Phase studies by diffusive interfacial transport using near-infrared analysis for water (DIT-NIR)

The Diffusive Interfacial Transport (DIT) method for performing phase studi es (in which all the phases that exist along an isotherm are produced by is othermal swelling and their compositions determined by using refractive ind ex data) was introduced in 1987.(1) While this method represented a signifi cant qualitative advance in phase studies methodology, its quantitative acc uracy has since been found to be unsatisfactory. We describe herein the DIT -NIR method in which execution of the study is the same as before, but peak -area data on the bend-stretch combination band of water are used to determ ine composition. Peak-area data are obtained by using Fourier transform nea r-infrared (FT-NIR) microspectroscopy. Calibration studies with octyldimeth ylphosphine oxide-water mixtures demonstrated that peak areas vary linearly with composition over the entire composition range. The slopes of calibrat ion lines follow a power-law dependence upon temperature. Phase studies of the C12E3-water system, using both DIT-NIR and isoplethal methods, validate the applicability of the DIT-NIR method to systems other than phosphine ox ides. These data suggest that DIT-NIR may represent a new general method fo r the complete and accurate quantitative determination of binary aqueous su rfactant phase diagrams. Further, the spectroscopic data can provide new in formation on the physical state of the material under study.