Energetics of self-assembly and chain confinement in silver alkanethiolates: Enthalpy-entropy interplay

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.