Solvents having the same refractive index and

Table 11.12 Solvents Having the Same Refractive Index and the Same Density at 25°C...

If paraffin permeates a white opaque paper, one can read letters through the paper because the paper becomes transparent. This phenomenon is based on the simple principle that micropores in the paper are filled with paraffin, which has a refractive index that is close to that of cellulose. If the porous PVA-PVAc composite film is soaked in organic solvents having the same refractive indices as that of PVA, the porous film is expected to become transparent again, according to the same principle as the phenomenon between paraffin and cellulose. On the basis of this consideration, subsequent experi-... 

For accuracy in light-scattering measurement the proper choice of solvent is necessary. The difference in refractive index between polymer and solvent should be as large as possible. Moreover, the solvent should itself have relatively low scattering and the polymer-solvent system must not have too high a second virial coefficient as the extrapolation to zero polymer concentration becomes less certain for high A2. Mixed solvent should be avoided unless both components have the same refractive index. 

This development assumes that for a polydispersed polymer in a single solvent, all polymer components have the same specific volume and the same refractive index and that dn/dw is independent of the molecular weight distribution. Generally the chemical potentials determined from this technique are slightly higher than values determined by other methods. The difference, however, is believed to be within the experimental error. The method is applicable over a concentration range from 0 to 80 weight percent polymer. 

Commercial instruments based on two different principles are available. One uses a vertically divided cell. The top view is shown in Figure 2.52a. One of the chambers contains a reference fluid, typically the pure solvent, and the other chamber contains the sample solution. A laser beam passes the divided cell twice before reaching a two-part photodetector. The detector is placed so that the beam hits the two parts equally when the two chambers of the cell have the same refractive index. A difference in the refractive index in the chambers deflects the beam, resulting in unequal intensities on the two parts of the detector. Thus the imbalance of the two intensities gives the refractive index difference An. 

Although cannot be measured in DLS, a closely related tracer diffusion coefficient Dj can be measured. In the tracer diffusion, the motion of a labeled solute called a probe or a tracer is traced selectively. A second solute called a matrix is added to the solution and its concentration is varied, whereas the concentration of the probe molecules is held low. The matrix must be invisible, and the probe must be visible. We can give a large contrast between the matrix and probe by choosing a pair of solvent and matrix that are nearly isorefractive, i.e., having the same refractive index. Then, the light scattering will look at the probe molecules only. For instance, we can follow the tracer diffusion of polystyrene in a matrix solution of poly(dimethyl siloxane) in tetrahydrofuran. 

Another technique for estimating the dipole moment in the excited state, suggested by Ito [35] is the study of the solvent shift in a mixture of two polar solvents that have practically the same refractive index, but quite different dielectric constants, and also, do not tend to form hydrogen bonds. Under these circumstances, when the interactions between the fixed dipole of the solvent and the induced dipole of the solute can be neglected, and when the contributions of the dispersion interactions are identical, the following interaction is operative [26, 35]. 

Diastercomers have different physical properties different melting points, boiling points, solubilities in a given solvent, densities, refractive indexes, and so on. Diastereomers differ in specific rotation they may have the same or opposite signs of rotation, or some may be inactive. 

While the technique of refractive index detection is less sensitive than UV or MS detection, it represents yet another useful, highly developed, quantitative online detector. RI detectors can perceive the difference in refractive index between a reference solvent and a solution with one or more analytes. Because of the comparative nature of the signal, a reference solution is necessary and gradient operations can be carried out only with very precise controllers to ensure that the reference cell and the sample cell have the same mobile-phase composition at any given time. 

The Hamaker constant can be evaluated accmately using the continuum theory, developed by Lifshitz and coworkers [40]. A key property in this theory is the frequency dependence of the dielectric permittivity, e( ). If this spectrum were the same for particles and solvent, then A=0. Since the refractive index n is also related to t ( ), the van der Waals forces tend to be very weak when the particles and solvent have similar refractive indices. A few examples of values for for interactions across vacuum and across water, obtained using the continuum theory, are given in table C2.6.3. 

The solvents used in analytical TREF are limited to chlorinated solvents, mainly ort/io-dichlorobenzene and 1,2,4 tri-chlorobenzene (perchloroethylene and a-chloronaphtalene have also been used), which can dissolve the polyolefins at high temperature and are transparent enough in the IR region of measurement. These solvents are the same as used in GPC/SEC analysis of polyolefins and are also appropriate for detection by refractive index, although this detector has not... 

Other important equilibria that affect absorbance involve acid-base forms of a species (such as pH indicators) and metal-ion complexes. With all equilibria there will be one wavelength, known as the isosbestic point, where the absorbances of both species are equal because they have the same e value. The nature of the solvent (and the sample cell) and refractive index changes can also cause a positive deviation and this should be checked as part of the measurement protocol for the blank. 

Thus the birefringence of the liquid crystal is much more critical in the fluorescence polarization studies. Measurements of the order parameter of excited molecules are possible if the rotational relaxation time of the solute is small compared with the lifetime of the emitting state. The birefringence of the liquid crystal is also of critical importance if one wishes to determine the solvent order from the absorption polarization spectrum of the solvent in its nematic phase. The complications arise from the strong refractive index dispersion in the wavelength regions of the solvent absorption bands. Saupe and Maier [104] have treated this case. The same difficulties arise of course if the solute and the solvent absorption overlap. 

In practice, surfrctant is always present during emulsification, and it is interesting to know whether coalescence then is a frequent phenomenon. To study this, a method for determining coalescence during emulsification has recently been developed in the authors laboratory. In brief, equal amounts of two OAV emulsions (I and 2) are mixed, which are the same in composition and droplet size distribution, except for the oil phases, which have different refractive indices, n, and n2-The mixture is then emulsified again. It is subsequently diluted with a solvent to yield for the continuous phase a refractive index of ( , -I- n2)/2. This dilute emulsion will have a certain turbidity if the oils 1 and 2 are unmixed, whereas turbidity will be virtually zero when the two oils have become completely mixed by... 

To deduce the weight distribution of the molecular weight, the chemical composition, and/or the size, Rh, from the distribution function G(T), we have to know the relationship of Rh to M and / as well as the relationship between M and / We need good models for these relations. To solve the At-/relation, the contrast change with different solvents may be helpful. An example of the trials for copolymers is as follows. We assume that the chemical composition is the same for molecules having the same molecular mass, so that we can write the weight fraction, w, and the refractive index increment as a function of M only, that is, w(M) and v(M), respectively. Furthermore, if eqn [99] holds for the copolymer, then we can evaluate the apparent weight fraction, u/app(M), expressed as... 

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