Cabannes factor

Experimentally the Rayleigh ratio for benzene at 90° has been observed to equal about 1.58 X 10 m" under the conditions described in this example. By Eq. (10.6), r = (167t/3) so the value of R corresponding to this calculated turbidity is Rg = 5.41 X lO" m". The ratio between the observed value of Rq and that calculated in the example is called the Cabannes factor and equals about 2.9 in this case. 

When using vertically (v) polarised incident light the Cabannes factor f is given by... 

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. 

Fig. 3. Cabannes factor as a function of molecular weight for solutions of polyethylene glycol in methanol33) at T = 25 °C and X0 = 546 nm. [Inset - refer to Section IV.2 Dependence of Cabannes factor on concentration for solutions indicated in Fig. 59)...

One of the most elegant LS studies on liquid mixtures is that of Sicotte and Rinfret17 and it will be instructive to summarise their approach solely with regard to that aspect which is concerned with molecular weight determination. Liquid 1 will be considered as solvent and liquid 2 (of ostensibly unknown molecular weight M2) as solute. The Rayleigh ratios implied are the isotropic ones, which are obtained for liquid 1 as well as for solutions (subscript 12) via the measured Cabannes factors [Eqs. (44) and (45)]. 

Cabannes factor analy chem An equational factor to correct for the depolarization effect of the horizontal components of scattered light during the determination of molecular weight by optical methods. ko banz. fak-tor cacodyl orgchem (CH3)2As A radical found in, for example, cacodylic acid, (CH3)2A-sOOH. kak-3 dil ... 

Compared to the colligative methods light scattering can yield information on a possible dissymmetry of the aggregates. For anisotropic particles the direction of the electric field associated with the incident light may not coincide with the shift of the electron cloud. The intensity of light scattered at (usually) 90° from anisotropic aggregates is increased over the value predicted on the basis of isotropy by the Cabannes factor. 

Another method for correcting for optical anisotropy has been described by Stacey (1956) and applied to lignosulfonates (Forss and Stenlund 1969). It involves essentially the multiplication of the measured Kc/AR(0) values by the Cabannes factor, expressed for the case in which the incident light is vertically polarized (Armizadeh and McDonnel 1982), as ... 

Cabannes factor 500 carbohydrate content of lignin pepara-tions 73, 306, 307, 343, 413, 421 carbon-13 nuclear magnetic resonance spectroscopy, C NMR 43, 146, 250-273,... 

Equations (54) and (55) neglect the effects of depolarization which if appreciable must be allowed for. If the depolarization is due to anisotropy alone then the corrections discussed in a previous section can be applied. However, additional contributions to the depolarization arise from the effects of size alone, so that the use of the Cabannes factor for a large refracting particle would result in an overcorrection. For a discussion of these complications, see Zimm, Stein and Doty (1945) and Lotmar (1938 a, b). Doty(i94< ) has given a helpful general discussion. 

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