For ionization or iontophoresis to work properly, the cream or lotion used on the face must be:

For ionization or iontophoresis to be effective, the cream or lotion applied to the skin must be water-soluble. This is because the process of iontophoresis involves the use of electrical currents to enhance the penetration of products into the skin. The products used must be able to dissolve in water so that they can be successfully delivered through the skin's barrier via the electrical current.

Water-soluble substances are able to conduct electricity, which is vital for the ionization process. These substances can also easily penetrate the stratum corneum, the outermost layer of skin, when aided by the current. On the other hand, oil-based, alcohol-soluble, or silicone-based options would not perform effectively during this procedure. Oil and silicone do not mix well with water and therefore cannot create the conducive environment necessary for effective iontophoresis, while alcohol-soluble products may evaporate too quickly to be effective during application. Hence, water solubility is a key characteristic that ensures proper functionality of the product during ionization or iontophoresis.