Mr. Bubble Has Never Been So Dangerous
Soap pops up a few times in the different time periods of Cloud Atlas. Both Vyvyan Ayrs and Timothy Cavendish mention people forced to eat soap. Neither of them realize that this is a premonition of things to come.
The fabricants of Nea So Copros, including Sonmi-451, have to ingest a chemical compound called Soap every night. Whether it's made of Dawn dish detergent or something else entirely isn't specified. What it does do, as soap usually does, is cleanse the body. With a fabricant, though, it does this in ways you wouldn't normally expect.
Like Ralphie in A Christmas Story, Timothy Cavendish is told by Nurse Noakes he'll have his dirty words washed out of his mouth with soap. For fabricants, soap washes words right out of their minds. Sonmi says, "Soap erases xtra words we acquire later" (5.1.26). It "cleans" the fabricants out, and leaves them fresh the next day. It's the perfect way to keep them ignorant and submissive. An ignorant slave is a happy slave.