Adsorption adsorber dyes

On standing, gelatinous aluminium hydroxide, which may initially have even more water occluded than indicated above, is converted into a form insoluble in both acids and alkalis, which is probably a hydrated form of the oxide AI2O3. Both forms, however, have strong adsorptive power and will adsorb dyes, a property long used by the textile trade to dye rayon. The cloth is first impregnated with an aluminium salt (for example sulphate or acetate) when addition of a little alkali, such as sodium carbonate, causes aluminium hydroxide to deposit in the pores of the material. The presence of this aluminium hydroxide in the cloth helps the dye to bite by ad sorbing it—hence the name mordant (Latin mordere = to bite) dye process. 

In the absence of dyes, APA- and AdPA-grafted silica bind La(III) with, respectively, 0.20 and 0.27 mmol/g sorption capacity, resulting in formation of 1 2 (La L) complexes. 50% of introduced cation is bonded at pH=5 (APA), pH=6.1 (AdPA) and complete adsorption occurs at pH=6 (APA), pH=6.5 (AdPA). The grafted support in absence of La adsorbs the chosen dyes at pH<4 due to the electrostatic interaction with the -NH, groups on the surface, present as a result of grafting procedure. The adsorption of dyes at pH>4 is insignificant. 

When the MLC preparation involves either adsorption of dye on the SiOj-Lj-La eomplex, or adsorption of La +-dye eomplex on SiO -L it proeeeds via eoordination of dye to pre-formed grafted eomplex SiO -Lq-La without its deeomposition. However, in ease of pre-adsorbing dye on the surfaee, it bloeks off the ehelating groups. This results in formation of monoligand La-dye... 

The adsorption spectra in the UV-visible range of the extracts exhibit different broad bands depending on the component of the mixtures photocurrent measurements show larger bands in comparison with those derived from adsorbed dyes. In particular, in the case of Mix, a bathochromic shift of about 40 nm was observed the proposed rationale is a band interruption between density states of Ti02 and the HOMO-LUMO in the dye. Moreover, the photocurrent response derives predominantly from the superposition of those of Morus nigra and carminic acid. 

Adsorption of azo dyes by the biomass is considered as the first step of their biological reduction [39]. Because of adsorption, the dye is concentrated onto the biomass until its saturation the amount of adsorbed dye is then proportional to the amount of biomass [4CM-2]. Steffan et al. [43] observed that 68% Ethyl Orange was rapidly adsorbed on a microbial consortium immobilized in alginate beads, but only after the addition of glucose or starch the dye was effectively degraded. 

Davey, E. P. Optical sensitization and adsorption of dyes on silver halide The state of the adsorbed dye. Trans. Faraday Soc. 35, 323 (1940). 

Many substances preferentially concentrate at interfaces, including liquid/liquid ones. Although TIRF is most easily adaptable to solid/liquid interfaces, Morrison and Weber(100) succeeded in observing the preferential adsorption of certain amphiphilic dyes at the interface between two immiscible and optically dissimilar liquids. Steady-state TIR fluorescence polarization in that system showed that the rotational diffusion of the interfa-cially adsorbed dye was restricted. 

The close correspondence between the absorption spectrum in solution and the photocurrent spectrum of the adsorbed dye is by no means found in all cases. The adsorbed state can be different in structure from the solution state which is seen in a different photocurrent spectrum. It has been found e. g. that polymers are formed in the adsorbed state. This is a well known phenomenon for cyanine dyes 50-5b where polymer bands are found in the absorption spectrum of the adsorbed molecules. An example is given in the photocurrent spectra of Fig. 15. One sees that with increasing amount of adsorbed dye — no equilibrium adsorption was reached during this experiment — the polymer absorption band appears in the photocurrent. 

Typical results are shown in Fig. 44. The spectral threshold of the proper photoconductivity and the photo-emf of PAC is situated at 520 nm. The spectral response for the photo emf of PAC itself is shown by curve 1. After PAC has been immersed in an ethanol solution of methylene blue and dried its spectral response is represented by curves 2 and 2. The photo-response appears in the range of the absorption maximum of the dye at 680 nm characteristic of the monomolecular form in the dilute initial solution (curve 3). The observed enhancement of the second maximum at 620 nm in comparison to the solution spectrum is obviously connected with the presence of dye dimers. The shift of the maximum photoresponse to the longer wavelength by 15 nm relatively to the solution is usually the case for the adsorbed state. The sign of the charge carriers both in the proper and sensitized spectra ranges is positive. As seen in Fig. 44 the adsorption of the dye also markedly changes the proper photosensitivity of the PAC. When the monomolecular form of the adsorbed dye dominates, the... 

Note El The color change and coagulation coincide in dil solns. With higher concns of chlorides, the ppt tends to flocculate about % before the end point Note F A disadvantage of adsorption indicators is that Ag halides are sensitized to the action of light by a layer of adsorbed dye. For best results, the titration should be carried out with minimum exposure to light... 

The incorporation of a cationic azobenzene derivative, p-( a> -dimethyl-ethanolammonioethoxyj-azobenzene bromide, into nanoporous silica films and the photochemical reactions of the adsorbed dye were investigated. The nanoporous silica films were prepared from tetramethoxysilane and octadecyltrimethyl-ammonium chloride by the rapid solvent evaporation method which we have reported previously. The adsorption of the cationic azo dye was conducted by casting an ethanol solution of the dye onto the nanoporous silica films. Upon UV light irradiation, trans-azobenzene isomerized photochemically to the c/s-form and photochemically formed c/ s-form turned back to the frans-form upon visible light irradiation. The nanoporous silica films were proved to be an excellent reaction media to immobilize organic photocromic species. 

The loading amounts of the dyes, the pore size and surface modification are expected to affect the photoprocesses of the adsorbed dyes. In order to construct molecularly designed functional host-guest systems from nanoporous silica films, further study on the adsorption and the photoprocesses of the dyes is now underway and will be reported subsequently. 

The applicability of this in situ method for the determination of surface areas depends not only on knowledge of the dye s molecular area in the adsorbed state but also on the assumption that the chosen spectral parameter measures the surface concentration of the dye. In order to test the relation between adsorption of dye to silver halide and its spectral characteristics in the bound state, the behavior of Pseudocyanine in a coarse silver halide suspension (Dispersion D) was studied. This particular dispersion was chosen because some of its relevant adsorption characteristics had already been examined (22, 23). Moreover, observations by Boyer and Cappelaere with Pseudocyanine adsorbed on AgBr powders (5) indicated that /-band intensity varied with the amount of adsorbed dye and was not sensitive to the concentration of Ag+ or Br" ions in the range pAg 3.3-8.7. 

A characteristic increase of 90 after the horizontal section is apparently more pronounced when the potassium oxalate K2C2O4 is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield ratio depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. 

In this respect adsorption of dyes has been popular, especially because their colour allows rapid and accurate spectroscopic detection in the supernatant. However, caveats (i) and (il) above also apply to these adsorbates. For a discussion the reader may consult the proceedings of a symposium devoted to surface area determination in general containing a contribution by Padday in which the outcome of a comparative survey over 19 different laboratories was reported, dealing with the adsorption of the dye 1,1 - diethyl-2,2 cyanine on silver bromide and a few other adsorbents. Giles et al. have given a list of recommended a -values ... 

G.. M. Walker and L. R. Weatherley, Adsorption of dyes from aqueous solution - the effect of adsorbent pore size distribution and dye aggregation, Chem. Eng. J., 83, 201-206, 2001. 

Relative adsorption power is generally defined as (amount of adsorbed dye on imprinted gel)/(amount of adsorbed dye on control gel) at 20 /tM equilibrium concentration. For further details of the equation see reference [32]. 

Adsorption of dyes onto a semiconductor surface allows for another mode of photoactivation [44-48]. The dye adsorbs a photon, generating an excited state in which a sufficiently higher-energy orbital is populated to allow direct injection of an electron into the conduction band edge [1]. The dye thus becomes oxidized and can either react chemically with nucleophiles by bond formation or can be restored to its original oxidation level by electron transfer. In the latter case, the reaction partner is oxidized, regenerating the ground state of the sensitizer ready to participate in... 

Adsorption isotherms of cyanine dyes on silver halide crystals are consistent with Langmuir behavior, and indicate a close-packed monolayer of dye on the surface of the silver halide crystal [163]. This is supported by the observation of large shifts in the absorption spectra of the adsorbed dyes compared with the absorption spectra of the dyes in dilute organic solution. An example is shown in Figure 46. 

The redox potential for the dye can shift upon adsorption from solution due to coulombic or stronger covalent interactions with the solid substrate. This potential change can amount to several hundreds of millivolts. While n-type semiconductors cannot be used generally to measure oxidation potentials of adsorbed dye sensitizers by conventional cyclic voltammetry, reduction potential i°(S/S ) is often more accessible. Assuming oxidation and reduction potentials of the dye ground state on the surface are linked by a constant relation ... 

Other important organic electrolytes are the dye molecules. The adsorption of dyes is of interest largely because they are pollutants frequently found in textile wastewaters and because some of them were proposed as molecular probes to characterize the pore texture of carbon adsorbents. However, this last apphcation should be viewed with caution [1] because dye adsorption is profoundly affected by the carbon surface chemistry and solution pH. Thus, Graham [40] found a good linear relationship between a decreased uptake of the anionic metanil yellow and an increased carbon surface acidity. This author concluded that acidic groups on the carbon surface tend to reduce the capacity for anionic adsorbates in general. The adsorption of dyes was subsequendy investigated by other authors [1]. For instance, Nandi and Walker [41] studied the adsorption of acid and basic dyes on different carbon materials and found that the area covered by a dye molecule depended on the nature of the solid surface. 

ELECTROM TRANSFER BETWEEN ADSORBED DYE MOLECULES AND ORGANIC CRYSTALS Model Character of the Adsorption System for Certain Aspects in Photosynthesis... 

In the present system adsorbed dye molecules are excited by picosecond laser light and electron transfer takes place from the valence band of the substrate molecular crystals to the excited dye and concomitantly quenches the fluorescence (6,7). We are able to determine the electron transfer rate by measuring the fluorescence decay dynamics of the adsorbed dye. Figure 2 shows the energy-gap (a) and temperature dependence (b) of the rate constant of electron transfer in the adsorption systems calculated according to eq.[1], which is Sarai s three-mode-variant (38) of Jortner s original equation (43) ... 

AS less than unity for nonadiabatic and greater than unity for adiabatic transfer. Three systems are discussed in which one might anticipate adiabatic electron transfer at van-der-Waals contact (1) the heterogeneous adsorption system of crystal and adsorbed dye, (2) the homogeneous analog-system comprised of the crystal melt and the dissolved dye, and (3) the capped porphyrin system. 

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