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Abstract: The solid state and lyotropic phase behavior of a series of nonionic diethanolamide amphiphiles with increasing saturated hydrocarbon chain length (lauroyl, myristoyl, palmitoyl, and stearoyl) has been examined. All four saturated diethanolamide amphiphiles form a crystalline solid with two or three different polymorphic crystalline forms at room temperature. Melting points and associated enthalpies for these four amphiphiles increased with increasing chain length. Approximate partial binary phase diagrams have been constructed for each amphiphile/water system by combining Cross-Polarized Optical Microscopy (POM) and Small-Angle X-ray Scattering (SAXS) results. In the presence of water, all four diethanolamides form an L α phase, between 10% and 50% water content, and an L 2 phase with decreasing hydration and increasing temperature. In addition to the L α and L 2 phases, the shorter chain diethanolamide amphiphiles (lauroyl and myristoyl) also display a normal micellar phase (L 1) at higher water contents, occurring to lower temperatures than the L α phase. By examining the effect of subtle molecular changes on both neat and lyotropic phase behavior, amphiphiles can be designed with properties tailored to a desired application. [Copyright &y& Elsevier]