Conformational molecular structure–surface force correlation of ethylenediaminetetracetic, nitrilotriacetic and (S,S)-ethylenediamine-N,N′-disuccinic acids in α-Al2O3 dispersions

The effects of adsorbed ethylenediaminetetracetic (EDTA), nitrilotriacetic (NTA) and (S,S)-ethylenediamine-N,N′-disuccinic (EDDS) acids on the yield stress of α-Al2O3 suspensions were evaluated to determine the nature of the inter- or intra-molecular forces responsible for the type of surface forces operating in the suspension. The conformational structures of the adsorbed additives obtained via molecular modeling subjected to MM2 energy minimization were used to explain the origin of these surface forces. At low additive concentrations and in the soluble pH region, all three additives perform as a steric agent as reflected by the large reduction in the maximum yield stress. Intra-molecular hydrogen bond was found to be present in all three additives at the pH of maximum yield stress. At higher additive concentration of 0.4 and 0.8 dwb%, an increasing maximum yield stress, now at a lower pH, was observed for both NTA and EDDS additives. This suggests the presence of an additional attractive force. All three additives were observed to form precipitate in this low pH region. Precipitate bridging is known to increase the maximum yield stress very significantly. EDTA precipitate was however the exception. It did not increase the maximum yield stress. We believe this is the first result ever reported where precipitate did not increase the maximum yield stress.

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► EDTA functions as a steric agent in suspension even in the insoluble pH region.
► NTA and EDDS bridged particles at low pH via precipitate bridging.
► Intramolecular hydrogen bond is essential for being an effective steric agent.