THERMODYNAMICS OF FORMATION AND ORDERING IN SILVER ALKANETHIOLATES by HC12110420165

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									                                             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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