The intrinsic persistence length of carboxymethyl cellulose (CMC) is determined by size exclusion chromatography in combination with multiangle 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 radius 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 nm, irrespective of the degree of substitution. A somewhat lower value (12 nm) is obtained when Odijk's theory for the description of polyelectrolyte dimensions is applied. The difference between L p0 assessed from both models is discussed briefly. Potentiometric titrations were carried out in NaCl solutions (ranging from 0.01 to 1 mol L-1). From the titrations the radius of the CMC backbone was obtained by application 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 groups (i.e., at zero degree of dissociation) amounted to 3.2. L p0 was also deduced from potentiometric titrations. A model developed by Katchalsky and Lifson, which relates the dissociation behavior of a polyelectrolyte to the stiffness of its chain, was applied to CMC. From analyses of the potentiometric titrations an intrinsic persistence length of 6 nm was deduced. The difference between L p0 assessed from SEC−MALLS and potentiometric titrations is discussed briefly.

The kinetics of polyelectrolyte adsorption has been investigated theoretically. In analogy with Kramers' rate theory for chemical reactions we present a model which is based on the assumption that a polyelectrolyte encounters a barrier in its motion towards an adsorbing surface. The height of the barrier, which is of electrostatic origin, is calculated with a self-consistent-field (SCF) model. The salt concentration strongly affects the height of the barrier. At moderate salt concentrations () equilibrium in the adsorption is attained; at low salt concentration () equilibrium is not reached on the time scale of experiments. The attachment process shows resemblances to the classical DLVO theory.

The adsorption behavior of carboxymethyl cellulose (CMC) on inorganic surfaces (TiO2 and R-Fe2O3) in aqueous solution has been studied systematically. The general trends are that the adsorbed amount decreases with increasing pH, whereas increasing the electrolyte (NaCl) concentration causes the adsorption to increase. The actual values, however, are influenced by the kinetics of the adsorption process. Near the point of zero charge of the oxidic surfaces the adsorption depends linearly on pH. This linearity is interpreted in terms of a molecular condenser, which is composed of the surface layer and the polyelectrolyte in the first layer near the surface. Adsorption is independent of the number of carboxylic groups per glucose unit (degree of substitution, ds). Neither at low (0.01 mol L -1 ) nor at high (1 mol L -1 ) electrolyte concentration does the adsorption depend on the chain length, which indicates a (rather) flat conformation of the adsorbed polymer. The apparent hydrodynamic thickness of the adsorbed layer is found to be substantial. Strong hysteresis is observed with respect to the pH dependence of adsorption: as compared to the adsorption measured directly at specified fixed pH values, at high pH values a substantially higher adsorbed amount can be obtained by initially adsorbing at low pH and subsequently increasing the pH value. Desorption of CMC only takes place after initially low pH values are increased substantially. The arduous desorption originates from a strong interaction with the surface and is enhanced by the chain rigidity of the CMC backbone.

Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately 20.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1-2.2 cm 3 ) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (,0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to 21.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr.

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