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The adsorption of the nuclein bases adenine, cytosine, thymine, and uracil from aqueous solutions exhibits several unusual features, especially if montmorillonites are used as the adsorbents. Adenine and cytosine are more strongly adsorbed than uracil and thymine. The amount adsorbed is sensitive to the layer charge and is at a maximum at layer charges of about 0.29 eq/(Si,Al)4O10. In the presence of adenine, the adsorption of thymine and uracil increases, probably due to base-pairing as in nucleic acids. The increased adsorption provides an instructive example of co-adsorption phenomena; however, the molar ratio of adsorbed adenine/thymine or adenine/uracil does not reach 1. Only some of the molecules are bound as base-pairs. Thymine and uracil are more strongly adsorbed if the smectite is previously loaded with adenine.

The adsorption of adenine and cytosine are highly dependent on the type and concentration of added salts. Adenine adsorption is strongly increased by salts in the following order: NaCl < NaNO3 < Nal; for cytosine adsorption, the order is: NaNO3 < Nal < NaCl. The influence of the salts is explained by their effect on the water structure.

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