The Porosity of Samples of the Disperse Dye, C.I. Disperse Blue 79

The Porosity of Samples of the Disperse Dye, C.I. Disperse Blue 79

KK. Unger et at. (Editors), Characterization of Porous Solids © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands THE POR...

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KK. Unger et at. (Editors), Characterization of Porous Solids © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands


R.R. MATHER Scottish College of Textiles, Galashiels, Scotland.


Nitrogen adsorption isotherms have been determined on commercial samples of the disperse dye, C.I. Disperse Blue 79. The samples contain of_ldye and dispersing agent. Low B.E.T. approximately equal amounts 2g surface areas (0.5 - 0.9 m ) and c-values (5 - 19) were obtained. Nearly all the isotherms exhibit hysteresis, although there are considerable variations in the character of the hysteresis loops from sample to sample. Frenkel-Halsey-Hill (F.H.H.) plots of the nitrogen isotherms have also been constructed. For some dye samples, these plots become linear at relative pressures of 0.6 - 0.7; for the other samples, the plots have no clear linear region at all. Analysis of the F.H.H. plots and of the hysteresis loops indicates the presence of mesopores, probably within aggregates of the dye samples. The mesoporous structures of some of the samples appear much more rigid than those of the others. This range of behaviour correlates well with variations found in the stabilities of dispersions of the dye samples in water. I NTRODUCTI ON

Disperse dyes are used commercially for colouring synthetic fibres, in particular polyester. However, these dyes have only a very low solubility in aqueous media and exist in the dyebath predominantly as fine dispersions. The nature of the dispersions is an important factor influencing dye application. For commercially acceptable performance, the dimensions of the suspended dye crysta 1s have to be of the order of on ly a few mi crons (1). If large crystals or crystal aggregates are present, performance of the dye in application is severely impaired.



Disperse dyes are often marketed in powder form. Commercial samples contain dispersing agent, included to assist incorporation into the dyebath and to confer stability to the resulting dispersion of dye crystals during dyeing (1). Indeed, the proportion of dispersing agent may well be as high Dispersing agents, therefore, have a strong as 50%, or even greater. influence on the physical properties of commercial disperse dyes, including Dispersion into the dyebath thus the state of aggregation of dye crystals. depends on the penetration of water into the pores of the aggregates and the subsequent separation of the dye crystals. This paper describes the investigation by nitrogen adsorption of the disperse dye, C.I. Disperse Blue 79, whose structural formula is:

(C.r. stands for Colour Index). This dye is the major blue dye applied commerc i a11y to polyester. The approach adopted fa 11 ows that used for exploring the aggregate properties of copper phthalocyanine pigment powders (2,3). Clear variatipns in the porous properties of the dye samples have been revealed, and there appears to be a good correlation of these variations with those found in the stabi 1ities of the dye dispersions in water.

MATERIALS AND METHODS The samples of C.I. Disperse Blue 79 studied were prepared and supplied Each sample contained about by Yorkshire Chemicals plc, Leeds, England. Both synthet ic, S, and equa1 parts of dye and di spers i ng agent. 1ignin-based, L, dispersing agents had been used in the preparation, as For comparison, a sample of the dye without indicated in Table 1. dispersing agent was also investigated.


TABLE 1 Properties of disperse dye samples Dye Sample

Dispersing Agent

1 2 3 4 7 8 9




B.E. T. - Plot S cn /nfg-1 0.80 0.64 0.68 0.47 0.91 0.61 0.72 0.66 6.7



19 12 5 7


14 12

F.H.H. -

Range of Linearity (p/po) None None None 0.6 0.7 0.6 0.72 0.7 0.7

Plot r

2.16 2.59 2.41 2.81 2.91 2.90

Nitrogen adsorption isotherms were determined at 77K by a volumetric method, with a semi-micro apparatus of the type designed by Harris and Sing (4). The nitrogen, of at least 99% purity, was dried by slow passage through a liquid nitrogen cold trap. Before the start of an isotherm determination, each sample of dye was evacuated for at least 18 hr. at room temperature. In many cases, prior freeze-drying was also found to be desirable. Equilibrium pressures were measured with the aid of a cathetometer (to :I: 0.002cm). The densities of the samples were assessed by means of displacement of hydrogen at room temperature. The densities of the commercial samples are 3 1.60 g cm- and that of sample A is 1.43 g cm- 3 (± 5%). Dispersions of the dye samples were prepared in deionised water at a concentration of O.lg dm- 3 • The procedure adopted closely resembles that recommended by Brossman et a1. (5). About 50 mg. of sample were weighed accurate 1y ina 250cm 3 beaker and were then transferred to a 500 em' volumetric flask. The beaker was washed several times with water and the wash ings were a1so transferred to the f1 ask. The f1ask now conta i ned a 3 dispersion of the dye in ca.100cm of deionised water. The dispersion, after vigorous shaking, was then made up to 500cm 3 and was again shaken vigorously. Immediately, a small portion was transferred to a sample cell, and the absorbance was determined at a wavelength of 600 nm in a Pye-Unicam SP6-500 spectrophotometer. The stability of the dispersion in the sample ce11 was assessed from subsequent measurements of absorbance at i nterva 1s over several days.


The dye samples were also examined by X-ray powder diffractometry, All of the samples showed using a Philips Debye-Scherrer powder camera. identical diffraction patterns. The samples thus appear to contain crystals of the same lattice structure.

RESULTS AND DISCUSSION Some of the nitrogen adsorption isotherms are shown in Figure 1. Although the adsorption of nitrogen by all the samples is low, the adsorption by Sample A (containing no dispersing agent) is much higher than by the other dye samples. Thus, the presence of dispersing agent considerab ly reduces the extent of samp 1e surface access ib le to nitrogen molecules. which (with one All the nitrogen isotherms exhibit hysteresis, It is noteworthy exception) extends down to very low relative pressures. too that at these low pressures the adsorption branches of the isotherms for many of the commercial samples have a slighUy wavy appearance. Nevertheless, Brunauer-Emmett-Teller (B.LT.) plots (6) are mostly sufficiently linear between relative pressures of 0.1 and 0.4 to permit estimates of nitrogen surface areas, Sn' Values of Sn' along with values of the B.E.T. parameter, cn' are given in Table 1. Because of the small values of c n and of the wavy nature of many of the isotherms at low relative pressures, the values of Sn reported in Table 1 can be taken only as The small values of c n' however, do indicate only a low approximate. adsorbent-adsorbate interaction with nitrogen. The Frenke1-Halsey-Hill (F.H.H.) equation has been used to analyse the It has been behaviour of the isotherms at higher relative pressures. applied in the form: r loge (po!p) ~ k!V , where Vis the vo 1ume (reduced to s . t. p.) of nitrogen adsorbed at the equilibrium relative pressure, p!po; k and r are constants. Plots of loge V against loge {loge (p/p}} (i.e. F.H.H. plots) were constructed for the isotherms, and Figure 2 shows the plots derived from the isotherms in Figure 1.


5 4-

.... I



0·1 0·+






o E


0·3 0·2 0·1 0·6

'Rela.tive pressure



Fig. 1. Nitrogen adsorption isotherms on some commercial samples of c.r. Disperse Blue 79. 00, adsorption; • • , desorption.


For samples 2 and 3, the F.H.H. plots display no clear linear region, whilst for samples A, 4 and 9, the plots do show some linearity. However, linearity begins generally only at relative pressures as high as 0.6 - 0.7. This behaviour contrasts strongly with that shown by many other adsorbents, such as oxides (7) and copper phthalocyanine pigments (8), where linearity of the F.H.H. plot (if obtained) usually starts at relative pressures of 0.35 - 0.5. The high relative pressures noted for the disperse dye samples provide further evidence for the low adsorbent-adsorbate interaction, suggested by the low values of c n' Values of r determined for those dyes which do exhibit linear F.H.H. plots are reported in Table 1. The values range from 2.2 to 2.9. The highest values are slightly above that found by Carrott et a1. (7) for They showed nitrogen isotherms on a variety of non-porous solids (r N 2.7). that the value of r exceeds 2.7 if micropores are present in the solid. The lower values may well be the result of capillary condensation of nitrogen in mesopores. For samples 1, 2 and 3 the F.H.H. plots display no clear linear region even at relative pressures above 0.7. The hysteresis loops displayed in the isotherms also indicate the Those isotherms which give presence of mesopores in the dye samples. linear F.H.H. plots at high relative pressures all exhibit some hysteresis, especially at low pressures. Hysteresis of this type has been observed for the adsorption of nitrogen on organic pigments (2, 9, 10). It has been associated with changes occurring in the structures of the aggregates formed by the pigment crystals, as adsorption of nitrogen proceeds (3). A similar Hysteresis can be attributed to the explanation may be advanced here. presence of a non-rigid mesoporous structure. The remaining samples, which do not give linear F.H.H. plots, display various types of hysteresis loops. Thus, as Figure 1 shows, sample 2 gives a Type IV adsorption isotherm in the B.D.D.T. classification (11), an indication of a rigid, compact structure. Sample 3 shows virtually reversible behaviour, which may indicate an even more tightly packed structure.




0·6 I

0'7 I

o·~ I

-0,8 -



> 1-1-6










-~. I U







0 0



0 0





--1,+ I






[J 0




° °!e '3 o 0 I






o -1' I





Fig. 2. F.H.H. plots derived from the adsorption branches of the isotherms in Fig. 1.


Table 2 compares the dispersions of the commercial dye samples in water The absorbances of each dispersion were at a concentration of 0.1 g dm -3. measured at a wavelength of 600 nm, immediately after preparation (values shown in Co lumn 2) and at subsequent times over severa 1 days. The absorbance of every dispersion was found to decrease. From the The measurements, the initial rate of loss of absorbance was determined. values, reported in Column 3, provide useful comparisons of the stabilities of the dispersions. Values of the F.H.H. - parameter, r , (where obtainable) are also included. It is noteworthy that the dye samples which do not give linear F.H.H. plots form dispersions with the lowest initial absorbances and highest initial rates of loss of absorbance. Those dyes which appear to have compact structures are apparently less readily dispersed in water. In addition, the dispersions which they do form appear to be the least stable.

TABLE 2 Properties of dye samples dispersed in water (0.1 g dm- 3 ) Dye Sample 1



4 7

8 9 10

Initial Absorbance at 600 nm

Initial Rate of Loss of Absorbance /103hr


2.39 2.95 3.18 1.06 1. 53 1.04 0.83 0.95

1. 710

1.642 1.868 1.821 1. 822 2.085 1.831

F.H.H.parameter r

2.16 2.59 2.41 2.81 2.91

ACKNOWLEDGEMENTS The help of the following is gratefully acknowledged: Dr. S. Partington and Dr. J.H. Varley for helpful discussions during the course of this work; Dr. M.J. Barrow for examination of the samples by X-ray powder diffractometry; Mrs. S.E. Brydon for help with the preparation of this paper.


REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

D. Blackburn, in D.M. Nunn (Editor), The Dyeing of Synthetic-polymer and Acetate Fibres, Dyers Company Publications Trust, 1979, pp 76-128 R.R. Mather and K.S.W. Sing, J. Colloid Interface Sci., 60 (1977) 60-66. J.R. Fryer, R.B. McKay, R.R. Mather and K.S.W. Sing, J. Chern. Tech. Biotechnol., 31 (1981) 371-387. M.R. Harris and K.S.W. Sing, J. Appl. Chern., 5 (1955) 223-227. R. Brossman, N. Kleinemeier, M. Krayer, H.-P. Oesch, B.-T. Groebel, R.G~ Kuehni and A.T. Leaver, J. Soc. Dyers Co1ourists, 103 (1987) 38-42. S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chern. Soc., 60 (1938) 309-319. P.J.M. Carrott, A.I. McLeod and K.S.W. Sing, in J. Rouquerol and K.S.W. Sing (Editors), Adsorption at the Gas-Solid and Liquid-Solid Interface, Elsevier, Amsterdam, 1982, pp 403-410. C.R.S. Dean, R.R. Mather, D.L. Segal and K.S.W. Sing, in S.J. Gregg, K.S.W. Sing and H.F. Stoeckli (Editors), Characterisation of Porous Solids, Society of Chemical Industry, London, 1979, pp 359-367. R.R. Mather, F.A.T.I.P.E.C. Congress XIV, 1978, 433-437. R.B. McKay, F.A.T.I.P.E.C. Congress XVIII, 2/B (1986), 405-425. S. Brunauer, L.S. Deming, W.E. Deming and E. Teller, J. Am. Chern. Soc., 62 (1949) 1723-