شيمي علمي براي زندگي امروز

When laser light impinges on a macromolecule, the oscillating electric field of the light induces an oscillating dipole within it. This oscillating dipole will re-radiate light, much like the antenna for a radio station sends out radio waves. The intensity of the radiated light depends on the magnitude of the dipole induced in the macromolecule. The more polarizable the macromolecule, the larger the induced dipole, and hence, the greater the intensity of the scattered light. Therefore, in order to characterize the scattering from a solution of such macromolecules, it is first necessary to know their polarizability. This may be determined from a measurement of the change, Dn, of the solution's refractive index n with the molecular concentration change, Dc, by measuring the dn/dc (=Dn/Dc) value using a so-called differential refractometer such as the Optilab rEX.

When there are many macromolecules in solution, each macromolecule scatters light via the aforementioned induced dipole mechanism. 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.

Consider now the important case where two monomers come together (aggregate) to form a dimer in solution. The initially separate monomers are constantly buffeted by solvent molecules, and undergo random motion known as Brownian motion. This imparts a randomness to the phase of the scattered light such that the light from the two separate monomers is incoherent. It adds as one expects classically: 1 + 1 = 2.

However, once the monomers form a dimer, the two monomers move together. The scattered light from one monomer now has a definite phase relationship with the scattered light from the other. In other words, the scattering is coherent. The net result is that the scattered light from the dimer is twice as intense as that from the separated monomers. Simply by doubling the mass, even while keeping the concentration the same, the intensity of the scattered light doubles. The intensity of light scattered by a molecule is directly proportional to the molar mass. Light scattering thus represents a powerful technique for monitoring the presence and formation of aggregates