Reflectometry Reveals Accumulation of Surfactant Impurities at Bare Oil/Water Interfaces

Abstract

Bare interfaces between water and hydrophobic media like air or oil are of fundamental scientific interest and of great relevance for numerous applications. A number of observations involving water/hydrophobic interfaces have, however, eluded a consensus mechanistic interpretation so far. Recent theoretical studies ascribe these phenomena to an interfacial accumulation of charged surfactant impurities in water. In the present work, we show that identifying surfactant accumulation with X-ray reflectometry (XRR) or neutron reflectometry (NR) is challenging under conventional contrast configurations because interfacial surfactant layers are then hardly visible. On the other hand, both XRR and NR become more sensitive to surfactant accumulation when a suitable scattering length contrast is generated by using fluorinated oil. With this approach, significant interfacial accumulation of surfactant impurities at the bare oil/water interface is observed in experiments involving standard cleaning procedures. These results suggest that surfactant impurities may be a limiting factor for the investigation of fundamental phenomena involving water/hydrophobic interfaces.

Towards a neutron and X-ray reflectometry environment for the study of solid–liquid interfaces under shear

“…Comparable films have been reported in other NR studies at interfaces with high and low interfacial energies, which are postulated to arise from fluid density depletion and/or gas present at the interface 35 – 38 . It has also been suggested that impurities within solvents and solutes could form similar layers at solid–liquid interfaces 39 , 40 . Possible contaminants at the iron oxide–dodecane interface are suggested to be gaseous molecules introduced from the atmosphere or polar contaminants that are native within the solvent, such as ambient dissolved water.…”

How Low Can You Go? Molecular Details of Low-Charge Nanoemulsion Surfaces

“…The center frequency of their IR pulses was higher (∼2800 cm –1 ) and they used a continuous D 2 O phase, so the lack of free OD could be due to IR absorption by the D 2 O aqueous phase. However, recent neutron and X-ray reflectivity experiments have identified that trace impurities occupy the “bare” alkane–water interface …”

“…This charge accumulation has not just been observed on the nanoemulsion surface, − but has also been measured at the air–water, − solid–water, and self-assembled monolayer–water interfaces. , Despite the ubiquitous nature of charge accumulation at aqueous–hydrophobic interfaces, there is no broad consensus on the identity of the charge carrier. Mainstream theories on the origins, illustrated in Figure , of the droplet’s charge include the interfacial adsorption of negatively charged ions, ,,− charge transfer mechanisms originating from asymmetric interfacial hydrogen-bonding environments, ,− and surface-adsorbed impurities. − …”

“…In recent years, it has increasingly become suspected that the interfacial charge originates from the accumulation of surface-active impurities. Recent experimental and computational approaches have provided evidence that this must be the case. − , If the highly charged nanoemulsions found in the literature are coated in surface impurities, then our fundamental understanding of the bare aqueous–hydrophobic interface is more lacking than previously thought.…”

“…Recent experimental and computational approaches have provided evidence that this must be the case. [26][27][28][29][30][31]40 If the highly charged nanoemulsions found in the literature are coated in surface impurities, then our fundamental understanding of the bare aqueous−hydrophobic interface is more lacking than previously thought. Recently, we have reported the creation of bare nanoemulsions possessing an average ZP of −10 mV and measured the vibrational spectrum of the interfacial water and hydrophobic phases using vibrational sum-frequency scattering spectroscopy (VSFSS) to directly probe the droplet interface.…”