Energetics of self-assembly and chain confinement in silver alkanethiolates: Enthalpy-entropy interplay
Energetics of self-assembly and chain confinement in silver alkanethiolates: Enthalpy-entropy interplay
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DOI：
10.1021/cm050961i
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发表时间：
2005-11-01
影响因子：
8.6
通讯作者：
Navrotsky, A
中科院分区：
文献类型：
--
作者：
Levchenko, AA;Yee, CK;Navrotsky, A
本文报道了一系列具有不同长链取代基（n = 9，11，15和17）的烷硫醇银化合物AgS（CH_2）（n）CH_3的原位直接合成量热法的生成规律。的量热结果支持的机制，逐步分级组装涉及初级定向之间的相互作用Ag和S形成的无机核心和二级堆叠促进形成的三维结构。生成焓数据是链长依赖性的，表明由于烷基链相互作用，每个甲基的能量为-4 +/-0.5 kJ/mol。与Ag和S之间的键合相关的焓的链独立分量为-137 +/-6 kJ/mol，这与这些和相关材料的先前实验数据和从头计算一致。一种新的重结晶方法提供了显着改善整个链长系列的结构一致性。较大的纯化晶体，通过这种方法制备的，用于探测的结构，相变的热力学，和热稳定性，使用差示扫描和溶液量热法，热重分析，演化气体傅里叶变换红外光谱，和温度依赖的X-射线衍射的组合。DSC数据表明，在131摄氏度的主要相变的温度是基本上独立的烷基链取代基的长度为重结晶样品。这种链长的独立性并不反映恒定的过渡焓，而是一个复杂的相互作用之间的相互作用的熵和熵的贡献。与以前的研究一致，这种相变被分配到一个完全可逆的转换，从层状晶体结构的柱状中间相，其特征在于由无机框架的结构重排和部分构象无序的链取代基。在甲苯中的原位扫描量热法在从室温缓慢加热到110摄氏度时，其中样品似乎溶解在接近100摄氏度的甲苯中，给出了对链组装和晶体生长的洞察。在DSC中在210 ℃下观察到的第二反应是不可逆转化为无定形衍生物，最终导致由烷基链的化学分解产生的银和银硫化物晶体的形成。
The formation enthalpies by in situ direct synthesis calorimetry for a series of silver alkanethiolates, AgS(CH2)(n)CH3, with various long chain-length substituents (n = 9, 11, 15, and 17) are reported. The calorimetric results support a mechanism of stepwise hierarchical assembly involving primary directional interactions between Ag and S forming the inorganic core and secondary stacking facilitating the formation of the three-dimensional structure. The formation enthalpy data are chain-length dependent, indicating an energy of -4 +/- 0.5 kJ/mol per methyl group due to alkyl chain interactions. The chain independent component of the enthalpy associated with bonding between Ag and S is -137 +/- 6 kJ/mol, which is consistent with previous experimental data and ab initio calculations for these and related materials. A new recrystallization method offers significantly improved structural consistency across the chain-length series. Larger purified crystals, prepared by this method, were used to probe the structure, thermodynamics of phase transitions, and thermal stability, using a combination of differential scanning and solution calorimetry, thermogravimetric analysis, evolved gas Fourier transform infrared spectroscopy, and temperature-dependent X-ray diffraction. The DSC data show that the temperature of the main phase transition at 131 degrees C is essentially independent of the length of the alkyl chain substituents for recrystallized samples. This chain-length independence does not reflect constant enthalpy of transition but rather a complex interplay between enthalpic and entropic contributions. In agreement with previous studies, this phase transition is assigned to a fully reversible transformation from the layered crystalline structure to a columnar mesophase, characterized by structural rearrangements of the inorganic framework and partial conformational disordering of the chain substituents. In situ scanning calorimetry in toluene upon slow heating from room temperature to 110 degrees C, where the sample appeared to dissolve in the toluene near 100 degrees C, gives insight into chain assembly and crystal growth. The second reaction seen in DSC at 210 degrees C is an irreversible transformation to an amorphous derivative, ultimately leading to the formation of silver and silver sulfide crystals resulting from the chemical decomposition of alkyl chains.