Anomalies in Solution Behavior of an Alkyl Aminopolycarboxylic
Chelating Surfactant
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Abstract
The solution behavior of a DTPA (diethylenetriamine
pentaacetic
acid)-based chelating surfactant, 4-C<sub>12</sub>-DTPA, has been
studied by tensiometry and NMR diffusometry. In the absence of metal
ions, the eight donor atoms in the headgroup are titrating, and the
charge of the headgroup can thus be tuned by altering the pH. 4-C<sub>12</sub>-DTPA changes from cationic at very low pH, over a number
of zwitterionic species as the pH is increased, and eventually becomes
anionic at high pH. Around the isoelectric point, the chelating surfactant
precipitated. The solution properties, studied above the solubility
gap, were found strongly pH dependent. When increasing the amount
of negative charges in the headgroup, by increasing the pH, the adsorption
efficiency was reduced and the cmc was increased. An optimum in surface
tension reduction was found at pH 5, due to a proper balance between
protonated and dissociated groups. Anomalies between surface tension
measurements and NMR diffusometry in determination of cmc revealed
a more complex relation between surface tension, surface coverage,
and cmc than usually considered, which is not in line with the common
interpretation of the Gibbs adsorption equation. At some of the investigated
pH levels, measurements of bulk pH could confirm the location of cmc,
due to the increased protonation of micelles compared to monomers
in solution. The adsorption of monomers to the air–water interface
showed unusually slow time dependence, evident from decreasing surface
tension for several hours. This is explained by rearrangements of
the large head groups to reduce the headgroup area and increase the
packing parameter