j. Cosmet. sd., 53, 229-236 (July/August 2002) Glycerin (glycerol): Current insights into the functional properties of a classic cosmetic raw material PAUL THAU, PaCar Tech, LLC, 181 Dogwood Lane, Berkeley Heights, NJ 07922. Accepted for publication March 15, 2002. INTRODUCTION Glycerin was discovered in the late 18th century by Carl Wilhelm Scheele (1742-1786) and has had over a century of use in cosmetic and personal care products. The primary features that account for its numerous uses are based upon its humectant or hygroscopic properties, solubility characteristics similar to water, its inherent lubricity, and glycer- in's capacity to prevent freezing and promote product shelf life. It is also a natural constituent of plants and is involved in physiological and biochemical processes (1). The beneficial cosmetic attributes of glycerin have been recognized for over 75 years. However, our understanding of the diverse mechanisms by which glycerin influences skin moisturization, accelerates healing, improves barrier properties, smoothes the skin surface, etc., had been limited up until the late 1970s. The purpose of this review article is to present an overview of recent research findings that provide a broader understand- ing of glycerin's multi-dimensional functionalities. A brief summary of the classic supportive literature for the above properties is provided however, the primary objective of this review article is to describe the biological func- tionalities of glycerin that have been discovered within the past twenty-five years. (Since glycerin is frequently referred to as glycerol in the scientific literature, the two descrip- tors are used interchangeably in this review.) Glycerin is recognized as an effective over-the-counter (OTC) skin protectant when used at 20% to 45 % in skin products (2). The OTC panel did not consider undiluted glycerin to be effective as a skin protectant. Undiluted glycerin can actually serve to dehydrate skin, based upon osmotic action. It has been demonstrated by numerous methods to be an effective moisturizer and skin conditioner when used at levels above 3%, although the choice of vehicle can influence performance. Table I, (3) shows the comparative per cent moisture holding ability of selected natural humectant materials at room temperature (RT) and 65-70% relative humidity (RH). In a study by Deshpande et al. (4), raw materials, contained in standard petri dishes, were evaluated at 20% RH over a saturated solution of potassium acetate. This RH value was 229

230 JOURNAL OF COSMETIC SCIENCE Table I Moisture Holding Ability of Selected Natural Humectants (% at RT, 65-70% RH) Material 1 Day 5 Days Sodium lactate 90% 25 % Sodium PCA 70% 30% Glycerin 50% 5 % Protein hydrolyzate 20% 10% Sodium hyaluronate 388% 98% maintained within (+) 2% for the length of the study. Results showing the comparative superiority of glycerin at this low RH condition are summarized in Table II. The researchers emphasized the need for agents that function effectively as humectants at a relative humidity of 20%. Only two compounds out of numerous materials tested, glycerin and sodium capryl lactylate, showed any appreciable activity at this humidity, which is often encountered in unhumidified indoor environments during winter. The i, vitro data shown in Tables I and II do not correlate with the capacity of these materials to perform z', vivo. The high molecular weight and ionic charge of the materials listed, with the exception of glycerin, limit their capacity to penetrate skin and to perform effectively. Research studies (11,12) presented in this review will document glycerin's capacity to penetrate skin and create a "reservoir." Studies conducted within the past twenty years have been conducted •, vivo with the aid of equipment and techniques that have been continuously refined. Research by both university and corporate R&D teams has enabled us to gain significant understanding of the complex interactions of glycerol with the epidermis. Results of some of these interactions, listed below, are supported by studies published within the past 25 years. Interaction of glycerol with the epidermis: 1. increases extensibility of the stratum corneum 2. increases the water gradient in the skin 3. reduces surface roughness (may not be caused solely by moisturization) 4. penetrates into the phospholipid bilayers 5. maintains the intercellular lipid cement in a fluid liquid crystal state, particularly under conditions of low temperature and low RH 6. accelerates recovery of barrier function i, vivo Table II Percentage of Moisture Absorbed or Regressed (*) at 20% Relative Humidity Material tested Day 1 (%) Day 5 (%) Day 11 (%) Day 15 (%) Glycerin 9.00 6.13 5.19 4.91 Sodium PCA (50%) 23.03* 36.48* 38.61' 39.04* Sodium lactate (60%) 3.05* 19.06' 27.36* 28.87* Methyl gluceth- 10 0.43 0.42 0.24' 0.43' Methyl gluceth-20 0.87 1.46 1.36 1.33 Sodium capryl lactylate 2.19 2.74 2.24 2.12