Molecular interactions at the hexadecane/water interface in the presence of surfactants studied with second harmonic generation

Abstract

It is important to investigate the influence of surfactants on structures and physical/chemical properties of oil/water interfaces. This work reports a second harmonic generation study of the adsorption of malachite green (MG) on the surfaces of oil droplets in a hexadecane/water emulsion in the presence of surfactants including sodium dodecyl sulfate, polyoxyethylene-sorbitan monooleate (Tween80), and cetyltrimethyl ammonium bromide. It is revealed that surfactants with micromolar concentrations notably influence the adsorption of MG at the oil/water interface. Both competition adsorption and charge-charge interactions played very important roles in affecting the adsorption free energy and the surface density of MG at the oil/water interface. The sensitive detection of the changing oil/water interface with the adsorption of surfactants at such low concentrations provides more information for understanding the behavior of these surfactants at the oil/water interface.

Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions

“…Often, the experimenter is limited to extended surfaces and has to assume that conclusions from the latter are transferable to droplet surfaces. In this article, we want to resolve effects of particle size on interface charging of hexadecane in water nanoemulsions where we decorate droplets with radii ranging from 80 to 270 nm by sodium dodecyl sulfate (SDS) surfactants and study their interfaces with second-harmonic generation light scattering (SHS). ,− We will provide much needed information on the transition regime between small nano and larger microparticles and show that the surface charge density changes drastically with particle size. Our results can be rationalized within a quantitative model for interface charging which we established and that builds on existing models for the critical charge on oil droplets (Rayleigh instability) as well as on counterion condensation at nanoscale interfaces.…”

Synthesis of Fe3O4 nanocomposites for efficient separation of ultra-small oil droplets from hexadecane–water emulsions

“…S4 in the ESI †), no obvious change the size of the oil droplets was observed by DLS analysis within 18 hours. 29 The zeta potential was between approximately À20 and À30 mV. This is due to the adsorption of hydroxide ions and deprotonated fatty acid impurity in 99% pure hexadecane on the surface of oil droplets.…”

The ionic strength dependent zeta potential at the surface of hexadecane droplets in water and the corresponding interfacial adsorption of surfactants

“…40 SDS (99%, Sigma-Aldrich) was recrystallized in the mixture of DI water and ethanol (99.7%, Tianjing Baishi Chemical Co. Ltd, China) three times. 21 Sodium hydroxide (NaOH, 99.99%, Sigma Aldrich) and hydrochloric acid (AR reagent, 37% solution in water, Tianjin Baishi Chemical Co. Ltd, China) were used to adjust the pH of the emulsions or water in the experiments. Sodium chloride (NaCl, 499%, Sigma Aldrich) was heated to 500 1C for 10 hours to remove the organic impurities, dissolved in DI water and then filtered two times using a membrane (MF-Millipore, 0.025 mm), as demonstrated in the literature.…”

“…1,2 Electric interactions are recognized as molecule-molecule interactions or the interaction between molecules and a static electric field, which can be induced by external sources or uneven distribution of interfacial ionic species, for example, the electric double layers at interfaces. 3 In order to understand the behavior of ions at interfaces, experimental techniques such as chemical titration, 4,5 surface tension, 6,7 surface/interface potential, [8][9][10] Brewster angle microscopy, 8,11,12 electron paramagnetic resonance spectroscopy, 13 neutron reflection 14 and second order nonlinear spectroscopic methods 4,7,[15][16][17][18][19][20][21] have been applied.…”