Substantivity and After‐Rinsing Hair Growth Promotion of Cationic Microparticles of Minoxidil

Abstract

Cationic minoxidil (MXD) particles were prepared by passing a suspension containing MXD and distearyldimethylammonium chloride (DSDMAC) through a high pressure microfluidizer, operating at 500 bar to 1000 bar. The size of the particles is a few micrometers and the surface charge was+42 mV to+44 mV. The cationic MXD particles were included in a hair cleansing shampoo, of which a major detergent is sodium lauryl ether sulfate (SLES). On an UV spectrophotometer, the turbidity of the cationic MXD particles suspension increased with increasing amount of the anionic surfactant. At the same time, the surface charge of the cationic MXD particles was neutralized around equi‐molar ratio of SLES/DSDMAC, and the value became negative in the excess amount of SLES. These mean that DSDMAC adsorbed on MXD particles is complexed with SLES in the hair shampoo by an ionic interaction. Interestingly, even though the MXD particles contained in a shampoo exhibited negative surface charge, the skin‐retentive amount of MXD was appreciable and the after‐rinsing hair growth promotion effect was remarkable. One of possible mechanisms is that SLES would be desorbed from the complexed MXD particles during the rinsing step, and the charge of the particles might change from a positive value to a negative one, leading to an ionic interaction between the cationic particles and negatively charged skin.