3.2.1 Attenuated total reflectance fourier transform infrared
ATR-FT-IR is a common tool to characterize the functional groups of a molecule. This technique is used to monitor the process of the synthesized iso - and n -fatty acid arginate compounds. ATR-FT-IR spectra of the feedstock, intermediate, and products were acquired and shown in Figure 3 . On the spectrum of the iso-oleoyl chloride, the peak at 1800 cm-1 is attributed to the carbonyl of fatty acid-chloride (-COCl ) (Fig. 3B ), and the peak at 1710 cm-1 that corresponds to the carbonyl of carboxylic group (-CO2H) completely disappears (Fig. 3A ). On the spectrum of the final ethyl fatty acid arginate products, a broad peak at 3100-3600 cm-1 is observed (Fig. 3C&D ). These are attributed to the amine group from the arginine. And the peaks at 1736 cm-1 and 1646 cm-1 indicate the carbonyl of ethyl ester group (-CO 2CH2CH3) and amide bond (-CH2CO NH-), respectively (Fig. 3C&D ). For the case of ethyl n -lauroyl arginate hydrochloride (Fig. 4 ), the n -lauroyl chloride intermediate (Fig. 4B ) correspond to the iso -oleoyl chloride (Fig 3B ). All these peaks of the arginate product (Fig. 4C ) are consistent with the commercial LAE (Fig 4D ). For the ethyl n -oleoyl arginate and its saturated arginate form, the ATR-FT-IR spectra show a similar pattern (SFig. 2, Supplementary Information ).