Photons Probe Entropic Potential Variation during Molecular Confinement in Nanocavities.

Vassilios Gavriil,Zoe Kollia,Vadim Semashko,Nikolaos Spyropoulos-Antonakakis,Alkiviadis-Constantinos Cefalas,Evangelia Sarantopoulou,Margarita Chatzichristidi

doi:10.3390/e20080545

Abstract

In thin polymeric layers, external molecular analytes may well be confined within tiny surface nano/microcavities, or they may be attached to ligand adhesion binding sites via electrical dipole forces. Even though molecular trapping is followed by a variation of the entropic potential, the experimental evidence of entropic energy variation from molecular confinement is scarce because tiny thermodynamic energy density diverseness can be tracked only by sub-nm surface strain. Here, it is shown that water confinement within photon-induced nanocavities in Poly (2-hydroxyethyl methacrylate), (PHEMA) layers could be trailed by an entropic potential variation that competes with a thermodynamic potential from electric dipole attachment of molecular adsorbates in polymeric ligands. The nano/microcavities and the ligands were fabricated on a PHEMA matrix by vacuum ultraviolet laser photons at 157 nm. The entropic energy variation during confinement of water analytes on the photon processed PHEMA layer was monitored via sub-nm surface strain by applying white light reflectance spectroscopy, nanoindentation, contact angle measurements, Atomic Force Microscopy (AFM) imaging, and surface and fractal analysis. The methodology has the potency to identify entropic energy density variations less than 1 pJm−3 and to monitor dipole and entropic fields on biosurfaces.

Highlights

Contrary to classical thermodynamics describing large-scale systems in equilibrium, the mean macroscopic properties of small systems were unfolded by the introduction of an additional thermodynamic potential, which provides a tool for recounting thermodynamic diversities in small size systems
The restriction of translational, vibration, or rotational degrees of molecular freedom within tiny space regions, either from the local confinement of molecules within tiny nanocavities or from electrical dipole binding between external molecules and ligand adhesive binding sites implies major diverging functional responses from bulk counterparts
An entropic jump trails the confinement of water vapor molecules adsorbates in either nanocavities or in electric polar binding sites crafted by 157 nm laser light on PHEMA surfaces

Summary

Introduction

Contrary to classical thermodynamics describing large-scale systems in equilibrium, the mean macroscopic properties of small (nanoscale) systems were unfolded by the introduction of an additional thermodynamic potential, which provides a tool for recounting thermodynamic diversities in small size systems. Diverging responses in micro/nano systems are coming up from surface localized heterogeneities and topological features, non-extensive fluctuations, boundary, or other constraints, including molecular confinement [13,14,15,16]. The restriction of translational, vibration or rotational degrees of molecular freedom within nanoscale regions, either from local confinement of molecules within tiny nanocavities [17], Entropy 2018, 20, 545; doi:10.3390/e20080545 www.mdpi.com/journal/entropy. Entropy 2018, 20, 545 or from electrical dipole interactions between external molecules and ligand adhesive binding sites in a matrix, mainly polymeric, implies major diverging functional responses. The outcome of thermodynamic energy flow across two-dimensional (2D) topologies entails the growth of unusual localized surface strain modes at interfaces. In two seminal articles, Gadomski [18]

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