The key challenge with vitamin C compounds in general is stability (oxygen sensitivity), particularly with ascorbic acid. Not only does oxidation lead to loss of the active material, there is also rapid product yellowing (an aesthetic negative for the consumer). Various stabilization strategies can be attempted to address the issue, such as exclusion of oxygen during formulation, oxygen impermeable packaging, encapsulation, low pH, minimization of water, and inclusion of other antioxidants. In spite of all those approaches, in general ascorbate stability remains a challenge, and some of these approaches (e. g., very low pH) can lead to unwanted aesthetic skin effects as noted above.
For the ascorbyl phosphates (Mg and Na salts), the resulting high content of salt in product can dramatically impact the thickener system, requiring increased use of thickener ingredients. These ascorbate derivatives are also considerably more expensive than other ascorbate compounds.
Another challenge is skin delivery. Ascorbic acid’s penetration across skin is in general poor (typically less than 1% of the topical dose entering skin). For the phosphate derivates of ascorbate, skin penetration can be an even greater challenge due to the
negative charges on the phosphate moiety. Thus, the use of skin penetration enhancement approaches is desired.