Dye absorption

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

Ion-Dipole Forces. Ion-dipole forces bring about solubihty resulting from the interaction of the dye ion with polar water molecules. The ions, in both dye and fiber, are therefore surrounded by bound water molecules that behave differently from the rest of the water molecules. If when the dye and fiber come together some of these bound water molecules are released, there is an increase in the entropy of the system. This lowers the free energy and chemical potential and thus acts as a driving force to dye absorption. 

In synthetic fibers the number of ionic groups or dye sites is relatively small, and may have been introduced dehberately to make the base polymer dyeable. The restrictions on dye absorption are therefore very great the dye molecule must find an available specific site from among the limited number of sites in the fiber. This situation follows a Langmuir isotherm, where the reciprocal of dye in fiber 1 /DF is direcdy proportional to the reciprocal of dye in the dyebath 1 /HT. A plot of 1/against 1/H therefore gives a straight line. 

Aufzieh geschwindigkeit,/. Dyeing) absorption rate, -krticke, /. Brewing) rouser. -ver-mdgen, n. Dyeing) absorptive power, sub-stantivity. 

After a few minutes irradiation with a high pressure mercury lamp at about 50 C, a rather complete cover of grafted acrylic acid, acrylamide and other vinyl monomers could be obtained. In later experiments a continuous grafting method has been developed where a tape or a fiber bundle after suitable pretreatment is grafted by UV irradiation for a few seconds. Homopolymer formed is removed by washing and grafted polymer analyzed by dye absorption, IR reflection and ESCA spectroscopy. 

Polo and Murakami Iha used anthocyanins extracted from jaboticaba (Myrciaria cauliflora Mart) and calafate (Berberis buxifolia Lam) as dyes for DSSCs. [46] The interaction between the dye molecules and Ti02 was identified by comparing the visible absorption spectra of the bare dye in solution with those acquired after dye absorption on the semiconductor a 15 nm red shift indicated the anchorage of the anthocyanin molecules on the Ti02 nanoparticles. The inorganic semiconductor layer was deposited on ITO and the electrolyte employed was I /I3 dissolved in acetonitrile. The photovoltaic cell obtained with the jaboticaba extract gave an IPCE value of 0.2 with a short-circuit current (/sc) of 7.2 mAcm 2, a Voc of 0.5 V and a fill factor of 54%. 

For comparison, the solution quantum yield was determined by the merocyanine dye technique. Acetonitrile solutions of triphenylsulfonium hexafluoroantimonate were irradiated with a 5 m. /cm2 dose. Dye solution was added and the acid content was determined by changes in dye absorption. The quantum yield for acid production was determined to be 0.8, which agrees reasonably well with the value (0.71) determined for the hexafluoroarsenate salt (8). 

Figure 2. Bleaching of merocyanine dye absorption on addition of fractional equivalents of trifluoroacetic acid.

Fig. 1 Spectra of QDs and organic dyes. Absorption (lines) and emission (symbols) spectra of representative QDs (a-c) and organic dyes (d-f). Reprinted by permission from Macmillan Publishers Ltd Nature Methods [1], copyright (2008)...

Each product is tested for interaction absorbance at 630 nm (the same wavelength used for green dye determination). If product interferes with the detection of green dye at 630 nm, the limit for green dye absorption may be adjusted to compensate for this interference. Acceptance criteria for green dye ingress are that the absorbance of the sample is less than or equal to zero or NMT 0.002 in absorbance difference between the negative control sample and the test sample. The acceptance specification was set at two times the sensitivity of the method per USP <1225> Validation of Compendial Methods. ... 

As already shown, the sensitized spectra follow the dye absorption. Most of the dyes have sufficiently narrow absorption bands. This does not permit us to obtain the panchromatic sensitivity in the sufficiently broad spectral range. It was proposed to use the polymers with conjugated bonds as sensitizers [21]. The broad diffuse absorption spectra are inherent to such compounds. One can expect higher thermal stability from such sensitizers. In addition the application of binder may be omitted from the preparation of the photosensitive layers, for example, in electrophotography. Polymers with triple bonds, polyphenylenes and polyoxiphenylenes were used as sensitizers [10, 14, 278-280]. The typical results are shown in Fig. 47. The main rules for photoconductivity sensitized by polymers were the same as for the dyes. Optimum sensitization was obtained at the concentration of the sensitizer of 10 1-10-2 g/cm3 relative to the polymeric photoconductor weight. 

Steady-state UV-visible absorption spectrum of the NKX-2311/ZnO film is also shown in Fig. 1. It was found that adsorption of the dye onto ZnO and TiC>2 (data not shown) leads to the spectral blue-shift of the dye absorption by 15 and 25 nm, respectively, and slight broadening compared with the spectrum in solution. When a bare ZnO film (without dye) was excited at 355 nm with the nanosecond laser, an absorption band shown by the dashed line in the same figure was observed in the near IR region. This band is assigned to intra-band transitions of electrons in the conduction band [10], Electrons in Ti02 showed weaker absorption in the near IR region. 

B-74MI11202 I. D. Rattee and M. M. Breuer The Physical Chemistry of Dye Absorption ,... 

For Class 2 dyes, absorption of a photon causes photoinjection of a hole into the valence band by transfer of an electron from the valence band to the now vacant Sq level of the excited molecule. The dye molecule retains the electron that has been excited to the level and is in effect a dye radical with an excess electron. In the absence of oxygen or other agent that could react with the radical, a thermally assisted transfer of the electron to the conduction band can occur. The time frame during which this transfer could occur is not limited by the normal lifetime of the excited state, as it is in the direct transfer of an electron in the Class 1 dyes. The time available could be much longer, limited only by the occurrence of some other reaction of the dye radical. 

Diffusion Rate is the rate at which dye is transported to the interior of the fiber material. A high diffusion rate leads to rapid establishment of dyeing equilibrium and rapid leveling out of irregularities in dye absorption. It also facilitates washing off the hydrolyzed dye. Increased dye substantivity is frequently combined with lower diffusibility. The diffusion rate doubles with a temperature increase of 10-20°C. 

Because of the strong temperature dependence of exhaustion and the poor migration properties, dye absorption must be as level as possible from the start. Correspondingly, attention should be paid to uniform packaging, package density, liquor flow or movement of textile material, and controlled machine conditions. 

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