Methanol adsorption concentration effect

The other method used for liquid surfaces is the flow method of Kenrick (14) in which a jet of one solution is passed down the center of a tube whose walls carry a flowing layer of a second solution. The potentials between the flowing liquids are monitored with a quadrant or other electrometer. This method has been used with good results by Randles (15) and Parsons (16). Case and Parsons (17) compared the Kenrick and radioactive electrode methods for methanol-water mixtures. They found good agreement except at elevated methanol concentrations where methanol adsorption at the air electrode probably occurs. Measurement of the null current (compensation) potential in the Kenrick method is suitable for determining the surface potentials of solutions where rapid surface equilibrium occurs, but it is not convenient for spread monolayers or adsorbed films that have slow time effects. 

For a methanol concentration smaller than ca. 0.3 M, we obviously have in all cases the effect of a to low rate of methanol adsorption. In the region of... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

Which reaction channel is followed depends on the availability of neighboring free places. This could explain the effect of methanol concentration on adsorbate composition. It has been observed that the initial rate of adsorption is strongly enhanced by increasing methanol concentration [14]. From the adsorption steps given above, the first one, Eq. (2.7), is directly affected by the bulk concentration. At high methanol concentrations the Pt surface becomes very quickly covered with species like CH2OH or CHOH. Further reaction to a more stable state such as COH is inhibited because of the lack of free adjacent sites. Under these conditions CO should be formed with a greater probability. 

Humic substances were concentrated more than 50-fold on the XAD-4 quaternary resin, but a saturated HCl/methanol solution was required for the desorption. This eluant was not concentrated further because the concentration of humic substances could be measured directly with a spectrophotometer. Total recovery of humic substances was higher in the bench-scale experiments than in the pilot plant studies. On the basis of the pilot plant results (see summary of experiment 3), it appears that the adsorption of humic substances was affected by the higher velocity or the loading capacity because 458 was recovered in the effluent water. The higher velocity in the pilot plant studies did not have a similar effect on other compounds such as quinaldic acid, for example, which was recovered at almost 100. It is believed that caffeine, which was concentrated during bench-scale studies, was also affected by the higher velocity in the pilot plant studies. 

We have also compared the determination of the adsorption isotherms with those from a standard gravimetric technique. The precision of the data obtained by the gravimetric technique was found to be lower than that obtained by the FT-IR technique, particularly at very low silane concentrations (about 5%, and 3% at 0.4 g/100 ml concentration for the gravimetric and the FT-IR techniques, respectively). This is because very small weights of the silanes cannot be measured accurately and the silane is volatile. The effects of suspended silica particles in the solution were also examined by centrifuging methanol solutions... 

Determinations of the adsorption isotherms for a number of organic solvent-water systems in contact with hydrocarbonaceous stationary phases have shown that a layer of solvent molecules forms on the bonded-phase surface and that the extent of the layer increases with the concentration of the solvent in the mobile phase. For example, methanol shows a Langmuir-type isotherm when distributed between water and Partisil ODS (56). This effect can be exploited to enhance the resolution and the recoveries of hydrophobic peptides by the use of low concentrations, i.e., <5% v/v, of medium-chain alkyl alcohols such as tm-butanol or tert-pentanol or other polar, but nonionic solvents added to aquo-methanol or acetonitrile eluents. It also highlights the cautionary requirement that adequate equilibration of a reversed-phase system is mandatory if reproducible chromatography is to be obtained with surface-active components in the mobile phase. 

Variations in the selectivity are sometimes observed with the change in the type of organic modifier due to the specifics of the analyte-solvent interactions (solvation) and the specific adsorption behavior of the organic modifier. In the following example the effect of type and concentration of methanol and acetonitrile modifiers on the retention of acidic, basic, and neutral analytes is discussed. 

The increase in the bulk concentration of methanol from 0.01 to 0.1 M (Fig. 30) has a clearly measurable effect on adsorption rates at short times up to 30 min at 0.2 V and 90 min at 0.1 V. Above these time values, adsorption tails at a similar rate, showing a surface state limitation rather than kinetic limitation. 

In the case of electrosorption, it is best to use a dimensionless scale of C/C(sat) when comparing the adsorjTtion of different solute molecules in the same solvent, or the same solute in different solvents. This scale permits us to compensate for the differences in the free energy of interaction between the solvent and the solute, and the effects seen arise from the different interactions of the solutes with the surface. A good example is the adsorption of phenol on mercury from two different solvents, shown in Fig. IJ. The solubility of phenol in water is much lower than in methanol. It takes therefore a much higher concentration in methanol to reach a given value of the fractional coverage 0 than in an aqueous solution. 

TPA characteristics of two adsorbers of honeycomb type for various hydrocarbons were evaluated. In this study, methyl alcohol, acetone, acetaldehyde, 224 trimethylpentane, n-octane and toluene were chosen as the hydrocarbons of cold start. The effect of the hydrocarbon components and oxygen concentration on TPA behavior was studied. According to the precious metal loading and the presence of Ch, the adsorption and desotptioit amount were decreased, while the conversion efficiency of hydrocarbons was increased. In case of hydrocarbons with oxygen, the thermal decomposition appeared to be in the order of methanol, acetaldehyde and acetone. 

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