Ester formation at the liquid–solid interface

A chemical reaction (esterification) within a molecular monolayer at the liquid–solid interface without any catalyst was studied using ambient scanning tunneling microscopy. The monolayer consisted of a regular array of two species, an organic acid (trimesic acid) and an alcohol (undecan-1-ol or decan-1-ol), coadsorbed out of a solution of the acid within the alcohol at the interface of highly oriented pyrolytic graphite (HOPG) (0001) substrate. The monoester was observed promptly after reaching a threshold either related to the increased packing density of the adsorbate layer (which can be controlled by the concentration of the trimesic acid within the alcoholic solution via sonication or extended stirring) or by reaching a threshold with regards to the deposition temperature. Evidence that esterification takes place directly at the liquid–solid interface was strongly supported.


S2
were compared to the corresponding experimental parameters to finalize the models.

S4
The model with a maximum of parameter matching to an experimentally observed structure is assigned to it. Optimized LP corresponding to no sonication is similar to LP0 and LP0_1 and that at higher sonication time is LP2. LP0 is energetically ≈ 150 meV more stable than LP2. That is LP0 is energetically more favorable thanLP2, which is observed spontaneously (reported before for TMA-undecanol mixture [3,4]).
LP2 is formed only by triggering with external stimuli namely high concentration of TMA at the interface. LP4 is a relatively stable pattern compared to LP2, however, it is slightly looser packed. Ester typeI is comparable with LP0 and typeII is related to LP2.
The relative energies and packing densities of these ester types are comparable with the corresponding LP.  clearly shows an adsorption structure of the monoester as found developing from the coadsorption pattern of TMA and undecanol as well as a further hexagonal structure (yellow ovals), which is not the object discussed in this paper.
In analogy to reference [5], benzene-1,3,5-tricarbonyl trichloride was treated in CH 2 Cl 2 solution with undecanol in the presence of pyridine as acid scavenger. After aqueous work-up and subsequent column chromatography the TMA-monoundecyl ester could S7 be obtained in analytically pure form. For further infomation cf. reference [6]. Below the 1 H NMR and ESIMS spectra of the TMA-monoundecyl ester are given.  calculated (below)). Spectrum was measured using acetonitrile as solvent.    has been freshly prepared by sonication. Approximately 85% of the area consists of S16 monoester and the rest is LP. The experiment has been continued and images have been acquired in intervals of one day. Interestingly the percentage of monoester decreases with increasing time. After 12 days the entire surface has been covered with LP. In addition, average A of LP has been decreased systematically with time. At time zero it was 3.9 nm and then reduced to 3.6 nm after 9 days, finally reduced to 3.4 nm after 12 days. That is the LP structure observed after 12 days is very similar to pure LP0.

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In our previous study, the polymorphs of TMA in octanoic acid driven by concentration have been observed to be always the same for several months (> 4 months), confirming the long-time stability of the solution [7,8]. That means sonication allows to fabricate a stable saturated solutions of TMA and octanoic acid. Unlike, the concentration driven polymorphs in TMA-octanoic acid mixture, TMA-undecanol mixtures shows a limited repeatability time for concentration driven products. That is TMA and alcohol molecules presumably form a supersaturated solution, which is not stable for longer time. This is possibly due to stronger interaction between undecanol and TMA compared to TMA and alkanoic acid. Sonication allows to increase the solubility of solute in solvent via breaking the solvent-solvent interactions [7,8]. These interactions are re-established within the solvent after given time. That is the concentration of TMA decreases as time evolves and therefore the ester formation at the interface vanishes.