Solubility as a model of sorption

Materials able to capture other substances on their surface are called sorbents, and the importance of their research is growing with increasing environmental pollution. A larger sorbent surface leads to a higher sorption efficiency, which is reflected in the scientific community's interest in nanostructured sorbents with very large surface area. One of the possibilities of further increasing the sorption efficiency is the surface modification of the sorbent with organic molecules exhibiting a high affinity to pollutant molecules. This procedure was used by experts from J. E. Purkyně University in Ústí nad Labem when they surface-modified the SiO2 layer on magnetic nanoparticles with octadecyl chains. The resulting sorbent, which can be easily manipulated due to its magnetism, was successfully used to remove harmful molecules of benzene, toluene, ethylbenzene and xylenes (BTEX) from water.

Synthesis, characterization and testing of sorbents are expensive and time-consuming. It is therefore advisable to use computer molecular modeling for the design of sorbents. Molecular modeling can determine cheaply and in a shorter time the strength of sorbent-pollutant interaction. The modeling was carried out in the Laboratory of Molecular Modeling and Functional Nanostructures at the Nanotechnology Centre belonging to CEET at VSB-Technical University of Ostrava. The interaction of BTEX with the sorbent in water was first studied using molecular dynamics. Although shorter compared to the experiment, these calculations are still too lengthy. The results agreed with the experiment, however, to obtain them it was necessary to build complex models of the real system, i.e. sorbent–BTEX–water.

Due to the binding of octadecyl chains to the sorbent surface by only one end, while the other end of the chain is free to move, the entry of BTEX molecules between the chains resembles the process of dissolving BTEX in octadecane. Simple and fast solubility calculations performed on models containing one octadecane and one BTEX molecule indeed led to results consistent not only with molecular dynamics but also with experiment. The view of sorption of this type as a dissolution process is unique and this research introduces it for the first time. The study was published in the international scientific journal Surfaces and Interfaces.