210 JOURNAL OF COSMETIC SCIENCE
solid determination. Figure 6D shows the FR-IT spectra of dopamine hydrochloride, GA,
ferrous chloride tetrahydrate, GA-Fe(II) solution, and GA-Fe(II)+DA solution. In the
FR-IT spectra of GA-Fe(II)+DA, we found that 1,611 cm−1 and 1,522 cm−1 are benzene
ring skeleton vibrational absorption peaks. Additionally, a peak at 1,286 cm−1 indicates
carbon-oxygen single bond stretching vibrations. Importantly, the strong absorption
peak between 1,100 cm−1 and 1,000 cm−1, consistent with ferrous chloride tetrahydrate,
confirms the complete formation of the GA-Fe(II) complex structure. In addition, the
spectrum of GA-Fe(II)+DA showed no bending vibration peaks of amine and stretching
vibration peaks of carbon and nitrogen bonds, which proved that DA was not involved in
the complexation in the coordination structure of GA-Fe(II), as DA was removed during
the freeze-drying process.
Although GA-Fe(II)+DA hair dye can dye commercial white hair, the dye molecules cannot
penetrate the cortical layer of hair through the protective layer of hair scales of natural white
hair without the help of external forces. This is because the maximum critical molecular
size that can be allowed to pass through the protective layer of hair scales is 1.4 nm. Figure
7A shows the van der Waals sphere model for calculating the molecular sizes of GA, DA,
and GA-Fe(II). The results show that the molecular size of GA is 0.965 nm ≤ 1.4 nm, and
when GA forms a complex with Fe(II), the molecular size increases to ≥ 1.4 nm. The larger
the molecular volume of the complex, the larger the molecular size, making it impossible
Figure 5. (A) Time screening of GA-Fe(II)+DA impregnated natural gray hair. (B) Molar ratio screening of
GA to Fe(II). (C), (D) pH screening of GA-Fe(II)+DA.

211 Polyphenol-metal Complex With Dopamine
for the complex molecules to penetrate the hair cortex layer (Figure 7B). By unfolding
the hair scales using ethanolamine emulsion, the maximum critical molecular size that
the protective layer of hair scales can allow to pass through increases, and large-size dye
molecules can also easily penetrate the hair cortex layer to achieve natural white hair
dyeing. Dopamine, which is free in the hair dye, also penetrates the cortical layer of hair
and is deposited and autoxidized to polymeric polydopamine, which increases dramatically
in size and cannot penetrate from the cortical layer to the hair surface through the hair
scales. Its adhesive properties effectively secure the dye molecules within the cortical layer
of the hair, accomplishing color fixation, enhancing the hair’s resistance to washing, and
resulting in a permanent dyeing effect.
In our study, we observed that natural white hair dyed solely with GA complexed with
Fe(II) dye molecules exhibited poor resistance to washing, with a significant color difference
occurring even after just 40 washes. Dopamine can adhere to the surface of the hair and
form an adhesive film that acts as a color fixation.31 Therefore, we introduced dopamine
to prepare a composite hair dye for improving the problem of poor washing resistance and
achieving permanent hair dyeing. However, in our study, we found that dopamine did not
adhere to the hair surface, but rather entered the hair cortical layer through the hair scales.
Based on the property that dopamine is prone to spontaneous oxidation and polymerization
into polydopamine,32 we hypothesized that dopamine accumulates in the cortical layer
and undergoes autoxidation and polymerization into polydopamine, which adheres to fix
Figure 6. UV-Vis absorption spectra of (A) DA, (B) GA, and (C) GA-Fe(II)+DA. (D) FT-IR spectra of DA,
GA, Fe(II), GA-Fe(II)+DA.