What is the role/function of SDS in SDS-PAGE?

SDS (sodium dodecyl sulfate) is an anionic detergent that unfolds and denatures proteins, coating proteins in negative charge. It is added in excess to the proteins, so that the proteins’ intrinsic charge is covered, and a similar charge-to-mass ratio is obtained for all proteins. In this way, the migration rate of proteins will be dependent on their size, but not their intrinsic charge.