Diffusion into hair

The rate of diffusion of mercaptans into human hair is undoubtedly influenced by steric considerations. For example, molecular size (effective minimum molecular diameter) of the mercaptan molecule should affect the rate of diffusion into hair. Therefore, the rate of reduction of human hair by ethyl mercaptan in neutral to alkaline media, where diffusion is ratedetermining, should be faster than that of higher homologs. (The possible effects of variation in the structure of cystinyl residues in hair on the rate of reduction was considered in the previous section on cystinyl residues of differing reactivities.)... 

Low-molecular-weight polypeptides [27] (M = 1,000) and polyethyl-eneimine (M = 600) [28] have been shown to diffuse into hair. Somewhat larger polypeptides (M = 10,000) [27] and polymer JR, with an average... 

Darkening or adding color to hair can be accomplished by several different treatments. The simplest process involves applying a dye of the desired color to the hair. However, because dye molecules are fairly large, they only coat the surface of hair strands the hair color is only temporary and will wash out after several shampoos. In more permanent coloring of hair, two or more components (usually smaller molecules) are added to hair. These molecules diffuse into hair strands and react to produce a larger, colored molecule. Because the molecule is formed within the hair strand, it tends to stay there, producing permanent color. 

The most widely accepted model for the incorporation of substances into hair has been proposed by Henderson [45] the incorporation of substances inside hair is primarily dependent on the diffusion of molecules from blood in the capillaries to the cells in growth. The drugs can also enter the hair fiber during its formation by spreading from the deep skin strata. Sebum and sweat, which themselves contain the substances, can carry them within the hair structure [46-51],... 

At low rhodamine concentrations, the rate of diffusion into the hair shaft was sufficiently slow to monitor. A cross section of Caucasian hair exposed to rhodamine at 1 pg/mL is shown in Figure 16. As exposure time was increased, more rhodamine reached the interior of the hair. At 120 min, even though the surface was much brighter than the interior, substantial fluorescence was observed in the interior when compared with unexposed hair or to the 30-min exposure. At a rhodamine exposure of 10 pg/mL for 2 h the rhodamine had penetrated throughout the hair sample (Figure 16). 

A third, potential route of entry across the skin involves diffusion down hair follicles and into sebaceous glands or via sweat glands. These are referred to as shunt pathways and are the biological equivalent of intergalactic wormholes both have been subject to intense debate but are of unknown practical relevance. 

In other words, for difficult-to-wave hair or at high pH, the concentration of mercaptide ion is so high that cleavage of the disulfide bond can occur faster than mercaptide can diffuse into the fibers. As the pH is decreased to the acid side, or for easy-to-wave hair, the rate of chemical reaction decreases faster than diffusion to the point at which the chemical reaction itself becomes rate-limiting. With many mercaptans [21], further lowering of the pH to about 2 freezes or stops the reduction reaction. 

Reese and Eyring [45] have demonstrated that the reaction of sulfite with hair is a pseudo-hrst-order reaction. In other words, the chemical reaction of sulfite with the disulfide bond of hair is slower than diffusion of sulfite into hair. Elsworth and Phillips [46, 47] and Volk [48] examined the sulfi-tolysis of keratin, demonstrating that the rate of cystine cleavage is optimal at acid pH. Wolfram and Underwood [49] found a broad optimum for cystine cleavage by sulhte at pH 4 to 6. The decrease in cystine cleavage at acidic pH (below pH 4) is due to a decrease in the concentration of the nucleophilic sulhte species. On the other hand, the decrease in cystine cleavage as pH is raised (alkaline pH) results from alkaline hydrolysis of the Bunte salt [50]. 

In fact, mercaptan in hair is capable of reacting with disulfide monoxide by nucleophilic displacement [91] or with most compounds that contain a group labile to nucleophilic displacement, if they are capable of diffusing into the hair. 

Diffusion or penetration of chemicals into hair is more concerned with permanent waves, hair straighteners, and hair dyes. However, given the recent evidence that shampoos over time can damage the nonkeratinous pathways for entry into hair and more recent evidence that some conditioner-shampoo interactions can damage the cell membrane complex, diffusion is also important to shampoos. 

The diffusion of anionic surfactants into hair is also very slow, and it takes days for an average-size surfactant to penetrate cosmetically unaltered hair completely. Although some penetration of surfactant can and does occur, the major interactions of the surfactants of shampoos and creme rinses occur at or near the fiber surface (i.e., near the first few micrometers or cuticle layers of the hair). 

Vickerstaff [87] noted that equations describing diffusion into an infinite cylinder (e.g., hair) or into a plane slab (e.g., skin) from a constant solute concentration (infinite bath), assuming a constant diffusion coefficient, are of the following general form ... 

Table 5-14 describes the describes the influence of cross-link density in different keratin fibers on diffusion rate. These data clearly show that the rate of diffusion into keratin fibers decreases with increasing cystine content and therefore with increasing cross-hnk density. One may conclude that reactions that decrease the cross-link density of hair (e.g., bleaching) will lead to hair that is more rapidly penetrated, and its penetrability will increase with increased bleaching. Decreasing cross-link density obviously increases the rate of transcellular diffusion. 

Shampooing and grooming actions cause the cuticle to be more susceptible to further abrasion/erosion and the lifting of scales and other types of hair damage described in this section. These actions can also lead to increased diffusion of chemicals into hair and to additional damage by penetrating chemicals or products. 

Semipermanent products consist of nitroaromatic amines or aromatic amino nitroanthroquinone dyes [4,5] that diffuse into and bind to the hair... 

Temporary dyes or color rinses are acid dyes [6-8] similar to those used in wool dyeing. However, because color rinses are used at room temperature, the dyes do not diffuse into the hair or bind firmly, and they may be removed by a single shampooing. 

The principles of this study by Wong are being utilized today in semipermanent hair dye products in the following manner. It is well known that diffusion into and removal of dye are faster in weathered tip ends than in the root ends of hair [5]. Thus, blends of dyes are used not only to obtain the right blend at root and tip but also to provide a more even wash fastness in both root and tip ends of hair. For example, blends of single ring dyes diffuse more readily into and are retained more readily in root ends, whereas blends of dinuclear and trinuclear dyes are retained more readily in tip ends [40]. Thus, the proper blending of mononuclear and dinuclear with trinuclear dyes will provide a more even wash fastness to both root and tip areas of hair. 

Figure 5 Two-dimensional plots showing the distribution of (A) suifur and (B) deuterium in the hair. (Reprinted with permission from Smith RW and Ciough AS (2002) Depth profiling of diffusion into cyiindricai matrices using a scanning micro-beam. Nuclear Instruments and Methods in Physics Research B188 126-129 Elsevier.)...

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