Abstract
Contrast-variation small-angle neutron scattering (CV-SANS) is a widely used technique for quantifying hydration water in soft matter systems, but it is predominantly applied in the dilute regime or for systems with a well-defined structure factor. Here, CV-SANS was used to quantify the number of hydration water molecules associating with three water-soluble polymers with different critical solution temperatures and types of water-solute interactions in dilute, semidilute, and concentrated solution through the exploration of novel methods of data fitting and analysis. Multiple SANS fitting workflows with varying levels of model assumptions were evaluated and compared to give insight into SANS model selection. These fitting pathways ranged from general, model-free algorithms to more standard form and structure factor fitting. In addition, Monte Carlo bootstrapping was evaluated as a method to estimate parameter uncertainty through simulation of technical replicates. The most robust fitting workflow for dilute solutions was found to be form factor fitting without CV-SANS (i.e. polymer in 100% D2O). For semidilute and concentrated solutions, while the model-free approach can be mathematically defined for CV-SANS data, the addition of a structure factor imposes physical constraints on the optimization problem, suggesting that the optimal fitting pathway should include appropriate form and structure factor models. The measured hydration numbers were consistent with the number of tightly bound water molecules associated with each monomer unit, and the concentration dependence of the hydration number was largely governed by the chemistry-specific interactions between water and polymer. Polymers with weaker water-polymer interactions (i.e. those with fewer hydration water molecules) were found to have more bound water at higher concentrations than those with stronger water-polymer interactions due to the increase in the number of forced water-polymer contacts in the concentrated system. This SANS-based method to count hydration water molecules can be applied to polymers in any concentration regime, which will lead to improved understanding of water-polymer interactions and their impact on materials design.
Highlights
Water plays a central role in the design and application of polymer-based materials in areas such as therapeutics,[1,2] antibiofouling,[3,4] enzyme catalysis,[5] and separations.[6]
For a polymer dissolved in a solvent blend of water and deuterium oxide, it is related to the neutron scattering length density (SLD) by eqn (1).[39] rp bp vp þ þ fnH bD2 O fnH vD2 O
The SLD of polymer is likewise obtained from small-angle neutron scattering (SANS), though the method of calculation depends on the concentration regime of the polymer solution, as illustrated in
Summary
Water plays a central role in the design and application of polymer-based materials in areas such as therapeutics,[1,2] antibiofouling,[3,4] enzyme catalysis,[5] and separations.[6]. In a system of water and polymer, there are often multiple populations of solvent. Water is typically divided into three populations: bound water, intermediate water, and bulk water.[8,9,16] These three populations have been identified by numerous groups based on mobility,[17,18,19] freezing point,[20,21,22,23]. Paper and thermal expansion.[16,24] Bound water consists of water molecules that are directly and strongly associated to the polymer via hydrogen bonds or electrostatic interactions.[25,26,27] These molecules can often stay bound to the polymer even through state changes, such as freezing[21] or macrophase separation.[28] Intermediate water is characterized by weaker interactions, such as dipole–dipole interactions or hydrophobic interactions.[26] This population often contributes to desirable properties such as anti-biofouling and protein repulsion.[8] The remainder of the water in the system is bulk water, whose molecules can diffuse around freely like molecules in pure water
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