Substrate isothermal adsorption measurements

As there is no rigorous theory that can predict adsorption isotherms, the most suitable method for investigating the adsorption of surfactants is to determine the adsorption isotherm. The measurement of surfactant adsorption is fairly straightforward a known mass m (in grams) of the particles (substrate) with known specific surface area Ag (m g ) is equilibrated at constant temperature with surfactant solution with initial concentration Cj. The suspension is kept... 

Early woik has shown that upon noble gas adsorption the surface potential (woik function) of the substrate is lowered by as much as 1 eV (Table 9). These large values can be ascribed to the large polarizabihty a of the noble gas atoms (in particular Xe) and possible chemical contributions to the surface bond. The woik function change has also been used to measure the surface coverage, e.g., in isothermal adsorption experiments. An analytic relation, accounting for dipole-dipole interactions and mutual depolarization effects, is given by the Topping formula ... 

To ascertain the differences in physical properties such as pore sizes, pore volumes, and surface areas between unmodified and modified samples, pore size analysis and measurements were performed. The pore size analysis also serves to confirm the anchoring of the aminosilanes on the silica substrate. All the measurements were taken using N2 adsorption/desorption isotherms at 77 K using a Micromeritics ASAP 2020 surface area and porosity analyzer. 

A key material property of powders and fibers in particular is the specific surface area. The extent of adsorption from the vapor and liquid states on a solid surface is determined in part by the specific surface area of the solid. Typically, to determine the specific surface area, a gas adsorption isotherm is measured for example, the adsorption of nitrogen is measured on the substrate of interest at 77 K, the boiling point of nitrogen. The experimental isotherm is then analyzed by the BET (Brunauer, Emmett and Teller) model [42,43] to determine the monolayer capacity of the substrate. The specific monolayer capacity multiplied by the cross-sectional area of the adsorbed gas molecule gives the specific surface area. Amorphous silica gel may have a specific area of 200-300 mVg while carbon fibers may have a value of around 0.1 mVg. 

Molinard pointed out that the ions introduced can serve as specific adsorption sites and that this type of modification allows fine-tuning of the PILC substrate for certain gas adsorption applications [18]. By introducing Sr + in Al-PILC, the gas adsorption isotherms at 273 K and 5 x 10 Pa equilibrium pressure showed that the Nj capacity doubled fi-om 0.06 nunol/g on Al-PILC to 0.12 mmol/g on Sr-Al-PILC. The amoimt of cations in the PILC influences the adsorption properties. A higher cation loading results in a lower capacity but a higher N2/O2 selectivity on Ca-Al-PILC, as was proven by adsorption measurements. For the anion exchanged Cl-Al-PILC, more O2 than N2 was adsorbed (0.09 mmol/g and 0.07 mmol/g, respectively) at 273 K and 5x10 Pa. This affinity for O2 should result from a specific interaction with the Cl"-ions. The CO2 capacity decreased in comparison with the unmodified substrate from 0.5 mmol/g on Al-PILC to 0.2 mmol/g on Cl-Al-PILC at 273 K and 0.5 bar. After modification with other anions, Hke F and PO, the same effect was observed. 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

Surface Acoustic Waves (SA Ws). The basic idea of this technique is to use the dependence of the frequency and propagation of surface acoustic waves on mass loading in a film. The porous film has to be deposited on a piezoelectric substrate (quartz), which is then placed into a physisorption setup to condense nitrogen at 77 K. Adsorption and condensation of N2 result in a shift of the oscillation frequency, and thus measurements of the oscillation frequency as a function of N2 partial pressure provide an adsorption-desorption isotherm.30 Although the technique has proven to provide a concise characterization of porous films,29,30 the requirement for the deposition directly onto the SAW piezoelectric substrate represents a certain restriction. 

We can determine how much of the dye (the adsorbate ) adsorbs on a plate (the substrate ) by devising a series of experiments, measuring how much adsorbate adsorbs as a function of concentration. We then analyse the data with the Langmuir adsorption isotherm ... 

Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective singlelayer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin fihns prepared by the surface sol-gel process on 2-D substrates." " ... 

Adsorption at liquid surfaces can be monitored using the Gibbs adsorption isotherm since the surface energy, y, of a solution can be readily measured. However, for solid substrates, this is not the case, and the adsorption density has to be measured in some other manner. In the present case, the concentration of adsorbate in solution will be monitored. In place of the Gibbs equation, we can use a simple adsorption model based on the mass action approach. 

Heats of adsorption can be experimentally measured by calorimetry, temperature programmed desorption (TPD), and adsorption isotherms taken at different temperatures. Calorimetry involves the direct measmement of temperature rise caused by the adsorption of a known amount of gas on to a well-characterized surface. TPD is the most coimnon method of determining the heats of adsorption. In this procedure, molecules are adsorbed on to a clean well-characterized substrate at a fixed temperature. The sample is then heated in a linear fashion while the pressure of the desorbing species is monitored with a mass spectrometer. The desorption rate E (t ) is given by... 

Another point to check is adequate use of substrate concentration in analysis. Since a Langmuir isotherm is derived on the basis of the adsorption equilibrium between species adsorbed and desorbed in solution, the isotherm is a function of concentration of the adsorbate (molecules to be adsorbed on surfaces) in solution, not concentration of the adsorbate in feed. Therefore, the actual concentration, which must be reduced from that in feed due to appreciable adsorption, must be measured at least before photoirradiation. 

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