# Advances in Permeation of Solutes into Hair: Influencing Factors and Theoretical Models
## 3 Main Factors That Affect the Permeation and Absorption of Solutes in Hair
### Solvent
#### 3.3.3 Solvent pH  
Changing the pH of the solvent can change the charge characteristics of ionic solutes, and it can also change the type and density of the charges carried by hair, thereby changing the interaction between hair and the solute molecules, which ultimately affects the permeation and absorption properties of the solute in hair. By combining scanning electron microscopy and micro-X-ray fluorescence analysis, [PERSON] et al. [23] found that the diffusion coefficients and diffusion activation energies of anionic surfactants in hair decrease with the increasing pH of the solvent, while that of cationic surfactants increase. Furthermore, they found that the diffusion coefficients and diffusion activation energies of cationic and anionic surfactants are approximately the same at pH = 3.5 (close to the isoelectric point of hair). Using solvents with pH of 5.1, 6.0, and 10.4, [PERSON] et al. [56] found that the amount of HC RED3 absorbed by hair decreases with increasing pH due to the difference in the degree of ionization of the cationic HC RED3 dye. [PERSON] et al. [28] found that under alkaline conditions, due to the force of the charge, the permeation distances of anionic, cationic, and neutral dyes in hair are different, and cationic dyes have the largest permeation distance and can penetrate the endocuticle and the \(\delta\)-layer of the CMC to the cortex. [PERSON] et al. [55] suggested that the permeation and absorption of solutes in hair are driven by both hydrophobic and charge forces. When the solvent pH is close to the isoelectric point of the hair, solute permeation is mainly driven by hydrophobic forces. Conversely, when the pH of the solvent is greater or less than the isoelectric point of the hair, solute permeation is mainly driven by the charge force.  
The pH of the solvent can also affect the swellability of hair. [PERSON] [11] and [PERSON] et al. [57] found that in an alkaline solvent, the degree of swelling of hair significantly increases, which leads to an increase in the pore size of the hair, which favors solute absorption in hair. [PERSON] et al. [58] pointed out that hair dyeing is usually performed under alkaline conditions, as the expanded hair fibers facilitate the rapid permeation of dyes.  
The wettability of the hair surface can be changed by absorbing solutes under different pH conditions. [PERSON] [59] and [PERSON] et al. [55] found that dyes mainly adsorb to the surface of hair fibers through charge or hydrophobic interactions depending on the pH of the solvent, which in turn changes the hydrophilic and hydrophobic properties of the hairsurface. For anionic dyes, when the pH is lower than the isoelectric point, hydrophobic fibers are formed on the hair surface after dye absorption. When the pH is higher than the isoelectric point, hydrophilic fibers are formed on the hair surface after dye absorption, and the reverse is true for cationic dyes.  
[PERSON] et al. [60; 61] found that the electrolyte components (e.g., sodium salt, potassium salt, and calcium salt) in the solvent occupy part of the adsorption sites of the cationic polymer on hair, resulting in competitive inhibition, thereby reducing the amount of adsorption under alkaline conditions.  
Thus, the solvent pH is a key factor that affects the permeation and absorption of solutes in hair, and changing the solvent pH causes changes in the physicochemical properties of the solute and hair, which in turn affect the permeation and absorption of the solute.  
In conclusion, the main factors that affect the permeation and absorption of solutes in hair are summarized in Table 1.