Understanding and controlling the crystal morphology of some ionic crystals

Powder

219

Technology, 65 (1991) 219-225

Understanding and controlling the crystal morphology some ionic crystals

of

P. Meenan, K. J. Roberts*, J. N. Sherwood Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow Gl 1XL (U.K.)

and K. R. Yuregir Unilever Research Laboratory, Port Sunlight, Quarry Road East, Bebington, Merseyside L63 3JW (U.K.)

Abstract Ccimputational techniques based on crystal structural information are described for predicting the equilibrium external morphology of particulate ionic solids. An understanding of the variation of surface chemistry between the various predicted crystal faces is outlined, by which a qualitative assessment of the effects of habit modification can be made. These approaches are applied to some ionic systems taken from carbonates, sulphates and phosphates, with reference to industrial applications as porous and high surface area crystalline builders.

Introduction Crystallisation and crystal morphology play a significant role in many industrial separation systems. Poorly defined crystal morphology can have serious detrimental effects on an industrial process: - Crystal morphology can reduce the efficiency of the separation of solid from mother liquor or washing processes, e.g., ‘plate-like’ crystals can block filtration processes. - Whenever a product is part of a formulation, the morphology is often crucial in altering the rheology of the formulation.

In the case of post-production, the potential enduser of a product may experience problems associated with crystal morphology: - Large crystals may adhere (caking) together, thus creating packing/storage problems. - Small crystals may disintegrate easily to dust particles, which may present a toxicity hazard. Habit modification techniques can be employed to alter crystal morphology where the growth morphology is not suitable. The ability to predict and control crystal morphology provides

*Also at SERC Daresbury Laboratory, Warrington WA4 4AD (U.K.)

0032-5910/91/$3.50

an important and increasingly essential tool in particle technology. This paper will look at some models for predicting equilibrium morphology and for assessing likely habit modification effects. The models will be applied in particular to some inorganic salts used as builders in detergents. The salts investigated are sodium tripolyphosphate hexahydrate and various compounds from the sodium carbonate/sulphate phase diagrams. Very little work has been reported on the observed morphology of these materials, necessitating the need for accurate morphology prediction models. These high surface area materials boost detergency by being able to hold the surfactants within a porous matrix structure. Simulations of equilibrium morphology using lattice geometry models

A crystal consists of an ordered solid array of atoms or molecules exhibiting long-range order in three dimensions. Hauy’s classic illustration [l] showed that by stacking/ordering cubes, various basic crystal morphologies could be obtained. These cubes or unit cells are the primary building blocks of a crystal. Gibbs [2] related the crystal morphology to energetic considerations, stating

0

Elsevier Sequoia/Printed in The Netherlands

220

Polyhedral

\

form

A

Nucleation

/

centre

‘Cusp’ minimum in surface energy

Fig. 1. Representation of the classical Wulff plot for the derivation of equilibrium form.

Sodium tripolyphosphate

hexahydrate

Sodium tripolyphosphate hexahydrate (Na5P,0,, - 6Hz0, hereinafter referred to as STP) crystallises with a triclinic crystal structure (spacegroup Pi) with unit cell parameters: a = 10.37A b = 9.224 A c = 9.455 A, a = 92.24” /I = 94.55” 0 = 90.87”, Z = 2 [lo]. The morphological importance based on the BFDH laws is summarised in Table 1 and the resultant morphology simulated in Fig. 2. It can be seen that the main face is a {lOO} type, which is of slightly more importance than the (010) forms. The effect of these forms predominating in the morphology causes the overall morphology to resemble a thick faceted plate, almost ‘block-like’ in nature. The {lOI}, (110) and (Oli> forms are of less importance, forming ‘edge’ facets on the morphology. This prediction is in good agreement with the observed morphology* reported by Troost [ 111. TABLE 1 BFDH analysis of sodium tripolyphosphate

hexahydrate

Form

that the equilibrium crystal morphology was one in which the total free energy was minimised. Wulff [3] defined the relationship between surface energy and equilibrium morphology. A Wulff plot is a 3-D polar representation of the surface energy, which reveals (see Fig. 1) several welldefined minima which correspond to the expected low surface energy directions for crystal growth. Constructing planes tangent to the cusp minimum for each growth vector, a regular polyhedron typical for a freely grown crystal results. Such plots can be prepared using the gnomonic projection [4] and several computer programs (e.g., [5]) are now available to draw these automatically. The Bravais/Friedel/Donnay/Harker (BFDH) rules [6,7] enable a measure of the relative growth along a specific crystal plane (hkl) to be related to the internal structure of a crystal using lattice geometry considerations. The BFDH law can be summarised: “The greater the interplanar spacing, the greater the importance of the corresponding crystal form (hkl), after allowance has been made for translational crystal symmetry elements such as Bravais lattice centring, glide planes and screw axes”, This model is -outlined in’ more detail in a recent review [8] and calculations using it can be made using the computer program MORANG [9]. Using the BFDH law, a morphological index can be defined as the reciprocal of the interplanar spacing (l/d,& and employed as an alternative to surface energy in the Wulff plot.

h

k

I

(4

1 0

0

0

1

0 :,

1 0 I

P 1 0 1 0

P T 1

01

1

:

0.096 15 0.101 64 0.131 80 0.139 03 0.141 61 0.157 17 0.163 19 0.169 66 0.169 69 0.185 23

0 1

roii)

A-------If\

-rnnr\

l-7

u

L0101

--

ciior

-----I

II II II II

\

‘\’

(loi)

(111)

Fig. 2. Predicted morphology of Na,P,O,,(H,O), the BFDH laws.

based on

*Troost employed a different axial system, interchanging the a and c crystallographic axes; thus, the {OOl}and {100) forms are equivalent to the predicted {lOO) and {OOl} forms respectively.

221

Nq CO,, 2 Na2 SO4 ----

Fig. 3. Phase diagram of the Na,CO,-Na,SO,-H,O

system.

Carbonate Jsulphate systems

Alternatives to sodium tripolyphosphate hexahydrate as a detergent builder have been extensively researched. It has been found that various carbonate/sulphate salts have exhibited comparable performances as builders for detergent applications. The phase diagram [ 121 for the Na,CO,Na,SO,-H,O system is shown in Fig. 3 and reveals the following phases: sodium carbonate monohydrate, Na, CO * H,&[ 131 sodium carbonate heptahydrate, Na,CO * 7H,O [14] - sodium carbonate decahydrate, Na,CO * lOH,O [15] - sodium sulphate Na*SO, [ 161 - sodium sulphate decahydrate Na,C03 * lOH,O [17] - Burkeite Na, CO3 (Na, SO)* [ 181 Crystallographic data for each of the phases are summarised in Table 2. The predicted morphologies for Na,CO, * H,O, (Na,CO, * 7H,O) and Na,CO, * IOH, are shown in Figs. 4 (a)-(c), respectively. It can be TABLE 2 Crystallographic data for phases present in Na,CO,-Na,SO,-H,O Phase

Unit cell dimensions a = 6.412, b = 10.724, c = 5.259

H,O

P2,ab

Na,CO,

7H,O

Pbca

Na,CO,

lo,0

cc

Na,SO

Fddd lOH,O

system

Space group

Na,CO,

Na,SO,

seen that the main face for the orthorhombic carbonates Na,C03 . H,O and (Na,CO, . 7H20) is (020) with both crystals having an essentially prismatic habit. The observed morphologies of Na,CO, . H,O [ 191 and Na,CO, . 7H20 [20] have been reported and show good correspondence to those predicted. Whilst the predicted morphology of the monoclinic Na,CO, . lOH,O is also prismatic, the distribution of forms is distinctly different with {lli} forms predominating over {liO} forms, in good agreement with the observed morphology [21]. Comparing the morphologies of Na,SO, (Fig. 4(e)) with Na,SO,. lOH,O (Fig. 4(f)), major differences can be observed. In the case of Na*SO,, the main forms are of {Iii} type, the morphology resembling a rhombic bipyramid, whereas in Na,SO, * lOH,O the morphology resembles a faceted ‘plate-like’ morphology, with (100) as the predominant form. The observed morphologies of Na, SO4 and Na,SO, . lOH,O [22,23] were found to be in good agreement with the predicted morphologies. In both the carbonates and sulphates, the complexity of the resultant morphology mirrors the complexity of the crystal structure. For the hydrates, the symmetry tends to be low as the structural packing in the unit cell has to accommodate the water molecules. Structures with the higher hydration levels tend to crystallise in monoclinic rather than orthorhombic structures. Burkeite [ 181-Na,CO, (Na, SO,), is the name given to the mineral discovered in 1935 by Foshag [24], with the formula Na2C0, * (Na,SO,),. It is a comparatively rare material, occurring mainly in natural evaporite situations. Recent work [ 181 has indicated that the stoichiometry of this compound can be variable and that the more general formulae of Na,SO,(CO,),(SO,)(, _ T) would be more appropriate for this material. Burkeite crystallises in an orthorhombic structure (space group Pmnm) and cell parameters of a = 5.17 A b = 9.217 A c = 7.05814 8, [ 181. How-

P2,ic

a=jj=r=9()” a = 14.492, b = 19.490, c = 7.017 a=b=y=90 u = 12.83, b = 9.026, c = 13.44 a = 90” /J = 123.0” y = 90” a = 5.859 6, b = 12.304 4, c = 9.817 a=B=v=9l)” a = 11.512, b = 10.370, c = 12.847 a = 90” B = 107.789” y = 90

1020)

ClZO,

(4

(4

(iii)

(511)

(2001

(910,

(100)

tzio)

(Olil

(4

IO101’

(200)

(cl

(0

Fig. 4. Predicted morphology of some compounds from the Na,CO,-Na,SO,-H,O Na,CO, H,O; (b), Na,CO, 7H,O; (c), Na,CO, . lOH,O;(d), Na,SO,; (e), Na,SO,

ever, the crystal structure shows substantial positional disorder, which in turn can give rise to structural modulations associated with a larger unit cell. Giuseppetti et al. [ 181 have classified 2X and 6X superstructures (see Table 3) in Burkeite with the variation in superstructure apparently

system using the BFDH

law. (4,

lOH,O;(0, Na,CO,(Na,SO,),.

being due to carbonate content. The equilibrium morphology simulated on the basis of the subcell is shown in Fig. 4(f) and it can be seen that the (010) forms predominate in the morphology, with the {lOi} forms giving distinct comer facets. The overall predicted morphology could be described

223

TABLE 3 Supercell formation

of Burkeite

A:6X subcell Pbnm

B:6X subcell Pbnm

C:2X subcell Pbnm

a = b = c = z=

a b c z

a = 5.195A b = 18.5118, c = 7.08SA z=4

5.llA 18.433A 21.173A 12

= 5.17lA = 18.447A = 21.199A = 12

as being ‘plate-like’ with distinct side and corner facets. This morphology sharply contrasts with that of Na,CO, * 7H,O and Na,CO, . H,O. The observed morphology has been measured from mineral samples and has been described as tabular, with the {lOO} forms predominating with {OlO}, {1lo} and (001) also present [24]. This differs from the predicted morphology, but may not be the equilibrium morphology due to growth in non-ideal conditions.\A detailed study is being undertaken to examine the growth of synthetic Burkeite crystals, from which we hope to provide better and more realistic experimental morphological data.

‘lattice match’ the additive to the crystal surface is not so strict. Polymeric additives are especially attractive for product modification, due to their ease of formulation and dispersion. Polymers containing at least three carboxylate groups, such as polyacrylates, have been found to have habit-modifying properties on detergent builders. Such a habitmodification process is shown schematically in Fig. 5, which shows the COO- groups in the polyacrylate to be coulombically attracted to the cation-rich surfaces, rather than those which have a mixed ionic environment. This face specific adsorption changes the morphology (see insert, Fig. 5) from ‘plate-like’ to ‘needle-like’. The selection of a suitable habit modifier can be greatly aided by examining the crystallographic structure of the material with the aid of molecular packing projections down the principal crystal axes using computer programs such as ORTEP [25]. In this way, we can examine the surface chemistry of the crystal faces predicted from the

r Assessing

likely habit modification

effects

The materials that have been examined have shown a variety of crystal morphologies, ranging from ‘block-like’ to largely prismatic in nature. However, dendritic morphologies are preferable for detergent builder materials, as these crystallise in a highly porous matrix which allow for the uptake of the various surfactants. Hence, habit modifiers are needed in product formulations to produce dendritic or needle-like crystal morphology to encourage the agglomeration of crystallites. Habit modifiers are tailor-made additives which adsorb on specific crystal faces. Mechanistically, it is thought that this surface adsorption blocks surface terraces and steps, thus reducing the growth rate of the modified face. As the crystal morphology is dominated by the slowestgrowing faces, a habit modifier can thus change the polyhedral shape of a crystal by slowing and hence enlarging speczjic crystal faces. The electrostatic interaction between modifier and substrate effectively determines whether the additive will adsorb. As a close topotactic relationship between adsorbate and substrate is needed for adsorption, ionic crystal surfaces which exhibit a mixed cation/anion environment are hence difficult to modify. Thus, modification usually takes place by adsorption onto crystal faces which are either anion or cation rich, for which the requirement to

I

L-l

Unmodified form

R-~-~--<-~-+-R

Habit B modification

Modified form

Attraction ,

Cation rich surface

‘Mixed caton /anion surface

Fig. 5. Schematic showing the interaction of a habit-modifying polyacrylate with a crystal surface; the insert illustrates the effect on morphology.

224

BFDH law and consider the chemical nature of the required additive. Sodium tripolyphosphate

hexahydrate

Figure 6 shows a molecular packing projection down the c crystallographic axis with the BFDH predicted crystal morphology superimposed. It can be seen that the surfaces chemistries of {lOO} and (010) faces are quite distinct. - On the (010) surfaces PO3 groups of the anions lie parallel to the surface and thus this growth surface comprises alternating layers of anions followed by cations and water molecules. - The {lOO} surfaces in contrast reveal both ionic types in the surface. This charge difference between the two forms offers the possibility of habit modification. Polyacrylates have been found to modify STP and one can presume that the COO- groups in a polyacrylate will be coulombically attracted to the cationrich (010) forms, rather than the (100) forms, which display a mixed ionic environment. Growth inhibition on the (010) forms will result in the formation of needle-like morphology with the needle axis presumably along the {lOO} growth normal. Sodium carbonate

monohydrate

The molecular packing of Na,CO, - H,O projected down onto the crystallographic a-axis is shown in Fig. 7 overlaid with its BFDH morphology. From this figure, the distinct surface chemistry of the (020}, (011) and (001) forms is clear: - The (020) surface comprises alternative layers; the first comprising anions and cations fol-

0-c

O-O

ODD-Na O--H

Fig. 7. Crystal structure/morphology superposition of Na,CO, . H,O as viewed down the a-axis, thus illustrating the surface chemistries of the {OOI), {020} and (011) forms.

lowed by the remaining cations together with the water of crystallisation. - The (01 l} surface contains anions, cations and water molecules in a repeating structure. The (001) surface is similar to that of the {020) but with the separation of the two layers less well defined. Polyacrylates will also habit modify Na,CO, * H,O with the COO- groups on the modifier being more likely to bond to forms rich in cations, e.g., (001) and (2001, than the more structurally homogeneous (01 l}. The better segregated cation structure of the (020) surface contrasts with that of the (001) where the cations appear to be ‘buried’ in between the surface anions. Coulombic repulsion and problems associated with steric hindrance between the COO- and the C03’- anions seem to preclude adsorption of the modifier on or close to anionic lattice sites. As with the case of habitmodified STP, the adsorption can be expected to give rise to a needle-like morpholgy along the (020) plane normal. Conclusions

O-0

@-Na

0-P

O-H

Fig. 6. Crystal structure/morphology superposition of Na,P,O,,(H,O), viewed along the u-axis, thus illustrating the surface chemistry of the {OlO} and { 100) forms.

It has been shown that the use of simple rules based on crystal lattice geometry and symmetry can be used to predict the morphology of some ionic salts. This morphology overlaid onto projections of the crystal structure gives a clear indication of the surface chemistries of the crystal faces

225

and can provide a tool for assessing likely habit modifiers. Current work is directed towards quantitative models for predicting habit modifiers for ionic systems. Such models require careful calculations of crystal lattice energies [26] and hence surface attachment energies. In such calculations, the crystallographic structure, together with suitable intermolecular energy functions (allowing both isotropic and electrostatic interactions), are needed. Morphologies of organic materials have been routinely predicted this way with great success [8, 27,281. However, this type of calculation [29-3 l] presents difficulties in its extension to inorganic systems due to: - the predominance of long range forces in inorganic systems which are not as important in organic systems; - the lack of suitable intermolecular potentials for molecular ions such as phosphates, carbonates and sulphates and their interaction, often, with water of crystallisation. Acknowledgements

We gratefully acknowledge Dr. P. Malinson (Chemistry Department, University of Glasgow) for his help with the ORTEP projections. One of us (P.M.) acknowledges SERC and Unilever Research for the financial support of a CASE studentship. References 1 R. J. Hauy, J. Phys., 19 (1792) 366. 2 J. W. Gibbs, Trans. Acad. Connecticut Acad., 3 (1875); see also The Equilibrium of Heterogeneous Substances, Scientific Papers, Vol. 1 (1906) and Collected Works, Longmans-Green, New York, 1928.

3 G. Wulff, Z. Krist., 34 (1901) 499. 4 F. C. Phillips, An Introduction to Crystallography, Longmans Green, London, 3rd edn.. 1963. 5 E. Dowty, Amer. Miner., 65 (1980) 465. 6 A. Bravais, Etudes Crystallographiques, Paris (1913). 7 J. D. H. Donnay and D. Harker, Amer. Miner., 22 (1937) 463. 8 R. Docherty, G. Clydesdale, K. J. Roberts and P. Bennema, J. Phys. D: App. Phys., (1991) in press. 9 R. Docherty, K. J. Roberts and E. Dowty, Compufer Physics Communications, 51 (1988) 423. 10 D. M. Weinch, M. Jansen and R. Hoppe, Z. Anorg. Allg. Chem., 488 (1982) 80. 11 S. Troost, Ph.D. Thesis, Univ. Groningen (1969). 12 W. Caspari, J. Chem. Sot., 125(1924) 2381. 13 K. K. Wu and I. D. Brown, Acta Cryst., B31 (1975) 890. 14 C. Betzel, W. Saenger and D. Loewus, Acta Crysr., 838 (1982) 2802. 15 T. Taga, Acta Crysf., B25 (1969) 2656. 16 A. G. Nord, Acta Chemica Scandinavia, 27 (1973) 814. 17 H. A. Levy and G. C. Lisensky, Acfa Cryst., 834 (1978) 3502. 18 G. Giuseppetti, F. Mazzi and C. Tadini, Neues. Jahrb. Mineral., Monatsch 5 (1988) 203. 19 Ch. de Marignac, Annales des Mines, 12 (1857) 55. 20 W. Haidingir, Poggendorfi Ann. d. Phys., 52 (1825) 369. 21 A. L. 0. L. Des Cloizeaux, Manual De Mineralogie, 2 (1874) 168. 22 A. L. 0. L. Des Cloizeaux, Mem. Sav. Etrang. Acad. Paris, 18 (1867) 610. 23 C. F. Rammelsberg, Handb. D. Physical.-Chemischen Krystall., Leipzig I (1881) 393. 24 W. F. Foshag, Amer. Miner., 20 (1935) 50. 25 C. K. Johnson, Oak Ridge National Laboratory Report ORNL-3794 (2nd revision) (1976). 26 P. Hartman and W. G. Perdok, Acta Cryst., 8 (1955) 49. 27 R. Docherty and K. J. Roberts, J. Crystal Growth, 88 (1988) 159. 28 G. Clydesdale, R. Docherty and K. J. Roberts, Comp. Phys. Comm. ( 1991) in press. 29 C. R. A. Catlow and W. C. Mackrodt, Computer Simulafion of Solids, No. 166, Springer-Verlag, Berlin, 1982. 30 C. R. A. Catlow, Ann. Rev. Mater. Sci., 16 (1986) 517. 31 C. R. A. Catlow and A. N. Cormack, Int. Rev. Phys. Chem., 6 (1987) 227.