A comparison of physicochemical properties of a selection of modern moisturizers: Hydrophilic index and pH

Objective: To quantify and compare the physiochemical properties of various topical emollients and to correlate these findings with the products' potential to maintain the stratum corneum (SC) acid milieu, while possessing the appropriate water content for skin rehydration, user adherence, and comfort. Material and Methods: The pH and hydrophilic fraction of 31 skin moisturizers sold in the US were measured. Hydrophilic Index (HI) was calculated using the "HI equation." The two parameters were charted using a scatter plot with quadrant divisions. Products with lower hydrophilicity were considered "more greasy" and assigned a lower HI as compared to their counterparts with a higher hydrophilicity. Results: Our findings are in good accordance with common clinical impressions: lotions generally have higher HI, while ointments have lower HI. The majority of the products tested fall into low HI, suggesting that a large percentage of the products may be rich in overall lipid content. The pH values range widely, from 3.7 to 8.2, with the majority of the products close to the physiologic skin pH of 4 to 6. Conclusion: This study introduces HI as a novel method of quantifying the aqueous content of topical emollients. When considered together with pH, the two indices can guide providers in choosing the most suitable emollients for patients with skin diseases involving altered acid mantle and barrier disruption, such as atopic dermatitis, irritant contact dermatitis, and ichthyosis vulgaris.