12 Langmuir 1986,2, 12-20

Monodispersed Colloids: Art and Science?

Egon MatijeviE

Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676

Received October 29, 1985

Procedures for preparation of inorganic, organic, and mixed colloids, consisting of particles uniform in size and shape, are described. The use of such well-defined systems in the characterization of their optical, magnetic, and electrokinetic properties is illustrated. Interactions of “monodispersed” systems with various solutes (adsorption and dissolution), with other particles (heterocoagulation), and with plane surfaces (adhesion) are discussed. A refined treatment of the electric double layer of unlike particles is described, and the experimental results are compared with the theoretical expectations.

Introduction Since time remembered colloid chemists have endeav-

ored to prepare uniform dispersions. Originally, their attempts were essentially considered as a challenge and the products as subject of curiosity. Before electron mi- croscopy was introduced, only indirect techniques (such as ultramicroscopy or the appearance of higher order Tyndall spectra) were available for the detection of par- ticles of narrow size distribution. With the development of high-resolution instrumentation the fascination with monodispersed colloids increased dramatically. Again, a t first the interest in such systems was mainly based on their usefulness as models for theoretical studies, but now the importance of uniform powders in many areas of modern technology is becoming more and more obvious. In this respect one may mention catalysts, ceramics, pigments, films, recording materials, coatings, etc.

While a number of well-defined colloids has been re- ported in the literature, the preparation procedures were the result of isolated efforts, without a common scientific basis. The only exception refers to polymer colloids (la- texes), which are being produced in large quantities by established processes.

Some years ago we initiated a comprehensive program which has resulted in a large variety of exceedingly uniform inorganic and organic colloids of single and mixed com- position. I t is of particular interest that it is now possible to obtain a number of metal (hydrous) oxides, sulfides, selenides, phosphates, and carbonates as finely dispersed particles of different shapes and modal sizes.

In this paper the techniques for the preparation of “monodispersed” colloids will be summarized and a few of such systems illustrated, as evidenced by electron mi- croscopy. The characterization of the resulting solids will be described. Furthermore, studies involving their inter- actions with solutes (adsorption, dissolution), other solids (heterocoagulation), and plane surfaces (adhesion) will be exemplified.

Preparation of Uniform Colloids Procedures which we have developed for the synthesis

of colloidal dispersions of narrow size distribution have been described in great detail in a number of papers and reviewed elsewhere.’J Only certain general principles will be offered in this presentation. The techniques can be classified into three categories: (1) precipitation from homogeneous solutions [(a) forced hydrolysis, (b) con- trolled release of anions, (c) controlled release of cations]; (2) phase transformation; (3) reactions with aerosols.

‘Supported by NSF Grants CHE-83 18196 and CPE-8420786.

0743-7463/86/2402-0012$01.50/0

1. Precipitation from Homogeneous Solutions. In homogeneous precipitation of inorganic compounds the precursors to the formation of a solid phase are, as a rule, one or more solute complexes. The adopted approach is based on the control of the kinetics of the complexation reactions in order to achieve only one burst of nuclei, which are then allowed to grow uniformly, resulting in particles of narrow size distribution. If the constituent solutes are generated at a proper rate their even diffusion onto existing nuclei leads to the least increase in the total free energy of the dispersion. Furthermore, the charge on the so produced particles prevents their aggregation.

Forced hydrolysis is employed in the preparation of colloidal metal (hydrous) oxides or basic metal compounds. Advantage is taken of the ability of many hydrated metal ions (especially of polyvalent cations) to readily de- protonate in aqueous solutions at elevated temperatures. Since hydrolyzed species are intermediates to precipitation of the corresponding hydroxides, it is possible to generate uniform particles simply by heating metal salt solutions. In these processes the pH and the nature of anions play a dominant role. The latter can be incorporated either in a stoichiometric ratio or as impurities in amorphous or crystalline solids. In some instances anions may just affect particle morphology without being a part of the solid phase.

I t should be mentioned that complexation reactions in metal salt solutions are greatly affected by minor changes in experimental parameters, such as temperature, pH, and the nature and concentration of anions. This sensitivity is the main reason that, with the exception of the rodlike P-FeOOH particle^,^ no uniform metal oxides were ob- tained by homogeneous precipitation in the past.

Figure 1 illustrates several kinds of metal (hydrous) oxide particles prepared by forced hydrolysis under con- ditions given in the legend. The electron micrographs in Figure la ,b are of amorphous chromium hydroxide4 and crystalline cerium oxide, respectively, both obtained in the presence of the sulfate ion. The unusually shaped boeh- mite (A100H) particles in Figure IC are formed by aging aluminum chloride solutions.6 Under similar conditions aluminum sulfate yields perfectly spherical amorphous a l ~ m i n a . ~ Finally, Figure Id shows ellipsoidal hematite

(1) MatijeviE, E. Annu. Reo. Muter. Sci. 1985, 15, 483. (2) MatijeviE, E. Acc. Chem. Res. 1981, 14, 22. (3) Watson, J. H. L.; Heller, W.; Wojtowicz, W. J. Chem. Phys. 1948,

(4) Demchak, R.; MatijeviE, E. J. Colloid Interface Sci. 1969,31, 257. (5) MatijeviE, E.; Ronnstad, L.; Hepel, M., unpublished results. (6) Scott, W. B.; MatijeviE, E. J. Colloid Interface Sci. 1978, 66,447. (7) Brace, R.; MatijeviE, E. J. Inorg. Nucl. Chem. 1973, 35, 3691.

16, 997.

0 1986 American Chemical Society

Langmuir, Vol. 2, No. 1, 1986 13

Figure 1. (a) Scanning electron micrograph (SEM) of chromium hydroxide particles obtained by aging a 4 X lo4 mol dm" cbrom alum solution at 75 O C for 24 h.' (b) SEM of CeOZ particles obtained by agin a solution of 1.5 X lU3 mol dm-3 Ce(SO,), and 5 x lUz mol dm-BH&30, at 90 "C for 24 hP (c) SEM of aluminum hydrous oxide particles obtained by aging a solution of 2 X lo-, mol dm' NC13 at 200 O C for 4 hP (d) SEM of hematite (a-Fe203) particles obtained by aging a solution of 1.9 x lo-' mol FeCb and 3 x lod mol dmJ NaHPO2 in ethanol/water (50%) mixtures of 100 O C for 7 days! (Longer bar in all micrographs corresponds to 1 rm.)

Figure 2. Transmission electron micrograph (TEM) of the llsme CeO, particles shown in Figure lb.

(a-FeZ03) which appears in this habitus when small amounts of phosphate ions are added to the aging solu- tion.8 Under other conditions hematite particles can be prepared in spherical? cuhic,'O and other shapes.*

~

(8) O d i , M.; Kratohvil, S.; Matijevit. E. J. Colloid Inter/aee Sei. I984,102,146.

Figure 3. (a) SEM of zinc sulfide (ZnS) particles obtained hy aging a solution of 0.024 mol dm3 Zn(NO&, 0.062 mol dm" HN03, and 0.11 mol dmJ thioacetamide (TU) first at 26 "C for 5 h and then at 60 'C for 4 h.l2 (b) SEM of lead sulfide (PhS) particles obtained by the addition of 0.50 cm3 of 5 X mol dm-3 TAA to 20 cm3 of a 'seed" sol and aging at 26 "C for 20 min.13 (c) SEM of cadmium selenide (CdSe) particles obtained by aging a solution of 0.0070 mol dmJ cadmium acetate, 0.0040 mol dm-3 selenourea, 0.0060 mol dm-3 acetic acid, and 0.0040 sodium acetate at 70 O C

for 8 h." (d) SEM of cadmium carbonate (CdC03) particles obtained by mixing 40 an3 of 10 mol dm" urea solution (preheated before addition at 80 "C for 24 h) and 40 cm3 of a 2.0 X mol dm-3 solution of CdCI, at rmm tempe~ature.'~

The high-resolution transmission electron micrograph (Figure 2) of the CrOz sample shown in Figure l b clearly indicates that these particles are composites of much smaller subunits. This finding makes sense; it would be difticult to explain the formation of spherical single crystals by direct homogeneous precipitation.

During forced hydrolysis the hydroxide ions are formed in situ by deprotonation of water molecules coordinated by the metal ions. The hydroxylated intermediates can also be produced by a controlled release of hydroxide ions on decomposition of organic molecules, such as urea or formamide. The same concept can be extended to other anions. Thus, sulfide ions can be generated from thio- acetamide and selenide ions from selenourea, yielding in metal salt solutions corresponding monodispersed sul- f i d e ~ " - ~ ~ or selenides."

Scanning electron micrographs in Figure 3 of ZnS" (a), P b S 3 (b), CdSe14 (e), and CdC0,'6 (d) are examples of colloidal particles obtained by homogeneous generation of anions. In all cases the X-ray analysis shows patterns

(9) MatijniE, E.; Scheiner. P. J. Colloid Inter/oco Sei. 1978,63,509. (10) Hamada, S.; Matijevii E. J. Colloid Inter/me Sei. 1981.84, n4;

(11) Matij&F E, Murphy Wilhelmy, D. J. Colbidlntorfon Sei. 1%

(12) Mumhy Wilhelmy. D.: MatijeviE. E. J. Chem. Soc.. Forodw

J. Chem. Soe.. Faraday Traru. 1 1982, 78,2147.

eS, 476.

Tmw. I 1984,-80,663. (13) Murphy Wilhelmy D.; Matijevi6, E. Colloid Sur/. 19.96. 16, 1. (14) Gobet, J.; MatijeviE, E. J. Colloid hter/eee Sei. 1'3&1.100,555. (15) JanekoviE, A.; MatijeviE, E. J. Colloid InteIfoee Sei. 1985.103.

436.

14 Langmuir, VoI. 2, No. 1, 1986

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Figure 4. (a) SEM of magnetite (Fe,O,) particles obtained by aging of a ferrous h droxide gel (2.5 X lo-, mol dm-3) containing

4 h.Ig (b) SEM of nickel ferrite particles obtained hy aging a mixed ferrousand nickel hydroxide gel, [Ni(OH),I/[Fe(OH) 1 - - 0 .2, in the presence o f 0 2 mol dmP KNO, at 90 "C for 4 h. (e) SEM of chromium ferrite particles obtained by aging a mixed gel of [Cr(OH),I/[Fe(OH),] = 0.2 in the presence of 0.0025 mol dm" NaOH and 0.025 mol dm+ KNO, at 150 'C for 16 h. (d) SEM of hematite (a-Fe20a) particles obtained by aging a Fe(OH), precipitate in solution of 4 x lo4 mol dm-3 NaH,PO, and 4 x lo-? mol

characteristic of known crystalline solids, e.g., ZnS of sphalerite, PbS of galena, and CdCO, of otavite. Again, ZnS spheres have a substructure of much smaller con- stituents.

In an analogous manner uniform colloids can be ob- tained by slow release of cations from organometallic complexes, which then react with hydroxide ions in solu- tion. This procedure yielded a variety of metal oxides including those of copper,16 iron," cobalt18 and nickel,'8 with particles of different shapes and sizes.

2. Phase Transformation. Frequently, colloidal particles are obtained in their final form through phase transformations. In these procedures one kind of solid precipitates first and is then changed through crystalli- zation, recrystallization, or dissolution-reprecipitation into another kind of dispersed matter. The most common type of such a process is the sol-gel transformation. As a rule the mechanisms of these reactions are not well understcod or readily predicted. Quite often it is not even recognized that a phase transformation has taken place since the initial state may be either short lived or so finely dispersed as to be missed. However, a number of well-defined col- loidal particles have been obtained by such techniques.

A scanning electron micrograph (Figure 4a) illustrates spherical magnetite particles prepared by first precipitating

1.0 X lo-? mol dm- 1 FeSO, and 0.2 mol dm" KNOs at 90 O C for

HCI at 100 O C for 2 days?,

(16) McFadyen. P.; MatijniE. E. J. Colloid Inferface Sei. 1973.44,95. (17) Sapiazko, R. S.; Mstijevif. E. J. Colloid Interface Sei. 1980. 74.

(18) Sapiessrko. R. S.; Matijevif, E. Corrosion 1980,36,522. 405.

Figure 5. la1 SEM of titanium dioxide particles obtained b

(h) SEM of titanium silicate particles ohtained hy hydrolysis of mixed titaniumtlVi ethoxide and SiCI, drnpleu with water vapor. (r) SEM of divinylknzene ethylvinylknzene copolymer panicles obtained by polymerization of liquid droplek using trifluoro- methanesulfonir arid as initiator.* (Iangest bar in this case only avr-ponds to IO um.1 (dl SKM of mixed polyurea-TiO, pwiirles olitained by reaction of hexamethylene diiwcyanate droplet.+ with ethylenediamine vapor with subsequent rvpmure to titnnium(lVJ ethoxide vapor and hydrolysis with watcr vapor."

Fe(OH), which was then crystallized into FelO, in the presence of a mild oxidizing agent (KNO3).I9 By admix- ture of corresponding metal hydroxides to Fe(OH),. Cw, Ni-, and Co-Ni-ferrites were obtained by the same pro. cedure.2p22 The so precipitated Ni-ferrite is shown in Figure 4b. Substitution of Cr(ll1) ions for Fe(ll1) in magnetite produced the crystalline particles of Figure 4c.

A different approach was taken in the preparation of spindle-type hematite particles of Figure 4d. Ferric hy- droxide was precipitated, dissolved, and then crystallized8 While the solid formed first was not crystalline, the final product was. This procedure allowed for a better control of the ionic strength and of the content in the chloride ion which greatly affect the precipitation process. By a se- quence of reduction-reoxidation processes the particles can be converted to magnetite without a change in their shape or size??

Larger magnetite particles could also be obtained by aging at elevated temperatures (-250 "C) colloidal he- matite dispersions in the presence of a chelating compound (e.g., triethanolamine) and a reducing agent (hydra~ine). '~

3. Reactions wi th Aerosols. This technique, devel- oped first in our laboratories, is based on the generation of uniform aerosol droplets of a liquid, which are then reacted with a vapor to yield a desired colloidal powder.

hydrolysis of titanium(lVj ethoride droplera with water vapor. 2

(19) SWmoto!T.; Mstijwif. 5 J. Colloid Intorloce Sri. 1Y80.74.227. (20) Regazwni. A. E.; Matijcnf. E. corrosion 19R2,38.212. (21) Tamura. H.: MatiieviE. E. Colloid lnterloce Sci. IYR2. 90. 100. (22) Regamni. A. E.; Mnatijwif, E. Colloids Surf. 1983.6, is$: ~ ~ ~

(23) Ozaki, M.; Matijevif, E. J. Colloid Interfme Sei. 1985,107, 199.

AI- I I

' 1 EXPERIMENTAI

- HEMATITE 0 I 0.10, -

I.

I

400 500 600 700

WAVELENGTH (nm) Figure 6. Experimental (upper part) and calculated (lower part) Mie extinction coefficients for spherical hematite particles of various diameters (D = 0.10, 0.12, 0.13, 0.15, and 0.16 pm) as a function of the incident ~avelength .~~

'The advantage of the procedure is that the shape and the size of the resulting particles are predictable; i.e., they are always spherical and the radius can be predetermined from the known size of the droplets. Other additives (surfac- tants, polyelectrolytes) are not required in the process. Inorganic, organic, or mixed matter in a variety of com- positions can be prepared by this technique.

A scanning electron micrograph (Figure 5a) shows TiOz particles obtained by interacting droplets of titanium(1V) ethoxide with water vapor.24 The so prepared colloidal spheres are amorphous but on calcination crystallize into anatase and eventually into rutile (at sufficiently high temperatures). Aluminum alkoxides yield spherical alu- mina" and by proper mixtures of alkoxides one may obtain alumina/titania particles of varying composition.26 Dro- plets obtained by cocondensation of titanium ethoxide and SiC1, vapors on reaction with water vapor give titanium silicate, which is illustrated in Figure 5b.

The same technique can be employed for the prepara- tion of polymer colloids. Thus, polystyrenez7 and the polymer of divinylbenzeneZ8 particles were generated by exposing droplets of p-tert-butylstyrene and of isomers of ethylvinylbenzene, respectively, to a vapor initiator (tri- fluoromethanesulfonic acid). In the latter case rather large particles could be obtained, which is not possible to achieve by the direct emulsion polymerizaton process. The elec- tron micrograph Figure 5c illustrates copolymer spheres of -30 pm in diameter.

Colloidal polyurea was prepared from droplets consisting of diisocyanate derivatives interacted with ethylenediamine

-- (24) Visca, M.; MatijeviE, E. J . Colloid Interface Sci. 1979, 68, 308. (25) Ingebrethsen, B. J.; MatijeviE, E. J . Aerosol Sci. 1980, 11, 271. (26) Ingebrethsen, B. J.; MatijeviE, E.; Partch, R. E. J . Colloid In-

terface Sci. 1983, 95, 228.

Polvm. Sci. Polvm. Chem. Ed. 1983. 21. 961. (27) Partch, R. E.; MatijeviE, E.; Hodgson, A. W.; Aiken, B. F. J.

(28) Nakamura, K.; Partch, R. E.; MatijeviE, E. J. Colloid Interface

(29) Partch, R. E.; Nakamura, K.; Wolfe, K. J.; MatijeviE, E. J. Colloid Sci. 1984, 99, 118.

Interface SCL. 1985, 105, 560.

Langmuir, Vol. 2, No. I , 1986 15

SCIS 6547- I -@

6

1 IGHT S C A T T E R I N G V O L U M E

_ _ _ _ _ _

1 I N C I D E N T L I G H T B E A M

5 , k I 4

Figure 7. Schematic diagram of the chdmber used in the study of the rate of hydrolysis of metal a l lwde droplets: (1) Inner tube for the liquid aerosol flow, (2) outer tube for the humidified helium, (3) cross-section of chamber, (4) variable reaction distance (X), (5) light beam of the photometer, (6) illuminated light- scattering volume, and (7) light-scattering cell.32

vapor.z8 By bringing these polyur ea aerosols into contact with metal alkoxides and subsequent treatment with water vapor, the mixed polymer-metal oxide particles are ob- tained.28 One such polyurea-Ti02 powder is shown in Figure 5d.

Characterization Dispersions of uniform colloids allow for various quan-

titative characterizations of their properties, such as op- tical, magnetic, electrochemical, etc. A few examples of these studies are offered below.

Having spherical particles of narrow size distribution is especially convenient as one can use light-scattering techniques for the evaluation of their size and optical properties in situ. Indeed, it is possible to detect the onset of precipitation and to follow the growth processes of such materials without disturbing the system.

Figure 6 compares the measured and calculated ex- tinction efficiency of spherical hematite particles of various diameters as a function of the incident wavelength^.^^ The study seems to be the first involving absorbing solids and the agreement with predictions of the Lorenz-Mie theory is excellent. Hematite dispersions, used in this work, were of sufficiently narrow size distribution to determine the color and its purity of such solids.J1

The application of light scattering in the evaluation of the rate of the formation of uniform colloidal particles is exemplified by the hydrolysis of metal alkoxide droplets.

(30) Hsu, W. P.; MatijeviE, E. Appl. Opt. 1985, 24, 1623. (31) Kerker, M.; Scheiner, P.; Cooke, D. D.; Kratohvil, J. P. J. Colloid

Interface Sci. 1979, 71, 176.

16 Langmuir, Vol. 2, No. I , 1986 ::MI I .oo

2.00r I I 1 ~ 1 . 4 7

1.00 2.00

~ 1 . 4 6 r:.201 %:.I9

1.00

i = 340 miec m = 1 . 4 9 ri.155 ut.14

. .

v/v.: 0.18 1.00

40 60 80 100 120 140

8 Figure 8. Polarization ratio against the light-scattering angle for the unreacted Al(sec-BuO), aerosol generated at 125 "C and the same aerosol reacted with water vapor at three distances X (Figure 7) corresponding to 13-, 39-, and 340-ms reacting time. m = refractive index of the particles and r is their radius.32

Figure I is a schematic presentation of the very simple experimental arrangement that allowed us to follow the change of droplet properties as a function of the reaction time;y2 The latter could be altered by using shorter and longer inner tubes through which the aeroaol droplets were introduced, to be then mixed with the water vapor supplied by means of the outer tube. The photometer was fixed in place and the size distribution and refractive index changes were monitored by light scattering using the po- larization ratio te~hnique.3~ The optical characteristics of the aluminum sec-butoxide droplets exposed to water vapor as R function of time are shown in Figure 8, which also lists the calculated decrease in the particle size and the increase in the refractive index. The entire process of transformation of liquid aluminum alkoxide into solid aluminum hydroxide is completed in -300 ms, whereby the volume is reduced by 82%. Obviously, the hydrolysis of this aerosol is an exceedingly fast process, yet i t can be followed by the described light scattering technique.

I t is also known that magnetic properties depend on particle size and shape. The magnetic susceptibility of colloidal hematite spheres smaller than 100 nm was shown to increase with decreasing particle size." Particles of 300 nm in diameter exhibit weak ferromagnetism superim- posed on antiferromagnetism. As a result, reversible or- dered agglomeration in a dispersion of such solids could be observed under the influence of a very weak magnetic field,"" illustrated in Figure 9.

Electrokinetic measurements of well-defmed colloids can be used for several purposes: (a) t o indicate sol stability (although a low <-potential does not necewarily mean that

(32) Ingebrethsen. B. J.; Matijevif, E. J. Colloid Interface Sei. 1984. loo. 1.

(33) Kerker. M.; Matijevif. E.; Espenaeheid, W. F.; Farone. W. A,;

(34) Muench, C. J.; Arajs, S.; Mstijevif. E. J . Appl. P h p . 1981.52. Kitani. S. J . Colloid Sei. 1964. 19, 213.

2493.

Interface Sei.. submitted for publication. (35) Oraki, M.; Suzuki. H.; TaLahashi, K., MatijwiC, E. J. Colloid

a h I

I

Figure 9. (a) Micrographs of spherical hematite particles (1.2 pm in diameter) taken after settling i n the ahsence of an applied magnetic field in an inverse metallurgical microscope. (b) Particle reorientation after a magnetic field of 100 Oe was applied.=

0 POLYUREA 0 POLYUREA - T i 0 2 . A POLYUREA . Alto,

I 2 3 4 5 6 7 8 9 10

PH Figure 10. Electrophoretic mobilities of polyurea particles prepared by the a e m l procedure (0) and of mixed polyurea/Ti& (0 ) (Figure 5d) and polyurea/Al O3 (A) particles dispersed in

the systems are unstable), (b) to detect surface impurities, or (e) to assess the surface composition of multicomponent particles.

The adsorption of anions can be detected by the shift in the isoelectric point (iep) and evaluated from the dif- ference of this quantity and the point of zero charge (pzc), as recently shown on a titanium dioxide dispersion;36

An example of the application of electrokinetics for surface identification is given in Figure 10, in which mo- bilities are plotted as a function of pH of polyurea particles obtained by the aerosol techniquem and of the same po- lyurea colloids doped with TiOp and A1203. Data for all three systems fall on the same curve. While it happens that the iep of pure polyurea and of Ti02 is rather close (at pH -4.5). the iep of pure A1208 is much higher (at pH -9.3). Obviously, in the preparation process of these mixed systems, the metal oxides must be in the interior of the polymer particles rather than forming a surface film. I t would follow that in the employed aerosol preparation technique, the alkoxide vapor penetrates into preformed polyurea with subsequent hydrolysis, caused by exposure to water vapor.

aqueous solutions of varying pH. b

(36) Kallay. N.; Babif. D.; Matijevif. E. Colloids Surf., submitted for publication.

Langmuir, Vol. 2, No. I , 1986 17

+ 7 5 I I : I I I 1

'50 > E 3 .25

i 9 0 k z E - 2 5 0 n

-50

-751 1 I I 1 I I I 3 5 7 9 I1 I3

PH Figure 11. Plots of adsorption potentials, $, as a function of pH calculated from adsorption data of oxalic acid on hematite41 assuming different combinations of adsorbed species. Circles give the values of {-potentials of hematite particles in the presence of oxalic acid (4 X low3 mol dm-3) and NaN03 (1 X lo-' mol dm-3).42

Similarly, it was observed that a series of metal ferrites consisting of spherical submicron particles varying in the Ni:Fe ratio showed the same dependence of electrophoretic mobility on pH, leading to the conclusion that the surface and bulk composition of these solids had to be different.20

Interactions with Solutes Uniform colloids of known surface properties lend

themselves well for the study of interactions with various solutes. Such investigations can involve adsorption of dissolved ionic or molecular species on dispersed particles with consequent change in the surface charge, as well as the dissolution of the solids under the influence of various complexing agents. The availability of well-defined dis- persions makes it possible to evaluate the behavior of particles of the same shape, but different chemical com- position and structure (e.g., crystalline vs. amorphous), or of the effect of varying particle shape.

Interactions of a number of solutes, including chelating (EDTA, HEDTA, NTA, etc.) and organic

(e.g., nicotinic, picolinic, dipicolinic, aspartic, oxalic, and citric), with spherical hematite, magnetite, and chromium hydroxide particles have been investigated in some detail.

While it is relatively easy to obtain reliable adsorption data, their interpretation depends to a great extent on the assumptions made. Recently, a different approach was adopted that eliminates the need to a priori postulate either an electric double layer model or specific surface e q ~ i l i b r i a . ~ ~ The procedure consists of taking into con- sideration all possible interfacial reactions and selecting those that best fit the experimental data. Furthermore, corresponding potentials are calculated from the adsorp- tion densities and compared to measured electrokinetic potentials under the same conditions, which then allow for drawing conclusions with respect to the kind of species that are actually adsorbed. Figure 11 illustrates the procedure as applied to the uptake of oxalic acid (H2X) by spherical

(37) Chang, H.-C.; Healy, T. W.; MatijeviE, E. J. Colloid Interface Sci.

(38) Chang, H.-C.; MatijeviE, E. J. Colloid Interface Sci. 1983,92, 479. (39) Pope, C. G.; MatijeviE, E.; Patel, R. C. J. Colloid Interface Sci.

(40) Kumanomido, H.; Patel, R. C.; MatijeviE, E. J. Colloid Interface

1983, 92, 469.

1981, 80, 74.

Sci. 1978, 66, 183. (41) Zhang, Y.; Kallay, N.; MatijeviE, E. Langmuir 1985, I, 201. (42) Kallay, N.; MatijeviE, E. Langmuir 1985, I , 195.

I " I " ' " ' " I H E M A T I TE O R G A N I C ACID 1000

n CL \ - 4

100 Y \a\ 0 W v)

W J W

a

a z 0 s

i

1 0 3 6 9 12

PH Figure 12. Specific amount of iron(II1) released from spherical hematite particles suspended in 2 X lo-' mol dm-3 oxalic and citric acid, respectively, after 17 h of aging at 25 and 60 "C as a function of pH.41

hematite particles a t constant ionic strength. Since the electrokinetic potential cannot be higher than the surface potential, only X2- and the combination H2X + X2- may be considered as adsorbing species. The coexistence in solution of the latter pair in the absence of HX- is im- possible, leaving the doubly charged oxalate ion as the only viable adsorbate. From adsorption isotherms it was es- tablished that a t saturation the area occupied per oxalate ion on hematite was 8.3 A2 which corresponds to a vertical orientation of the adsorbed species with two carboxyl groups attached to the surface.

Analogous analysis for the hematite-citric (H,Y) acid system yielded HY2- and Y3- as adsorbing species with a surface area per molecule of 33 A2.

It is known that both oxalic and citric acids can dissolve iron oxides. One would expect the latter to be a more efficient agent in this process as the citrate ion forms stronger complexes with iron than the oxalate ion and it is the iron complex that is released into solution. Figure 1 2 shows, however, that the oxalic acid dissolves up to 2 orders of magnitude more hematite than does citric acid under otherwise identical condition^.^' The explanation for such a behavior is found in the adsorption data. First, there are more oxalate than citrate ions adsorbed for unit area, and, second, fewer bonds between iron and constit- uent ions of the crystal lattice have to be broken to remove from the interface the oxalate/iron rather than the ci- trate/iron complex. Obviously, more energy is required in the second case to release the complex, making citric acid a less powerful additive for dissolution of iron oxides. The latter was indeed experienced in the chemical de- contamination of water-cooled nuclear power plants.

Interactions of Mixed Solids The problem of interactions of unlike particles is con-

siderably more involved than that of identical dispersed solids. Certain phenomena in mixed systems, that cannot occur with monodispersed identical particles, are of par-

18 Langmuir, Vol. 2, Wo 1 , 1986

0 5 0 ii

0 ' w i

0 !- a

o l : 5 X I O 'm = 2 5 m V // a 2 - 2 X 1 0 'm q 2 = 50mV _.

w -100 I L L - 0 0 0 2 c ,I 0 6 O B 10

SEPARATION (RH)

Figure 13. Electrostatic energy (in kT units) as a function of scaled separation (KH) for the spheres of radii a, = 5 X m and a2 = 2 X m with wrface potentials 4, = 25 mV and 4z = 50 mV, for K = 1 x l Q X m ' calculated by using the models of Hogg, Healy, and F 1 1 w ~ b i i a i i (HHF)47 and of Barouch and MatijeviF (RM) 44

ticular interest. For pxaniple, Parsegian and Ginge1143 have shown that two plates of the same sign, but different yet constant magnitude of surface potential, can electrostat- ically attract each other at wifficiently small separations. Physically this effect can hp explained in terms of inter- dependence of the potential and the charge distribution. When two surfaces approach each other in ionic media, the charge density o v the surface and in the interfacial liquid layer is altered hading to a change in sign. More recently it was demonstratrd that the same effect can be observed between int nrarting spheres, if the two-dimen- sional Poisson-Boltzinann (PB) equation is taken into consideration. The total electrostatic repulsion between two spheres of differwit size and potential (of the same sign) decreases due to the fact that portions of the surface can be attractive a t s i i f Ciciently small separation^.^^^^^ Indeed, under proper conditions net attraction may result as illustrated in Figure 13. It is essential to note that this effect is due strictly to electrostatic forces despite the fact that the potential on both surfaces bears the same sign. The dispersion force contributes additional attraction. The solid line is calculated using the model based on the two-dimensional PB equation, while the dashed line rep- resents the well-known approximation of Hogg, Healy, and F u e r ~ t e n a u . ~ ~ In the latter case no reversal of repulsion to attraction is observed even a t very small separations.

The attraction of different particles of the same sign of potential has been experimentally verified. For example, negatively charged silica particles have been found to readily deposit on a negatively charged polymer latex.48

Another significant consequence is discovered when the effect of the ionic strength on interactions in mixed dis- persions is investigated. Figure 14 shows that at close separations the repulsion between unlike particles may increase as the electrolyte content becomes higher. No such effect is possible with a system of identical dispersed solids where the addition of electrolytes always causes a decrease in the repulsion barrier and, consequently, de-

(43) Parsegian, V. A.; Gingell D. Biophys. J. 1972, 12, 1192. (44) Barouch, E.; MatijeviE, E. J. Chem. Soc., Faraday Trans. 1 1985,

81. 1797. .~ ~ ~~

(45) Barouch E.; MatijeviE, E.; Wright, T. H. J. Chem. Soc., Faraday

(46) Barouch E.; MatijeviE, E. J. Colloid Interface Sci. 1985,105, 552. (47) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday SOC.

Trans. 1 1985, 81, 1819.

1966, 62, 1638. 148) Bleier, A,; Matijevif. E. J. Chem. SOC., Faraday Trans. 1 1978,

74, 1346.

~. ... . . .. . .

..

J : I / 1 250

w -15 0 50 100 150 200

SEPARATION, h/% Figure 14. Electrostatic energy (EBM) for al = 1 X = 2 x m, +, = 15 mV, +z = 75 mV, and K = 2 X 5 x (---) , 1 X 108 (-.-), 1.5 X lo8(---) , and 3 X loam-'

m, a2 (-),

4 6 8 IO 12

Figure 15. Fraction of spherical hematite (a-Fe,O,) particles deposited on steel as a function of pH at 25 "C (full line) as determined by using a packed-column technique.49 Dashed and dotted lines give corresponding electrophoretic mobilities of hematite and steel, respectively.

stabilization. The above finding, related to unlike particles only, explains why in some cases, on addition of salts, mixed aggregates are formed, while under somewhat dif- ferent conditions selective coagulation is observed. In the same system heterocoagulation may take place at lower ionic strength, while separation of coagula of the same kind of solids occurs at higher ionic strength.45 Such behavior can be understood only if partial attraction of unlike particles of the same sign but different magnitude of po- tential is taken into consideration.

PH

Particle Adhesion A special case of the model discussed in the previous

section arises when there is a large difference in the size of two spheres. Such geometry approximates the plate/ sphere configuration, which can be applied to problems of adhesion, i.e., particle deposition on solid surf*I!.es and their detachment from substrates.

Adhesion phenomena have been extensively stuc: ed with "monodispersed" inorganic colloids, using the packed- column technique. Specifically, deposition of spherical hematite and chromium hydroxide particles from aqueous

Langmuir, Vol. 2, No. I , 1986 19

0 8- I I $ ~ - 4 0 m V . a,* 0 1pm

+E = - 5 0 m V , at= I O p m

s, A = 14X10-'oJ

w > 0

- 0.6 A a

I hdi, 3 K 0 4 z

I a-Fe,O, '

a m w

6 .20-

x 5: .10

E

cn

z I- 9

,30-AADSORPTlON pH 6-7 ' RINSE VOLUME 300 cm3

25°C '

3 I

4 6 8 10 12 14

Figure 16. Fraction of spherical hematite particles detached from two different steel surfaces on continuous elution of the packed column into solutions of different pH.49 The particles were first deposited as described in Figure 15.

PH

dispersions on steel and glass and of rodlike p-FeOOH particles on steel was investigated as a function of various parameters. Furthermore, their removal from these car- riers was followed once the composition of the liquid medium was altered.

It was shown that the particle uptake is strongly influ- enced by the charge of the interacting surfaces; the most efficient deposition occurs when the charges are of opposite sign49 (Figure 15). The effect of ionic strength on the kinetics of particle deposition was analyzed50 and it was found that in the absence of a potential barrier the results agree with the calculations based on the convective dif- fusion

The removal of deposited particles, in the absence of chemical bonds between them and the substrate, can be affected by altering the pH and the ionic strength of the rinsing solution. However, the fraction of particles de- tached is also greatly influenced by the structure of the adsorbent; the particle escape is much less efficient from rough surfaces. In Figure 16 the fraction of spherical hematite particles removed from two different steels is plotted as a function of pH. The particle escape takes place only over a narrow pH range. It may appear unex- pected that a t the highest pH values little or no removal occurs despite the fact that under these conditions both surfaces have high negative potentials (Figure 15). Thik finding is understood, if the change in the ionic strength is taken into consideration. The large difference in the behavior of the two steels is due to their surface charac- teristics.

Particle deposition and detachment cannot be treated as a reversible process, because an adhered solid is subject to different forces than a particle approaching the surface from the solution in bulk. The problem of particle escape from a plane surface can be analyzed theoretically by taking into consideration the double-layer forces and the short-range Born repulsion. Once the interaction energy is known, the probability of particle escape can be esti-

(49) Kuo, R. J.; MatijeviE, E. J . Colloid Interface Sci. 1980, 78, 407. (50) MatijeviE, E.; Kallay, N. Croat. Chem. Acta 1983, 56, 649. (51) Ruckenstein, E. Chem. Eng. Sci. 1964, 19, 131. (52) Ruckenstein, E.; Westfried, F. C. R. Hebd. Acad. Sci. 1960, 251,

2467.

mated by using a treatment introduced by Dahneke.53 A rather comprehensive review of the problem of particle

removal is given elsewhere.54 Here, only the effect of the ionic strength on the entrainment of adhered solids will be illustrated. As mentioned above, particle escape may be initiated by the adjustment of the pH in order to in- crease the surface potential of the same sign on both surfaces and consequently causing them to repel each other. Intuitively, one would expect the addition of neutral electrolyte to inhibit the particle removal. The opposite effect was established experimentally, as shown by circles in Figure 17, which refers to detachment of spherical he- matite particles from glass.55 The solid lines represent calculated trends for various distances of closest approach, if Dahneke's expression for particle escape is combined with the double-layer energy evaluated for the case of constant potential. The best f i t is for separations of 6-7 A, which is a reasonable value, corresponding to about 2-3 water molecules between the particle and the surface.

In contrast, the constant-charge model requires the particle removal to be inhibited with increasing salt con- centration. This analysis shows that the relaxation of the double layer during the particle separation is responsible for the observed phenomena. Obviously, studies on the escape of adhered solids can be used in gaining information on the interfacial properties involving the electric double layer.

Concluding Remarks The purpose of this paper, based on the Langmuir lec-

ture, is to demonstrate that procedures are now available for the preparation of a large variety of inorganic, organic, and mixed inorganic/organic dispersions, consisting of particles uniform in size and shape. A number of studies using such dispersions have been described to illustrate their usefulness in the elucidation of certain properties and interactions and to compare the obtained results with

(53) Dahneke, B. J . Colloid Interface Sci. 1975, 50, 194. (54) Kallay, N.; Barouch, E.; MatijeviE, E. Adu. Colloid Interface Sci.,

(55) Kallay, N.; BiSkup, B.; TomiE, M.; MatjeviE, E. J . Colloid Inter- submitted for publication.

face Sci., submitted for publication.

20 Langmuir 1986,2, 20-23

theoretical predictions. Obviously, in a review of this kind, it was impossible to discuss any of the examples in suf- ficient detail.

A comment is in order with respect to the title of the article. There are two ways one may consider the “art”. Nature has produced opals, whose beautiful iridescence is caused by their composition, consisting of monodispersed colloidal silica spheres. Artificial “opals” have been syn- thesized by aggregating uniform spheres of latex and other materials. Opals and similar gems can certainly be clas- sified in the domain of arts. In addition, some electron micrographs of monodispersed systems have a genuine artistic appeal.

In considering the technological aspect of the prepara- tion of well-defined colloids, one may fairly state that the

Articles

procedures employed in the past could be described mostly as “art”. While presently there is still some “art” involved in all of this work, it is gratifying that the scientific aspect of such colloids is steadily increasing. It is the hope that this paper has contributed somewhat to the ultimate aim, which is to develop solid fundamental principles for the formation and interactions of colloidal dispersions.

Acknowledgment. I acknowledge the fruitful collab- oration with my colleagues Professors E. Barouch and R. E. Partch (Clarkson University) and N. Kallay (University of Zagreb) in various aspects of this work. I am also deeply indebted to my many associates and students whose names appear in the cited publications. Without their hard work and enthusiasm this program would not be possible.

Ordered/Disordered Packing in Mixed Chemisorbed Layers: Vertically Oriented Polyphenolic Compounds at Smooth

Polycrystalline Platinum Electrodes

Dian Song, Manuel P. Soriaga,*t and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 931 06

Received April 1, 1985. I n Final Form: J u n e 19, 1985

Packing density measurements, based on thin-layer coulometry, have been made of polyphenolic com- pounds chemisorbed from binary mixtures in aqueous media onto smooth polycrystalline platinum electrodes. The compounds studied were hydroquinone, 1,4-naphthohydroquinone, phenylhydroquinone, and 2,2’,5,5’-tetrahydrobiphenyl. Chemisorption from single-adsorbate solutions of sufficiently high concentration led to formation of ordered, close-packed, edge-oriented layers in which the molecular area requirements of each subject polyphenol were similar. In contrast, adsorption from binary mixtures resulted in various degrees of disordered (inefficient) packing. Ordered packing in mixed chemisorbed layers correlated with edgewise structures which promoted intermolecular hydrogen bonding.

Introduction Metal surfaces in contact with electrolytic solutions tend

to adsorb a mixture of species derived from the solvent, supporting electrolyte, and redox-active material. The adlayer surface composition is dependent on the concen- trations and surface activities of the solution componentS,1r2 as well as the electrode p ~ t e n t i a l . ~ Studies based on thin-layer coulometry, Auger electron spectroscopy, and infrared reflection-absorption spectroscopy have revealed that, in the absence of other strongly surface-active ma- terials, polyphenolic, and quinonoid molecules are irre- versibly adsorbed at smooth (annealed) polycrystalline and single-crystal Pt electrodes in specific orientational states, to the virtual exclusion of water and electrolyte from the compact l a ~ e r . ~ - ~ Extensive intermolecular hydrogen bonding was indicated in the infrared spectrum of pure edge-chemisorbed hydroqu in~ne .~ ,~ The presence of a strongly coordinating electrolyte (such as C1-, Br-, or or solvent (various organic media) ,9-11 however, led to mixed chemisorbed l a y e r ~ . ~ - ~ , ~ - l l The competitive ad- sorption of two different diphenolic compounds has re- cently been studied.12

* T o whom correspondence should be addressed. Present address: Department of Chemistry, Texas A&M Univ-

ersity, College Station, T X 77843.

The present investigation is an extension of previous Packing density work on mixed aromatic adlayers.12

(1) Gibbs, J. W. “The Collected Works of J. W. Gibbs“; Longmans, Green: New York, 1931.

(2) Langmuir, I. J. Am. Chem. SOC. 1918,40, 1361. Langmuir, I. Proc. R . SOC. London. Ser. A 1939, 170, 1. Adam, N. K. ”The Physics and Chemistry of Surfaces”; Oxford University Press: London, 1941.

(3) Frumkin, A. N.; Damaskin, B. B. In “Modern Aspects of Electrochemistry”; Bockris, J. O’M., Conway, B. E., Eds.; Butterworths: London; 1964; Vol. 3, p 149. Butler, J. A. V. “Electrical Phenomena at Interfaces”; Methuen: London, 1951.

(4) Soriaga, M. P.; Binamira-Soriaga, E.; Hubbard, A. T.; Benziger, J. B.; Pang, P. K-W. Inorg. Chem. 1985, 24, 65.

(5) Soriaga, M. P.; White, J. H.; Song, D.; Chia, V. K. F.; Arrhenius, P. 0.; Hubbard, A. T. Inorg. Chem. 1985, 24, 73.

(6) Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Rosasco, S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. J . Electroanal. Chem. 1984, 168, 43.

(7,) Chia, V. K. F.; Stickney, J. L.; Soriaga, M. P.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Benziger, J. B.; Pang, P. K-W. J . Elec- troanal. Chem. 1984, 163, 407.

(8) Pang, P. K.-W.; Benziger, J. B.; Soriaga, M. P.; Hubbard, A. T. J . Phys. Chem. 1984,88, 4583.

(9) Song, D.; Soriaga, M. P.; Vieira, K. L.; Hubbard, A. T. J . Elec- troanal. Chem. 1985,184, 171.

A. T. J. Phys. Chem. 1985,89, 3999. (10) Song, D.; Soriaga, M. P.; Vieira, K. L.; Zapien, D. C.; Hubbard,

(11) Song, D.; Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem., in press.

1985, 1, 123. (12) Soriaga, M. P.; Song, D.; Zapien, D. C.; Hubbard, A. T. Langmuir

0743-7463/86/2402-0020$01.50/0 0 1986 American Chemical Society