Adsorption of water vapor on MgCl2 x 6H2O salt surface

The adsorption of water vapor on dry solid films of MgCl2 x 6H(2)O salt was studied as a function of partial pressure of water over the temperature range 240-340 K using a flow reactor coupled to a modulated molecular beam mass spectrometer. The uptake of water was found to be completely reversible. The adsorption data are well described by a Freundlich isotherm with the heterogeneity parameter close to 0.5. The isosteric enthalpy of adsorption was found to be -(44.7 +/- 1.2) kJ mol(-1), independent of the salt surface coverage in the range (0.8-30) x 10(15) molecule cm(-2). An empirical equation is proposed for the amount of water adsorbed on MgCl2 x 6H(2)O as a function of relative humidity. Uptake of H2O on MgCl2 x 6H(2)O is observed to be much higher than on NaCl indicating that the amount of surface adsorbed water on mixed salts will be drastically dependent on the salt sample composition. The observed results support previous considerations that under atmospheric conditions sea salt particles are probably enveloped by a MgCl2 x 6H(2)O brine. (C) 2011 Elsevier Ltd. All rights reserved.