Faraday Discussion Meeting 2017

Sheffield played host to the three-day Faraday Discussion meeting, between the 24th-26th of July 2017 with Dr Natalia Martsinovich presenting her research.

Faraday Discussions have a special format where scientists meet to discuss research papers related to the common theme, whereby this discussion forms the majority of the meeting (i.e. no lectures). The aims of this Faraday Discussion was to promote interdisciplinary research between researchers in the area of constructing molecules; surface reactivity; characterization; and measuring and modeling of properties.

Dr Natalia Martsinovich was one of the scientists who presented work on the self-assembly of substituted terephthalic acids. Below is a summary of the paper written by Natalia.

Molecular Self-Assembly of Substituted Terephthalic Acids at the Liquid/Solid Interface: Investigating the Effect of Solvent

A. Della Pia,a D. Luo,a R. Blackwell,b G. Costantini,a and N. Martsinovichb
a. Department of Chemistry, University of Warwick, Coventry, UK.
b. Department of Chemistry, University of Sheffield, Sheffield, UK.

Molecules are able to assemble into extended ordered structures thanks to specific intermolecular interactions. These interactions are what holds, for example, DNA and protein molecules together and enables them to maintain their shape, which is essential for life on Earth. We want to exploit such interactions to be able to build a variety of molecular assemblies, for possible use in such diverse fields as biomedicine, molecular electronics, sensors and even molecular machines.

In some cases it is easy to predict the self-assembled structure of molecules simply by analysing the interactions of functional groups in the molecule. For example, carboxylic acids can form hydrogen bonds, (Figure 1). The key interaction is the attraction between the electronegative oxygen and electropositive hydrogen atoms. However, the more functional groups are present in the molecule, the more possibilities there are for interactions, so that the same type of molecules can form different assemblies – polymorphs.

The chain of terephthalic acid formed from hydrogen bonds

A collaboration between the Universities of Sheffield and Warwick looked at polymorphism in self-assembly of organic molecules. The group of Prof. Giovanni Costantini at the University of Warwick used scanning tunnelling microscopy (STM) experiments to visualise self-assembled structures formed by substituted terephthalic acid molecules (Figure 2). The molecules were adsorbed on a flat surface of graphite immersed in organic solvents. Surprisingly, slightly different arrangements were observed not only for different molecules but also for the same molecules in different solvents. In the most dramatic case, terephthalic acid molecules did not adsorb at all when phenyloctane was the solvent, but they adsorbed and formed self-assembled structures when heptanoic acid was used as a solvent.

STM image of terephthalic acid

Dr. Natalia Martsinovich and MChem student Robert Blackwell at the University of Sheffield used molecular modelling methods, taught in Level 3 of the Sheffield Chemistry degree, to create a detailed model of all interactions in this system. The aim was to understand the balance between two states of the molecules: dissolved in solution, of adsorbed into one or several self-assembled structures at the solid-liquid interface. This computer modelling of intermolecular interactions showed that subtle effects, such as good or poor solvation of the molecules, can tip the balance in favour of either dissolved state (no self-assembly) or adsorbed self-assembled state.

This study shows us that by varying the solvent – an essential ingredient in chemical experiments – we can control intermolecular assemblies and can either build or destroy self-assembled structures. Therefore, this very simple organic molecular system teaches us some useful lessons for future construction of molecular machines.