PERSISTENCE LENGTH OF CARBOXYMETHYL CELLULOSE AS EVALUATED FROM SIZE-EXCLUSION CHROMATOGRAPHY AND POTENTIOMETRIC TITRATIONS

The intrinsic persistence length of carboxymethyl cellulose (CMC) is d etermined by size exclusion chromatography in combination with multian gle laser light scattering (SEC-MALLS) as well as from potentiometric titrations. Samples with degree of substitution (ds) ranging from 0.75 to 1.25 were investigated. The relation between molar mass M and radi us of gyration R-g as obtained by SEC-MALLS is determined in 0.02, 0.1 , and 0.2 mol L-1 NaNO3. Using the electrostatic wormlike chain theory a bare (intrinsic) persistence length L-p0 of CMC is assessed at 16 n m, irrespective of the degree of substitution. A somewhat lower value (12 nm) is obtained when Odijk's theory for the description of polyele ctrolyte dimensions is applied. The difference between L-p0 assessed f rom both models is discussed briefly. Potentiometric titrations were c arried out in NaCl solutions (ranging from 0.01 to 1 mol L-1). From th e titrations the radius of the CMC backbone was obtained by applicatio n of the model of a uniformly charged cylinder. The radius amounts to 0.95 nm for CMC ds = 0.75, and increases to 1.15 nm for CMC with ds = 1.25. The pK for the intrinsic dissociation constant of the carboxyl g roups (i.e., at zero degree of dissociation) amounted to 3.2. L-p0 was also deduced from potentiometric titrations. A model developed by Kat chalsky and Lifson, which relates the dissociation behavior of a polye lectrolyte to the stiffness of its chain, was applied to CMC. From ana lyses of the potentiometric titrations an intrinsic persistence length of 6 nm was deduced. The difference between L-p0 assessed from SEC-MA LLS and potentiometric titrations is discussed briefly.