j. Cosmet. Sci., 52, 169-184 (May/June 2001) Secondary ion mass spectrometric investigation of penetration of coconut and mineral oils into human hair fibers: Relevance to hair damage S. B. RUETSCH, Y. K. KAMATH, AARTI S. RELE and R. B. MOHILE, TRI/Princeton, Princeton, NJ 08540 (S.B.R., Y.K.K.), and Marico Industries Ltd., Andheri, Mumbai 400 058, India (A.S.R., R.B.M.). Accepted for publication March 15, 2001. Synopsis An attempt has been made to show the difference in the penetrability of coconut oil and mineral oil in human hair. We have used secondary ion mass spectrometry (SIMS) in combination with a time-of-flight (TOF) mass spectrometer. Characteristic ions formed by the pure components when bombarded with gallium ions have been identified with their m/z values. The distribution of the ion, characteristic of the particular treatment, has been established in the cross sections of hair treated with coconut and mineral oils. The results show that coconut oil penetrates the hair shaft while mineral oil does not. The difference may be due to the polarity of the coconut oil compared to the nonpolar nature of the mineral oil. The affinity of the penerrant to the protein seems to be the cause for this difference in their behavior. This study also indicates that the swelling of hair is limited by the presence oil. Since the process of swelling and deswelling of hair is one of the causes of hair damage by hygral fatigue, coconut oil, which is a better penetrant than mineral oil, may provide better protection from damage by hygral fatigue. INTRODUCTION In an earlier study (1) the beneficial effect of coconut oil used as a pre-wash conditioner was investigated. Use of coconut oil was found to significantly reduce cuticular damage during combing. This was attributed to the hydrophobicity of the oil, which reduced the swelling of the cuticle that otherwise would have damaged the scale structure, especially in wet grooming. The lubricating effect of the oil further contributed to the reduction in damage. In Asian and African countries, vegetable oils are extensively used as hair dressings, and are known to reduce hair damage. Anecdotal accounts also suggest that the beneficial effects of oils accrue from the penetration of oils into hair and skin. Although conven- tional concepts of diffusion doubt the penetration of high-molecular-weight compounds such as polymeric conditioners (above a molecular weight of-1000) beyond the cuticu- lar sheath, claims have been made (2) to the contrary. However, in the case of coconut 169

170 JOURNAL OF COSMETIC SCIENCE oil, which is principally a laurie acid triglyceride, the molecular weight is likely to be well below 1000 Da and, therefore, diffusion into the hair is a distinct possibility. What has been lacking is a study involving diffusion of compounds as a function of molecular weight, and a reliable method of identifying them in the fiber, especially at low con- centrations. Recently, mineral oils have been promoted for use as hair dressings. Although the external effect of these oils is essentially one of lubricating the hair surface, their penetrability into the cortex of hair is likely to be different because of the differences in the polarity of the two materials, namely coconut and mineral oils. This study attempts to demonstrate the penetration of coconut and mineral oils by mapping their presence in the hair fiber cross section. The technique used for this work is time-of-flight secondary ion mass spectrometry, TOF-SIMS for short (Figure 1). The TOF-SIMS method makes use of the secondary ion mass spectra, which are obtained when the sample surface is bombarded with a positively charged gallium ion beam. The positive/negative ion mass spectra are obtained by the time-of-flight method. First, characteristic positive/negative ions (peaks) are isolated in the mass spectra of the pure materials, namely neat coconut and mineral oils (reference spectra) used for treatment. The observed characteristic positive/negative ions, which are unique for the pure com- pounds, are then traced/mapped in cross sections of untreated and oil-treated hair fibers. EXPERIMENTAL MATERIALS Pure coconut oil (Parachute brand from Marico Industries Ltd., Mumbai, India) and mineral oils (viscosity - 1 P) were used for this study. The hair sample was black Indian hair obtained from individuals who did not use coconut or mineral oils as hair dressings. TOF-SIMS .... The Basic Principle Ga + Gun (Vac•O I yIon | Fli•t Tube, l=2m Hold Ex•action Plate V•a.-q = Eun.::•l• mv z accl. V: 3180 V ' Tim•f•ight = 'llv Detector Figure 1. Schematic representation of the basic principle of TOF-SIMS.