Confined Water Molecules in Binary Mixtures of Water and 2,6-Lutidine Near Lower Solution Critical Temperature

Unraveling the interplay of temperature with molecular aggregation and miscibility in TEA-water mixtures

In the phase diagram of binary liquid mixtures, a miscibility gap is found with the concomitant liquid-liquid phase separation, wherein temperature is a key parameter in modulating the phase behavior. This includes critical temperatures such as the lower critical solution temperature (LCST) and upper critical solution temperature (UCST). Using a comprehensive approach including molecular dynamics (MD) simulation, graph theoretical analysis and spatial inhomogeneity measurement in an LCST-type mixture, we attempt to establish the relationship between the molecular aggregation pattern and phase behavior in TEA-water mixtures. At lower temperatures of binary liquid mixtures, TEA molecules tend to aggregate while simultaneously interacting with water forming a homogeneous solution. As the temperature increases, these TEA aggregates tend to self-associate by minimizing the interaction with water, which facilitates formation of two distinct liquid phases in the binary liquid. The spatial distribution analysis also reveals that the TEA aggregates compatible with water promote uniform distribution of water molecules, maintaining a homogeneous solution, while the water-incompatible ones generate isolation of water H-bond aggregates, leading to liquid-liquid phase separation in the binary system. This current study on temperature-induced molecular aggregation behavior is anticipated to contribute to a critical understanding of the phase behavior in binary liquid mixtures, including UCST, LCST, and reentrant phase behavior.

Direct Probing of Vibrational Interactions in UiO-66 Polycrystalline Membranes with Femtosecond Two-Dimensional Infrared Spectroscopy

UiO-66 is a benchmark metal-organic framework that holds great promise for the design of new functional materials. In this work, we perform two-dimensional infrared measurements on polycrystalline membranes of UiO-66 grown on c-sapphire substrates. We study the symmetric and antisymmetric stretch vibrations of the carboxylate groups of the terephthalate linker ions and find that these vibrations show a rapid energy exchange and a collective vibrational relaxation with a time constant of 1.3 ps. We also find that the symmetric vibration of the carboxylate group is strongly coupled to a vibration of the aromatic ring of the terephthalate ion. We observe that the antisymmetric carboxylate vibrations of different terephthalate linkers show rapid resonant (Förster) energy transfer with a time constant of ∼1 ps.