Communication pubs.acs.org/cm
Thermoresponsive Colloidal Crystallization Using Adsorption of Ionic Surfactants Akiko Toyotama,† Masaaki Yamamoto,† Yuki Nakamura,† Chizuru Yamazaki,† Ayumi Tobinaga,† Yoshiaki Ohashi,† Tohru Okuzono,† Hiroshi Ozaki,‡ Fumio Uchida,‡ and Junpei Yamanaka*,† †
Faculty of Pharmaceutical Sciences, Graduate School of Nagoya City University, 3-1 Tanabe, Mizuho, Nagoya City, Aichi 467-8603, Japan ‡ Fuji Chemical Company, Ltd., 1-35-1, Deyashiki-Nishi, Hirakata City, Osaka 573-0003, Japan S Supporting Information *
U
thermally induced crystallization. Figure 1a illustrates the crystallization mechanism anticipated for the aqueous dis-
niformly shaped charged colloidal particles dispersed in water are arranged into ordered “crystal” structures when the Coulombic interaction acting between the particles is sufficiently strong.1−4 These colloidal crystals usually have Bragg wavelengths in the visible to near-infrared regimes, and thus are potential photonic materials.5,6 By immobilizing the crystal structures in polymer gel7,8 or polymer matrixes, we can obtain self-standing materials. To fabricate high-quality colloidal crystals, the ability to control the crystallization by temperature T would be valuable because ingenious crystal growth techniques for atomic/molecular systems are then available.9 However, generally temperature has little influence on colloidal crystallization.1−3 Herein, we present a versatile method for constructing colloidal crystals that melt and freeze with changing temperature, by using adsorption of ionic surfactants onto particle surfaces. We report the thermoresponsive crystallization of hydrophobic polystyrene and hydrophilic silica (SiO2) colloids, as well as those of titania (TiO2) and gold colloids, which have been anticipated to be useful photonic and plasmonic materials.10,11 Crystallization of hard sphere colloids is governed by only their particle volume fraction (ϕ). For crystallization of charged colloids major experimental parameters include the charge number of the particle (Z) and the salt concentration in the medium (Cs), in addition to ϕ.1−3,12,13 The Coulomb interaction is stronger at higher Z and lower Cs because ions screen the interaction. Generally, temperature T is not an effective variable for crystallization of charged colloids.14 However, one can control the crystallization based on the T dependence of ϕ, Z, and Cs. We reported thermally induced crystallization of silica colloids + a weak base pyridine by using the T dependence of Z.15 Based on this, we have succeeded in unidirectional crystallization15 and zone-melting of silica colloids.16 However, this method was applicable only to colloidal particles having pH-dependent charges. Surfactants are frequently used as dispersants of colloids, which adsorb onto particles to prevent coagulation. Chargeinduced crystallization using strongly adsorbing fluorinatebased ionic surfactants has been reported.17 The adsorption behavior is generally endothermic because molecules lose translational entropy upon adsorption and the resulting reduction in free energy is released as heat. When cooled, the adsorption equilibrium shifts toward heat generation; that is, the adsorbed amount is larger at lower T. For colloids containing ionic absorbents, this process should bring about © 2014 American Chemical Society
Figure 1. Illustration of the thermoresponsive crystallization of charged colloids. (a) Aqueous dispersions of hydrophobic PS particles and (b) hydrophilic silica colloidal particles.
persion of hydrophobic polystyrene (PS) particles and ionic surfactants (sodium alkylsulfates CnH2n+1SO4−Na+; Na+ ions are not shown). Here, we chose a surfactant concentration (Csurf) below the critical micelle concentration (cmc). The surfactant molecules are partly adsorbed onto the particle surfaces resulting in an increase in Z, while the rest of the surfactant molecules remain in water, resulting in an increase in Cs. Upon cooling, more surfactant molecules are adsorbed, i.e., Z increases and Cs decreases; both changes promote crystallization. On the other hand, the interaction becomes weaker on heating, resulting in a melting of crystals. The characteristics of the colloid samples used in the present study are compiled in Table 1. Here, ap is the particle radius as determined by the dynamic light-scattering method, and Z0 is the charge number in the absence of surfactant, estimated by performing electrical conductivity (for PS and silica) and zetapotential (for titania) measurements. The PS#1 particles were Received: February 18, 2014 Revised: July 6, 2014 Published: July 7, 2014 4057
dx.doi.org/10.1021/cm500580q | Chem. Mater. 2014, 26, 4057−4059
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crystal phases were identified by detecting the Bragg peaks 10 min after homogenizing the sample by shaking. We determined the melting point Tm at various values of Csurf below cmc (= 230 μM at 40 °C for SOS). The data for the sample without surfactant are shown in the red curve in Figure 2c for comparison; in this case, we tuned the interparticle interaction using NaCl. T m dropped sharply on increasing NaCl concentration (CNaCl) at around 10 μM. However, in the presence of the surfactants, we clearly observed crystallization upon cooling. For larger values of n, Csurf at a given Tm was higher, due to the adsorption of larger amounts of surfactants. We then examined colloidal silica (SiO2). Because silica surfaces are almost completely water-wet, the adsorption of alkylsulfates was negligible. On the other hand, poly(oxyethylene)-based surfactants strongly adsorb onto the silica surface in water via hydrogen bonds with silanol groups (Si− OH).19 We first added nonionic poly(oxyethylene) phenyl− nonyl ether (EPC) to the silica colloids to introduce alkyl chains on the silica, and then we added SOS [Figure 1b]. Figure 3a is the phase diagram for silica (the concentrations of EPC,
Table 1. Characteristics of Colloid Samples sample
ap (nm)
Z0
ϕ
PS#1 PS#2 PS#3 silica titania#1 titania#2 gold
215 62 66 53 270 105 50
−2990 −754 −1055 −180 −5210 −1110
0.010 0.010 0.010 0.035 0.072 0.150 0.004
obtained from Thermo Scientific (MA). PS#2 and #3 were synthesized by emulsifier-free polymerizations.18 Silica particles were purchased from Japan Catalyst Co., Ltd., Japan. All these samples were purified by dialysis and the ion exchange method. Titania#1 and #2 particles were synthesized by hydrolysis of titanium alcohoxides in Fuji Chemical Co., Ltd. (Japan). The refractive index determined by using Abbe refractometer was 2.66 at 589.3 nm and at 25 °C. Colloidal gold was purchased from Tanaka Kikinzoku Kogyo K.K. (Japan). Figure 2a shows optical micrographs of the PS#1 colloid in the presence of 4 μM sodium n-octadecylsulfate (SOS). At T =
Figure 3. (a) Crystallization phase diagram of silica colloid defined by T and CSOS. The filled symbols indicate the observed phase boundaries; the curves are simply visual guides. (b) The adsorption isotherms and the crystallization phase diagram.
CEPC = 350 μM) + SOS. Without SOS, the sample did not crystallize at any value of T because of the low Z0. At moderate CSOS, the sample exhibited crystallization upon cooling, but it reentered the liquid state at CSOS > 80 μM at low T. To verify the crystallization mechanism shown in Figure 1b, we determined the amounts of EPC and SOS adsorbed on silica particles.20 The experimental details are described in Supporting Information B. The adsorption isotherms (free surfactant concentration, C, vs adsorbed surfactant concentration, S, curves) for SOS at the three temperatures are shown in Figure 3b, which clearly indicates that the value of S increased with decreasing temperature. We note that Z and Cs of silica colloids are independently variable by additions of NaOH and NaCl.12 In Figure 3b, we superimposed a phase diagram for silica, defined by CNaOH and CNaCl, which corresponds to S and C, respectively. The hatched region in Figure 3b is the observed crystallization phase boundary. The critical values of Csos for the crystallization at the three values of T are in good agreement with the phase boundaries determined in the T vs Csos coordinate. This implies that the observed thermally induced crystallization is quantitatively explainable in terms of the temperature dependence of the adsorption of the ionic surfactant. Various functional particles are currently attainable. Colloidal crystals of titania (TiO2) have been anticipated as efficient photonic materials6,21−23 because of their high refractive index. To the best of our knowledge, only close-packed opal and inverse opal crystals of titania have been reported22 thus far.
Figure 2. (a) Optical micrographs of PS#1 colloids + SOS at two temperatures. (b) Reflection spectra of the PS#2 colloids. (c) Crystallization phase diagram of PS#3 colloids.
5 °C, the sample was in a crystalline state, whereas it melted when T was increased to 40 °C. Figure 2b is the reflection spectra of PS#2 + 100 μM SOS as a function of T (each spectrum is shifted vertically for clarity). Overviews of the samples are shown in the insets. At T = 5 °C, a sharp Bragg peak from the crystal state was observed at approximately 560 nm. Detailed spectra are provided in Supporting Information A. The crystal structure was attributable to the body-centeredcubic lattice structure. Upon heating to T = 40 °C, the Bragg peak disappeared, which implies crystal melting. These transitions were thermoreversible. We determined the crystallization phase diagram of polystyrene colloid (PS#3) in the coexistence of sodium alkylsulfates having carbon numbers n = 6−18. [Figure 2c]. The 4058
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Optical micrographs of the resulting samples are shown in Figure 4a. By adding EPC and SOS, we were able to provide
Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
The present study was partly supported by Practical Application Study, Plaza Tokai, Japan Science and Technology Corporation (JST), and by A-Step program, JST. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors sincerely thank Dr. Tsutomu Sawada, Dr. Masanobu Iwanaga, and Dr. Kazuaki Sakoda, National Institute of Materials Science, Japan, for fruitful discussion on gold colloids. Thanks are due to Sachiko Onda, Yukihiro Sugao, Ai Murakado, Manami Okachi, and Naoko Sato, Nagoya City University, for their help in the experiments.
Figure 4. (a) Optical micrograph of charged colloidal crystals of titania#1. (b) Successive change of reflection spectra of titania#2. Each spectrum is shifted vertically for clarity. (c) Image of the colloidal crystals of gold particles.
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temperature responsivity. Figure 4b shows the reflection spectra of titania#2 (CEPC = 600 μM; CSOS = 60 μM) as a function of temperature. Thermoreversible crystallization was also confirmed. Gold particles have attracted considerable attention in plasmonics.24 Opal-type two-dimensional (2D) and threedimensional (3D) crystals of nanosize gold particles have been reported.25 Hachisu et al. studied 3D crystals of submicrometersized charged gold particles in a concentrated region.26 Herein, we modified the gold particle surfaces with hexadecanethiol, which is used to prepare self-assembled monolayers on gold substrates in order to introduce hydrophobic alkyl chains. Then, we added SOS to provide surface charges. A micrograph of the colloidal gold crystal is shown in Figure 4b. Despite their small size, the locations of individual particles were detectable by optical microscopy because of the surface plasmon resonances. At CSOS = 40 μM and ϕ = 0.01, the gold colloid crystallized at T = 30 °C and melted at T greater than 50 °C. A detailed study on the phase diagram is ongoing. In this study, we demonstrated that a variety of colloidal particleshydrophobic polymers, hydrophilic metal oxides, and metal particlesexhibit crystallization upon cooling because of the adsorption of ionic surfactants. The present method should be versatile because the adsorption behavior is relatively easy to control by tuning the affinities among the particles, surfactants, and medium. On the basis of these thermoresponsive crystallizations, we can apply various crystal growth techniques developed for atomic and molecular crystals9 to colloids. Thus, the present findings would be useful to fabricate practical colloidal materials. We also expect that these colloids are valuable as models to study phase transitions in general.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The reflection spectra of PS colloidal crystal and details of the adsorption experiments. These materials are available free of charge via the Internet at http://pubs.acs.org. 4059
dx.doi.org/10.1021/cm500580q | Chem. Mater. 2014, 26, 4057−4059
