Beyond Additivity: A mixture of glucose and NaCl can influence skin hydration more than the individual compounds

The barrier function of the skin mainly relies on its outermost layer, the stratum corneum (SC), which is a lipid-protein composite biomaterial. In addition, SC contains a mixture of small polar compounds, the natural moisturizing factor (NMF). Most of the NMF components are solid at ambient relative humidities (RHs). We herein raise the question of what the effects of adding a mixture of chemicals to the SC are as compared to adding the single components. We use mixtures of glucose and NaCl, both present in NMF and in skin formulations, and the effects on SC were studied using solid-state NMR, wide angle X-ray diffraction, and sorption microbalance. The deliquescence RHs of the glucose-NaCl mixtures are lower than the individual chemicals. The same trend is also seen when these chemicals are added to SC, as the dissolution occurs at lower RH for the mixture than for the single chemicals. Notably, the mixtures can also increase the mobility of SC lipids and proteins more than the individual compounds. These data illustrate a synergistic effect by adding a mixture rather than a single polar compound to the SC, which may also have implications for the complex mixture NMF inside SC in dry conditions.