Liquid−Liquid Phase Separation and Microscopic Structure in Rubidium−Ammonia Solutions Observed Using X-ray Absorption Spectroscopy

Abstract

The phase behavior and microscopic structure of rubidium−ammonia (Rb−NH3) solutions have been measured below the consolute temperature (TC = 209 K), using the extended X-ray absorption fine structure (EXAFS) technique. The use of an X-ray video camera provides us with the first direct evidence for liquid−liquid-phase separation of metallic (concentrated) and nonmetallic (dilute) Rb−NH3 solutions. Our real-time X-ray imaging also gives an extraordinary glimpse of the dynamic nature of the interior liquid−liquid interface, from which bubbles of ammonia vapor “boil” without disturbing the external meniscus. Analysis of the EXAFS structure measured above the Rb K-edge absorption at ∼15.2 keV shows that the first solvation sphere of the cation is relatively broad, with ∼6.4 N atoms being located at a distance of 2.9−3.5 Å. The location of the K-edge absorption for our solution is comparable with that measured for solid rubidium oxide, which is consistent with a Rb valency of +1 in our solution.