Silica particles were modified with a poly(4-vinylpyridine) (P4VP) for efficient separation of oil-in-water emulsions. Using a 3-(aminopropyl) triethoxysilane (APTES)-based molecule coupled to an appropriate radical initiator, P4VP chains were built from silica surfaces under mild reaction conditions using surface-initiated supplemental activator and reducing agent atom transfer radical polymerization. P4VP-modified silica particles were successfully used as a pH-switchable barrier for oil/water separation and proved to be efficient in removing oil from an oil-in-water nanoemulsion.