Rayleigh scattering ratio

A modified fonn of equation (B 1.9.16) is usually used to express the scattering power of a system in tenns of the Rayleigh ratio defined as... 

The second method to calculate the scattered intensity or R the Rayleigh ratio) is to square the sum in I... 

Hence, the value of I/Iq is dependent on relative molar mass of the molecules involved in the light scattering. The Rayleigh ratio, Rg, may be defined as ... 

Equation (6.12) is important it shows that we can determine the relative molar mass of the molecule from the experimental measurement of the Rayleigh ratio in fight scattering. 

Values of are obtained partly by previous calibration using a series of standard light scatterers whose Rayleigh ratios have been precisely determined. Typical standards used in practice are poly(methyl methacrylate) blocks, colloidal silica suspensions, or tungsto-silicic acid, H4SiW 2O40-... 

The variable quantities in the K term, i.e. rig, (AnMcf, and X, must be determined. Values of are available for most solvents from the literature X is obtained by dividing the value of X by the refractive index of the solution. The refractive index increment, (dn/dcj, must be determined to within 10 in dn using a differential refractometer. The choice of solvent is limited if dn/dc = 0, there is no scattering if dn/dc is greater than 0.3 cm g the Rayleigh ratio is no longer proportional to (dn/dc). ... 

If the scattering intensity can be measured with sufficient accuracy down to very small angles, the Rayleigh ratio at zero angle may be... 

The total polarisability per unit volume of solute plus solvent has been invoked in Eq. (22) so that the dimensions of the Rayleigh ratio R0 in Eq. (23) are not cm2 as it appears, but actually cm 1. For pure liquids and polymers of small-moderate molecular weight, scattering at an angle of 90° is used and the Rayleigh ratio in such instances is given simply as... 

The expressions for scattered light intensity (and Rayleigh ratio) must be corrected by dividing by the appropriate Cabannes factor. Effectively this is equivalent to replacing the optical constant K as defined in Eq. (24) by Kf and by 2 Kfj for unpolarised and vertically polarised incident light respectively. 

Here the symbols R, P and G denote respectively the Rayleigh ratio, particle scattering function and instrument scattering reading. It is possible to take other angles such as 60° and 120°, which are also symmetrical about 90°. However, the angles 45° and 135° are most frequently selected, and the widely used Brice-Phoenix photo-... 

Rayleigh ratio of scattering intensity at scattering angle 0 particle scattering factor = normalized molecular structure factor... 

Note If the scattering intensity is not reduced to the Rayleigh ratio, the difference between the scattering intensities for a dilute solution and that for pure solvent is named excess scattering . 

Note Several modifications of the Zimm plot are in frequent use the most common one uses the excess scattering instead of the excess Rayleigh ratio. 

The observed ratio of detector intensities has to be corrected by a few factors in order to obtain the two Rayleigh ratios [10,15-18,41,42], Typical phenomena that must be corrected for are [10,41] (1) scattering volume, (2) refraction correction, (3) reflection correction, (4) absorption, (5) fluorescence, (6) polarization, and (7) sensitivity of photomultiplier. 

Light Scattering Photometry. We will limit our discussion to LALLS because it constitutes a net improvement as compared to other methods of Mw determination. It has the advantage of providing absolute values of the Rayleigh ratio by direct comparison of scattered and transmitted light. 

---