Langmuir 1999, 15, 4447-4453
4447
Latex Particle Self-Assembly and Particle Microchemical Symmetry: PS/HEMA Latex Particles Are Intrinsic Dipoles Andre´ Herzog Cardoso,† Carlos Alberto Paula Leite, and Fernando Galembeck* Instituto de Quı´mica, Universidade Estadual de Campinas, 13083-970 Campinas SP, Brazil Received November 17, 1998. In Final Form: April 7, 1999 Poly[styrene-co-(2-hydroxyethyl methacrylate)] copolymer latex (PS/HEMA) self-assembles easily, forming both macrocrystals and colloidal crystals. Two types of microchemical data were obtained for these particles: high-resolution scanning electron microscopy using a field-emission gun (FESEM) in the BEI (backscattered electron imaging) and SEI (secondary electron imaging) modes as well as energy-loss spectroscopy imaging in the transmission electron microscope (ESI-TEM). BEI and SEI particle pictures for the same macrocrystal domain are not coincident, showing that the symmetry of the chemical domains distribution in the particle differs from the overall morphological symmetry. The ESI images show that the distribution of groups bearing electrical charges within the particles (sulfate residues and potassium ions) is also asymmetrical. The PS/HEMA particles are thus intrinsic electrical dipoles (or multipoles), regularly aligned within the macrocrystals. Particle polarity was previously acknowledged in the literature (but only as a response to an external electrical field), and we propose that this is a decisive factor for the ease of PS/HEMA latex particle self-arraying.
Introduction Latex dispersions form two types of self-assembly systems: colloidal crystals and macrocrystals. The first are solid-liquid dispersions, in which the particles arrange themselves in ordered arrays, often found in equilibrium with randomly dispersed particles, in a “solid-liquid” equilibrium.1 Colloidal crystals are formed by monodisperse or bidisperse latex particles in a low ionic strength medium, at the appropriate volume fraction; different crystalline structures have been found, and their phase transitions were examined by different authors.2-8 The forces responsible for the formation and stability of colloidal crystals in disperse liquids are still controversial,9 although the prevailing idea is that the entropy plays a decisive role in these disorder-to-order transition phenomena.1,6,10 Macrocrystals are dry solids formed by regularly packed particles, which maintain their identities. Their formation has received much attention from many authors, and recent publications assign to capillary adhesion a decisive role in the formation of polymer latex dry macrocrystals.11-15 However, the fabrication of macroscopic two- or * Towhomcorrespondenceshouldbeaddressed.E-mail: fernagal@ iqm.unicamp.br. † Permanent address: Departamento de Cie ˆ ncias Fı´sicas e Biolo´gicas, Universidade Regional do Cariri URCA, Crato Ce, Brazil 60100-000. (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions, Cambridge University Press: New York, 1989. (2) Hachisu, S.; Kobayashi, Y.; Kose, A. J. Colloid Interface Sci. 1973, 42, 342. (3) Ottewill, R. H. Langmuir 1989, 5, 4. (4) Pusey, P. N.; van Megen, W. Nature 1986, 320, 340. (5) van Megen, W.; Underwood, S. M. Nature 1993, 362, 616. (6) Phan, S.-E.; Russel, W. B.; Cheng, Z.; Zhu, J.; Chaikin, P. M.; Dunsmuir, J. H.; Ottewill, R. H. Phys. Rev. E 1996, 54, 6633. (7) Ise, N. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 841. (8) Okubo, T. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 109, 77. (9) Dinsmore, A. D.; Crocker, J. C.; Yodh, A. G. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (10) Bartlett, P.; Ottewill, R. H.; Pusey, P. N. Phys. Rev. Lett. 1992, 68, 3801. (11) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; and Nagayama, K. Nature 1993, 361, 26.
three-dimensional latex particle ordered arrays is not yet regularly performed with a high success ratio16 and using many different latexes, which suggests that current understanding of latex self-arraying mechanisms is still limited. We have recently found that a copolymer latex made out of styrene and 2-hydroxyethyl methacrylate forms colloidal crystals and macrocrystals very easily, without using any specially designed device or equipment.17 We have not yet found an explicit report of an analogous behavior in the literature. A remarkable feature of this system is the formation of opalescent dispersions and dry solids over large domains, without any special pretreatment. Indeed, even as-prepared dispersions containing the initiator residues yield brilliant opalescent solids upon drying. In this case, we conclude that latex ordered arrays made by large monocrystalline domains could be obtained provided the particles used have highly repulsive hydrophilic coating surfaces. In a recent work,18 we made a detailed examination of this latex by using the technique of electron spectroscopy imaging (ESI), which revealed that negatively charged sulfate initiators are dispersed throughout the latex particles, so the dry particles can be described as large multipoles. We hypothesized that the easy formation of liquid and dry self-arrayed PS/HEMA is due to the existence of some peculiar interparticle interaction, which is in turn associated to the distribution of chemical constituents within the particle. For this reason, we decided to obtain detailed morphological and microchemical evidence concerning the symmetry of elemental distribution within the particles, (12) Nagayama, K. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 109, 363. (13) Velev, O. D.; Denkov, N. D.; Paunov, V. N.; Kralchevsky, P. A.; Nagayama, K. Langmuir 1993, 9, 3702. (14) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (15) Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996, 8, 245. (16) Nagayama, K. Phase Transitions 1993, 45, 185. (17) Herzog Cardoso, A.; Leite, C. A. P.; Galembeck, F. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 144, 207. (18) Herzog Cardoso, A.; Leite, C. A. P.; Galembeck, F. Langmuir 1998, 14, 3187.
10.1021/la9816149 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/29/1999
4448 Langmuir, Vol. 15, No. 13, 1999
using a field-emission, high-resolution scanning electron microscope (FESEM) as well as energy filtered transmission electron microscopic images (EFTEM), which is reported in this paper. Experimental Section Synthesis of the Latex. The latex was prepared by batch surfactant-free emulsion copolymerization of styrene (S) and 2-hydroxyethyl methacrylate (HEMA) following procedures similar to those developed by Okubo19 and Suzawa.20 The detailed description of the procedures used in this preparation is described elsewhere.18 Following the synthesis, ca. 200 mL of the nondialyzed PS/ HEMA latex was stored in a 250 mL glass bottle. After storage for 5 months, the latex decanted yielding a homogeneous opalescent layer (height ∼15 mm) at the bottom of the bottle. The mean particle diameter in this bottom layer is (420 ( 5) nm, as determined by DLS (dynamic light scattering) using a Brookhaven ZetaPlus instrument. The fractionation of PS/ HEMA latex by colloidal crystallization under gravity settling will be described in greater detail in a forthcoming report. Film Preparation. The PS/HEMA latex film was obtained by sedimentation and drying in a Petri dish within a desiccator at 25 °C, using an aliquot of the opalescent bottom fraction of a settled PS/HEMA dispersion. The detailed description of the film preparation and film morphology are described elsewhere.17 FESEM Images. Secondary and backscattered electron images were obtained in an ultra-high-resolution “semi-in-lens” JEOL JSM-6340F field emission scanning electron microscope operating at 15 kV, which corresponds to an 1.2 nm nominal resolution. The films were placed on a metal stub and sputter coated with carbon prior to examination. Secondary electron images as well as backscattered electron images were obtained. The former are much more common in the literature than the latter, but these contain more explicit information on chemical differences between different domains in a sample. Backscattered electrons are primary beam electrons that have collided with electrons in the sample and bounced back through the sample surface. Backscattered electrons vary in their amount and direction with the composition, surface topography, crystallinity, and magnetism of the specimen domains. The contrast of a backscattered electron image depends on (i) the backscattered electron generation rate, which in turn increases with the mean atomic number of the specimen; (ii) the angle dependence of backscattered electrons at the specimen surface; and (iii) the change in the backscattered electron intensity when the electron incidence angle on a crystalline specimen is changed. The backscattered electron image contains information on specimen chemical composition as well as on specimen topography. To separate these two types of information in a JEOL JSM-6340F microscope, a paired semiconductor detector is mounted symmetrically with respect to the optical axis. Addition of the signals from the two detector elements gives a composition image while subtraction gives a topography image. The width of the domain generating backscattered electrons is several tens of nanometers, larger than that for secondary electrons. Therefore, backscattered electrons images have a poorer spatial resolution than secondary electron images. On the other hand, backscattered electrons have a higher energy than secondary electrons, for which reason they are less influenced by specimen charging-up and contamination. In BSE images (BEI), areas of higher mean atomic number will appear brighter than areas of lower mean atomic numbers. Images generated by using the BSE signal provide nonspecific compositional information to a depth of 1-2 µm (depending on the element under analysis) in the sample, and differences in (19) Kamei, S.; Okubo, M.; Matsuda, T.; Matsumoto, T. Colloid Polym. Sci. 1986, 264, 743. (20) Tamai, H.; Fujii, A.; Suzawa, T. J. Colloid Interface Sci. 1987, 116, 37.
Herzog Cardoso et al. the mean atomic number as low as 0.1% are distinguishable in topographically flat specimens.21 Atomic Force Microscopy. The dry latex film obtained as described above was examined by atomic force microscopy (AFM) in order to observe individual latex particle morphology. The AFM measurements were carried out with a Topometrix Discoverer instrument operating in the true noncontact mode and using a silicon probe tip. Energy-Loss Spectroscopy Image (ELSI). The elemental distribution within latex particles was observed using a Carl Zeiss CEM 902 transmission electron microscope, equipped with a Castaing-Henry-Ottensmeyer energy filter spectrometer within the column. The spectrometer uses inelastically scattered electrons to form element specific images. When the electron beam passes through the sample, interaction with electrons of different elements results in characteristic energy losses. A prism-mirror system deflects electrons with different energies to different angles so that only electrons with a well-defined energy are selected. If elastic electrons only are chosen (∆E ) 0 eV), a transmission image with reduced chromatic aberration is obtained. When monochromatic inelastically scattered electrons are selected, electron spectroscopic images (ESI) are formed in which contrast is dependent on the local concentration fluctuations of a particular chosen element. Clear areas correspond to element-rich domains. For individual latex particle examination, one drop of the latex dispersion (1% solids content) was deposited on carbon-coated parlodion films supported in 400 mesh copper grids (Ted Pella). To make sure that the whole particles were not excessively thick, they were first observed using ∆E ) 0 eV electrons, then observed again at ∆E ) 20-50 eV. Image contrast inversion was always obtained, showing that a significant number of electrons were transmitted throughout the particles.22 This observation is understood, considering that the 80 keV electron mean free path within these latex particles is greater than 160 nm for elastic scattering,23 and is estimated as many hundreds of nanometers for inelastic scattering.24 Elemental images were observed for the relevant elements found in this sample, using monochromatic electrons corresponding to the carbon K-edge, oxygen K-edge, sulfur L-edge, and potassium L-edge with an energy-selecting slit of 15 eV. The energy-selecting slit was set at 278 ( 6 eV for C, 532 ( 6 eV for O, 165 ( 6 eV for S, and 292 ( 6 eV for K. The images were recorded by a MTI-Dage SIT-66 camera and digitized (512 × 480 pixels, 8 bits) by an IBAS image analyzer software from Kontron running on an IBM PC-AT compatible microcomputer. The threewindow technique was used to perform the background subtraction for each elemental image.25 Image processing and densitometric line scan particle analysis were performed in an IBM PC microcomputer using the ImagePro Plus 3.0 image analyzer program (Media Cybernetics). The line scans are plots of pixel gray level as a function of position, and the gray level scale used varies from 0 (full black) to 255 (full white). For each particle elemental distribution picture, two perpendicular line scans were obtained.
Results FESEM and AFM High Magnification Images. Secondary electron image (SEI) as well as AFM images of dry macrocrystal surfaces reveal that the particles of PS/HEMA latex have irregular surfaces, which appear in (21) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray Microanalysis; Plenum Press: New York, 1992. (22) We are grateful to Dr. W. Probst (LEO-Zeiss Elektronenmikroskopie Gmbh) for this valuable private communication. (23) Newbury, D. E. In Principles of Analytical Electron Microscopy; Joy, D. C., Romig Jr., A. D., Goldstein, J. I., Eds.; Plenum Press: New York, 1986. (24) Ibid., p 20. (25) Reimer, L.; Zepke, U.; Moesch, J.; Schulze-Hillert, St.; RossMessemer, M.; Probst, W.; Weimer, E. EELS Spectroscopy: A Reference Handbook of Standard Data for Identification and Interpretation of Electron Energy Loss Spectra and for Generation of Electron Spectroscopic Images; Carl Zeiss: Oberkochen, 1992.
Latex Particle Self-Assembly
Langmuir, Vol. 15, No. 13, 1999 4449
Figure 3. Low-contrast SEI image of a macrocrystal surface. The arrows indicate areas where the particles were not completely deformed by capillary adhesion.
Figure 1. Secondary electron images of a PS/HEMA macrocrystal surface, obtained in an ultra-high-resolution field emission scanning electron microscope (FESEM).
Figure 2. Noncontact atomic force microscopy (AFM) image of PS/HEMA macrocrystal surface.
the SEI as irregularly shaped, clear spots, as in the raspberry particle model26 (Figures 1 and 2). Particle Deformation. A low-contrast image of a macrocrystal surface is presented in Figure 3. Most particles appear as hexagons rather than spheres. It is also possible to observe incompletely deformed particles (indicated by arrows in the Figure 3), thus ruling out that the observed hexagons are the result of some imaging artifact. Particle deformation is expected, considering the strong capillary forces acting on drying macrocrystals. Comparison of SEI and BEI Images. Figure 4 is a pair of SEI and BEI (composition mode) images of a film surface area. The SEI image presents very well arranged (26) Okubo, M. Makromol. Chem., Makromol. Symp. 1990, 35/36, 307.
Figure 4. SEI and BEI images of the same area of a selforganized, iridescent PS/HEMA dry film surface. The SEI picture shows a line defect and a point defect. The BEI image shows the line defect and two point defects. The pictures were taken in the order SEI, BEI, SEI. The second SEI picture (not shown) coincided with the first.
particles, together with a line defect (across the picture, from top left to bottom right) and a point defect (in the lower left quadrant). A comparison of the corresponding SEI and BEI pictures reveals many important differences (Figure 4): (i) The predominating particle shape is hexagonal in SEI, but these same particles appear as diamond-shaped in BEI. The ordered interparticle voids seen in the SEI pictures are small, typically 1/10 of the particle diameter, and they are hexagonally distributed around each particle. This is quite different from what is observed in the BEI
4450 Langmuir, Vol. 15, No. 13, 1999
pictures: these show sets of larger and smaller darker areas, which alternate as we travel along a line (e.g., normal to the macrocrystal line defect in Figure 4a). The larger darker areas around each particle have a trigonal symmetry, alternating with the smaller darker areas. As a result, the majority of the clear particle domains seen in the BEI picture (Figure 4b) are diamond-shaped rather than hexagonal. (ii) One particle that is clearly seen in SEI appears very dark in the BEI picture (upper right quadrant). Its chemical composition is thus different from its neighbors; (iii) In the SEI picture (lower left quadrant) there is a point defect, in which a particle is missing. The surfaces of the particles surrounding this defect are bright and well-rounded, indicating the absence of deformation, as expected in the case of weaker capillary forces associated to larger interparticle distances. In the BEI image, the defect appears larger, because another particle appears very dark in this picture, and the other neighboring particles appear strongly deformed. Of course, one particle could have jumped from this defect in between the acquisition of the SEI and BEI images. However, after recording of the BEI image the same area was again examined in the SEI mode, but then the image was identical with the first image obtained (which confirms that one only particle was missing). (iv) There is also a large line defect in the SEI and BEI pictures. However, there is a major difference in the aspect of this defect between both pictures: in the first, the particles on both sides of the line appear to touch each other, but in the latter the contacting areas of the particles are dark and they seem not to touch each other. Moreover, the particles around the line defect in the BEI image (Figure 4b) display another interesting feature: in the lower line, they are diamond-shaped, but in the upper half they look more like ellipsoids. This means the contacting domains along the line defect have chemical compositions different from the other particle domains. Summing up, the PS/HEMA particles show two types of chemical heterogeneities: (i) there are differences among particles and (ii) each of the dominating, more uniform, hexagonally shaped particles is made out of at least two well-differentiated domains: a clear (in the BEI picture), diamond-shaped domain and darker domains on the sides of the diamond tip. This demonstrates a polarity in the distribution of chemical constituents of each particle, following a lower-than-hexagonal symmetry. The diamondshaped clear domains have a higher average atomic number, since they are brighter than the surroundings. For this reason, we conclude that they are richer in O (and probably also S, K) than the darker particle domains. Consequently, the darker domains are richer in C and thus in styrene-rich polymer chains. ESI-TEM Images. The differences in the SEI and BEI images led us to look for asymmetries in the elemental chemical distributions in isolated particles, using the ESI technique in the transmission microscope. The elemental images corresponding to carbon, oxygen, sulfur, and potassium are shown in Figure 5, and the corresponding line scans taken at two different particle orientations are in Figure 6. Perpendicular line scans for carbon and oxygen are well superimposed, for which reason we conclude that these elements are distributed symmetrically throughout the particles. As observed in our previous report,18 carbon signal intensity is maximum in the particle center and decreases along its radius. The amount of carbon is thus maximum in the center of each particle image and decreases toward the surface, as expected assuming that carbon distribution is quasi-uniform within spherical
Herzog Cardoso et al.
particles. Oxygen is also distributed throughout the particles, but it is more concentrated in the vicinity of the particle surfaces than in its center. This conclusion is reached observing that the oxygen elemental map line scan is flatter than the carbon line scan. Counterion potassium is found at the particle surfaces, while sulfur (from sulfate initiator residues) is distributed throughout the particles. We conclude that negative charges are scattered throughout the particles and physically separated from the neutralizing positive charges. Contrary to the symmetry observed in the distribution of carbon and oxygen, we find that sulfur and to a lesser extent potassium are not symmetrically distributed in the particles, as shown by the crossing over of the line scans presented in Figures 6c,d. Sulfur and potassium derive exclusively from the initiator potassium persulfate used in the latex synthesis. It is well-known that sulfur is covalently incorporated to the particles during the synthesis, mainly in the form of terminal sulfate groups (-SO4-). The sulfate groups are normally expected to lie at the particle surface, but there is previous evidence showing that part of the sulfate groups are buried within the particles.18,27-29 Potassium ions are expected to be concentrated at the particle surfaces, to account for the electroneutrality of the dry particles. We conclude that there are electrical charge asymmetries in these latex particles, due to sulfur and potassium asymmetric distribution. Consequently, PS/ HEMA particles are dipolar, or multipolar. Discussion The BEI images of PS/HEMA reveal a hitherto unexpected asymmetry of distribution of the latex particle chemical constituents, within the particles in a macrocrystal. Since this asymmetry is necessarily associated with an asymmetry of electrical charge distribution, the latex particles are necessarily dipoles (or higher multipoles) orderly patterned in the dry macrocrystal. The diamond-shaped particle domains are all aligned, so that each diamond tip fits very well between neighboring particles from an adjacent row. This excellent spatial correlation between similar domains from different particles is strong evidence for a long-range interaction between them. Independent evidence for the multipolar nature of the particles was obtained from the asymmetry of S and K (respectively associated to negative and positive charges) distribution in the isolated PS/HEMA particles, as demonstrated by the mismatch of ESI picture line scans, for the same particle. There is also heterogeneity30 of chemical composition among different PS/HEMA particles, also evidenced by the comparison of SEI and BEI pictures. Many particles appear identical in SEI but not in BEI mode, and this is particularly true in the case of particles in defective crystal regions. These observations lead us to the following proposal: macrocrystal formation is not only the result of the entropy-driven packing of hydrophilic paucidisperse particles, enhanced by capillary adhesion during the drying stage. It depends also on the cooperative association of (27) van den Hul, H. J.; Vanderhoff, J. W. Br. Polym. J. 1970, 2, 121. (28) Gilbert, R. G. Emulsion Polymerization: a Mechanistic Approach; Academic Press: New York, 1995. (29) Machtle, W.; Ley, G.; Rieger, J. Colloid Polym. Sci. 1995, 273, 708. (30) Galembeck, F.; Souza, E. F. Latex Particle Heterogeneity: Origins, Detection and Consequences In Polymers Interfaces and Emulsions; Esumi, K. Ed.; Marcel Dekker: New York.
Latex Particle Self-Assembly
Langmuir, Vol. 15, No. 13, 1999 4451
Figure 5. Elemental maps of PS/HEMA copolymer latex particles, obtained by energy-loss spectroscopy imaging (ESI): (a) brightfield elastic image, (b) carbon map, (c) oxygen map, (d) sulfur map and (e) potassium map.
dipolar (or multipolar) particles. Particle electrical polarity is well-known in the cases of colloids which develop strong birefringence in an electrical field, where dipoles may appear as a result of the polarization of the particle double layer in the presence of an external electrical field. Recent results published in the literature by Ottewill31 suggest the nonuniformity of surface charge distribution in ellipsoidal latex particles. Dipolar particle association was also invoked by Yeh et al.32 to account for the assembly of ordered colloidal aggregates by electric-field-induced fluid flow, as a result of the polarization of the particle bead double layer in the external field. The same assumption is the basis for the (31) Ho, C. C.; Ottewill, R. H. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 141, 29. (32) Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 386, 57.
electrorheological effect, in which the particles under an electrical field acquire an induced electric dipole. They attract one another along the axis of the dipole, but repel each other in the other directions. As a result, particles aggregate into chains in which all the dipoles are aligned.33-35 The origins of the particle multipoles that are being proposed in the present work are open to discussion, but one relevant factor was demonstrated in this work: ionic groups within the particles are asymmetrically distributed around the particle centers. The results presented in this paper allow us to derive completely new information from (33) Ball, P. Made to Measure: New Materials for the 21st Century; Princeton University Press: Princeton, 1997; p 133. (34) Halsey, T. C. Science 1992, 258, 761. (35) Block, H.; Kelly, J. P. J. Phys. D. Appl. Phys. 1988, 21, 1661.
4452 Langmuir, Vol. 15, No. 13, 1999
Herzog Cardoso et al.
Figure 6. Line-scan profiles of pixel intensity across elemental maps in Figure 5. A couple of line scans (one horizontal, another vertical) were taken for each particle, and data are given for two particles. Note that the couples of line scans for S and K cross over, showing an asymmetry of charge distribution within the particles.
Latex Particle Self-Assembly
the particle elemental distribution maps, which is the polarity of the chemical distribution throughout the particles. In future work, quantitative data on chargebearing particle distribution may allow us to calculate dipolar moments for the latex particles. The nonspherically symmetrical distribution of chemical constituents in each particle is understood considering two main factors: (i) These particles contain domains of different hydrophilicity, made out of mers for which the respective homopolymers are immiscible; assuming a monomer distribution throughout the chains, there should be some degree of chain segregation within the particles, moving some chains away from others and thus imparting chemical polarity to the particles. This has been acknowledged in the literature, e.g., as the “acorn-” or “sandwichlike” latex particle shapes. (ii) The overall charge distribution in these particles is of the core-and-shell type, as shown by the energy-loss elemental images:17 the core is negative (as indicated by the concentration of the element sulfur) and the shell is positive, due to the potassium element concentration. The nonspherical symmetry of the chemical constituent distribution and thus of electrical charge implies that each particle is a multipole and its plane projection may be approximated by an electrical dipole. When the particles approach each other, neighboring dipoles interact forming an orderly array of dipoles and consequently organizing the particles. Following this argument, the macrocrystal
Langmuir, Vol. 15, No. 13, 1999 4453
defective areas are indeed those in which defective particles were concentrated, during their approximation. Until now, a defective particle would be one with a too large or too small diameter to allow for its regular packing. The evidence given in this paper shows that defective particles are also those with normal sizes, but with a peculiar distribution of the chemical elements, or topochemistry. Our pictures show also significantly smaller particles in defective areas, as expected. These results allow us to understand why this PS/HEMA latex forms ordered arrays in both the liquid and dry states, very easily: not only the usual entropic considerations apply, but also there are strong mutual interactions among the latex particle multipoles, which contribute to mutual particle alignment. To sum up, particle microchemistry and polarity are revealed as new, additional factors in macrocrystal and colloidal crystal formation. Beyond that, they suggest that the chemical symmetry of the particles is an important latex feature. If our present results are verified in the case of many other particles, we may develop newer, more sophisticated approaches for colloidal particle self-arraying in the liquid and dry states. Acknowledgment. F.G. thanks Fapesp, CNPq, and Pronex/Finep/MCT for support. LA9816149
