J. Cosmet. Sci.J 56, 283-295 (September/October 2005) Investigation of penetration abilities of various oils into human hair fibers K. KEIS, D. PERSAUD, Y. K. KAMATH, and A. S. RELE, TRI Princeton, Princeton, NJ 08542 (K.K., D.P., Y.K.K.), and Marica Industries Ltd., Andheri, Mumbai 400 058, India (A.S.R.). Accepted for publication June 8, 2005. Synopsis In this work we have explored capillary adhesion between hair fibers treated with different types of oils. With coconut, olive, and sunflower oils the capillary adhesion was found to decrease with time, but not with mineral oil. Application of heat reduced the capillary adhesion further for coconut and sunflower oils. Again, this was not observed with mineral oil. Based on an earlier study, where coconut oil was found to penetrate hait while mineral oil was unable to do so, it was hypothesized that the reduction in capillary adhesion resulted from the penetration of oil into the fiber, leaving a thinner oil film on the surface. Such a reduction in capillary adhesion can be explained by changes in Laplace pressure and in the areas of liquid bridges formed between the fibers. The thinning of oil films on the surface of hair has been confirmed independently by goniophotometric measurements on single hair fibers treated with coconut, sunflower, and mineral oils. Thick films of oil (thicker than -0.5 µm) are known to mask the scale structure. As the oil is absorbed into the hait, the film thins with time and application of heat, and the scale structure reappears. This change can be conveniently determined by measuring the scale angle, using the well established goniophotometric protocol. The agreement between the two methods supports the concept that the reduction in capillary adhesion between hait fibers is most likely due to thinning of oil films by absorption of oil into the hair. INTRODUCTION Various natural oils are increasingly used in skin and hair care products, especially in ethnic hair products. In Asian and African countries, vegetable oils are commonly used as hair pomades and are known to lead to healthy-looking hair. Deposition of oils on hair has been claimed to have a beneficial protective effect. Oil-based hair conditioners are thus believed to help prevent moisture loss from hair, which causes dryness and loss of elasticity. Specifically, the beneficial effect of coconut oil on the prevention of cuticular damage during combing, when used as a prewash conditioner, has been demonstrated previously by protein loss and water retention measurements (1). In addition to pro­ viding a lubricating film, this effect was explained by the hydrophobicity of coconut oil, which reduces the water penetration into the fiber. For full beneficial effect, oil pen­ etration throughout the hair fiber cortex is desirable, as mechanical properties of the hair fiber are determined by the cortex. However, the moisture-retention effect of oil is 283
284 JOURNAL OF COSMETIC SCIENCE dominated by surface-deposited oil. Recently, Ruetsch et al. (2) showed that coconut oil penetrates into the hair cortex and reduces the swelling of the hair fiber. The presence of oil films on the surface of fibers leads to capillary adhesion between the fibers and increased specular reflection of light from the surface. The magnitude of these two effects depends on the thickness of the oil film, determined by the amount of oil applied. Penetration of oil into the fiber reduces the film thickness, which affects both capillary adhesion and light reflection. In this paper, the penetrability of various oils is studied by means of the changes in interfiber adhesion and light reflection measure­ ments. The force of adhesion between fibers plays an important role in describing fiber assembly behavior and is measured using a recently described dynamic fiber pull-out method (3). The capillary adhesion force measured on a single fiber due to its contact with other fibers in the assembly is sensitive to the amount of oil on the fiber surface. Therefore, it is possible to determine which oils are more readily diffused into the hair fiber, thereby leaving thinner films of oil on the surface without "clumping" of the hair assembly. A goniophotometer records the angular profile of reflected light and is frequently employed to quantify luster. This method is also known to serve as a sensitive probe of the hair surface, mostly used for detection of structural changes caused by cosmetic products and mechanical grooming (4-6). In this study we demonstrate that the changes in reflected light intensity, angular position for specular reflectance, and calculated scale angle can all be successfully used to detect the changes in the thickness of the surface oil film as a result of penetration. EXPERIMENT AL MATERIALS The hair used in this study was black hair of Indian origin supplied by Marica Industries Ltd., Mumbai, India. Pure coconut oil, mineral oil, and ricebran oils were also provided by Marica Industries Ltd .. Cold-pressed extra-virgin olive oil (Filippo Berio, Italy), expeller pressed sunflower oil (Hain Pure Foods, Inc., New York), pure sesame oil (Kadoya, Summit Import Corp., New York), and pure mustard massage oil (KTC Edibles Ltd. England) were obtained from commercially available sources (health and food stores). MEASUREMENT OF INTERFIBER ADHESION Hair tress preparation. Prior to treatment, the hair tress was kept overnight at 21 °C and 65% RH. A hair tress with a weight of 2 grams was gently combed in order to ensure parallel alignment of the hair fibers. Then 0.2 cc of the appropriate oil was directly applied to the tress with a syringe. The oil was gently massaged with gloved fingers into the tress for five minutes to distribute it uniformly into the hair assembly. In the application procedure a small amount of oil is left on the glove, but is considered to be negligible to have an effect on the measurement. Packing density and hair fiber mounting. A detailed description of the method is given in reference 3. The hair tress (about 0.5 grams of hair per cm3) was packed into a specially
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