.2023 Apr 26;9(5):1019-1024.

doi: 10.1021/acscentsci.3c00208. eCollection 2023 May 24.

Design of Soft Material Surfaces with Rationally Tuned Water Diffusivity

Audra J DeStefano¹, My Nguyen¹, Glenn H Fredrickson^{1 2 3}, Songi Han^{1 4}, Rachel A Segalman^{1 2 3 4}

Affiliations

¹ Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States.
² Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States.
³ Department of Materials, University of California, Santa Barbara, California 93106, United States.
⁴ Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States.

PMID:37252353
PMCID: PMC10214527
DOI: 10.1021/acscentsci.3c00208

Design of Soft Material Surfaces with Rationally Tuned Water Diffusivity

Audra J DeStefano et al. ACS Cent Sci.2023.

.2023 Apr 26;9(5):1019-1024.

doi: 10.1021/acscentsci.3c00208. eCollection 2023 May 24.

Authors

Audra J DeStefano¹, My Nguyen¹, Glenn H Fredrickson^{1 2 3}, Songi Han^{1 4}, Rachel A Segalman^{1 2 3 4}

Affiliations

¹ Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States.
² Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States.
³ Department of Materials, University of California, Santa Barbara, California 93106, United States.
⁴ Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States.

PMID:37252353
PMCID: PMC10214527
DOI: 10.1021/acscentsci.3c00208

Abstract

Water structure and dynamics can be key modulators of adsorption, separations, and reactions at soft material interfaces, but systematically tuning water environments in an aqueous, accessible, and functionalizable material platform has been elusive. This work leverages variations in excluded volume to control and measure water diffusivity as a function of position within polymeric micelles using Overhauser dynamic nuclear polarization spectroscopy. Specifically, a versatile materials platform consisting of sequence-defined polypeptoids simultaneously offers a route to controlling the functional group position and a unique opportunity to generate a water diffusivity gradient extending away from the polymer micelle core. These results demonstrate an avenue not only to rationally design the chemical and structural properties of polymer surfaces but also to design and tune the local water dynamics that, in turn, can adjust the local activity for solutes.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

**Figure 1**
Sequence-defined polypeptoids enable spatialmapping of polymerand water properties. (a) Polypeptoids containing a hydrophobic 5-merblock and a hydrophilic 20-mer block self-assemble into sphericalmicelles. (b) Approximately one polypeptoid with a paramagnetic spinlabel is incorporated into each micelle. Precisely controlling thespin label position within the polypeptoid chain enables the characterizationof average segmental motion and local water dynamics throughout themicelle.

**Figure 2**
Distributions of the monomer position relativeto themicelle core(R –R_core) aredetermined by coarse-grained MD simulations. Distributions are plottedfor each hydrophilic peptoid monomer, where C6 refers to the firsthydrophilic monomer at the hydrophobic/hydrophilic transition andC25 represents the terminal hydrophilic monomer. The darker-blue distributionsrepresent distinct monomer positions within the polypeptoid chainsat which local water dynamics are measured by ODNP. Because C26 utilizesa spin label attached to the hydrated chain end, C25 is used as anapproximate position.

**Figure 3**
Water diffusivity is experimentally mapped throughout the micellecorona using seven distinct spin-label positions.D_local is slowest within about 1 nm of the hydrophobicsurface and approaches a diffusivity about one-third that of bulkwater toward the outside of the corona. Water diffusivity correlatesmore closely with the water volume fraction (⟨ϕ_water⟩) within the corona than with the distance from the surface.This suggests that excluded volume predicts water behavior near nonpolarhydrophobic surfaces (the hydrophobic micelle core). The average distancefrom the core is calculated by coarse-grained MD simulations, andthe average water volume fraction is calculated by ⟨ϕ_water⟩ = 1 –ϕ_polymer (⟨R⟩). The diffusivity of bulk wateris obtained from ref (23). Dark-blue shading represents water with buried character, whilelight-blue shading denotes surface-like behavior.

**Figure 4**
Spin label mobility serves as a proxy for thedistance from thesurface. Monomer distance from the micelle core (calculated by coarse-grainedMD simulations) correlates linearly with the rotational correlationtime (τ_c) of EPR spin probes at seven spin labelpositions. Because spin probes are incorporated into the polymer backbone,changes in rotational correlation times are expected to reflect changesin segmental motion. Close to the micelle core, polymer chains arehighly hindered (large τ_c) due to dense packing inthe core, while more water-rich regions experience higher mobility(short τ_c). For each spin label position, τ_c is normalized to the range between the largest (edge of thecore, τ_c,core) and shortest (hydrophilic chain end,τ_c,C26) τ_c values.

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References

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Design of Soft Material Surfaces with Rationally Tuned Water Diffusivity

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Design of Soft Material Surfaces with Rationally Tuned Water Diffusivity

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