Journal article

Anomalies in Solution Behavior of an Alkyl Aminopolycarboxylic Chelating Surfactant

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Publication Details

Author list: EDLUND H, SVANEDAL I, NORGREN M, SVANEDAL I, Persson G, NORGREN M, EDLUND H

Publication year: 2013

Start page: 13708

End page: 13716

Number of pages: 9

ISSN: 0743-7463

DOI: http://dx.doi.org/10.1021/la4024508

View additional information: View in Web of Science


Abstract

The solution behavior of a DTPA (diethylenetriaminepentaacetic acid)-based chelating surfactant, 4-C12-DTPA, has beenstudied by tensiometry and NMR di?usometry. In the absence of metalions, the eight donor atoms in the headgroup are titrating, and thecharge of the headgroup can thus be tuned by altering the pH. 4-C12-DTPA changes from cationic at very low pH, over a number ofzwitterionic species as the pH is increased, and eventually becomesanionic at high pH. Around the isoelectric point, the chelatingsurfactant precipitated. The solution properties, studied above thesolubility gap, were found strongly pH dependent. When increasing theamount of negative charges in the headgroup, by increasing the pH, theadsorption e?ciency was reduced and the cmc was increased. Anoptimum in surface tension reduction was found at pH 5, due to aproper balance between protonated and dissociated groups. Anomaliesbetween surface tension measurements and NMR di?usometry in determination of cmc revealed a more complex relationbetween surface tension, surface coverage, and cmc than usually considered, which is not in line with the common interpretationof the Gibbs adsorption equation. At some of the investigated pH levels, measurements of bulk pH could con?rm the location ofcmc, 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 isexplained by rearrangements of the large head groups to reduce the headgroup area and increase the packing parameter.


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