Try adding the contents of a Vitamin E capsule to the lotion, and/or a pinch of borax.

Vitamin E won't help keep bacteria out of water. Vitamin E helps slow the rate at which oils go rancid. Bacteria growth is another matter. To keep bacteria growth down in a water infusion, you'll need something like phenonip or Germall Plus. Or, refrigerate the lotion and use within 2 weeks.