A Comparison of Physicochemical Properties of a Selection of Modern Moisturizers: Hydrophilic Index and pH

May 2012 | Volume 11 | Issue 5 | Original Article | 633 | Copyright © May 2012


Vivian Y. Shi BS, a Khiem Tran PhD, b and Peter A. Lio MD c

a University of Illinois, College of Medicine, Chicago, IL b University of Illinois College of Medicine, Department of Pharmacology, Chicago, IL c Northwestern University Feinberg School of Medicine, Department of Dermatology, Chicago, IL

Abstract
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.

J Drugs Dermatol. 2012;11(5):633-636.

INTRODUCTION

Avast array of choices are available for skin moisturizers, making the selection process an overwhelming task for both consumers and health care practitioners. Despite this problem, few studies have compared the physicochemical properties between various brands in an independent fashion.

The Acid Mantle

Healthy human skin lies in the pH range of 4 to 6. 1-4 An increase in stratum corneum (SC) pH can disrupt the activity of enzymes involved in keratinization, barrier restoration, and anti-microbial function. 5-6 This phenomenon is seen in atopic dermatitis 7-9 and other xerotic skin diseases 10-11 and correlates with disease severity of dryness, 9 pruritus, 8 and total skin involvement. 3,6,8 Chemicals applied to the skin are an important exogenous factor that may stabilize the skin's acid mantle. 3 Therefore, topical products with near-physiologic pH are considered best in prevention and treatment of these same skin abnormalities. 3

Formulations of Moisturizers

Topical products are traditionally divided into very limited and general classes, namely ointment, cream, lotion, gel, and foam.12 There have been many modern additions and changes to these standard excipients. The "lotion"from one manufacturer may be significantly texturally more viscous than the "cream" of another manufacturer. This system creates obvious confusion in selecting an emollient with the desirable texture and reflects the need for a more precise and standardized classification. In an attempt to address this problem, we introduce Hydrophilic Index (HI) as a novel methodology to measure the aqueous component of a topical product, and utilize it as an indirect quantification of "greasiness." In this study, we assessed the pH and HI of 31 skin moisturizers sold in the US, and then combined these two parameters to determine which products have the most potential to restore SC acid milieu and provide sufficient hydration, yet possess the appropriate aqueous content to suit user preference.

METHODS AND MATERIALS

pH Measurement

A pea sized amount of moisturizer was placed on Parafilm® wrap. pH was measured using a glass flat tip electrode (Hanna® HI 99191) with accuracy: pH = ± 0.02 at 25°C. All measurements were performed 5 times to achieve their standard deviations.
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