THERMODYNAMICS OF FORMATION AND ORDERING IN SILVER Thermochemistry ALKANETHIOLATES: THREE DIMENSIONAL ANALOGUES TO Facility SELF-ASSEMBLED MONOLAYERS Alexandra Navrotsky, Thermochemistry Facility and NEAT ORU, University of California at Davis Energetics of Microporous Materials, DMR-0101391 Silver alkanethiolates are 3D analogues to self-assembled monolayers (SAM), SAMs are applicable to microelectronics, sensors and data storage. Silver alkanethiolates undergo transitions from a layered low temperature 3D phase to a high temperature mesophase, characterized by 2D packing and hydrocarbon chain disordering. The transition temperature is chain length independent, which reflects a complex interplay (synergy) between enthalpic and entropic contributions, each of which varies linearly with chain length due to bonding at the organic-inorganic interface and confinement of the organic chains, pinned onto the inorganic framework. In situ reaction calorimetry together with solution calorimetry supports a proposed mechanism of enthalpy-driven step- wise hierarchical assembly, which involves primary directional interactions between Ag and S forming the inorganic core and secondary stacking through van der Waals interactions facilitating the formation of the 3D structure. The formation enthalpy data, inferred by systematically varying the alkyl chain length, are chain length dependent indicating an energy contribution due to alkyl chain interactions. The chain independent component of the enthalpy associated with bonding between Ag and S is consistent with energetics of SAM formation and our ab initio calculations. Figure 1. Structure of silver alkanethiolates. (a) low temperature layered phase with silver Figure 2. Comparison of energetics of the phase layers separated by thiol groups, and (b) high temperature mesophase consisting of cylindrical transition to mesophase for silver alkanethiolates ● stacks of silver clusters surrounded by thiol groups and melting of hydrocarbons ■ a Enthalpy, kJ/mol b 60 interlayer d-spacing 40 20 0 50 100 150 200 Entropy, J/K mol THERMODYNAMICS OF FORMATION AND ORDERING IN SILVER Thermochemistry ALKANETHIOLATES: THREE DIMENSIONAL ANALOGUES TO Facility SELF-ASSEMBLED MONOLAYERS Alexandra Navrotsky, Thermochemistry Facility and NEAT ORU, University of California at Davis Energetics of Microporous Materials, DMR 01-01391 Education and Outreach: This study was done in collaboration with Prof Atul N. Parikh (Applied Science Department, University of California at Davis). One high school student (Saliem Than) and two postdocs (Andrey Levchenko and Channel Yee) contributed to this work. Saliem Than had received a fellowship within the NSF SEED program to participate in this project. This program enables low-income high school students to have hands-on experience in the scientific laboratory environment. This is what Saliem wrote in the essay based on her experience in the Thermochemistry Facility: “… the time I spent as a student in the SEED program has left me with something priceless, something I will always remember.” It made her consider a profession as a chemical engineer as an attractive career. After the SEED program is over all the students involved gave talks on the work done in the lab, therefore, disseminating knowledge about chemistry and related disciplines to the prospective collage students and, also, emphasizing important notion that they can be part of the frontier research.
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