Light Scattering: Solutions of small molecules

The fluctuations are larger if a solute is present. If the solute molecules are small enough so that they can be considered point sources (which requires that they are smaller than lambda/20), the focus is on the excess scattering due to the presence of solute. The polarizability is replaced by the excess polarizability,

alpha = (n₀/2 pi) (d n/ d c) (M/L)

where n₀ is the refractive index of the solvent, and L is Avogadro's number. The Rayleigh ratio is

R_theta = K (1 + cos² theta) (N/V) (M²/L)

K = (2 pi²n₀²/ L lambda⁴) (d n/ (d c)²

Nonideality is introduced via the virial coefficients. This procedure measures M_w for a polydisperse sample.

R_theta = K (1 + cos² theta)[M_wc/(1 + 2 gamma₂c + ....)]

The scattering envelope, PC Fig. 10.8, shows equal intensities for forward and backward scattering.

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