Using Crystallographic and Morphological Data for the Design and Selection of Additives as Stabilizers of Solid-Liquid Dispersions: The Case of Tin Dioxide Roger J. Davey* and Alison M. M. J. Rebello Colloids, Crystals and Interfaces Group, Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 3 187-189
Received January 31, 2001
ABSTRACT: We report the application of molecular recognition principles to the design and selection of dispersing agents for colloidal suspensions. As an illustrative example we have chosen aqueous dispersions of tin dioxide and demonstrate how the methodology yields a new and potent dispersant. The use of additive molecules (dispersants) as stabilizers for solid-liquid dispersions is widespread across a range of consumer products, including paints, pharmaceutical liquid and aerosol preparations, dispersable granules for agrochemical use, and foods. The active solid phases involved typically have sizes below a few micrometers and hence constitute colloidal suspensions. With increasing emphasis on nanosized materials this size range is likely to extend downward toward 0.01 µm with a concomitant increase in surface area and hence more stringent demands on dispersant efficacy. Classically a dispersant is selected on the basis of a number of “global factors”: viz., its solubility in the continuous liquid phase, its potential to adsorb strongly at high coverage on the particle surface, and its likely ability to confer stabilization of the dispersion through the introduction of charge or steric barriers to flocculation. The likely extent of surface adsorption is generally assessed by considering the polarity of the surface and judging whether the adsorbed moiety would best be polar or nonpolar.1 Thus, for example, block or grafted copolymers containing both hydrophilic and hydrophobic domains are typically used, as are anionic, cationic, and nonionic surfactants.2 Current selection criteria pay no regard to the detailed ionic or molecular packing and functionality of the adsorbate surfaces and, hence, make no attempt to utilize specific interactions between substrate and dispersant. In other fields, however, notably that of crystal growth and nucleation, the understanding of the principles governing molecular recognition and binding of additive molecules to growing crystal surfaces has received significant attention in recent years. The ways in which functionality, stereochemistry, and packing may be utilized in order to design additives as crystallization modifiers is now well understood.3-5 The objective of the current study has been to explore the extent to which such learning may be transferred to the design and selection of dispersant molecules and hence find application in the important area of colloid stabilization. Tin oxide (cassiterite) was chosen as a target material since it is commercially significant, may be prepared in nanoparticulate form, and, being an oxide, has wellunderstood surface chemistry in aqueous suspension.6 Colloidal dispersions were prepared by hydrolysis of acidic tin(IV) chloride (SnCl2‚5H2O) solutions, under reflux conditions, using the methodology of Ocana and Matijevic.7 The concentration of tin(IV) chloride was varied from 0.2 to 0.0005 mol L-1 and hydrochloric acid was added at 0.3 mol * To whom correspondence should be addressed. E-mail: r.j.davey@ umist.ac.uk.
Figure 1.
L-1, giving starting pHs in the range 1-2. The resulting oxide suspensions were washed and sonicated twice in water to remove unreacted materials and stored as aqueous dispersions. The sizes of individual crystallites in these dispersions were assessed from direct observation by transmission electron microscopy and from line broadening of their associated powder X-ray diffractograms. With increasing reactant concentration sizes fell from approximately 15 × 15 × 60 nm to 2.5 × 2.5 × 5.5 nm. Crystals were always elongated along the 4-fold c axis of the morphology so that the most important crystal surfaces bounding the particles were the symmetry-related {110} faces. The atomic arrangement of tin and oxygen in this surface was then used as the basis for selecting potential dispersants. Figure 1 shows such a {110} surface, constructed by terminating the known crystal structure,8 and it is clear that it contains two types of oxygen atomssthose which are simultaneously bound to two tin atoms and those bound to single tin atoms. For both, the smallest interoxygen separation is 0.3187 nm. In selecting molecules as dispersants, it was decided to search for molecules with the potential to replicate these oxygens and bind directly to the surface tin atoms. The criteria for selection was, first, functional in that it was restricted to molecules containing two nucleophilic oxygens capable of supplanting the surface oxygens and, second, stereochemical in that these oxygen atoms should be able to access the lattice spacing of 0.3187 nm. It was acknowledged that two carbonyl oxygens may be preferred since these would have lone pair electrons in sp2 orbitals having appropriate geometry to bond simul-
10.1021/cg010289b CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
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Figure 2. Effect of potential dispersants on the ζ potential of aqueous tin oxide dispersions.
taneously to a single surface tin atom, that conformational rigidity may be an advantage, and that aqueous solubility is essential. The search was made using the Quest functionality within the Cambridge Structural Database.9 Seven molecules meeting the structural criteria were found, two of which, malonic acid (CH2(COOH)2) and 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, C2O2(CH)2(OH)2), were chosen for their simplicity and commercial availability. From their crystal structures it is evident that in malonic acid the carbonyl oxygens can access a separation of 0.317 nm,10 while for squaric acid the hydroxyl oxygen separation is 0.317 nm and the carbonyl oxygen separation 0.326 nm.11 Squaric acid is a planar, rigid molecule, while malonic acid has significant flexibility, and both show acceptable water solubility. As dispersants it was anticipated that the combination of conformational rigidity and potential steric effects imposed by the cyclobutene ring would make squaric acid the better dispersant of the two. For completeness the behavior of these molecules was compared to that of a “classical” dispersant, sodium hexametaphosphate (Na8P6 O198-, Calgon), which is essentially a highly charged linear phosphate anion with an interoxygen separation of 0.499 nm. To assess the potential for these three dispersants to adsorb to tin oxide surfaces and stabilize dispersions, electrophoretic mobilities were measured with a DELSA 440 (Coulter Electronic) instrument. The dispersions used typically contained 0.2 mg of SnO2 mL-1 with 0.01 mol L-1 NaCl as the conducting medium and pH adjusted using 0.1 mol L-1 HCl or NaOH. The selected dispersants were added at the level of 0.025 mg mL-1 (10% on the weight of SnO2 in the sample). Typical data are shown in Figure 2, in which electrophoretic mobilities have been converted to ζ potentials.12 Mobilities were found to be independent of preparative conditions of SnO2 with the effect of pH typical for a metal oxide in an aqueous environment and with the isoelectric point (iep) at a pH of 4, in good agreement with previous work of Ocana and Matijevic.7 The addition of malonic acid to these dispersions has little effect at high pH but does reduce the ζ potential at low pH and slightly lowers the iep. Squaric acid and Calgon, on the other hand, have a significant effect on the ζ potential across the entire pH range, conferring negative charge on the particles, lowering the iep outside the measurable range of pH, and hence, stabilizing dispersions. Visual observations confirmed this stability, with dispersions prepared using squaric acid or Calgon showing virtually no tendency to settle. In aqueous environments the oxide surfaces will be hydrated and charged depending on pH according to the
Sn-OH + H+ S Sn-OH2+
(1)
Sn-OH + OH- S Sn-O- + H2O
(2)
Thus, at low pH the surface is positive, at high pH it is negative, and at the iep it is uncharged and dominated by the hydroxyl functionality. The binding of a dispersant may then be considered in terms of its ability to bind to the prevailing surface functionality or to exchange with surface hydroxyls, a process which releases OH- ions and is favored at low pH. The behavior of Calgon can be explained in terms of the nonspecific electrostatically driven adsorption of a highly negatively charged anion onto the positively charged protonated surfaces at low pH, imposing a negative charge and reducing the iep. The small effect seen at high pH is presumably due to displacement of remaining hydroxyl ions. Malonic acid has pKa values for its two hydroxyl protons of 2.85 and 5.70,13 indicating that at pH values above 6 this molecule is essentially present as a mixture of monoand dicarboxylate anions which are evidently unable to displace hydroxyl groups from the oxide surface. As the pH falls, the concentration of the singly ionized form increases and some replacement of surface hydroxyls occurs, restricting the extent to which surface protonation can occur by reaction 1 above. This explains the reduction in magnitude of the positive surface charge and the slight lowering of the iep. The relative ineffectiveness of malonic acid is consistent with its conformational flexibility, which may restrict its ability to form surface complexes, and its pKa values, which limit its charge state. Squaric acid has pKa values for its hydroxyl protons of 0.59 and 3.48,14 so that over the pH range 4-8 there is always a significant proportion of dianionic species present. Unlike the poly-charged Calgon anion, coordination of squaric acid to the surface must involve specific binding via its carbonyl oxygens, since this is the only way in which the deprotonated hydroxyl oxygens would be available to determine the surface charge. Thus, at low pH values this molecule competes more successfully than malonic acid for the surface hydroxyls, completely preventing protonation of the surface and imposing a negative charge, while at high pH values it is able to reduce the surface potential further, confirming its ability to coordinate with surface tin atoms. This specific activity compared to those of both malonic acid and Calgon is totally consistent with the selection criteria usedsit was expected that, due to its geometry and availability of carbonyl oxygen lone pairs, squaric acid would effectively substitute for surface hydroxyls and coordinate with surface tin atoms. The additional importance of the acidity of its ionizable groups and their ability to control the surface charge is now also revealed as a further important aspect of the design and selection procedure. Overall, it is clear from these data that the application of molecular recognition principles, through use of crystallographic data to define binding sites and search for possible surface ligands, has significant potential in the important field of dispersant design and selection. Acknowledgment. A.M.M.J.R. acknowledges the support of the EPSRC and ICI plc through a CASE studentship and significant scientific discussions with Dr. H. F. Lieberman.
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References (1) Black, W. In Dispersions of Powders in Liquids, 2nd ed.; Parfitt, G. D., Ed.; Applied Science: Essex, U.K., 1973; Chapter 4, pp 132-174. (2) Aveyard, R. In Solid/Liquid Dispersions; Tadros, T. F., Ed.; Academic Press: London, 1987; Chapter 5, pp 111-129. (3) Davey, R. J.; Black, S. N.; Bromley, L. A.; Cottier, D.; Dobbs, B.; Rout, J. E. Nature 1991, 353, 549-550. (4) Davey, R. J.; Williams-Seton, L.; Lieberman, H. F.; Blagden, N. Nature 1999, 402, 797-799. (5) Davey, R. J.; Blagden, N.; Potts, G. D.; Docherty, R. J. Am. Chem. Soc. 1997, 119, 1767-1772. (6) Stumm, W. Chemistry of the Solid-Water Interface; Wiley: New York, 1992. (7) Ocana, M.; Matijevic, E. J. Mater. Res. 1990, 5, 1083-1091. (8) Bauer, W. H. Acta Crystallogr. 1956, 9, 515-519.
(9) The Cambridge Crystallographic Database, Cambridge Crystallographic Data Centre, Union Street, Cambridge, U.K. (10) Jagannathan, N. R.; Rajan, S. S.; Subramanian, E. J. Chem. Crystallogr. 1994, 24, 75-78. (11) Wang, Y.; Stucky G. D.; Williams, J. M. J. Chem. Soc., Perkin Trans. 1974, 35, 2-6. (12) Shaw, D. J. An Introduction to Colloid Science and Surface Chemistry, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 1992. (13) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1992. (14) Schwartz, L. M.; Howard, L. O. J. Phys. Chem. 1970, 74, 4374-4377.
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