228 JOURNAL OF COSMETIC SCIENCE From the slopes of the plane in both directions, the related rate constants for the reactions (95% confidence limits) were determined: k t = 0.011 + 0.0164 min -• (1.83 + 2.73 * 10 -4 S -1) and k, = 0.408 + 0.0896 CONCLUSIONS DSC analysis of human hair in water yields results for the denaturation temperature T o and the related enthalpy AH o. The enthalpy depends on the structural integrity of the or-helical material in the intermediate filaments (IF), while T o is kinetically controlled by the cross-link density of the matrix (IFAPs) in which the IFs are embedded. Against the background of this view, a detailed description and interpretation of the changes, or rather the structural damage bleaching and permanent waving impart to human hair, can be given, including kinetic considerations. REFERENCES (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (1.4) (15) (16) (17) (18) R. D. B. Fraser, Keratins: Their Composition, Structure, and Biosynthesis, (C. C. Thomas, Springfield, IL, 1982). M. Feughelman, Mechanical Properties and Structure of Alpha-Keratin Fibres (UNSW Press, Sydney, 1997), pp. 28-59. M. Feughelman, A two-phase structure for keratin fibers, Text. Res. J., 29, 223-228 (1959). H. Zahn, F.-J. Wortmann, G. Wortmann, and R. Hoffman, "Wool," in Ullmann's Encyclopedia of Industrial Chemistry (VCH Verlagsges, Weinheim, 1996), Vol. A28, pp. 395-421. F.-J. Wortmann and I. Souren, Extensional properties of human hair and permanent waving, J. Soc. Cosmet. Chem., 38, 125-140 (1987). G. Ebert, Anwendungsm/Sglichkeiten der Differentialkalorimetrie zur Untersuchung von Keratin- fasern, Melliand Textilber., 48, 87-90 (1967). A. R. Haly and J. W. Snaith, Differential thermal analysis of wool--The phase transition endotherm under various conditions, Text. Res. J., 37, 898-907 (1967). J. S. Crighton and E. S. Hole, A study of wool in aqueous media by high pressure differential thermal analysis, Proc. 7th Int. Wool Text. Res. Conf, Tokyo, I, 283-292 (1985). F.-J. Wortmann and H. Deutz, Characterizing keratins using high-pressure differential scanning calorimetry (HPDSC),J. AppL Polym. Sci, 48, 137-150 (1993). F.-J. Wortmann and H. Deutz, Thermal analysis of ortho- and para-cortical ceils isolated from wool fibers,J. Appl. Polym. Sci., 68, 1991-1995 (1998). H. Schmidt and F.-J. Wortmann, High pressure differential scanning calorimetry and wet bundle tensile strength of weathered wool, Text. Res. J., 64, 690-695 (1994). M. Spei and R. Holzem, Thermoanalytical investigations of extended and annealed keratins, Colloid Polym. Sci., 265, 965-970 (1987). F. Leroy, A. Franbourg, and J. L. L6v&que, Thermoanalytical investigations of reduced hair, 8th lnt. Hair-Science Symp. German Wool Res. Inst., Kiel (1992). J. F/Shles, K. J/Srissen, and M. Spei, Aminos•iureanalysen von thermisch behandelten Faserkeratinen im Zusammenhang mir R/Sntgenstrukturuntersuchungen, Proc. 6th Int. Wool Text. Res. Conf, Pretoria, II, 159-172 (1980). J. Cao, Origin of the bimodal "melting" endotherm of alpha-form crystallites in wool keratin,J. Appl. Polym. Sci., 63, 411-415 (1997). A. Franbourg, F. Leroy, J. L. L•v•que, and J. Doucet, Synchroton light: A powerful tool for the analysis of human hair damage, iOth Int. Hair-Science Symp. German Wool Res. Inst., Rostock (1996). N. Nishikawa, Y. Tanizawa, S. Tanaka, Y. Horiguchi, and T. Sakura, Structural change of keratin protein in human hair by permanent waving treatment, Polymer, 39, 3835-3840 (1998). S. Ogawa, K. Fujii, K. Kaneyama, K. Arai, and K. Joko, A curing method for permanent hair straightening using thioglycolic and dithioglycolic acids, J. Cosmet. Sci., 51,379-399 (2000).

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