Hair conditioner
Hair conditioner is a hair care product that alters the texture and appearance of human hair.

# History
For centuries, Natural oils have been used to condition human hair.  These natural products are still used today, including essential oils such as tea tree oil and carrier oils such as jojoba oil.  A conditioner popular with men in the late Victorian era was Macassar oil, but this product was quite greasy and required pinning a small cloth, known as an antimacassar, to chairs and sofas to keep the upholstery from being damaged by the greasy Macassar oil.
Modern hair conditioner was created at the turn of the century, when well-known perfumer Ed. 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 mustaches.  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 allow the benefits of hair conditioner without feeling greasy or heavy.

# Ingredients
There are several types of hair conditioner ingredients, differing in composition and functionality:
- Moisturizers, whose role is to hold moisture in the hair. Usually these contain high proportions of humectants.
- Reconstructors, usually containing hydrolyzed protein. Their role is to penetrate the hair and strengthen its structure through polymer crosslinking.
- Acidifiers, acidity regulators which maintain the conditioner's pH at about 2.53.5. In contact with acidic environment, the hair's somewhat scaly surface tightens up, as the hydrogen bonds between the keratin molecules are strengthened.
- Detanglers, which modify the hair surface by pH as acidifiers, and/or by coating it with polymers, as glossers.
- Thermal protectors, usually heat-absorbing polymers, shielding the hair against excessive heat, caused by, e.g., blow-drying or curling irons or hot rollers.
- Glossers, light-reflecting chemicals which bind to the hair surface. Usually polymers, usually silicones, e.g., dimethicone or cyclomethicone.
- Oils (EFAs - essential fatty acids), which can help dry/porous hair become more  soft and pliable.  The scalp produces a natural oil called sebum. EFAs are the closest thing to natural sebum (sebum contains EFAs).
- Surfactants - Hair consists of approximately 97% of a protein called keratin. The surface of keratin contains negatively-charged amino acids. Hair conditioners therefore usually contain cationic surfactants, which don't wash out completely, because their hydrophilic ends strongly bind to keratin. The hydrophobic ends of the surfactant molecules then act as the new hair surface.
- Lubricants, such as fatty alcohols, panthenol, dimethicone, etc
- Sequestrants,  for better function in hard water.
- Antistatic agents
- Preservatives

# Types
- Pack conditioners, heavy and thick, with high content of surfactants able to bind to the hair structure and "glue" the hair surface scales together. These are usually applied to the hair for longer time. The surfactants are based on long straight aliphatic chains similar to saturated fatty acids. Their molecules have tendency to crystallize easily, giving the conditioner higher viscosity, and they tend to form thicker layers on the hair surface.
- Leave-in conditioners, thinner, containing different surfactants, adding only a little material to the hair. They are based on unsaturated chains, which are bent rather than straight. This shape makes them less prone to crystallizing, making a lighter, less viscous mixture and providing significantly thinner layer on the hair. The difference between leave-in and pack conditioners is similar to the difference between fats and oils.
- Ordinary conditioners, combining some aspects of both pack and leave-in ones.
- Hold conditioners, based on cationic polyelectrolyte polymers, holding the hair in a desired shape. These have both the function and the composition similar to diluted hair gels.

# pH
Conditioners are frequently acidic, as low pH protonates the amino acids, providing the hair with positive charge and thus more hydrogen bonds between the keratin scales, giving the hair a more compact structure. Organic acids such as citric acid are usually used to maintain acidity.

# Sources
- www.bbc.co.uk/dna/h2g2/A851627
- www.carefair.com/Hair/How_to_Choose_Natural_Oils_1311.html
- www.bookrags.com/research/hair-care-woi/