The linearity of the data at constant temperature is consistent with the Cauchy expression, which predicts that the refractive index will be proportional to l -2. The figure below shows the wavelength dependence of the refractive index values for pure water and toluene at 20 C. The dependence of the refractive index on density is described by the Lorentz-Lorenz formula shown below, where R is the molar refraction, M is the molecular weight, and r is the density. Hence, the refractive index of the solvent is strongly correlated with the solvent density. If the solvent molecules are more densely packed, the magnitude of the decrease in light velocity is proportional. On a molecular level, the decrease in the velocity of the radiation in the solvent arises from the electromagnetic interactions of the light wave with the solvent molecules. Therefore, since the velocity of light in the solvent is wavelength dependent, the measured solvent refractive index will be dependent upon the same.
![malvern zetasizer add refractive index malvern zetasizer add refractive index](https://media.biocompare.com/m/37/product/10394241-400x300.jpg)
According to Snell’s law, the magnitude of the refraction, and hence the velocity of the radiation in the solvent, is dependent upon the wavelength of the incident beam.
![malvern zetasizer add refractive index malvern zetasizer add refractive index](https://nano.indiana.edu/wp-content/uploads/2016/01/image3-3.jpg)
As the light passes through an interface into a medium of higher density, say for example from a vacuum into a solvent, the decreased velocity of the radiation in the solvent causes the light to be refracted or bent. The refractive index (ñ) of a medium is defined as the ratio of the speed of light in a vacuum to that in the medium. It should also be noted, that since the error is additive, multi-component mixtures can be particularly problematic.įigure 2: Effects of additives on the medium refractive index and the subsequent error in the diffusion coefficient arising from use of the solvent refractive index value. As evident in the figure, the error in the diffusion coefficient can become significant at higher additive concentrations. Included in the figure is the calculated %error in the diffusion coefficient arising from the use of the refractive index of the base solvent water (ñ = 1.333), rather than the refractive index of the medium. W% (in water) data for typical additives, extracted from the 77 th edition of the CRC Handbook of Chemistry and Physics. The figure below shows a compilation of ñ D 20 vs. For aqueous solutions, with the water base having a refractive index of 1.333, a 0.5% error is equivalent to +/- 0.007, which suggests that one needs 2 decimal precision in the medium refractive index. So if one were willing to accept a 1% error in the diffusion coefficient and hence a 1% error in the derived particle size, we would need to ensure that we had no more than 0.5% error in the refractive index. As shown in the derivation below, the relative error in D is twice the relative error in. In order to quantify the effects of additives on the medium refractive index, it is useful to examine the error in the diffusion coefficient arising from error in the refractive index. Theoretically then, the refractive index that should be used is that for the medium within which the particle is diffusing, i.e.
![malvern zetasizer add refractive index malvern zetasizer add refractive index](https://blog.sysmex.nl/hubfs/Zetasizer_Ultra_Background.jpg)
The scattering vector is dependent upon the dielectric properties of the medium, with ñ o entering the expression via the Clausius-Mosetti equation, which relates the polarizability to the permittivity of a molecule. As seen in this expression, the solvent refractive index dependence of the particle diffusion coefficient and the subsequently calculated size is introduced through the scattering vector.Ī common misconception is that the subscript in ñ o indicates that the refractive index to be used is that for the pure/base solvent. The particle diffusion coefficient is related to the lifetime ( G = 1/ t ) of the exponential correlogram and the scattering vector as shown in the expression given below, where l o is the vacuum wavelength of the incident light, q is the scattering angle, and ñ is refractive index of the medium within which the particle is diffusing. An example correlation curve and the exponential fitting expression is shown in Figure 1, where I is the measured intensity, t is the time, t is the delay time, B is the baseline in the limit of t = ¥, A is the amplitude or intercept of the correlation curve, D is the translational diffusion coefficient, and q is the scattering vector.įigure 1: Example correlation curve showing exponential fitting expression. In a dynamic light scattering (DLS) measurement, the hydrodynamic size is calculated from the particle diffusion coefficient, which is extracted from an exponential fitting of the measured correlation curve.
![malvern zetasizer add refractive index malvern zetasizer add refractive index](https://data2.manualslib.com/first-image/i31/153/15249/1524845/malvern-mastersizer-3000.jpg)
Is_it_OK_to_estimate_the_solvent_refractive_index_ Is it OK to estimate the solvent refractive index?