Aluminum hydrous oxide sols—I

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 3691-3705. Pergamon Press. Printedin Great Britain.

ALUMINUM

HYDROUS

OXIDE

SOLS--I

SPHERICAL PARTICLES OF NARROW SIZE DISTRIBUTION R O G E R BRACE* and E G O N MATIJEVI(~

Institute of Colloid and Surface Science and Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676 (Received 15 January 1973) Abstract--The methods for preparation, purification, and stabilization of aluminum hydrous oxide sols consisting of spherical particles of narrow size distribution are described in detail. To produce such sols, aluminum salt solutions containing complexing anions (i.e. sulfate) are aged at 98°C for hours or days. Once the particles are formed the constituent sulfate ions can be removed by exchange with hydroxyls of an added base. As a result the particle diameter shrinks by ~ 10 per cent. The size distribution and the number concentration of these sols are determined by light scattering and by electron microscopy. Electrophoretic mobilities indicate a great effect of the presence of sulfate ions and of pH on the surface charge of the particles. INTRODUCTION

THE ABILITY to prepare "monodispersed" colloidal sols offers certain distinct advantages in studies of particle formation and growth. Specifically, if the particles are spherical and sufficiently uniform in size, light scattering can be employed, to determine their size distribution and the number concentration in situ. This in turn permits the study of the kinetics of the formation of the new phase. Additional information may result from these investigations, such as the value of the refractive index or any change in the particle size distribution as a consequence of the interactions with some species from the dispersion environment. Relatively few inorganic hydrosols consisting of spherical particles of narrow size distribution have been prepared heretofore. Those reported were primarily sols of elements, such as colloidal gold, sulfur, selenium, etc., although some compounds (silica, lead iodate) have also been obtained as uniform spheres. A few years ago a method was described for preparation of"monodispersed" chromium hydrous oxide sols having spherical particles[l, 2]. The procedure essentially consisted of aging at elevated temperatures for several hours solutions of chromium salts containing certain complexing ions, notably sulfate or phosphate. Some evidence was provided that basic chromium sulfate complexes acted as the precursors to formation of nuclei. * On leave from Unilever Research Laboratory, Port Sunlight, England. 1. R. Demchak and E. Matijevi~, J. Colloid Interface ScL 31, 257 (1969). 2. E. Matijevi~, A. D. Lindsay, S. Kratohvil, M. E. Jones, It. I. Larson and N. W. Cayey, J. Colloid Interface Sci. 36, 273 (1971). 3691

3692

ROGER BRACE and EGON MATIJEVI(~

The availability of these sols has opened up new avenues for the studies of the formation of meta[ hydroxides, which certainly are much more complicated systems than ionic precipitates. Since aluminum ions are also known to form basic sulfate complexes, it was assumed that it should be possible to prepare "monodispersed" aluminum hydrous oxide sols consisting of spherical particles. Indeed, after considerable efforts, procedures have been developed which yield such stable dispersions. The methods of preparation and purification are described in this paper in detail. In addition optical properties of these sols have been utilized to obtain the particle size distribution for systems prepared under various conditions. Finally, the electrokinetic characteristics have been studied as a function of the method of preparation and of pH. The similarities and the differences with corresponding chromium hydrous oxide sols are discussed. EXPERIMENTAL

1. Materials The stock solutions of aluminum salts, sodium sulfate, acids, and bases were prepared from "Baker Analyzed" reagents, without further purification. Water was doubly distilled, the second distillation being carried out in Pyrex glassware. Only concentrated solutions (> 10- 2 F) of aluminum salts were kept for longer periods of time (weeks); dilute solutions were made as required just prior to experiments to avoid slow aging which may take place at room temperature. To remove any solid contaminants, all solutions were filtered through 0-45 #m Millipore filters. This was particularly important in order to prevent heterogeneous nucleation during the sol formation and to avoid interference of foreign particles in the light scattering analysis.

2. Analytical The stock solutions and the sol particles were analyzed for aluminum content using the method based on extraction of the 8-hydroxyquinoline complex as described in detail by Riley[3]. In this procedure a sample was acidified to destroy any aluminum hydrolysis complexes[4] then buffered to pH 4.9 with sodium acetate, and complexed with 8-hydroxyquinoline. The complex was extracted with chloroform and its concentration determined from absorption at 2 -- 410 nm. The concentration of aluminum was calculated from an appropriate calibration line. Sulfate ion content was determined turbidimetrically after precipitation with a barium salt[5]. The unknown sample contained in 5 ml was mixed with 5 ml of a barium reagent. The latter consisted of I ml 10 per cant BaCI2 solution, 1 ml 4 N NaCI in 0.32 N HCI, 2 ml glycerol/ethanol mixture (1:2 by volume) and I ml water. The optical density of the resulting sol was measured at a wavelength ,~ = 436 nm using 1 cm cells in a Beckman D U Spectrophotometer 3 rain after mixing the reacting components. This was compared with a calibration curve obtained using blank samples of known sulfate concentrations. The presence of aluminum was found not to influence the results. To determine the molar ratios of aluminum to sulfate in the sol particles, the sols were filtered through 0.45 #m Millipore filters and washed free of the mother liquor with three 10 ml portions of distilled water. The last rinse gave a negative test for sulfate with barium chloride. The particles were dissolved in nitric acid (0.5 N) which was added onto the filter and allowed to interact for 5 min before the resulting solution was drawn through by vacuum. The acid solution and washings were made up to 10 ml and analyzed for aluminum and sulfate as d~'z~scribedabove. Approximately 150 ml of the sol was necessary to give enough particles for a reliable analysis. In some cases the aluminum content of the supernatant solution of the sols was also determined in order to obtain the fraction of the original amount of aluminum incorporated in the solid phase. 3. J. P. Riley, Analytiea Chim. Acta 19, 413 (1958). 4. T. Okura, K. Goto, T. Yotuyanagl, Analyt. Chem. 34, 581 (1962). 5. A. I. Vogel, Textbook of Quantitative Inorganic Analysis, p. 850. Wiley, New York (1961).

Aluminum hydrous oxide sols--I

3693

3. Techniques (a) Electron microscopy. A Phillips 100C electron microscope was used to examine the particle shape and size. As the sols contained high concentrations of electrolyte and relatively low particle numbers ( ~ 10 7 partides/ml) the most satisfactory method of preparing the samples involved a centrifugation technique. A collodion coated copper grid was placed at the bottom of a test tube containing a few milliliters of the sol and spun for 2 min in a laboratory centrifuge. The grid, with the collection of a reasonable number of particles, was removed, dried with filter paper, and examined in the microscope. Particle size histograms were constructed from electron micrographs with the aid of a Zeiss TGZ 3 particle size analyzer. Since the stability of the samples on the grid varied with the method of sol preparation, it was essential that in some cases the electron microscope examination be made as soon as possible. Chromium shadowing and carbon replication was carried out with a JEOL vacuum evaporator type JEE. The replication procedure consisted of centrifuging the sol particles onto a freshly cleaved mica disc and subsequent coating with evaporated carbon at an angle of 90°. After stripping the carbon film, the sol particles were dissolved in dilute nitric acid, the replica was mounted on the copper grid and photographed in the electron microscope. In some cases samples were shadowed with chromium at a low angle ( ~ 30*) prior to the carbon coating step. (b) Light scattering. The size distribution and number concentration of the aluminum hydrous oxide sols were determined in situ by light scattering. A Brice Phoenix light scattering photometer Series 2000 outfitted with Glan-Thomson prisms instead of the original polarizers[6] was used in this work. Diaphrams 4 mm wide and 10 mm high were placed at the collimator, at the cell table, and immediately in front of the photomultiplier to adjust the beam height to the dimensions of the prisms and the beam width to the fiat window of a cylindrical cell (Brice Phoenix cell C 101). The photomultiplier nose cone had a 6 × 3 mm opening. In some measurements the outside back curved surface was painted with mat black paint in order to reduce reflections. The instrument constants were determined following the procedure described by Kratohvil and Smart[7]. Prior to measurements the sols were kept in the cell for a period of 15-30 min to allow for any large extraneous particles to settle. For particle size distribution and number concentration analyses, the vertical and horizontal scattering intensities were measured at 0° and at nineteen angles at 5° intervals between 40 and 130". In some cases the data were obtained at two wavelengths, 546 and 436 nm, in order to check for the internal consistency of the method. To avoid multiple scattering effects, the sols were diluted when necessary a~ much as the instrument sensitivity would allow. (c) Electrophoresis. Electrophoretic measurements were measured in the van Gils cell[8] using an apparatus described in detail previously[9]. The pH values of the sols were adjusted with nitric acid or sodium hydroxide and the samples were allowed to stand ten minutes before the pH and/or mobility measurements were made. For each determination at least twenty particles were followed (in both directions and at the upper and lower stationary levels in the capillary cell) and an average mobility calculated. (d) pH Measurements. All pH determinations were made with combination glass electrodes using the Radiometer Model 26 pH meter. The electrodes were calibrated by means of appropriate buffer solutions.

PREPARATION OF SOLS OF NARROW SIZE DISTRIBUTION

1. Procedures an'd conditions

In principle the aluminum hydrous oxide sols consisting of spherical particles of narrow size distribution can be obtained if aluminum salt solutions containing certain anions (notably sulfate ions) are aged at elevated temperatures. However, it is necessary that the aluminum salt concentration, the sulfate concentration, the initial pH, the final temperature, and the mode of heating be controlled within narrow 6. 7. 8. 9.

M. Kerker and E. Matijevid, J. Opt. Soc. Am. 50, 722 (1960). J. P. Kratohvil and C. Smart, J. Colloid Sci. 20, 875 (1965). G. E. van Gils and H. R. Kruyt, Kolloid-Beih. 45, 60 (1937). E. Matijevid, K. G. Mathai, R. H. Ottewill and M. Kerker, J. Phys. Chem. 65, 826 (1961).

3694

ROGER BRACEand EGON MATIJEVIC

limits. When these requirements are met, the self-hydrolysis of aluminum ions combined with sulfate complexing may result in spherical particles of remarkable uniformity. The method is essentially the same as described for the preparation of "monodispersed" chromium hydrous oxide sols(!, 2]. A typical procedure for the preparation of a "monodispersed" aluminum hydrous oxide sol is to place a solution which is, for example, 2 x 10-a F in aluminum as either aluminum sulfate, or one of the alums (e.g. aluminum potassium sulfate), or aluminum nitrate plus sodium sulfate into a preheated (98 _+ 2°C) laboratory constant temperature oven. Samples amounting to between 5 and 150 ml are kept in Pyrex tubes sealed with Teflon lined caps. The solutions, when prepared at room temperature, show no Tyndall cone if properly illuminated but begin to scatter light after approximately 6 hr of aging at 98 _ 2°C. If the sols thus formed are of sufficiently narrow size distribution, rather intense higher order Tyndall spectra (HOTS) will appear. After approximately 24 hr of aging the scattering properties of the sols rt,main unchanged. The pH of freshly prepared solutions (2 x 10 -a F) is "~4.1 and requires no adjustment in order to obtain uniform sol particles. Upon completion of aging and cooling to room temperature the sols have a pH of ~ 3-1. Figure l(a) shows the electron micrograph of aluminum hydrous oxide particles obtained by aging a 2 x l0 -3 F solution of AI2(SO4) 3 under conditions described in the legend. Fig. l(b) presents a replica of the shadowed particles formed in a 1 x 10 -3 F solution of KAI(SO4)2. I n a number of experiments various parameters leading to the formation of "monodisPersed" aluminum hydrous oxide sols have been examined. Solutions of A12(SO4)3, KAI(SO4)2, or containing mixtures of AI(NOa)a-A12(SO4)a and AI2 (SO4)a-Na2SO4 in various ratios were heated and inspected in bright illumination. The appearance of HOTS was taken as the first indication of particle uniformity. The concentration of aluminum ions in these experiments ranged from 10 -4 to 10-2F and that of sulfate ion from 10 -4 to 3 x 10-2F. The sols of narrow size distributions were obtained only when aluminum ion concentration was between 2 x 10 -4 and 5 x 10 -3 F provided that the aluminum to sulfate molar ratio ranged from 1:1 to 1:2. With [SO2-]:[AI 3+] somewhat less than unity no sols formed despite the fact that aluminum ions underwent hydrolysis, as evidenced by a decrease in pH of test solutions upon aging at elevated temperatures. At ratios [SO 2-] : [AIa ÷] higher than two the sols did form but the particles were polydisperse and settled rapidly. Figure 2 gives the sol formation domain in terms of aluminum and sulfate molarities. The sols exhibiting HOTS are obtained only under conditions bounded by the two dashed lines. Changing the sulfate concentration at constant content of the aluminum ions has an effect upon the particle size; the higher the sulfate concentration the larger is the modal diameter of the spherical sol particles. Figure 3 illustrates this effect on two sets of data; circles designate a series of sols prepared under otherwise identical conditions at constant aluminum concentration, but varying sulfate concentration, whereas by squares are given data for sols of constant [A13+]:[SO~ -] but different aluminum concentrations. The temperature of aging represents a rather critical parameter; no sols could be produced below 90°C in contrast to the behaviour of chromium salt solutions,

Fig. 1. (a) Electron micrograph of a n aluminum hydrous oxide sol prepared by aging a 2 x 10-3 F solution of A12(SO4)3 at 98°C for 84 hr. (b) Electron micrograph of a carbon replica of aluminum hydrous oxide particles obtained by aging 1 x 10-3 F solution of KAI(SO4)2 at 98"C for 24 hr. (c) Electron mierograph of a sol prepared by aging of a 1 x 10 -3 F solution of A12(SO4)3 for 24 hr at 98"C followed by adjustment of pH to 9-7 by NaOH, deionization, repeptization at pH ~8, and additional aging at 98°C for 24 hr. (d) The same as (c) but at higher magnification. The lines indicate 1/~m in all cases.

3694

Aluminum hydrous oxide sols--I

3695

-2"0

A 0 • O

LL

Clear solutions Stable sols (no hots) Stable sols (hofs) Uns'foble sols

.-2'5

[AI~]:[S04]/St /f s I,I ~ / . 1:2 A ~ • , O n

A I

SIS

.0,"

~a ~11 &

s r~ •

•

0

O

s/ _

/

,, o

~" • f

,/ -3.5

,,{0~ •s

-4.0

"eo

o o

//

~ a~ s•

s

,JD"

0

/.

Aluminum hydrous oxide

IS

-4.0

-3.5

-3.0

-2'5

-2.0

log c0nc. of sulfate ions,

-I. 5

F

Fig. 2. Aluminum vs sulfate ion concentration domain for solutions aged for longer than 24 hr at 98°C showing compositions resulting in no sol formation (A); sols of particle size too small to show HOTS (©); stable sols showing HOTS ( 0 ) ; and coagulated and settled sols (Fq). The dashed lines indicate systems of molar ratio of [AI 3+] to [SO j - ] of 1 : I and 1 : 2, respectively.

in which sols formed at temperatures as low as 65°C[1, 2]. The best results with aluminum salt solutions were obtained at 98 + 2°C. There was some evidence that the rate of heating had effect upon the sol formation, but this has not been 0"8

I

r

Aluminum hydrous oxide O [AI3+] = 10-3F O [AI 3÷] = 0 ' 6 6 x [S0~] 0'6 -

98°C, 4 0 hr

-

E ::k

6

0,4

~ o

0"2

0

/

-3'2

/ -3'0

./ -2.8

log conc. of sulfete ions,

/,

-2.6

-2.4

F

Fig. 3. Modal particle size obtained by electron microscopy of aluminum hydrous oxide sols formed by aging at 98"C for 40hr solutions containing 1 x 10 -3 F aluminum ions in the presence of various sulfate concentrations (O). Squares represent data obtained using solutions of different aluminum concentrations but of a constant aluminum/sulfate ratio (F]).

3696

ROGER BRACEand EGON MATIJEVI(~

examined in any detail at the present time. The rate of temperature rise in the test solutions studied was ~0.5°C/min during the first 2-3 hr of aging.

2. Sol stability The stability of the sols prepared as described above was influenced by two factors. Firstly, the particles formed at high temperature slowly redissolved upon cooling. Secondly, if kept at high temperature for extended periods of time the particles were lost to the container walls due to adsorption. The latter is most likely caused by the weak positive charge on the particles generated at the lower pH values. For these reasons all measurements on the original sols (light scattering, electrophoresis, electron microscopy, etc.) had to be carried out shortly after cooling the systems to room temperature. The dissolution of particles could be prevented and consequently the life time of the dispersions markedly enhanced by rising pH of the sols at the completion of a given aging period. The optimum pH to produce such stabilization was found to be around 9.7. The explanation is simple since at this pH the particle charge is reversed to negative and the unreacted aluminum species are complexed as aluminate ions. Thus no further aluminum hydroxide precipitation takes place. In practice the sols, still hot, were slowly added to a cold dilute sodium hydroxide solution of pH 9.7. During this mixing additional amounts of base were introduced by means of an automatic titrator to maintain the pH unchanged. A further step in purification and stabilization consisted of removal of the excess electrolyte from the environment. This was accomplished by centrifugation of sols, to which base was added, in a laboratory centrifuge at 3000 rpm and subsequent dispersion in distilled water. The repeptized sols showed intense HOTS and remained suspended over a broad pH range (acidic and basic) except near the zero point of charge (ZPC ~ 9.3). Flocculated sols which settled out, could readily be redispersed by adding dilute acid or base (particularly with the aid of ultrasonication) indicating that no permanent particle bridging took place in the aggregates. Electron microscopy showed that in purified sols the particles remained spherical. If repeptized, purified sols were reheated a significant change in particle shape took place. This is illustrated in Fig. l(c, d) which was obtained from a sol which was aged for 24 hr at 98°C following the purification procedure. It is evident that a considerable fraction of particles lost their spherical habit and acquired a variety of crystalline forms. At present, it is not known if reheating for very extended periods of time would result in complete recrystallization of all sol particles.

3. Chemical composition of the sol particles The analytical data on the composition of sol particles are given in Table 1, which shows the molar ratios of aluminum to sulfate ions in the solid phase as well as the fraction of the total aluminum concentration which precipitated in course of the sol formation. It is obvious that the original sols contained considerable quantities of sulfate ions ([A13+]/[SO24-] = 2-9 -i- 0.6). Significantly, the particles in sols which had been treated with base and deionized were essentially free of sulfate. This will be discussed in more detail later on.

3697

Aluminum hydrous oxide sols--I Table 1. Chemical composition of aluminum hydrous oxide sols of narrow size distribution Conc. of aged A12(SO4) 3 solution

(expressed in molarity of AI)

Period of aging (days)

A1 precipitated ( ~ of the total)

[AI3+]/[SO~ + ]

10 -3 10 -3 10 -3 10 -3 10 -3 10 -3

6 6 4 2 1 1

12 12 25 13

3.2 2-2 3.0 2.9 2.5 3.5

1 x 10 -3 1 x 10 -3 1 x 10 -3 1 x 10 -3

4 4 4 1

29

3.3

33 24

3.5 3.5 2"5

in solid

(A) Original sols 2 2 2 2 2 2

x x × x x x

* *

*

(B) Sols treated with base to p H 9.7 and electrolytes removed 2 × 10 -3 1 * 2 x 10 -3 1 * 2 x 10 -3 1 *

>>.100 >100 >100

* Not analyzed.

Only a fraction of the total aluminum in the original solutions is used up on the formation of hydrous oxide particles. It would seem that this fraction is higher when the concentration of the aged aluminum sulfate solution is lower; this is possibly due to the change in pH with dilution. In several experiments it was not possible to account for the total aluminum content by adding the quantities of aluminum determined in the solid particle phase and in the electrolyte environment. The difference amounts to ~ 10 per cent and could have been lost to the walls of the reaction vessel[lO]. DETERMINATION

OF PARTICLE SIZE DISTRIBUTION

BY L I G H T

SCATTERING

1. Method

Electron microscopy showed that the particles were spherical, sufficiently uniform and in the proper size range to make these sols available to particle size distribution analysis in situ by light scattering using the Mie theory. This was further substantiated by the fact that the sols exhibited brilliant higher order Tyndall spectra (HOTS). Therefore, the polarization ratio method was employed to obtain the particle size distribution of aluminum hydrous oxide sols. This method was described earlier[11, 12], and it has been applied successfully to a variety of colloidal disper10. L. Gordon, M. L. Salutsky and H. H. W i l l a r d , Precipitationfrom Homogeneous Solutions, pp. 12-15. Wiley, New Y o r k (1959). 11. M. Kerker, E. Matijevir, W. F. Espenscheid, W. A. Farone and S. Kitani, J. Colloid Sci. 19, 213 (1964). 12. M. Kerker,

The Scattering of Light. Academic Press, New

Y o r k (1969).

3698

ROGER BRACE and EGON MATIJEVI(~

sions[1, 2, 12-18]. The procedure consists in measuring the scattered intensities of the vertically and horizontally polarized light at various angles using a monochromatic incident beam (i.e. of wavelengths 2 = 436 or 546 nm in oacuo). For a sol of frequency distribution p(ot), the Rayleigh ratios (Vo,v and Ho,h) of the radiant scattered intensity for unit solid angle from an incident beam of unit irradiance are represented by Vo.~ = N(2/2~z)2

io o (il)op(~t) de(

(1)

[to, h = g ( 2 / 2 n ) 2

(i2)op(ot) dot.

(2)

The subscripts v and h indicate the polarization of the incident radiation, Vo and Ho are the scattered intensities of the vertically and horizontally polarized components at an angle 0, N is the number of particles per unit volume, 2 is the wavelength in the medium, (il)o and (i2)0 are the Mie angular intensity functions, and ot = nD/2 is the size parameter in which D = particle diameter. The polarization ratio, Po, is given by S (i2)oP(ot) dot Po = S (il)oP(ot) dot"

(3)

A computer program was employed to compare the polarization ratios at different angles of observation (Po) with a large number of theoretical values corresponding to various distributions characterized by the modal size parameter otM = nDu/2 (DM = modal particle diameter) and the distribution width parameter ao in the so called zeroth order logarithmic distribution" p(ot) = exp [ - ( l n ot'- In

GtM)2/2tT02]

2X//2~--pp0otMexp (a02/2)

(4)

The number concentration (N) can also be obtained from Eqns (1) and (2). The method requires but a simple measurement of the Rayleigh ratio and a knowledge of the size distribution parameters derived from any method. In the present case these were obtained from the polarization ratios. Despite the fact that the sols met the requirements necessary to successfully apply the polarization ratio method to size distribution analysis, early attempts failed to produce satisfactory results. Either no fits between the measured and calculated data could be achieved or the distributions obtained by light scattering disagreed considerably with the electron microscopic histograms. Eventually, it was established that the cause of the difficulties was due to two factors. Firstly, the 13. 14. 15. 16. 17. 18.

E. Matijevir, S. Kitani and M. Kerker, J. Colloid Sci. 19, 223 (1964). W. F. Espenscheid, E. Matijevi6 and M. Kerker, J. Phys. Chem. 68; 2831 (1964). R. T. Jacobsen, M. Kerker and E. Matijevir, J. Phys. Chem. 71,514 (1967). R. L. Rowell, J. P. Kratohvil and M. Kerker, J. Colloid Interface Sci. 27, 501 (1968). Chao-Ming Huang, M. Kerker, E. Matijevi6 and D. D. Cooke, J. Colloidlnterface Sci. 33, 244 (1970). T. P. Wallace and J. P. Kratohvil, J. Polymer Sci. Part A-2, 10, 631 (1972).

3699

Aluminum hydrous oxide soIs--

scattering. characteristics of the aluminum hydrous oxide sols are such to show extraordinarily strong dissymmetries. This made it necessary to consider carefully the reflections in the light scattering cell. To eliminate these, one can either blacken portions of the cell or carry out numerical corrections. Blackening of the cell has the disadvantage that it prevents direct measurements of absolute scattering intensities. Numerical corrections can be made as described in detail by Kratohvil[l9]. Both methods were applied in this work to alleviate reflection effects. Figure 4 gives an example of measured Rayleigh ratios (V,,, and H,,,) in a clear cell (circles) and in a blackened cell (squares). The solid curves are the theoretically calculated intensity functions which best fit the scattering data for the sol in question. The high dissymmetries are readily observed if one recognizes that the ordinate is on a logarithmic scale. The improvement in the agreement between the calculated and measured intensities due to elimination of reflections in the blackened cell is apparent, particularly for H,,, at large angles. Numerical corrections for data obtained in the clear cell proved in this case to be inadequate. Despite all precautions, the nature of the scattering properties of these sols were such that in some cases at angles > 115” the results were not reliable. Thus, all the calculations for the puvose of size distribution determination were carried out with angular intensities measured over the range 45” (5’) 115”.

I

I 40

60

53

I

100

I 120

I

140

/

9'

Fig. 4. Light scattering 5 x 10m4F solution of measured in a clear cell (0, W) are compared

data for an aluminum hydrous oxide sol prepared by aging a AI,(SO,), at 98°C for 5 days. The Rayleigh ratios, b,, and f-f,,,, (0, l ), and in a cell with blackened back surface and exit window with corresponding theoretical curves (A = V,,,., B = HB.h)which best fit the experimental data.

19. J. P. Kratohvil, J. Colloid Interface Sci. 21, 498 (1966).

3700

ROGER BRACE and EGON MATIJEVI(~

The second difficulty was encountered in determining the correct refractive index (n) of the aluminum hydrous oxide particles. The value of n ~ 1"58 available in the literature refers to crystalline aluminum hydroxides (i.e. Gibbsite or Bayerite) and when applied to sols used in this work never produced a satisfactory fit between calculated and measured light scattering data. It was concluded, therefore, that the sols prepared as described had different optical properties and the correct refractive index had to be established. This is possible to accomplish by light scattering as shown elsewhere[15]. In practice, experimental polarization ratios were compared with calculated theoretical functions for a range of refractive indices and for systematically varied distributions characterized by a range of atu and tro values. It was found that all sols of sufficiently narrow size distribution prepared at the low pH could be analyzed by light scattering if a relative refractive index of m = 1.125 + 0.005 was employed in the calculation of the theoretical Mie functions. This uncertainty in m does not affect the particle size distribution analysis by light scattering[20]. Figure 5 gives an example of the method employed and the results obtained. Bottom diagram on the left compares the experimental angular dependence of the polarization ratios (circles) with the theoretically calculated p--0 curve which best fits these data. The corresponding contour error map is shown above this diagram. 0'15

i

i

i

m=1.125, aM= 5.30 h-436nm,~=O.09 0-20 0.25

0-~

30

0.10 -

5 0.05

~ 5"5

5"0

I0

6.0

GtM o

f

40

'

'

60

80

~ 20 '

O

I00

120

0.4

o.5 Particle

0.6 dia.,

o.7 /zm

Fig. 5. Example of the size distribution analysis of an aluminum hydrous oxide sol prepared by aging of a 1 × 1 0 - 3 F solution of A12(SO4)~ at 98°C for 10hr. Lower left: A comparison of experimental p(O) values at 436 n m (O), with the theoretical curve which best fit the data. Upper left : Contour error map for comparison of the experimental values of the polarization ratio with theoretical values for various distributions of the same sol as below. A unique fit is found at ctM = 5.3 and cro = 0.09. The contour levels (A) correspond to the r.m.s, deviation of the experimental polarization ratios for the theoretical values. Right: Comparison of the histogram obtained by the electron microscopy (160 particles counted) of the same sol with the distribution from light scattering at ;t = 436 nm, ~tu = 5.3, and ~ro = 0.09. 20. M. Kerker, E. D a b y , G. L. Cohen, J. P. Kratohvil and E. Matijevi6, J. Phys. Chem. 67, 2105 (1963).

Aluminum hydrous oxide s o l s - - I

3701

The contour lines in the au--a0 domain represent equal r.m.s, deviations, A, between experiment and calculation[l 1]. A unique pair of size distribution parameters (0~M = 5.30 and ao = 0.09) results from this analysis. Using these values the actual size distribution is given as solid curve on the right and compared with a histogram obtained by sizing particles from electron micrography of the same sol. The agreement is as reasonable as one would expect from this type of work.

2. Applications The characteristics of several aluminum hydrous oxide sols in terms of their corresponding size distribution parameters are listed in Table 2. Columns 3 and 4 Table 2. Particle size distribution parameters for aluminum hydrous oxide sols

Hours of aging

M

at 98°C

aM

ao

c~M

a0

1 × 10 -3

30 24 22 29 25

3.4 4.65 5.5 5.9 8.0

0.14 0.13 0-15 0.09 0.10

3.1 4.35 4.3 5.3 7.5

0.15 0.13 0.17 0.11 0.11

1 2 2 2

x x × x

10 -3 10 -3 10 -3 10 -3

Original sols

Deionized sols at pH 9-7

[AI3+] *

* In all these experiments A12(SO4)3 stock solutions were used; the concentration is weighed-in molarity of the salt in terms of aluminum ions.

give this information for the sols as originally obtained by heating aluminum sulfate solutions without further treatment after the termination of the aging process. Several observations are in order. Although the aging of the solutions almost invariably resulted in sols of narrow size distribution, exhibiting HOTS, the modal diameter of the sol particles obtained from aged solutions of the same concentration varied considerably. It would seem that the nucleation stage is very sensitive to small changes in experimental conditions. This aspect has not been investigated in detail at this stage of the work. However, there is evidence that the walls of the containers (and their pretreatments) may have an effect upon the particle size. Furthermore, small variations in pH and even more, somewhat different rates of heating may affect the reproducibility of the modal diameters. Data in Table 2 indicate that the average particle size becomes larger with increasing sulfate concentration, as was earlier shown by electron microscopy (Fig. 3). Similar behavior was observed in the case of chromium :hydrous oxide sols of narrow size distribution[ 1, 21 ]. Purification by addition of base and subsequent deionization brought about a definite charge in the optical properties of the sols. Rather comprehensive light scattering studies showed that these changes were due to two reasons, i.e. to a decrease in particle size and to a lowering of the refractive index. The last two columns of Table 2 give the ~u and a o values of deionized sols at pH 9.7 which, before puri21. E. Matijevir, A. Bell, R. Brace, P. McFadyen, J. electrochem. Soc. In press. Also, In Proc. Syrup. on Oxide Electrolyte Interfaces, pp. 45-64. The Electrochem. Soc. (1973).

3702

ROGER BRACEand EGON MATIJEVI(~

fication, had size distribution parameters as shown in columns 3 and 4. In order to obtain fits between the experimental and calculated light scattering data, the refractive index had to be changed. Unlike the original sols, which all showed the same relative refractive index, the purified systems required the use of m values ranging between 1.110 and 1.125. Although the variations are reasonably small there seems little doubt that the purification process causes a decrease in the refractive index of aluminum hydrous oxide particles. The results of the chemical analyses described earlier showed,that the "purification" process caused a removal of sulfate ions from the sol particles. Sulfate is known to act as the "penetrating" ion in the case of basic aluminum species[22]. Obviously the addition of base causes an exchange of the sulfate ions incorporated in the particles during the sol formation with the hydroxyl ions from the electrt~lyte environment. This exchange apparently is the reason for the particle "shrinkage" and the lowering of the refractive index. It is a rather remarkable fact that such an exchange process can be followed so closely by light scattering as a result of the changes in the optical properties of the sols.

3. Particle number concentration

Equations (1) and (2) show that number concentration N can also be obtained from absolute intensities of the scattered light once the size distribution of the dispersed system is established. The latter permits the calculation of the integral in Eqns (1) and (2) which 'in turn will give the value N. The number concentration can be computed for each angle and the values averaged out, but this procedure is subject to significant deviations at some angles. Alternatively, a graphical procedure may be employed which consists in finding a single value of N which best matches the experimental results with the calculated theoretical curves of absolute intensities of the horizontally and vertically polarized light as a function of the scattering angle for a known particle size distribution. Using this procedure the number concentration of the sol described in Fig. 5, N = 3 x 107 ml. ELECTROKINETIC PHENOMENA Electrophoretic mobilities were used to establish the charge on the aluminum hydrous oxide particles as a function of the method of sol preparation and of pH. For this purpose the mobility vs pH curves were determined for: (a) the sol particles as formed in the presence of the mother liquor, (b) sols centrifuged and resuspended in aqueous N a O H solutions of various concentrations to give a range of pH values, and (c) sols to which N a O H was added first to adjust pH to 9.7 followed by centrifugation and resuspension in solutions of dilute HNOa or N a O H to adjust the pH over a broad range. The results are shown in Fig. 6. The triangle gives the mobility of the original sol, whereas squares and circles designate sols treated as described under (b) and (c), respectively. 22. E. Matijevi~and L. J. Stryker, J. Colloid Interface Sci. 22, 68 (1966).

3703

Aluminum hydrous oxide sols--I 4

5

!

2 E u

I > o.

o

0

E

=L

.o

-I

-2 Alu0r~'i~n~lhydr0us oxide sols -5

-4

D 0eionized 0 Treafed with baseand deionized 3

5

7

9

,

II

pH

Fig. 6. The electrophoretic inabilities of aluminum hydrous oxide sols prepared by aging a 1 x 10-3 F solution of A12(SO4)3 at 98°C for 24 hr and treated as follows: A, original sol at room temperature; I-q, sol freed of the original electrolyte by centrifugation and redispersed in dilute NaOH solutions; O, pH of the sol adjusted to 9-7 by NaOH, followed by deionization, and redispersion of various pH va.lues.

Sols deionized at low pH (b) have a zero point of charge (ZPC) at pH 7.0 whereas the sols deionized at high pH (c) have a ZPC at pH 9.3. The latter value is in agreement with the "best values" for hydrous aluminum oxide as reviewed by Parks[23]. A lowering of ZPC due to modification of the surface of aluminum hydroxides in the presence of sulfate ions was observed by Modi and Fuerstenau[24]. Indeed the chemical analyses of the particles used in this work showed that the sols purified at high pH (9.7) were essentially sulfate free, whereas the sols originally prepared contained appreciable amounts of sulfate ions. Thus, the differences in mobility curves are obviously caused by the different content of sol particles in sulfate ions. Prior to any purification or pH adjustments of the sols, the particles exhibit a positive charge. This explains their adsorptivity on glass walls of the container. The charge of redispersed sols depends on the pH, and so does their colloid stability. As a rule they flocculate readily at pH values in the vicinity of ZPC and remain dispersed when the pH is sufficiently different from the ZPC to make particles either positively or negatively charged. However, if the pH is made too low (< ~ 3) the sols will redissolve, although the rate of dissolution may vary depending on the method of the sol preparation. 23. G. A. Parks, Chem. Rev. 65, 177 (1965). 24. H. J. Modi and D. W. Fuerstenau, J. phys. Chem. 61,640 (1957).

3704

ROGER BRACE and EGON MATIJEVI(~ DISCUSSION

When chromium hydrous oxide sols consisting of spherical particles of narrow size distributions were first prepared it was suggested that coordinated basic sulfate or phosphate complexes acted as precursors to embryo formation. It was reasonable to expect that one should be able to obtain such sols with ions of other metals which are capable of forming similar complexes. The fact that aluminum salt solutions containing sulfate ions upon aging indeed yield "monodispersed" sols indicates that the original assumptions have been correct. Ample evidence is available that aluminum ions readily hydrolyze on aging[25] and that sulfate ions tend to coordinate with hydroxylated aluminum ions to form complex species[22]. In addition, under certain conditions, the aluminum basic sulfates are less soluble than either the aluminum hydroxide or aluminum sulfate alone[26]. Thus, the mechanism of the formation of aluminum hydrous oxide sols by the procedure described in this work seems to consist in the formation of basic aluminum sulfate during the aging of aluminum salt solutions at elevated temperatures. These species then precipitate as nuclei upon which condense the hydrolyzed sulfated products which are continuously formed while the solutions are being heated. There are several notable differences between the chromium and the aluminum systems studied. Firstly, to produce aluminum hydrous oxide sols, solutions have to be aged at temperatures higher than for the corresponding chromium sols. Unless treated, the aluminum hydrous oxide particles redissolve upon cooling whereas this is not the case with chromium hydrous oxides, indicating more strongly bonded complexes in the latter case. This is further substantiated by the fact that sulfate ions can readily be removed from the aluminum hydrous oxide particles by simply adding base to the sol. As a result particle diameters are reduced. Contrary to this, the size distribution of chromium hydrous oxide sols remains unchanged upon addition of base and subsequent deionization[2]. It should be pointed out, however, that the content of sulfate in chromium hydrous oxide particles was found to be substantially less[27] than in corresponding aluminum sols. Thus, the role of sulfate ions may be different in these systems. Whereas the nucleation in both cases seems to be due to metal basic sulfate formation, in chromium hydrous oxide sols, the sulfate ions apparently play an additional role. They may act as bridges between hydrolyzed polynuclear species and thus they become tightly bound and difficult to remove. No such bridging is taking place in the case of aluminum hydrous oxides making the sulfate ion easily exchangeable. Two findings merit special consideration: firstly, "monodispersed" sols form only in solutions of rather well defined compositions in regard to [A13÷] to ESO2- ] ratio, and secondly, the particles originally formed have reasonably constant chemical composition. This would further indicate that one or more well defined aluminum basic sulfate complexes act as precursors in the solid phase formation. The complex of the composition AI4(OH)lo(SO4) has been suggested repeatedly[26, 28]. In addi25. E. Matijevi6, G. E. Janauer and M. Kerker, J. Colloid Sci. 19, 333 (1964). 26. S. S. Singh, Can. J. SoilSci. 49, 383 (1969). 27. E. Matijevi6 and A. Bell, Particle Growth in Suspensions, pp. 179-193. Proe. Soc. Chem. Ind., London (1973). 28. C. Brosset, G. Biedermann and L. G. Sill6n, Acta chem. scand. 8, 1917 (1954).

Aluminum hydrous oxide sols--I

3705

tion, Willard[10] has indicated that the molar ratio AI/SO4 in basic aluminum sulfates approaches 4 as the pH is decreased. This ratio is in qualitative agreement with the analyses given in Table 1. It is noteworthy that Willard also found that basic aluminum sulfate precipitates lose almost the entire amount of sulfate upon digestion of the solid at pH 9-4. Our findings are in excellent agreement with the above observation. The fact that aluminum hydroxide sols of narrow size distribution can now be readily prepared offers the opportunity for a detailed study of the mechanisms of metal hydroxide formation in general and that of aluminum hydroxide in particular. Such study is now in progress. Acknowledgements--One of us (R.B.) is greatly indebted to Unilever Research Laboratory, Port Sunlight for a leave of absence. The authors are pleased to acknowledge the assistance of Professor J. P. Kratohvil in some aspects of the light scattering analysis.