The Dyeing of Nylon and Cotton Cloth with Azo Dyes: Kinetics and Mechanism

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

180, 605–613 (1996)

0342

The Dyeing of Nylon and Cotton Cloth with Azo Dyes: Kinetics and Mechanism J. JUSTIN GOODING,* RICHARD G. COMPTON,* ,1 COLIN M. BRENNAN,†

AND

JOHN H. ATHERTON†

*Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom; and †Zeneca Limited, P.O. Box 42, Blackley, Manchester M9 3DA, United Kingdom. Received September 11, 1995; accepted January 9, 1996

The mechanism of the dyeing of cotton and nylon cloth by the azo dyes Orange G and Sunset Yellow FCF was investigated using a channel flow cell. The variation in dyeing with flow rate was found to proceed via a mechanism in which the flux of dye entering the cloth relative to the flux of dye to the cloth surface decreased with increasing flow rate. A mechanism is deduced in which the dye passes from bulk solution, through a porous surface layer within the cloth, before passing into the bulk cloth. Adsorption onto surface sites in this porous layer blocks the passage of further dye into the cloth. Kinetic parameters for such a mechanism are given. q 1996 Academic Press, Inc.

INTRODUCTION

The enormous industrial significance of dyes and dyeing has lead to a substantial body of research related to the application of dyes to cloth (1–13). The rate at which dye molecules are transferred to the cloth may be influenced by the transport of dye through the bulk solution to the surface of the fiber, the possible adsorption of dye molecules onto this surface, and the diffusion of the dye from the surface to the interior of the fiber (4, 7). The diffusion of the dye into the substrate is often considered to be the slowest and therefore the limiting factor governing the dyeing rate (7), although an understanding of the kinetics of each individual step is clearly important in understanding the mechanism of dyeing and its optimization. Once the dye is dissolved and dispersed evenly throughout the dye bath, the processes occurring at the surface and within the cloth are the most important provided that agitation of the solution and mass transport to the surface are both efficient. The better methods of studying dyeing reactions will ideally probe these surface processes. Nevertheless, most dyeing kinetics research has monitored changes in bulk concentrations of dye in the dye liquor (5–9) to make indirect inferences about the surface processes. Alternative methods have made assumptions about the rate of dyeing 1

To whom correspondence should be addressed.

by the extent and/or amount of dye penetration into a cylinder film roll, filament, or sheet of substrate with time (4, 7, 8, 10, 11). Measurements of bulk concentrations however, provide little information on the surface reaction mechanism due to the remote and insensitive nature of the detection of the reaction. The channel flow cell approach pioneered by Compton et al. (2, 3, 14–19) has been successful in probing surface kinetic phenomena at the insulator/solution interface and this has been applied to the study of dyeing kinetics (2, 3). In previous work the reactions between cotton cloth and a dichlorotriazinyl reactive anthraquinone dye and between cotton and a dichlorotriazinyl reactive azo dye were investigated. In both cases the rate of heterogeneous reaction was controlled by a surface process which was first order with respect to the surface concentration of the dye. In this paper a similar approach is used to investigate the reaction of Orange G and Sunset Yellow FCF (Scheme 1) as model nonreactive dyes with cloth substrates. The bonding of these dyes to the cloth is via attractive forces, such as van der Waals forces and hydrogen bonding, rather than by forming a covalent bond between the cloth and the dye. Hence the mechanism of dyeing may be significantly different since the strength of these attractive forces dictates the dye fastness as distinct from reactive dyes which do not necessarily have a high intrinsic affinity for the cloth (12). In the work reported here the two dyes are detected in the channel flow cell using a mercury plated copper electrode; the reduction mechanism of azo dyes under a range of pH conditions has been investigated previously (20). MATERIALS AND METHODS

A perspex channel flow cell (approximate dimensions 60 1 0.6 1 0.1 cm) was used with a modified cover plate to incorporate the cloth as in Fig. 1. The cloth was adhered to the inside of a duct such that the surface of the cloth was flush with the cover plate surface. A rubber-based glue, Evo stick (Evode Ltd. Stafford, UK), was used to fix the cloth to the cover plate, as its high viscosity ensured glue did not

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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

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SCHEME 1

pass through the cloth and block reaction sites or hinder dye diffusion in the cloth. The cloth at the edge of the cover plate, not exposed to the channel, was blocked with melted wax to prevent leakage of solution. The length of cloth used in the cell was approximately 1 cm but was measured accurately for each experiment using a travelling microscope. The electrodes were prepared from copper foil (Goodfellows Metals Ltd., Cambridge) plated with mercury prior to use according to the standard protocol of Daly et al. (21). The dimensions of the electrodes were typically 4 mm 1 4 mm with a thin adjoining ‘‘tail.’’ The tail was used to provide electrical connection by feeding it through a hole in the cover plate. Araldite epoxy resin was used to cement the electrodes onto the inside face of the cover plate. The cemented electrode was then polished to a smoothness of 0.25 mm using progressively decreasing sizes of diamond lapping compound (Engis, Maidstone). The flow cell was formed by mating the channel and cover plate using mechanical pressure. An o-ring around the channel formed a leak proof seal. The cell was plumbed into a flow system which comprised a glass reservoir (250 cm3 ) and several meters of glass tubing, of 2 mm internal diameter, through which solution ran to waste. A silver pseudoreference electrode was located upstream of the cell and a platinum gauze counter electrode downstream. Flow was achieved by gravity feed; deoxygenated solution was fed from the reservoir via one of several calibrated glass capillaries capable of delivering a total flow rate range of 10 04 – 10 01 cm3 s 01 . Finer adjustment of flow rate within the range of each capillary was achieved by varying the height between the reservoir and the collection vessel. Thermal control of the experiments was maintained by locating the cell and 1 m of glass tubing, blown into a coil, within a temperaturecontrolled enclosure. Experiments were conducted at 36 { 17C unless otherwise stated. The concentration of the dye solution was varied from 0.25 to 2 mmol dm03 with a background salt concentration of 0.1 mol dm03 NaCl, 0.0275 mol dm03 disodium hydrogen orthophosphate, and 0.0275 mol dm03 potassium dihydrogen orthophosphate. The orthophosphate buffer kept the solution pH constant at 6.8. The orthophosphate buffer system was chosen because dyes of the type employed are normally applied at or around pH 7 (22). At pH 6.8 the rotating disc electrode (RDE) voltammograms for both Orange G and

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Sunset Yellow FCF showed single 4 electron reduction waves (20). The dyes Sunset Yellow FCF and Orange G were supplied by Zeneca Specialties (Manchester) and all salts were obtained from BDH (Poole). The mercury and mercuric chloride were supplied by BDH (Poole). All solutions were made in Elgastat (High Wycombe, Bucks) UHQ grade water. The cloth used was either scoured/bleached unmercerized flat-woven Indian Head cotton or nylon 66, both of which were supplied by Zeneca Specialties. In a typical experiment voltammograms were recorded with the cloth located upstream of the electrode (Fig. 1) at a variety of flow rates. Between voltammograms the cell was flushed clean with background electrolyte until there was no visible sign of dye on the cloth. Once the dyeing voltammograms were completed the cell was plumbed into the flow system with the electrode upstream of the cloth so that currents, Ino cloth , were recorded corresponding to solution which had not been partially depleted of dye by reaction with the cloth. The ratio of the transport limiting current with the cloth upstream of the electrode relative to the cloth downstream gave the shielding factor, S f , Sf Å

Icloth Ino cloth

[1]

The cell height for each experiment was determined from the data where the electrode was upstream of the cloth using the Levich equation (23). The copper rotating disc electrodes were obtained from Oxford Electrodes Ltd. The electrode had an approximate diameter of 7 mm and was insulated within a Teflon sheath. It was polished to a mirror finish using diamond lapping compound and then coated with mercury using the same procedure as for the channel flow cell electrodes. The elec-

FIG. 1. Schematic diagram of the channel flow cell used in dyeing experiments.

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DYEING OF NYLON AND COTTON CLOTH WITH AZO DYES

FIG. 2. Comparison of Levich plots for 0.5 mmol dm03 Orange G when the cotton cloth precedes ( l ) and supercedes ( 1 ) the detector electrode. The experimental parameters are 2h Å 0.109 cm, cloth length Å 0.982 cm, and electrode length Å 0.327 cm.

trode was used in conjunction with standard control equipment (Oxford Electrodes). RESULTS

Prior to conducting channel flow cell experiments the diffusion coefficients of the dyes used were determined at 367C using a mercury-plated rotating disc electrode. The RDE Levich analysis (24) gave diffusion coefficients of 5.69 1 10 06 cm2 s 01 for Orange G and 5.90 1 10 06 cm2 s 01 for Sunset Yellow FCF. Turning to the flow cell experiments, the effect of the dyeing of cloth on the voltammetric behavior of Orange G is shown in Fig. 2 for the dyeing of cotton cloth with 0.5 mmol dm03 Orange G at 367C. The data display the expected cube root dependence of the transport-limited current on the solution flow rate (‘‘Levich behavior’’) when the cloth is downstream of the electrode. The cell height calculated from this data was 2h Å 0.109 cm. The plot further shows that when the cloth is upstream of the electrode a reduced current is observed and the data deviate from Levich behavior. The deviation is more pronounced at higher flow rates as shown more clearly in a plot of the variation in shielding factor (see above) with flow rate (Fig. 3). The variation in shielding factor with flow rate is qualitatively different to that observed with previous channel flow cell studies of dyeing using reactive dyes (2, 3). This earlier work with reactive rather than nonreactive dyes showed a lower shielding factor at slower flow rates. The size of the shielding factor was consistent with a first order heterogeneous rate law, J Å k1[Dye]0 ,

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are unity at slow flow rates and decrease as the flow rate increases. The dependence of the shielding factor on flow rate for these dyes suggests a more complex mechanism than the simple heterogeneous rate law observed for reactive dyes. The reduction in shielding factor with increased flow rate indicates either that adsorption or dye uptake onto the surface is effectively hindered at slower flow rates or alternatively that an adsorption/desorption equilibrium is established under those transport conditions where the rate of adsorption is matched by the rate of desorption. With the first alternative an increase in bulk concentration would be expected to result in a higher surface concentration and the filling of more of the surface sites. Hence a higher bulk concentration would be expected to increase the shielding factor at a given flow rate relative to a lower dye concentration. However, with an adsorption/desorption mechanism, the rate of adsorption would increase with the surface concentration of dye and hence, an increase in bulk dye concentration should yield a decrease in the measured shielding factor at a given flow rate. A plot of limiting current versus (flow rate) 1 / 3 for a 2 mmol dm03 Orange G solution showed no apparent deviation from Levich behavior when the cotton cloth was upstream of the detector electrode. Significantly, a 0.25 mmol dm03 Orange G showed a similar variation of the shielding factor with flow rate as for the 0.5 mmol dm03 Orange G illustrated in Fig. 3. These observations support the adsorption/desorption hypothesis. The influence of temperature on the dyeing process was also investigated as a qualitative mechanistic indicator. With an increase in temperature to 387C or a decrease to 257C there was no significant departure from Levich behavior observed. Two factors may operate that explain this. At 257C the lack of deviation may be attributed to the rate of dye adsorption onto the cloth being too slow. This is not surprising considering that dyes of the type studied are not generally applied to cotton at room temperature. More commonly the dye bath temperature is between 407C and 807C (12, 22). At 387C the lack of deviation may be attributed to the increase in

[2]

where [Dye]0 is the surface concentration of the dye. Thus the shielding factor increases with flow rate since the enhanced transport serves to maintain [Dye]0 at a higher value. By contrast the data in Fig. 3 show shielding factors which

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FIG. 3. Variation in shielding factor with flow rate for 0.5 mmol dm03 Orange G on cotton cloth.

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where D is the dye diffusion coefficient, Vx is the solution velocity in the x-direction, [Dye] is the dye concentration, and x, y, and z are Cartesian coordinates (Fig. 1). The xyplane through the cell is subdivided into a finite difference grid, as shown in Fig. 6, and Eq. [3] is expressed in finite difference form, gj,k Å 0 lj gj 01,k/1 / (2lj / 1)gj,k/1 0 lj gj /1,k/1 , [4]

03

FIG. 4. Variation in shielding factor with flow rate for 1.0 mmol dm Sunset Yellow FCF on cotton cloth where 2h Å 0.115 cm, cloth length Å 1.019 cm, and electrode length Å 0.340 cm.

temperature reducing the amount of dye that is adsorbed onto the fiber. An increase in temperature decreases the amount of dye adsorbed by the fiber at equilibrium. Increasing the thermal energy of the molecules also hastens the speed at which equilibrium is attained (12). Thus at a concentration of 0.5 mmol dm03 at 387C all the surface sites appear to be filled within the mass transport regime investigated. The cotton cloth was dyed with Sunset Yellow FCF rather than Orange G to identify whether the variation in shielding factor with flow rate was unique to Orange G. The same solution conditions were used, the dye solution concentration was 1 mmol dm03 , and the measured cell height was 0.115 cm. Again the usual decrease in shielding factor with increasing flow rate for direct dyes was observed (Fig. 4). Similarly, the substrate to which Orange G was applied was changed to identify whether there was a change in the dyeing behavior, in regard to either the mechanism or the extent to which the variation in shielding factor depended on the flow rate. The Levich plot when the detector electrode precedes the cloth gives a cell height, 2h, of 0.101 cm. The variation in shielding factor is shown in Fig. 5. The plot shows that the dyeing of nylon under these solution and temperature conditions proceeded by a mechanism similar to that used for the cotton cloth.

where gj,k is the concentration of dye normalized to its bulk value and lj is lj Å

DD x(2h) 3 d , 6Vf j( Dy) 3 (2h 0 jDy)

[5]

where h is half the cell height, V f is the volume flow rate, d is the channel width, D x and Dy are increments in the x and y directions, and j is the grid position in the y direction. In the model of the grid mesh was defined separately for the regions over the cloth and the electrode. In this way the convective diffusion equation was solved sequentially for the xy-space over the cloth, with the incorporation of the heterogeneous rate law, and then over the electrode. To calculate the shielding factor, the current when the cloth was present was divided by the calculated current when the cloth was absent. The models assessed differed in the nature of the process(es) assumed to be taking place at the solution/cloth interface. These included: (i) Constant surface concentration model: In this model the concentration of dye at the surface of the cloth is assumed constant at all flow rates. Thus the boundary condition in the region of the cloth is 0 õ x õ x1 ; y Å 0;

[Dye] Å [Dye]0 ,

where [Dye]0 is the surface concentration of the dye. In finite difference form the boundary condition becomes

DISCUSSION

To elucidate the mechanism by which the unusual variation in shielding factor with flow rate occurs, several different models were investigated to explain the observed dyeing results before the successful model was determined. In all cases the steady-state convective diffusion equations, describing mass transport of dye with the flow cell, were solved using the BI method (25) with adaptations of the surface boundary condition over the cloth (14–17, 19) describing the candidate dyeing mechanism. The steady-state convective diffusion equation for the transport of the dye is Ì 2[Dye] Ì[Dye] D 0 Vx Å 0, 2 Ìy Ìx

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FIG. 5. Variation in shielding factor with flow rate for 0.5 mmol dm03 Orange G on nylon 66 cloth where 2h Å 0.101 cm, cloth length Å 0.999 cm, and electrode length Å 0.383 cm.

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609

FIG. 6. The finite difference grid used in the modelling of the dyeing process.

g0,k/1 Å a,

where a is the normalized surface concentration such that a Å [Dye]0 /[Dye]bulk . (ii) Flux model: Here the dyeing mechanism was viewed as a constant flux into the cloth which was independent of flow rate (as might arise from slow dye adsorption): D

S

Ì[Dye] Ìy

D

Å kad .

[7]

(iii) Langmuir adsorption model: In this model the flux is described as the rate of transport into the cloth of material adsorbed onto the surface sites on the cloth. The adsorption mechanism is defined by the Langmuir isotherm. Thus the flux equation becomes

S

D

Ì[Dye] Ìy

D

J Å k *T [Dye]0 (1 0 Nu ),

[6]

Å kT u,

[8]

[10]

where J is the flux, N (where N ú 1) is a factor to describe the reduction in availability of surface sites with increased adsorption, and the other variables have the same meanings as previously described. All these four models gave an increase in shielding factor with increasing flow rate, rather than the experimentally observed decrease in S f . This suggested the alternative interpretive protocol to which we next turn. The actual flux of dye into the cloth required to reproduce the experimentally observed shielding factors can be determined using the flux model given above by simply adjusting the input flux until the experimentally observed shielding factor is obtained. A plot of the variation in flux with flow rate is shown in Fig. 7 for the dyeing of nylon with 0.5 mmol dm03 Orange G. The flux model can also be used to calculate the surface

where kT is the rate of transport of material into the cloth and u is the fraction of filled surface sites, as given by uÅ

S

K[Dye]0 1 / K[Dye]0

D

,

[9]

where K is the equilibrium constant for the adsorption/desorption process. (iv) Surface blocking model: With this model the surface is assumed to be modified as material adsorbs onto surface sites, making the adsorption of further material more difficult. If the adsorption is described by the Langmuir isotherm then the rate of uptake of dye is given by

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FIG. 7. Variation in calculated flux with flow rate for the 0.5 mmol dm03 Orange G on nylon 66 data. Cell dimensions are the same as in Table 1.

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FIG. 8. Variation in average surface concentration with flow rate for 0.5 mmol dm03 Orange G on nylon 66; dimensions are the same as in Table 1.

concentration corresponding to the observed shielding factor. The dependence of the average surface concentration (see Eq. [12]) with flow rate is shown in Fig. 8 for the dyeing of nylon with 0.5 mmol dm03 Orange G. The interesting feature of this plot is the decrease in surface concentration with increased flow rate. This is a particularly unusual flow rate dependence as the higher the flow rate the more rapidly material is brought to a reacting interface. A plot of the flux versus the average surface concentration for the 0.5 mmol dm03 Orange G on nylon experimental data is shown in Fig. 9. The surprising aspect of this figure is that the flux is reduced at higher surface concentrations. Such an observation again suggests a blocking mechanism where the more material that is adsorbed onto the cloth, the harder it is for subsequent material to be taken up by the cloth. Also of interest in Fig. 9 is the linear variation in flux with surface concentration. The linear regression gives the equation Flux, J Å 2.8 1 10 03 0 3.1 1 10 03 gV ,

[11]

where gV is the normalized surface concentration averaged across the surface of the cloth given by kc

gV Å (kc ) 01 ( ∑ g0,k );

[12]

kÅ1

kc is the number of boxes in the x-direction over the cloth as in Fig. 5. Equivalent plots to those given in Fig. 9 were also linear for other experimental conditions as summarized in Table 1. Also shown in the table are the linear regressions

FIG. 9. Plot of flux of dye into cloth versus the average surface concentration for 0.5 mmol dm03 Orange G on nylon; dimensions are the same as in Table 1.

for the plots. As can be seen from the latter, in all cases the value of the intercept and the gradient are approximately equal. Thus in all four cases Eq. [11] is very close to the general form, J Å k * [1 0 gV ].

[13]

In terms of real concentrations this equation becomes J Å k * ([Dye]bulk 0 [Dye]0 )/[Dye]bulk ,

[14]

where [Dye]bulk and [Dye]0 are the bulk and surface concentrations of the dye. Evidently a mechanism that obeys this equation is required. Consider a model for the dyeing process, as shown in Fig. 10, where dye is transported from the bulk solution across a thin porous surface layer formed within the cloth immediately adjacent to the solution. From the surface layer, dye penetrates into the bulk cloth. The concentration of the dye near the surface region approximates to the bulk concentration, [Dye]bulk , but the dye concentration at the surface of the bulk cloth is [Dye]0 . Dye molecules must then diffuse through the surface layer to the bulk cloth. However, we suggest that adsorption of dye into the surface layer blocks the channels through which the dye is transported from the bulk solution environment to the bulk cloth surface. Thus with progressive adsorption the rate of diffusion through the surface layer will be decreased. However, as the experiments are conducted under steady-state conditions, at any particular

TABLE 1 Experimental Conditions and Linear Regression Data for the Different Dyeing Experiments (mM, mmol dm03) Dye

Cloth

[Dye] (mM)

x1 (cm)

xtot (cm)

2h (cm)

Orange G Orange G Orange G Sunset

Nylon Cotton Cotton Cotton

0.5 0.5 0.25 1.0

0.999 0.982 0.982 1.019

1.382 1.309 1.309 1.359

0.101 0.109 0.109 0.115

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Linear regression J J J J

Å Å Å Å

0.0028–0.0031 0.0020–0.0021 0.0018–0.0019 0.0012–0.0013

gV gV gV gV

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no net loss of material into the cloth. Hence, in the mass transport regime in which the experiments are being conducted, the surface is near saturation and it may be assumed that K[Dye]bulk @ 1. Thus

(1 0 u ) É

FIG. 10. Schematic of the model describing the dyeing mechanism with azo direct dyes.

flow rate the diffusion coefficient for diffusion through the surface region, Dfilm , will be constant. Thus if the thickness of the surface region is d, then the net flux through the surface region is JÅ

Dfilm {[Dye]bulk 0 [Dye]0 }. d

JÅ

Dfilm {[Dye]bulk 0 [Dye]0 } Å kI[Dye]0 , d

[16]

where kI is the rate constant for transfer of dye from the porous surface layer into the bulk cloth. As Dfilm decreases as the amount of adsorption into the surface region increases, a simple ‘‘blocking’’ mechanism in which the reduction is directly in proportion with the uptake gives Dfilm Å Dfree (1 0 u ),

[17]

where Dfree is the diffusion coefficient through the surface region if no dye was adsorbed onto the surface sites and u is the fraction of filled sites. The latter may be supposed to be described by the Langmuir isotherm uÅ

K[Dye]bulk , 1 / K[Dye]bulk

[18]

where K is the equilibrium constant for the adsorption/desorption process. Therefore, 1 (1 0 u ) Å . 1 / K[Dye]bulk

At slow flow rates the shielding factor is effectively unity, which implies that all the surface sites are full, as there is

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Dfilm Å

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[21]

Substituting Eq. [21] into Eq. [16] gives

JÅ

Dfree dK

H

10

[Dye]0 [Dye]bulk

J

Å kI[Dye]0 .

[22]

Equation [22] has the same form as Eq. [14] where k * Å (Dfree / dK). It follows from Eq. [22] that the surface behaves as if it were reacting in a first order manner with the dye, with a rate constant for the uptake of dye into the bulk cloth from the surface region, kI kI Å k *

H

1 1 0 [Dye]0 [Dye]bulk

J

.

[23]

Thus kI can be determined, as k * is found from the gradients of the flux versus surface concentration plots (the values of k * are given in Table 2); [Dye]0 is calculated using the flux model and [Dye]bulk is the concentration of the dye solution used. The essential feature of Eq. [23] is that kI is dependent on the surface concentration of dye, which is in turn dependent on the flow rate in the channel flow cell. The dependence of kI with the surface concentration is shown in Fig.

TABLE 2 The Values of k* Determined from Plots of Flux versus Surface Concentrations Calculated Using the Flux Model (mM, mmol dm03) Experiment

[19]

[20]

So Dfilm becomes

[15]

Now under steady-state conditions J must be constant and thus the flux of material through the cloth must be equal to the flux of material that is transported from the surface region into the bulk cloth. Therefore,

1 . K[Dye]bulk

0.5 mM Orange G on nylon 0.5 mM Orange G on cotton 0.25 mM Orange G on cotton 1.0 mM Sunset Yellow FCF on cotton

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k* (103 mol cm02 s01) 3.0 2.0 2.0 1.2

{ { { {

0.2 0.2 0.2 0.2

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FIG. 11. Plot of kI versus surface concentration for 0.5 mmol dm03 Orange G on nylon where k * Å 3.0 1 10 03 mol cm02 s 01 .

11 for a value of k * and [Dye]bulk for the 0.5 mmol dm03 Orange G on nylon 66 data. The plot clearly shows the large increase in flux as the surface concentration becomes smaller. The variation in kI determined for the different sets of experimental data is shown in Fig. 12. Note that the concentration range over which each experiment is conducted is much smaller than the range shown in Fig. 11. Figures 11 and 12 show that for a given bulk concentration, as the dye surface concentration increases, the value of the rate constant for the passage of dye across the porous surface layer/bulk cloth interface, kI , decreases. However, an increase in bulk concentration results in an increase in kI . Furthermore, Fig. 9 shows that as the flow rate increases the surface concentration decreases, and hence the rate of dyeing increases. Thus the dyeing of cotton and nylon cloth with azo dyes such as Orange G and Sunset Yellow FCF is a delicate balance between bulk dye concentration and the amount of agitation in the dye bath; the results presented in this paper therefore have industrial ramifications. The rate of material crossing the surface layer/bulk cloth interface is dependent on the difference between the bulk concentration and the dye concentration at this interface, as shown in Eq. [14]. Increasing the rate is not as simple as just increasing the bulk concentration. Increasing the bulk concentration will have two adverse effects on the rate of dyeing the cloth. First, a higher bulk concentration will result in a concomitant increase in the surface concentration. Second, the diffusion through the porous surface layer is decreased for a greater bulk concentration, as can be seen from Eq. [19]. The consequence of large increases in the bulk concentration is shown by the 2 mmol dm03 Orange G on cotton cloth data. With this higher concentration no net flux of material into the cloth was observed. Thus, in effect the dyeing of the cloth has been ‘‘turned off.’’ Based on the form of the kinetic model just elucidated, the turning off of the dyeing is due to adsorption onto surface sites in the porous surface layer restricting the passage of further material through this layer. The shielding factor versus flow rate plots indicate that

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not only does the bulk concentration has an effect on the rate of dyeing, but so also does the flow rate; the faster the flow rate, the greater the rate of dyeing. An alternative to increasing the bulk dye concentration in order to increase the rate is to increase the amount of agitation in the dye bath (i.e., the rate of mass transport): the greater the rate of agitation, the greater the difference in the surface to bulk concentrations and hence the greater the rate of dyeing. The dependence of the rate of the dyeing of both cotton and nylon cloth on the surface concentration and the amount of agitation in the dye bath has implications for commercial dyeing methods with high affinity dyes. The above discussion highlights the necessity to obtain the right balance between dye concentration and the level of agitation in the dye bath. All other things being equal, dyeing conditions should be designed to operate at relatively high rates of mass transport. Naturally there is an economic trade off between the improved dyeing with higher agitation rates and the energy input. CONCLUSIONS

The dyeing of cotton and nylon cloth with model nonreactive dyes Orange G and Sunset Yellow FCF was shown to proceed by an unusual mechanism whereby the greater the flow rate in the channel flow cell the lower the shielding factor. A mechanism, consistent with the experimental data, is proposed where the dye is transported from bulk solution, across a porous surface layer within the cloth next to the solution, and then into the bulk cloth. Adsorption of dye within the surface layer blocks transport through into the bulk cloth. The flux across this surface layer is thus dependent on the concentration of dye at the surface of the bulk cloth relative to the bulk dye concentration and also the diffusion coefficient through the surface layer. At high flow rates the surface concentration was shown to be low, relatively to slower flow rates, and hence the amount of dye entering the cloth was greater. For the different experimental

FIG. 12. The variation of kI with surface concentration for ( h ) 0.5 mmol dm03 Orange G on nylon 66, ( 1 ) 0.5 mmol dm03 Orange G on cotton, ( / ) 0.25 mmol dm03 Orange G on cotton and ( l ) 1 mmol dm03 Sunset Yellow FCF on cotton.

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conditions investigated kinetic parameters are given. The physical causes of the surface blocking, as described by Eq. [18] remain open to speculation and will require nonkinetic means for their elucidation. ACKNOWLEDGMENTS We thank Zeneca Ltd. for support via the Strategic Research Fund. J.J.G. thanks Merton College, Oxford for financial support.

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11. Lin, S. H., J. App. Polymer Sci. 44, 1743 (1992). 12. Trotman, E. R., ‘‘Dyeing and Chemical Technology of Textile Fibres, 6th Ed.’’ Griffin, New York, 1984. 13. Peters, R. H., ‘‘Textile Chemistry Vol. III: The Physical Chemistry of Dyeing.’’ Elsevier, Oxford, 1975. 14. Compton, R. G., and Pritchard, K. L., J. Chem. Soc. Faraday Trans. I. 86, 129 (1990). 15. Compton, R. G., Harding, M. S., Atherton, J. H., and Brennan, C. M., J. Phys. Chem. 97, 4677 (1993). 16. Compton, R. G., and Unwin, P. R., in ‘‘New Techniques For the Study of Electrodes and Their Reactions,’’ (R. G. Compton and A. Hamnett, Eds.), Comprehensive Chemical Kinetics, Vol. 29, p. 173, Elsevier, Amsterdam, 1989. 17. Compton, R. G., and Unwin, P. R., Phil. Trans. R. Soc. London A 330, 1 (1990). 18. Compton, R. G., and Sanders, G. H. W., J. Coll. Interface Sci. 158, 439 (1993). 19. Compton, R. G., Harding, M. S., Pluck, M. R., Atherton, J. H., and Brennan, C. M., J. Phys. Chem. 97, 10,416 (1993). 20. Gooding, J. J., Compton, R. G., Brennan, C. M., and Atherton, J. H., Electroanalysis, in press. 21. Daly, P. J., Page, D. J., and Compton, R. G., Anal. Chem. 55, 1191 (1983). 22. Aspland, J. R., Textile Chemists and Colourists 23, 41 (1991). 23. Levich, V. G., ‘‘Physicochemical Methods.’’ Prentice–Hall, Englewood Cliffs, NJ, 1962. 24. Southampton Electrochemistry Group, ‘‘Instrumental Methods in Electrochemistry.’’ Ellis Horwood, Chichester, England, 1985. 25. Compton, R. G., Pilkington, M. B. G., and Stearn, G. M., J. Chem. Soc. Faraday Trans. I. 84, 2155 (1988).

coida

AP: Colloid