Thermoresponsive Colloidal Crystallization Using Adsorption of Ionic Surfactants
Thermoresponsive Colloidal Crystallization Using Adsorption of Ionic Surfactants
复制标题
利用离子表面活性剂吸附的热响应胶体结晶
DOI：
10.1021/cm500580q
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发表时间：
2014
期刊：
影响因子：
8.6
通讯作者：
J. Yamanaka
中科院分区：
文献类型：
--
作者：
A. Toyotama;M. Yamamoto;Y. Nakamura;C. Yamazaki;A. Tobinaga;Y. Ohashi;T. Okuzono;H. Ozaki;F. Uchida;J. Yamanaka
当粒子之间的库仑相互作用足够强时，分散在水中的均匀形状的带电胶体粒子排列成有序的“晶体”结构。1−4这些胶体晶体通常在可见光到近红外区域具有布拉格波长，因此是潜在的光子材料。5、6通过将晶体结构固定在聚合物凝胶7、8或聚合物基质中，可以获得自立性材料。为了制备高质量的胶体晶体，通过温度T控制结晶的能力将是有价值的，因为这样就可以获得用于原子/分子系统的巧妙的晶体生长技术。9但一般情况下，温度对胶体结晶影响不大。1−3在本文中，我们提出了一种通用的方法，通过在颗粒表面吸附离子表面活性剂来构建随温度变化而熔化和冻结的胶体晶体。我们报道了疏水性聚苯乙烯和亲水性二氧化硅(SiO_2)胶体以及二氧化钛(TiO_2)和金胶体的热响应结晶，这些胶体有望成为有用的光子和等离子体材料。硬球胶体的结晶只受其颗粒体积分数(ϕ)的控制。对于带电胶体的结晶，除了ϕ外，主要的实验参数还包括粒子的电荷数(Z)和介质中的盐浓度(Cs)。1−3，12，13库仑相互作用在较高的Z和较低的Cs时较强，因为离子屏蔽了这种相互作用。一般来说，温度T不是带电胶体结晶的有效变量。然而，人们可以基于ϕ、Z和Cs的T依赖关系来控制结晶。利用Z.15的T依赖性，我们报道了硅胶+弱碱吡啶的热诱导结晶。在此基础上，我们成功地实现了二氧化硅胶体的定向结晶和区域熔融。16然而，这种方法仅适用于具有pH依赖电荷的胶体颗粒。表面活性剂经常被用作胶体的分散剂，胶体吸附在颗粒上以防止凝结。报道了强吸附氟基离子表面活性剂的荷电诱导结晶。17吸附行为通常是吸热的，因为分子在吸附时失去平动熵，由此产生的自由能减少以热的形式释放。当冷却时，吸附平衡向生热方向移动，即在较低的温度下，吸附的量较大。对于含有离子吸收剂的胶体，这一过程应该会导致热诱导结晶。图1a说明了疏水性聚苯乙烯(PS)粒子和离子表面活性剂(烷基硫酸钠CnH2n+1SO4−Na+；未显示Na+离子)水分散体的预期结晶机理。这里，我们选择了低于临界胶束浓度(CMC)的表面活性剂浓度(Csurf)。表面活性剂分子部分被吸附到颗粒表面，导致Z增加，而其余表面活性剂分子留在水中，导致Cs增加。冷却后，更多的表面活性剂分子被吸附，即Z增加，Cs减少；这两个变化都促进了结晶。另一方面，加热时相互作用变弱，导致晶体熔化。本研究中使用的胶体样品的特性汇编在表1中。这里，Ap是通过动态光散射法确定的颗粒半径，Z0是在没有表面活性剂的情况下通过进行电学…估计的电荷数
Uniformly 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 gel 7, 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 crystallization 15 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 thermally induced crystallization. Figure 1a illustrates the crystallization mechanism anticipated for the aqueous dispersion 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, ie, 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 …