Four sorbitan monoesters: XRDP and DSC studies on bulk material and ellipsometric studies on Langmuir-Blodgett monolayers

The thicknesses of Langmuir-Blodgett monolayers of four different surfactan ts, Span 20 (sorbitan monolaurate), Span 40 (sorbitan monopalmitate), Span 60 (sorbitan monostearate) and Span 80 (sorbitan mono-oleate) were measured via an ellipsometric technique. Structural studies of surfactant bulk mate rial as a function of temperature were performed by means of X-ray powder d iffraction (XRPD) and differential scanning calorimetry (DSC). The XRPD stu dies confirmed that Span 60 was the most crystalline, stable and the most s trongly packed surfactant. Results of Span 40 differed only slightly from t hose obtained with Span 60. There were no phase transitions in the DSC meas urements in the temperature range covered (22-60 degreesC). The ellipsometr y measurements confirmed that the hydrocarbon chains in the longer-chained Span 40 and Span 60 monolayers are packed perpendicular to the substrate pl ane, the corresponding monolayer thicknesses being 39 and 41 Angstrom. With Span 20 and Span 80, the chains are more curved and layers are thinner wit h the approximate monolayer thicknesses being 28 Angstrom.