Behentrimonium chloride is an anti-static ingredient which helps to condition the hair.
Not only conditioning it also acts as a preservative by preventing the growth of bacteria in liquid solutions, and therefore help products to last longer. It is also a surfactant: partly water-soluble and partly oil-soluble, it allows oil and water to become dispersed and give products a good consistency and a gentle cleansing foam.
Behentrimonium chloride is a quaternary ammonium salt, its concentration in cosmetics is highly controlled by state regulations.
It is very similar to another conditioning ingredient called cetrimonium chloride. Indeed, only the number of carbon atoms they contain differs, which slightly changes the feel of the conditioner on the hair when you apply it, cetrimonium chloride being very light, almost imperceptible when the hair is still wet, whereas, with behentrimonium, you can really feel the product in the hair.