EFFECT OF DYES ON THE THERMAL STABILITY OF SILVER AZIDE

E. S. Kurochkin, G. G. Savel’ev, Yu. A. Zakharov, O. I. Chizhikova

(Presented by the scientific seminar of the Department of Radiation Chemistry, KhTF)

In previous works [1–4] we studied the effect of heterogeneous inorganic semiconductor additives on the thermal stability of silver salts. The mechanism of their action was discussed from the standpoint of processes occurring at the phase boundary between the semiconductor and the decomposing substance, while catalytic activity was associated with the contact potential difference. Publications on the catalysis of the thermolysis process by organic semiconductors are limited to a single report [5]. The authors related the effect of the influence to the features of electron redistribution between the additive and the decomposing substance. However, the causes of the influence of organic additives on thermolysis remain unclear. To establish the mechanism of catalysis by organic semiconductors, it is necessary at the present stage to accumulate experimental data. For this purpose we investigated a number of dyes from various classes as heterogeneous impurities in studying their effect on the thermal stability of silver azide.

Since the behavior of azide–organic dye systems with respect to heating proved to be complex, additional microscopic and mass-spectrometric studies were required.

This work presents results on the action of organic semiconductor additives on the thermal stability of silver azide. As catalysts we tested a number of dyes from the classes of arylmethane polycyclic vat dyes, quinone imine dyes, azo dyes, cyanine dyes, and copper phthalocyanine. The additive was introduced in amounts of 5 and 10 wt.%; experiments were carried out on a gravimetric vacuum apparatus with automatic recording of the change in weight, described in [6]. The results of kinetic experiments at 235°C are presented in Figs. 1, 2, 3; the plots are given in the coordinates $\alpha$—$\tau$ (degree of decomposition—time). From the class of azo dyes, introduced in an amount of 5 wt.%, the following were studied: acid scarlet, direct diazo sky blue lightfast, direct diazo blue, and fat orange. From Fig. 1 it is seen that all of them accelerate thermal decomposition; moreover, the accelerating effect increases with an increase in the number of azo groups in the dye molecule. Acid scarlet somewhat retards thermal decomposition.

From the class of arylmethane dyes the following were studied: erythrosine B, rhodamine C, fluorescein, belonging to the xanthene group; auramine, malachite green, belonging to the diaryl- and triarylmethane groups.

Fig. 1. Kinetics of the thermal decomposition of $\mathrm{AgN_3}$ at 235°C: pure (1) and with additions of 5 wt %: brilliant green cubo dye (2), acid scarlet (3), golden-yellow cubo dye (4), direct diazo lightfast deep blue (5), direct diazo blue (6), erythrosin B (7), fat orange (8), trypaflavin (9), acridine yellow (10), and fluorescein, rhodamine C, auramine, malachite green (11–14)—flash.

respectively, as well as acridine yellow and trypaflavin from the acridine group. It is evident from Fig. 1 that acridine dyes and erythrosin B markedly accelerate the thermolysis of $\mathrm{AgN_3}$; the addition of all the other dyes listed causes a flash of the azide.

Fig. 2. Kinetics of thermal decomposition at 235°C of pure $\mathrm{AgN_3}$ (1) and with dye additions of 10 wt %: copper phthalocyanine (2), azo carmine G (3), brilliant green cubo dye (4), cubo blue K (5), anthanthrone (6), and neutral red (7).

Among the polycyclic cubo dyes (Figs. 1, 2) introduced in amounts of 5 and 10 wt %, anthanthrone noticeably accelerates the thermal decomposition of the azide, whereas the dyes golden-yellow cubo dye, brilliant green cubo dye, cubo blue K, and brilliant violet cubo dye noticeably do not …

Fig. 3. Thermal decomposition at 235°C of pure AgN₃ (1) and with additions of 5 wt.% cyanine dyes: imidocarbocyanine (2), indocarbocyanine (3), thiocarbocyanine (4), neocyanine (5), and neocyanine introduced in an amount of 20 wt.% (6).

change the rate of thermal decomposition or slightly reduce it. Of the quinone-imine dyes tested, introduced up to 10 wt.%, azocarmine G has almost no effect on the thermolysis of silver azide, while neutral red gives a flash (Fig. 2). All cyanine dyes (Fig. 3) noticeably accelerate the thermal decomposition of the azide. The strongest accelerating effect is produced by neocyanine, whose introduction in an amount of 20% leads to a flash of the azide.

From the data obtained it is evident that lengthening the chain in the dye molecule (the group of azo dyes) leads to some increase in their catalytic activity. The introduction of electron-acceptor substituents Cl, Br, I (compare, for example, erythrosin B and fluorescein; golden-yellow vat anthanthrone and bright-green vat dye from the group of polycyclic vat dyes; and also neocyanine and indocarbocyanine from the group of cyanine dyes, and certain others) leads to a marked decrease in the catalyzing ability of the compound.

It was then of unquestionable interest to investigate in greater detail the influence on the thermal stability of the azide of those dyes that substantially accelerate the process of thermal decomposition or produce a flash. We investigated the temperature dependence and carried out a kinetic analysis for AgN₃ and silver azide with 5 wt.% auramine, neocyanine, fluorescein, rhodamine C, and malachite green. The experimental curves were processed by the method of curve transformation. Sections of the curves from 10 to 60% were satisfactorily transformed into a single curve. Figure 4 presents the dependence

\[ \lg \varkappa - \frac{1}{T}, \]

where \(\varkappa\) is the transformation coefficient. The calculated activation energy is: for pure silver azide, \(24 \pm 2\) kcal/mol; \(25 \pm 2\) kcal/mol; \(40 \pm 2\) kcal/mol; \(24 \pm 2\) kcal/mol; \(62 \pm 2\) kcal/mol; \(51 \pm 2\) kcal/mol for the azide with admixtures of auramine, neocyanine, fluorescein, rhodamine C, and malachite green, respectively. Calculation of the activation energy from the rate constant found from the equation

Fig. 4. Dependence of $\lg \chi$ for pure silver azide 2 and for silver azide containing 5 wt % of neocyanine, rhodamine C, auramine, fluorescein, and malachite green (curves 1, 3, 4, 5, 6, respectively)

\[ \frac{d[\mathrm{N}_2]}{dt}=K\left[\mathrm{AgN}_3\right]^{\frac{2}{3}}, \]

gives exactly the same values for the activation energy. From these data it is evident that dyes, which significantly accelerate the thermolysis of silver azide, noticeably increase the activation energy of the process.

Additional data on the behavior of the mixture components, obtained by means of microscopic observations, showed that, with respect to thermal decomposition, dyes in contact with azide can be divided into 2 groups. The first includes cubane dyes, acid scarlet, fluorescein, acridine yellow, copper phthalocyanine, azocarmine G, and neocyanine, which do not melt during the decomposition of the azide and do not change their appearance. In the region of contact of these dyes with azide, rapid darkening is observed, which most likely can be attributed to the silver being formed. The second group includes all the remaining dyes, which melt, noticeably change their color and, evidently, decompose as a result of a chemical reaction with the azide.

In order to study the question of possible thermal decomposition of the dyes of group I, or the presence of chemical interaction between them and the azide under the conditions of thermal decomposition, we performed a mass-spectrometric analysis of the products of thermal decomposition of a mixture of $\mathrm{AgN}_3 + 10$ wt % neocyanine, using the apparatus described in [7]. The experiments were carried out at a temperature of 270° C for pure silver azide and 220° C for the azide–neocyanine mixture and for pure neocyanine.

As regards the question of the nature of the action of additives of the second group, because of the complexity of the system this question requires further experiments.

Analysis of the mass-spectrometric data showed that pure neocyanine under the above conditions releases only a small amount of decomposition products. After heating, it retains its properties. On heating the mixture of azide and neocyanine, only the amount of the products of thermal decomposition of the azide increases. The invariability of neocyanine after complete decomposition of the azide can also

it may be concluded from the fact that, upon dissolution, it gives the same coloration as the corresponding amount of the unheated preparation.

Similar experiments were carried out with several other dyes as well. In this case, the investigated dyes of the first group did not change the color of the solution after heating in a mixture with the azide, whereas the dyes of the second group completely lost the ability to color the solvent.

These data provide grounds for asserting that, in the case of dyes of the first group (which undergo no chemical change in the course of the catalytic process), we are dealing with catalysis of a solid by a solid.

Thus, as a result of the experiments performed, a group of dye catalysts has been identified which can subsequently be used to study the mechanism of catalysis. This mechanism is evidently associated with processes occurring at the boundary between two phases, as was clear from microscopic observations of the localization of the reaction in the contact region, and may be connected with the contact potential difference [1], or with the formation of a “bridge” for electron transfer [3], or, finally, with the formation of an activated complex between the azide and the dye.

The study of the possible mechanism of catalysis will be continued by us in subsequent work.

REFERENCES

1. Yu. A. Zakharov, G. G. Savel’ev. Kinetika i kataliz, 7, 55 (1966).

2. G. G. Savel’ev, Yu. A. Zakharov. Izv. vyssh. uchebn. zaved., ser. Khimiya i khimicheskaya tekhnologiya, 7, 768 (1964).

3. Yu. A. Zakharov, Yu. S. Kurochkin, G. G. Savel’ev, Yu. N. Rufov. Kinetika i kataliz, 7, 425 (1966).

4. Yu. A. Zakharov. Izv. vyssh. uchebn. zaved., Khimiya i khimicheskaya tekhnologiya, 8, No. 2 (1967).

5. V. V. Boldyrev, I. V. Shmidt, V. I. Pismenko, M. S. Shvartsberg, L. P. Kotlyarevskii, V. N. Andrievskii, V. F. Komarov. Kinetika i kataliz, 6, 766 (1965).

6. G. G. Savel’ev, V. V. Bordachev. Izv. TPI, 158, 80, 1968.

7. R. N. Isaev. This collection.