An Approach to the Estimation of Adsorption Enthalpies of Polycyclic Aromatic Hydrocarbons on Particle Surfaces

作者：Cochran Richard E; Kubatova Alena; Kozliak Evguenii I^*

来源：Journal of Physical Chemistry A, 2016, 120(30): 6029-6038.

DOI：10.1021/acs.jpca.6b03611

Current atmospheric models incorporate the values of vaporization enthalpies, Delta H-vap, obtained for neat standards, thus disregarding the matrix effects on volatilization. To test the adequacy of this approximation, this study measured enthalpies of vaporization for five polycyclic aromatic hydrocarbons (PAHs) in the form of neat standards (Delta H-vap) as well as adsorbed on the surface of silica, graphite, and graphene particles (Delta H-vap(eff)), by using simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Measurement of the corresponding activation energy values, E-a(vap) and E-a (eff)(vap), by TGA using a derivative method was shown to be the most reliable and practical way to assess Delta H-vap and Delta H-vap(eff). Enthalpies of adsorption (Delta H-ads) were then calculated from the differences between E-a(vap) and E-a (eff)(vap), thus paving a way to modeling the solid-gas phase partitioning in atmospheric particulate matter (PM). The PAH adsorption on silica particle surfaces (representing n-pi* interactions) resulted in negative values of Delta H-ads, indicating significant interactions. For graphite particles, positive Delta H-ads values were obtained; i.e., PAHs did not interact with the particle surface as strongly as observed for PM. PAHs on the surface of graphene particles evaporated in two stages, with the bulk of the mass loss occurring at temperatures lower than those with the neat standard, just as on graphite. Yet, unlike graphite, a small PAH fraction did not evaporate until higher temperatures compared to case of the neat standards and other particle surfaces (37.4-145.7 K), signifying negative, more PM-relevant values of Delta H-ads, apparently reflecting pi-pi* interactions and ranging between -7.6 and +32.6 kJ mol(-1), i.e., even larger than for silica, -3.3 to +8.3 kJ mol(-1). Thus, current atmospheric models may underestimate the partitioning of organic species in the particle phase unless matrix adsorption is taken into account.

• 出版日期2016-8-4