Simon Mazengarb, George A. F. Roberts 1

Transcription

1 Studies on the Diffusion of Direct Dyes in Chitosan Film Simon Mazengarb, George A. F. Roberts 1 Department of Textiles & Fashion, Nottingham Trent University, Notts. NG1 4BU, UK 1Biomedical Materials Engineering Unit, Rm 312, Wolfson Building, University of Nottingham, Notts. NG7 2RD, UK (Corresponding author) Abstract The diffusion of two direct dyes in chitosan film under neutral conditions has been followed using the Sekido roll technique. Analysis of the concentration profiles confirms that the diffusion process is similar to that observed with direct dyes on cellulose and that the dependence of the diffusion coefficient on dye concentration follows the same general form, D C = D 0 (1 + βc), on both substrates. Key words: Chitosan; direct dyes; diffusion behaviour. Progress on Chemistry and Application of Chitin and Its..., Volume XIV,

2 Simon Mazengarb, George A. F. Roberts 1. Introduction One major dye class used on cellulosic polymers is that of direct dyes. These dyes are adsorbed by a diffuse adsorption mechanism, there being no suitable specific dye sites in cellulose, and rely on a combination of non-specific forces of attraction chiefly van der Waals (dispersion) forces together with minor contributions from hydrogen bonding and hydrophobic bonding. Direct dyes are therefore designed as large planar molecules in order to maximise the cumulative strength of the van der Waals forces between dye and fibre by maximising the surface areas in contact and ensuring the closest possible approach of the dye and polymer chain surfaces. Chitin and chitosan also have a β-(1 4)-linked anhydro-d-glycopyranose repeat unit in the molecular chain leading to a flat, ribbon-like structure very similar to that of the cellulose chain in fibres such as cotton, flax and viscose rayon. Hence it is reasonable to assume that chitin and chitosan would also be dyeable with direct dyes, and there are a number of studies showing that this is indeed the case. However there has been very little research carried out on the diffusion process itself in order to compare the dyeing process on the two substrates. Hudson and co-workers [1] determined the diffusion coefficients of four direct dyes in chitosan film at 60 C and found them to be between and cm 2. s -1, which is within the range for direct dyes on cellulose film, but no investigation of the diffusion mechanism was carried out. In the current work the diffusion mechanism is investigated through the nature of the concentration profile, the effect on it of the concentration of neutral electrolyte present, and the concentration dependence of the diffusion coefficient D C. 2. Materials and methods 2.1. Materials The chitosan used was prepared in the laboratory from crab chitin obtained from India. It had an F A value of 0.38, determined by dye adsorption [2], and an <M V > value of [3]. The chemicals used were GPR grade and the dyes were commercial samples, purified where necessary. All water used was distilled. The dyes were commercial samples and were purified by recrystallisation Chitosan films Preparation: Chitosan films were prepared from a 4% solution in 0.2 M acetic acid after filtering to remove swollen gel particles and other insoluble matter, and standing under vacuum to remove any entrapped air bubbles. The bubble-free solution was then poured onto a levelled glass plate and skimmed to the required depth (0.9 mm) using a doctor blade. The films were then allowed to dry at room temperature in a clean, dust-free environment. Once dry, the films were immersed in a tank containing a dilute solution of Na 2 CO 3, then rinsed in 26 Progress on Chemistry and Application of Chitin and Its..., Volume XIV, 2009

3 Studies on the Diffusion of Direct Dyes in Chitosan Film distilled water, trimmed to the required size using a scalpel, and dried. Characterisation: A film produced as described above was divided into ten segments, using a 30 mm 30 mm template to ensure that each segment had the same surface area. All 10 segments were weighed individually (W s ) and the total weight (W t ) calculated. The ten segments were then placed on top of one another and the thickness (T t ) of the combined stack of segments measured using a digital micrometer. The dry thickness of each individual segment (T s,d ) was then calculated using the equation: T s,d = T t (W s /W t ) (1) The wet thickness of the segments (T s,w ) was determined in the same way after steeping the original 10 segments in distilled water for 3 hours and the ratio of T s,w /T s,d was found to be Equipment The absorbance values of solutions were measured using a Unicam 8625 uv/visible spectrophotometer. Dyeing was carried out in an Ahiba Texomat laboratory dyeing machine Dyeing Procedure The chitosan film was wound round a metal cylinder, the ends sealed, and a blanking plate placed along the outer edge of the film and clamped tightly. The plate prevents dye from penetrating into the substrate directly beneath it, and sealing the ends prevents interlayer diffusion of dye liquor from the edges. The prepared roll was then placed in an infinite dyebath set at 80 C containing purified dye, together with any required chemical additive. Dyeing was carried out at 80 C for the specified period of time Analysis of the dyed film The rinsed and dried film was cut into segments using the standard 30 mm 30 mm template, each separate layer accurately weighed, and its wet thickness calculated. It was found impossible to dissolve dyed samples using 0.1 M aqueous acetic acid as solvent, due to ionic interactions between the cationic protonated amine groups and the anionic sulphonic acid groups of the dyes. Each 30 mm 30 mm sample was therefore placed in a flask containing 20 ml of 5% sodium formaldehyde bisulphite [4] and allowed to stand, with occasional agitation, until the film was completely dissolved. The solutions were then transferred to 50 ml volumetric flasks, made up to volume with distilled water, and dye concentration measured by spectrophotometry, dilutions being made where necessary. Progress on Chemistry and Application of Chitin and Its..., Volume XIV,

4 3. Results and discussion Simon Mazengarb, George A. F. Roberts The diffusion of dyes in polymers follows Fick s 1 st Law: F = -D C * (dc/dx) (2) where: F = rate of transfer of dye across unit area at a given point in the substrate; dc/dx = concentration gradient at that point in the substrate; D C = the concentration-dependent diffusion coefficient for that concentration of dye. The major problem in applying this equation is determining the value of dc/dx at different points within the substrate. Currently this can only be carried out when the substrate is in the form of film rather than fibre or powder. Several methods are available but the Sekido roll technique [5], which is a development of the multiple membrane method [6], is the most convenient. The main characteristics of the diffusion of direct dyes in cellulose have been shown to be [6-8]: the presence of an added neutral electrolyte reduces the rate of diffusion of the dye ions in the cellulose substrate, although it increases the equilibrium adsorption; the concentration profile, a plot of dye concentration versus distance from the substrate surface, is rectilinear over most of the plot, unlike acid dyes on wool, whose concentration profile is strongly convex to the distance axis, or disperse dyes on cellulose acetate or polyester, where the concentration profile is concave to the distance axis; the diffusion coefficient is concentration dependent, initially increasing linearly with concentration at the point measured, then gradually showing a decrease in the rate of increase as a function of concentration. First the effect of neutral electrolyte concentration on the equilibrium uptake of C.I. Direct Yellow 12 was studied using NaCl concentrations of 0 g. L -1, 5 g. L -1 and 10 g. L -1 and a dyeing temperature of 80 C. This showed that the concentration of dye adsorbed at equilibrium increased with increase in NaCl concentration, which is similar to the adsorption behaviour of direct dyes on cellulose. Next, the effect of neutral electrolyte on the diffusion of C.I. Direct Yellow 12 in chitosan at 80 C was studied at the same three levels of added NaCl for a dyeing time of 120 minutes, using the Sekido roll technique. Following dyeing and drying the strips were examined and the number of layers penetrated determined. This number decreased with increase in NaCl concentration (Figure 1), showing that the diffusion rate decreases with increase in added electrolyte, in agreement with the known behaviour of direct dyes on cellulose film. Also, for each of the dyeings the concentration profile over the majority of the plot was rectilinear, with a negative slope, and with a final less steep, curved portion. Again that is similar to the behaviour shown by direct dyes on cellulose film [6-8]. 28 Progress on Chemistry and Application of Chitin and Its..., Volume XIV, 2009

5 Studies on the Diffusion of Direct Dyes in Chitosan Film Figure 1. Effect of neutral electrolyte on the rate of diffusion of C.I. Direct Yellow 12 in chitosan film. In much of the previous work on diffusion of direct dyes in cellulose film [6-8] the dye used has been C.I. Direct Blue 1 (CI DB1). As this dye was not available the structurally related dye, C.I. Direct Blue 15 [CI DB15], was used in the current work. This dye is structurally the same as CI B1 except for the positions occupied by the two sulphonic acid groups present on each of the two terminal naphthalene residues; these are in positions 5 and 7 for CI DB1 and 3 and 6 for CI DB15. They also have the same molecular volumes. A strip of chitosan film was wound and sealed as described in the Materials and Methods section and dyed at 80 C for 2 hours with C.I. DB15 and 5% NaCl. After rinsing and drying the film 30 mm 30 mm sections were cut out from the centre of each panel on the strip, dissolved in a 5% aqueous solution of sodium formaldehyde bisulphite, and the concentration of dye determined. Analysis of the Dye concentration versus Distance from surface plot (see Figure 2) to obtain values of D c at different dye concentrations is most readily carried out using Matano s equation [9]: D C1 = -(1/2t) * (dx/dc) * x * dc (2) where: D C1 = diffusion coefficient at a dye concentration of C1 t = time of dyeing in seconds dx/dc = reciprocal of the concentration gradient where dye concentration = C1 and the integral x * dc is determined graphically between the limits C = 0 and C = C1. Progress on Chemistry and Application of Chitin and Its..., Volume XIV,

6 Simon Mazengarb, George A. F. Roberts Figure 2. Concentration profile for chitosan film dyed with C.I. Direct Blue 15 under neutral conditions for 120 minutes at 80 C. The values of D C for C.I. DB15 on chitosan film, calculated from Figure 2, were then plotted against the dye concentration (Figure 3). Figure 3. Dependence of D C on dye concentration for C.I. Direct Blue 15 on chitosan. 30 Progress on Chemistry and Application of Chitin and Its..., Volume XIV, 2009

7 Studies on the Diffusion of Direct Dyes in Chitosan Film The plot is similar to that found for C.I. DB1 on cellulose film [7, 8], with the D C values ranging from cm2. s -1 at C = 3 g. kg-1, to cm2. s -1 at C = 17 g. kg-1. Extrapolation of the initial rectilinear portion of Figure 3 back to D C=0 gives the value of the constant D 0, while the relationship between diffusion coefficient and dye concentration over the rectilinear portion can be expressed as: where β = (slope/d 0 ). D C = D 0 (1+βC) (4) This is a similar relationship to that found for C.I. DB1 in cellulose film [8], and may be considered as proof that direct dyes follow the same mechanism of diffusion, that is simultaneous diffusion and adsorption, in both cellulose and chitosan films, although the values of the constants in equation (4) differ somewhat for the two substrates (Table 1). Table 1. Values of D 0 and β in equation 3 for chitosan and cellulose films. Substrate D 0 /cm 2. s -1 β Cellulose film Chitosan film The values for D 0 for the two substrates are similar and any difference can be attributed to the fact that that the cellulose film was subjected to stretching during extrusion and processing, whereas the chitosan film was used as cast. Hence the latter would be more amorphous and so be more easily penetrated by the dye ions, leading to a greater rate of diffusion of the dye in chitosan. The results also show that the measured diffusion coefficient, D C, is more dependent on the dye concentration at the point at which it is measured when the substrate is chitosan, and this again may be due to the more amorphous nature of this substrate. Analogous differences found in the results of diffusion experiments with cellulose films, that were carried out in the 1930s and the 1960s [6, 8] were attributed [8] to the differences in the physical structures of the films produced in the two eras. The average value for the diffusion coefficient is of the order of 10-9 cm 2. s -1, which is considerably greater than that found by Hudson and co-workers [1]. However they carried out their measurements at 60 C, rather than 80 C, and this considerable difference in temperature offers a logical explanation for the smaller diffusion coefficients recorded by these earlier workers. 4. Conclusions The diffusion of direct dyes in chitosan film is similar to that shown by direct dyes in cellulose film, both systems showing the following characteristics: Progress on Chemistry and Application of Chitin and Its..., Volume XIV,

8 Simon Mazengarb, George A. F. Roberts the rate of diffusion decreases with increase in the concentration of neutral electrolyte (NaCl) present in the dyebath, although the equilibrium adsorption increases with increasing NaCl concentration; the concentration profiles for dye diffusion shown by the two systems are similar; the concentration dependence of dye diffusion is similar for both systems, with the measured diffusion coefficient D C increaseing with increase in dye concentration at the point measured in both systems. Differences in the absolute values of D 0 and β are most probably due to the physical characteristics of the two substrates rather than to any inherent differences in the diffusion mechanism in the two substrates. 5. References 1. Carlough M., Hudson S., Smith B., Spadgenske D.; Diffusion coefficients of direct dyes in chitosan, J. Appl. Poly. Sci., 42 (11), 1991, pp Roberts G. A. F.; Determination of the degree of N-acetylation of chitin and chitosan, Chitin Handbook, R A A A Muzzarelli and M G Peter eds.), European Chitin Society, 1997, pp Roberts G. A. F., Domszy J G.; Determination of the viscometric constants for chitosan, Int. J. Biol. Macromol., 1982, 4, pp Roberts G. A. F.; A novel solvent system for chitosan, Advances in Chitin Science, Vol. VIII (H Struszczyk, A Domrd, M G Peter and H Pospieszny eds.), Poznan, 2005, pp Sekido M., Matsui K.; Vat dyeing (I) Determination of surface concentration and diffusion coefficient by cylindrical cellophane film roll, Kogyo Kagaku Zasshi, 1965, 68, pp Garvie W. M., Neale S. M.; Adsorption of dyestuffs by cellulose (VII) Analysis of the diffusion of Sky Blue FF through single and double membranes, Trans. Faraday Soc., 1938, 34, pp Crank J.; The diffusion of direct dyes into cellulose. II The interpretation of rate of dyeing measurements, J Soc Dyers Col., 1948, 64, pp Peters R. H., Petropoulos J. H., McGregor R.; A study of diffusion of dyes in polymer films by a microdensitometric technique, J. Soc. Dyers & Col., 1961, 77, pp Matano C.; Diffusion coefficients and concentrations of solid metals, Jap. J. Physics, 1932, 8, pp Progress on Chemistry and Application of Chitin and Its..., Volume XIV, 2009