Ultrafast Intermolecular Dynamics of Nanoconfined Water in Swollen Lipid Cubic Mesophases

Understanding the structure and dynamics of the hydrogen-bond network of water in topologically distinct swollen lipidic mesophases, is fundamental for their application in biomedical, pharmaceutical, and food science fields. Here, a positive and non-linear correlation between water hydrogen-bond dynamics and interfacial water population is uncovered in inverse bicontinuous swollen mesophases across an extended temperature range (298-340 K). Particularly, small-angle X-ray scattering determines the mesophase's structural features, uncovering a temperature-driven re-entrant phenomenon (reappearance) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>P</mml:mi> <mml:mi>n</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$Pn\bar{3}m$</mml:annotation></mml:semantics> </mml:math> phase upon heating. This topologically rich environment, however, has no detectable impact on the temperature dependence of the intermolecular modes of water, as revealed by terahertz absorption spectroscopy. Specifically, these modes show distinct dynamics: the stretching mode exhibits longer lifetimes than the libration mode, yet with a higher temperature-dependence, with approximately two-fold lower Arrhenius activation energies. In contrast, both stretching and libration modes exhibit a monotonic decrease in lifetime with increasing temperature, due to the increasing disruption of the hydrogen-bond network. Atomistic molecular dynamics simulations enable the quantification of interfacial water population, which shows a positive correlation with intermolecular lifetimes in a nonlinear manner, revealing a non-additive coupling between interfacial water population and water hydrogen-bond network dynamics within these systems.