In honor of Don Paul, we present herewith our contributions to structural studies of ultra-thin cellulose nano-fibers in suspension and in dried form by using synchrotron X-ray scattering/diffraction, together with other techniques. Corresponding applications to separation membranes as a barrier layer and as a substrate support will also be presented. Breakthrough concepts on water purification, especially for oil-water separations [1, 2] have been carried out. Non-woven fibrous mats have unique properties, such as interconnected pores, very large surface-to-volume ratios, high porosity, and high capacity for surface modifications. These advantages have been utilized in using nano-fibrous materials composed of different fiber cross sections and surface properties, especially with cellulose ultra-thin nano-fibers having fibrous cross-sections down to less than 10 nm by means of a variety of sustainable sources, such as wood from trees, cotton, and Jute. Figure 1 shows typical synchrotron small angle X-ray scattering of cellulose suspensions at four different concentrations, after background correction. 1. Hongyang Ma, Christian Burger, Benjamin S. Hsiao and Benjamin Chu, “Highly Permeable Polymer Membranes containing Directed Channels for Water Purification,” ACS Macro Letters, Viewpoint, 1(6), 723-726, (2012). 2. “Surface Coating on Nanofibrous Scaffolds by Interfacial Polymerization for Ultrafiltration and Desalination Applications.” Inventors: Benjamin Chu, Benjamin S. Hsiao and Kyunghwan Yoon, U.S. Provisional Application Serial Nos.: 60/872,891 (August 4, 2006) and 60/873,086 (December 06, 2006); or “Articles Comprising a Fibrous Support”, U.S. Patent No. 8,231,013 B2 (2013). 2013 Long Island Patent of the Year Award in the category of Innovation Industry.