Gap Attack Platform 1

Emergent Properties of Fluids and Interfaces. Molecular Design of Surfaces to Control and Tune Water Properties at Interfaces.

GAP Co-Leads:   Songi Han, Nate Lynd

GAP Co-Investigators:   Ethan Crumlin, Alex Hexemer, Lynn Katz, Nate Lynd, Rachel Segalman, Scott Shell, and Mukul Sharma

Problem Statement:

Interaction of water with interfaces is crucial for separation membranes; it influences membrane surface properties and mediates interactions between the membrane surface and solutes in aqueous mixtures. Despite progress in characterizing water and dilute solute behavior at idealized interfaces, water’s interactions with and structure near complex heterogeneous surfaces remains poorly understood. Even less is known of interfacial water at surfaces in complex aqueous fluids (e.g., concentrated ionic solutions containing emulsified oil droplets at high solute concentrations common in energy applications). The primary challenge is understanding the nature of hydration layer water, which is influenced or “programmed” in complex ways by surface chemical organization. In turn, hydration layer structure and properties directly impact solute/ion adsorption on membrane surfaces and pore walls. Thus, such knowledge is critical to understanding selective adsorption which impacts, for example, membrane fouling. The objective of GAP 1 is to gain fundamental knowledge of the influence of molecular-scale membrane surface structural and chemical topology on hydration water thermodynamics and dynamics at/near complex aqueous mixture/membrane the interfaces. This knowledge will allow tuning surface characteristics to elicit desired hydration properties, including eviction of surface hydration water (the hydration “barrier”), and control of solute/ion adsorption/desorption and surface reorganization. Deliverables include rules to: (1) design membrane surfaces with hydration properties of desired solute affinity, (2) identify conditions to optimize solute/ion adsorption (fouling, separation) and desorption (membrane cleaning) kinetics, and (3) inform inverse design of membranes with tailored properties.

Research Questions:

Research questions focus on: (1) surface programming of hydration water structure and dynamics, (2) surface tuning of solute affinity (adsorption, desorption) in the thermodynamic limit, and (3) solute and ion transport mechanisms from bulk water to the membrane/water interface and interior, and vice versa, in the kinetic limit (adsorption vs. desorption pathways).

1.    What is the surface property that systematically tunes surface hydration water structure, thermodynamics, and dynamics? How does the polymer surface influence the structure and dynamics of the hydration layer? What is the effect of charged chemical groups, polymer flexibility, and chemical topology (atomic-level roughness or spatial clustering of chemical groups) on hydration water? We propose that: (1) experimentally-measured surface water diffusivity; and (2) computationally-measured surface hydrophobicity via a model hydrophobe’s surface chemical potential are strongly correlated and provide robust, accessible, predictive signatures of hydration water behavior across surface types. We also ask when the hydration layer vs. the dielectric properties of a charged polymer surface dominates.

2.    How does surface hydrophilicity tune equilibrium solute affinity and, in turn, solute concentration profiles at the membrane surface? Do measures of surface hydration water properties and desolvation barriers established in Question 1 correlate with solute affinity to surfaces? Do such correlations depend on solute and ion type? Do they break down at high solute/ion concentration, especially when dielectric effects dominate? Fundamentally, how does ion activity vary in simple or complex aqueous mixtures and at membrane surfaces?

3.    What dynamic bottlenecks limit solute or ion passage from bulk solution to the membrane-water interface and the membrane interior? How does solute or ion hydration change along the adsorption trajectory, and to what extent does surface hydration water impede or facilitate adsorption kinetics? How do desolvation barriers depend on solute and surface characteristics (e.g., uncharged vs. ionic solutes and hydrophilic vs. hydrophobic surfaces)?

Research Approach:

We pursue an integrated approach entailing systematic synthesis of model polymer surfaces with designed hydrophilicity and chemical heterogeneity, novel characterization methods to study water structure and solute adsorption, and new theories and simulation approaches to describe the water-solute-membrane interface. We will begin with simple solutions and model polymers with controlled surface structures, then extend to complex fluid mixtures and higher concentrations within the MFP, and guide UMCP polymer design in GAPs 2 & 3.


Project #1: Surface Programming of Hydration

Project Goals:

Learn and test design principles to tune surface chemistry and topology to modulate the UMCP surface hydration water thermodynamics and dynamics at the nanometer scale verified by an empirical hydration layer index (HLI).

Potential Impact:

Predictive understanding of relationships between surface structure, hydrophobicity, and hydration water properties useful for surface design.

Project #2: Programming Solvent Affinity

Project Goals:

Verify whether UMCP surfaces with systematically varying HLI systematically tunes the adsorption barrier, and if so, how does this correlation depend on the solute type and solvent type?

Knowledge to precisely engineer surfaces to influence dilute adsorption.

Potential Impact:

Predictive understanding of relationships between surface structure, hydrophobicity, and hydration water properties useful for surface design.

Project #3: Solute and Ion Transport to/from Surface

Project Goals:

Test whether variation in HLI (rooted in surface modification and hydration water effects at the nanometer scale) will modulate transport and dynamics of solutes from the molecular to macroscopic (mm – cm) scales. Study how choices in the UMCP and MFP tunes this relationship.

Knowledge to precisely engineer surfaces to influence solute adsorption.

GAP Attack Platform 1

Research Highlights - GAP 1

The Donnan Potential Revealed

Using tender-APXPS, we have directly measured the Donnan potential at the membrane/solution interface for CR-61 membranes equilibrated with NaCl and MgCl2 solutions.

Effects of Amphiphilic Polypeptoid Side Chains on Polymer Surface Chemistry and Hydrophilicity

In situ ambient pressure X-ray photoelectron spectroscopy (APXPS) reveals that polypeptoid-modifications can alter polymer interactions with water and instigate surface restructuring.

Sequence modulates polypeptoid hydration water structure and dynamics

Demonstrated that changes in both the number and placement of nonpolar monomers in the polypeptoid sequence affects both local water structure and dynamics and that such effects are correlated.

Quantifying polypeptoid conformational landscapes through integrated experiment and simulation

Proposed and cross-validated experimental and computational workflows to analyze structural populations of disordered polypeptoids.

Molecular Orientation and Structure in Model Ultrathin Layer-by-Layer Polyamide RO Membranes

Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy reveals molecular orientation varies with film thickness in layer-by-layer grown model RO membranes.

Simulations Reveal Complexity of Solute Affinity to Surfaces

Phase-field simulations demonstrate that mass-transfer-driven spinodal decomposition, thermal fluctuations, and structural arrest are essential to the formation of asymmetric polymer membranes in the Nonsolvent-Induced Phase Separation process.

Quantifying Polypeptoid Conformational Landscapes Through Integrated Experiment and Simulation

Proposed and cross-validated experimental and computational workflows to analyze structural populations of disordered polypeptoids.

End-to-end Distance Measurements of Aqueous Low-Molecular-Weight Polyethylene Oxide

Fully resolved end-to-end distance probability distribution measurements and simulations for aqueous polyethylene oxide (PEO) establish semi-flexible, excluded volume polymer.

Directly Probing Polymer Thin Film Chemistry and Counterions Influence on Water Sorption

Tender APXPS results provide direct quantification of hydration on polymers in situ at the molecular level.