Pantene's Smooth & Sleek Conditioner provides this week's product example. Hair serum is usually a leave-on product with volatile silicones and high molecular weight silicone-based gums. Once applied to hair, it begins to lose volatile silicones, leaving the non-volatile gums to deposit as a thin hydrophobic layer on the hair shaft. Hair conditioner, unlike serum, is a wash-off product whose active constituents like quaternary ammonium salts, cationic polymers and silicones are deposited onto hair. Hair oil, on the other hand, is often used in an oiling cycle of application overnight followed by washing.