The modern concept of the dissolved organic matter (DOM) “colloid” is that of a supramolecular aggregate of molecules and ions of different sizes. Solid OM (SOM) also can be described as supramolecular aggregates that are insoluble in water or strongly attached to mineral surfaces. An important question is the nature of the molecular forces holding OM molecules together into supramolecular aggregates in the colloidal and solid states. While metal ion bridging and the hydrophobic effect are surely important, we postulate that another important force, overlooked until now, is negative charge-assisted hydrogen bonding, (-)CAHB, a subset of the Low Barrier Hydrogen Bond. The (-)CAHB is written (X...H...Y)- and can form between X-Y pairs of carboxylate, phenolate, and enolate groups on OM having similar pKa values. The (-)CAHB is among the strongest hydrogen bonds known—far stronger than ordinary hydrogen bonds and more competitive against water molecules. Examples of such bonds in the adsorption of simple compounds on carbonaceous surfaces will be presented. Evidence for (-)CAHB holding OM molecules together in SOM, included, a) water-solubility enhancement of SOM in the presence of added simple carboxylate salts (RCO2Na) compared to other polar nonionic solutes, as well the predicted pH-dependence of solubility enhancement; and b) the release of hydroxide ions when SOM is eluted with solutions of simple carboxylate ions due to the reaction: OM-O- + H2O + RCO2- = (OM-O...H...O2C-R)- + OH-.

Forces holding organic matter molecules together: Involvement of exceptionally strong hydrogen bonds

253rd National Meeting (2017)

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