Pack conditioners are heavy and thick, with a 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 a longer time. The surfactants are based on long straight aliphatic fatty acid chains similar to saturated fatty acids. Their molecules have a tendency to crystallize easily, giving the conditioner higher viscosity, and they tend to form thicker layers on the hair surface.
Leave-in conditioners are thinner and have different surfactants which add only a little material to the hair. They are based on unsaturated fatty acid chains, which are bent, not straight. This shape makes them less prone to crystallizing, making a lighter, less viscous mixture and providing a significantly thinner layer on the hair. The difference between leave-in and pack conditioners is similar to the difference between fats and oils. Leave-in conditioner is designed to be used in a similar way to hair oil, preventing the tangling of hair and keeping it smooth. Its use is particularly prevalent by those with naturally curly or kinky hair.