Microhydration and Interfacial Activity of Triarylmethane Dyes at the Air-Water Interface

Interfacial adsorption and aggregation of triarylmethane (TAM) dyes are central to their photophysics and to spectroscopic signals measured at the air-water interface. However, the molecular mechanisms controlling how chemical substitution modulates interfacial behavior remain underexplored. In this work, we report a multiscale computational study of the interfacial behavior of a series of TAM dye cations at the air-water interface. Our results demonstrate that TAM dyes exhibit strong interfacial affinity and adsorb through a two-step process: entering the interface region in a predominantly vertical orientation followed by reorientation into a surface-bound state, with a mostly planar orientation. Moreover, the preference for planar interfacial orientation becomes more pronounced as dye size increases. We further show that at higher concentrations dyes could form long-lived small aggregates that exhibit strong attractive interactions due to dispersion that can overcome Coulombic repulsion. We also show that the qualitative adsorption and orientational trends are robust across many classical water models and temperature regimes. Finally, ensemble-averaged TDDFT spectra computed from MD snapshots with explicit solvation indicate that bulk and interfacial spectra are similar under symmetric parametrization, whereas quinone-like parametrization yields non-negligible bulk-interface shifts.