Elucidating molecular transport mechanisms through atomistic simulations

Water scarcity is one of the grand challenges of our time.  With less than 1% of the available water supply in the form of drinking water, new materials and technologies are needed to tap unconventional water sources (e.g., brackish and seawater, and wastewater). To this end, membrane processes such as reverse osmosis (RO) offer high removal of suspended solids, organic carbon, and inorganic ions compared to conventional treatment methods based on flocculation, sedimentation, and granular media filtration. In particular, RO holds promise in augmenting the water supply through membrane-based desalination and advanced wastewater treatment. However, polyamide (PA) membranes – the cornerstone of RO – allows passage of Small (i.e., molecular weight <70 Da), charge-Neutral Contaminants (SNCs, Fig. 1), many of which endanger human health and biota; removal of SNCs requires additional treatment stages that increase RO energy costs by up to 20%. This project aims to discover highly selective membranes for (waste)water treatment by elucidating the mechanisms governing aqueous SNC sorption and transport using molecular dynamics (MD) simulation and free energy calculations. The project will focus on boric acid (BA) and NDMA (shown in Fig. 2a), two ubiquitous and toxic SNCs.

Improving the selectivity of PA RO membranes towards SNCs requires new insights into the thermodynamics of SNC sorption into membranes, as well as a molecular-level description of the transport processes driving SNCs through the membrane. Specifically, we need molecular-level insight into the hydration layer at the polyamide-water interface, to understand how interfacial water molecules, narrowly confined within ~1 nm from the membrane, determine SNC sorption and transport (Fig. 2a-b). Further, we need to elucidate the role of interfacial chemistry in SNC sorption to polyamide, to formulate surface coatings to bolster SNC rejection, and establish structure-property-performance relations linking coating composition with SNC rejection. Finally, we need to elucidate the transport mechanisms of SNCs in polyamide, to formulate transport models and quantify the trade-off between contaminant rejection and water permeance. 

The specific objectives of this project are:

Objective 1. To gain molecular-level insight into the hydration layer at the polyamide-water interface, to understand how interfacial water molecules determine SNC sorption and transport.

Objective 2. To elucidate the role of interfacial chemistry in SNC sorption to polyamide, in order to computationally develop surface coatings to bolster SNC rejection, and thus establish structure-property-performance relations linking coating composition with SNC rejection.

Objective 3. To characterise the transport mechanisms of SNCs through polyamide, to enable transport models to quantify the trade-off between contaminant rejection and water permeance.

Simulation insights emerging from this project will enable membrane manufacturers to develop highly selective RO membranes. These materials will lower the cost of seawater desalination and wastewater recycling by RO, in addition to producing safer product water for humans and ecosystems.

Research and Training

The successful applicant will conduct research in the School of Engineering at the University of Edinburgh, under the co-supervision of Dr Santiago Romero-Vargas Castrillón and Dr Rohit Pillai. The student will have access to a wide range of computational facilities, including ARCHER2, the UK’s national supercomputer. The exceptional educational and research opportunities afforded by this project include:

• training in state-of-the-art molecular simulation techniques;

• close mentoring through weekly meetings;

• close interactions with an interdisciplinary network of researchers at the Institute of Multiscale Thermofluids (IMT) and the Institute for Infrastructure and Environment (IIE) at Edinburgh;

• the opportunity to attend national and international scientific conferences to disseminate your results;

• strong emphasis and support to publish research results in leading scientific journals, which will kickstart your career in academia or industry.


Further Information: 

The University of Edinburgh is committed to equality of opportunity for all its staff and students, and promotes a culture of inclusivity. Please see details here: https://www.ed.ac.uk/equality-diversity

Closing Date: 

Wednesday, June 12, 2024
Figure 1. Summary of trace organic contaminant rejection as a function of molecular weight (MW) (data for polyamide RO membranes). Small (i.e., low MW), charge-Neutral Contaminants (SNCs), such as NDMA, exhibit lower rejection compared to charged compounds of similar molecular weight. Data from Werber et al. Environ. Sci. Technol. Lett. 2016, 3, 4, 112–120
Figure 2. (a) Molecular structure of polyamide (PA). Sorption of contaminants (e.g., boric acid and NDMA) into polyamide is driven by water density fluctuations, which create solute-sized cavities in water (H2O molecules not shown). (b) Distribution of the number of water molecules, N, inside a solute-sized probe volume placed at hydrophobic (red squares) and hydrophilic interfaces (blue circles). The probability of observing a solute-sized cavity (log p(N = 0)) is amplified at hydrophobic interfaces, thus enabling solute sorption. At the hydrophilic interface, cavities are suppressed and solute sorption is hindered.

Principal Supervisor: 


This is a challenging and scientifically ambitious project, requiring a student who is dedicated and enthusiastic about asking, and tackling, fundamental questions. The successful applicant will have been awarded an undergraduate degree at the time of appointment (2:1 or above, preferably supported by an MSc) in chemical engineering, mechanical engineering, chemistry, physics, materials science, or a cognate field. A strong background in mathematics and physics is required, as well as interest in molecular simulation. Prior research experience in modeling and simulation is highly desirable. Further information on English language requirements for EU/Overseas applicants.


Applications are welcomed from self-funded students, or students who are applying for scholarships from the University of Edinburgh or elsewhere, as explained below.

PhD studentships managed by the School of Engineering at the University of Edinburgh are available every year through a competitive process.

Applicants interested in applying for a University-administered award should e-mail the supervisors (Santiago@ed.ac.uk, R.Pillai@ed.ac.uk) as soon as possible to begin discussions, explaining how your experience meets the Applicant Requirements given above. Application deadlines vary from mid-January to late March.

Further information about funding options may be found in the following link:


Please note that most studentships are available only to Home Students (International students not eligible.)

To qualify as a Home student, you must fulfil one of the following criteria:

• You are a UK student

• You are an EU student with settled/pre-settled status who also has 3 years residency in the UK/EEA/Gibraltar/Switzerland immediately before the start of your Programme.

Further information and other funding options.

Informal Enquiries: