Langmuir 1995,11,563-567
563
Polymer Adsorption Effects on Structures and Rheological Properties of Silica Suspensions Masami Kawaguchi,"??Yoshinobu Kimura,? Tohru Tanahashi,? Jun Takeoka,? Tadaya Kato,' Jun-ichi Suzuki,S and Satoru Funahashi$ Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514,Japan, and Neutron Scattering Laboratory, Department of Materials Science and Engineering, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-11,Japan Received April 20, 1994. I n Final Form: October 27, 1994@ By a combination of small angle neutron scattering (SANS) and rheological measurements, characterization of silica suspensions in aqueous (hydroxypropy1)methylcellulose (HPMC) solutions and their shear stresses and dynamic moduli measurements were performed as a function of concentrations of silica, HPMC, and isopropyl alcohol, which plays a role to displace molecules of HPMC. Aerosil and Snowtex C silicas were used: the former is easily to aggregate in water and the latter is stable in water, and the respective structures were confirmed by a SANS technique. From the SANS measurements, adsorption of HPMC induced a small portion of the Snowtex C silica particles to form a cluster, whereas it did not influence the self-similar structures of Aerosil silicas suspensions. The amounts of HPMC adsorbed on the silica surfaces decreased with an increase in the displacer concentration. In the flow curves, plots of steady-state shear stress against the shear rate of the Aerosil silica suspensions, there was a bump, which is more like a plateau around 0.1 s-l, in shear rate and this bump did not disappear with addition of isopropyl alcohol. However, the shear stress of the Snowtex C increased monotonically with an increase in the shear rate and its magnitude decreased with increasing displacer concentration. Upon addition of isopropyl alcohol the dynamic moduli of the respective silica suspensions decreased with the amounts of HPMC desorbed from the silica surfaces. For the Aerosil silica suspensions the storage modulus was more sensitive to the adsorbed amounts of HPMC than the loss modulus.
Introduction For inorganic particles dispersed in polymer solutions, adsorption of polymer on the particles should play a role in the stability and dynamic properties of the dispersions and influence changes in their structures. Although many studies on various dispersions were reported concerning their stability, dynamic properties, and structural changes, the effect of polymer adsorption on characteristic behavior of the dispersions was not well under~tood.l-~ Recently, several reports have been published that describe the rheological behavior of silica suspensions by taking account of polymer a d ~ o r p t i o n . ~An - ~ extensive study on the effects of particle sizes on the rheological properties of flocculated silica suspensions by large molecular weight was investigated by O t s ~ b o , who ~,~ reported t h a t the silica suspensions showed elastic behavior and that their elasticity was increased in relation to the adsorbed amount of polymer. Tadros and coworkers6 found that the rheology of silica suspensions in nonaqueous solution in the presence of the low molecular weight block copolymer poly(2-vinylpyridine)-co-poly(tertbutylstyrene) as a function of the adsorbed amount of the block copolymer. The most effective stabilization of the silica suspensions was observed in the full surface coverage of the silica particles by adsorption of the smallest molecular weight of the block copolymer. + Mie
University.
* Japan Atomic Energy Research Institute.
@Abstractpublished in Advance A C S Abstracts, January 15, 1995. (1)Krieger, I. M.Adu. Colloid Interface Sci. 1972,3,111. (2)Tadros, Th. F.Adv. Colloid Interface Sci. 1980,12, 141. (3)Tadros, Th. F.In SolidlLiquid Dispersions;Tadros, Th. F., Ed.; Academic Press, London, 1987,pp 1-16; pp 225-274. (4)Otsubo, Y.Langmuir 1990,6, 114. (5)Otsubo, Y.J . Colloid Interface Sci. 1992,153,584. (6)De Silva, D. P. H. L.; Luckham, P. F.; Tadros, Th. F. Colloids Surfaces 1990,50,263. (7) Kawaguchi, M.; Ryo, T.; Hada, T. Langmuir 1990,7,1340.
We have recently described the rheological properties of Aerosil silica suspensions in aqueous solutions of (hydroxypropy1)methyl cellulose (HPMC)that adsorbs on the silica particles as functions of the concentrations of silica and HPMC and its molecular ~ e i g h t . ~ - l O We found a bump in the plots ofthe steady-state shear stress against shear rate at higher silica content.6 Furthermore, the viscoelastic behavior of the silica suspensions changed from liquidlike to solidlike upon increasing the silica content in combination of HPMC adsorption and the gelling character of the silica particles in aqueous dispersion medium. In this paper, in order to have additional and complementary information on the silica suspensions dispersed in HPMC solutions, small angle neutron scattering (SANS) and rheological measurements have been performed for two type silica suspensions by changes in concentration ofisopropyl alcohol, which behaves as a displacer ofHPMC. Furthermore, adsorption isotherms of HPMC on the silica surfaces from aqueous HPMC solution involving the displacer were also determined.
Experimental Section Materials. One HPMC sample of 65SH-400 was kindly supplied by Shin-EtsuChemical Co., Ltd. It was purified by the same method as that described previo~sly.~-l~ The molecular weight of the sample was determined by intrinsic viscosity measurements in aqueous 0.1 N NaCl solution at 25.0 f 0.05 "C using an Ubbelohde viscometer.'l The molecular weight (Mw) of 65SH-400 was 317 x lo3. The degree of substitution (DS)of OCH3group and the molar substitution (MS)ofOC&OH group were 1.8and 0.15, respectively, according to the manufacturer. (8)Ryo, Y.;Nakai, Y.; Kawaguchi, M. Langmuir 1992,8, 2413. (9) Kawaguchi, M.;Ryo, T. Chem. Eng. Sci. 1993,49,393. (10)Nakai, Y.;Ryo, T. Kawaguchi, M. J . Chem.Soc.,Faraday Trans. 1993,89,2467. (11)Kato, T.; Tokuya, T.; Takahashi, A. KobunshiRonbunshu 1982, 39,293.
0743-7463/95/2411-0563$09.00/0 0 1995 American Chemical Society
564 Langmuir, Vol. 11, No. 2, 1995
Kawaguchi et al.
Water purified by a Millipore Q-TM was used. Pure grade quality NaN3 was used as a preservative for HPMC. Spectroscopic grade quality isopropyl alcoholwas used without further purification. NonporousAerosill30 silica (JapanAerosil,Yokkaichi, Japan) with a surface area of 141mVg, a particle diameter of 16nm, and a silanol density of 2.5/nm2was used; it was received as a powder and dried in a vacuum oven at 200 "C before use.12 The preparation method of the Aerosil silica suspensions in aqueous HPMC solutions was the same as that described previously:7-10 Aerosil 130 silica was dispersed in water (pH = 5.5) to form a silica gel-like slurry by aggregation of the silica particles due to the catalytic action of the hydroxyl ions;13the resulting slurry was mixed with an aqueous solution of HPMC to prepare a silica suspension. The stability of the Aerosil silica suspensions was roughly estimated from the visual observation of how long the silica suspensions are suspended without sedimentation of silica after homogeneous mixing. We used such an Aerosil silica suspension in which no sedimentation of silica was observed over 1month for the rheological measurements. The amounts of added silica and HPMC were expressed as weight percentages in the final mixture. Colloidal silicaparticles, SnowtexC,which was kindly supplied from Nissan Chemical Industries, Ltd. (Tokyo,Japan), are stable sols due to the electrostatic repulsion between the negative charges on the silica surface in the range pH = 8-10.13 The Snowtex C particles were used without further purification. The diameter of the silica particles measured by the manufacturer was 15 & 3 nm. The solid content in the bulk colloidal silica suspension was determined to be 21.7 wt % by evaporation ofthe dispersed medium (water)in a weighed colloidal silica suspension and by drying of the residue under vacuum. For preparation of the colloidal silica suspensions in aqueous HPMC solutions, a weighed amount of Snowtex C was mixed with a preprepared aqueous HPMC solution with a known concentration in a glass bottle. The resulting suspensions were subjected t o ultrasonic irradiation and mechanical shaking to obtain a homogeneous mixture. The pH of the colloidal silica suspension, which was not adjusted, was around 8.2. Adsorption of HPMC. The amounts of HPMC adsorbed on the silicas were determined as follows. A constant amount of the silicas was mixed with 20 mL of aqueous HPMC solution (involving a displacer, for which concentration is expressed by volume percentage), the mixture was agitated with stirring by a magnetic ship to allow equilibration at 27 f 0.1 "C, and the supernatant was removed to separate the silica using a Kubota 6700 centrifuge. The HPMC concentration (Cp) in the supernatant was determined by a gravimetric method: silica particles were sedimented at 3000 rpm for 10 min and at 15000 rpm for half a day for the Aerosill30 and the Snowtex C silica particles, respectively, followingthe evaporation of the solvent for a define amount of the supernatant weighed, and the residue was dried under vacuum and weighed. SANS Measurements. SANS experiments were performed using the Japan Atomic Energy Research Institute (JAERI) 20m SANS instrument. The wavelength (1)was selected to be 0.625 nm using a velocity selector with variable speeds and pitches, and the wavelength resolution was Ad4 = 10%. The monochromatic beam was collimated by a series of circular apertures having a 10 mm diameter. The 5 mm circular aperture was suited before the sample cell to define the sample illuminated. The samples were transferred to quartz cells of path length 2 mm. The scattered neutrons were detected by a 3Hearea detector of 58 cm diameter circle containing 128 pixel elements. The sample to detector distances of 3 and 10 m correspond to the wave vector ( q ) range from 0.03 to 0.8 nm-'. Data taken at a 3 m sample to detector distance cover the q range from 0.09 to 0.8 nm-l and data at 10 m range from 0.03 to 0.25 nm-l. Data were corrected for empty cell background. The resulting scattering intensity was obtained by normalization to the isotopic scattering intensity of protonated water. The experiments were performed at ambient temperature (ca. 25 "C). ~
(12)Kawaguchi, M.; Hayakawa, K.; Takahashi, A. Polymer J . 1980,
00
O
0.10
. ' 0.05 h
0.5 1.o Cp / g/lOOmL
1.5
Figure 1. Adsorption isotherms of HPMC on Aerosil silica for various silica concentrations: ( 0 )0.08 g, (m) 0.16 g, (A)0.24 g, (0)1.0g, (0) 1.5 g.
1
0.15 - 1
-s .o . l o ~
'
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o
A
+
a
, A0 +
~
* +
0 s
*
0
O.O8.0
0.1
0.2
0.3
0.4
Cp / gilOOmL
Figure 2. Adsorption isotherms of HPMC on Aerosil silica from aqueous HPMC solutions with various isopropyl alcohol concentrations (Cal): (0)at Cal = 0%, ( 0 )at Cal = 3%, ( A ) at Cal = 5%,(+) at Cal = 7%. RheologicalMeasurements. Steady-state shear stress and dynamic measurements were performed using an MR-300 Soliquid Meter produced by Rheology Co. Ltd. (Kyoto, Japan). The steady-state shear stress measurements were carried out in the shear rate range from 0.02 to 148 s-l and the dynamic measurements were performed in the frequency range from 0.05 to 12.4 s-l using a cone and plate geometry (plate diameter, 30 mm; cone angle, 5") at 27 f 0.5 "C. In general, it is well-known that suspensions and dispersions show nonlinear viscoelastic responses to large strains. The dynamic measurements were carried out at a strain of 0.03 in the linear range. These measurement systems were modified with an aluminum cover to limit solvent evaporation. For every shear stress measurement, the silica suspensions were presheared at the highest shear rate of 148 s-l for 5 min to maintain the same initial condition.
Results and Discussion Aerosil Silica. Adsorption ofHPMC. HPMC did not adsorb on hydrophobic silica surfaces,14 such as R972 (Nippon Aerosil), with a modification of silanol groups by dimethyldichlorosilane. Thus, HPMC should be adsorbed on silanol groups through hydrogen bonding. Isopropyl alcohol can hydrogen bond to silanol groups and its addition should lead to a smaller adsorbed amount of HPMC.I5 It can be expected that isopropyl alcohol behaves as a displacer without any change in the pH of the dispersion medium. We measured the amounts of HPMC adsorbed on the silica surfaces for various concentrations of isopropyl alcohol. However, above 30% isopropyl alcohol, HPMC did not dissolve since the solvent is not a solventfor HPMC. Figure 1 shows the adsorption isotherms of HPMC at various concentrations of Aerosil silica. The adsorbed amount at the plateau region is almost independent of the silica content. The adsorbed amount of HPMC for the respective silica suspensions subjected to the rheological experiments corresponds to that in the plateau region of the adsorption isotherm. Figure 2 shows the adsorption isotherms of HPMC at concentrations of isopropyl alcohol of 0, 3, 5, and 7%, in
12, 265.
(13)Iler, R. K. In The Chemistry ofSilica;John-Wiley & Sons: New
York,1978, Chapter 4 and 5.
(14)Tanahashi, T.; Kawaguchi, M. Unpublished data. (15)Kawaguchi, M. Adu. Colloid Interface Sci. 1990, 32, 1.
Properties of Silica Suspensions
Langmuir, Vol. 11, No. 2, 1995 565 I
10'1
10'
10'
-
IO0
0 v
IO' 10' 0
IOlb. q / l/nm
Figure 3. Double-logarithmicplots ofthe scatteringintensity, Z(q), against wave vector, q , for the 7.5%Aerosil silica slurry
(0) and the 7.5% Aerosil silica suspension (0).
which the silica concentration was 0.24 g. The adsorbed amounts of HPMC decreased with a n increase in isopropyl alcohol. From the figure, however the amount of HPMC desorbed from the silica surfaces does not much increase a t 5 and 7% isopropyl alcohol concentrations. The shape of the adsorption isotherms is rounded because of the polydispersity (Mw/Mn 3) of the HPMC. SANS Measurements. SANS is useful to investigate the structures of the condensed matter. The shape of the plot of scattering intensity, I(q), vs wave vector, q , gives a clue to information on the structures ofthe objects. Figure 3 illustrates typical scattering intensity curves for the Aerosil silica slurry of 7.5%silica content and its suspension. The scattering intensity of the slurry a t low q range decreases with an exponent of -2.1, namely corresponding to the fractal dimension, and a t high q region, I ( q )tends toward Porod's law. This indicates that Aerosil silicas dispersed in water make porous aggregates with an open structure, Le., a rather fractal object and its structure are self-similar. Similar results have been obtained for Cab0-Si1 silicas.16 As seen from the figure, the scattering intensity for the silica suspension is superimposed on that for the silica slurry. Thus, adsorption of HPMC on the Aerosil silica surfaces brings no changes in the aggregated structures of the Aerosil silicas dispersed in water. Shear Stress Measurements. Before discussing effects of the displacer molecule, isopropyl alcohol, on the rheological properties of the silica suspensions, we should address the effect of isopropyl alcohol on the HPMC solution viscosity. The viscosity of HPMC solution with isopropyl alcohol content lower than 7% showed shear thinning, and the zero-shear rate viscosity slightly increased from 0.135 to 0.156 P a s in accordance with 0-7% isopropyl alcohol concentrations. A plot of the steady-state shear stress against the shear rate of a n aqueous HPMC solution of 1.5%concentration is displayed in Figure 4. The adsorbed amounts of HPMC for the 7.5% Aerosil silica suspensions in 1.5%HPMC solutions with various concentrations of isopropyl alcohol were well within the plateau region in the corresponding adsorption isotherms. Figure 4 shows plots of the steady-state shear stress against the shear rate of the Aerosil silica suspensions in aqueous HPMC solutions with various isopropyl alcohol concentrations. The steady-state shear stress of the dispersion medium is much less than those for the silica suspensions in the entire shear rate range and it increases monotonically with the shear rate. On the other hand, for the silica suspension a bump more like a plateau is observed around ashear rate of 0.1 s-l, and beyond the shear rate the shear stress gradually
10,
100
IO
102
.1;
Shear rate / l/s
Figure 4. Plots of the steady-state shear stress against the shear rate of the Aerosil silica suspensions in aqueous HPMC solutions with various isopropyl alcohol concentrations (Cal): (0) at Cal = 0%, (0)at Cal = 3%, (A) at Cal = 7%. The filled circles indicate the steady-state shear stress of an aqueous HPMC solution with 1.5%concentration.
-
(16) Sinha, S.K.;Freltoft, T.; Kjems, J. In Kinetics ofAggregation and Gelation; Family, F., Landau, D. P., Eds.; Elsevier: Amsterdam, 1984; p 87.
Frequency / lis
Figure 5. Storage ( G ) and loss ( G )moduli as a function of frequency of the Aerosil silica suspensionswith various isopropyl concentrations (Call: G(0)and G ( 0 )at Cal = 0%, G ( 0 )and G ( 0 )at Cal = 3%, G(A) and G ( A ) at Cal = 7%. increases with an increase in the shear rate. As seen from the figure, the plateau (bump) in the flow curves is observed around a t the same shear rate when isopropyl alcohol is added. This means t h a t the appearance of the plateau should strongly depend on the kinetics of formation and breaking of the silica aggregation under shear flow. At a shear rate ~ 0 . s-l 1 the homogeneous silica suspensions produced by preshearing a t a shear rate of 124 s-l preferentially aggregated to form clusters due to the gelation character of the silica particles, whereas, for shear rates '0.1 s-l, breakage of the clusters generated occurred, the cluster size became smaller, and the number of clusters increased. In addition, a 7.5% silica slurry must show some stress response, although it may be out of the measurement range for the instrument. At 3 and 7% concentrations of isopropyl alcohol the steady-state shear stress below the shear rate of 30 s-l increases with an increase in isopropyl alcohol concentration and this is attributed to an increase in HPMC concentration in the dispersion medium. Beyond the shear rate of 30 s-l the shear stress was lower than t h a t without isopropyl alcohol, which reflects on the somewhat weaker structure of the aggregated silica due to the less adsorbed amount of HPMC. Dynamic Measurements. Figure 5 displays the storage ( G ) and loss ( G )moduli of the suspensions a s a function of frequency. The dynamic moduli without isopropyl alcohol have a pseudo-second plateau, which indicates the existence of the internal structure in the silica suspensions, such as aggregated fractal-like structures due to the gelation character of the Aerosil silica. No dynamic responses were observed, however, for the 7.5% silica slurry (without HPMC) in our instrument. Thus, adsorption of HPMC leads to bridging interactions on silica surfaces, and such interactions should support the elasticity of the silica suspensions. Since the loss modulus is smaller than the storage modulus in the entire frequency range, the silica suspension behaves as a solidlike viscoelastic material.
Kawaguchi et al.
566 Langmuir, Vol. 11, No. 2, 1995
I
0.2 0.4 Cp / g/100mL
0.6
Figure 6. Adsorption isotherms of HPMC on Snowtex-C silica for various silica concentrations: ( 0 )0.5 g, (0)1.0 g, (U) 1.5 g, (0)2.0 g . 0.151
IU
10' q I llnm
10'
1OD
Figure 8. Double-logarithmic plots of the scatteringintensity, I(q), against wave vector, q , for the 7.5%Snowtex C silica (0) and the 7.5% Snowtex C silica suspension (0).
i
. '"10'
Figure 7. Amount of HPMC adsorbed on Snowtex-C silica as a function of isopropyl alcohol concentration. Desorption of HPMC by addition of isopropyl alcohol should be expected to lead to a less dynamic moduli than those of the silica suspensions without the displacers. Figure 5 realizes this speculation. At 7% concentration of isopropyl alcohol there is no second plateau, because of a reduced structure, i.e., a much open structure. The G" value seems to be less sensitive to desorption of HPMC than G . Snowtex C . Adsorption ofHPMC. Figure 6 displays adsorption isotherms of HPMC on the Snowtex C silicas for various silica contents. The adsorbed amount increases with a n increase in HPMC concentration of the supernatant solution. At a constant HPMC concentration, the lower the silica concentration is, the greater the amount of HPMC adsorbed on the silicas is. This means that HPMC chains more easily approach to the silica surface at lower silica concentrations. An increase in silica concentration leads to formation of a semicrystal-like ordered structure, and it should somewhat prevent adsorption of HPMC. The presence of such an ordered structure in concentrated silica suspensions has been confirmed by SANS m e a ~ u r e m e n t s land ~ the similar results will be described below. In Figure 7 the adsorbed amount of HPMC for the 7.5% Snowtex C in 1.5%HPMC solution is plotted as a function of isopropyl alcohol concentration. The degree of desorption of HPMC from the Snowtex C surfaces is somewhat higher than t h a t for the Aerosil silicas a t a given concentration of isopropyl alcohol, indicating that HPMC chains are in stronger contact with the Aerosil silica than the Snowtex C. SANS Measurements. Figure 8 shows the scattering curve of a 7.5% Snowtex C, and a peak around q = 0.17 nm-l is attributed to a correlation between the position of the silica particles due to the electrostatic repulsion between the negative charges on the silica particles. The separation distance ( D ) calculated from the Bragg's relation, D = 2rt/q,is 36.9 nm, and its value is comparable with the average distance d = 39.8 nm calculated from * ~ on a simple model, where the relation d = ( 4 n ~ / 3 C )based e is the density of silica (2.2 g/cm3) and C is the silica (17) Wong, K.;Cabane, B.; Duplessix, R. J . Colloid Interface Sci. 1988,123,466.
10'
IO0
10'
10'
10'
Shear rate / lis
Figure 9. Plots of the steady-state shear stress against the shear rate ofthe Snowtex C silica suspensionsin aqueous HPMC solutions with various isopropyl alcohol concentrations (Cal): (0)at tal= 0%, (0)at Cal = 3%)(A) at Cal = 5%. The filled circles indicate the steady-state shear stress of an aqueous HPMC solution with 1.5%concentration. concentration. Furthermore, the peak position shifts to high q systematically with a n increase in the silica content.l8 The scattering curve of a 7.5% Snowtex C silica suspension in 1.5% aqueous HPMC solution is also displayed in the figure. A slight rise in scattering intensity a t low q (below q = 0.1 nm-l) implies that the long wavelength fluctuations of density are increased by reduction of the electrostatic repulsion by adsorption of HPMC, showing formation of a cluster participated in by a small portion of the silica particles. However, there is no precipitation of the silica particles. Shear Stress Measurements. Figure 9 shows the effect ofisopropyl alcohol concentration on the shear flow curves of 7.5% Snowtex C silica suspension in 1.5% HPMC solution. For the entire shear rate, the silica suspension shows a larger stress response than the 1.5% aqueous HPMC solution. This is attributed to adsorption ofHPMC on the colloidal silica particles, where adsorbed HPMC chains mainly play a role in formation of a dense polymer layer and increase the effective hydrodynamic volume of the particles. Thus, addition of isopropyl alcohol leads to a decrease in the effective hydrodynamic volume, resulting in a depression in the steady shear stress. Of course, some portions of the adsorbed HPMC chains behave as cross-linkers among the silica particles. Such cross-linkers also partially desorb from the silica surface by addition of isopropyl alcohol. Furthermore, the silica suspension showed strong shear thinning, indicating rearrangements of the ordered structures of silica particles as well as some portions of the cross-linkers under higher shear flows. Dynamic Measurements. Figure 10 shows the dynamic moduli for the Snowtex C silica suspension in the presence of isopropyl alcohol: there is no second plateau in the dynamic moduli and the dynamic moduli monotonically (18) Kimura, Y.; Kawaguchi, M.; Suzuki, J.; Funahashi, S.; Izumi,
Y.Pro. Fifth Inter. Symp. Adv.Nuclear Ener. Res. 1993, pp 647.
Properties of Silica Suspensions
Langmuir, Vol. 11,No.2, 1995 567 dynamic moduli of the Snowtex C silica than those of the Aerosil silica suspension was observed. Furthermore, the dynamic moduli decreased a t 7% isopropyl alcohol concentration due to a decrease in the number of HPMC adsorbed on the silica surfaces.
7-201
Conclusions
b
1u
10.'
1oo
10'
Frequency i lis
Figure 10. Storage (a, G ) and loss (b, G )moduli as a function of frequency of the Snowtex C silica suspensions in aqueous HPMC solutions with various isopropyl alcohol concentrations at Cal = 3%, (A) at Cal = 7%. (Call: (0)at Cal = 0%, (0) increased with an increase in frequency. Both the absence of the second plateau and the larger G than G indicate little existence of the internal structure in the suspensions and liquid-like character. These facts are consistent with the steady shear responses described above. At 3% isopropyl alcohol concentration a larger depression in the
SANS measurements showed not only a discrete difference in the structures of the Aerosil and Snowtex silica suspensions but also the structural changes ofthe Snowtex silica suspension by HPMC adsorption. Such differences in the structures induce their characteristic steady-state flow curves: the Aerosil silica suspension showed a plateau a t shear rates lower than 1 s-l; the steady-state shear stress of the Snowtex C silica suspension increased with the shear rate. Addition of isopropyl alcohol yielded desorption of HPMC from the silica surfaces and influenced the rheological properties of the respective silica suspensions: for the Snowtex C silica suspension, a decrease in the steady-state shear stress as well as in dynamic moduli, whereas for the Aerosil silica suspension, a n increase in the steady-state shear stress at shear rates than lower than 30 s-l and a decrease in dynamic moduli without any changes in the aggregated structure.
Acknowledgment. M.K. expresses appreciation to the partial financial support of a Grant-in-Aid for Scientific Research (No. 03453109) from the Ministry of Education, Science, and Culture, Japan. LA9403284
