Capturing Even the Invisible: A Comprehensive Understanding of Methyl Viologen Adsorption Distribution on Clay Nanosheets

Abstract

Fully understanding how dye molecules distribute over a solid surface demands three things at once: capturing the entire population─including a nonemissive component invisible to both steady-state fluorescence and fluorescence-lifetime measurements─pinpointing the interactions that govern it, and proving that it can be deliberately controlled. Here, we achieved all three for the system of methyl viologen (MV2+) and synthetic saponite (SSA). First, using the difference between the emission-intensity ratio obtained by steady-state fluorescence and that obtained from fluorescence lifetimes, we quantified the adsorption distribution─including the nonemissive H-aggregate that does not appear in fluorescence measurements─as three states: (i) isolated adsorbed monomers, (ii) closely spaced monomers self-quenched by intermolecular electron transfer, and (iii) nonemissive H-aggregates. The H-aggregate population reached about 70% of all adsorbed species at a loading of 50% relative to the CEC. Second, a van’t Hoff analysis of the equilibrium between the isolated and closely spaced monomers separated ΔH (electrostatic attraction, which stabilizes the isolated arrangement) and ΔS (hydrophobic interaction, which stabilizes the closely spaced arrangement), showing that ΔS dominates at 293 K. Third, lengthening the alkyl chain from C1 to C8, which strengthens the hydrophobic interaction without changing the cationic charge, shifted the closely spaced (short-lifetime) fraction from 63% to 88%, thereby demonstrating control of the adsorption distribution. Through quantification including the invisible component, thermodynamic understanding, and demonstrated control, this work provides a comprehensive understanding of a complex adsorption distribution and a guideline for designing two-dimensional dye arrays on inorganic surfaces.