1 research outputs found
Macromolecular Structure of Dodecyltrimethylammonium Chloride at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy
Adsorption
of the cationic surfactant dodecyltrimethylammonium
chloride at the silica/water interface was studied using sum frequency
generation (SFG) spectroscopy under high ionic strength (100 mM NaCl)
and at pH values ranging from 3 to 11, which are conditions relevant
to hydraulic fracturing in enhanced oil recovery operations. At surfactant
concentrations above the critical micelle concentration, SFG spectra
of the CH stretching region indicate a more noncentrosymmetric structure
for the surfactant aggregate is formed at the interface under acidic
or basic conditions compared to neutral conditions. The SFG spectra
also indicate a change in the packing/ordering of the surfactant hydrophobic
tails with pH as well. In addition, the observed changes in the SFG
spectra of water upon the addition of surfactant vary depending on
the pH. At pH 7 and 11, the SFG intensity decreases in the OH stretching
region, indicating a decrease in the magnitude of the electrostatic
potential at the interface when the cationic surfactant is adsorbed
at the negatively charged silica/water interface. At pH 3, an increase
in the SFG intensity in the OH stretching region is attributed to
an increase in the electrostatic potential at the silica/water interface
due to the adsorption of a positively charged surfactant at a pH value
close to the point of zero charge for the silica surface. These results
demonstrate how the pH can influence the macromolecular structure
of surfactants at mineral/water interfaces through the corresponding
changes in the interfacial charge density and interfacial potential.
In particular, we discuss how an unequal density of surfactants on
each side of the interfacial bilayer or the adsorbed micelles may
exist under either acidic or basic pH conditions