Provided is an interfacial polymerization process for preparation of a thin film composite membrane, which can be used for nanofiltration, forward osmosis, or reverse osmosis, particularly for use with brackish water or seawater. The process includes contacting a porous support membrane with an aqueous phase containing a polyamine to form a coated support membrane, and applying an organic phase containing a polyfunctional acyl halide to the coated support membrane to interfacially polymerize the polyamine and the polyfunctional acyl halide to form a discrimination layer of a thin film composite membrane, where during formation of the membrane, the polyfunctional acyl halide is purified in situ by removal of hydrolyzed acyl halide through addition of a salt rejection-enhancing additive that includes a biguanide compound, dicarbonate compound, pentathiodicarbonate compound, or salt thereof. Also provided are the membranes prepared by the methods and reverse osmosis modules containing the membranes.