Structure and Stability of Concentrated Boehmite Sols BY J. D. F. RAMSAY AND S. R. DAISH Chemistry Division, AERE, Harwell AND C. J. WRIGHT Materials Physics Division, AERE, Harwell Received 7th December, 1977 The viscoelastic properties of dispersions of rnicrocrystalline boehmite (A100H) particles, covering a range of high concentrations (> 10% w/w) and containing different electrolytes, have been measured under oscillatory shear with a Weissenberg rheogoniometer. Stable dispersions were highly elastic and often thixotropic-properties which are ascribed to short range (<lo nm) inter- particle repulsion forces. When destabilised by addition of certain counterions (103-, Br03-, F-, S042-) this elasticity was lost and plastic properties developed. Interparticle repulsion is attributed to extensively solvated polynuclear aluminium cations, formed at the boehmite surface during acid peptisation, whose presence was consistent with quasielastic neutron scattering and other evidence.Light scattering measurements on dilute dispersions (t2% w/w) showed that the latter contain large and very open aggregates of primary sol particles-the number of primary particles per aggregate being dependent on the electrolyte concentration. In more concentrated dispersions (with volume fractions, Q, > 0.1) a stable and coherent structure analogous to the individual aggregates is proposed. In recent years the preparation of stable concentrated (>20% w/w) sols of several metal oxides has been descI5bed.l Such sols, which are composed of small (254 nm to ~ 0 .1 pm) primary particles have been prepared for example by peptisation of hydrous oxide precipitates and fine powders, usually with dilute mineral acids.2 As well as their remarkable stability at such high concentrations, many of these sols have other properties which are not typical of lyophobic colloids. Thus on further concentration they become progressively less fluid (sometimes being thixo- tropic) and are finally transformed into rigid solids, known as " gels ", when the volume fraction of the dispersion, q, exceeds ~ 0 . 4 . Such gels can generally be re- dispersed in water to yield stable sols once again. Indirect evidence, which suggests that the colloidal particles are partially ordered during the transformation of sols to gels has been obtained from gas adsorption studies: which show that many gels have a very uniform porous structure after evacuation.An explanation of some of these exceptional, and hitherto unexplored, features may possibly result from a better understanding of the forces of interaction between the colloidal particles. Accordingly, the stress response of concentrated dispersions of well characterised boehmite particles has been examined under oscillatory shear, with a Weissenberg rheogoniometer. Furthermore, to obtain an insight into the possible role of the water-surface interaction, the quasielastic neutron scattering of water in the dispersions has been measured. Light scattering measurements have also been made on more dilute sols ((2% w/w), to obtain details of possible structure in the concentrated dispersions.66 STABILITY OF CONCENTRATED SOLS EXPERIMENTAL MATERIALS Sols were prepared from a commercially produced (Condea, Petrochemie-Gesellschaft) microcrystalline boehmite powder which was characterised by electron microscopy and X-ray line broadening.The powder consisted of thin ( ~ 4 nm) plate-like particles ~ 2 0 - 30 nm across, and had a specific surface area, SBET, of 190 mz g-l. Concentrated sols were formed by peptising slurries of powder (-10 - 30% w/w) with dilute nitric acid (t0.1 mol dm-3). The minimum amount of acid required for peptisation corresponded to a mole ratio, [HN03]/ [AlOOH], of 2 x 10”; the resulting sol, depending on its concentration, had a pH in the range 3.9 to 4.2. Sols were also examined which had higher mole ratios than 2 x When dried in air at room temperatures, all the sols formed opaque, glassy gels which could be readily redispersed in water to give dispersions of any concentration desired.MI C R 0 EL E C TR 0 P H ORE S I S Electrophoretic mobilities, u, of dilute ( x g cm”) aggregated sols were measured over a range of pH (-3 to 11) with an instrument (Rank Bros., Cambridge) fitted with a capillary cell. LIGHT SCATTERING Concentrated sols (-5 - 20% w/w) were centrifuged (-2 x lo5 m s-~) and light scattering measurements (Sofica, model 42000) were made on diluted samples ( 5 2 x g ~ m - ~ ) at a wavelength of 546 nm using vertically polarised light. Results were treated using the scattering equation based on the Rayleigh-Debye theory : (1) where Re is the Rayleigh ratio corresponding to a scattering angle 8, P(8) is a particle scatter- ing factor, K* is an optical constant, B is the second osmotic virial coefficient and mW is the weight average molecular weight of the dispersed particles of concentration c.The optical K’clRe = (l/Bw)P-l(8) + 2Bc constant is given by: K* = 2n2 fig(d,ii/d~)’A-~ N-I where Ti,, is the refractive index of the dispersion medium, d,ii/dc is the refractive index incre- ment, and 13. and N are respectively the wavelength of incident light and Avogadro’s number. Experimental data were examined using the Zimm plot method4 to obtain ATw, B and the radius of gyration of the particles, R,, by the standard Guinier proced~re.~ OSCILLATORY SHEAR MEASUREMENTS The stress response of dispersions to oscillatory strains ( y = yo sin wt) of progressively increasing peak amplitudes, yo (from z 2 x lo” to 2 x lO-l), was measured with a Weissen- berg rheogoniometer (model R 19; fitted with a cone and plate), within a frequency (2nv = w ) range from z Dispersions (many were thixotropic) were allowed to age for several days and then gently placed on the rheogoniometer platens before making measurements.The response was recorded on a U.V. oscillograph and also analysed vectori- ally (Solartron 1170 frequency response analyser) to obtain storage, G’, and loss, G”, moduli. to lo3 rad s-l. NEUTRON SCATTERING Quasielastic neutron scattering data, obtained with the cold neutron time-of-flight spectrometers (4H5 and 6H) at AERE, Harwell,6 were converted to scattering laws using standard routines.’ The scattering laws were analysed using a simple diffusion model (which holds for pure water when Q2 < 2 A-’),J .D . F . RAMSAY, S . R . DAISH AND C . J . WRIGHT 67 where Q and m are the momentum and energy transfers respectively and F the diffusion constant. S(Q, m) is a Lorentzian function with a FWHM, AE, which is equal to 2hDQ2. D values were then obtained from the limiting slopes at small Q, of plots of AE against Q2. Scattering law half widths were obtained from the FWHM of the experimentally measured quasielastic peaks by deconvoluting their Voigt profiles (using standard tables) and the gaussian instrument resolution curve. Numerical convolution techniques also gave similar half widths. Due to the appreciable number of protons in the boehmite and their associated incoherent scattering, special care was taken to subtract the background scattering from the total produced by the dispersions.RESULTS MICROELECTROPHORESIS Sols were positively charged [u = (4 -I: 0.5)/102 pm s-l V-l m at pH 41, the mobil- ity changing insignificantly in the pH range 253 to 256. At pH > 6, u decreased and an isoelectric point was reached at pH 8 to 9. Measurements at different concentra- tions of potassium nitrate and 10-1 mol dm-3) showed no significant variation. LIGHT SCATTERING Light scattering measurements on diluted samples (<2 x g ~ r n - ~ ) showed that the primary particles in the Concentrated sols formed colloidal aggregates. Aggregation was increased when excess acid was used to peptise the powders and when dilute electrolytes (e.g., KN03, KClQ,, KC104) were added to the sols.For the dispersion corresponding to the Zinim plot shown (fig. l), which was prepared with just sufficient acid to effect peptisation ([HNO3]/[A100H] -2 x the extent I I I I L A 0.4 0.8 1.2 sin2(+) + 14.0 c FIG. 1.-Zimm plot for boehmite sol; [HN03]/[AIOOH] = 2 x Concentrations, CI, Ct, C3, Cq, C,/g ~ r n - ~ are respectively 2.41, 1.93, 1.45, 0.96,0.48, X68 STABILITY OF CONCENTRATED SOLS I I I 1 of aggregation was only limited; it would correspond to 2 to 3 units per aggregate, based on the dimensions of the primary particles obtained from electron microscopy. Data derived from fig. 1 are given in table 1. Results for several other dispersions either prepared with more acid or containing electrolytes are summarised in fig.2 to TABLE LIGHT SCATTERING RESULTS FOR A BOEHMITE SOL Kv R,lm B/mol m3 kg-2 (d2/dc)/rn3 kg-l 1.18 x 107 43, (31)" 1.74 x 10-9 1.04 x 104 * Zero extrapolation of Zimm plot from sin2(8/2) > 0.3. illustrate the extent and variability of aggregation. Larger aggregates were obtained as the concentration of anion in the concentrated sols was increased either during or after peptisation (from 252 x to 2 x 10-1 mol dm-3 for the examples in fig. 2). Measurements, made on diluted samples, withdrawn at intervals after peptisation, showed that the growth of aggregates was initially rapid and then continued more slowly, often for several days, before a stable size was attained; this rate was increased as the boehmite concentration in the concentrated sols (which was normally in the range 1 to 4 mol dm--3) was increased.lo8 i 1, tnm FIG. 2.-Dependence of m,., on R, for aggregated boehmite sols. Broken line shows RJL against XNJeqn (5)] forf = 3. * Refers to a single boehmite particle calculated for dimensions of 25 x 25 x 4nm3.J . D . F . RAMSAY, S . R. DAISH A N D C. J . WRIGHT 69 It can be showng that if logarithmic plots of nw and R,, such as in fig. 2 approach linearity in the limit of high nW (i.e. nW = KR,”), then information on the structure of the aggregates can be obtained from the value of the exponent x. Since the points in fig. 2 fall on a line for which x = 3, the aggregates can be considered to have a structure in which each unit (considered as a mass point) is separated, by a distance L, from f similar units.If the units are arranged in layers, confined by concentric shells having a difference in radii of L, the number of units in each layer being cf - 1) times that of the preceding layer, then the radius of gyration of such an aggregate is given by: Ri = r i N J 3 N, (4) n = l n = l where rn is the distance of the mass points in the nth layer to the centre of gravity and Nn the number of mass points in this layer. Assuming the first “ layer ” contains only one unit, substitution for r, and N,, gives In fig. 2 a plot of R,/L against 237, forf= 3 shows that the experimental data are in satisfactory accord with such a model and, moreover, indicates the low density of the aggregates (viz. L - 30 nm).Additions of some electrolytes (e.g., KF, KI03, KBrO,) at similar concentrations did not, however, give open aggregates, but resulted in destabilisation (uiz. coagula- tion) of sols and rapid sedimentation of boehmite particles. OSCILLATORY SHEAR EXPERIMENTS The effects of progressively increasing strain amplitude, yo, on the storage, G’, and loss, G”, moduli (LO = 99 rad s-l) of boehmite dispersions at several concentra- tions are illustrated in fig. 3. In the lower range of concentration ((32% w/w) sols had viscoelastic properties [cf. fig.3(a)]. As yo was increased however, G’ decreased steadily whereas G” remained almost unchanged-behaviour, which under conditions of steady shear would be consistent with a predominantly viscous fluid. At slightly higher concentrations (33.8 % w/w) the behaviour changed markedly [fig.3(b)]. Thus at low strains the response of the dispersions was almost entirely elastic (G’ 9 G”), until a particular value of yo ( ~ 3 x loA2) was exceeded. A perceptible phase dif- ference between the strain and stress response then began to occur, which corres- ponded to the marked increase in G” and fall of G’ shown; the response still remained linear however. As yo was increased further e 7 x the response became pro- gressively non-linear ; this feature, which coincided with a gradual reduction in peak stress (both G’ and G” decrease), was probably due to the onset of structural break- down in the dispersions. Thereafter G‘ and G” were derived from an analysis of the stress fundamental into the in phase and quadrature components of the strain (ie., neglecting other odd harmonics), a procedure only considered justifiable for yo < 0.15.Measurements of moduli at small strains ( y o = 1.6 x extending to lower fre- quencies showed no significant changes for LO > 1 rad s-l (fig. 4). Below this fre- quency a gradual increase in G” occurred which probably reflected the onset of structural relaxation. The dependence of the initial storage modulus, G;O+O, on concentration, is compared for other types of boehmite dispersion in fig. 5. An important feature is70 STABILITY OF CONCENTRATED SOLS N ‘E 0 m e c VI I 10” lo-* lo-’ 1.0 strain amplitude, Yo FIG. 3.-Dynamic shear moduli, G’, G” of boehmite dispersions ([HN03]/[A100H] = 2 X at different strain amplitudes, yo (a = 99.3 rad s-I). Concentrations, boehmite % w/w, (a) 31.9, (b) 33.8, (c) 37.1, ( d ) 42.6, (e) 49.0.Solid symbols denote G”, open symbols G’. radial frequency, w/ rad s-’ FIG. 4.-Storage and loss moduli of boehmite dispersion (34% w/w) at different test frequencies, ~ ( 7 0 = 1.19 x lov2). [NO~]/[AIOOH] = 4.2 X 10”. (0) G’, (a) G”. the marked rise in G;o-+o which results from an increase in the mole ratio of nitric acid (from 2 x to 4 x loA2) used for peptisation [cf. fig. 5(a) and (b)]. This effect, which was most striking at lower dispersion concentrations, was apparently due to the increase in the nitrate ion concentration, because additions of KNO, solution to sols peptised at low acid ratios had a similar effect [fig. 5(c)]. Extensive disruption of the dispersions under oscillatory shear began at a strain (yc 2 6 x which corresponded to a maximum in peak stress ad in phase withJ .D. F. RAMSAY, S . R . D A I S H A N D C. J . W R I G H T lo4- P) lE lo3- 7 \ ruu p lo2- 0) In z .- " 10 c;' Y E , I 1- - I l a volume fraction, Ib 0.1 0.2 a3 1 I I / b t 0 71 ** 1.2 1.L 1.6 1 .a log m.( % "W FIG. 5.--Storage moduli, G$o+o, of boehmite dispersions at different concentrations. [HN03]/- [AlOOH]: (a), 2 x (c) [NO?] = 0.4 mol dm-3, [HN03]/[A1001-I] = 2 X lo-'. (b), 4 x n.b. ro = 99 rad s-'. the strain. The work required to disrupt the dispersions, equivalent to their cohesive energy, E,, is therefore given by: E, = /I a'dy. Values of E, increased markedly with increases in dispersion concentration (fig.6)72 STABILITY OF CONCENTRATED SOLS and were also enhanced on further additions of both nitric acid and potassium nitrate solutions. Study of the effects of electrolyte concentration at a fixed dispersion concentration (28% w/w; [HNO,]/[ALOOH] = 2 x 1W2) showed that a similar and progressive increase in E, occurred with KNO,, KClO, and KC10, (e.g., E, w 6 and ~ 2 7 J me3 at 5 x and lo-' mol dm-3, respectively) up to ~ 0 . 1 5 mol drn-,; thereafter (up to w0.4 mol drn-,) little further change took place. Solutions of KI03, KF, K2S04, of low Concentration ( ( 5 x mol dm-3) also produced initial increases in G' and E,, but thereafter, on further increases in concentration, the dispersions lost their elasticity and developed plastic properties. This was typified by a marked non- linearity in the stress response, which contained significant contributions from the third and fifth harmonics, and approached a square wave.Eventually ([anion] > TABLE 2.-cONCENTRATiONS OF POTASSIUM IODATE SOLUTIONS REQUIRED TO DESTABILISE BOEHMITE SOLS* boehmite KI03 conc. (% w/w) conc./mol dm-3 [103]/[AlOOH] 11.7 0.04 2.0 x 20.8 0.08 2.0 x 27.9 0.12 1.8 x 33.8 0.15 1.6 x * [HNOJ[AlOOH] = 2.0 X 10-1 mol dm-3) sedimentation occurred due to a coagulation of the sol particles. Similar effects were observed with KBrO, solutions at higher concentrations (20.3 mol dm-3). The concentration of electrolyte (KI03, KF, KBrO,) required for destabilisation (i.e., to cause rapid sedimentation) was dependent on the dispersion concentration, as is illustrated for KIO, solutions in table 2.It will be noted that at the point of instability, the ratio [IO;]/[AlOOH] is close to the mole ratio of peptising acid. QUASI-ELASTIC NEUTRON SCATTERING Scattering law half widths, AE, for dispersions of several concentrations (at 296 K) are plotted in fig. 7 as a function of the square of momentum transfer, Q2 (a plot at 49% w/w has been omitted for the sake of clarity). Diffusion coefficients of water, D, obtained from the limiting slopes of these plots (table 3), decrease progressively as the dispersion concentration is increased. In contrast, for a concentrated (55% w/w) paste of unpeptised boehmite powder, D was very similar to that of bulk water. As Q increases, and the diffusion event is observed on a diminishing time scale ( Z 10-l1 - lo-', s), the curves deviate from the limiting slope, an effect which is most marked for the highest concentration.This feature has been ascribed'' to the water diffusion becoming less continuous and more like jump diffusion, since then whereJ . D . F . RAMSAY, S . R . DAISH A N D C. J . WRIGHT 73 0 1 2 3 lI2/ FIG. 7.-Dependence of quasi-elastic scattering, aE, on square of momentum transfer, Q2, for boehmite dispersions of different concentration, % wlw, (a) 28, (b) 34, (c) 43, (d) 52. where zo is the mean time between jumps of length 1. Further analysis on the basis of this model is unfortunately precluded until the contribution of rotational diffusion to the scattering is known. The present calculations of D are, however, valid because it has been shown that such a contribution becomes insignificant at low Q.lf TABLE 3.-DIFFUSION CONSTANTS, D, OF WATER IN BOEHMITE DISPERSIONS DERIVED FROM QUASI-ELASTIC NEUTRON SCATTERING MEASUREMENTS dispersion conc.(% wlw) (P D x 109/m2s-l water - 2.14 28 8.0 x 2.02 34 1.45 x 10-l 1.83 43 1.99 x 10-1 1.67 49 2 . 4 3 x 10-1 1.38 52 2.69 x 10-1 1.07 55 * 2.92 x 10-1 2.27 _ ~ _ ~ ~ * Paste of unpeptised boehmite powder. DISCUSSION Many of the properties of the dispersions can be ascribed to a short range (< 10 nm) interparticle repulsion which is different from the electrostatic repulsion encountered with lyophobic colloids. This interpretation is required by the presence of the open aggregates of particles observed by light scattering. These increase in size as the anion concentration in the dispersions is increased (> 10-1 mol dm-3) and the electro- static repulsion becomes negligible, resulting in a loosely flocculated network, whose structure is determined by a balance between van der Waals attraction (VA) and a short range repulsion.If at higher concentrations than those normally employed for light scattering (q7 > a similar but coherent network extends throughout the74 STABILITY OF CONCENTRATED SOLS dispersions, then the cohesive energies, E,, will be related to the work required to disrupt the network, which in turn will reflect the depth of a P.E. minimum resulting from attraction and repulsion interactions. In table 4 the interaction energy per particle of boehmite, e,, has been estimated from E, at different dispersion concentra- tions by assuming a number concentration, N, based on a particle volume derived TABLE 4.-cOHESIVE ENERGIES, E,, OF BOEHMITE DISPERSIONS AND VAN DER WAALS INTERACTION, v ~ , BETWEEN PARTICLES dispersion conc. e, x 1021/J (% wlw) P EJJ m-3 per particle (VAs)* x 1OZ1/J t/nm 33.7 1.45 x lo-' 2.05 x 10 3.53 x 10-1 2.00 10.0 37.1 1.65 x 10-1 3.00 x 10 4.54 x 10-1 2.62 9.2 40.1 1.82 x 10-1 1.52 x lo2 2.09 3.38 8.5 42.7 2.00 x 10-1 2.74 x lo2 3.42 4.38 7.8 49.0 2.43 x 10-1 1.42 x lo3 1.46 x 10 8.13 6.2 * Area of interaction per particle, s, is 6.25 x m2.n.b. [HN03]/[A100H] = 2 x from electron microscopy (uiz. 4 x 25 x 25 nm3). At lower concentrations, where the dispersions exhibit thixotropic behaviour, e, is comparable with kT (4 x J), a feature which probably indicates the beginning of a continuous structure due to association of aggregates.Values of e, at different dispersion concentrations can be compared with calculations of VA for a pair of parallel boehmite particles, of thickness 6, if a hypothetical structure is assumed in which all the particles are arranged in cubic close packed arrays and are separated on all sides by a distance t, where V, is given by12 A VA = - 4s [(t/2)" + (t/2 + s)-2 - 2(t/2 + S/2)-". (9) Values of Va (table 4) calculated with a Hamaker constant, A , of 4.2 x J, as reported for alumina in water,13 and 6 of 4 nm, are in better agreement with e, as the dispersion concentration is increased, which possibly suggests that the particles become more aligned as their separation is decreased. This arrangement would be consistent with the properties of the dispersible boehmite gels,14 which contain slit- shaped pores of width ~4 nm.Using a somewhat similar approach to that previously employed for other col- loidal dispersions l5 having elastic properties, the magnitude of the interparticle repulsion, PR, can also be estimated in principle from the dependence of G& on dispersion concentration, if the separation and arrangement of the particles is known with some confidence. Although such estimates can only be tentative because the arrangement of the boehmite particles is uncertain, calculations based on the hypo- thetical structure described show that PR rises rapidly and far exceeds the van der Waals attraction, V,, when p > 2 x lO-l, a feature which accords with the lyophilic properties and stability of the concentrated dispersions.A possible reason for this short range repulsion could arise from the presence of polymeric aluminium cations, which are formed at the surface of boehmite particles during peptisation with dilute nitric acid. These would be similar to the polynuclear ions, present in solutions of partially hydrolysed aluminium salt solutions, which have indeed been reported 1 6 9 1 7 to be highly effective in stabilising lyophobic colloids such as polymer latices. Adsorption at the colloid surface has been shown to beJ . D . F . RAMSAY, S . R . DAISH AND C . J . WRIGHT 75 particularly enhanced17 with the more extensively hydrolysed ions [e.g., Al,(OH)ii, A11304(0H)72i], which would be expectedls at the relatively high pH (>3.7) of the boehmite sols.In the present case, however, it is possible that the ions remain chemic- ally bound to the underlying boehmite after peptisation of a surface layer. Although the surface concentration of polynuclear cations required for stabilisation is evidently quite low (viz. one cation per 7 nm2, assuming one mol of Al,,O,(OH)\$ is formed for every 7 mol of HN03 added) the amount of water associated with each ion in solution is probably considerable. The presence of the polynuclear ions on the surrounding water probably contributes to the progressive reduction in the diffusion constant, D, as the dispersion concentration is increased-an effect, which although more marked, has been reportedl1*l9 for concentrated electrolytes, containing other " structure promoting " cations (e.g., Li+, Mg2+, Ca2+). Reductions in the diffusion constant of water in dispersions of ultrafine fumed silica powders of similar concentration have been reportedlo to be of similar magnitude to those found here.This effect has been ascribed to a structuring of water promoted by the silica surface. However, under the preparative conditions which were em- ployed it is likely that the silica surface was covered with a layer of polysilicic acid20 and that a similar mechanism for a reduction in D to that discussed for boehmite dispersions could also be appropriate. The stabilising role of polynuclear ions at the boehmite surface is also consistent with the powerful coagulating effects of anions, such as IO;, Br03, F- and SO$-, which form insoluble complexes with hydrolysed aluminium ions,21 e.g., Na [Al,,04(OH)2,(H,0)12(S04)4].Since most metal ions of valency +3 and +4 readily hydrolyse in solution to give polynuclear ions, the stabilising mechanism discussed here may also apply to a wide range of other concen- trated metal oxide sols, which have lyophilic properties typical of those described for boehmite. We thank Mr. R. G. Avery and Mr. G. H. Hearn for experimental assistance in part of this work. R. M. Dell, 7th International Symposium on the Reactivity of Solids, ed. J. S . Anderson et al. (Chapman and Hall, London, 1972), p. 553. C. J. Hardy, Sol-gel Processes for Ceramic Nuclear Fuels (IAEA, Vienna, 1968), p. 33. J. D. F. Ramsay, Chromatography of Synthetic and Biological Polymers, ed. R. Epton (Ellis Horwood, Chichester, 1977), p. 339. 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Bunsenges. phys. Chem., 1971, 75, 366. 2i G. Johansson, Acta Chem. Scand., 1960, 14, 771. p. 267. See, for example, J. H. Paterson and S. Y. Tyree, J. Colloid Interface Sci., 1973, 43, 389. J. A. Kitchener, Disc. Faraday SOC., 1971,52, 379.