The Group of Thermooptical Spectroscopy of Dr. M. Proskurnin

1. Direct determination of adsorbed substances at glass surfaces.
2. Adsorbtion kinetics
3. Adsorption isoterm
4. Brief conclusions

Direct Determination of Adsorbed Substances at Glass Surfaces

The possibilities of TLS in studying the adsorption of iron(II) tris-(1,10-phenanthrolinate) (ferroin) am nickel(II) dimethylglyoximate at sorbents with low capacities (chemical glass, Pyrex, and quartz) are shown. The surface concentration of ferroin adsorbed from its concentrated solution calculated from thermal lensing, 1.25×10^–10mol×2, agrees well with the value from the model of an adsorbed monolayer of ferroin molecules. The detected absolute amount of ferroin (at the area irradiated by the excitation beam) is 2.4×mol. The surface of a ferroin molecule calculated from the experiment is 1.31 nm², which is in a good agreement with the calculated by MOPAC resulting in 1.32 nm². The limit of detection of ferroin under these conditions was estimated as 2×10^–11 mol×cm^–2 absolute 4.1 10^–15mol).Glass samples were analysed in parallel using scanning electron-probe microanalysis. It is shown that thermal lensing can detects changes in the adsorption at the glass surface at the level of a monolayer of ferroin, which is comparable to the sensitivity of electron-probe microanalysis in these conditions. A combination of these two methods for characterization glass surfaces with adsorbed (and also bonded substances) seems promising as they give complementary information: electron-probe microanalysis gives a whole picture of a sample with qualitative map of adsorbed substance, while thermal lensing provides a way of semiquantitative or even quantitative information on the concentrations of the adsorbed substance with the locality determined by the spatial dimensions of the excitation beam. For instance, thermal lensing can be used for estimating the uniformity of the adsorbed (bonded) layer in the scale corresponding the linear dimensions of the thermal lens. Measurements were made using a CamScan 44 scanning electron microscope with a thermal-emission tungsten cathode. Due to very low conductivities of the samples, measurements were made using under acceleration voltage of 2 kV. Secondary electron images were obtained with magnification from 200 to 1500

Adsorbtion Kinetics

At the first stage, we studied the kinetics of sorption of ferroin on laboratory glass. It was found that the time of attainment of sorption equilibrium depends on the starting concentration of the test substance in the solution (see table).Dependence of time of attainment of absorption equilibrium on the starting concentration of iron(II) as 1,10-phenanthrolinate in the test solution (n = 3).

At low concentrations only the most thermodynamically favourable active surface groups participate in the adsorption process. As a result, the concentration of the chelate near the surface rapidly decreases. Thus, the rate of the adsorption process is determined by diffusion only. With an increase in the starting concentration the adsorption process involves less and less favourable surface groups, the ion exchange becomes the rate-determining step, which increases the time of attainment of the equilibrium. While the shape and the time of attainment of the equilibrium changes from test-tube to test-tube, but the equilibrium concentration corresponding for the given starting concentration of ferroin almost does not depend on the certain test-tube. This confirms that the kinetic of the process depends on the properties of the surface of the given test-tube, while thermodynamic parameters depend on the macrocomposition of the glass, which is the same for all the test-tubes. An increase in the starting concentration of ferroin increases the deviation of the equilibrium concentration, which also confirms that less favourable surface groups start participating in the adsorption process.Fig. 1. Dependence of the concentration of iron(II) tris-(1,10-phenanthrolinate) in solution; T = 20°С, l = 514 nm, Р = 80 mW, 1 is adsorption on a newly used test-tube, 2 is adsorption on a previously used surface. A repeated procedure of adsorption on the surface of the same test-tubes shows much lower adsorption (see Fig. 1). The repetition of this process for four times (the initial concentration of iron(II) is 50 ng/mL) results in a final constant value of adsorption (surface concentrations of ferroin is 3×mol×cm2, which is about 0.1 of a monolayer. Thus, treatment of glass surface with ferroin for four times results in a twofold decrease in adsorption, which probably results from irreversible saturation of the part of adsorption centres at the glass surface. It should be noted that the sensitivity of thermal lensing in studying adsorption at glass and quartz surfaces are 100-fold higher than diffuse-reflectance measurements under the same conditions. The limiting value of the analytes adsorbed at the surface that can be measured by thermal lensing under these conditions is about 1×10^–11mol×cm2 for ferroin and 3×10^–11mol×2 for nickel(II) chelate, which corresponds approximately to 0.01 of a monolayer.

Adsorption Isoterm

As expected, an increase in the starting concentration of iron(II) chelate results in a decrease in the percentage of the adsorbed substance (Fig. 2). We used several models for monomolecular adsorption and the BAT model of polymolecular adsorption. The isotherm is most closely described with the Freundlich equation (see Fig. 3).>Fig. 2. Dependence of adsorption a (%) of iron(II) tris-(1,10-phenanthrolinate) on the surface of glass test-tubes on its initial concentration in solution; T = 20°С, l = 514 nm, Р = 80 mW. Freundlich isotherms correspond to the case of monomolecular adsorption at a surface with an exponential distribution of adsorption centres by adsorption heats, and, as a rule, this model is used for describing adsorption processes with medium concentrations of the adsorbate in solution. Thus, it also confirms our hypothesis about the participation of adsorption centres with various values of adsorption heat in the total process. However, as a whole, adsorption on the surface of laboratory glassware is a more complicated process than that described by the Freundlich isotherm.We think that low-capacity adsorbents hardly obey the only process of monomolecular sorption. The existing data are still insufficient for making more detailed conclusions, however, some qualitative aspects can be discussed. For instance, an increase in the slope of the function of adsorption on the starting concentration when changing from low initial concentrations of adsorbate to higher levels may result from a change from a monomolecular-type adsorption to polymolecular adsorption. Fig. 3Adsorption isotherm of iron(II) tris-(1,10-phenanthrolinate) on the surface of glass test-tubes; T = 20°С, l = 514 nm, Р = 80 mW, the range of starting concentrations of iron(II) tris-(1,10-phenanthrolinate) in solution is 10–50 ng/mL (calculated as iron(II). >We have found that the total adsorption have reversible and irreversible constituents. The irreversible adsorption takes place during only the first measurement of a new sample, and the further measurements with the same test-tube showed only reversible adsorption. This suggestion is supported by the fact that the first adsorption curve is the most pronounced (Fig. 1), and all of the subsequent curves for the same sample showed much lower and reproducible adsorption value just about 0.5 ng×. A procedure was developed for glassware treatment for elimination of adsorption. The 24-hour treatment by a concentrated nitric acid was enough to destroy adsorption ability of glass so our samples showed no adsorption later. The practical result of this stage is a procedure for glassware treatment for eliminating sorption effects in thermal-lens determination of nanogram amounts of iron in aqueous solutions.

Brief Conclusions

• Thermal lensing can be used for non-destructively investigating the sorption on glass surfaces, which is currently studied only by several, and specific, methods like radionuclide etc. Thermal lensing can give us a rather large set of information, both fundamental and practical.
• Thermal lensing was used for the characterisation of adsorption at laboratory glassware surface at nanogram level of analytes in solution. It was found that the initial concentration of the adsorbate and the surface properties of individual test-tubes affect the time of attainment of the adsorption equilibrium.
• The sorption capacity of the glass for test substances (ferroin and nickel dimethylglyoximate) was characterized, which was found to be several ng×cm^–2
• Possible models of sorption are proposed, which is most closely detailed with the Freundlich equation for monomolecular adsorption.