Macromolecular Structure of Dodecyltrimethylammonium Chloride at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy

Abstract

Adsorption of the cationic surfactant dodecyltrimethylammonium chloride (DTAC) 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 (CMC), SFG spectra of the CH stretching region indicate a more non-centrosymmetric 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 electrostatic potential at the silica/water interface due to the adsorption of positively charged surfactant at a pH value close to the point of zero charge for the silica surface. These results demonstrate how pH can influence the macromolecular structure of surfactants at mineral/water interfaces through the corresponding changes in 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.

Cationic-Surfactant-Coated Mica Surfaces below the Critical Micellar Concentration: 1. Patchy Structures As Revealed by Peak Force Tapping AFM Mode

“…In agreement with direct force measurements both on mica ,, and on glass or silica, − all of the reflectometry studies support the idea that C 16 TAB adsorption occurs via the formation of mono/bilayer-like fragments, and the surface is not fully covered when the substrate is immersed in surfactant solutions well below the cmc. Interestingly, the same conclusions about solids covered heterogeneously by surfactant patches are obtained not only on small platelets but also on curved surfaces, either colloidal particles or those with the inverse geometry, such as pores in solid materials, using different techniques ranging from small-angle scattering , and X-ray diffraction, spectroscopic techniques (infrared, − fluorescence quenching, Raman, ,, and sum frequency generation (SFG) spectroscopy) to NMR techniques (refs − and references therein). Thus, the structural information gained from dynamic NMR leads to the same structural picture as obtained from reflectometry techniques: there is the formation of anisotropic surface aggregates whose shape is disc-shaped micelles or bilayer-like patches. ,, For these high-charge-density surfaces such as mica and silica, calorimetric studies are also consistent with the interpretation of small discrete aggregates at the lowest adsorption densities and bilayer formation in the later stage of adsorption. , Remarkably, the surface aggregate shape does not change significantly as a function of surface coverage: only the number of defects in the distorted and disrupted bilayer-like film increases as the surface coverage becomes low, and conversely a complete bilayer is found at the adsorption plateau.…”

“…54 In agreement with direct force measurements both on mica 45,51,71 and on glass or silica, [72][73][74] all the reflectometry studies support the idea that C16TAB adsorption occurs via the formation of mono/bi-layered-like fragments, and the surface is not fully covered when the substrate is immersed in surfactant solutions well below the cmc. Interestingly, the same conclusions about solids covered heterogeneously by surfactant patches are obtained not only on small platelets, but also on curved surfaces, either colloidal particles, or of the inverse geometry, such as pores in solid materials, using different techniques ranging from small angle scattering 75,76 and X-ray diffraction, 77 spectroscopic techniques (infrared, [78][79][80][81][82] fluorescence quenching, 83 Raman, 82,84,85 sum frequency generation (SFG) spectroscopy 86 ) to NMR techniques (see ref. 87-89 and references therein).…”

Dependence of the Surface Structure of Polystyrene on Chain Molecular Weight Investigated by Sum Frequency Generation Spectroscopy

“…SFG Experimental Procedure. As described previously, 24 the scanning SFG system (EKSPLA) consists of a PL2241 series picosecond laser that generates fundamental (1064 nm) and second harmonic (532 nm) beams that pump a PG501/DFG parametric generator, which produces a tunable IR beam. The 532 nm and tunable IR laser are then overlapped in time and space on the polystyrene surface.…”

Understanding the Different Steps of Surfactant Adsorption at the Oil–Water Interface with Second Harmonic Generation

“…This should originate from the effect of counterions in the water phase, that is, the interaction of Na + with the interface after the adsorption of DS – and the attraction of Br – to the interface after the adsorption of CTA + . A similar interaction between ionic surfactants and counterions has been studied by SFG-VS and SHG. ,− The effect of monovalent ions on the headgroup of lipid layers at the water interface has also been experimentally and theoretically investigated. , The significant increase of the interfacial potential with increasing concentration at very low surfactant concentrations, as well as the following gradual decrease, not only reveals the initial adsorption step of the hydrophobic part of the surfactants, which has not been described before, but also provides information about the electric charge and molecular structure of the interface during the adsorption process following this initial step.…”