Impact of nanoconfinement on acetylacetone Equilibria in Ordered Mesoporous Silicates

Abstract

Nanoconfinement is one of the most intriguing nanoscale effects and affects several physical and chemical properties of molecules and materials, including viscosity, reaction kinetics, and glass transition temperature. In this work, liquid nuclear magnetic resonance (NMR) was used to analyze the behavior of 2,4-pentadienone in ordered mesoporous materials with a pore diameter of between 3 and 10 nm. The liquid NMR results showed meaningful changes in the hydrogen chemical shift and the keto-enol chemical equilibrium, which were associated with the pore diameter, allowing the authors to observe the effects of nanoconfinement. An interesting phenomenon was observed where the chemical equilibria of 2,4-pentadienone confined in a mesoporous material with a pore diameter of 3.5 nm was similar to that obtained with free (bulk) 2,4-pentadienone in larger pore materials. Another interesting result was observed for the enthalpy and entropy of the tautomeric equilibria of 2,4-pentadienone confined in mesoporous materials with a 5.5 nm pore diameter being −7.9 kJ mol−1 and −15.9 J mol−1.K. These values are similar to those obtained by dimethyl sulfoxide. This phenomenon indicates the possible use of ordered mesoporous materials as a reaction substitute in organic solvents. It was further observed that while the values of enthalpy (ΔH) and entropy (ΔS) had been modified by confinement, the Gibbs free energy (ΔG) value remained closer to that observed in free (bulk) 2,4-pentadienone. It is expected that this study will help in understanding the effects of nanoconfinement and provide a simple method to employ NMR techniques to analyze these phenomena.

Effects of hard nanoconfinement on polystyrene synthesized inside a complex mesoporous material KIT-6

“…Detailed characterization of the KIT-6 materials has been reported in a previous work. 5 Several material characterization data can be found in our previous work dedicated to KIT-6 characterization; representative results are shown in Fig. 2.…”

“…KIT-6 was synthesized as described in a previous article, 5 and the general procedure was as follows: 4 g of Pluronic P123 was dissolved at 35 °C, under vigorous stirring, in 144 g of distilled water and 7.9 g of HCl solution (35 wt%). After complete dissolution, 4 g of n -butanol (99.4 wt%) was added.…”

“…4 The nanoconfinement effect can be explained as any system in which a small group of atoms or molecules is spatially restricted up to 100 nm in one dimension, and meaningful modifications occur in one or more chemical or physicochemical properties of the inner molecules, atoms, or complex materials. 5 Although there is no official definition of the term, it covers most nanoconfinement systems, such as polymer thin films, [6][7][8] nanoreactors, [9][10][11] and micelles. [12][13][14] Nanoconfinement can be classified according to the number of dimensions in which confinement occurs (one, two, or three) and the rigidity of confinement, which is classi-fied as soft or hard confinement.…”

“…4 The nanoconfinement effect can be explained as any system in which a small group of atoms or molecules is spatially restricted up to 100 nm in one dimension, and meaningful modifications occur in one or more chemical or physicochemical properties of the inner molecules, atoms, or complex materials. 5 Although there is no official definition of the term, it covers most nanoconfinement systems, such as polymer thin films, 6–8 nanoreactors, 9–11 and micelles. 12–14…”