Terahertz microfluidic sensor for in situ exploration of hydration shell of molecules

Abstract

Hydration is the interaction between water and other molecules in aqueous solutions. Understanding the hydration mechanisms is crucial because they are involved in many biological phenomena. When a non-charged molecule is solvated in water, hydration is made through weak bonds such as hydrogen ones between the molecule and water molecules. As a consequence, the behavior of water molecules at the vicinity of the solvated molecule is changed. These molecules are called “bound water” and form the hydration shell of the molecule. Far from the molecule, the so-called “bulk water” behavior is the same as pure water. Whereas many studies have been led on this topic, many questions remain concerning the hydration shell extent. By probing directly the hydrogen bond network in water, Terahertz (THz) spectroscopy can give additional knowledge on hydration shell structure. However, because of the huge water absorption, THz measurements on liquid samples are not easy. To perform transmission measurements, powerful sources have to be used or sample volumes have to be reduced and well-controlled. This is the reason why we present here an original microfluidic THz sensor to characterize hydration of molecules in a chip. The aim of this study is to show a fruitful example of lab-on-chip/chemometrics combination with the example of subterahertz hydration probing of ethanol/water solutions in a chip. The MCR-ALS approach is used to extract the contributions of the chemical compounds. Influence of the chemical rank is also discussed. Contrary to the well-established classical model, the hydration shell seems to be composed of two different contributions. We think that this two-component behavior could be the result of the hydration shell structure composed of two bound-water layers. This interpretation is also supported by molecular dynamics simulations.