Hair conditioner
Hair conditioner is a hair care cosmetic product used to improve the feel, texture appearance and manageability of hair. Its main purpose is to reduce friction between strands of hair to allow smoother brushing or combing, which might otherwise cause damage to the scalp. Various other benefits are often advertised, such as hair repair, strengthening, or a reduction in split ends. Conditioners are available in a wide range of forms including viscous liquids, gels, and creams, as well as thinner lotions and sprays. Hair conditioner is usually used after the hair has been washed with shampoo. It is  applied and worked into the hair, and may either be rinsed out a short time later, or left in. History. For centuries, natural oils have been used to condition human hair. A conditioner popular with men in the late Victorian era was Macassar oil, but this product was quite greasy and necessitated the pinning of a small cloth, known as an antimacassar, to the headrests of chairs and sofas to preserve the upholstery from being damaged by the oil. Modern hair conditioner was created at the turn of the 20th century when perfumer Édouard Pinaud presented a product he called  Brilliantine at the 1900 Exposition Universelle in Paris. His product was intended to soften men's hair, including beards and moustaches. Since the invention of Pinaud's early products, modern science has advanced the hair conditioner industry to include those made with silicone, fatty alcohols, and quaternary ammonium compounds. These chemical products have the benefits of hair conditioner without feeling greasy or heavy. Mechanism of action. The outermost layer of a hair follicle is called the cuticle and is composed largely of keratin. This is rich in cysteine groups which are mildly acidic. When the hair is washed these groups  can deprotonate, giving the hair a negative charge. The ingredients in conditioner, especially positively charged quaternary ammonium species, such as behentrimonium chloride or polymers that are known as Polyquaternium-XX, where XX is an arbitrary number, can then become attached to the hair via electrostatic interactions. Once attached these compounds have several effects. Their long hydrocarbon backbone helps to lubricate the surface of each hair follicle, reducing the sensation of roughness and assisting combing. The surface coating of cationic groups means that hairs are repelled from each other electrostatically, which reduces clumping. The compounds can also act as antistatic agents , which helps to reduce frizzing. Ingredients. There are several types of hair conditioner ingredients, differing in composition and functionality: pH. Conditioners are frequently acidic, as low pH protonates the keratin's amino acids. The hydrogen ions give the hair a positive charge and create more hydrogen bonds among the keratin scales, giving the hair a more compact structure. Organic acids such as citric acid are usually used to maintain acidity.