Rhodamine B diffusion in hair as a probe for structural integrity
Introduction
The condition of hair affects the uptake and the diffusion of external substances to the inside. A rule of thumb is that diffusion is faster into altered or damaged hair than into unaltered hair [1]. Three distinct stages are recognized in this process. In the first stage, molecules are transported to the fiber/water interface by a combination of aqueous diffusion and agitation, if any. The molecules are then adsorbed onto the outer layers of the hair and in the final step they diffuse into the shaft where they may or not subsequently participate in reactions with the fiber or with themselves. The final state is characterized by the number of available reactive sites in hair, and must ensure that the chemical potentials of the molecules in solution and the molecules in the hair have the same value [1].
Diffusion processes may be considered as three types: free diffusion, forced diffusion and obstructed diffusion. Free diffusion applies to the transport of matter by random thermal motion. Forced diffusion involves transport by forces other than random molecular motion. Obstructed diffusion consists of a hindered diffusion in a media containing obstacles, such as dextrans, membranes or phospholipids whose size and concentration influence the rate of diffusion. Diffusion coefficients involving only free diffusion are called true or intrinsic diffusion coefficients; processes involving both free and forced diffusion are called mutual diffusion processes [2], [3], [4]. Experimentally, one cannot evaluate free diffusion in kinetic studies on keratin fibers. Therefore, the usual practice is to apply equations derived from Fick's law for free diffusion to data involving mutual diffusion. This provides apparent or approximate diffusion coefficients, instead of intrinsic diffusion coefficients. In this work, no attempt is made to distinguish between free and mutual diffusion: the term “diffusion” is used loosely.
Morphologically, a hair fiber contains three and sometimes four different units. Its surface is formed by a thick protective covering consisting of layers of flat overlapping scalelike structures called the cuticle. The cuticle layers surround the cortex, which constitutes the major part of the fiber mass. The cortex consists of spindle-shaped cells that are aligned along the fiber axis. Porous regions called medulla are located near the center of the fiber and sometimes are absent. The fourth unit is the cell membrane complex that glues or binds cuticle and cortical cells together [1].
There are few methods of measuring diffusion coefficients, and fewer data for diffusion coefficients in hair, in the literature. Since this parameter can give valuable information about hair structure, this work intended to study the diffusion of Rhodamine B, a well-known dye, into human hair. Different types of hair submitted to different chemical or physical damage were investigated. Techniques based on spetrophotometric and fluorescence principles were used and allowed validating the method.
Section snippets
Materials
Standard Caucasian medium brown hair (Standard hair) was purchased from De Meo Brothers, New York. The hair tress was ca. 25 cm long. A mixture of phytoceramide-6 and glycosylated β-sitosterol named “ceramide” was supplied by Laboratoires Sérobiologiques and used as received. The following chemicals were used without further purification: diethyl ether (Merck, Nuclear), commercial hydrogen peroxide (0.81 mol L−1), concentrated ammonium hydroxide (Vetec), potassium persulfate (Merck), cetrimonium
Spectrophotometric results for the diffusion coefficient of Rhodamine B
Fig. 1 shows Rhodamine solution spectra in contact with one BC sample. Similar spectra, with maximum absorbance at 554 nm, were obtained for all hair samples. Saturation values, which are illustrated in Table 3, were obtained after 24 h contact with the Rhodamine solution and allowed the calculation of the maximum sorption (C∞) of Rhodamine by the fibers.
Table 3 shows that after saturation, the Rhodamine concentration in the diffusion media decreased from 1.04 × 10−4 mol L−1 to 58–74% of this value.
The methods
Comparing the results given in Table 4, Table 5, it can be seen that the imaging method gives apparent diffusion coefficients roughly an order of magnitude higher than the colorimetric method. Since was assumed that the quantum yield was equal to one and that the integrated area corresponds not only to the fluorescence emitted by Rhodamine molecules on the surface of the section, but also by those distributed along the thickness of the section (300 nm), higher results were expected.
The diffusion model
During the
Conclusions
Diffusion experiments produced data that clarified the mechanism of dye penetration into hair fibers. The curve that represents the ratio between the amount of solute sorbed in time t (Qt) and the maximum amount of solute sorbed by the hair (Q∞) as a function of the square root of time (t1/2) is decomposed into two straight segments with different slopes. The first segment is assigned to the penetration of Rhodamine into the cuticle and the other refers to the cortex. From these, apparent
Acknowledgements
The authors thank FAPESP (Grants 98/05252-6 and 01/14161-9) for financial support.
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