Surface concentrations of dyes

Tt is the purpose of this paper to describe methods for determining and A interpreting dye spectra in aqueous dispersions of silver halides and other substrates. Such spectra can be utilized for the direct measurement of surface concentrations of dyes from which, in turn, the surface area of the substrate can be derived. The techniques involved are not limited to a specific dye class but will be illustrated in this paper by the behavior of cyanine dyes. 

Whereas the optical method indicates attainment of saturation coverage by the dye in its /-state, results obtained with the phase-separation technique show that, after reaching apparent saturation, further adsorption occurs as the dye concentration in solution is increased. The horizontal step observed with the phase separation measurement is in approximate agreement with the maximum surface concentration of dye in its /-state as determined by the optical method. Hence, it is concluded that, contrary to earlier suppositions (70), only the first layer of this dye is adsorbed in its /-state subsequent dye layers must be adsorbed in different states. 

It remains to be determined to what extent the dye adsorption technique is applicable to other substrates. No evidence was obtained for Pseudocyanine adsorption to Mn02, Fe2Os or to pure silver surfaces, although this dye can be bound to mica, lead halides, and mercury salts with formation of a /-band (61). Not only cyanines but other dye classes can yield surface spectra which may be similarly analyzed. This is specifically the case with the phthalein and azine dyes which were recommended by Fajans and by Kolthoff as adsorption indicators in potentio-metric titrations (15, 30). The techniques described are also convenient for determining rates and heats of adsorption and surface concentrations of dyes they have already found application in studies of luminescence (18) and electrophoresis (68) of silver halides as a function of dye coverage. 

Fig. 4.9. (a, top) The 8iph/iph vs. v 1 dependence for W03 electrode sensitized by Dye II in monomeric form ( ) partially aggregated by coprecipitation with PD IV (O). The excitation wavelength 560 nm. / = 20 s. The total surface concentration of Dye II 10 8 mol cm 2. Electrolyte 0.25 M Na2S04. (b, bottom) The potential-time programme and corresponding photocurrent-time curves used for x evaluation. Hatched areas indicate the exposure periods. 

Orientation of porphyrin molecules has not much influence on efficiency of the DSSCs. The PCE of the 3-AB-type porphyrin was higher than that of the trans-2A2B-type porphyrin and the ZnO semiconductor was superior to Ti02. However, the PCE values of all cells were similar, and raffier low, being 0.27 and 0.23 % for the cell 1 and 2, respectively (Scheme 29). This difference might originate from different surface concentrations of dyes on the MO surface. The 1 2 dye surface concentrations ratio was 1.25. 

For preparing lakes, a solution of aluminium sulfate (or chloride) is mixed with sodium carbonate, forming fresh alumina Al(OH)3. The colorant is then added and adsorbed on the surface of alumina. Usually the content of colorant in the lake ranges from 10 to 40%." The product is filtered, washed with water, dried, and milled. The product is allowed to contain unreacted alumina but must not contain more than 0.5% HCl-insoluble matter and not more than 0.2 % ether-extractable matter. - Lakes are insoluble in most solvents used for pure dyes, and they have high opacity and better stability to light and heat. Lakes impart their color by dispersion of solid particles in the food. The coloring properties of lakes depend on particles, crystal structures, concentrations of dye, etc. 

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. 

Hou (1992) used a simple screening test to determine whether acid and direct dyes precipitate at calcium concentrations typical of hard waters of the SE Piedmont region of the U.S. Of the 52 dyes tested, only three direct dyes (Direct Black 19, Direct Black 22, and Direct Blue 75) and seven acid dyes (Acid Red 88, Acid Red 114, Acid Red 151, Acid Brown 14, Acid Black 24, Acid Orange 8, and Acid Blue 113) precipitated. Although the Ca salts of acid and direct dyes were thought to be the most likely metal salts to precipitate after dye discharge to natural waters, the precipitation is not likely to occur unless dye concentrations exceed 0.02 to 0.6 mg/L, a level far greater than reported concentrations of dyes in surface waters. 

The rate of vatting depends not only on the concentration of dye and reducing agent but also on the crystal form, surface, and dispersion of the pigment (i.e., on its finish quality [51]). Leuco compounds are soluble in alkali. In the case of anthraquinoid vat dyes, the pH of the vat is about 13. At lower values the risk of vat acid sediments exists. Reduction is usually performed at 50-60°C. At higher temperature, over-reduction of certain dyes can occur (i.e., reductive destruction of the dye molecule). 

Expression (2.24) gives at least a qualitative description of the experimentally observed dependence of the initial quantum yield cpo of reaction on the concentration of MO acceptor. Indeed, the number (concentration) of dye ions MOad adsorbed at the surface of a colloidal particle before the light illumination, is a function of the MO concentration in a the solution and follows the dye adsorption isotherm at the same surface. Since MOad increases with increasing MO concentration, at surpassing of a certain concentration [MO], the ratio kj" K MO can appear to be much less that unity. If the Dad value is fixed, the second term of sum (24) becomes constant, and hence... 

The testing system (Fig. 1) was a 1.2 volume pressure apparatus made of metaplex (1). The har support covered with the membrane (2) of an effective surface area of 49.2 cm was fixed in the lower part of the apparatus. To maintain the dye concentration on the level required, continuous circulation of the permeate between the feeding tank (5) and the apparatus was applied. The solution was mixed with a magnetic stirrer (3) which prevents excess concentration of dye on the membrane surface. Pressure was generated by feeding the apparatus with an inert gas (nitrogen) from a cylinder (8). Samples for flow rate measurements and determinations of dye concentration in the permeate were taken through a stub pipe (4). 

Measurements of surfactant concentrations on travelling capillary waves is complicated by the rapid decay rate of these waves, necessitating measurements close to the source of wave generation. To avoid this complication, we utilized a field of standing capillary waves. The wave tank was a circular (6.99 cm, inner diameter) glass vessel. The inner wall was coated with paraffin to avoid loss of the surfactant to the tank side walls. Triply distilled water was used as the substrate. The tank was overflowed to clean the surface prior to spreading the insoluble hemicyanine surfactant mono-layer at a surface concentration of 0.288 pg cm"2. Hemicyanine, 4-[4-(dimethylamino)styrl]-l-docosyl-pyridinium bromide, is a stilbazolium dye molecule to which is attached on one end a saturated twenty two car-... 

The ions are delivered to the membrane by diffusion and electromigration. Because the membrane is not permeable for large dye molecules the concentration of dye increases near the membrane surface and reaches the limit of its solubility. Then, dye crystallization occurs and microcrystals get induced dipole moments. 

---