In recent years, the electric potential and water molecular orientation at oil/water interfaces, as well as the origin of the interfacial potential, have been intensively investigated. For a pristine oil/water interface, or more generally, a hydrophobic/water interface, whether there is negative electric potential induced by enrichment of negative ions such as hydroxide ion or not? Such discussion has attracted researchers’ attentions in the physical chemistry, the interface and colloids, and the spectroscopy communities.
Prof. GAN Wei’s group from Xinjiang Technical Institute of Physics and Chemistry (XTIPC), Chinese Academy of Sciences revisited this system using another second order nonlinear spectroscopic technique, Second Harmonic Generation (SHG). Researchers showed that the SHG signal radiated from the hexadecane/water interface can very sensitively reflect the structure of water molecules at this oil/water interface. By using this technique, the influence of long chain carboxylic acid at concentration as low as micromolar on molecular structure at the oil/water interface could be evaluated.
Based on this characterization method, researchers confirmed the existence of long chain carboxylic acid impurities in hexadecane samples generally used in previous studies. Also, it was showed that the hexadecane sample can be further purified by alumina columns and the carboxylic acid can be substantially reduced so that there was no influence on the oil/water interface.
Researchers confirmed the adsorption of hydroxide ions at the oil/water interface at basic aqueous solution by employing the purified hexadecane sample. This study, which provided more evidence for the understanding of the molecular structure at the oil/water interfaces, were published on RSC Advances, 2015, 5, 23578-23585.
This work drew great attention soon after its online publication. A scientist in the Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology in Germany read the article and emailed Dr. GAN Wei. He mentioned that they observed similar effect at Teflon particle surface, which represents another type of hydrophobic/water interface. They would offer the sample they are using for cooperative investigations.
The work was supported by the Thousand Talent Program, the National Natural Science Foundation of China, and the Western Action Plan from Chinese Academy of Sciences etc.
Figure: The orientational ordering of water molecules at the oil/water interface is influenced by the presence of electric charges（Image by XTIPC）
Contact:
Prof. GAN Wei
Email: ganwei@ms.xjb.ac.cn
Xinjiang Technical Institute of Physics and Chemistry
