J. Cosmet. Sci., 65, 37–48 (January/February 2014) 37 Preliminary analysis of the distribution of water in human hair by small-angle neutron scattering YASH KAMATH, N. SANJEEVA MURTHY, and RAM RAMAPRASAD, Kamath Consulting Inc., Monmouth Junction, NJ (Y.K.), Center for Biomaterials Rutgers University, Piscataway, NJ (N.S.M), and TRI/Princeton, Princeton, NJ (R.R.). Accepted for publication November 17, 2013. Synopsis Diffusion and distribution of water in hair can reveal the internal structure of hair that determines the pen- etration of various products used to treat hair. The distribution of water into different morphological compo- nents in unmodifi ed hair, cuticle-free hair, and hair saturated with oil at various levels of humidity was examined using small-angle neutron scattering (SANS) by substituting water with deuterium oxide (D2O). Infrared spectroscopy was used to follow hydrogen–deuterium exchange. Water present in hair gives basically two types of responses in SANS: (i) interference patterns, and (ii) central diffuse scattering (CDS) around the beam stop. The amount of water in the matrix between the intermediate fi laments that gives rise to interfer- ence patterns remained essentially constant over the 50–98% humidity range without swelling this region of the fi ber extensively. This observation suggests that a signifi cant fraction of water in the hair, which contrib- utes to the CDS, is likely located in a different morphological region of hair that is more like pores in a fi brous structure, which leads to signifi cant additional swelling of the fi ber. Comparison of the scattering of hair treated with oil shows that soybean oil, which diffuses less into hair, allows more water into hair than coconut oil. These preliminary results illustrate the utility of SANS for evaluating and understanding the diffusion of deuterated liquids into different morphological structures in hair. INTRODUCTION Hair is a part of skin, a protective organ of most mammals. It consists of mainly keratin, a structural protein that is also a major constituent of hoofs, nails, and horns. Although hair has a molecularly complex hierarchical structure (Fig. 1) (1), three distinct parts can be identifi ed in the cross sections of human hair fi bers: the cuticle, the cortex, and the medulla (2). The cuticle sheath, which is the outermost protective layer, consists of sev- eral layers of cuticle cells arranged like shingles on a roof. The cuticle envelopes the cor- tex, which is made of cortical cells that comprise the major part of the α-keratin in hair. Cells in both the cuticle and the cortex are held together by cell membrane complexes. Cells in the cortex occur as an assemblage of macrofi brils with rodlike intermediate fi laments Address all correspondence to Yash Kamath at yashkamath@verizon.net.

JOURNAL OF COSMETIC SCIENCE 38 (IFs, sometimes referred to as microfi bril) embedded in an amorphous matrix of keratin- associated proteins cross-linked by the amino acid cystine. The IFs are made of eight protofi laments, each with a pair of coiled-coil α-keratin molecules, and thus contains 32 α-helical chains. The medulla, which can be sometimes found in the cross section, is a structure made of incompletely developed hollow cells. IF and the surrounding matrix constitute about 85% of the hair shaft, but the properties of hair are determined by all the constituents that make up hair (3). This paper presents observations about the ingress of water and its distribution into the different structures of hair. The mechanical properties of hair, such as tensile strength, bending, and torsional rigid- ity, are sensitively dependent on the amount of water in the fi ber (4). Hair, with a hydro- philic core and a hydrophobic surface, absorbs water from the vicinity, the amount depending on the relative humidity (RH) of the environment. The longitudinal swelling of hair with moisture content is the basis of hygrometer used for the measurement of RH in the environment (2). Water molecules diffuse into the fi ber through intercellular path- ways, such as the cell membrane complexes, rather than transcellular pathways. The fi ber equilibrates rather slowly to the partial pressure of water vapor in the environment. This behavior has been studied and quantifi ed in detail, using dynamic vapor sorption analyzer (Surface Measurements Systems, Alperton, Middlesex, U.K.) (5). This apparatus gravi- metrically determines the amount of water absorbed by hair from the moisture surround- ing the hair sample as a function of RH. Desorption data can also be obtained in the same experiment by reversing the steps. From the data, a sorption–desorption isotherm com- bination is generated by plotting the water uptake (% w/w) against %RH. A typical sorption–desorption isotherm is shown in Fig. 2. A sorption isotherm gives information about the water held in different forms at varying RHs. For example, water is held in the form of an adsorbed monolayer up to ~20 %RH, in the form of adsorbed multimolecular layers from ~20 to 65 %RH, and in the form of free water held in relatively large pores (capillary condensation) above 65 %RH. Water held by capillary condensation leads to a sharp increase in the swelling of the fi ber. For most hydrophilic materials such as hair, water content as measured during desorption is higher than that during the sorption Figure 1. (A) Schematic of the longitudinal section of hair shaft, (B) A model for the internal organization of the keratin molecules in the microfi bril.