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Accueil du site > Thèmes de recherche > Atmosphère & environnement > 2.2 Interactions espèces gazeuses -surfaces > 2.2.3 Interaction of water vapour with MgCl2 × 6H2O et NaCl surfaces

2.2.3 Interaction of water vapour with MgCl2 × 6H2O et NaCl surfaces

This work deals with the laboratory study of interactions of water vapour with sea salt particles and includes the measurements of the uptake of water to dry solid films of MgCl2×6H2O and NaCl over the temperature range 240 to 340 K using a flow reactor coupled to a modulated molecular beam mass spectrometer. The following Arrhenius expression (calculated with specific BET surface area) was obtained for the initial uptake coefficient of H2O on MgCl2×6H2O films : γ0 (MgCl2) = (6.5 ± 1.0)x10-6 exp[(470 ± 40)/T]. The rate of H2O adsorption on NaCl was found to be much lower than on MgCl2×6H2O, and only an upper limit was determined for the corresponding uptake coefficient : γ (NaCl) ≤ 5.6x10-6 at T = 300K. The experimental data on H2O adsorption to MgCl2×6H2O salt surface was found to be well described by Freundlich isotherm with the heterogeneity parameter close to 0.5. The isosteric heat of adsorption was determined to be (44.7 ± 1.2) kJ mol-1 independent of the salt surface coverage in the range (0.8 – 30) x 1015 molecule cm-2. An empirical equation is proposed for the amount of water adsorbed on MgCl2×6H2O as a function of relative humidity. The observed results suggest that the rate of H2O adsorption to salt surfaces is drastically dependent on the salt sample composition and that under atmospheric conditions sea salt particles are probably enveloped by a MgCl2×6H2O brine (ACL103).