This invention relates thin film nanocomposites (TFNCs) and methods of preparing the same by molecular layer-by-layer assembly. The TFNCs comprise a porous nanofibrous support first layer coated with a mid-layer having an outer separating layer, wherein the out separating layer has one or more bilayers or trilayers. The TFNCs can be particularly suitable for use as filtration membranes for the separation of dissolved components from fluids such as ultrafiltration, nanofiltration, and reverse osmosis. Thus, embodiments of the invention also include filtration systems and methods of filtering.