Phase transitions in magnesium nitrate thin films: A transmission FT-IR study of the deliquescence and efflorescence of nitric acid reacted magnesium oxide interfaces

In this study, transmission FT-IR spectroscopy is used to investigate the role of water in nitric acid uptake on MgO(100) and water uptake on nitric acid reacted MgO(100) at 296 K. Under dry conditions, nitric acid uptake is limited to the topmost surface layer and saturates at a nitrate coverage of (2.3 +/- 0.1) x 1015 ions cm(-2) to form a single layer of magnesium nitrate. In the presence of water vapor at 25% relative humidity (RH), subsurface layers can react and the extent of nitric acid uptake and the formation of magnesium nitrate is significantly enhanced on MgO(100) without evidence of saturation. Following reaction with nitric acid, water adsorption/desorption isotherms have been measured in the presence of 0.2-20 Torr water vapor pressure corresponding to 1-95% RH on surfaces with varying nitrate coverages. The infrared spectra clearly show two phase transitions in these thin nitrate films as a function of increasing RH. One transition occurs at low relative humidity, <10% RH, corresponding to the formation of crystalline magnesium nitrate hydrates, Mg(NO3)(2).nH(2)O, 4 < n less than or equal to 6. The second phase transition occurs between 49 and 55% RH and corresponds to the deliquescence of crystalline Mg(NO3)(2).6H(2)O to an aqueous salt solution. Thinner films deliquesce at 49 +/- 2% RH, whereas thicker nitrate films deliquesce at 54 2% RH which is within experimental error of the value for pure Mg(NO3)(2).6H(2)O crystals. The efflorescence RH, i.e., crystallization as a function of decreasing RH, depends to an even greater extent on the film thickness. Thinner films of magnesium nitrate show no evidence of efflorescence whereas thicker films show efflorescence at 48 +/- 3%. Atomic force microscopy (AFM) images of nitric acid reacted MgO(100) surfaces show that the surface roughness significantly increases relative to the unreacted surface. The rougher surface and the size of the features, on the order of nanometers in dimension, provide nucleation sites for crystallization to occur.