Molecular modelling of monolayers for evaporation suppressing materials
Creators: name: Plazzer, Michael
Publisher: name: RMIT University
Year: 2024

description: Self-assembling thin films known as monolayers have been identified as an effective and inexpensive solution to mitigate the evaporative loss of water from dams and reservoirs. A typical monolayer molecule consists of a hydrocarbon chain that is hydrophobic, and a hydrophilic headgroup with a strong affinity to water. When applied to water, these amphiphilic molecules quickly spread across the surface and assemble into a one-molecule thick film that constitutes a physical barrier to evaporation. Previously, field trials of these materials were conducted to assess their efficacy, and while some evaporation suppression was observed, their overall performance was considered poor, especially in the presence of wind. A new generation of monolayer materials have been proposed with the aim of increasing their robustness to the deleterious effects of wind, and to increase their overall evaporation mitigation properties. Computational modelling techniques such as Molecular Dynamics (MD) used in this thesis allow us to observe the interactions occurring at the monolayer/water interface at the atomistic level. We can then relate molecular structural and interaction properties with experimental observables such as evaporation resistance and monolayer stability.<br><br>Monolayers of octadecanoic acid (C18OOH) were previously proposed as an evaporation suppressant, demonstrating strong evaporation resistance under laboratory conditions. However, they were found to be ineffective in the external environment. We undertook MD simulations on this monolayer using the validated OPLS force field to establish its lack of efficacy at the molecular level. We found that although C18OOH was able to form close packed monolayers, it was unable to anchor to the water subphase via hydrogen bonding. This was followed by a more comprehensive comparative MD study on several possible evaporation suppressants. We were able to relate atomistic structural and chemical properties with experimental evaporation suppression and wind resistance. Specifically, we found that effective material must be able to form a tightly packed monolayer over a wide range of surface pressures, while maintaining strong anchoring to the water subphase via hydrogen bonding. We found that ethylene glycol-monooctadecyl-ether (C18E1) was the only monolayer molecule to meet this criteria. Multi-component monolayers have demonstrated higher than expected evaporation mitigation performance and we have conducted MD simulations in order to identify the underlying mechanism. C18E1 was blended with octadecanol (C18OH) at a range of different ratios, and its evaporation suppressing performance was observed to dramatically increase around ratios of 1:1. MD simulations showed that the uneven surface geometry of the mixed monolayer encourages the formation of water-ether oxygen hydrogen bonds, which is not possible in pure octadecanol and negligible in pure C18E1. The additional hydrogen bonding increases anchoring of the monolayer to the water, and explains the improvement in performance of these mixtures. The work performed in this thesis has provided for the first time, a molecular level understanding of the structural and dynamical features of water evaporation suppressing monolayers. Based on this we have also made a significant contribution to the development of the fundamental requirements for the design of stable and efficient material for water evaporation mitigation. <br><br>