3.2.3. Colorimetric Study

Table 1 showed the (*L\**, *a\**, *b\**, *C\**, *h*, *K/S*) values of blank leather and leather/SeNPs. The results revealed that the *K/S* value of leather/SeNPs was significantly higher than the blank leather. The (*L\**, *a\**, *b\**) values for leather/SeNPs were varied considerably from blank leather. The color of blank leather is cream-white with a relatively high *L\** value (74.9), low *a\** value (8.6), and low *b\** value (17.7). The lightness (*L\**) value of leather/SeNPs was decreased due to the presence of SeNPs, resulting in the coloration of leather. The *a\** values of leather/SeNPs was increased, the increment in *a\** value indicated more redness color for the treated leather. The rise in *b\** value of leather/SeNPs was observed as yellow/brownish, which confirming the coverage of leather with SeNPs. It can be concluded from the equation 1 and Table 1 that the total color difference (Δ*E*) between the two leathers is ~42.



Effect of Treatment *pH* on Color Strength (*K/S*)

To investigate the color changes of leather/SeNPs at different *pH* values under an ultrasonic water bath, Figure 4 showed the effect of *pH* on *K/S* of leather/SeNPs. The *pH* was adjusted from 3 to 8 to reflect the different colors expressed in leather/SeNPs. As shown, the maximum *K/S* value appeared at *pH* 6; this might be caused by overcrowding of SeNPs molecules into the leather matrix in this *pH* value due to the ultrasonic power [48]. The results indicated that the *K/S* of leather/SeNPs can be fine-tuned by controlling the *pH*. According to Lee and Kim et al. [49], the correlation between SeNPs and the leather can be attributed to *pH* of treatment bath [48].

#### Effect of Treatment Temperature on Color Strength (*K/S*)

Figure 5 demonstrated the *K/S* values of leathers/SeNPs at different temperatures. The results showed that the maximum *K/S* value was found to be 18.7 at 65 ◦C. In addition, when the temperature was increased above 65 ◦C, the *K/S* value was simultaneously decreased. This may be attributed to the tough muscle fiber of leather which did not allow the nanoparticles to enter the leather matrix, causing a lower SeNPs uptake. Moreover, high temperatures can cause the leather to shrink, harden, stiffen, and become brittle. It can be concluded that the temperature of 65 ◦C was set to be the optimum temperature for binding SeNPs into the leather [48].
