![]() ![]() The intensity of light scattered by a molecule, measured by means of a DAWN ® or miniDAWN ® multi-angle light scattering (MALS) detector, is directly proportional to the molar mass. Simply by doubling the molar mass, even while keeping the mass/volume concentration the same, the intensity of the scattered light doubles. The net result is that the scattered light from the dimer is twice as intense as the total light scattered from two independent monomers. In other words, the scattering is coherent. The scattered light from one monomer now has a definite phase relationship with the scattered light from the other. However, once the monomers form a dimer, the two monomers move together. Averaged over time, the scattered intensities add as one expects classically: 1 + 1 = 2. This imparts a randomness to the phase of the scattered light such that the light from the two separate monomers is incoherent. The initially separate monomers are constantly buffeted by solvent molecules, and undergo random motion known as Brownian motion. Hence, the intensity of the scattered light is proportional to the concentration of the macromolecules in solution twice as many molecules scatter twice as much light.Ĭonsider now the important case where two monomers aggregate to form a dimer in solution. When there are many macromolecules in solution, each macromolecule scatters light via the aforementioned induced dipole mechanism. This may be determined from a measurement of the change, Δn, of the solution's refractive index n with the molecular concentration change, Δc, by measuring the dn/dc (=Δn/Δc) value using an Optilab ® differential refractometer. Therefore, in order to analyze the scattering from a solution of such macromolecules, it is necessary to know their polarizability relative to the surrounding medium (i.e., the solvent). The more polarizable the macromolecule, the larger the induced dipole, and hence, the greater the intensity of the scattered light. The intensity of the radiated light depends on the magnitude of the dipole induced in the macromolecule. This oscillating dipole will re-radiate light, much like the antenna for a radio station sends out radio waves. When laser light impinges on a macromolecule, the oscillating electric field of the light induces an oscillating dipole within it. The following theory section contains a brief overview of the following topics related to multi-angle light scattering: The overall intensity carries information about the molar mass, while the angular dependence within the horizontal plane carries information about the size of the macromolecule. The electric field of the polarized light beam is preferably produced perpendicular to the plane in which the intensity and angular dependence of the subsequently scattered light is to be measured (by convention, the polarization direction is denoted ‘vertical’ and the measurement plane ‘horizontal’). In a typical light scattering experiment, a well collimated, single frequency, polarized light beam (i.e., from a laser) is used to illuminate a solution containing a suspension of the macromolecules or nanoparticles of interest. The extension of Rayleigh’s theory to describe the scattering of light by larger macromolecules in solution is called the Rayleigh-Gans-Debye (RGD) theory of light scattering. His insights were based on the fundamental equations describing light and its interaction with matter, laid out by James Clerk Maxwell in 1865 in one of the most important achievements of theoretical physics. In the 19th century, Lord Rayleigh (John William Strutt) offered the first explanation for the sky's brilliant blue color (on a clear day!). Dynamic & Electrophoretic Light Scattering.
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