Properties of Crystalline Sulfuric Acid in Ultrahigh Vacuum: Incongruent Melting to the Liquid Azeotrope and Its Slow Recrystallization

The growth and characterization of ultrathin (≈80 monolayers) films of pure, crystalline sulfuric acid and of the liquid H 2 SO 4 + H 2 O azeotrope in ultrahigh vacuum (UHV) are described. The films, which were deposited on a Pt(111) substrate, were synthesized in vacuo via the reaction of co-condensed mixtures of H 2 O and SO 3 . Fourier transform infrared reflection−absorption spectroscopy (FTIRAS) shows that the pure, crystalline acid forms when such mixtures are briefly annealed at 260 K. Crystalline H 2 SO 4 is stable in UHV until 285 K, at which temperature FTIRAS measurements suggest the melting occurs. The melting temperature is equal, within experimental error, to the melting point of a macroscopic sample of crystalline H 2 SO 4 . Analysis of the gas phase composition during melting reveals that SO 3 is expelled from a film as it melts. The liquid is identified as the azeotrope of H 2 SO 4 and H 2 O, formed in the incongruent melting of H 2 SO 4 . The H 2 SO 4 + H 2 O mixtures can be supercooled to 255 K. Freezing results in the formation of pure, crystalline H 2 SO 4 and is accompanied by the expulsion of H 2 O into the gas phase. To our knowledge, this work provides the first observation of melting of a molecular solid under UHV.