Probing Functional Group Specific Surface Interactions in Porous Solids Using ESR Spectroscopy as a Sensitive and Quantitative Tool

Abstract

The interactions between molecular compounds and functionalized surfaces are omnipresent in many areas. For example, they are of crucial importance in all chromatographic processes using porous hosts. Developing a thorough understanding about the complex interplay of molecular scale processes involved in surface interactions is an issue of current scientific activity. In the present paper we discuss ESR spectroscopy as a powerful and easy to apply tool for examining host–guest effects within mesoporous materials. The method is tested for a series of modified, mesoporous materials comprising different surface functionalities like amine groups, carboxy groups, and others. There is a characteristic influence on the rotational characteristics of a molecular nitroxide spin probe. From temperature dependent data one can derive key thermodynamic parameters like interaction enthalpies. It is possible to differentiate (qualitatively and quantitatively) between mobile spin probes interacting mainly with the solvent and immobile guests interacting mainly with the surfaces. The polarity difference Δpv between solvent and pore-surface determines if molecular guests are in a free and highly mobile, interacting but still mobile, or interacting and immobilized state. It is interesting to note that immobilization takes place even for moderate Δpv values no matter if the surface is polar or unpolar. However, the behavior changes if there is a chemically specific interaction between the functional groups of the guests with a functionalized surface. Based on the systematic effect of the pore size on the rotational dynamics of the spin probe a qualitative model is proposed that includes a characteristic distance between confined guest species and pore surface as a function of the interaction strength.