Effect of polyvinylpyrrolidone on the physical properties of titanium dioxide suspensions

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

267

E£Eect of polyvinylpyrrolidone on the physical properties of titanium dioxide suspensions Tatsuo Sato*, Shigeru Kohnosu Monsanto Agricultural Research Station for Asia Pacific 1-5-1 Arakawaoki-ffigashi Tsuchiura, Ibaraki 300-1171, JAPAN The effect of polyvinylpyrrolidone (PVP) on the physical properties of aqueous and non-aqueous (methanol-based) TiOg suspension was studied by measuring PVP adsorption and the rheological properties and dispersion stabiUty of the suspensions. The amount of PVP adsorbed onto TiOg from the methanol solution is much larger than that firom the aqueous solution and the effect on the dispersion StabiUty of the suspension is much greater in the methanol-based suspension than in the aqueous suspension. The amount of adsorption of PVP is independent of the molecular weight, while the thickness of the adsorbed layer increases as the molecular weight increases. As the PVP concentration increases, the viscosity of the PVP solution increases, while the relative viscosity and the dynamic rheological parameters(G', G") of the suspension decrease, indicating that the network structure formed by dispersed TiOg particles in the absence of PVP diminishes with the increase in the PVP concentration. The dispersion stabiUty of the methanol-based suspension increases with the increase in the concentration of PVP of high molecular weight, while the dispersion stabiUty of the aqueous suspension is not significantly improved by PVP. It appears firom the conformation of the adsorbed PVP molecules, determined by the measurement of the amount adsorbed and the thickness of the adsorbed layer, that the stabiUzation of the methanol-based suspension by PVP is due to entropic repulsion caused by the interaction of protruded tails of the adsorbed PVP molecules. It also appears firom the measurement of the adsorption isotherm and the rheological properties that the increase in the dispersion stabiUty with PVP at high PVP concentration is due to the increase in the concentration of non-adsorbingfireePVP molecules.

Corresponding author.

268

1. EXPERIMENTAL 1.1.

Materials

1.1.1. PVP Table 1 Agrimer® supplied by ISP AjL5 MW

8 XIO^

A30

A60

A 90

58 XIO^

400 XIO^

1300 XIO^

1.1.2. TLO2 Rutile type TiOg supplied by Ishihara Industry Co., Tokyo. Surface area : 8 m / g Average diameter : 0.21 // m Specific gravity : 4.2 1.2.

Procedures

1.2.1. Determination of adsorption of PVP onto TiOs. The amount of adsorption was calculated from measurements of the differences in concentrations of the polymer solutions before and after preparation of the suspension. The PVP concentration was determined by measuring the intensity of the adsorption band at 212 nm by UV spectrophotometer. 1.2.2. Determination of the adsorbed layer thickness. The hydrod5niamic thickness of the adsorbed layer was determined by measuring the viscosity of the suspensions by an Ubbelobde viscometer. The thickness was calculated from the Guth-Gould equation. 1.2.3. Kheological measurement. Rheological parameters were measured by using Rheometrics Spectrophotometer RFS 11 equipped with Couette geometry.

Fluid

269

2. RESULTS AND DISCUSSIONS 2.1. Adflorptioii of PVP onto IIO2 and the thicknesa ci adsorbed layers. The following results were obtained. i ) The amount of adsorption is independent of the molecular weight of PVP as shown in Fig. 1. ii) The adsorption of PVP from the methanol solution is much greeter than that from the aqueous solution as shown in Fig. 1.

E C 0 •H 4-1

Ck M 0 CO V

< >

iao

0

Afi

1

AQ

I

2

J-

_L

3

4

E q u i l i b r i u m PVP C o n c e n t r a t i o n ,

5 %wt

Fig. 1. Adsorption isotherms of PVP of various molecular weights from aqueous and methanol solutions at 25 °C: • , A 15-methanol; A, A 30-methanol; • , A 60-methanol; • , A 90-methanol; O, A 15-water; A, A 30-water.

2.2. Rheological properties As the PVP concentration increases, the viscosity of the methanol and the aqueous solutions increases as shown in Fig. 2, while the viscosity (or relative viscosity) of TiOg suspensions decreases markedly as shown in Fig. 3.

270

0

5

10

15

20

PVP Concentration, %wt Fig.2. Change in Newtonian viscosity of Aqueous and methanol solution with PVP concentration at25*C: 9 ^ 15-methanol; A^SO-methanol; 0 ^ 1 5 - w a t e r ; A^30-water.

2

4

6

8

10

PVP Concentration, %wt Fig.3. Change in relative viscosity of TiOj suspensions with PVP concentration at 25*^ # ^ 15-methanol; • ^ 30-methanol; 0 , A 15-water, A.A 30-water.

2.3. Dispersion stability The stability of the methanol-based suspensions increases markedly as the PVP concentration increases, while the stabiUty of the aqueous suspensions in not significantly influenced by PVP. It appears that the stabilization of the methanol-based suspensions by PVP is due to entropic repulsion caused by the interaction of protruded tails of the adsorbed PVP molecules that the increases in the dispersion stabiUty at high PVP concentration is due to the increase in the concentration of non-adsorbing fi'ee PVP molecules.

REFERENCES 1. T. Sato, S. Kohnosu, CoUoids Surf A, 88 (1994) 197. 2. T. Sato, A. Sato, T. Arai, Colloids Surf. A, 142 (1998) 117. 3. T. Sato and R. Ruch, StabiUzation of Colloidal Dispersions by Polymer Adsorption, Surfactant Science Series Vol.9, Marcel Dekker, N.Y., 1980.