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UV light sources could lead to the reduction of the intensity absorption of the materials that have been dyed and this phenomenon can be termed as the photo fading. Simply, it can be stated as the elimination of the absorbed or excess alternative energy and following are the ways through which the fading can occur due to the following factors:
1. Radiation emission or the interaction of the phosphorescence or the fluorescence
2. Photochemical are other possible causes those could lead to the fading (Oster and Wotherspoon 1957)
3. The factors associated with the internal energy conversion or intersystem crossing could lead to the radiationless transitions are another major ways those could lead to the fading.
Methylene blue, dimers, or aggregates containing a sm2~11 number of molecules are the molecules those could fade away due to the interaction of the ultra-violate rays or any other chemical reaction. “In connection with the basophilic staining of wool fibers by Methylene Blue to disclose cortical differentiation, the persistence of the intensity of the initial staining of the orthocortex has been suspect (Dong et al. 2011).” It can be instructive that there is the possibility of the relationship between the light intensity and the rate of fading considering the assumption of the existence of threshold intensity.
Fading occurs due to excited single states that is being produced when one of the molecule absorbs a photon of light that has been intended to be short-living for the conventional photochemical reactions. The photo degradation occurs due to the absorption of the 1 out of 100, 000 through the typical quantum resulting in the textile dye fading. Fading occurring due to the singlet oxygen as the dye molecules might undergo triplet-triplet annihilation with the oxygen having triplet ground state that alternatively results in the production of the singlet oxygen resulting in the destruction of the dye (Barka, Abdennouri and Makhfouk 2011). Superoxide might also result in the fading of the dye molecules.
Following are the known reaction mechanismsthose havebeen evaluated for the molecules that fade way:
Capacity-fading and reaction mechanisms of the tin nanoparticles in potassium-ion batteries, reaction of ozone with indigos results fading of the natural organic colorants, the mechanism of the photofading of the azo dye within the hydrazine and azo forms through the UV irradiation (Franca, Oliveira and Ferreira 2009). The mechanism of the TiO2-coated photoluminescent materials. Another study focused on the capsanthin fading in vitro being induced due to the reactive oxyzen spices.
The kinetics of fading can be explained as following:
For a reaction:
aA + bB ---> cC + dD
The reactions’ rate can be defined in terms of the change in the concentration of the product or reactants per unit of the time:
Reaction rate = -D[A]/(a Dt) = -D[B]/(b Dt) = D[C]/(c Dt) = D[D]/(d Dt)
The relationship between the instantaneous concentration and rate of the reaction of the reactants can be defined as:
Rate of reaction = k [A]m[B]n
Following are the known relationships considering different order of reaction as:
Order of the reaction ----- Plot that will yield a straight line
Second order (m=2) ----- [A]-1 vs. time
First order (m=1) ----- ln [A] vs. time
Half-order (m=1/2) ----- [A]1/2 vs. time
Zero order (m=0) ----- [A] vs. time
Excitation of the dye to the triplet state results in the triplet-triplet annihilation with the oxygen that results in the production of the singlet oxygen that will be alternatively resulting in the dye destruction as:
(Source:Mowry and Ogren 1999)
The phthalocyanine and copper are both the methylene blue those are efficient singlet oxygen generators however, the efficiency will be depending upon the solvent utilized because of the variations in the singlet oxygen lifetime as demonstrated in the following figures:
(Source: Mowry and Orgen 1999)
Mechanism of fading of thiazole orange, Methylene blue are capable of damaging the cellulose that absorbs the energy intensive part of the visible spectrum and thus, the general dyes have been also known exhibiting the phototendering. The dye-sensitised oxidative degradation’s mechanism explains that the very initial step in the cellulose degradation is the hydrogen atom removal through the excited dye molecule.
1. Oakes, J., 2001. Photofading of textile dyes. Review of Progress in Coloration and Related Topics, 31(1), pp.21-28.
2. Mowry, S. and Ogren, P.J., 1999. Kinetics of methylene blue reduction by ascorbic acid. Journal of chemical education, 76(7), p.970.
3. Franca, A.S., Oliveira, L.S. and Ferreira, M.E., 2009. Kinetics and equilibrium studies of methylene blue adsorption by spent coffee grounds. Desalination, 249(1), pp.267-272.
4. Dong, Y., Lu, B., Zang, S., Zhao, J., Wang, X. and Cai, Q., 2011. Removal of methylene blue from coloured effluents by adsorption onto SBA?15. Journal of Chemical Technology & Biotechnology, 86(4), pp.616-619.
5. Barka, N., Abdennouri, M. and Makhfouk, M.E., 2011. Removal of Methylene Blue and Eriochrome Black T from aqueous solutions by biosorption on Scolymus hispanicus L.: Kinetics, equilibrium and thermodynamics. Journal of the Taiwan Institute of Chemical Software Engineers, 42(2), pp.320-326.
6. Oster, G. and Wotherspoon, N., 1957. Photoreduction of Methylene Blue by Ethylenediaminetetraacetic Acid1a, b. Journal of the American Chemical Society, 79(18), pp.4836-4838.