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1- what is the fading? why that happen?
2- Molecules that are known to fade, them explain Methylene blue
3- condition under which molecules are fading, and the chemical processes that lead to fading (other factors may also play a role in the fading process)
4- Are any reaction mechanism known for any of the molecules that fade, for example, there are already some studies of fading of studies of fading of Thiazole Orange, DODC and Methylene blue,
5- explain the kinetics of this fading process
6- explain with figure the effect of oxygen (singlet oxygen) generation is often used to explain photobleaching processes in biological stains.
7- the kinetic insights into the mechanism of fading of thiazole orange, Methylene blue,

Color Fading and its Causes

The molecular attraction of the pigment in the garment loses the molecular attraction with the fabric that alternatively results in color fading. The chemical reaction results in incorporating the dye with the fabric as a part of the fabric as a layer of the fabric.

UV light sources could lead to the reduction of the intensity absorption of the materials those 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 energy and following are the ways through which the fading can occur due to the following factors:

Radiation emission or the interaction of the phosphorescence or the fluorescence

Photochemical are other possible causes those could lead to the fading (Oster and Wotherspoon 1957)

The factors associated with the internal energy conversion or intersystem crossing could lead to the radiation less transitions are another major ways those could lead to the fading.  

The very low or high pH can also result in the breakage of the azo bond that could be an instance process.

Jablonski Scheme: The following diagram represents the difference between the phosphorescence and fluorescence or in other words, it can explain the photochemistry of textile dyes:

                                                                 

                                                                          Figure 1: Jablonski Scheme

                                                                                            (Source: )

The above diagram explains the essential transition between the electronic states of the fiber molecule and dyes components. The absorption of the light by the ground state molecules (S0) raises to excited electronic states (S1 or S2). The excited states tend to be short-lived as that of the molecules tending to be at the ground state.

Photochemical reactions: the conventional photochemical generally originate from the triplet states those lives till 10s from 100ns. The excited singlet tends to live for much shorter lifetime of 1-1000 ps. During the process of the degradation reaction, there are possibilities that the molecule might undergo intersystem results crossing in the corresponding triplet state that lives longer than the excited molecules. The accelerated reaction might occur due to the cases that the molecules could get to the high vibrational level.

Radiationless transitions: It could occur because of the internal energy conversion or intersystem crossing right behind the vibrational relaxation. The above mentioned diagram indicates the process using wavy lines those could be macroscopically observed due to heat evolution. The major cause behind the radiationless transition is because of the internal energy conversion or intersystem crossing those can be described as the iso-energic process or can be represented as no change in overall energy.

Jablonski Scheme

Emission of radiation: this radiation occurs from the excited states’ lowest vibrational levels of the ground state that could be stated as the fluorescence. The radiation being emitted to the ground state from the lowest triplet state T, is known as phosphorescence.

Photochemical reactions: The triplet state contributes in the most conventional photochemical reaction those have limited lifetime ranging between the age 10 s and 100 ns. The excited singlet state span between 1-1000ps and lives much shorter than compared to that of the ground state element that elads to the efficient chemical reaction. The relaxation of the higher excited singlet generally relaxes in condensed phase that alternatively results in the loss of thermal energy to the singlet those have been lowest excited and leading to the insignificance of the chemical reactions in general. “Although singlet excited states, HD*, produced when a molecule absorbs a photon of light, tend to be too short-lived for conventional photochemical reactions.”

Photo-oxidation via singlet oxygen: 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. The triplet-triplet annihilation could possibly occur with the oxygen for the instance when dye is excited to the triplet state that could alternatively result in the production of the singlet oxygen (scheme 2) that in turn initiates the dye destruction (scheme 3).

3HD* + 3O2 - HD + 1O2

1O2 + HD ------à Decomposition

The efficient singlet oxygen generators include the methylene blue and copper (ii) phthalocyanine because of the variations in the singlet oxygen lifetimes.

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 mechanisms those have been 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.

Photochemical reactions

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:

                                                                     

                                                                                Figure 1

                                                             (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:

                                                     

                                                                                          Figure 2

                                                                            (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.

References:

Oakes, J., 2001. Photofading of textile dyes. Review of Progress in Coloration and Related Topics, 31(1), pp.21-28.

Mowry, S. and Ogren, P.J., 1999. Kinetics of methylene blue reduction by ascorbic acid. Journal of chemical education, 76(7), p.970.

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.

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.

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 Engineers, 42(2), pp.320-326.

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.

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