On the Mechanism of Lake Formation in Alizarin Dyeing

L. P. Kulev and T. N. Gorizontova

A considerable number of works have been devoted to elucidating the structure of alizarin lakes; however, the question of the mechanism of lake formation has not yet received sufficiently complete clarification.

Among the most interesting and, to a certain extent, experimentally substantiated schemes for the structure of the aluminum–calcium alizarin lake are the schemes of Krasnovsky, Galler, Val’ko, Fierz-David, and Rutishauser.

A. Krasnovsky \[1\] believes that the molecule of the alizarin lake should not have unsaturated valences; the absence of these, in his opinion, explains the high light fastness and low chemical activity of the lake.

In the interaction of alizarin with calcium salts and aluminum hydroxide, the author considers the first phase of the reaction to be the adsorption of alizarin and the calcium ion on the surface of aluminum hydroxide molecules; at the same time alizarin dissociates with the formation of the colored alizarate ion. Upon heating, the second phase of the reaction begins, accompanied by the transition of the system into a stable chemical compound in which two aluminum atoms and one calcium atom replace the hydrogens of the hydroxyl groups of alizarin, while the coordinative unsaturation of the oxygen of the second carbonyl group of dioxyanthraquinone is satisfied by the addition of aluminum hydroxide molecules:

\[ \begin{array}{c} \mathrm{Al(OH)_2} \qquad\qquad\qquad \mathrm{Al(OH)_2} \\[2mm] \begin{array}{c} \mathrm{O}\quad \mathrm{O}\\ \| \quad | \end{array} \quad \begin{array}{c} \text{anthraquinone nucleus} \end{array} \!-\mathrm{O{-}Ca{-}O}-\! \begin{array}{c} \text{anthraquinone nucleus} \end{array} \quad \begin{array}{c} \mathrm{O}\quad \mathrm{O}\\ | \quad \| \end{array} \\[2mm] \mathrm{O}\cdots n\mathrm{Al(OH)_3} \qquad\qquad\qquad \mathrm{O}\cdots n\mathrm{Al(OH)_3} \end{array} \]

According to Galler \[2\], during mordanting the mordanted fabric gives, with hydrates of the oxides of aluminum and calcium, soaps which, upon interaction with the dye, form alizarates of the following structure:

\[ \begin{array}{cc} \begin{array}{c} \mathrm{O}\quad \mathrm{O{-}Ca}\\ \| \quad | \quad |\\ \text{anthraquinone nucleus}\!-\!\mathrm{O}\\ \|\\ \mathrm{O} \end{array} & , \qquad \begin{array}{c} \mathrm{O}\quad \mathrm{O{-}Al(OH)}\\ \| \quad | \quad |\\ \text{anthraquinone nucleus}\!-\!\mathrm{O}\\ \|\\ \mathrm{O} \end{array} \end{array} \]

These alizarates are present as a mixture on the surface of the fabric. The subsequent steaming after dyeing promotes the formation of complexes of aluminum–calcium alizarate, possessing very high fastness. The author assumes that fatty acids do not enter into the composition of the complex, but play an extremely important role in the dyeing process, constituting a dispersion medium in which the formation of alizarates and complexes takes place.

Walco \[3\] considers the process of interaction of the mordant with the dye as a colloid-chemical process. At first, in his opinion, as a result of the interaction of the dye with the mordant, an unstable addition product arises, which then, upon heating, passes into a salt of the inner-complex type.

Fierz-David and M. Rutishauser \[4\], studying the conditions for the formation of the alizarin lake in water and pyridine, established that the lake is a complex of alizarin, calcium, and aluminum. To this complex, in the authors’ opinion, water and pyridine may be added. In pure form, a black modification of the complex with two molecules of water and a red one with five were obtained.

The variety of schemes for the structure of the alizarin lake indicates that the question of its actual structure still remains open. For a complete and comprehensive study of this question, further investigation is required, involving various methods, including physicochemical ones. The potentiometric method used by us \[5\] for determining the reactivity of oxyanthraquinones may be of great service here, since it makes it possible to follow the dynamics of lake formation in the process of dyeing cotton fabric.

When mordanted fabric is immersed in the dye bath, an e.m.f. arises, reaching, for quinizarin, 341 mV, for alizarin 340 mV, and for xanthopurpurin 273 mV \[5\].

The curve of change in e.m.f. during dyeing with quinizarin proceeds smoothly, with one maximum 50 minutes after the moment of immersion of the fabric in the solution. A very similar picture was given under the same conditions by alizarin. The curve of change in e.m.f. during dyeing with xanthopurpurin has a different character. In contrast to all other oxy derivatives of anthraquinone, here the appearance of two maxima is observed—after 15 and 50 minutes from the beginning of the experiment. If one takes into account that the electromotive force arises as a result of the interaction of the aluminum mordant with the hydroxyl groups of the dioxyanthraquinones and, probably, is connected with the complex-forming ability of these compounds, one may imagine the following order of reaction. Quinizarin, having hydroxyl groups in positions 1 and 4, reacts with aluminum hydroxide with the formation of an inner-complex compound of symmetrical structure:

\[ \begin{array}{c} \text{quinizarin structure} \end{array} \quad + 2\mathrm{Al(OH)_3} \longrightarrow \quad \begin{array}{c} \text{inner-complex aluminum compound structure} \end{array} \quad + 2\mathrm{H_2O} \]

Both hydroxyl groups of quinizarin are equivalent and must react simultaneously at the same rate.

In alizarin the hydroxyl groups are adjacent, but here too they participate in the formation of two new rings containing residues of \(\mathrm{Al(OH)_3}\):

\[ \ce{ \text{alizarin} + 2Al(OH)3 -> \text{alizarin--O--Al(OH)2\ (chelate)\ ...\ Al(OH)2} + 2H2O } \]

Xanthopurpurin can give only a mononuclear intracomplex compound, since its second hydroxyl group (in position 3) is remote from the carbonyl groups and from the first hydroxyl. With aluminum hydroxide, one must suppose, the hydroxyl group that has no neighboring complex-forming groups reacts first of all, i.e., the hydroxyl in position 3; the mobility of the hydrogen of the hydroxyl group situated in the \(\alpha\)-position must be lower as a result of ring closure through a hydrogen bond:

\[ \ce{ \text{xanthopurpurin with intramolecular H-bond} + Al(OH)3 -> \text{xanthopurpurin--O--Al(OH)2 with intramolecular H-bond} + H2O } \]

\[ \ce{ \text{xanthopurpurin--O--Al(OH)2 with intramolecular H-bond} + Al(OH)3 -> \text{xanthopurpurin--O--Al(OH)2 with Al(OH)2 chelate} + H2O } \]

Both reactions proceed at different rates, which is reflected in the appearance of two maxima on the curve of change in e.m.f. The difference in the reactivity of the hydroxyl groups as a function of their position is also confirmed by experiments with \(\alpha\)- and \(\beta\)-oxyanthraquinones. The first of these gives the maximum value of e.m.f. only after 60 minutes, whereas for the second it is reached already after 30 minutes, i.e., twice as fast. As in the preceding cases, this phenomenon is connected with the structural features of the oxy-derivatives of anthraquinone:

\[ \begin{aligned} &\text{\(\alpha\)-oxyanthraquinone} + \mathrm{Al(OH)_3} \longrightarrow \text{aluminum derivative} + \mathrm{H_2O},\\[4pt] &\text{\(\beta\)-oxyanthraquinone} + \mathrm{Al(OH)_3} \longrightarrow \text{aluminum derivative} + \mathrm{H_2O}. \end{aligned} \]

The \(\beta\)-methyl ether of alizarin, in which only one hydroxyl group remains free, should react according to the type of \(\alpha\)-oxyanthraquinone, i.e.

\[ \text{\(\beta\)-methyl ether of alizarin} + \mathrm{Al(OH)_3} \longrightarrow \text{aluminum derivative} + \mathrm{H_2O}. \]

Indeed, the maximum e.m.f. is reached here also after 60 minutes. Since the e.m.f. arises as a result of the interaction of aluminum hydroxide with the hydroxyl groups of the dye, its magnitude must depend on the number of these groups. It is natural, therefore, that purpurin, which has three hydroxyls, showed the greatest activity of all the oxy-derivatives of anthraquinone that we tested. All the hydroxyl groups of purpurin probably enter into the reaction simultaneously, which accounts for the appearance of one maximum of the e.m.f. 30 minutes after the start of the experiment.

\[ \text{purpurin} + 3\,\mathrm{Al(OH)_3} \longrightarrow \text{aluminum derivative of purpurin} + 3\,\mathrm{H_2O}. \]

The nature of the change in the e.m.f. upon immersion of dyed fabric samples in the fixing bath makes it possible to express the following considerations concerning the role of calcium salts in alizarin dyeing. If the β-oxy group participates in the formation of calcium alizarate, as follows from Krasnovskii’s data, then compounds not possessing this group—for example, α-oxyanthraquinone, the β-methyl ether of alizarin, and quinizarin—should differ greatly in e.m.f. value from those oxy derivatives in which it is present. In fact, the following order of change in e.m.f. is observed:

Table 1

It is evident from this that, except for xanthopurpurin and β-oxyanthraquinone, all the other compounds under the indicated conditions gave a maximum e.m.f. after one and the same interval of time, i.e., after 30 minutes. To this must be added the absence of any substantial difference in the behavior of alizarin and quinizarin, for which the maximum e.m.f. values lie within the limits of experimental error (±1 mV). On the basis of these data we come to the conclusion that compounds capable of ring closure with the participation of aluminum react with calcium acetate according to the following scheme:

\[ \cdots \mathrm{Al} \begin{matrix} & \mathrm{OH} \\ < & \\ | & \mathrm{OH} \end{matrix} \quad +\quad \begin{matrix} \mathrm{CH_3COO} \\ \\ \mathrm{CH_3COO} \end{matrix} >\mathrm{Ca} \;\longrightarrow\; 2\,\mathrm{CH_3COOH} \;+\; \cdots \mathrm{Al} \begin{matrix} & \mathrm{O} \\ < & >\mathrm{Ca} \\ | & \mathrm{O} \end{matrix} \]

Thus, complex aluminates of this type should be formed by oxy derivatives of anthraquinone having a disposition of hydroxyl groups favorable for the formation of intracomplex compounds. For example, for alizarin and quinizarin the reactions in the fixing bath at low temperature proceed in this direction (p. 188).

The aluminates of α-oxyanthraquinone and of the β-methyl ether of alizarin, having one residue of \(\mathrm{Al(OH)_3}\) each, give almost identical e.m.f. maxima in one and the same interval of time. It should be noted that dyeings with the β-methyl ether noticeably deepen their color in a calcium acetate solution, which indicates the presence of a reaction with this salt even in the absence of a free hydroxyl group in the β-position (see the structural scheme of alizarin lake according to A. Krasnovskii).

Consequently, for oxy derivatives with the same number of hydroxyl groups participating in the formation of internal complexes, a reaction of one and the same order takes place; this also explains the similarity noted above in the behavior of these compounds. Xanthopurpurin and β-oxyanthraquinone differ substantially in structure from all the other compounds we tested.

\[ \begin{array}{c} \text{[alizarin complex with two } \mathrm{Al(OH)_2}\text{ groups]} + 2(\mathrm{CH_3COO})_2\mathrm{Ca} \longrightarrow \text{[Al–Ca chelate complex of alizarin]} + 4\mathrm{CH_3COOH} \\[1.5em] \text{[alizarin complex with two } \mathrm{Al(OH)_2}\text{ groups]} + 2(\mathrm{CH_3COO})_2\mathrm{Ca} \longrightarrow \text{[Al–Ca chelate complex of alizarin]} + 4\mathrm{CH_3COOH} \end{array} \]

oxy derivatives of anthraquinone. Therefore reactions of another type are possible for them:

\[ \begin{array}{rcl} \mathrm{I.}\quad \text{[anthraquinone complex with } \mathrm{Al(OH)_2}\text{ and }{-}\mathrm{OAl(OH)_2}\text{]} + \mathrm{H_2O} &\longrightarrow& \text{[anthraquinone complex with } \mathrm{Al(OH)_2}\text{ and }{-}\mathrm{OH}\text{]} + \mathrm{Al(OH)_3} \\[1.5em] \mathrm{2.}\quad \text{[anthraquinone complex with } \mathrm{Al(OH)_2}\text{ and }{-}\mathrm{OH}\text{]} + 3(\mathrm{CH_3COO})_2\mathrm{Ca} &\longrightarrow& \text{[dimeric Al–Ca complex joined through }{-}\mathrm{O{-}Ca{-}O}{-}\text{]} + 6\mathrm{CH_3COOH} \end{array} \]

\[ \begin{array}{rcl} \mathrm{I.}\quad \text{[oxy-anthraquinone }{-}\mathrm{OAl(OH)_2}\text{ derivative]} + \mathrm{H_2O} &\longrightarrow& \text{[oxy-anthraquinone }{-}\mathrm{OH}\text{ derivative]} + \mathrm{Al(OH)_3} \\[1.5em] \mathrm{2.}\quad 2\,\text{[oxy-anthraquinone }{-}\mathrm{OH}\text{ derivative]} + (\mathrm{CH_3COO})_2\mathrm{Ca} &\longrightarrow& \text{[dimeric calcium salt, }{-}\mathrm{O{-}Ca{-}O}{-}\text{]} + 2\mathrm{CH_3COOH} \end{array} \]

Here the first stage of the reaction consists in the hydrolytic cleavage of the metal, weakly bound to the oxygen of the phenolic hydroxyl, and

then the ring systems are bound with the aid of calcium. In xanthopurpurin, in addition, interaction occurs between calcium acetate and another residue of aluminum hydroxide included in the inner complex. The possibility of the indicated transformations is confirmed by the appearance on the fabric of a pink coloration characteristic of the calcium salt of β-oxyanthraquinone of the structure given above.

Purpurin, which combines the structural features of alizarin and quinizarin, reacts according to the same scheme as the latter:

\[ \begin{array}{c} \text{purpurin--Al(OH)}_{2}\text{ complex} \end{array} \quad +\; 3(\mathrm{CH_3COO})_2\mathrm{Ca} \;\longrightarrow\; \begin{array}{c} \text{calcium--aluminum complex of purpurin} \end{array} \quad +\; 6\mathrm{CH_3COOH}. \]

Deepening of the color of the dyeings, especially upon heating, is associated with a further complication of the dye molecule. Since a transition to a deeper tone is also observed in the absence of calcium ions in the solution—for example, upon prolonged boiling of samples of dyed fabric in distilled water—the binding of individual molecules into a more complex aggregate may take place through the hydroxyl groups of aluminum hydroxide residues:

\[ \cdots \mathrm{Al} \begin{matrix} \diagup \mathrm{OH}\\[-2mm] \diagdown \mathrm{OH}\\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \quad \mathrm{HO} \begin{matrix} \diagdown\\[-2mm] \diagup\\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \mathrm{Al}\cdots \;\longrightarrow\; \cdots \mathrm{Al} \begin{matrix} \diagup \mathrm{O}\\[-2mm] \diagdown \mathrm{O}\\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \mathrm{Al}\cdots \]

In the presence of calcium salts the reaction is accompanied by the formation of a bond with participation of the metal:

\[ \cdots \mathrm{Al} \begin{matrix} \diagup \mathrm{OH}\\[-2mm] \diagdown \mathrm{OH}\\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \;+\;2(\mathrm{CH_3COO})_2\mathrm{Ca} \;+\; \]

\[ \mathrm{HO} \begin{matrix} \diagdown\\[-2mm] \diagup \end{matrix} \mathrm{Al} \begin{matrix} \\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \cdots \;\longrightarrow\; \cdots \mathrm{Al} \begin{matrix} \diagup \mathrm{O{-}Ca{-}O}\\[-2mm] \diagdown \mathrm{O{-}Ca{-}O}\\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \mathrm{Al} \begin{matrix} \\[-2mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{matrix} \;+\;2\mathrm{CH_3COOH}. \]

During steaming of the dyed fabric, which is a necessary operation in alizarin dyeing, the possibility is not excluded of rupture and rearrangement of bonds, as a result of which the intermediate reaction products,

for example, the complex calcium aluminates mentioned above interact in the same direction:

\[ \begin{array}{ccccccccc} \ldots A & \overset{O}{\diagup} & & & \overset{O}{\diagup} & & & O{-}Ca{-}O & \\ & & Ca + Ca & & & Al\ldots & \longrightarrow & \ldots Al & \diagdown Al\ldots \\ & \underset{O}{\diagdown} & & & \underset{O}{\diagdown} & & & O{-}Ca{-}O & \\ & \big| & & & & \big| & & \big| & \big| \\ & O & & & & O & & O & O \\ & \big| & & & & \big| & & \big| & \big| \end{array} \]

Thus, individual molecules of intracomplex salts are firmly linked with one another by intermediate bridges and form a macromolecular system, i.e., an alizarin lake. Apparently, in its final form the dye is not an individual compound, but a mixture of condensation products of varying degrees of complexity, which explains the large discrepancies in the analytical data given by different authors for one and the same alizarin lake.

Conclusions

1. From the character of the change in the e.m.f. arising when mordanted fabric is immersed in a dye solution and dyed fabric in a solution of calcium acetate, one may judge the dynamics of the process of lake formation. All hydroxyl groups of oxyanthraquinone react with aluminum hydroxide, irrespective of their position in the molecule.

2. When dyed fabric is treated in the fixing bath, interaction occurs between calcium acetate and the residue of aluminum hydroxide that is part of the inner complex; moreover, individual molecules are linked with one another by intermediate bridges of oxygen and metal.

3. The reaction of lake formation is a multistage process accompanied by gradual complication of the molecule, and the lake itself consists of a mixture of condensation products of varying degrees of complexity.

Literature

1. Krasnovskii A. Promyshlennost’ organicheskoi khimii, 10, 597, 1938.
2. Gailer. Helv. Chim. acta, 30, 844, 1938.
3. Valko E. Kolloidno-khimicheskie osnovy tekstil’noi tekhnologii, part II. Moscow, 471, 478, 1940.
4. Fierz-David, M. Rutishauser. Helv. Chim. acta, 23, 1298, 1940.
5. Kulev L. and Gorizontova T. Trudy ZSФAN SSSR, issue 4, 3, 1950.