Aqueous dispersions of colloidal gibbsite platelets: synthesis, characterisation and intrinsic...

COLLOIDS AND A

Colloids and Surfaces SURFACES

A: Physicochemical and Engineering Aspects 134 (1998) 359-371 E L S E V I E R

Aqueous dispersions of colloidal gibbsite platelets: synthesis, characterisation and intrinsic viscosity measurements

Anieke M. Wierenga, Tjerk A.J. Lenstra, Albert P. Philipse * Van' t Hoff Laboratory for Physical and Colloid Chemistry, Debye Research Institute, Utrecht University, Padualaan 8,

3584 CH Utrecht, Netherlands

Received 8 June 1997; accepted 4 August 1997

Abstract

We report preparation and properties of model platelets for the study of rheology and structure tbrmation. The colloids are well-defined, regularly shaped and fairly monodisperse hexagons with an average diameter of 160 nm and a thickness of 13 nm. Various characterisation techniques demonstrate that the particles consist of gibbsite. The isoelectric point of the edges (pH ~7) differs from that on the faces (pH ~ 10). At pH <4, addition of lithium chloride induces face-to-face aggregation, which causes settling to dense sediments. In the range 7 <pH < 10, low density gels are formed, probably due to the formation of a fractal network of edge-to-face aggregates. For stable gibbsite dispersion, low-shear viscosity measurements yield the intrinsic viscosity [r/] = 23 and the Huggins coefficient kH = 0.4. © 1998 Elsevier Science B.V.

Keywords: Gibbsite; Colloidal platelets; Disks; Shear viscosity; Gelation; Flocculation

I. Introduction

In many colloidal systems, such as drilling fluids [1 ], or pharmaceutical products, like ointments or toothpaste, 1 the rheological behaviour and structure formation is determined largely by dispersed anisotropic clay particles. The rheology of such dispersions is complicated by the presence of con- taminants, adsorption of (organic) species on the clay surface, clay-swelling and the flexibility of the particles. To study exclusively the effect of particle shape on the rheology and structure formation, well-defined model dispersions of anisotropic colloids are required. Recently, we reported a number

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of studies of low-shear rheology and structure formation of dispersions of boehmite needles [2- 4]. As many clay particles are plate-like, it would be interesting to compare the behaviour of the boehmite rod-systems with that of platelet dispersions. However, there is a lack of well-documented synthesis procedures of well-defined, non-swelling, colloidal platelets.

Buining et al. [5] described the preparation of boehmite rods from aqueous aluminium alkoxide solutions by hydrothermal treatment at 150°C. At lower temperatures the formation of plate-like particles was observed [6]. Plate-like particles were also formed at room temperature when the original aluminium alkoxide solutions were stored for longer than 3-4 weeks. The hexagonal shape of the particles suggests that the colloids formed are gibbsite platelets. The regular shape of the plate-

360 A.M. Wierenga et al. / Colloids" SurJaces A. Physicochem. Eng. Aspects 134 (1998) 359 371

lets, the purity of the dispersions after dialysis and the convenient preparation method make these particles promising as model colloids.

The aim of this work is to study the preparation and properties of these platelet dispersions and, as a first application, to investigate the low-shear rheology and structure formation of these systems. We investigate the surface chemistry of the platelets as a function of pH by electrophoresis and potentiometric titration. Colloidal gold was used as marker for positive surface charges, following Thiessen [7, 8]. In addition, the stabilising effect of the addition of aluminium chlorohydrate (ACH) was studied. (ACH was found to be an effective stabilising agent in boehmite dispersion [9].) We report results of low-shear viscosity measurements on dilute dispersions of stabilised particles and the gelation and flocculation behaviour in various pH regimes.

This paper is organised as follows. In Section 2.1 we summarise the chemistry of the gibbsite synthesis and crystal structure. Some theoretical predictions for the low-shear rheology of dilute dispersions of colloidal disks are given in Section 2.2. Experimental methods of preparation, characterisation and viscosity measurements and results are described in Sections 3 and 4 respectively. In Section 5 the observed structure formation and the viscosity data are compared with the predictions of Section 2 and results for aqueous dispersions of boehmite rods [3].

2. Background

2.1. Gibbsite platelets." structure and chemistry

Gibbsite (AI(OH)3) may be formed by hydrolysis of Al(OH/)3+-groups at temperatures below 100-150°C. At higher temperatures, hydrolysis of the same solutions yields boehmite (A1OOH) [10,11]. The gibbsite structure is a stacking of A1-OH layers, in which each A13+ is surrounded by six hydroxyl groups, as shown in the two- dimensional projection [12, 13] of the gibbsite crystal structure in Fig. 1. Gibbsite forms hexagonal platelets, which do not swell in water, due to the

E t- O.- O~ O

O AI ~

~OH

Fig. 1. Two-dimensional projection of the crystal structure of gibbsite [13].

polarisation of the A10 bonds by the A1 ions [ 14]. When dispersed in water, the gibbsite surfaces will be charged according to the following reactions:

(I): A1-OH; + OH - ¢~A1-OH + H20 (1)

(II): A 1 - O H + H2Oc~A1-O- +H3 O÷

At a pH lower than the isoelectric point (iep), the platelet will be positively charged, following reaction (I) in Eq. (1). At pH >iep, reaction (II) of Eq. ( 1 ) will induce a negative surface charge. When specific adsorption of electrolytes can be neglected, the iep coincides with the experimentally accessible point of zero charge (pzc) [15]. The structure of the gibbsite crystals suggests that the acidity of the A1 OH groups at the edges is different from that on the faces. Therefore, the iep of the edges is likely to be different from that on the faces. The presence of two distinct ieps causes an inhomogeneous charge density on the gibbsite platelets. Depending on the pH, three different types of flocculation may be distinguished: face-to-face aggregation, edge-to-face flocculation and edge-to- edge flocculation. The type of flocculation will determine the particle structure in the dispersions: face-to-face bonds will predominantly yield thicker platelets which sediment, whereas both edge-to- edge and edge-to-face links will favour the formation of voluminous network structures which cause gelation [ 16 ]. Another effect of pH on the stability of the dispersions is the increased solubility of gibbsite at low pH. In acid dispersions, the adsorption of H ÷ at the surface groups gives rise to

hydration. This will eventually lead to dissolution of A13 +, and disappearance of the gibbsite crystal. In alkaline dispersions, the solubility of gibbsite is much lower [17].

2.2. The shear viscosity of a dispersion of platelets

For particle concentrations far below the maxi- mum packing density of the colloids, the shear viscosity of a colloidal dispersion can generally be written as a power series in the volume fraction

Fig. 2. Hexagonal disk with diameter D and thickness b.

A.M. Wierenga et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359-371 361

?]d r/~ = - - = 1 +[q]~ + C2c/,2 (2)

r/o

For uncharged spheres, Einstein [18,19] showed that [r/]=2.5, Batchelor and co-workers [20,21] calculated that the low-shear limit C2=6.2. For platelets, both [r/] and C2 are shear dependent, because the degree of alignment of the particles depends on the shear rate. Here, we only consider the viscosity in the low-shear limit, where Brownian motion dominates the orientation distribution of the platelets. For this limit, Kuhn and Kuhn [22-24] found for the intrinsic viscosity of thin oblate spheroids

Brownian rods in the limit of zero-shear:

c~ =k.[~l ~ (5)

with the Huggins coefficient kn=0.4. In the absence of a more appropriate description, we will compare this prediction with our results.

3. Experimental

In all cases deionised water and p.a.-grade chem- icals were used. The pH of the dispersions was measured with a Schott-Ger/ite pH meter.

[,fl = - + - 1

2 15rt

_06 8( 1 -0.075p/

0 < p < 1 (3)

Here p is the particle aspect ratio, defined as

b p = -- (4)

D

with D the diameter of the disk and b the thickness or height of the particle (see Fig. 2). Although Kuhn and Kuhn neglect the effects of finite particle size, Eq. (3) yields the hard-sphere value, [r/] = 2.5 for p = 1. To our knowledge, no calculations of the second-order term of the low-shear viscosity of oblate spheroids have been reported. Using a far- field approximation, Berry and Russel [25] calculated the two-particle interaction term for two

3.1. Synthesis and charaeterisation

The gibbsite synthesis route is based on the boehmite preparation method, which is described in detail in Refs. [5,26]. An aqueous solution of aluminium-iso-propoxide ( 15.2 g 1 - 1), aluminium- sec-butoxide (20 g 1 -1) and HC1 (0.86 M) is stirred in a closed vessel at room temperature for 10 days. The resulting turbid solution is heated (in closed Teflon containers of 180 ml) in an autoclave for 72 h at 85°C, and then dialysed for 1 week against a continuous flow of deionised water in cellophane tubes. After dialysis the dispersion was filtered and stored in closed polyethylene bottles to prevent contamination by dissolved silicates from a glass wall. The procedure yielded a platelet concentration of 5-8 g 1 - 1 in the final dispersions. Higher particle concentrations were obtained by solvent- evaporation under a continuous nitrogen-flow at 60°C. Small quantities (either 0.05 g1-1 or

362 A.M. Wierenga et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359-371

0.1 g1-1) of ACH (Hoechst) were added to two dialysed and filtered dispersions.

Transmission electron micrographs of the colloids (see Fig. 3) show hexagonal platelets. Image analysis revealed that the diameter D of the platelets is approximately 160 nm (standard deviation 50 nm). For some tilted platelets, the particle thickness b could be estimated to be 14 nm. The particle dimensions in the different batches did not vary within experimental error.

The solid content of one dispersion was dried under a continuous nitrogen-flow. The specific surface area of the dried gibbsite was determined from nitrogen adsorption isotherms and BET analysis to be 76 m 2 g -< The solid product was characterised by powder X-ray diffraction (XRD) using monochromatic Cu K~I radiation. Further- more, differential thermal analysis (DTA, heating rate 20°C rain- ~) and thermal gravitational analysis (TGA, heating rate 10°C m i n - b were carried out under a continuous nitrogen flow.

3.2. Analysis o f colloidal stability and surface chemistry

A few millilitres of a colloidal gold dispersion were added to gibbsite dispersions (with and with-

Fig. 3. TEM micrograph of gibbsite platelets. The bar denotes 200 nm.

out ACH) at different pH values. Transmission electron microscopy (TEM) micrographs of the samples with added gold demonstrate the charge distribution in the sample, as colloidal gold is negatively charged and, therefore, adsorbed at positive surfaces [7,8].

The iep of the gibbsite particles was determined by potentiometric titration and by measurement of the electrophoretic mobility of the gibbsite colloids as a function of pH. For the potentiometric titration, samples of 50 ml of 2.4 g 1-1 gibbsite and 2.5 g 1 - 1 gibbsite with 0.1 g 1 - 1 ACH were used. NaOH solution (0.01 M) was added slowly to the dispersions and the pH was measured after an equilibration time of 4min. Electro- phoretic mobilities of pure gibbsite dispersions with varying NaOH-concentration were measured with a Coulter DELSA (Doppler electrophoretic sonic amplitude) 440SX apparatus. The pH was increased gradually (successive pH increments of about 0.5) by addition of the required volume of a 1 M NaOH solution. The sample was then stirred vigorously, and the pH and electrophoretic mobility were measured after a few minutes.

The effect of the addition of ACH on the colloidal stability of the dispersions was evaluated by following the effect of addition of lithium chloride (LiCI) on the appearance in 50 samples of varying ACH and LiC1 concentrations. Also, ten samples with varying pH in the range 2-12, without LiC1, were prepared. All samples were stored (undisturbed) in a thermostatic room (21.5 + 0.1 °C) and inspected for changes in appearance on a regular basis over a period of 3 months.

3.3. Viscosity measurements

All rheology measurements were carried out on a Contraves LS40 rheometer, using a DIN412 couette-geometry. A thermostatic bath was used to keep the temperature at 25°C. For each measurement, 3 ml of a sample was inserted into the geometry using a Finn-pipette. Two types of measurement were carried out. To study the colloidal stability of the particles in a shear field, the time dependence of the shear stress was measured. A constant shear rate (50 s -~) was applied and the shear stress was followed in time for 25 min. In

A.M. Wierenga et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359-371 363

addition, a flow-curve was measured for the dispersions that did not show orthokinetic flocculation. Here, the shear stress was measured for 15 shear rates equally spaced between 0.5 and 25 s-1. Each shear rate was applied for 2 min, after which the shear stress was measured. The slope of the (low- shear) Newtonian part of the flow curve was interpreted as the zero-shear viscosity. The viscosity of dispersions with A C H was corrected for the contributions of dissolved ACH.

4. Results

4.1. Characterisation o f the platelets

Table 1 lists the X R D results of the solid product. These values agree reasonably with reference data for gibbsite and do not match with reference values for bayerite and boehmite [28]. Analysis of the relation between the peak position and the half-height width of the 002-peak with the Scherrer equation [27] yields an average platelet thickness of b = 31.2 nm. The thickness may also be calculated from the specific surface area obtained from BET analysis (see Appendix A); this yields a lower limit of b = 12.6 nm. Both values are in reasonable agreement with values estimated from TEM micro-

graphs (14 nm). In Fig. 4, DTA and T G A results of the ground solid product are compared with results for boehmite prepared by hydrothermal treatment at 150°C of the same starting solution of aluminium-sec-butoxide and aluminium-iso- propoxide. The product formed at 85°C clearly differs from boehmite. The measured DTA curve agrees with the DTA curve of fine-grade commer- cial gibbsite [11 ]. In both the T G A and the DTA curves the same transitions can be observed: at T--- 100°C intercrystalline water evaporates, and at T > 300°C gibbsite transforms to A1203 (presumably of the type z-AI203 I l l ] ) . The weight loss of about 35% between 100°C and 1000c'C, as observed in the T G A curve, indeed corresponds to the change in weight resulting from the net reaction

2Al(OH)3 ~A1203 + 3H20 (7)

4.2. Gibbsite-water interface

4.2.1. Dispersions without A C H Fig. 5 shows TEM micrographs of gibbsite plate-

lets mixed with small gold colloids at three different pH values. At pH---4 the platelets are well dispersed and the gold colloids are adsorbed on the gibbsite particles, with a preference for the edges of the platelet. The platelets are less well dispersed

Table 1 XRD results of the hexagonal AI(OH)3 platelets

20 (rad) D (/k) hkl Height (%) Half-height width d (nm) a

18. 346 4.8358 002 100 0.59 13.22 20.390 4.3554 110 9 0.4 19.57 26.741 3.3336 200 4 1 7.92 28.134 3.1717 112 3 1 7.94 37.162 2.4193 004 12 1.4 5.80 41.672 2.1674 312 2 1.1 7.49 44.260 2.0464 313 4 1.2 6.80 45.648 1.9874 023 4 1.1 7.52 47.357 1.9196 123 2 0,80 10.5 50.793 1.7975 322 3 1.2 7.38 52.269 1.7502 0.24 3 1.1 8.08 54.502 1.6865 124 3 1.3 6.6 63.903 1.4567 324/300 1 0.51 18 66.960 1.3975 316 2 0.86 11

a Calculated from the Scherrer equation [27].

364 A.M. Wierenga et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359 371

2 -

8= o~ m ~ - 2 -

"{3 t-~ _4~ E

-6~

- 8

- 1 0

(a)

f

. . . . ]

V [ . . . . . boehmite !

' ' , ; o ' ' ,o'oo

Temp (C)

. ~ 8 0

1 0 0

70

60

(b)

•• ! platelets . . . . boehmite

i i r

200 400 600 800 Temp (C)

T

1000

Fig. 4. (a) DTA curve of gibbsite compared with result for boehmite [5]. (b) TGA curve of gibbsite compared with result for boehmite [5].

at pH = 7. At this pH the gold is adsorbed preferentially on the faces of the crystals. At higher pH ( p H = 9 ) , the platelets are clearly flocculated and the gold particles are not as well spread over the gibbsite surfaces as at the lower pH values. Note that for pH = 4 many platelet surfaces are dam- aged, and that irregularly shaped small (fibrous) particles are present. This is presumably due to partial dissolution of gibbsite in the acid environment. These fibrils are probably formed from (dissolved) aluminates during preparation of the TEM grid. The results of the potentiometric titration and the electrophoresis measurements are shown in Figs. 6 and 7. As the dispersions are purified by dialysis, specific adsorption of ions

Fig. 5. TEM micrographs of gibbsite platelets mixed with gold colloids (the black 'dots') at: (a) pH =4; (b) pH=6.5; (c) pH= 9. The bar denotes 200 nm. Gold particles are preferentially adsorbed on positively charged surfaces.

A.M. Wierenga et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359 371 365

1 2

x. ix. ix- ix, i x . . x - i x - . x ×. .x. x

x.X 10 ~ x

I 9 - ~< cu

/

6 i i i i i / 0 5 10 15 2 0 25 3 0

ml NaOH (O.01M)

Fig. 6. Potentiometric titration of 50 ml of a 2.38 g 1 - 1 gibbsite dispersion with NaOH solution.

12 i I

"I-- CL

1 0 -

8 -

l-e . . . . ~ p • - _ . . . . . . . . . . . . . . . . . . . . . . . p u < t - ? ! = ? p

I

pH(t=O)=2.74 I

. . . 0 _ 0 _ 0 ~ - - . . . . 0 . . . . . . 0 . . . . . . . . . . 0 . . . . .

I I I 2 0 0 4 0 0 6 0 0

t ime (hours)

Fig. 8. The equilibration of the pH of a 5 g 1 - 1 gibbsite dispersion after addition of a concentrated NaOH solution and a concentrated HC1 solution.

6 ¸

E ~ 4 '

0

II

i

6 7

o ,

o',

i i i

8 9 10 11

pH

Fig. 7. Electrophoretic mobility as a function of pH.

other than OH- and H + is negligible. We may therefore assume that pzc = iep. The shapes of both curves indicate that the platelets have two ieps. From the curvature of the plots, we can estimate the ieps: p H i 7 (iep(l)) and p H i l 0 (iep(2)). Instantaneous formation of flocs was visible for pH>8. Nevertheless, for p H = l l , well above iep(2), the electrophoresis measurements suggest that the platelets are still positively charged. This observation may be explained by the slow equilibration of the potential-determining reactions at the gibbsite surface. Fig. 8 shows how, after addition of either an acid or alkaline solution, the pH of the dispersion only approaches an equilibrium value on a time scale of days to months. We also

observed that, in time, the pH of a dialysed dispersion decreased slowly. From the potentiometric titration graph, we may estimate the surface charge density a of the gibbsite colloids from the equation

non- F 0 " - - - - ( C m -z) (8)

mGl B S

with non the molal number of NaOH added before reaching the iep (pH = 10.1 ), F the Faraday constant, morn the total mass of gibbsite in the sample and S the specific surface area of the platelets. For our dispersions this yields a~0.8 C m -2, which corresponds to five protonated groups per square nanometre. The hydroxyl density of the boehmite surface [11,26,29] is approximately eight per square nanometre. If the hydroxyl density for gibbsite does not differ too much from this value, approximately 63% of the hydroxyl groups on the gibbsite surface are protonated at pH = 6.5.

4.2.2. Dispersions with ACH TEM micrographs of gibbsite dispersion with

added ACH (0.1 g 1-1) indicated that small, predominantly fibrous particles (aspect ratio r~8.5) were formed, which were not attached to the platelets. Irrespective of pH (4, 7 or 9), gold particles are preferentially adsorbed on these


fibrils. It is likely that these particles are formed from dissolved species (ACH, A1) by evaporation of the solvent during preparation of the TEM grid. Fig. 9 shows the potentiometric titration curve of a gibbsite dispersion in the presence of ACH. Comparison with the titration curve of pure gibbsite (Fig. 6) reveals that addition of ACH decreases ~ 0.~ the pH of the dispersions. After addition of ACH, [ the particles still show two distinct ieps, but the I 0.2~/J pH values at which they occur ( p H i 5 . 5 and p H i 9 . 5 ) are significantly lower than found for the pure gibbsite dispersions. 0.4~

4.3. Colloidal stability

When not subjected to a shear flow, dialysed gibbsite dispersions show colloidal stability on a time scale of months. In time, a sedimentation concentration gradient developed, but the dispersions were easily homogenised by gentle shaking. Addition of small amounts of LiC1 or NaOH induced flocculation or gelation.

In dispersions with p H > 7 gelation occurred, and the gel volume decreases with pH. Decreasing the pH did not induce flocculation, but in time the turbidity of the samples with p H = 2 - 3 disap- peared, due to dissolution of the colloids. The remaining solid formed a thin layer of dense white sediment.

The addition of ACH increases the colloidal stability of the gibbsite dispersions. In Fig. 10 an overview is given of the behaviour observed in the

12-

10-

8-

6-

4

2

, / / /-

f X

× X

X

x x X X X X X X X

i i

o ,'0 2'0 30 40

ml NaOH (0.01 M)

Fig. 9. Potentiometric titration with an NaOH solution of a gibbsite dispersion (2 g l i) with 0.1 g 1-1 ACH.

[LiCI] )

1M 0.5M 0.1M O.05M O.01M

0.8ga

1.0g/1

E]fll D[fl

;qlq

<O.01M

sedimentation gradient

Fig. 10. Appearance of test tubes with varying ACH and LiC1 concentrations 2 months after preparation. (The major changes in appearance occurred during the first week.)

test tubes with varying ACH and LiCI concentration. Three types of behaviour can be distinguished. Below a certain flocculation concentration, a sedimentation concentration gradient develops. The volume of clear supernatant expands with increasing [LiCI ], and the critical flocculation concentration is 0.01 M LiC1 for dispersions without ACH, and 0.5 M LiC1 for dispersions with ACH. When [LiC1] exceeds this critical flocculation concentration, two different types of flocculated layer are observed: a dense, white sediment in the dispersions with [ACH]>0.1 g1-1, and a gelled bottom layer in the samples without ACH and in the sample with 0.1 g 1-1 ACH. The dense sediments did not flow when the test bottle was tilted, but the gelled sediments did. Apparently, the yield stress of the gels formed is very low: the samples are easily homogenised by gentle shaking. For high [LiC1], a space-filling gel is formed at very low particle concentrations, for instance at a weight concentration of 23.4 g 1-1 gibbsite in a dispersion of 5 M LiC1.

4.3.1. Colloidal stability under shear The effect of pH and the addition of ACH on

the colloidal stability of gibbsite is clearly demon-

A.M. Wierenga et aL/ Colloids Surfaces A." Physicochem. Eng. Aspects 134 (1998) 359-371 367

strated in the constant shear experiments (50 s -~) shown in Fig. 11. The dispersion is stabilised by decreasing the pH to values below pH--7, and increasing the pH decreases the stability. Addition of ACH increases the colloidal stability; at least 0 .1gl-~ ACH is needed to stabilise the dispersions.

The addition of ACH decreases the pH; as the surface charge on gibbsite is highest in acid dispersion, this may explain the stabilising effect of ACH. To test this hypothesis, a dispersion with 0.1 g 1- ~ ACH was purified by ultracentrifugation and redispersion in deionised water. Fig. l l(c) shows that this redispersed sample was still more stable than the sample without ACH, but only marginally.

4. 4. Intrinsic viscosity of stable platelets

Inspection if Fig. 11 reveals that stable dispersions can be obtained when 0.1 gl -~ ACH is added. Stable flow curves were also measured for samples without ACH, in which the pH was brought to p H = 4 by addition of HC1. As a reference, samples with 0.05 g 1 - ~ ACH were also measured, but these samples flocculated when the particle volume fraction exceeded 1.2%. The results are shown in Fig. 12, in which the volume fraction q~ is calculated from the mass fraction, using Pg ibbs i t e = 2.42 g ml - ~. Note that for q~ < 0.3%, the variation of the data points measured for the different dispersions is within experimental error. Extrapolation of the data point yields values for the intrinsic viscosity of the gibbsite platelets (p = 0.08), which are listed in Table 2. For all samples the intrinsic viscosity is [r/]~23. This is about 2.5 times the value predicted by Eq. (3) for a disk with the dimensions of the gibbsite platelets. In Fig. 12 the data are also compared with Eq. (2) with a two-particle interaction term as predicted by Berry and Russel [25] for hard prolate spheroids (Eq.(5), with [r/]=23 and kn=0.4 ). The agreement with the data points of the samples with 0.1 g 1 - x ACH is very good, but the deviation of the data points of the sample with p H = 4 (without ACH) suggests that in that dispersion there are other colloidal interactions, like Van der

0.08

~0.07

0.06

0.05

o pH=7.1

• pH=6.1

x p H = 8 . 2 x

x

x x~x,~ t x - - × ~,,(

time (min)

( a )

40

0.060 -

v

0.056- (D

0.052.

• n o A C H

• x 0.05gf l A C H

i o 4'0 6'0 time (min)

(b)

0.060 '

.~ 0.056,

0.052

no A C H

• 0.1g/I A C H

o red ispersed 0.1g/I A C H (c)

-10 O 10 20 30 40 50 60 70 60 90

time (rain)

Fig. 1 l. Behaviour of shear stress in time at constant shear rate (50 s 1): (a) for three different pH values; (b) for various ACH concentrations; (c) for a dispersion with 0.1 g l - 1 ACH, redispersed after centrifugation, compared with dispersions with 0.1 g 1-1 ACH and without ACH (pH=7.1) .

368 A.M. Wierenga et al. / Colloids Surfaces A: Physicochern. Eng. Aspects 134 (1998) 359-371

~ , • x ~ S

~-~X

1

i i i

0,00 0.01 0.02 0.03 0.04 0.05

volume fraction gibbsite

Fig. 12. Relative viscosity of stable gibbsite dispersions: ( • ) no ACH; pH=4, (©) 0.05 gl -~ ACH, (x) 0.1gl -~ ACH. The line represents Eq. (5) with [q]=23 and kH=0.4.

Table 2 The intrinsic viscosity of the stabilised gibbsite dispersions

Sample [q] Standard deviation

Gibbsite, 0.1 g l -~ ACH, pH=4 22.0 3 Gibbsite, 0.05 g 1-1 ACH, pH=4 24.5 2.5 Gibbsite, no ACH, pH=4 25.4 2.3

Waals attractions or electric double layer repul- sions, which also play a role.

5. Discussion

The chemical characterisation of the solid product clearly confirms that the platelets formed consist of gibbsite. The characterisation results of the preceding sections will be employed here to interpret the rheology results and the structure formation (aggregation and gelation) in the gibbsite dispersions. First we consider the colloidal stability in some more detail.

5.1. Colloidal stability

5.1.1. Gibbsite without added ACH." surface charge The pH dependence of the adsorption of gold

colloids on the gibbsite surface and the results of the potentiometric titration and electrophoresis

measurements indicate that the charge density on the edges differs from that on the faces. The iep of the edges is approximately pH = 7, and Fig. 5(a) suggests that in acid solutions the charge density at the edges is higher than at the faces. The constant shear experiment in Fig. 11 (a) shows that at a pH higher than this first iep the colloidal stability is decreased. At pH = 4 the charge density on the gibbsite surfaces is sufficient to yield stable dispersions. An important drawback of stabilising gibbsite dispersions by decreasing the pH is the increased solubility of gibbsite in an acid environment.

The behaviour of gibbsite in an alkaline environment is more complicated. The exact value of the iep of the faces of the crystal is difficult to assess. Both potentiometric titration and electrophoresis measurements were carried out on samples that were allowed to equilibrate with added O H - for only a few minutes. Those measurements both suggest iep (2) ~ 10. Nonetheless, the electrophoresis measurements indicate that, at this pH, not all protonated sites of the gibbsite surface have reacted with OH- on a time scale of a few minutes. Fig. 8 shows that, in time, more OH- reacts with the surface (the pH decreases). So, it is conceivable that the surface charge of the platelets will be reversed in time. Parks [15] calculated the iep for AI(OH)3 crystals to be pH=9.2. Considering the complexity and slow kinetics of the charge-determining reactions at the gibbsite surface, it is likely that the experimental iep decreases when longer equilibration times between addition of NaOH and measurement of pH and electrophoretic mobility are allowed. This indicates that the iep is only of limited significance for the characterisation of the gibbsite surface chemistry.

5.1.2. Effect of A CH on the colloidal stability Van Bruggen and Lekkerkerker [9] found that

for boehmite dispersions the addition of ACH greatly increased the colloidal stability. This stabilising effect on boehmite dispersions can be explained by the adsorption of ACH-polymers on the boehmite surface. For gibbsite the stabilising effect of ACH is less pronounced, and the decreased stability of the ACH-treated colloids after redispersion in demi-water suggests that the

A.M. Wierenga et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 134 (1998) 359 371 369

major part of ACH is not chemically bound to the gibbsite surface. However, there are also indic- ations that some ACH is adsorbed on the crystal. For instance, comparison of the potentiometric titrations in Figs. 5 and 8 reveals that the ieps of the dispersions have shifted, in particular iep(1). Furthermore, the Huggins coefficient kn of the dispersions without ACH (pH =-4) is higher than 0.4, the value measured for ACH-stabilised gibbsite at the same pH.

5.2. Sediment structure and density

Fig. 10 shows that, depending on the ACH concentration, flocculated particles either form "open" gel structures or "compact" white sediments. When the dispersions are destabilised by increase of pH, the flocculated platelets always form gels. The TEM micrographs in Fig. 5, together with the results of the potentiometric titration and electrophoresis measurements (Figs. 6 and 7), demonstrate that for pH <iep(1) the positive charge density on the gibbsite faces is lower than on the edges, and that for pH >iep(1 ) this situation is the opposite. This pH dependence of the charge distribution is consistent with the observed gelation behaviour. Addition of ACH decreases the pH of the dispersions. This favours face-to-face aggregation, yielding dense sediments of parallel clusters of platelets. When pH > iep( 1 ), the charges on the edges are probably opposite to those on the faces. This will induce edge-to-face clustering and the formation of open, voluminous network structures. However, on approach to the iep of the faces (iep(2)), the sign and density of the surface charges on the faces will approximate that on the faces. The degree of edge-to-face clustering will then decrease, and the gel volume will shrink, as observed.

In dispersions with pH ~ 7, an open structure of presumably edge-to-face clusters is formed. Space- filling gels are formed for ~ / p ~ O . 1 , with p the aspect ratio of the platelets. Philipse and Wierenga [30] estimated that a space-filling structure of homogeneously distributed rods does not occur for q~L/D < 0.7. It is not clear whether this estimate also applies for platelets. However, the low value of ~/p will be difficult to explain with a homogen-

eous microstructure in which platelets are evenly distributed in space. A low (rescaled) density ~ / p is easily accounted for by a fractal aggregation [31], as is described in detail for rods elsewhere [30]. Note that, although the type of interparticle bond renders these networks comparable with the well-known house-of-cards structure, the density of the networks is much lower than the descriptions that van Olphen [16] suggested. This would be a consequence of an inhomogeneous (or fractal) interparticle structure. The turbidity of the gibbsite-gels obstructed the verification of a fractal structure by light scattering measurements, as reported for boehmite [3] and montmorillonite [32] gels.

In contrast with the behaviour observed in boehmite [3] and montmorillonite [ 1 ] dispersions, the gel strength in the gibbsite sediments increased continuously with the electrolyte concentration. The yield stress of the gibbsite gels is usually very small in comparison with that of the boehmite gels. In boehmite-gels, we attribute [3] the pec- uliar gelation behaviour to "polydispersity in interactions". At moderate ionic strength (10 mM<[LiCI]<75 mM) only some of the rods are destabilised, and the gel consists of a "back- bone" of flocculated rods which is strengthened by the high viscosity of the interspacing dispersion of stable boehmite rods. When the ionic strength is increased the gel strength is reduced. This is because more rods flocculate, which causes a decrease of the volume fraction and viscosity of the interspacing dispersion. In contrast with boehmite dispersions, however, dialysed gibbsite dispersions have a low-shear viscosity that is only slightly higher than the solvent viscosity (see Fig, 12). In a gibbsite gel, the interspacing dispersion between the network branches will, therefore, only contribute marginally to the gel strength.

5.3. Low-shear viscosity measurements

The intrinsic viscosity of the gibbsite platelets is 2.5 times higher than the expression of Kuhn and Kuhn [22] (Eq. (3) predicts. This discrepancy may be due to several factors. Eq. (3) is valid for thin disks, and for platelets with p = 0.08 we expect that end effects will increase the intrinsic viscosity [33]. In addition, it is plausible that the intrinsic viscos-


ity of hexagonal platelets with sharp vertices is higher than that of smooth disks. The presence of aggregates will also increase the intrinsic viscosity. This effect, however, is expected to be small. At infinite dilution the intrinsic viscosity corresponds to the number average of the particle dimensions, and Eq. (3) predicts [r/] to depend linearly on p -L TEM micrographs of dispersions at p H = 4 though (see Fig. 5(a)), do not give any indication for the formation of a large number of aggregates with diameters _2.5D. (Recent Atomic Force Microscopy measurements suggest that the gibbsite platelets may be roughly twice as thin as concluded from our XRD and TEM measurements [36]). In analogy with calculations for charged spheres [34] and rods [35], we expect that the contribution of the primary electroviscous effect to the intrinsic viscosity is small.

The validity of the two-particle interaction term calculated for hard rods is remarkable, but not surprising. Berry and Russel [25] approximate the two-particle interactions by pre-averaging the rod orientations. For ~ = 0, the pre-averaged excluded volume of a rod is a sphere, just like for disks. This approximation apparently renders the calcu- lation of Berry and Russel also appropriate for gibbsite dispersions.

6. Summary and conclusions

Aluminium hydroxide platelets with an average diameter of 160 + 50 nm and an average thickness of 13 nm were synthesised from a solution of aluminium alkoxide precursors. Various characterisation techniques, such XRD, TGA and DTA, demonstrate that the particles consist of gibbsite. The gibbsite particles are hexagonal platelets with two distinct ieps: pH ~ 7 and pH ~ 10 for the edges and faces respectively. The value of p H = 10 is higher than calculations of Parks [ 15] predict. This is probably due to the long equilibration time of the potential-determining reactions at the gibbsite surface.

The pH determines the type of clustering in the dispersions. For pH < 7 the surfaces are sufficiently charged to stabilise the colloids. However, for pH <4 the solubility of gibbsite becomes too large to consider the dispersions well-stabilised. In the

range 7 < p H < 1 0 , gels with a very low particle density are formed. The low particle density results from the preference for edge-to-face interparticle links. When the pH is increased further, the sign and density of charges on the faces is no longer very different from the edges; consequently, the degree of edge-to-face clustering decreases and the gel density increases.

Addition of ACH increases the colloidal stability of the gibbsite dispersions; at least 0.1 g l-1 ACH is needed for stabilisation. Probably only a small portion of ACH is adsorbed on the gibbsite surface, and the stabilising effect is mainly due to the decrease in pH caused by the ACH addition. The experiments indicate that, partly due to the complexity of the aluminium hydroxide-water interface, it is difficult to obtain long-term stability of gibbsite platelets in aqueous dispersions. (Recently, van der Kooij and Lekkerkerker [36] prepared sterically stabilised gibbsite particles in toluene, which show the long-term stability needed for more elaborate studies.)

The intrinsic viscosity of stable gibbsite platelets is [~/] =23, and the Huggins coefficient kH=0.4. The intrinsic viscosity is 2.5 times higher than calculated for smooth circular disks. This discrepancy is probably caused by neglect of the end effects in the calculations of Kuhn and Kuhn [22] and the angular shape of the crystals. The Huggins coefficient equals the value calculated for Brownian rods, due to the similarly shaped excluded volume.

Acknowledgment

Dr. Paul Buining is thanked for providing the gold dispersions and helpful comments on the boehmite/gibbsite synthesis. C. Pathmamanoharan is acknowledged for the explorative work on gibbsite synthesis, and we thank Michel van Bruggen for fruitful discussions on the stabilising effect of ACH.

Appendix A

7.1. Plate thickness from specific surface area

Consider a platelet with arbitrary shape with thickness b, circumference L and face surface area

A.M. Wierenga et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 359-371 371

O, so the to ta l surface area is A = 2 0 + L b . The vo lume o f the disk is V = Ob. W h e n the col lo ida l p la te le ts are no t aggregated , the specific surface a rea S (for a ma te r i a l wi th mass densi ty p) can be wr i t ten as

A 2 L s = - + ( A . 1 )

Vp bp Op

For hexagona l p la te le ts wi th d iamete r D:

L 4

O D

2

4 S p - - -

D

(A.2)

F o r the gibbsi te plate le ts D ~ 160 nm, p = 2 . 4 2 g c m -3 and S = 7 6 m 2 g - t , and combina -

t ion with Eqs. ( A . I ) and (A .2 ) yields b = 12.6 nm. I r regu la r surface shapes increase the ra t io L/O and, therefore , Eq. (A .2 ) co r re sponds to the lower b o u n d a r y o f b.

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Aqueous dispersions of colloidal gibbsite platelets: synthesis, characterisation and intrinsic...

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