Tools to Investigate Structure at Hydrocarbon–Air Interfaces: Field-Induced Droplet Ionization Mass Spectrometry and Sum-Frequency Generation with Pulsed Lasers

An overview is presented relating to the use of field-induced droplet ionization (FIDI) and sum-frequency generation (SFG) to unravel questions pertaining to interfacial structure in general and real-world processes in particular. The present contribution focuses on the study of air crude oil interfaces as addressed by a comparison of electrospray ionization spectra and interface-sensitive FIDI methodology. FIDI reveals clear trends toward the specific orientation of select crude components at the interface. We provide evidence that the determining factor for what places itself at the interface is the unpolar nature of air. SFG as a well-established technique shows the sensitivity toward molecular alignments, and molecular interactions for a few model polar and nonpolar systems. The implications of being able to address interfacial structure are discussed in the context of determining molecular orientation in sum-frequency generation experiments as model examples. The combined information identifies factors that play a major role in impacting macroscopic properties, such as interfacial tension, which is a key player in efforts that focus on enhanced oil recovery.