Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 16

The first and main objective of all developments of pharmaceutical and cosmetic formulations is the stability of the active principle and the formulation as a whole. Decisive factors are the physicochemical compatibility of all substances used in the formulation. In terms of activity and stability, the pH and the buffering capacity of the hydrophilic phase also play a key role. Based on the pH partition hypothesis, that is, the nonionized species of an acid or a basic molecule is more permeable across biological barriers than the ionized formulators will pH-optimize the formulation to increase the non-ionized species of the active principle [59, 60]. Furthermore, the activity (e.g., preservatives) and stability of many vehicle ingredients are pH dependent. To maintain an optimal pH in the formulation and hence to ensure product safety and stability during shelf live, buffering systems are added.

Because product development is generally focused on product stability, it may happen that the optimal product pH is different from the physiologic skin surface pH (4.5–5.5) [61–63]. Shi et al. [62] determined the pH of hydrophilic phase of 31 skin care products sold in the United States (2012). Eighteen products from important brands showed a pH between 5.5 and 8.2. No information on the buffering capacity was given and hence it is difficult to estimate the potential impairment of the skin surface pH. However, it is understandable that the long-term use of skin care products (leave-on product) with elevated pH impairs the skin. In a recent investigation, Wohlrab and Gebert [61] determined the pH and the buffering capacity of the hydrophilic phase of 66 cosmetic skin care products from the German market. Only 43% of the evaluable products showed a pH of <5.5 and may therefore be considered appropriate for the treatment of skin with impaired barrier function or for topical skin acidification. Three products specifically developed for the acidification of the skin showed a buffering capacity >1. The majority of products had a low or very low buffering capacity.

The significance of pH and buffering capacity for product stability and quality is well known and part of every galenic product development plan. However, the targeted development of products with a specific pH and buffering capacity for skin acidification to accelerate barrier recovery to maintain or to enhance barrier integrity has gained only some attention from developers, patients, and consumers. Exceptions exist; see the section above: Topical Influencing of the Skin Surface pH [52, 56].