Gravitational effects on the hydrogen bond network of water and ionic solutions revealed by near infrared spectroscopy under simulated microgravity

The near-infrared spectra of ultrapure water and five types of ionic aqueous solutions were recorded under normal and simulated microgravity (< 0.1 G). The water band at approximately 1450 nm, corresponding to the combination of O–H symmetric and antisymmetric stretching vibrations, was analyzed. The results showed that the hydrogen-bond network (HBN) weakened in both ultrapure water and all ionic solutions under microgravity. This weakening is physically reasonable when considered in terms of a slight decrease in hydrostatic pressure leading to volumetric expansion of water. Furthermore, weakening of the HBN by gravity changes was smaller than that caused by modest temperature changes of approximately 2 °C and varied depending on the type of ions present. Specifically, gravity-induced changes in the HBN of water were less pronounced in solutions containing kosmotropic anions than in solutions containing chaotropic anions. These findings suggest that even subtle changes in HBN could disrupt finely tuned biochemical reactions, potentially influencing human health in altered gravity environments.