]. 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

136 JOURNAL OF COSMETIC SCIENCE to examine how the sorption of water vapor is affected by various oils applied to hair in relation to film thickness, which was controlled by the amount of oil applied to hair. No attempt has been made to determine film thickness on the hair surface. For this study, three oils, namely coconut, mineral, and sunflower oils, were chosen based on their performance in an earlier study (1). Among these, coconut oil was known to penetrate the whole fiber, whereas sunflower oil showed limited penetration, especially into the cortex (3). Mineral oil was found not to penetrate the hair (2,3). The sorption and desorption of water vapor 24 hours after oil treatment, with the application of heat, and after removal of the oil film from the surface were studied. Additionally, hair samples were treated with various amounts of oil in order to demonstrate the effect of the film thickness on water vapor sorption behavior. It is known that oils have different abilities to penetrate into the cortex of the hair (1,3). In this study, we are attempting to see how the sorption of the water vapor is affected by the oil in the interior of the fiber and the oil film on the surface. EXPERIMENTAL All treatments were performed on dark brown European hair (purchased from DeMeo Brothers Inc., New York), which was washed with 1 % sodium lauryl sulfate solution and rinsed thoroughly. The oils used for this study were commercially available mineral oil, coconut oil, and sunflower oil. Oils were applied to hair tresses in a ratio of 0.1 ml of oil per gram of hair. The oils were gently massaged into the hair swatch, and the samples were kept for 24 hours at room temperature (-25°C) before testing. Another set of tresses was treated with each of the oils as described above and then exposed to heat after the 24-hour time period. The samples were heated with a hair dryer on medium heat (-60-80°C) and air flow for 15 minutes before testing. Additionally, samples were treated with a larger amount of coconut and mineral oil to form a thicker layer on the hair surface. This was achieved by applying oil in a ratio of 1.2 ml of oil per gram of hair. Finally, measurement on a coconut oil-treated sample was performed after the removal of the surface oil. This tress was treated with O .1 ml of coconut oil, and kept for 24 hours. Then a Kimwipe® was wetted with acetone and the hair sample was pulled through the wipe multiple times until the fibers no longer showed any visual signs of the coconut oil treatment. Sorption and desorption of water vapor was determined gravimetrically using a dynamic vapor sorption analyzer (DVS-1 from Surface Measurement Systems, NA, Allentown, PA). The description of the apparatus has been covered elsewhere (5). All experiments were conducted at 25°C and a total gas flow of 200 ml/min. Approximately 25 mg of hair snippets cut from the hair tress were dried overnight at 0% RH under dry nitrogen gas. Relative humidity was then increased by steps of 10% up to 90%, and then to 95%. Relative humidity was then decreased through the same steps back to 0%. The samples remained at each stage until its mass reached "equilibrium" (arbitrarily defined by a change in mass of less than 0.0015% per minute for 20 minutes). For the studies on the thicker oil coatings, only the sorption half of the experiment was performed.