Static and dynamic light scattering studies of dextran from leuconostoc mesenteroides in the dilute region

Abstract

AbstractDextrans with mass‐average molar masses 8.104 g/mol ⩽ M̄w ⩽ 108 g/mol have been studied in water by means of static and dynamic light scattering at different temperatures. Static light scattering (SLS) yields the z‐average mean‐square radius of gyration, 〈S2〉z and the second virial coefficient A2. It is found that the dependences of 〈S2〉 and A2 on the mass‐average molar mass, M̄w, can be well described by the power laws 〈S2〉 = KS · M̄ and A2 = KA · M̄. The exponent vs is independent of the temperature T, while vA decreases as T is raised. Dynamic light scattering (DLS) yields the apparent diffusion coefficient, Dapp (q,c), and the hydrodynamic radius, Rh, where q is the wave vector and c the polymer concentration. For small q, a plot of Dapp (q, c) versus q2 · 〈S2〉z gives a straight line. The intercept $ D_{\rm z} (c) = \mathop {\lim }\limits_{q \to 0} D_{{\rm app}} (q,c) $ represents the centre‐of‐mass translational diffusion coefficient. Its dependence on the concentration, c, can be well simulated by the relation \documentclass{article}\pagestyle{empty}\begin{document}$ D_{\rm z} (c) = D_{{\rm z,}0} \left[ {1 + k_{{\rm D},2} \cdot c} \right] $\end{document}. Here kD, 2 is the second hydrodynamic virial coefficient and Dz,0 the z‐average of the translational diffusion coefficient at infinite dilution. The analysis of the kD,2 ‐data shows that dextran molecules behave rather as interpenetrable than as non‐interpenetrable spheres. The density p = (3 M̄w)/(NA · 4 π R) proves to be a measure for the degree of penetration; p decreases with increasing M̄w, indicating that penetration becomes easier at higher molar masses.