Colloidal properties of single component naphthenic acids and complex naphthenic acid mixtures

Tensiometry was used to provide estimates of the critical micelle concentration (cmc) values for three sources of naphthenic acids (NAs) and three examples of single component NAs (S1-S3) in aqueous solution at pH 10.5 and 295 K. Two commercially available mixtures of NAs and an industrially derived mixture of NAs obtained from Alberta oil sands process water (OSPW) were investigated. The three examples of single component NAs (CnH2n+zO2) were chosen with variable z-series to represent chemical structures with 0-2 rings, as follows: 2-hexyldecanoic acid (z = 0; S1), trans-4-pentylcyclohexanecarboxylic acid (z = 2; S2) and dicyclohexylacetic acid (z = -4; S3). The estimated cmc values for S1 (35.6 mu M), S2 (0.545 mM), and S3 (4.71 mM) vary over a wide range according to their relative lipophile characteristics of each carboxylate anion. The cmc values for the three complex mixtures of NAs were evaluated. Two disctinct cmc values were observed (second listed in brackets) as follows: Commercial sample 1; 50.9 mu M (109 mu M), Commercial sample 2; 22.3 mu M (52.2 mu M), and Alberta derived OSPW; 154 mu M (417 mu M). These results provide strong support favouring two general classes of NAs in the mixtures investigated with distinct cmc values. We propose that the two groups may be linked to a recalcitrant fraction with a relatively large range of cmc values (52.2-417 mu M) and a readily biodegradable fraction with a relatively low range of cmc values (22.3-154 mu M) depending on the source of NAs in a given mixture. (C) 2013 Elsevier Inc. All rights reserved.