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]. Cosmet. Sci., 58, 135-145 (March/April 2007) Effect of oil films on moisture vapor absorption on human hair K. KEIS, C. L. HUEMMER, and Y. K. KAMATH, TRI/Princeton, Box 625, Princeton, NJ 08542. Accepted for publication March 6, 2007. Synopsis In this paper sorption and desorption of water vapor on hair fibers treated with various oils is investigated, using a dynamic vapor sorption (DVS) apparatus. Results show lower "equilibrium" sorption of moisture for various oil-treated samples compared to the untreated sample. Coconut oil-treated hair had a higher regain than mineral oil-treated hair. Although treating the hair samples with oil reduced moisture pickup, a considerable amount of moisture vapor was still able to penetrate into hair fibers. Calculated hysteresis plots show that the samples treated with different oils have slightly higher moisture retention at low relative humidities compared to that of the untreated sample, which suggests a beneficial effect. The calculated moisture diffusion coefficients for oil-treated samples were much lower compared to the untreated hair fibers, suggesting that surface oil films and penetrated oil molecules form a diffusion barrier. A moisture diffusion model is discussed in terms of the possible role of fiber swelling on restrictive narrowing of the cell membrane complexes (CMCs), which form the diffusion pathways in the fiber. The effect of film thickness on moisture absorption and the reverting of the sorption isotherm to that of the untreated hair after removal of the oil film shows that oil film is the main resistance to moisture diffusion. The lowering of the diffusion coefficient of water vapor by oil films will slow the loss of moisture, an effect similar to "moisturization" of hair. INTRODUCTION In many countries, vegetable oils are used as pre- or post-wash dressings for hair. Oils are known to lubricate the hair surface to prevent damage during grooming and to improve the luster of the hair by forming a thin coating on the surface. It has been shown that coconut oil prevents wet combing damage due to its hydrophobicity and the resulting reduction in fiber swelling (1,2). In earlier studies by Ruetsch et al. (2), it was shown that coconut oil penetrates into hair whereas mineral oil does not. In a later study by Hornby et al. (3 ), penetrability of different vegetable oils was investigated. Penetra­ bility of oils into hair is an important aspect in this study because once the oil is applied to hair, the film thickness of the oil depends on the ability of the oils to penetrate hair. Oils that do not penetrate, such as mineral oil, will leave a thicker film on the surface as compared to coconut oil, which is known to penetrate hair. Such films left on the surface of hair can have a considerable effect on interfiber adhesion due to capillarity. This was investigated by Keis et al. (4) using several vegetable oils. This study attempts 135