## Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor

## Abstract

The properties of water under confinement are of practical and fundamental interest. Here in this work we study the properties of water in the self-assembled lyotropic phases of gemini surfactants with a focus on testing the standard analysis of quasi-elastic neutron scattering (QENS) experiments. In QENS experiments the dynamic structure factor is measured and fit to models to extract the translational diffusion constant, D _{T} , and rotational relaxation time, τ _{R}. We test this procedure by using simulation results for the dynamic structure factor, extracting the dynamic parameters from the fit as is typically done in experiments, and comparing the values to those directly measured in the simulations. We find that the decoupling approximation, where the intermediate scattering function is assumed to be a product of translational and rotational contributions, is quite accurate. The jump-diffusion and isotropic rotation models, however, are not accurate when the degree of confinement is high. In particular, the exponential approximations for the intermediate scattering function fail for highly confined water and the values of D _{T} and τ _{R} can differ from the measured value by as much as a factor of two. Other models have more fit parameters, however, and with the rangemore »

- Authors:

- Univ. of Wisconsin, Madison, WI (United States). Theoretical Chemistry Inst. and Dept. of Chemistry

- Publication Date:

- Research Org.:
- Univ. of Wisconsin, Madison, WI (United States)

- Sponsoring Org.:
- USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)

- Contributing Org.:
- Extreme Science and Engineering Discovery Environment (XSEDE); the UW-Madison chemistry department cluster; and the UW-Madison Center for High Throughput Computing (CHTC).

- OSTI Identifier:
- 1433957

- Alternate Identifier(s):
- OSTI ID: 1239441; OSTI ID: 1341620

- Report Number(s):
- DOE-UWMadison-46938-2nd product

Journal ID: ISSN 0021-9606; DE-FG02-13ER46938

- Grant/Contract Number:
- SC0010328; TG-CHE090065; CHE-0840494

- Resource Type:
- Accepted Manuscript

- Journal Name:
- Journal of Chemical Physics

- Additional Journal Information:
- Journal Volume: 144; Journal Issue: 8; Journal ID: ISSN 0021-9606

- Publisher:
- American Institute of Physics (AIP)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 36 MATERIALS SCIENCE; quasielastic neutron scattering; QENS; lyotropic; double gyroid; decoupling approximation

### Citation Formats

```
Mantha, Sriteja, and Yethiraj, Arun. Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor. United States: N. p., 2016.
Web. doi:10.1063/1.4942471.
```

```
Mantha, Sriteja, & Yethiraj, Arun. Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor. United States. doi:10.1063/1.4942471.
```

```
Mantha, Sriteja, and Yethiraj, Arun. Wed .
"Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor". United States. doi:10.1063/1.4942471. https://www.osti.gov/servlets/purl/1433957.
```

```
@article{osti_1433957,
```

title = {Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor},

author = {Mantha, Sriteja and Yethiraj, Arun},

abstractNote = {The properties of water under confinement are of practical and fundamental interest. Here in this work we study the properties of water in the self-assembled lyotropic phases of gemini surfactants with a focus on testing the standard analysis of quasi-elastic neutron scattering (QENS) experiments. In QENS experiments the dynamic structure factor is measured and fit to models to extract the translational diffusion constant, DT , and rotational relaxation time, τR. We test this procedure by using simulation results for the dynamic structure factor, extracting the dynamic parameters from the fit as is typically done in experiments, and comparing the values to those directly measured in the simulations. We find that the decoupling approximation, where the intermediate scattering function is assumed to be a product of translational and rotational contributions, is quite accurate. The jump-diffusion and isotropic rotation models, however, are not accurate when the degree of confinement is high. In particular, the exponential approximations for the intermediate scattering function fail for highly confined water and the values of DT and τR can differ from the measured value by as much as a factor of two. Other models have more fit parameters, however, and with the range of energies and wave-vectors accessible to QENS, the typical analysis appears to be the best choice. In the most confined lamellar phase, the dynamics are sufficiently slow that QENS does not access a large enough time scale and neutron spin echo measurements would be a valuable technique in addition to QENS.},

doi = {10.1063/1.4942471},

journal = {Journal of Chemical Physics},

number = 8,

volume = 144,

place = {United States},

year = {2016},

month = {2}

}

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