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Electrokinetic studies of modified cellulosic fibres
Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 462–468
Electrokinetic studies of modified cellulosic fibres A. Ramesh Kumar ∗ , M.D. Teli Department of Fibres and Textile Processing Technology, University Institute of Chemical Technology, Matunga, Mumbai 400019, India Received 23 December 2005; received in revised form 15 August 2006; accepted 17 January 2007 Available online 20 January 2007
Abstract The electrokinetic phenomenon is the one involving electricity connected with tangential movement of two phases along with each other. The present paper deals with changes in the electrokinetic properties of cotton fibres with varying degrees of purification and modification of the cellulose with dimethylol dihydroxy ethylene urea (DMDHEU) and DMDHEU–triethanol amine hydrochloride (TEA). The electrokinetic properties such as zeta potential, surface charge density and surface conductivity were determined in the presence of direct dyes in the streaming the solution. The paper also discusses the behaviour of cotton cellulose with respect to extent of purification and modification of cellulose and dye uptake in the light of electrokinetic properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Cellulose; Dimethylol dihydroxy ethylene urea; Direct dye; Surface charge density; Surface conductivity; Triethanol amine; Zeta potential
1. Introduction Ion adsorption of dyes, surfactants and other species on the textile surface plays a prominent role in dyeing and finishing. It is assumed that ions are adsorbed in an electrical double layer, existing at the interface of charged textile surface and aqueous solution. The electrokinetic or zeta potential is an electrokinetic phenomenon that is developed when one of these two charged surface moves relative to the other [1]. A knowledge of zeta potential gave added theoretical base for many important wet processes of fibrous materials such as dyeing and detergency to which the textile materials were subjected. Zeta potential of textile materials was usually measured using streaming potential and electroosmosis. The zeta potential for fibres such as cotton, rayon, and polyester in aqueous solution shows a negative value. For amphoteric fibres such as wool, silk and polyamide, the isoelectric point is determined from the effect of pH on zeta potential of the fibre. In manufacturing nylon fibres, drawing and heat setting processes result in changes in the fibrous structure, especially in the fine structure. The effect of these processes on the isoelectric point of the fibres was determined by the
∗
Corresponding author. Present address: KG Denim Ltd., Thenthirumalai, Mettupalayam, Coimbatore 641302, India. Tel.: +91 4254 304349. E-mail address: rameshkumar [email protected] (A. Ramesh Kumar). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.01.021
zeta potential method. When the zeta potential of fibres such as cotton and rayon were measured in aqueous solutions of direct dyes, it was observed with increasing concentration of the dye, there was an increase in negative value of the zeta potential of the fibre which passed through as maximum value and thereafter decreased. Polyamidines were investigated by electrokinetic measurements for its surface properties in dependence on the pH value [2]. The adsorption of copolymers and its effect on electrokinetic potential depends on the degree of modifications such as grafting and pH etc have been investigated earlier [3,4]. Influence of polyacrylic acid (PAA) and polyacrylamide (PAM) molecular weight on adsorption and electrokinetic properties of Fe2 O3 –polyelectrolyte solution interface was studied. Zeta potential changes are as a function of polymer molecular weight, pH and concentration of the polymer solution [5]. The interaction of alumina with ammonium poly (methacrylate) (APMA) has been investigated through adsorption and electrokinetic measurements by Santhiya et al. Electrokinetic studies indicate specific adsorption between alumina and APMA resulting in a shift of the isoelectric point to acidic pH values [6]. PETP fibres exhibit high negative zeta potential value, due to their strong hydrophobic and non-ionic character. The application of the surfactant changes the electrokinetic properties with respect to the adsorption process conditions [7]. Application of zeta potential in flocculation and surface deposition have been studied by many researchers [8,9].
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The reactivity and adsorption properties of cellulose fibres were critical for successful treatment because behaviour during the finishing process is determined by both structure and surface properties. The natural fibres have the smaller hydrophilic character and they are less reactive than the regenerated ones and hence zeta potential of cotton and contact angle was the highest as compared to regenerated fibres [10]. Relationships between formulation, bulk properties, and surface properties were investigated on series of copolymers prepared with hydroxyethylmethacrylate (HEMA), methylmethacrylate (MMA), and ethylmethacrylate (EMA) monomers and on the homopolymers PMMA and PHEMA by Hermitte [11–13]. Based on recent progress in the measurement and interpretation of ion accumulation and motion in the stagnant part of the electrical double layer (DL), a generalized standard electrokinetic model (GSEM) was proposed by Dukhin. The suggested model reflected a new standard for the modeling of electrokinetic transport [14]. The accessibility of free adsorption places in the less ordered regions of cellulose, i.e. viscose, modal and lyocell fibres, which in turn was investigated by determining of electrokinetic properties. An excellent correlation between structure, adsorption and electrokinetic properties existed for the regenerated fibres [15]. Alternative ways to describe the success of different processes in fibre purification which result in distinct surface charge and hydrophilicity was the determination of electrokinetic properties and the water uptake of textile fibres. The zeta potential was determined by streaming potential measurement as a function of the pH [16]. The electrokinetic properties of bleached cotton in aqueous solutions of direct dye s have been studied [17]. It was observed that the negative zeta potential of bleached cotton dyed with direct dyes, which differ only in sulphonic group content of their chemical structure, increases with dye concentration [18]. The zeta potential of cotton treated with different cross linking agents dimethylol ethylene urea (DMEU), dimethylol dihydroxy ethylene urea (DMDHEU), when measured in aqueous solutions of various inorganic salts, the zeta potential of the treated fibre decreased but the surface charge density increased with increasing electrolyte concentration [19]. Hence electrokinetics studies have been found to be excellent tool to understand the dye fibre interaction [20–22]. It throws light on the functional groups present on the surface of the fibre, as it is a surface phenomenon. There has been very little work reported as far as electrokinetic properties of cotton fibres passing through various stages of pretreatment and finishing with resin and additives were concerned [23]. The present paper deals with the electrokinetic studies of the cotton fibres subjected to various processing treatments like scouring, scouring–bleaching, scouring–bleaching–resin treatments and scouring–bleaching and resin treatment with triethanol amine hydrochloride as an additive. The treated fibres were studied for their electrokinetic properties like zeta potential, surface charge density and surface conductivity when CI Direct Yellow 28 dye was streamed through plug of fibres at various pH. The treated fibres were also dyed with the direct
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dye with and without using the electrolyte, for different pH and the K/S values were evaluated. 2. Materials and experimental methods 2.1. Materials 2.1.1. Fibres The grey cotton fibres were obtained from Tata Mills, Mumbai, India. 2.1.2. Chemicals and auxiliaries Superfinish EUC was supplied by Supertex Chemicals, Mumbai, India. Triethanol amine hydrochloride (TEA), sodium hydroxide, hydrogen peroxide (50%), sodium sulphate was obtained from SD Fine chemicals, Mumbai, India. All the chemicals were of chemically pure grade. 2.1.3. Dye CI Direct Yellow 28 dye was supplied by Indokem Ltd., Mumbai, India. 2.2. Experimental methods 2.2.1. Scouring The cotton fibres were scoured for 1 h at 100 ◦ C with caustic soda (10 g/l) and non-ionic detergent (0.5%) keeping material to liquor ratio 1:20. 2.2.2. Bleaching The scoured cotton fibres were bleached for 1 h at 85 ◦ C with 3% hydrogen peroxide (50%), 4.5% sodium silicate, 1% sodium carbonate and 0.5% EDTA keeping material to liquor ratio 1:20. 2.2.3. Resin treatment The bleached cotton fibres was given resin finishing treatment by soaking the same for 15 min in the resin containing DMDHEU (100 g/l) and MgCl2 (10%) and keeping material to liquor ratio 1:20. at room temperature. It was then hydroextracted, dried at 80 ◦ C and cured at 150 ◦ C for 3 min in a hot air curing chamber. It was then washed and dried. 2.2.4. Cationisation of cotton The bleached cotton fibres was soaked in the bath containing DMDHEU (100 g/l), TEA·HCl (6%) and MgCl2 (10%) on the weight of resin and keeping material to liquor ratio 1:20 for 15 min at room temperature. It was then hydroextracted, dried at 80 ◦ C and cured at 150 ◦ C for 3 min in a hot air curing chamber. It was then washed and dried. 2.2.5. Dyeing of cotton fibres Dyeing of cotton fibres was carried out in a laboratory Rotadyer machine supplied by RBE Company, Mumbai, India. The samples of known weight were dyed in the dye pots containing required amount of dye and sodium sulphate (optional) for the first three batches and dye, acetic acid and sodium sulphate (optional) for resin and resin and TEA treated samples keeping material to liquor ratio 1:20. Dyeing was started at 40 ◦ C
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followed by heating the dye bath up to 95 ◦ C in 30 min. Dyeing was then carried out for further 45 min at that temperature. After the dyeing was over samples were removed and washed with water. The samples were again washed with hot water at 80 ◦ C for 10 min and air-dried.
2.3.5. Measurement of pH of the streaming solution The pH of the distilled water as well as dye solutions was measured by using Equip-tronics Digital pH meter, Type MKVI.
2.3. Determination of streaming potential
2.3.6. Measurement of K/S value The K/S values were measured in the Spectra Flash SF 300, Data colour international 3.1 computer colour matching system.
2.3.1. Measurement of zeta potential The determination of electrokinetic properties by streaming potential method has been described. While measuring these properties, plug of 7 g of cotton fibres was used and the streaming solution contained various concentration of pure direct dye, CI Direct Yellow 28. The detailed methods of estimation have been reported in the literature [17–23]. Zeta potential was calculated by using the Helmholtz– Smoluchouskii equation as shown below: ξ = 6.75 × 107 ×
4πηVSTR × Cs EP
(1)
where ξ is the zeta potential (mV); η the viscosity of the solution (poise); Cs the specific conductivity of the solution in the fibres (−1 cm−1 ); E the dielectric constant of the solution; P the pressure of the solution streaming through the cell (cm of Hg); VSTR is the streaming potential (mV). The surface charge density was calculated using the following equation: √ zeξ σ = 2NKTDπ sin H (2) 2KT
3. Results and discussion 3.1. Effect of zeta potential The results from Fig. 1 and Table 1 show the zeta potential values for raw cotton, scoured cotton, scoured–bleached cotton, scoured–bleached–resin treated and scoured–bleached–resin and TEA treated cotton fibres. The results indicate that when the solutions of varied concentrations of the purified CI Direct Yellow 28 dye was streamed through the plug of grey cotton fibres, as the concentration of dye in the streaming solution increased, the negative zeta potential of the fibre was found to be progressively increasing. This may be attributed to the increase in the adsorption of anionic dye as its concentration in the fibre–dye solution interphase increased. The dye being a direct dye having high affinity
where σ is the surface charge density (esu/cm2 ); N the number of anions or cations per cm3 in the bulk of solution; K the Boltzmann constant (1.37 × 10−16 ergs/degree); T the absolute temperature (K); D the dielectric constant of the solution; e the electronic charge (4.77 × 10−10 esu); ξ the zeta potential (mV); Z is the valency of ions (z = 1 for univalent ions). 2.3.2. Measurement of surface conductivity Specific conductivities of the streaming solution (Ks ) and the plug (Kc ) were also determined. Surface conductivity of the fibres (Kf ) was obtained by taking the difference between Kc and Ks : Kf = Kc − Ks
(3)
Fig. 1. Comparison of zeta potential of different treated cotton samples for different concentrations of direct dye at pH 6.98. Table 1 The zeta potential values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton
All the conductivities mentioned above bear the units −1 cm−1 .
Concentration of dyes (×10−6 mol)
Grey cottona
Scoured Resin cottona treatedb
Bleached cottona
Resin and TEA treatedc
2.3.3. Measurement of potential difference The potential difference was measured on Equip-tronics Dual Channel Digital Potentiometer EQDGD, having an accuracy of 0.1 mV.
0 10 20 40 60 80 100
−9.66 −13.40 −19.96 −30.47 −34.26 −55.37 −64.83
−4.13 −7.15 −10.79 −17.55 −24.04 −34.39 −41.62
−2.25 −3.34 −5.87 −12.36 −18.74 −29.68 −35.99
31.75 28.52 23.44 19.45 15.92 8.05 2.99
2.3.4. Measurement of resistance and conductivity The resistance and the conductivity of the plug as well as that of the solution were measured by using Equip-tronics Digital Conductivity meter, Model no. EQ-DCM-P.
a b c
pH 6.98. pH 4.5. pH 4.1.
−7.12 −11.55 −18.03 −29.07 −33.54 −42.90 −61.29
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gets increasingly adsorbed on the fibre. When its concentration in the streaming solution increased, each of the anionic dye molecule containing two sulphonic acid groups on adsorption further contribute in enhancing the negative zeta potential of the fibre. It is to be noted however that similar trend was also followed when the fibre was scoured or scoured and bleached. These were successive stages of purification of cotton fibres. While in scouring absorbency of cotton increases and in case of bleaching its whiteness also increases. It has been found that the accessibility and moisture regain of cotton increases as one scoured or bleaches the fibre. Hence increase in direct dye concentration in the streaming solution resulted in increased zeta potential due to progressive increase in adsorption of the dye by the fibre, giving almost much parallel trend for all the fibre samples. However, at any given concentration of dye in the streaming solution zeta potential of grey cotton was maximum followed by scoured one and least was of bleached material. This is attributed to increase in hydrophilicity and accessibility of the fibre as a result of pretreatment. Also these pretreatment mainly bleaching was found to increase distinctly the carboxyl groups of fibre sample. Hence the lowering of zeta potential as a result of pretreatment was expected. In practice when actual dyeing with direct dye was carried out, it is clear that at any given shade, the dye uptake (K/S) values increased as the fibre was increasingly purified. In other words, the dye uptake values of bleached cotton were more than the scoured cotton, which in turn were more than grey cotton. In case of resin treated fibres similar trend was seen, i.e. the dye concentration in the streaming solution increased, the negative zeta potential also increased. However, it is to be noted that the negative zeta potential of DMDHEU resin treated cotton showed sharp increase in its magnitude. Hence the curve representing this sample traces almost similar path as that of grey cotton. In other words at any given concentration of the dye in streaming solution, following was the order in which magnitude of the negative zeta potential varied. 3.1.1. Grey cotton > resin treated cotton > resin and TEA treated cotton > scoured cotton > scoured and bleached cotton This may be attributed to DMDHEU resin forming covalent linkage at the hydroxyl groups in cellulose and thus resin treated cellulose will have more such hydroxyl groups blocked in the reaction with resin and thus the resin treated cotton always is less hydrophilic and has lower moisture regain. The increased hydrophobicity and cross-linking due to resin treatment brings about increased negative zeta potential. Such resin treated cotton infact also showed distinct decrease in dye uptake values which is in consonant with this observation. The increased zeta potential will thus account for the increased hydrophobicity of cotton due to resin treatment. However, when resin finishing was done using TEA·HCl as an additive, its original zeta potential in absence of dye was negative. The positive zeta potential decreases as the dye concentration increases in the streaming solution at acidic pH. This is because the nitrogen present in the TEA gets activated at
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acidic pH and hence attracts the dye anions more and henceforth decreasing the positive zeta potential of the fibres. However, when the cotton fibre gets cationised as the concentration of the anionic direct dye in the streaming solution increased, the adsorption of increased dye anions brought down progressively this positive zeta potential once again sharing the parallel trend. In other words, the general behaviour of resin finished cotton giving much lower dye uptake and the one finished with resin and TEA·HCl additive giving enhanced dye uptake is very clear from these results. 3.2. Effect of surface charge density In case of surface charge density of the fibre, since it depends on the zeta potential of the fibre and since the surface charge density calculation involves zeta potential factor, the trend of increase in the surface charge density with increase in dye concentration was observed similar to the trend found in the case of zeta potential measurement. The results from Table 2 indicates that the surface charge density being dependent on the increase in concentration of the dye in the streaming solution may be because of the progressive increase in the concentration of the dye anions in the fibre–liquid interphase and, as the negatively charged dye anions adsorb more and more amount of the dye from the streaming solution at a higher concentration, the surface of the fibre was likely to show an increase in the surface charge density. It is to be noted that each of the dye molecules carried two sulphonic acid groups, which contributed in building up the negative surface charge density. 3.3. Effect of surface conductivity The results from Fig. 2 and Table 3 indicate that in the case of surface conductivity also, the trend observed was similar as that in the case of zeta potential and surface charge density, but the actual values for individual fibre samples did change a lot. It is to be noted that an increase in the concentration of the dye shows an increased in the surface conductivity of the fibre. This is attributed to the higher extent of polarity of the fibre surface due to the increased surface charge density. The adsorption of the dye anion was enhanced when the concentration of the dye in the streaming solution was increased and it itself resulted in increase in surface conductivity. As higher and higher concentration of
Fig. 2. The effect of surface conductivity with different treatment of cotton at various concentration of dyes.
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Table 2 The surface charge density values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton Concentration of dyes (×10−6 mol)
Grey cottona
Scoured cottona
Resin treatedb
Bleached cottona
Resin and TEA treatedc
0 10 20 40 60 80 100
−2.57 −27.9 −59.5 −133 −185 −386 −539
−1.1 −14.8 −31.6 −73.6 −125 −215 −300
196 61.6 70.5 81.9 81.5 47 19.5
−0.596 −6.87 −17.1 −51.3 −96.5 −182 −253
196 79.6 70.5 61.9 51.5 47 19.5
a b c
pH 6.98. pH 4.5. pH 4.1.
Table 3 The surface conductivity values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton Concentration of dyes (×10−6 mol)
Grey cottona
Scoured cottona
Resin treatedb
Bleached cottona
Resin and TEA treatedc
0 10 20 40 60 80 100
3.651 4.342 7.501 12.238 15.89 24.181 26.945
3.355 3.948 6.119 8.784 10.462 15.101 17.371
3.454 4.145 7.106 10.462 14.311 19.049 25.859
2.961 3.651 3.257 5.329 6.612 7.205 11.646
−5.132 −4.935 −3.553 −2.862 −2.368 −1.875 −1.776
a b c
pH 6.98. pH 4.5. pH 4.1.
The pH of the streaming solution plays a prominent role in determining the electrokinetic properties of the cotton fibres. At neutral pH the zeta potential of raw cotton was higher than that of the scoured and bleached cotton. The resin treated cotton showed higher zeta potential than scoured but lower than that of grey cotton and the zeta potential and surface charge density values for resin treated cotton at different pH have been shown in Figs. 3 and 4. The negative zeta potential of the cotton resin treated with TEA additive was also lowest of all other samples of cotton fibres.
When the pH was reduced to 4.1, the zeta potential values increased towards the positive side due to the cationisation of TEA additive. It is quite natural that as the anionic dye adsorption increased, the fibre would get its positive charge neutralised to an increasing extent and thus with higher concentration of anionic dye in the streaming solution, positive values of zeta potential will consequently decrease, giving further indication of enhanced dye uptake under such conditions. It is for this reason, suppressed dye uptake due to resin treatment, once again enhanced to a great extent when additives such as TEA·HCl were present and dyeing was carried out in acidic condition so that cationisation of nitrogen additives takes place. At this pH 4.1, the zeta potential of cationised cotton however showed a decrease in its positive zeta potential values, when the dye concentration of the direct dye in the streaming
Fig. 3. The effect of zeta potential for various concentration of direct dye at different pH for resin treated samples.
Fig. 4. The effect of zeta potential for various concentration of direct dye at different pH for resin and TEA treated samples.
dye anions in the fibre–liquid interphase and also on the fibre surface the conductivity of the fibre increases. 3.4. Effect of pH of streaming solution
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Fig. 5. The effect of surface charge density of resin treated cotton at various concentration of direct dye for different pH. Fig. 8. The effect of K/S on various treatments of cotton at pH 6.98 for bleached cotton, pH 4.5 for resin treated cotton and pH 4.1 for resin and TEA treated cotton.
3.5. Effect of K/S values
Fig. 6. The effect of surface charge density of resin and TEA treated cotton at various concentrations of direct dye for different pH.
solution was increased as shown in Fig. 5. Similar trend is observed also in case of surface charge density as shown from Fig. 6. The zeta potential values of resin treated cotton at pH 4.1 shows increase of negative zeta potential when the dye concentration is increased in the solution as like grey cotton.
The K/S values of the dyed cotton fibres were given in Figs. 7 and 8. The results indicate that the K/S values increased from grey cotton to scoured and scoured to bleached cotton. The K/S values also increased from grey to bleached cotton with the addition of electrolyte than without electrolyte. The K/S values also increased for all the treatments with increase in shade from 0.5% to 2% and 2% to 4% which was quite obvious. The K/S values of cationised cotton were found to be higher as compared to resin treated cotton but less than bleached cotton in neutral pH. However, the K/S values of cationised cotton was higher than resin treated cotton at acidic pH. 4. Conclusions Electrokinetic properties exhibited variation in their behaviour subsequent to chemical modification and purification of the fibres. Cotton on purification showed lowering of the zeta potential and thus scouring, and scouring followed by bleaching brought down a progressive decrease in the negative zeta potential at any given dye concentration in the streaming solution. The trend of surface charge density was parallel to that of the zeta potential. Surface conductivity also increased with increasing dye concentration in the streaming solution. The K/S values of resin treated cotton with TEA as additive was higher than with the resin treated cotton. Also the purification of cotton enhances the dye uptake values and hence bleached cotton showed higher K/S values than the scoured and grey cotton. The K/S values of the dyed cotton samples and the electrokinetics properties of the cotton fibres were found to go hand in hand. References
Fig. 7. The effect of K/S on various treatments of cotton in neutral pH with and without electrolyte.
[1] A.M. Grancaric, T. Pusic, I. Sabjacic, Text. Chem. Color. 29 (12) (1997) 33. [2] F. Bohme, C. Klinger, C. Bellmann, Colloid Surf. A: Physicochem. Eng. Aspects 189 (2001) 21–27.
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[3] S. Ivan, M. Ognjen, P. Velimir, J. Colloid Interf. Sci. 189 (1997) 66–73. [4] B.V. Eremenko, M.L. Malysheva, O.D. Rusina, W.V. Kutsevol, T.B. Zeltonozskaya, Colloid Surf. A: Physicochem. Eng. Aspects 98 (1995) 19–24. [5] M. Chibowski SWi´sniewska, Colloid Surf. A: Physicochem. Eng. Aspects 208 (2002) 131–145. [6] D. Santhiya, S. Subramanian, K.A. Natarajan, S.G. Malghan, Colloid Surf. A: Physicochem. Eng. Aspects 164 (2000) 143–154. [7] S. Majda Sfiligoj, K. Stana-Kleinschek, V. Ribitsch, B. Pointner, Zipper S P., K. Stakne, M. Bele, Mater. Rev. Innov. 6 (2002) 19–23. [8] K. Juha, S. Per, Colloid Surf. A: Physicochem. Eng. Aspects 156 (1999) 357–372. [9] K.D. Kalyan, P. Somasundaran, Colloid Surf. A: Physicochem. Eng. Aspects 182 (2001) 25–33. [10] K. Stana-Kleinschek, V. Ribitsch, T. Kreze, L. Fras, Mater. Res. Innov. 6 (2002) 13–18. [11] L. Hermitte, F. Thomas, R. Bougaran, C. Martelet, J. Colloid Interf. Sci. 272 (2004) 82–89. [12] C. Werner, R. Zimmermann, T. Kratzmuller, Colloid Surf. A: Physicochem. Eng. Aspects 192 (2001) 205–213.
[13] A. Bismarck, M.E. Kumru, J. Springer, J. Colloid Interf. Sci 217 (1999) 377–387. [14] S.S. Dukhin, R. Zimmermann, C. Werner, Colloid Surf. A: Physicochem. Eng. Aspects 195 (2001) 103–112. [15] K. Stana-Kleinschek, K. Tatjana, V. Ribitsch, S. Strnad, Colloid Surf. A: Physicochem. Eng. Aspects 195 (2001) 275–284. [16] K. Stana-Kleinschek, V. Ribitsch, Colloid Surf. A: Physicochem. Eng. Aspects 140 (1998) 127–138. [17] V.F. Androsov, E.N. Golovanov, V.I. Karslieva, I.D. Tugai, H.T. Lokhande, Legka Promst. 35 (1967) 19–21. [18] P.D. Kale, H.T. Lokhande, K.N. Rao, M.H. Rao, J. Appl. Polym. Sci. 19 (1975) 461–480. [19] H.T. Lokhande, V.F. Androsov, E.N. Golovanov, Ind. J. Technol. 8 (1970) 89–91. [20] H.T. Lokhande, M.D. Teli, Colloid Polym. Sci. 262 (1984) 127–130. [21] M.D. Teli, Text. Dyer Print. 18 (1985) 17. [22] M.D. Teli, N.M. Prasad, U.V. Vyas, J. Appl. Polym. Sci. 50 (1993) 449– 457. [23] M.D. Teli, B.R. Rao, Text. Res. J. 65 (1995) 693–696.
Electrokinetic studies of modified cellulosic fibres A. Ramesh Kumar ∗ , M.D. Teli Department of Fibres and Textile Processing Technology, University Institute of Chemical Technology, Matunga, Mumbai 400019, India Received 23 December 2005; received in revised form 15 August 2006; accepted 17 January 2007 Available online 20 January 2007
Abstract The electrokinetic phenomenon is the one involving electricity connected with tangential movement of two phases along with each other. The present paper deals with changes in the electrokinetic properties of cotton fibres with varying degrees of purification and modification of the cellulose with dimethylol dihydroxy ethylene urea (DMDHEU) and DMDHEU–triethanol amine hydrochloride (TEA). The electrokinetic properties such as zeta potential, surface charge density and surface conductivity were determined in the presence of direct dyes in the streaming the solution. The paper also discusses the behaviour of cotton cellulose with respect to extent of purification and modification of cellulose and dye uptake in the light of electrokinetic properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Cellulose; Dimethylol dihydroxy ethylene urea; Direct dye; Surface charge density; Surface conductivity; Triethanol amine; Zeta potential
1. Introduction Ion adsorption of dyes, surfactants and other species on the textile surface plays a prominent role in dyeing and finishing. It is assumed that ions are adsorbed in an electrical double layer, existing at the interface of charged textile surface and aqueous solution. The electrokinetic or zeta potential is an electrokinetic phenomenon that is developed when one of these two charged surface moves relative to the other [1]. A knowledge of zeta potential gave added theoretical base for many important wet processes of fibrous materials such as dyeing and detergency to which the textile materials were subjected. Zeta potential of textile materials was usually measured using streaming potential and electroosmosis. The zeta potential for fibres such as cotton, rayon, and polyester in aqueous solution shows a negative value. For amphoteric fibres such as wool, silk and polyamide, the isoelectric point is determined from the effect of pH on zeta potential of the fibre. In manufacturing nylon fibres, drawing and heat setting processes result in changes in the fibrous structure, especially in the fine structure. The effect of these processes on the isoelectric point of the fibres was determined by the
∗
Corresponding author. Present address: KG Denim Ltd., Thenthirumalai, Mettupalayam, Coimbatore 641302, India. Tel.: +91 4254 304349. E-mail address: rameshkumar [email protected] (A. Ramesh Kumar). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.01.021
zeta potential method. When the zeta potential of fibres such as cotton and rayon were measured in aqueous solutions of direct dyes, it was observed with increasing concentration of the dye, there was an increase in negative value of the zeta potential of the fibre which passed through as maximum value and thereafter decreased. Polyamidines were investigated by electrokinetic measurements for its surface properties in dependence on the pH value [2]. The adsorption of copolymers and its effect on electrokinetic potential depends on the degree of modifications such as grafting and pH etc have been investigated earlier [3,4]. Influence of polyacrylic acid (PAA) and polyacrylamide (PAM) molecular weight on adsorption and electrokinetic properties of Fe2 O3 –polyelectrolyte solution interface was studied. Zeta potential changes are as a function of polymer molecular weight, pH and concentration of the polymer solution [5]. The interaction of alumina with ammonium poly (methacrylate) (APMA) has been investigated through adsorption and electrokinetic measurements by Santhiya et al. Electrokinetic studies indicate specific adsorption between alumina and APMA resulting in a shift of the isoelectric point to acidic pH values [6]. PETP fibres exhibit high negative zeta potential value, due to their strong hydrophobic and non-ionic character. The application of the surfactant changes the electrokinetic properties with respect to the adsorption process conditions [7]. Application of zeta potential in flocculation and surface deposition have been studied by many researchers [8,9].
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The reactivity and adsorption properties of cellulose fibres were critical for successful treatment because behaviour during the finishing process is determined by both structure and surface properties. The natural fibres have the smaller hydrophilic character and they are less reactive than the regenerated ones and hence zeta potential of cotton and contact angle was the highest as compared to regenerated fibres [10]. Relationships between formulation, bulk properties, and surface properties were investigated on series of copolymers prepared with hydroxyethylmethacrylate (HEMA), methylmethacrylate (MMA), and ethylmethacrylate (EMA) monomers and on the homopolymers PMMA and PHEMA by Hermitte [11–13]. Based on recent progress in the measurement and interpretation of ion accumulation and motion in the stagnant part of the electrical double layer (DL), a generalized standard electrokinetic model (GSEM) was proposed by Dukhin. The suggested model reflected a new standard for the modeling of electrokinetic transport [14]. The accessibility of free adsorption places in the less ordered regions of cellulose, i.e. viscose, modal and lyocell fibres, which in turn was investigated by determining of electrokinetic properties. An excellent correlation between structure, adsorption and electrokinetic properties existed for the regenerated fibres [15]. Alternative ways to describe the success of different processes in fibre purification which result in distinct surface charge and hydrophilicity was the determination of electrokinetic properties and the water uptake of textile fibres. The zeta potential was determined by streaming potential measurement as a function of the pH [16]. The electrokinetic properties of bleached cotton in aqueous solutions of direct dye s have been studied [17]. It was observed that the negative zeta potential of bleached cotton dyed with direct dyes, which differ only in sulphonic group content of their chemical structure, increases with dye concentration [18]. The zeta potential of cotton treated with different cross linking agents dimethylol ethylene urea (DMEU), dimethylol dihydroxy ethylene urea (DMDHEU), when measured in aqueous solutions of various inorganic salts, the zeta potential of the treated fibre decreased but the surface charge density increased with increasing electrolyte concentration [19]. Hence electrokinetics studies have been found to be excellent tool to understand the dye fibre interaction [20–22]. It throws light on the functional groups present on the surface of the fibre, as it is a surface phenomenon. There has been very little work reported as far as electrokinetic properties of cotton fibres passing through various stages of pretreatment and finishing with resin and additives were concerned [23]. The present paper deals with the electrokinetic studies of the cotton fibres subjected to various processing treatments like scouring, scouring–bleaching, scouring–bleaching–resin treatments and scouring–bleaching and resin treatment with triethanol amine hydrochloride as an additive. The treated fibres were studied for their electrokinetic properties like zeta potential, surface charge density and surface conductivity when CI Direct Yellow 28 dye was streamed through plug of fibres at various pH. The treated fibres were also dyed with the direct
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dye with and without using the electrolyte, for different pH and the K/S values were evaluated. 2. Materials and experimental methods 2.1. Materials 2.1.1. Fibres The grey cotton fibres were obtained from Tata Mills, Mumbai, India. 2.1.2. Chemicals and auxiliaries Superfinish EUC was supplied by Supertex Chemicals, Mumbai, India. Triethanol amine hydrochloride (TEA), sodium hydroxide, hydrogen peroxide (50%), sodium sulphate was obtained from SD Fine chemicals, Mumbai, India. All the chemicals were of chemically pure grade. 2.1.3. Dye CI Direct Yellow 28 dye was supplied by Indokem Ltd., Mumbai, India. 2.2. Experimental methods 2.2.1. Scouring The cotton fibres were scoured for 1 h at 100 ◦ C with caustic soda (10 g/l) and non-ionic detergent (0.5%) keeping material to liquor ratio 1:20. 2.2.2. Bleaching The scoured cotton fibres were bleached for 1 h at 85 ◦ C with 3% hydrogen peroxide (50%), 4.5% sodium silicate, 1% sodium carbonate and 0.5% EDTA keeping material to liquor ratio 1:20. 2.2.3. Resin treatment The bleached cotton fibres was given resin finishing treatment by soaking the same for 15 min in the resin containing DMDHEU (100 g/l) and MgCl2 (10%) and keeping material to liquor ratio 1:20. at room temperature. It was then hydroextracted, dried at 80 ◦ C and cured at 150 ◦ C for 3 min in a hot air curing chamber. It was then washed and dried. 2.2.4. Cationisation of cotton The bleached cotton fibres was soaked in the bath containing DMDHEU (100 g/l), TEA·HCl (6%) and MgCl2 (10%) on the weight of resin and keeping material to liquor ratio 1:20 for 15 min at room temperature. It was then hydroextracted, dried at 80 ◦ C and cured at 150 ◦ C for 3 min in a hot air curing chamber. It was then washed and dried. 2.2.5. Dyeing of cotton fibres Dyeing of cotton fibres was carried out in a laboratory Rotadyer machine supplied by RBE Company, Mumbai, India. The samples of known weight were dyed in the dye pots containing required amount of dye and sodium sulphate (optional) for the first three batches and dye, acetic acid and sodium sulphate (optional) for resin and resin and TEA treated samples keeping material to liquor ratio 1:20. Dyeing was started at 40 ◦ C
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followed by heating the dye bath up to 95 ◦ C in 30 min. Dyeing was then carried out for further 45 min at that temperature. After the dyeing was over samples were removed and washed with water. The samples were again washed with hot water at 80 ◦ C for 10 min and air-dried.
2.3.5. Measurement of pH of the streaming solution The pH of the distilled water as well as dye solutions was measured by using Equip-tronics Digital pH meter, Type MKVI.
2.3. Determination of streaming potential
2.3.6. Measurement of K/S value The K/S values were measured in the Spectra Flash SF 300, Data colour international 3.1 computer colour matching system.
2.3.1. Measurement of zeta potential The determination of electrokinetic properties by streaming potential method has been described. While measuring these properties, plug of 7 g of cotton fibres was used and the streaming solution contained various concentration of pure direct dye, CI Direct Yellow 28. The detailed methods of estimation have been reported in the literature [17–23]. Zeta potential was calculated by using the Helmholtz– Smoluchouskii equation as shown below: ξ = 6.75 × 107 ×
4πηVSTR × Cs EP
(1)
where ξ is the zeta potential (mV); η the viscosity of the solution (poise); Cs the specific conductivity of the solution in the fibres (−1 cm−1 ); E the dielectric constant of the solution; P the pressure of the solution streaming through the cell (cm of Hg); VSTR is the streaming potential (mV). The surface charge density was calculated using the following equation: √ zeξ σ = 2NKTDπ sin H (2) 2KT
3. Results and discussion 3.1. Effect of zeta potential The results from Fig. 1 and Table 1 show the zeta potential values for raw cotton, scoured cotton, scoured–bleached cotton, scoured–bleached–resin treated and scoured–bleached–resin and TEA treated cotton fibres. The results indicate that when the solutions of varied concentrations of the purified CI Direct Yellow 28 dye was streamed through the plug of grey cotton fibres, as the concentration of dye in the streaming solution increased, the negative zeta potential of the fibre was found to be progressively increasing. This may be attributed to the increase in the adsorption of anionic dye as its concentration in the fibre–dye solution interphase increased. The dye being a direct dye having high affinity
where σ is the surface charge density (esu/cm2 ); N the number of anions or cations per cm3 in the bulk of solution; K the Boltzmann constant (1.37 × 10−16 ergs/degree); T the absolute temperature (K); D the dielectric constant of the solution; e the electronic charge (4.77 × 10−10 esu); ξ the zeta potential (mV); Z is the valency of ions (z = 1 for univalent ions). 2.3.2. Measurement of surface conductivity Specific conductivities of the streaming solution (Ks ) and the plug (Kc ) were also determined. Surface conductivity of the fibres (Kf ) was obtained by taking the difference between Kc and Ks : Kf = Kc − Ks
(3)
Fig. 1. Comparison of zeta potential of different treated cotton samples for different concentrations of direct dye at pH 6.98. Table 1 The zeta potential values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton
All the conductivities mentioned above bear the units −1 cm−1 .
Concentration of dyes (×10−6 mol)
Grey cottona
Scoured Resin cottona treatedb
Bleached cottona
Resin and TEA treatedc
2.3.3. Measurement of potential difference The potential difference was measured on Equip-tronics Dual Channel Digital Potentiometer EQDGD, having an accuracy of 0.1 mV.
0 10 20 40 60 80 100
−9.66 −13.40 −19.96 −30.47 −34.26 −55.37 −64.83
−4.13 −7.15 −10.79 −17.55 −24.04 −34.39 −41.62
−2.25 −3.34 −5.87 −12.36 −18.74 −29.68 −35.99
31.75 28.52 23.44 19.45 15.92 8.05 2.99
2.3.4. Measurement of resistance and conductivity The resistance and the conductivity of the plug as well as that of the solution were measured by using Equip-tronics Digital Conductivity meter, Model no. EQ-DCM-P.
a b c
pH 6.98. pH 4.5. pH 4.1.
−7.12 −11.55 −18.03 −29.07 −33.54 −42.90 −61.29
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gets increasingly adsorbed on the fibre. When its concentration in the streaming solution increased, each of the anionic dye molecule containing two sulphonic acid groups on adsorption further contribute in enhancing the negative zeta potential of the fibre. It is to be noted however that similar trend was also followed when the fibre was scoured or scoured and bleached. These were successive stages of purification of cotton fibres. While in scouring absorbency of cotton increases and in case of bleaching its whiteness also increases. It has been found that the accessibility and moisture regain of cotton increases as one scoured or bleaches the fibre. Hence increase in direct dye concentration in the streaming solution resulted in increased zeta potential due to progressive increase in adsorption of the dye by the fibre, giving almost much parallel trend for all the fibre samples. However, at any given concentration of dye in the streaming solution zeta potential of grey cotton was maximum followed by scoured one and least was of bleached material. This is attributed to increase in hydrophilicity and accessibility of the fibre as a result of pretreatment. Also these pretreatment mainly bleaching was found to increase distinctly the carboxyl groups of fibre sample. Hence the lowering of zeta potential as a result of pretreatment was expected. In practice when actual dyeing with direct dye was carried out, it is clear that at any given shade, the dye uptake (K/S) values increased as the fibre was increasingly purified. In other words, the dye uptake values of bleached cotton were more than the scoured cotton, which in turn were more than grey cotton. In case of resin treated fibres similar trend was seen, i.e. the dye concentration in the streaming solution increased, the negative zeta potential also increased. However, it is to be noted that the negative zeta potential of DMDHEU resin treated cotton showed sharp increase in its magnitude. Hence the curve representing this sample traces almost similar path as that of grey cotton. In other words at any given concentration of the dye in streaming solution, following was the order in which magnitude of the negative zeta potential varied. 3.1.1. Grey cotton > resin treated cotton > resin and TEA treated cotton > scoured cotton > scoured and bleached cotton This may be attributed to DMDHEU resin forming covalent linkage at the hydroxyl groups in cellulose and thus resin treated cellulose will have more such hydroxyl groups blocked in the reaction with resin and thus the resin treated cotton always is less hydrophilic and has lower moisture regain. The increased hydrophobicity and cross-linking due to resin treatment brings about increased negative zeta potential. Such resin treated cotton infact also showed distinct decrease in dye uptake values which is in consonant with this observation. The increased zeta potential will thus account for the increased hydrophobicity of cotton due to resin treatment. However, when resin finishing was done using TEA·HCl as an additive, its original zeta potential in absence of dye was negative. The positive zeta potential decreases as the dye concentration increases in the streaming solution at acidic pH. This is because the nitrogen present in the TEA gets activated at
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acidic pH and hence attracts the dye anions more and henceforth decreasing the positive zeta potential of the fibres. However, when the cotton fibre gets cationised as the concentration of the anionic direct dye in the streaming solution increased, the adsorption of increased dye anions brought down progressively this positive zeta potential once again sharing the parallel trend. In other words, the general behaviour of resin finished cotton giving much lower dye uptake and the one finished with resin and TEA·HCl additive giving enhanced dye uptake is very clear from these results. 3.2. Effect of surface charge density In case of surface charge density of the fibre, since it depends on the zeta potential of the fibre and since the surface charge density calculation involves zeta potential factor, the trend of increase in the surface charge density with increase in dye concentration was observed similar to the trend found in the case of zeta potential measurement. The results from Table 2 indicates that the surface charge density being dependent on the increase in concentration of the dye in the streaming solution may be because of the progressive increase in the concentration of the dye anions in the fibre–liquid interphase and, as the negatively charged dye anions adsorb more and more amount of the dye from the streaming solution at a higher concentration, the surface of the fibre was likely to show an increase in the surface charge density. It is to be noted that each of the dye molecules carried two sulphonic acid groups, which contributed in building up the negative surface charge density. 3.3. Effect of surface conductivity The results from Fig. 2 and Table 3 indicate that in the case of surface conductivity also, the trend observed was similar as that in the case of zeta potential and surface charge density, but the actual values for individual fibre samples did change a lot. It is to be noted that an increase in the concentration of the dye shows an increased in the surface conductivity of the fibre. This is attributed to the higher extent of polarity of the fibre surface due to the increased surface charge density. The adsorption of the dye anion was enhanced when the concentration of the dye in the streaming solution was increased and it itself resulted in increase in surface conductivity. As higher and higher concentration of
Fig. 2. The effect of surface conductivity with different treatment of cotton at various concentration of dyes.
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Table 2 The surface charge density values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton Concentration of dyes (×10−6 mol)
Grey cottona
Scoured cottona
Resin treatedb
Bleached cottona
Resin and TEA treatedc
0 10 20 40 60 80 100
−2.57 −27.9 −59.5 −133 −185 −386 −539
−1.1 −14.8 −31.6 −73.6 −125 −215 −300
196 61.6 70.5 81.9 81.5 47 19.5
−0.596 −6.87 −17.1 −51.3 −96.5 −182 −253
196 79.6 70.5 61.9 51.5 47 19.5
a b c
pH 6.98. pH 4.5. pH 4.1.
Table 3 The surface conductivity values for various concentration of direct dye for grey, scoured, bleached, resin treated and resin and TEA treated cotton Concentration of dyes (×10−6 mol)
Grey cottona
Scoured cottona
Resin treatedb
Bleached cottona
Resin and TEA treatedc
0 10 20 40 60 80 100
3.651 4.342 7.501 12.238 15.89 24.181 26.945
3.355 3.948 6.119 8.784 10.462 15.101 17.371
3.454 4.145 7.106 10.462 14.311 19.049 25.859
2.961 3.651 3.257 5.329 6.612 7.205 11.646
−5.132 −4.935 −3.553 −2.862 −2.368 −1.875 −1.776
a b c
pH 6.98. pH 4.5. pH 4.1.
The pH of the streaming solution plays a prominent role in determining the electrokinetic properties of the cotton fibres. At neutral pH the zeta potential of raw cotton was higher than that of the scoured and bleached cotton. The resin treated cotton showed higher zeta potential than scoured but lower than that of grey cotton and the zeta potential and surface charge density values for resin treated cotton at different pH have been shown in Figs. 3 and 4. The negative zeta potential of the cotton resin treated with TEA additive was also lowest of all other samples of cotton fibres.
When the pH was reduced to 4.1, the zeta potential values increased towards the positive side due to the cationisation of TEA additive. It is quite natural that as the anionic dye adsorption increased, the fibre would get its positive charge neutralised to an increasing extent and thus with higher concentration of anionic dye in the streaming solution, positive values of zeta potential will consequently decrease, giving further indication of enhanced dye uptake under such conditions. It is for this reason, suppressed dye uptake due to resin treatment, once again enhanced to a great extent when additives such as TEA·HCl were present and dyeing was carried out in acidic condition so that cationisation of nitrogen additives takes place. At this pH 4.1, the zeta potential of cationised cotton however showed a decrease in its positive zeta potential values, when the dye concentration of the direct dye in the streaming
Fig. 3. The effect of zeta potential for various concentration of direct dye at different pH for resin treated samples.
Fig. 4. The effect of zeta potential for various concentration of direct dye at different pH for resin and TEA treated samples.
dye anions in the fibre–liquid interphase and also on the fibre surface the conductivity of the fibre increases. 3.4. Effect of pH of streaming solution
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Fig. 5. The effect of surface charge density of resin treated cotton at various concentration of direct dye for different pH. Fig. 8. The effect of K/S on various treatments of cotton at pH 6.98 for bleached cotton, pH 4.5 for resin treated cotton and pH 4.1 for resin and TEA treated cotton.
3.5. Effect of K/S values
Fig. 6. The effect of surface charge density of resin and TEA treated cotton at various concentrations of direct dye for different pH.
solution was increased as shown in Fig. 5. Similar trend is observed also in case of surface charge density as shown from Fig. 6. The zeta potential values of resin treated cotton at pH 4.1 shows increase of negative zeta potential when the dye concentration is increased in the solution as like grey cotton.
The K/S values of the dyed cotton fibres were given in Figs. 7 and 8. The results indicate that the K/S values increased from grey cotton to scoured and scoured to bleached cotton. The K/S values also increased from grey to bleached cotton with the addition of electrolyte than without electrolyte. The K/S values also increased for all the treatments with increase in shade from 0.5% to 2% and 2% to 4% which was quite obvious. The K/S values of cationised cotton were found to be higher as compared to resin treated cotton but less than bleached cotton in neutral pH. However, the K/S values of cationised cotton was higher than resin treated cotton at acidic pH. 4. Conclusions Electrokinetic properties exhibited variation in their behaviour subsequent to chemical modification and purification of the fibres. Cotton on purification showed lowering of the zeta potential and thus scouring, and scouring followed by bleaching brought down a progressive decrease in the negative zeta potential at any given dye concentration in the streaming solution. The trend of surface charge density was parallel to that of the zeta potential. Surface conductivity also increased with increasing dye concentration in the streaming solution. The K/S values of resin treated cotton with TEA as additive was higher than with the resin treated cotton. Also the purification of cotton enhances the dye uptake values and hence bleached cotton showed higher K/S values than the scoured and grey cotton. The K/S values of the dyed cotton samples and the electrokinetics properties of the cotton fibres were found to go hand in hand. References
Fig. 7. The effect of K/S on various treatments of cotton in neutral pH with and without electrolyte.
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