Volume 10 Number 19 21 May 2014 Pages 3349–3514
Soft Matter www.softmatter.org
ISSN 1744-683X
HIGHLIGHT Junpei Yamanaka et al. Exclusion of impurity particles in charged colloidal crystals
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Exclusion of impurity particles in charged colloidal crystals Koki Yoshizawa, Akiko Toyotama, Tohru Okuzono and Junpei Yamanaka*
Received 19th November 2013 Accepted 6th January 2014
Uniformly shaped, charged colloidal particles dispersed in water form ordered “crystal” structures when the interaction between the particles is sufficiently strong. Herein, we report the behavior of “impurity” particles, whose sizes and/or charge numbers are different from those of the bulk, on addition to the charged colloidal crystals. These impurities were excluded from the crystals during the homogeneous
DOI: 10.1039/c3sm52912f
crystallization, crystal grain growth, and unidirectional crystallization processes. Such systems will be
www.rsc.org/softmatter
useful as models for studying the refinement of materials and crystal defects.
1. Introduction Colloids have been used as models for studying the phase behavior of condensed matter.1 In particular, phase transitions, e.g., crystallization and glass transition of uniform, submicronsized dispersed particles have been studied extensively.2 Because the interaction between the colloidal particles is easily tunable over a broad range, the experimental conditions corresponding to extremely high/low pressures and temperatures in atomic systems are readily accessible. The large sizes of the colloidal particles relative to atoms enable direct visualization of individual particles and their spatial organizations by means of conventional optical microscopy. Furthermore, long characteristic times of colloids facilitate in situ observations of dynamic processes. The phase behavior of hard sphere (HS) colloids, wherein the particles interact via HS repulsions, was purely entropic and
Faculty of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe, Mizuho, Nagoya, Aichi 467-8603, Japan. E-mail: [email protected]
Koki Yoshizawa is a researcher at L'Oreal Research & Innovation. Aer earning a PhD in pharmaceutical sciences from Nagoya City University (Japan), he joined Nihon L'oreal K.K. He leads scalp care projects and is in charge of the development of scalp care products. His doctoral project involved the study of impurity exclusions in colloidal crystals.
This journal is © The Royal Society of Chemistry 2014
governed only by the particle volume fraction f. Upon increasing f to 0.494, HS colloids undergo phase transition from a disordered “uid” state to an ordered “crystal” state.3 In the case of charged colloids, wherein the colloidal particles bear surface charges, the phase behavior can be well described by so interactions. The charged colloids crystallize at much lower f compared to that of HS colloids. Apart from f, other major governing parameters of the charged colloids include the particle charge number (Z) and salt concentration (Cs). The interaction is weaker at higher Cs because the salts dissociate into small ions, which screen the electrostatic interaction. Binary colloids have attracted considerable attention as models to study phase separation. Crystallization kinetics4 and size-fractionation upon crystallization have been studied for HS colloids.5 The phase behavior of binary colloids in combination with polymer systems owing to the depletion of attraction has also been extensively studied.6 Recently, phase separations were also found in charged binary colloids having asymmetry in particle sizes and/or charge numbers.7
Akiko Toyotama earned her PhD in pharmaceutical sciences from Nagoya City University. She pursued her post-doctoral research at the National Institute of Material Sciences (Japan) on a project funded by JAXA and JST; she is currently employed as a lecturer at Nagoya City University. Her research interests include self-organization of so matter, in particular the crystallization of charged colloids.
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When the quantity of the minor component in binary colloids is signicantly small, one may regard it as an “impurity”. Herein, we report the results of our recent studies on the exclusions of such impurity particles from colloidal crystals. We describe the exclusion of impurities during crystallization, grain growth, and controlled crystallization processes. Nozawa et al. recently studied impurity partitioning during crystallization based on the crystal growth theory.8 Impurity exclusions in the ordered structure of block copolymer micelles have been reported by Ghofraniha et al.9 Sh¨ ope et al. reported that tiny quantities of impurities signicantly affected the crystal sizes and crystallization rates.10
2.
Colloid samples
We used colloidal crystals formed in dilute aqueous dispersions of silica (SiO2) particles [diameter d ¼ 100 nm; f ¼ fS ¼ 0.03– 0.05], and uorescent-labeled charged polystyrene (PS) (Z ¼ 1680; d ¼ 333 nm; excitation and emission wavelengths of 468 nm and 510 nm, respectively) as impurity particles.11 We chose very low PS concentrations (f ¼ fPS ¼ 5 105 to 1 104) to ensure that the phase behavior of the colloidal silica would not be signicantly inuenced by the presence of the impurities. Silica particles were slightly charged because of the dissociation of silanol groups on their surface (^Si–OH 4 ^Si–O + H+).12 Because the silanols are weak acids, the Z value of the silica particles increased upon the addition of a base such as pyridine, Py (^Si–OH + PyH+OH / ^Si–O PyH+ + H2O; here PyH+ represents the pyridinium ion). We controlled the interaction between the particles by tuning Z, due to variation in Py concentration ([Py]). The puried colloidal silica samples were in the uid state because of low Z (¼ 220) while they crystallized upon addition of bases under appropriate conditions.13,14 We noted that the dissociation degree of Py increased with increasing temperature T. In other words, the concentration of basic species (PyH+) increased with heating.15,16 Thus, silica + Py colloids crystallized upon heating as well as with increasing [Py].
Tohru Okuzono is an Associate Professor of pharmaceutical sciences at Nagoya City University. Aer earning a PhD in theoretical physics from Kyushu University (Japan), followed by several post-doctoral stints, he worked as a lecturer at Hiroshima University (Japan) and the University of Tokyo (Japan). His main research interests include dynamics of charged colloids and polyelectrolytes under non-equilibrium conditions.
3358 | Soft Matter, 2014, 10, 3357–3361
3. Exclusion of impurities from colloidal crystals 3.1
Homogeneous crystallization
First, we examined the spatial distribution of the impurity particles during the crystallization process in a metastable uid (melt) state using a uorescent confocal laser scanning microscope (LSM) ([Py] ¼ 100 mM; fPS ¼ 1 104).11 The samples did not crystallize at fS # 0.03 while sub-millimeter sized polycrystals grew over time at larger fS values. At fS ¼ 0.035, which was close to the freezing concentration, the crystallization rate was very slow, and volume-lling polycrystals were formed at t ¼ 500 s, aer the original crystals had melted due to vigorous shaking at t ¼ 0 s. During this crystallization process, the impurity particles migrated from the crystalline region and were distributed throughout the network structure [Fig. 1(a)]. This process was analogous to the exclusion of impurity atoms/molecules in crystalline materials. On the other hand, at fS $ 0.04, the volume-lling crystals were formed within 10 s. Fig. 1(b) shows a micrograph obtained at fS ¼ 0.05 (t ¼ 7.9 s), which demonstrates that the impurities were almost randomly distributed in the polycrystals.
Fig. 1 LSM images recorded in fluorescence mode for silica + fluorescent polystyrene (PS) colloids. (a) fS ¼ 0.035, t ¼ 506 s; (b) fS ¼ 0.05, t ¼ 7.9 s. fPS ¼ 0.0001 for both cases. Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
Junpei Yamanaka is a Professor of pharmaceutical sciences at Nagoya City University. He graduated from the Department of Physics, Kanazawa University (Japan). Aer earning a PhD in polymer chemistry from Kyoto University (Japan), he held the position of an Assistant Professor at Fukui University (Japan) followed by a research position in the ERATO Hashimoto polymer phasing project. His research interests include phase behavior of charged colloids and their material applications.
This journal is © The Royal Society of Chemistry 2014
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LSM images of silica (fS ¼ 0.05) + PS (fPS ¼ 0.0001) colloid. t ¼ 1260 min after homogenization. (a) Reflection and (b) fluorescence images. (c) Superposition of (a) and (b). Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
Fig. 2
3.2
Crystal grain growth
As mentioned above, the colloidal crystals are usually polycrystals that consist of crystal grains (domains). In general, the sizes of the crystal grains of crystalline materials increase with time owing to grain growth.17 This results from a reduction in the free energy of the system through a decrease in the total grain boundary area. A physically equivalent growth process is observed in metals and froth.18
We examined the exclusion process of impurity particles from the colloidal crystals during the grain growth.11 The sample used had fS ¼ 0.05 ([Py] ¼ 100 mM; fPS ¼ 1 104), wherein the impurities were almost randomly distributed initially. Fig. 2(a) and (b) show the LSM images depicting the spatial distribution of the impurity particles in refraction and uorescent modes, respectively, at t ¼ 1260 s. Fig. 2(c) is a superposition of Fig. 2(a) and (b). It became clear that most of the impurity particles were distributed along the grain boundaries.
Fig. 3 (a–i) Time-resolved LSM images during the grain growth process. fS ¼ 0.05; fPS ¼ 0.0001. Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
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Fig. 3(a)–(i) show the time evolution of the impurity distributions. At t ¼ 15 min [Fig. 3(a)], the exclusions were detected, only to a small extent. Hereinaer, we refer to the three crystal grains indicated in Fig. 3(a) as I, II, and III. Over time, the grain boundaries migrated because of grain growth. At t ¼ 265 min, grain III increased in size, while grain I shrank and eventually disappeared [Fig. 3(d)]. During this process, the impurity particles accumulated at the grain boundaries. Grain II then shrank with time [Fig. 3(b–f)] and vanished within 435 min [Fig. 3(g)]. The impurity particles were removed from the region swept away by the grain boundaries. In other words, the impurity exclusion was triggered by grain boundary migration. A similar exclusion behavior was found for the impurity having a diameter of 183 nm, but not observed for particles with a diameter of 2420 nm because of their lower mobility.19 3.3
Highlight
Three-dimensional LSM image of a striped pattern observed for unidirectional crystallization under a pH gradient. From ref. 21; courtesy of the Chemical Society of Japan.
Fig. 5
Controlled crystallization
By utilizing heat-induced crystallization of silica + Py colloids, we have previously reported the unidirectional crystallization of silica under temperature gradients.15 The impurity exclusion was also observed during directed crystallization.16 We used a binary colloid (fS ¼ 0.035, fPS ¼ 5 105; [Py] ¼ 65 mM; crystallization temperature ¼ 20 C). The resulting crystal was partially melted at T ¼ 5 C, and then kept at T ¼ 25 C to allow recrystallization. Fig. 4(a) shows micrographs, which reveal a variation in the impurity distribution with time. The crystal region was formed within t < 10 min and grew from the le to right. We found that the impurity particles were swept toward the crystal/melt boundary and accumulated in the boundary regions. In Fig. 4(b),
we demonstrate the impurity concentration estimated as a function of location x, along the direction of the crystal growth. The impurity concentrations were calculated using the uorescence intensities, which were averaged perpendicularly in the x direction in each micrograph. From the rate of migration of the peak position in Fig. 4(b), the exclusion rate was estimated to be about 0.2 mm s1. Exclusions were not observed at higher crystallization rates. When the heating temperature was varied in a stepwise manner, the impurities assembled in a striped pattern.20 This was analogous to the striation patterns observed in most of the crystalline materials.21 Colloidal silica exhibits unidirectional crystallization also due to diffusion of Py from the Py reservoir through semipermeable membranes.14,22 At sufficiently slow growth rates, the crystals are composed of thin, lamella-shaped grains, which grow along the crystallization front in a direction perpendicular to the crystal growth.23 When the crystallization rate is small, the coexisting impurity particles accumulate at the grain boundaries to form a striped pattern.23 Fig. 5 shows the 3D LSM image of such a pattern (fS ¼ 0.05, fPS ¼ 1 104; [Py] in the reservoir ¼ 1 mM).
4. Conclusions In this paper, we reported various modes of impurity exclusion in charged colloidal crystals, which were analogous to those in the atomic and molecular crystalline materials. The proposed modes will be useful as models for studying the renement of materials and crystal defects. Unlike atomic crystals, colloidal crystals are made up of particles with non-uniform sizes. In addition, colloid samples essentially include particle aggregates, which act as impurities during the crystallization process. Thus, the present ndings will be potentially useful for fabricating high quality colloidal crystals. Fig. 4 (a) Fluorescence micrographs showing spatial distributions of
impurities during unidirectional crystallization under temperature gradients. (b) Time dependent concentration profiles of impurity particles. Reprinted with permission from ref. 16. Copyright (2013) American Chemical Society.
3360 | Soft Matter, 2014, 10, 3357–3361
Acknowledgements We express our sincere gratitude to Dr C. Patrick Royall at Bristol University (United Kingdom), and Professor Satoshi Uda
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and Jun Nozawa at Tohoku University (Japan) for suggestions and discussions. We also thank Yukihiro Sugao at Nagoya City University (Japan) for performing some of the experiments described in Section 3.3.
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References and notes 1 (a) W. B. Russel, D. A. Saville and W. R. Schowalter, Colloidal Dispersions, Cambridge University Press, New York, 1989; (b) A. K. Sood, Solid State Phys., ed. H. Ehrenreich and D. Turnbull, Academic Press, New York, 1991; (c) V. J. Anderson and H. N. W. Lekkerkerker, Nature, 2002, 416, 811; (d) A. Yethiraj and A. van Blaaderen, Nature, 2003, 421, 513. 2 (a) Ordering and Phase Transition, in Charged Colloids, ed. A. K. Arora and B. V. R. Tata, VCH, New York, 1996; (b) Phase Transitions, ed. B. J. Ackerson, 1990, vol. 21 (2–4), pp. 73– 249; (c) T. Palberg, J. Phys.: Condens. Matter, 1999, 11, R323; (d) N. Ise and I. Sogami, Structure Formation in Solution, Springer, Berlin, 2005; (e) U. Gasser, J. Phys.: Condens. Matter, 2009, 21, 203101. 3 (a) P. N. Pusey and W. van Megen, Nature, 1986, 320, 340; (b) P. Bartlett and W. van Megen, in Granular Matter, ed. A. Metha, Springer, New York, 1994, pp. 195–257; (c) T. Zykova-Timan, J. Horbach and K. Binder, J. Chem. Phys., 2010, 133, 014705. 4 (a) S. Martin, G. Bryant and W. van Megen, Phys. Rev. E: Stat., Nonlinear, So Matter Phys., 2005, 71, 021404; (b) V. W. A. de Villeneuve, L. Derendorp, D. Verboekend, E. C. M. Vermolen, W. K. Kegel, H. N. W. Lekkerkerker and R. P. A. Dullens, So Matter, 2009, 5, 2448; (c) V. W. A. de Villeneuve, R. P. A. Dullens, D. G. A. L. Aarts, E. Groeneveld, J. H. Scherff, W. K. Kegel and H. N. W. Lekkerkerker, Science, 2005, 309, 1231. 5 (a) P. Bartlett, J. Chem. Phys., 1998, 109, 10970; (b) A. Kozina, P. Diaz-Leyva, E. Bartsch and T. Palberg, So Matter, 2012, 8, 627; (c) A. Stipp, H. J. Sch¨ ope, T. Palberg, T. Eckert, R. Biehl and E. Bartsch, Phys. Rev. E: Stat., Nonlinear, So Matter Phys., 2010, 81, 051401; (d) M. Leocmach, C. P. Royall and H. Tanaka, Europhys. Lett., 2010, 89, 38006. 6 H. N. W. Lekkerkerker and R. Tuinier, Lecture Notes in Physics 833, Colloids and the Depletion Interaction, Springer, Berlin, 2011.
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Soft Matter
7 (a) N. J. Lorenz, H. J. Sch¨ ope, H. Reiber, T. Palberg, P. Wette, I. Klassen, D. Holland-Moritz, D. Herlach and T. Okubo, J. Phys.: Condens. Matter, 2009, 21, 464116; N. J. Lorenz, H. J. Sch¨ ope and T. Palberg, J. Chem. Phys., 2009, 131, 134501; (b) K. Yoshizawa, N. Wakabayashi, M. Yonese, J. Yamanaka and C. P. Royall, So Matter, 2012, 8, 11732. 8 J. Nozawa, S. Uda, Y. Naradate, H. Koizumi, K. Fujiwara, A. Toyotama and J. Yamanaka, J. Phys. Chem. B, 2013, 117, 5289. 9 N. Ghofraniha, E. Tamborini, J. Oberdisse, L. Cipelletti and L. Ramos, So Matter, 2012, 8, 6214. 10 A. Engelbrecht and H. J. Sh¨ ope, So Matter, 2012, 8, 11034. 11 K. Yoshizawa, T. Okuzono, T. Koga, T. Taniji and J. Yamanaka, Langmuir, 2011, 27, 13420. 12 R. K. Iler, The Chemistry of Silica, Wiley, New York, 1976, ch. 6. 13 J. Yamanaka, H. Yoshida, T. Koga, N. Ise and T. Hashimoto, Phys. Rev. Lett., 1998, 29, 5806. 14 J. Yamanaka, M. Murai, Y. Iwayama, M. Yonese, K. Ito and T. Sawada, J. Am. Chem. Soc., 2004, 126, 7156. 15 A. Toyotama, J. Yamanaka, M. Yonese, T. Sawada and F. Uchida, J. Am. Chem. Soc., 2007, 129, 3044. 16 M. Shinohara, A. Toyotama, M. Suzuki, Y. Sugao, T. Okuzono, F. Uchida and J. Yamanaka, Langmuir, 2013, 29, 9668. 17 (a) Physical Metallurgy, ed.R. W. Cahn and P. Haasen, Elsevier, Amsterdam, 4th edn, 1996, vol. 3, ch. 28; (b) For discussion of the kinetics and the power-law growth see, K. Binder, Rep. Prog. Phys., 1987, 50, 783; A. Heymann, A. Stipp and K. Sch¨ atzel, Il Nouvo Cimento D, 1994, 16, 1149. 18 D. Weaire and S. Hutzler, The Physics of Foams, Oxford University Press, Oxford, 2000. 19 A. Stipp and T. Palberg, Philos. Mag. Lett., 2007, 87, 899. 20 Y. Sugao, K. Yoshizawa, A. Toyotama, T. Okuzono and J. Yamanaka, Chem. Lett., 2012, 41, 1163. 21 Springer Handbook of Crystal Growth, Part B, ed. G. Dhanaraj, K. Byrappa, V. Prasad and M. Dudley, Springer, Heidelberg, 2010. 22 M. Murai, H. Yamada, J. Yamanaka, S. Onda, M. Yonese, K. Ito, T. Sawada, F. Uchida and Y. Ohki, Langmuir, 2007, 23, 7510. 23 K. Yoshizawa, S. Onda, T. Sawada and J. Yamanaka, Chem. Lett., 2012, 41, 322.
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Soft Matter www.softmatter.org
ISSN 1744-683X
HIGHLIGHT Junpei Yamanaka et al. Exclusion of impurity particles in charged colloidal crystals
Soft Matter View Article Online
Published on 05 February 2014. Downloaded by Georgia Institute of Technology on 31/10/2014 01:27:35.
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Cite this: Soft Matter, 2014, 10, 3357
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Exclusion of impurity particles in charged colloidal crystals Koki Yoshizawa, Akiko Toyotama, Tohru Okuzono and Junpei Yamanaka*
Received 19th November 2013 Accepted 6th January 2014
Uniformly shaped, charged colloidal particles dispersed in water form ordered “crystal” structures when the interaction between the particles is sufficiently strong. Herein, we report the behavior of “impurity” particles, whose sizes and/or charge numbers are different from those of the bulk, on addition to the charged colloidal crystals. These impurities were excluded from the crystals during the homogeneous
DOI: 10.1039/c3sm52912f
crystallization, crystal grain growth, and unidirectional crystallization processes. Such systems will be
www.rsc.org/softmatter
useful as models for studying the refinement of materials and crystal defects.
1. Introduction Colloids have been used as models for studying the phase behavior of condensed matter.1 In particular, phase transitions, e.g., crystallization and glass transition of uniform, submicronsized dispersed particles have been studied extensively.2 Because the interaction between the colloidal particles is easily tunable over a broad range, the experimental conditions corresponding to extremely high/low pressures and temperatures in atomic systems are readily accessible. The large sizes of the colloidal particles relative to atoms enable direct visualization of individual particles and their spatial organizations by means of conventional optical microscopy. Furthermore, long characteristic times of colloids facilitate in situ observations of dynamic processes. The phase behavior of hard sphere (HS) colloids, wherein the particles interact via HS repulsions, was purely entropic and
Faculty of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe, Mizuho, Nagoya, Aichi 467-8603, Japan. E-mail: [email protected]
Koki Yoshizawa is a researcher at L'Oreal Research & Innovation. Aer earning a PhD in pharmaceutical sciences from Nagoya City University (Japan), he joined Nihon L'oreal K.K. He leads scalp care projects and is in charge of the development of scalp care products. His doctoral project involved the study of impurity exclusions in colloidal crystals.
This journal is © The Royal Society of Chemistry 2014
governed only by the particle volume fraction f. Upon increasing f to 0.494, HS colloids undergo phase transition from a disordered “uid” state to an ordered “crystal” state.3 In the case of charged colloids, wherein the colloidal particles bear surface charges, the phase behavior can be well described by so interactions. The charged colloids crystallize at much lower f compared to that of HS colloids. Apart from f, other major governing parameters of the charged colloids include the particle charge number (Z) and salt concentration (Cs). The interaction is weaker at higher Cs because the salts dissociate into small ions, which screen the electrostatic interaction. Binary colloids have attracted considerable attention as models to study phase separation. Crystallization kinetics4 and size-fractionation upon crystallization have been studied for HS colloids.5 The phase behavior of binary colloids in combination with polymer systems owing to the depletion of attraction has also been extensively studied.6 Recently, phase separations were also found in charged binary colloids having asymmetry in particle sizes and/or charge numbers.7
Akiko Toyotama earned her PhD in pharmaceutical sciences from Nagoya City University. She pursued her post-doctoral research at the National Institute of Material Sciences (Japan) on a project funded by JAXA and JST; she is currently employed as a lecturer at Nagoya City University. Her research interests include self-organization of so matter, in particular the crystallization of charged colloids.
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When the quantity of the minor component in binary colloids is signicantly small, one may regard it as an “impurity”. Herein, we report the results of our recent studies on the exclusions of such impurity particles from colloidal crystals. We describe the exclusion of impurities during crystallization, grain growth, and controlled crystallization processes. Nozawa et al. recently studied impurity partitioning during crystallization based on the crystal growth theory.8 Impurity exclusions in the ordered structure of block copolymer micelles have been reported by Ghofraniha et al.9 Sh¨ ope et al. reported that tiny quantities of impurities signicantly affected the crystal sizes and crystallization rates.10
2.
Colloid samples
We used colloidal crystals formed in dilute aqueous dispersions of silica (SiO2) particles [diameter d ¼ 100 nm; f ¼ fS ¼ 0.03– 0.05], and uorescent-labeled charged polystyrene (PS) (Z ¼ 1680; d ¼ 333 nm; excitation and emission wavelengths of 468 nm and 510 nm, respectively) as impurity particles.11 We chose very low PS concentrations (f ¼ fPS ¼ 5 105 to 1 104) to ensure that the phase behavior of the colloidal silica would not be signicantly inuenced by the presence of the impurities. Silica particles were slightly charged because of the dissociation of silanol groups on their surface (^Si–OH 4 ^Si–O + H+).12 Because the silanols are weak acids, the Z value of the silica particles increased upon the addition of a base such as pyridine, Py (^Si–OH + PyH+OH / ^Si–O PyH+ + H2O; here PyH+ represents the pyridinium ion). We controlled the interaction between the particles by tuning Z, due to variation in Py concentration ([Py]). The puried colloidal silica samples were in the uid state because of low Z (¼ 220) while they crystallized upon addition of bases under appropriate conditions.13,14 We noted that the dissociation degree of Py increased with increasing temperature T. In other words, the concentration of basic species (PyH+) increased with heating.15,16 Thus, silica + Py colloids crystallized upon heating as well as with increasing [Py].
Tohru Okuzono is an Associate Professor of pharmaceutical sciences at Nagoya City University. Aer earning a PhD in theoretical physics from Kyushu University (Japan), followed by several post-doctoral stints, he worked as a lecturer at Hiroshima University (Japan) and the University of Tokyo (Japan). His main research interests include dynamics of charged colloids and polyelectrolytes under non-equilibrium conditions.
3358 | Soft Matter, 2014, 10, 3357–3361
3. Exclusion of impurities from colloidal crystals 3.1
Homogeneous crystallization
First, we examined the spatial distribution of the impurity particles during the crystallization process in a metastable uid (melt) state using a uorescent confocal laser scanning microscope (LSM) ([Py] ¼ 100 mM; fPS ¼ 1 104).11 The samples did not crystallize at fS # 0.03 while sub-millimeter sized polycrystals grew over time at larger fS values. At fS ¼ 0.035, which was close to the freezing concentration, the crystallization rate was very slow, and volume-lling polycrystals were formed at t ¼ 500 s, aer the original crystals had melted due to vigorous shaking at t ¼ 0 s. During this crystallization process, the impurity particles migrated from the crystalline region and were distributed throughout the network structure [Fig. 1(a)]. This process was analogous to the exclusion of impurity atoms/molecules in crystalline materials. On the other hand, at fS $ 0.04, the volume-lling crystals were formed within 10 s. Fig. 1(b) shows a micrograph obtained at fS ¼ 0.05 (t ¼ 7.9 s), which demonstrates that the impurities were almost randomly distributed in the polycrystals.
Fig. 1 LSM images recorded in fluorescence mode for silica + fluorescent polystyrene (PS) colloids. (a) fS ¼ 0.035, t ¼ 506 s; (b) fS ¼ 0.05, t ¼ 7.9 s. fPS ¼ 0.0001 for both cases. Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
Junpei Yamanaka is a Professor of pharmaceutical sciences at Nagoya City University. He graduated from the Department of Physics, Kanazawa University (Japan). Aer earning a PhD in polymer chemistry from Kyoto University (Japan), he held the position of an Assistant Professor at Fukui University (Japan) followed by a research position in the ERATO Hashimoto polymer phasing project. His research interests include phase behavior of charged colloids and their material applications.
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LSM images of silica (fS ¼ 0.05) + PS (fPS ¼ 0.0001) colloid. t ¼ 1260 min after homogenization. (a) Reflection and (b) fluorescence images. (c) Superposition of (a) and (b). Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
Fig. 2
3.2
Crystal grain growth
As mentioned above, the colloidal crystals are usually polycrystals that consist of crystal grains (domains). In general, the sizes of the crystal grains of crystalline materials increase with time owing to grain growth.17 This results from a reduction in the free energy of the system through a decrease in the total grain boundary area. A physically equivalent growth process is observed in metals and froth.18
We examined the exclusion process of impurity particles from the colloidal crystals during the grain growth.11 The sample used had fS ¼ 0.05 ([Py] ¼ 100 mM; fPS ¼ 1 104), wherein the impurities were almost randomly distributed initially. Fig. 2(a) and (b) show the LSM images depicting the spatial distribution of the impurity particles in refraction and uorescent modes, respectively, at t ¼ 1260 s. Fig. 2(c) is a superposition of Fig. 2(a) and (b). It became clear that most of the impurity particles were distributed along the grain boundaries.
Fig. 3 (a–i) Time-resolved LSM images during the grain growth process. fS ¼ 0.05; fPS ¼ 0.0001. Reprinted with permission from ref. 11. Copyright (2011) American Chemical Society.
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Fig. 3(a)–(i) show the time evolution of the impurity distributions. At t ¼ 15 min [Fig. 3(a)], the exclusions were detected, only to a small extent. Hereinaer, we refer to the three crystal grains indicated in Fig. 3(a) as I, II, and III. Over time, the grain boundaries migrated because of grain growth. At t ¼ 265 min, grain III increased in size, while grain I shrank and eventually disappeared [Fig. 3(d)]. During this process, the impurity particles accumulated at the grain boundaries. Grain II then shrank with time [Fig. 3(b–f)] and vanished within 435 min [Fig. 3(g)]. The impurity particles were removed from the region swept away by the grain boundaries. In other words, the impurity exclusion was triggered by grain boundary migration. A similar exclusion behavior was found for the impurity having a diameter of 183 nm, but not observed for particles with a diameter of 2420 nm because of their lower mobility.19 3.3
Highlight
Three-dimensional LSM image of a striped pattern observed for unidirectional crystallization under a pH gradient. From ref. 21; courtesy of the Chemical Society of Japan.
Fig. 5
Controlled crystallization
By utilizing heat-induced crystallization of silica + Py colloids, we have previously reported the unidirectional crystallization of silica under temperature gradients.15 The impurity exclusion was also observed during directed crystallization.16 We used a binary colloid (fS ¼ 0.035, fPS ¼ 5 105; [Py] ¼ 65 mM; crystallization temperature ¼ 20 C). The resulting crystal was partially melted at T ¼ 5 C, and then kept at T ¼ 25 C to allow recrystallization. Fig. 4(a) shows micrographs, which reveal a variation in the impurity distribution with time. The crystal region was formed within t < 10 min and grew from the le to right. We found that the impurity particles were swept toward the crystal/melt boundary and accumulated in the boundary regions. In Fig. 4(b),
we demonstrate the impurity concentration estimated as a function of location x, along the direction of the crystal growth. The impurity concentrations were calculated using the uorescence intensities, which were averaged perpendicularly in the x direction in each micrograph. From the rate of migration of the peak position in Fig. 4(b), the exclusion rate was estimated to be about 0.2 mm s1. Exclusions were not observed at higher crystallization rates. When the heating temperature was varied in a stepwise manner, the impurities assembled in a striped pattern.20 This was analogous to the striation patterns observed in most of the crystalline materials.21 Colloidal silica exhibits unidirectional crystallization also due to diffusion of Py from the Py reservoir through semipermeable membranes.14,22 At sufficiently slow growth rates, the crystals are composed of thin, lamella-shaped grains, which grow along the crystallization front in a direction perpendicular to the crystal growth.23 When the crystallization rate is small, the coexisting impurity particles accumulate at the grain boundaries to form a striped pattern.23 Fig. 5 shows the 3D LSM image of such a pattern (fS ¼ 0.05, fPS ¼ 1 104; [Py] in the reservoir ¼ 1 mM).
4. Conclusions In this paper, we reported various modes of impurity exclusion in charged colloidal crystals, which were analogous to those in the atomic and molecular crystalline materials. The proposed modes will be useful as models for studying the renement of materials and crystal defects. Unlike atomic crystals, colloidal crystals are made up of particles with non-uniform sizes. In addition, colloid samples essentially include particle aggregates, which act as impurities during the crystallization process. Thus, the present ndings will be potentially useful for fabricating high quality colloidal crystals. Fig. 4 (a) Fluorescence micrographs showing spatial distributions of
impurities during unidirectional crystallization under temperature gradients. (b) Time dependent concentration profiles of impurity particles. Reprinted with permission from ref. 16. Copyright (2013) American Chemical Society.
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Acknowledgements We express our sincere gratitude to Dr C. Patrick Royall at Bristol University (United Kingdom), and Professor Satoshi Uda
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and Jun Nozawa at Tohoku University (Japan) for suggestions and discussions. We also thank Yukihiro Sugao at Nagoya City University (Japan) for performing some of the experiments described in Section 3.3.
Published on 05 February 2014. Downloaded by Georgia Institute of Technology on 31/10/2014 01:27:35.
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