| Abstract
| - The formation enthalpies by in situ direct synthesis calorimetry for a series of silver alkanethiolates,AgS(CH2)nCH3, with various long chain-length substituents (n = 9, 11, 15, and 17) are reported. Thecalorimetric results support a mechanism of stepwise hierarchical assembly involving primary directionalinteractions between Ag and S forming the inorganic core and secondary stacking facilitating the formationof the three-dimensional structure. The formation enthalpy data are chain-length dependent, indicatingan energy of −4 ± 0.5 kJ/mol per methyl group due to alkyl chain interactions. The chain independentcomponent of the enthalpy associated with bonding between Ag and S is −137 ± 6 kJ/mol, which isconsistent with previous experimental data and ab initio calculations for these and related materials. Anew recrystallization method offers significantly improved structural consistency across the chain-lengthseries. Larger purified crystals, prepared by this method, were used to probe the structure, thermodynamicsof phase transitions, and thermal stability, using a combination of differential scanning and solutioncalorimetry, thermogravimetric analysis, evolved gas Fourier transform infrared spectroscopy, andtemperature-dependent X-ray diffraction. The DSC data show that the temperature of the main phasetransition at 131 °C is essentially independent of the length of the alkyl chain substituents for recrystallizedsamples. This chain-length independence does not reflect constant enthalpy of transition but rather acomplex interplay between enthalpic and entropic contributions. In agreement with previous studies, thisphase transition is assigned to a fully reversible transformation from the layered crystalline structure toa columnar mesophase, characterized by structural rearrangements of the inorganic framework and partialconformational disordering of the chain substituents. In situ scanning calorimetry in toluene upon slowheating from room temperature to 110 °C, where the sample appeared to dissolve in the toluene near 100°C, gives insight into chain assembly and crystal growth. The second reaction seen in DSC at 210 °C isan irreversible transformation to an amorphous derivative, ultimately leading to the formation of silverand silver sulfide crystals resulting from the chemical decomposition of alkyl chains.
- The energetics of silver alkanethiolates with various long-chain lengths, formed through 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, were directly assessed by in situ synthesis calorimetry. The thermal evolution of the materials is discussed in terms of confinement and synergy between enthalpy and entropy factors.
|
