# Probing the Molecular Dynamics of Aqueous Binary Solutions with THz Time-Domain Ellipsometry

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

## 3. Results and Discussion

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## 4. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**The horizontal THz-TDSE setup used to carry out the measurements of the binary liquid solutions.

**Figure 2.**The frequency dependence of (

**a**) refractive index n and (

**b**) absorption coefficient k obtained from TDSE measurements for aqueous binary solutions with different volume concentrations $Vc$ of isopropyl alcohol. Dashed curves represent the results of a fitting procedure performed on pure water (red continuous line) and pure 2-propanol (black continuous line) using the generalized Debye relaxation model. The volume concentration increases from $0\%$ (water, □) to $100\%$ (2-propanol, ∘) according to the values listed in Table 1. The arrow indicates the increasing trend for $Vc$.

**Figure 3.**(

**a**) n and (

**b**) k plot vs. $Vc$ showing the comparison between experimental (red circles) and theoretical values expected from the Maxwell–Garnet model (dotted line) for noninteracting mixtures and from the effective Debye relaxation model (dashed line) using the dielectric parameters of the pure constituent liquids, water, and isopropyl alcohol. In the graph, the results (open black squares) directly obtained by fitting (see text) the ellipsometric data at 0.35 THz with the effective Debye relaxation model are also shown.

**Figure 4.**Comparison between the values of the effective relaxation time ${\tau}_{1,eff}$ calculated (black open squares) and directly extracted (red full circles) from the effective Debye model fit of the experimental values (n and k vs. frequency) as a function of the volume concentration Vc of the binary mixture. The dashed lines are used as guides for the eye only.

Water Volume (mL) | Isopropyl Alcohol Volume (mL) | Vc (% v/v) |
---|---|---|

20 | 0 | 0 |

20 | 0.2 | 1 |

20 | 0.6 | 3 |

20 | 0.8 | 4 |

20 | 2.2 | 10 |

20 | 6.5 | 25 |

20 | 20 | 50 |

0 | 20 | 100 |

Sample | ${\mathit{\u03f5}}_{\mathit{\infty}}$ | ${\mathit{\u03f5}}_{\mathit{s}}{\phantom{\rule{0.166667em}{0ex}}}^{\mathit{a}}$ | ${\mathit{\u03f5}}_{2}{\phantom{\rule{0.166667em}{0ex}}}^{\mathit{a}}$ | ${\mathit{\u03f5}}_{3}{\phantom{\rule{0.166667em}{0ex}}}^{\mathit{a}}$ | ${\mathit{\tau}}_{1}$ (ps) | ${\mathit{\tau}}_{2}$ (ps) | ${\mathit{\tau}}_{3}$ (ps) |
---|---|---|---|---|---|---|---|

water | 3.06 | 78.36 | - | 4.00 | 6.1 ± 0.5 | - | 0.20 ± 0.02 |

isopropyl alcohol | 2.05 | 19.42 | 4.75 | 3.86 | 350${\phantom{\rule{0.166667em}{0ex}}}^{a}$ | 5.5 ± 0.9 | 1.0 ± 0.4 |

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**MDPI and ACS Style**

Mazaheri, Z.; Papari, G.P.; Andreone, A.
Probing the Molecular Dynamics of Aqueous Binary Solutions with THz Time-Domain Ellipsometry. *Sensors* **2023**, *23*, 2292.
https://doi.org/10.3390/s23042292

**AMA Style**

Mazaheri Z, Papari GP, Andreone A.
Probing the Molecular Dynamics of Aqueous Binary Solutions with THz Time-Domain Ellipsometry. *Sensors*. 2023; 23(4):2292.
https://doi.org/10.3390/s23042292

**Chicago/Turabian Style**

Mazaheri, Zahra, Gian Paolo Papari, and Antonello Andreone.
2023. "Probing the Molecular Dynamics of Aqueous Binary Solutions with THz Time-Domain Ellipsometry" *Sensors* 23, no. 4: 2292.
https://doi.org/10.3390/s23042292