Combined UV-Vis-Absorbance and Reflectance Spectroscopy Study of Dye Transfer Kinetics in Aqueous Mixtures of Surfactants

Noname manuscript No. (will be inserted by the editor)

Combined UV-Vis-absorbance and Reﬂectance Spectroscopy Study of Dye Transfer Kinetics in Aqueous Mixtures of Surfactants

Carlos G. Lopez · Anna Manova · Corinna Hoppe · Michael Dreja · Peter Schmiedel · Mareile Job · Walter Richtering · Alexander B¨oker · Larisa Tsarkova

Received: date / Accepted: date

Abstract We report an analytical approach to study the kinetics of desorption and exhaustion of a hydrophobic dye in a multicomponent washing-model environment. The process of dye transfer between an acceptor textile (white polyamide), detergent micelles and a donor textile (red polyester) was quan- tiﬁed by a combination of colorimetric analyses. UV-Vis absorbance and UV- reﬂectance spectroscopy were used to follow the concentration of the solubilised dye in the micelles and the amount of dyer transferred to the acceptor textile, respectively, as a function of time. Up to ' 10 min of the washing process, the released dye is predominantly solubilised in surfactant micelles. At later times, the adsorption of the dye on the hydrophobic surface of the acceptor textile is energetically favoured. The shift of the desorption equilibrium in the presence of the acceptor textile results in ' 30% increase in the release of the dye. The reported methodology provides insight into the competition between solubilisation of hydrophobic molecules by amphiphiles and dye adsorption on solid substrates, important for designing novel concepts of dye transfer inhibition.

Carlos G. Lopez · Anna Manova · Corinna Hoppe · Walter Richtering Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany Anna Manova DWI-Leibniz Institute for Interactive Materials, Forckenbeckstrasse 50, 52056 Aachen. Michael Dreja · Peter Schmiedel · Mareile Job Henkel AG & Co. KGaA Henkelstrasse 67, D-40191, Düsseldorf,Germany Alexander Böker Fraunhofer-Institut fürAngewandte Polymerforschung, Lehrstuhl fürPolymermaterialien und Polymertechnologie, UniversitätPotsdam, Geiselbergstraße 69, 14476 Potsdam-Golm, Germany Larisa Tsarkova Faculty of Chemistry, Chair of Colloid Chemistry, Moscow State University, 119991,Moscow 1, GSP-1, 1-3 Leninskiye Gory, Russia. E-mail: [email protected] 2 Carlos G. Lopez et al.

Keywords Dye transfer kinetics · Disperse dyes · Surfactants · Detergent · Colorimetric analysis · Laundry · Dye Transfer Inhibition

1 Introduction

Dye transfer between textiles during the washing is a complex and generally undesirable process [1–3]. Different strategies exist to inhibit dye transfer, for example by use of dye bleachers or catchers [1, 2, 4]. Whatever the approach used for dye transfer inhibition, it is necessary to understand the relevant timescales for dye migration [5] which include desorption from the donor textiles, solubilisation into the washing medium (containing solubilisation agents such as surfactants) and adsorption onto the acceptor textile. Disperse dyes are non-ionic, organic, colouring compounds with low solubility in water, which are used for the dyeing of hydrophobic fibres such as cellulose ester, cellulose acetate, Nylon and polyester fibres. Owing to their limited solubility in water, disperse dyes are used along with auxiliary dispersing agents, typically surfactants and polymers, to stabilise their aqueous dispersion during the dyeing process. [6–18]. The kinetics of dye adsorption onto a number of fibre types, in particular polyester [19,20], have been the subject of experimental [21–27] and theoretical [28–39] studies. The rate of dyeing of polyester is limited by the diffusion of dye molecules into the textile fibres [28,33,40]. Dyeing kinetics can be modelled from first principles, following solutions for the diffusion equation under different conditions [31,35], or according to a range of empirical equations [38]. Desorption kinetics of disperse dyes from textile fibres remains less studied [41,42] than the process of dyeing, despite a direct relevance of the former for dye transfer [5]. Fastness is the ability of textiles to withstand colour reduction during treatment with surfactant-containing alkaline water solutions. Systematic studies on the staining of undyed materials in contact with coloured specimens are valuable for understanding of equilibrium and kinetic reactions involving poorly water-soluble organic compounds. Given that washing is performed at temperatures much lower than the temperature of the dyeing process, it is unlikely that during the limited time of a washing cycle (' 2hrs) an equilibrium between the dye concentration inside and outside the fibre will be reached [10]. It is then reasonable to assume that dye release during washing tests proceeds dominantly from the outer fibre surface, which contains a higher concentration of dye as compared to the bulk of the fibres (see references [32] and [36] and references therein). Therefore, dye diffusion through the fibre’s densely packed crystalline structure can be neglected in considerations of the release kinetics. The role of surfactants in coloration of textiles with disperse dyes is an area of active research due to its industrial relevance both in textile and in laundry technologies. Choi et al studied the dyeing of polyamide and polyester fibres in the presence and absence of surfactants [6, 7]. The addition of sur- Title Suppressed Due to Excessive Length 3 factants to the dyeing solution resulted in a decrease in the dyeing rate and in the final dyeing value due to solubilisation and stabilisation of molecularly dissolved dyes in solution. Qualitatively similar behaviour was observed for different surfactants, but the magnitude of change with respect to the surfactant free case was dependent on the type of surfactant used. Regarding laundry applications, surfactants and polymers (commonly present in detergents) are envisaged to play a role in promoting or inhibiting dye transfer to undyed materials (acceptors) [1, 6, 43–45]. Standard tests on dye transfer are based on the analysis of the colorimetric data of acceptor samples, since the loss of the colour intensity of the coloured fibres after a standard washing treatment is undetectable. Quantitative spectroscopic analysis of aqueous solutions, as a media for dye transfer, is generally not feasible due to extremely low solubility of the hydrophobic dyes in water. Here we report an analytical approach which combines UV-Vis absorbance and -reflectance spectroscopy in order to follow the concentration of the des- orbed dye in solution and on the acceptor textile with time. We considered two sources of the molecularly dissolved dye in solution: (i) a technical coloration powder and (ii) a donor textile. These two sources of the dye have been ana- lyzed in a system containing a fixed mass of acceptor textile which shifts the dissolution equilibrium by constantly removing the molecular-dissolved dye from the solution. The mass of dye in solution and absorbed onto the acceptor textile are quantified by UV-Vis and UV-reflectance respectively. The goal of the present study is to correlate within the same experimental system the kinetics of the dye desorption from a donor polyester (PES) textile and the kinetics of the dye transfer onto hydrophobic acceptor polyamide (PA) textile in an aqueous detergent liquor. In particular, we resolved the competition between the solubilisation of dye within micelles and dye adsorption onto the hydrophobic fabric. The analytical protocols reported here are applica- ble to other types of dispersed dyes, as well as other types of fibres, as long as the solubilisation of the respective coloration powder in aqueous media is achievable with the aid of auxiliary dispersing additives.

2 Background Theory

Here we outline some basic theoretical considerations that help us describe and quantify the rate of dyeing of acceptor textiles. During a typical dyeing experiment, textile fibres are immersed in a dye bath containing an initial concentration of dye C(0). In the absence of solubilising agents such as surfactants, disperse dyes will be suspended as large particles, with only a small fraction being molecularly dissolved. The dyeing of the textile occurs through this latter fraction and not through the particles, which act only as a reser- voir [33,46]. Under strong stirring (also referred to as strong flow), the rate of dyeing is limited by the diffusion of dye molecules into the textile fibres. The fraction of dye adsorbed onto the textile at an given time is called the exhaustion E(t). For dyebaths with low concentrations of dye, the concentration of 4 Carlos G. Lopez et al.

dye in the fibre at equilibrium (F∞) follows a Nernst type relation F∞ ∝ C∞ where C∞ is the equilibrium concentration of dye in the bath [33]. The full solutions to the diffusion equation for a cylinder or slab of absorbing material [31] are complex. It is therefore advantageous to use limiting forms which can easily be fit to experimental data [47, 48]. Since our experiments are carried out under constant stirring, we focus on the case of ‘strong flow’, modelled by Crank for different geometries of the absorbing material. Equation 1 models the absorption of solute by a slab of material for finite bath under such strong flow conditions [31] (see reference [33] for a discussion of its application to experimental data for the dyeing of polyester fibres):

βt/α2 2 1/2 M(t) = M∞(1 + α)[1 − e erfc(βt/α ) ] (1)

where M(t) and M∞ are the mass of dye in the fibre at time t and at equilibrium respectively, 1/(1 + α) is the fraction of dye inside the textile at equilibrium and β = D/l2 where D is the diffusion coefficient of the dye inside the 2 R x −z2 fibre and l is the thickness of the textile sheet. erfc(x) = 1 − π1/2 0 e dz is the complementary error function. For high values of α and low values of t, equation 1 can be expanded as:

2 M(t)/M = (βt)1/2 (2) ∞ π1/2 For a cylindrical instead of a slab geometry, l becomes the fibre diameter and the prefactor in equation 2 increased by a factor of 2. The M(t) ∝ t1/2 at early times has been observed in a number of studies [22, 46,49]. A number of empirical equations have been employed to fit the kinetics of dye absorption onto textile fibres [32]. The physical meaning of the empirical fit parameters have been discussed by Shibusawa [32]. A simple, general form to fit dye kinetics is given by:

−kta b M(t) ' M∞[1 − (e )] (3) where k is a rate constant and a and b are ﬁtting parameters. A number of empirical equations are particular cases of 3, for example for b = 1/2 and a = 1 it reduces to the Cegarra-Puente formula [38] and for a = 1/2 and b = 1 to the Etters-Urbanik [32, 50] formula. Equation 3 at short times reduces to the functional form of equation 2 when for example a = 1 and b = 1/2.

3 Methods

Chemicals, solvents and textiles

De-ionised water was obtained from a mili-Q source. Salts were reagent grades purchased from Sigma-Aldrich. The surfactants and the technical paste were Title Suppressed Due to Excessive Length 5

provided by Henkel. The aqueous media used in this study contained NaHCO3 (0.0025 M), CaCl2 (0.002 M) and MgCl2 (0.0007 M). The total ionic strength is 11 mmol/L, including a concentration of divalent ions of ' 2.65 mmol/L, equivalent to a water hardness of 15 dGH. The surfactant mixture contains two anionic surfactants and one non-ionic surfactant: linear alkylbenzenesulfonate (LAS), sodium lauryl (C12-C14) ether sulphate (FAEOS) and fatty alcohol (C12-C18) with 7 mole ethoxylation degree ethylene oxide (FAEO). The pH of the liquor was adjusted to 8.5±0.1. The surfactant concentrations are tenfold above their respective critical micelle concentrations. Standard dye solutions were prepared from the technical powder (Dystar Dianix Red EF-B, consisting of the dye Disperse Red 60 and dispersing agents) by dissolution of weighted amounts of the powder in detergent liquor upon stirring at 50◦C. The technical powder does not display detectable solubility in deionised (DI) or in dGH15◦ water. Since the precise amount of the dye in the powder is not known, all concentrations are expressed in the arbitrary units related to the mass of the powder. Red polyester (PES, E-477 from CFT) donor textile and undyed polyamide (PA, EMPA 406) acceptor textiles were provided by Henkel.

UV-Vis spectroscopy

The UV-Vis absorbance spectroscopy in aqueous dye solutions was possible due to the solubility of the technical powder Disperse red 60 in the detergent liquor at employed conditions (50◦C, pH 8.5). Concentration calibration curves according to the Lambert-Beer law were established at λ = 544 nm and λ = 593 nm by measuring the UV absorbance (A) of solutions with a known concentration at 50◦C: A(544nm) = 12.08c, and A(544nm) = 12.64c where the dye concentration c is in g/L. Examples of spectra and the calibration curves are shown in Figure S1 (Supporting information). The measurement temperature was shown to be important since at room temperature the calibration solutions exhibit an ageing behaviour within several hours in a form of a coloured residue. The residue can be re-dispersed by stirring at 50◦C, resulting in the same absorbance values that of as a freshly prepared warm solution. Therefore, all UV-Vis spectroscopic measurements have been done either on freshly prepared or on re-dispersed solutions at 50◦C. Filtering of hot solutions trough filter paper or through membrane filters did not affect the absorbance spectra indicating insignificant adsorption of the dye by the filter media.

Measurement of dye absorbed onto acceptor textiles

A spectrophotometric method based on calculating colour differences from vis- ible spectra in the CIELab colour space was used to monitor the coloration process of the acceptor textile. Colorimetric reflectance measurements were performed with a Spectraflash SF 600 PLUS CT reflectance spectrophotome- ter (Fa. Datacolor, Marl), using the L*-value of the CIELAB L*a*b colour 6 Carlos G. Lopez et al. coordinates, illumination with standard light-type D65 at 10◦ observation an- gle with a UV cut-off filter at 420 nm and a 20 mm aperture. Each textile sample has been measured three times at different places with three repeated measurements at each place, and an averaged value was estimated. The reflectance of polyamide acceptor textiles was measured at least three times at different sample areas. As a reference value, undyed polyamide sample was measured.

Wet treatment tests of donor textile

We consider three systems, shown schematically in Figure 1. The experimental setup for each system is as follows: System I: Technical powder is added to 20 mL wash liquor and stirred at 50◦C in a closed vial until it is dissolved. 0.2 g of acceptor textile is then added to the solution and stirred at 50◦C for a given time. The textile is then taken out, rinsed with D.I. water and dried overnight. Solution is pipetted out and measured using UV-Vis as described for System II. The coloured textiles are measured using UV-reflectance as described above. Knowledge of the initial and final concentration of dye in solution allows us to calculate the amount of dye absorbed onto the acceptor textile. A calibration curve for UV-reflectance as a function of dye absorbed can thus be established. See supporting information for further details. System II: The donor textile (red polyester) is cut into pieces of ' 3 × 2 cm. 20 mL of wash liquor is heated to 50◦C under stirring in a closed vial. The donor textile is then added and the system is kept under stirring for the required time. In order to quantify the concentration of dispersed dye, hot solution is pipetted out of the vial an transferred into a cuvette for UV-Vis measurements. The solutions are either kept at 50 ◦C or reheated before the measurement to ensure full dye dispersion. For each experiment, three UV-Vis measurements are taken and averaged. System III: 20 mL of wash liquor are heated up to 50◦C in a closed vial. 0.2 g of acceptor textile and a given amount of donor textile are added and the system is kept at the same temperature under stirring for a required amount of time. The acceptor textile is then removed, rinsed with D.I. water, dried overnight and measured with UV-reflectance. The concentration of dye in solution is quantified with UV-Vis as described for systems I and II.

4 Results and discussion

General results on dye solubility and coloration

Addition of either the pure dye, technical powder or donor textile distilled water or 15 dGH at 50◦C does not result in a measurable absorption spectra in the solvent, conﬁrming the low solubility of the dye (' 10−6 mole fraction [51]) Title Suppressed Due to Excessive Length 7

Fig. 1 Schematic of complete model system used to study dye transfer: Acceptor textile (white polyamide), donor textile (red polyester), disperse red dye, which can be molecularly dissolved in solution or inside micelles. The marked shapes and corresponding numbers indicate diﬀerent experiments carried out to elucidate the dye transfer mechanism: I. Dashed border: Coloration of acceptor textile. II. Dashed-point line border: Desorption from donor textile. III. Solid line: Full system: Donor and acceptor textiles in the presence of wash liquor.

in aqueous media on one side, and a good fastness of the coloured PES textile to the employed wet treatment. However, washing of a donor textile together with the acceptor textile in 15 dGH at 50◦C leads a measurable coloration on the latter, indicating that the finite solubility of the dye is sufficient for the dye to migrate from the donor to acceptor textile in significant quantities. The fastness of the donor textile drops significantly when micelles are introduced into the aqueous media. As is well established, the solubility of hydrophobic dyes is considerably increased by solubilisation of dyes within micelles, which results in significant coloration of the solution upon wet treatment.

System I: Coloration of the acceptor textile.

Coloration of the acceptor textile, as described in section 3 was carried out for diﬀerent initial concentrations of the technical powder in the wash liquor as a function of time. The results of the UV-Vis spectroscopy measurements of the dye solutions are plotted in Figure 2. The mass of dye absorbed onto the textile can be calculated from the decrease of dye concentration in solution. 8 Carlos G. Lopez et al.

Fig. 2 Dye concentration in solution as a function of time for diﬀerent starting concentrations of dye for system I. 0.1 g/L (◦), 0.05 g/L (), 0.01 g/L (), 0.005 g/L (4). Lines correspond to ﬁts to equation 3 with a = 1. Fit parameters are collected in Table 1.

Reflectivity measurements on the acceptor textiles then allows us to establish a master curve between dye adsorbed and UV-reflectance, as detailed in the S.I. The data are reasonably well modelled by Equation 3 for a = 1 and b ' 0.3 − 0.5, see Table I. Ovejero et al report values for a and b in the same range for the dyeing of polyester [21] at higher temperatures. Fits to Equation 1 are presented in the Supporting Information. We focus on the time-scales relevant to standard laundry procedures, and due to the lack of data at long times, we cannot reliably estimate the equilibrium exhaustion, particularly for high initial dye concentrations. This makes the estimation of the diffusion coefficient of dyes inside the fibres difficult, see the S.I. for further details.

−1 C(0) (g/L) C∞ (g/L) k (s ) b 0.1 8.6 ± 1.9 × 10−3 4 ± 2 × 10−4 0.52 ± 0.04 0.05 4.4 ± 1.1 × 10−3 6 ± 4 × 10−4 0.52 ± 0.07 0.01 8.6 ± 0.4 × 10−4 1.9±0.7×10−3 0.45 ± 0.05 0.005 4.1 ± 0.2 × 10−5 3.2±1.4×10−3 0.35 ± 0.06

Table 1 Kinetic parameters from ﬁts to Equation 3 with a = 1. See Figure 2 for ﬁts. Title Suppressed Due to Excessive Length 9

System II: Release of dye from the donor textile.

Fig. 3 System II. a: Concentration of dye in solution as a function of time for 1.5g of red textile washed in 20 mL of wash liquor. Circles correspond to an average over 6 experiments and crosses to single experiments. b: Mass of dye in 20 mL of solution for t ' 60 min as a function of the mass of donor textile. Line is a best ﬁt linear relation forced through the origin.

Figure 3a plots the concentration of dye in solution as a function of time for 1.5g of donor textile washed in 20 mL of wash liquor. The solid circles are averages from ' six repeated experiments (for each sample three measurements are taken) and the crosses are individual experiments (again with three 10 Carlos G. Lopez et al. measurements taken per sample). The error bars are calculated as twice the standard deviation from the different experiments. We would then expect the error bars for single experiments to be on average approximately twice larger. The scatter in the data however suggests our calculation overestimates the error bars. A rapid release of dye into the solution occurs at early times (t . 2 − 3 min), which is assigned to fast desorption of unbound dye from the PES fibres and its solubilisation within micelles [52]. This is followed by a steady increase in dye concentration over the next two hours. These results are similar to the data of Hazenkamp et al [5] for the dyeing of cotton with Reactive Black 5. Figure 3b plots the mass of dye in solution at t ' 60min as a function of the mass of PES donor textile. For 1.5g and 3g, the data corresponds to an average over multiple (' 6) measurements at t = 60 min. For lower textile masses we fit data in the 30 ≤ t ≤ 120 min range to a straight line and take the fit value at 60 min. An approximately linear relation is observed, with ' 0.06 mg of dye per gram of textile being released.

System III: Washing experiments with both donor and acceptor textiles.

Displayed in Figure 4a is a plot of mass of dye released from the donor textile as a function of PES mass after 60 min of washing, analogous to the dependence shown in Figure 3b for system II. A fit to a straight line yields a value of 0.1 mg of dye released per gram of textile, almost twice as much as in the absence of the acceptor. The higher dye release value compared to system II is expected as the acceptor textile removes the dissolved dye from the solution, thus shifting the equilibrium towards higher values of released dye. The total mass of dye leaving the donor textile is shown in Figure 4b. With the approach used here, it is possible to resolve the concentration of the released dye in solution and absorbed onto the acceptor textile within the same experimental system as a function of time. As for the case without acceptor textile, a rapid release of the dye is observed at early times. Absorption of the dye onto the acceptor textile on the other hand is a slower process which increases in an almost linear manner over the timescale studied, while the dye concentration in solution reveals a peak around t ' 15 min. Data for different amounts of donor textile as a function of time are plotted in supplementary Figure 3. The dye desorption kinetics from the donor textile in the presence and absence of the acceptor textile are compared in Figure 4c. In the presence of the donor textile, dye release occurs more rapidly, presumably because PA contin- ually removes dye from solution, hence producing a shift in the equilibrium. Figure 5 compares the mass of dye in solution (filled symbols) and on the acceptor textile (open symbols) for systems I and III, that is, using the technical powder and the donor textile as a source for the dye respectively. In the latter case the PES fibres represent a source of constant release of additional dye and the coloration of the acceptor textile happens more rapidly. Title Suppressed Due to Excessive Length 11

Fig. 4 System III. a: Mass of dye released from donor textile at t ' 60 min as a function of textile mass along with linear best ﬁt line, forced through the origin. b: Mass of dye as function of time for washing experiment with 1.5 g of donor textile, 0.2 g of acceptor textile in 20 mL of wash liquor. Black circles: mass released from donor textile, red triangles, mass absorbed onto the acceptor textile, green squares, mass in solution. Lines are guides to the eye. c: Mass of dye leaving donor textile. Blue symbols: System II, 1.5g of donor textile. Circles are averages over six experiments and crosses are single experiments. Black symbols: System III, same as part b, mass released from donor textile as a function of washing time. Lines are guides to eye. 12 Carlos G. Lopez et al.

This result suggests that the auxiliary dispersing agents present in the paste provide more eﬀective solubilisation and attenuation of the dye within micelles as compared to synthetic wash liquor. At long times of ' 2 hours, with a similar concentration of dye in solution, the amount of dye adsorbed on acceptor textile is twice larger when the PES textile is used a source. This observation indicates that for an eﬃcient inhibition of dye transfer an adjustment of the solubilisation capacity of micelles or other types of solubilisation agents is essential.

Fig. 5 Comparison between system I, initial concentration of dye C(0) = 0.005 g/L and System III, same as Figure 4b. Open symbols: mass of dye absorbed onto the acceptor textile. Full symbols: Mass of dye in solution as a function of washing time. Lines are guides to the eye.

5 Conclusions

We report a methodology to study the process of dye transfer between a donor and an acceptor textile, red polyester and white polyamide respectively in an aqueous mixture of nonionic and anionic surfactants. Using Disperse Red 60 coloration powder for standard dye solutions, we established a master curve Title Suppressed Due to Excessive Length 13 relating the textile reflectance data to the amount of the adsorbed dye. Ap- plication of two colorimetric analysis, UV-Vis spectroscopy and reflectometry, also allowed us to follow the change in the concentration of the stabilised dye in solution and the amount of dye adsorbed on the acceptor textile with time, thus addressing the role of the solubilisation efficiency in the inhibition of dye transfer. We conclude that at a given surfactant concentration the transfer of the dye obeys a two-stage transfer mechanism which is defined by the solubilisation capacity of the amphiphiles: within the first 15 minutes, dye is released from the donor textile and solubilised in the surfactant micelles, at longer times, the concentration of dye is solution decreases as more dye is adsorbed onto the acceptor textile.

Acknowledgements

The research was partially funded by Henkel, as part of the Innovation Campus for Advanced Sustainable Technologies (HICAST) project. The authors declare no conﬂict of interest.

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