Molecular light scattering, theory

The work represents an application of core-shell light scattering theory to polymer latex suspensions and addresses the separate identification of light scattering by dust, latex particles and low molecular weight solutes. 

R. W. Hellwarth. Theory of molecular light scattering spectra using the linear dipole approximation. J. Chem. Phys., 52 2128-2138 (1970). 

Barron L (1982) Molecular Light Scattering and Optical Activity. Cambridge Cambridge University Press. Caldwell DJ and Eyring H (1971) The Theory of Optical Activity. New York Wiley-Interscience. 

Molecular weight was calculated using Astra 5.3.4.14 software (Wyatt Technology), according to the basic light scattering theory equation, in a simplified form ... 

The current frontiers for the subject of non-equilibrium thennodynamics are rich and active. Two areas dommate interest non-linear effects and molecular bioenergetics. The linearization step used in the near equilibrium regime is inappropriate far from equilibrium. Progress with a microscopic kinetic theory [38] for non-linear fluctuation phenomena has been made. Carefiil experiments [39] confinn this theory. Non-equilibrium long range correlations play an important role in some of the light scattering effects in fluids in far from equilibrium states [38, 39]. 

Equations (10.17) and (10.18) show that both the relative dielectric constant and the refractive index of a substance are measurable properties of matter that quantify the interaction between matter and electric fields of whatever origin. The polarizability is the molecular parameter which is pertinent to this interaction. We shall see in the next section that a also plays an important role in the theory of light scattering. The following example illustrates the use of Eq. (10.17) to evaluate a and considers one aspect of the applicability of this quantity to light scattering. 

Analysis of polyelectrolytes in the semi-dilute regime is even more complicated as a result of inter-molecular interactions. It has been established, via dynamic light-scattering and time-dependent electric birefringence measurements, that the behavior of polyelectrolytes is qualitatively different in dilute and semi-dilute regimes. The qualitative behavior of osmotic pressure has been described by a power-law relationship, but no theory approaching quantitative description is available. 

Chapter C deals with molecular dimensions of interrupted helices. Typical theories for mean-square radius of gyration and mean-square end-to-end distance are reviewed. Important predictions from theory are compared with the results of recent light-scattering measurements. Complications attendant upon the analysis of light-scattering data for polypeptides in the helix-coil transition region are discussed. 

An alternative (but equivalent) approach is the so-called fluctuation theory, in which light scattering is treated as a consequence of random non-uniformities of concentration and, hence, refractive index, arising from random molecular movement (see page 26). Using this approach, the above relationship can be written in the quantitative form derived by Debye140 for dilute macromolecular solutions ... 

In contrast to osmotic pressure, light-scattering measurements become easier as the particle size increases. For spherical particles the upper limit of applicability of the Debye equation is a particle diameter of c. A/20 (i.e. 20-25 nm for A0 600 nm or Awater 450 nm or a relative molecular mass of the order of 10 ). For asymmetric particles this upper limit is lower. However, by modification of the theory, much larger particles can also be studied by light scattering methods. For polydispersed systems a mass-average relative molecular mass is given. 

The optical measurements presented in the previous chapters can be used to either characterize local, microstractural properties or as probes of bulk responses to orientation processes. In either case, it is normally desirable to make the connection between experimental observables and their molecular or microstractural origins. The particular molecular properties that are probed will naturally depend on the physical interaction between the light and the material. This chapter explores molecular models and theories that describe these interactions and identifies the properties of complex materials that can be extracted from measurements of optical anisotropies. The presentation begins with a discussion of molecular models that are applied to polymeric materials. Using these models, optical phenomena such as birefringence, dichroism, and Rayleigh and Raman scattering are predicted. Models appropriate for particulate systems are also developed. 

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