Miscibility of Hydrogen Sulfate Planetary Ionic Liquids with Water

Abstract Hydrogen sulfate protic ionic liquids may exhibit exceptional stability under warm, low-pressure planetary conditions, where conventional solvents would evaporate, making them plausible persistent fluids on sulfur-rich planetary surfaces. Yet their physicochemical behavior under hydration remains poorly constrained. We investigate glycinium hydrogen sulfate ionic liquid [GlyH + ][ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">HSO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> ] mixed with 0%–80% water by volume using infrared spectroscopy, cryogenic electron microscopy (cryo-EM), and molecular dynamics simulations. We identify two distinct phases separated by a critical transition at ∼3 water molecules per ion pair (35% v/v water). Below this threshold, water molecules integrate into ionic liquid polar domains without disrupting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>HSO</mml:mi> <mml:mn>4</mml:mn> <mml:mrow> <mml:mspace width="0.25em"/> <mml:mspace width="0.25em"/> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">HSO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> hydrogen-bonded networks, maintaining a homogeneous phase visible by cryo-EM. Above this threshold, the mixture undergoes phase inversion, forming 10 ± 2 nm ionic liquid aggregates dispersed in continuous water. Infrared spectroscopy reveals a sharp decrease in the strong/weak hydrogen bonding ratio at this transition point. At 70% water, cryo-EM shows uniform spherical nanodomains, each containing ∼3100 ion pairs with hydrated shells surrounding neat ionic liquid cores. Molecular dynamics simulations confirm spontaneous segregation into pure water regions and intact ionic liquid domains separated by interfacial zones. These findings demonstrate that hydrogen sulfate ionic liquids exposed to episodic water on planetary surfaces would persist as functional nanoscale compartments rather than fully mixing and dissolving in water. Our work has implications for planetary solvent availability, solute aggregation, and prebiotic chemistry in nonaqueous brines.