posted on 2024-01-17, 20:37authored byK.W. Hipps, Ursula Mazur
The
density functional theory (DFT) is used to investigate the
conversion from a solvent incorporated pseudopolymorph into a single-component
monolayer. Calculations of thermodynamic properties for the surfaces
in contact both with the gas phase and with the solvent are reported.
In the case of wetted surfaces, a simple bond-additivity model, first
proposed by Campbell and modified here, is used to augment the DFT
calculations. The model predicts a dramatic reduction in desorption
energies in the solvent as compared to the gas phase. Eyring’s
reaction rate theory is used to predict limiting desorption rates
for guest (solvent) molecules from the pockets in the pseudopolymorph
and for cobalt octaethylporphyrin (COEP) molecules in all structures.
The pseudopolymorph studied here is a nearly rectangular lattice (REC)
composed of two CoOEP and two molecules of either 1,2,4-trichlorobenzene
(TCB) or toluene (TOL) supported on 63 atoms of Au(111). At sufficiently
high initial concentrations of CoOEP, only a hexagonal unit cell (HEX)
with two molecules of CoOEP, supported on 50 atoms of gold, is observed.
Experimentally, the TCB-REC structure is more stable than the TOL-REC
structure existing in the solution at initial mM concentrations of
CoOEP in TCB as opposed to the initial μM concentration of CoOEP
in toluene. Calculations here show that the HEX structure is the thermodynamically
stable structure at all practical concentrations of CoOEP. Once the
REC structure forms kinetically at low concentrations because of the
vast excess of solvent on the surface, it is difficult to convert
it to the more stable HEX structure. The difference in stability is
primarily due to the difference in electronic adsorption energy of
the solvents (TOL or TCB) and to the very low desorption rate of CoOEP.
The adsorption energy of TCB has two important contributors: the adsorption
energy onto Au alone and the intermolecular interactions between TCB
and the CoOEP host lattice. Neither factor can be neglected. We also
find that the planar adsorption of both TOL and TCB on Au(111) is
the energetically preferred orientation when space is available on
the surface. Rates of desorption are very sensitive to the solvent
free activation energy and to the thermodynamic parameters required
to convert the solvent free activation energy to one for the solvated
surface. Small changes in the computed energy (of the order of 5%)
can lead to a 1 order of magnitude change in rates. Further, the solvation
model used does not provide the barrier to adsorption in the solution
needed to determine values for the desorption activation energy in
solution. Thus, the rates computed here for desorption into the solvent
are limiting values.