Inhaltszusammenfassung

The molecular basis for many of humic substances (HS) properties and processes still has to be investigated. Besides advanced analytical methods, molecular modeling offers the opportunity to investigate various features of HS. We investigated the wetting process of model nanopore segments (trimers of polyacrylic acid) in humic substances as function of the distance. At shorter distances between the two trimer chains an outer solvation was most stable. However, with increasing distance between...The molecular basis for many of humic substances (HS) properties and processes still has to be investigated. Besides advanced analytical methods, molecular modeling offers the opportunity to investigate various features of HS. We investigated the wetting process of model nanopore segments (trimers of polyacrylic acid) in humic substances as function of the distance. At shorter distances between the two trimer chains an outer solvation was most stable. However, with increasing distance between two trimeric segments the water molecules penetrated into the formed free space, connecting the two chains by means of a hydrogen-bonded network, which had a significant stabilization effect. The model strongly supports the hypothesized bridging function of water molecules in humic substances. Another setting with four undecanoid fatty acids served as model for spatially fixed aliphatic chains in HSs terminated by a polar (carboxyl) group. The rigidity of the oligomer chains is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. The interactions of Al3+, Ca2+ and Na+ with carboxylate and carboxyl groups and the concomitant proton transfer reactions induced by these interactions were investigated and described. The strongest effect on the local environment was observed for Al3+ cation. The calculations reflect the amphoteric character of the hydrated Al3+ complex showing in most cases its acidic character via proton transfer from the water molecules of the hydration shell to the carboxylate group. In addition, dry aluminum cation acts as a strong Lewis acid initiating electron transfers.» weiterlesen» einklappen

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