the zero point of charge (zpc). The suspension flocculates when the absolute value o f the ~ potential becomes smaller than i0 m V (f~-f < 10 m V ) and the stability reaches a minim u m at the zpc. These results indicate that the destabilization by the adsorption of the surfactant in this concentration range is caused by the reduction o f electrical repulsion between particles. The p H of the suspension varies from 7.3 to 8.0 in the surfactant concentration range used in the present work. No correlation with surfactant concentration and ~- potential was observed. The adsorption isotherm is shown in Fig. 2. The amounts adsorbed are plotted against the equilibrium surfactant concentration on the normal axis of abscissa and also against logarithm of the initial surfactant concentration (before the adsorption takes place) to show the correlation between adsorption, stability, and ~"potential. Figure 2 shows that as the concentration increases, the adsorption increases and reaches a plateau at about 1.0%, indicating that the adsorption follows the Langmuir isotherm. It can be seen by comparing Figs. 1 and 2 that the dispersion stability reaches a m a x i m u m Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

where E is the dielectric constant of the medium, r is the radius of particles, ~ is the Debye-Hueckel reciprocal length parameter, and H is the shortest distance between two particles. In computation, K-1 = 10 -6 cm is used for 1% ( 10 -2 mole/liter) aqueous surfactant solution (13). The dielectric constant, e, is expressed as the product of DD0, where D is the usual dimensionless dielectric constant and is 80 for water at 25°C and Do is the permeability of free space and has the value of 1.11 X 10 -12 C V -1 cm -1 (14). The potential energy curves at I0 and 15 m V are shown in Fig. 3, which are calculated from Eq. [ 1] under the conditions e = 80 X 1.11 X 10 -12 C V - I c m - l , r = 10-5 cm, K-1 = 10-6 cm. The van der Waals attractive potential energy between particles

Equllibrium concentzation, % 0.1

0.2

0.3

0.4

1.0

30

25

20

15

10

5

%

~2

-1

i 0

Log C (log %)

FIG. 2. Adsorption isotherm of POE-DETADAA on TiO2 at 20% (w/w).

SURFACTANT EFFECT ON DISPERSION

intrinsic viscosity. Flow's theory predicts that the intrinsic viscosity is related to the mean square radius of gyration, ( a 2) 1/2 by

140

120

[r/] = q563/2

~00

80

60

.

40 20 4

y

437

50

100 H (2a),

-20 40

-60 -80

MOO

FIG. 3. Potential energies between TiO2 particles. (a) EAwith no adsorbed layer. (b) EA with adsorbed layer of thickness 50 A. (1) E~ at 15 inV. (2) E~ at 10 mY. (3) Total potential energy at 15 mV (1 + b). (4) Total potential energy at 10 mV (2 + b).

at small distance ( H ~ mated by ( 1 )

(a)3/2/M2

[31

where ch should be a universal constant independent of the nature of macromolecule and the solvent m e d i u m ( 18, 19 ). Using the result of the calculation of Aver and Gardner (20) or Z i m m (21 ), ch should have the value of 2.1 × 10 -21 ' The intrinsic viscosity, [ rl], obtained for the surfactant is 0.55 dl g 1. The mean square radius of gyration calculated from the equation is 25 A. The value 25 A seems too large for the radius of a single molecule of the surfactant. It is likely that the surfactant molecules form micelles in the solution. The thickness of the adsorbed layer (diameter of the molecule) is approximately 50 A. The thickness of the adsorbed layer calculated directly from the amount of adsorption, assuming that the surfactants are adsorbed in a closely packed layer, is 40 A. Since the surfactant molecules are adsorbed somewhat loosely, the 50-A thickness seems to be appropriate.

0) can be approxiA?"

EA --

12H

[2]

where A is the H a m a k e r constant. According to the calculation by Fowkes ( 15, 16), A for rutile in water is 8.1 × 10-3 erg. An adsorbed layer reduces the attraction potential energy. Vold (17) developed an expression for Ea taking into account the presence of an adsorbed layer on the particles. The thickness of the adsorbed layer can be estimated to be about equal to the diameter of gyration of the surfactant molecule in the aqueous solution, assuming that the molecules adsorb on the particle in the same form as they exist in solution. The mean size of an isolated polymer molecule in solution m a y be estimated from the

FIG. 4. Model for calculation of osmotic attraction between two particles containing adsorbed layer in a solution of spherical surfactant molecules. Journal of Colloid and Interface Science, Vol. 143, N o 2, May [ 991

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SATO AND KOHNOSU

The potential energy curves for electrical repulsion and van der Waal's attraction and the total potential energy curves obtained by combining rep;Ulsion and attraction are shown in Fig. 3. Curves 1 and 2 are the electrical repulsive energy with ~-potentials of 15 and 10 mV, respectively. Curve a is the attractive energy with no adsorbed layer and curve b is the attractive energy with an adsorbed layer 50 thick obtained from Vold's equation (17). Curves 3 and 4 are the total potential energies obtained by combining curves 1 and b and curves 2 and b, respectively. The total potential energy curves show that when the ~"potential is 15 mV, the electrical repulsive energy is sufficiently high to overcome the attractive energy and the maximum energy is as high as 67 kT, while when the ~potential is 10 mV, the maximum energy is as low as 8 kT. According to DLVO theory, an energy barrier of 15 kT is sufficient to produce a highly stabilized dispersion system. The experimental results show that the titanium dioxide dispersion is stable when the f potential is 15 mV but not stable when the ~"potential is 10 mV (as shown in the preceding section). The results are well accounted for by DLVO theory.

Destabilization in High-SurfactantConcentration Solutions It was found that stability decreases when the surfactant concentration becomes higher than the concentration where the adsorption reaches a maximum, as shown in Fig. I. As the surfactant concentration increases beyond this concentration, free (nonadsorbing) surfactant molecules in solution increase. Flocculation by polymers is usually explained by charge neutralization or by a bridging mechanism (23, 24). However, the destabilization of titanium dioxide dispersion in the high surfactant concentration cannot be explained by these mechanisms because there is no change in the ~"potential or the adsorption in this surfactant concentration range, as shown in Figs. 1 and 2. In addition, it is very unlikely that bridging of the surfactant takes Journal of Colloid and Interface Science.

Vol. 143,No. 2, May 1991

place in this dispersion system because bridging flocculation usually occurs when the polymer concentration is low and the molecular weight of the polymer is very large (over 10,000) ( 1, 23, 24). It was found fairly recently that free (nonadsorbing) polymer can affect colloid stability. Flocculation by free polymers is termed depletion flocculation. The mechanism of depletion flocculation has been extensively studied by several investigators in the past decade and their theories were reviewed by Napper (6). It is likely that the destabilization of TiO2 dispersion observed in the high surfactant concentration is caused by the depletion effect of the free surfactant molecules in the solution. CONCLUSIONS

1. Destabilization of TiO2 dispersion with POE-DETADAA in low-concentration solutions (0.05-0.2%) is due to the reduction of electrical repulsion between the particles caused by the reduction of negative charge on the particles. The dispersion is not stable when the maximum of the total potential energy is lower than 15 kT. 2. Stabilization with POE-DETADAA in medium-concentration solutions (0.2-1.0%) is due to the increase in electrical repulsion caused by the increase in positive charge on the particles. The dispersion is stable when the maximum of the total potential energy is higher than 60 kT. 3. Destabilization with POE-DETADAA in high-concentration solutions (1.0-3.0%) seems to be due to the depletion effect caused by free surfactant molecules in the solutions. ACKNOWLEDGMENT The authors thank Dr. J. L. Killmer, director of Monsanto Agricultural Research Station, Kawachi, Ibaraki, Japan, for his helpful guidance in this project. REFERENCES 1. Sato, T., and Ruch, R., "Stabilization of Colloidal Dispersions by Polymer Adsorption," Marcel Dekker, New York, 1980.

SURFACTANT EFFECT ON DISPERSION 2. Lyklema, J., Adv. Colloid Interface Sci. 2, 65 ( 1968 ). 3. Vincent, B.,Adv. Colloid Interface Sci. 4, 193 (1974). 4. Tadros, Th. F., Adv. Colloid Interface Sci. 12, 141 (1980). 5. Parfitt, G. D., in "Dispersions of Powders in Liquids" (G. D. Parfitt, Ed.). Applied Science, London, 1981. 6. Napper, D. H., "Polymeric Stabilization of Colloidal Dispersions." Academic Press, London/New York, 1983. 7. Fleer, G. J., Seheutjens, J. M. H. M., and Stuart, M. A. C., in "Polymers in Colloid Systems" (Th. F. Tadros, Ed.). Elsevier, Amsterdam, 1988. 8. Scheutijens, J. M. H. M., and Fleer, G. J., Adv. Colloid lnte(ace Sci. 16, 361 (1982). 9. Sato, T., J. Appl. Polym. Sci. 15, 1053 (1971). 10. Sato, T., a~ AppL Polym~ Sci. 23, 1693 (1979). 11. Sato, T., and Sieglaff, C. L., J. Appl. Polym. Sci. 25, 1781 (1980). 12. Nabzar, L., Pefferkorn, E., and Varoqiu, R., Colloid Surf. 30, 345 (1988).

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13. Hachisu, S., Shikizai (Japan) 38, 523 (1965). 14. Wiese, G. R., and Healy, T. W., Trans. FaradaySoc. 66, 490 (1970) 15. Fowkes, F. M,, in "Chemistry and Physics of Interface" (S. Ross, Ed.), p. 7. Washington, DC, 1965. 16. Moriyama, N., ColloidPolym. Sci. 254, 726 (1976). 17. Vold, M. J., J. Colloid Interface Sci. 16, 1 (1961). 18. Billmeyer, F. W., Jr., in "Textbook of Polymer Science," Chap. 3, p. 84. Interscience, New York, 1962. 19. Flory, P. J., in "Principles of Polymer Chemistry," Vol. 2 (S. Oka and K. Kanamaru), Transl., Chap. 14, p. 557. Maruzen, Tokyo, 1970. [in Japanese] 20. Aver, P. L., and Gardner, C. S., .Z Chem. Phys. 23, 1546 (1955). 21. Zimm, B. H., J. Chem. Phys. 24, 269 (1956). 22. Asakura, S., and Oosawa, F., J. Polym. Sci. 33, 183 (1958). 23. Ishikawa, M., J. Colloidlnterface Sci. 56, 596 (1976). 24. Gregory, J., Z Colloid Inte(ace Sci. 42, 448 ( 1973 ).

Journal of Colloid and Interfoce Science, VoL 143, No. 2, May 1991