Reduced adsorption Refractive index

The detection limit is generally limited by electronic and mechanical noise, thermal drift, light source instabilities and chemical noise. But the intrinsic reference channel of the interferometric devices offers the possibility of reducing common mode effects like temperature drifts and non-specific adsorptions. Detection limit of 10 in refractive index (or better) can be achieved with these devices which opens the possibility of development of highly sensitive devices, for example, for in-situ chemical and biologically harmful agent detection. 

It may not be appropriate to consider the scattering coefficient to be constant, and this point may become important in evaluation of trends monitored during a catalytic reaction experiment. Kortiim et al. (1963) pointed out that the scattering behavior depends on the ratio of refractive indices of the sample and the surrounding medium. As an example for a change in the refractive index of the sample, Kortiim described the adsorption of water, which reduces the scattering coefficient. Hence F(p) will increase, which could erroneously be interpreted as an increase in absorption. 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

The data presented in Sections 1.8 and 2.5.4 show that immersion of the powder into a transparent liquid with a refractive index close to that of the IRE can substantially increase the SNR of the ATR spectrum of the interfacial layer due to the increase of the MSEF at the interface. This approach also reduces scattering and so is particularly effective in studying in situ adsorption on powders. If the r/k < 10 condition is met, the refractive index of the composite layer (powder plus the immersion medium) may be evaluated within the framework of the EMT (Section 1.9), which allows one to calculate the value of cpc and simulate the ATR spectrum [175, 176]. 

If the small molecule has a lower refractive index than the carrier solvent and the probe a higher refractive index than the carrier solvent, the calculated level of amount of probe adsorbed will be greater than it really is. If however, both the probe and the small molecule have lower refractive index than the carrier solvent, the amount of probe adsorbed will be smaller than it really is. The extent to which this problem occurs also depends upon the solubility of the small molecule in the carrier solvent. This effect has cansed problems in a recent study [13] of adsorption of various carboxylic acids on to metal hydroxides. Figure 3.4 shows both the heat of adsorption and amount of carboxylic acid adsorbed. It is evident that adsorption of stearic acid (octadecanoic acid) and isostearic acid (16-methyl heptadecanoic acid) from toluene affords apparently reduced levels of adsorption, relative to when adsorbed from heptane. This is due to both the probe and the water having lower refractive indices than toluene. DRIFTS analysis of the respective filler samples retrieved from the FMC cell, however, indicates similar levels of adsorption. Only in the case of adsorption from heptane is the level of adsorption in concordance with theoretical values. This is due to reduced solubility of water in heptane, relative to toluene. 

Fig. 12 sketches the model for the interface. At lower surface coverage most of the ions are spread out within the diffuse layer leading to a pronounced refractive index profile. The situation is sketched in Fig. 12a. At higher concentration some ions enter the adsorption layer and form ion pairs with the headgroups accounted with a lower dn/dc-value as compared to the bulk phase. The formation of this Stern layer effectively reduces the surface charge and as a consequence the extension and the magnitude of the refractive index profile of the diffuse layer decrease. This situation is sketched in Fig. 12b.

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