Anomalies in Solution Behavior of an Alkyl Aminopolycarboxylic Chelating Surfactant

The solution behavior of a DTPA (diethylenetriamine pentaacetic acid)-based chelating surfactant, 4-C₁₂-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-C₁₂-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

Liquid exfoliation (LE) methods for solution processing of MoS₂ indicating the solvent used, if surfactant was employed, the final dispersion concentration, exfoliation time, and range or the average lateral nanosheet dimension reported according to AFM or TEM statistical measurements.

<p>Liquid exfoliation (LE) methods for solution processing of MoS₂ indicating the solvent used, if surfactant was employed, the final dispersion concentration, exfoliation time, and range or the average lateral nanosheet dimension reported according to AFM or TEM statistical measurements.</p

Nanosheets characterization for MoS₂ dispersions in water.

<p>Fig 1-A, SEM image of vacuum filtrated dispersion onto polycarbonate membrane, Fig 1-B, TEM image of nanosheets, Fig 1-C, AFM image of a nanosheet deposited on silicon wafer with the thickness profile on the inset. Fig 1-D, is a photograph of the dispersion before exfoliation. Fig 1-E, Fig 1-F, and Fig 1-G are the photographs shortly after, 30 days and 4 months after exfoliation respectively.</p

FTIR spectrum for MoS₂.

<p>The data for measurements were offset on the vertical axis for clarity.</p

Electrophoretic mobility measurements and estimated zeta potential for water dispersion before exfoliation on the left, dispersion measured right after liquid exfoliation on the middle and, on the right dispersion measured after 30 days of liquid exfoliation.

<p>The number of particles exceeded 1 million on each measurement. The same measurement cell and sample were used for the PSD measurements.</p

XRD measurements for the dispersion in water vacuum filtrated onto a cellulose membrane, the bulk powder deposited on a silicon wafer, the cellulose membrane filter and the modelling clay used to fix the sample onto the measurement container.

<p>The data for measurements were offset on the vertical axis for clarity. The Miller indexes for each reflexion are indicated for the bulk powder measurement [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154522#pone.0154522.ref039" target="_blank">39</a>].</p

A probability decay equation (Eq 1) was applied to the stability data reported by other authors in the referenced papers and the stability data for the present work.

<p>A probability decay equation (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154522#pone.0154522.e001" target="_blank">Eq 1</a>) was applied to the stability data reported by other authors in the referenced papers and the stability data for the present work.</p

Particle size distribution (PSD) measurements for water dispersion before exfoliation on the left and, after liquid exfoliation on the right.

<p>Particle size distribution (PSD) measurements for water dispersion before exfoliation on the left and, after liquid exfoliation on the right.</p

UV-Vis absorption spectrum for MoS₂ dispersion in water.

<p>UV-Vis absorption spectrum for MoS₂ dispersion in water.</p

Statistical analysis for the average dimensions of nanosheets.

<p>Fig 2-A, width Fig 2-B, length and Fig 2-C, thickness for 50 nanosheets. The apparent average value is written on the left corner of each histogram.</p