Amorphous supramolecular structure of carboxymethyl cellulose in aqueous solution at different pH values as determined by rheology, small angle X-ray and light scattering

Highlights

• The supramolecular structure of CMC was represented as a set of subspaces with different densities.

• CMC is most expanded, has the largest R_g, persistence length, and size of heterogeneous regions at pK_a.

• Most CMC molecules reside in a few high density subspaces below pK_a.

• Above the pK_a the CMC molecules are approximately uniformly distributed.

• SAXS, SLS, DLS, microscopy and viscosimetry give coherent view of CMC structure from 1 nm to 100 μm scale.

Abstract

Carboxymethyl cellulose (CMC) is one of the most widely used thickening agents in industry. The combination of small-angle X-ray scattering (SAXS), static and dynamic light scattering, as well as viscosity measurements and microscopy at different pH values was utilized to explore the physicochemical properties of CMC on a scale ranging from individual macromolecules to supramolecular assemblies. The supramolecular structure of CMC was represented as a set of characteristic sample subspaces based on SAXS data utilizing the string-of-beads model. The results indicate that at pH 7.0 individual CMC molecules are approximately uniformly distributed in a supramolecular structure owing to strong intra- and intermolecular repulsive interactions. The structure of CMC is most expanded at the value of pK_a, where it has the largest radius of gyration, persistence length, and size of heterogeneous regions. Below pK_a the majority of the CMC sample volume belongs to the low density subspaces. Most of CMC molecules, however, reside in a few high density subspaces. Dynamically, supramolecular structure of CMC is composed of fast diffusive relaxation processes embedded in a background of non-diffusive slow relaxation process at high pH and mostly slow relaxation processes at low pH. The rheological properties of CMC at different pH values were directly related to the CMC supramolecular structure in the aqueous environment.

Introduction

The term supramolecular structure refers to the structural organization of molecules, i.e. the structure beyond the individual molecule. One way to obtain detailed insight into the supramolecular structure of carbohydrate polymer systems is to model each individual polymer molecule as an assembly of monomers held together by covalent bonds, and to numerically simulate a system with a number of such polymer molecules involving intermolecular interactions. For polysaccharides where local molecular order is expected to be intermediate between the randomly coiled chain and the rigid helix, simple models do not give an acceptable result. To remedy this, a more complex modelling of broken rod-like chains may be applied. In such models individual segments (i.e. coils or helices) are linked together in a random orientation assuming equal averaged lengths, which is an obvious simplification. To improve modelling realism further the length of the rod-like chain segment should be shortened ultimately to the size of an individual monomer. In this work monomers were modelled as spheres with the dimensions of anhydroglucose unit (i.e. 5.2 Å). Individual spheres were connected to a string of beads. We have demonstrated in our previous work a good correspondence between the string of beads model and the atomistic model of short oligomer polysaccharides (Dogsa, Štrancar, Laggner, & Stopar, 2008). Such an approach implies that the ideas, which have been very successful in describing the structure of individual polymer molecules, may also be successful in describing supramolecular structures of carbohydrates (Turro, 2005). It has been suggested that cellulose derivatives (e.g. cellulose trinitrate) can form supramolecular structures in solution (Schurz, 1983). To the best of our knowledge, there has so far been no detailed study of the supramolecular structure of carboxymethyl cellulose (CMC) in aqueous solutions at different pH values.

CMC is a modified water soluble cellulose with unparalleled applicability in different branches of industry (e.g. food processing, pharmaceuticals, adhesives, paints, leather, paper, textiles, ceramics, agricultural products, detergents, as well in oil well drilling, foundry work and mineral processing). In industrial applications CMC is primarily used for controlling product viscosity and at the same time avoiding gelling. It therefore acts as a thickener, phase and emulsion stabilizer, and/or suspension agent. CMC is biocompatible and it was recently shown that it can be used to form pectin/carboxymethyl cellulose/microfibrillated cellulose composite scaffolds for tissue engineering (Ninan et al., 2013). CMC has a large water-holding capacity, which is high even in samples with low viscosities (Cummings & Stephen, 1979). It is an anionic linear polyelectrolyte and its molecular conformation in aqueous solution strongly depends on the concentration, ionic strength and pH value (Wandrey, Bartowiak, & Harding, 2009). Above the overlap concentration thermo-reversible hydrogels are formed in aqueous systems, as for example in carrageenan (Yuguchi, 2009). The critical overlap concentration of polymer coils, denoted c*, is one of the most important characteristic values of a polymer solution. It is generally accepted that at concentrations c/c* < 1 the steric and frictional interactions of neighbouring polymer coils are negligible and the rheological response of the fluid is solely governed by the sum of the deformation and hydrodynamic interactions of the isolated polymer coils and solvent which comprise the polymer solution. When these conditions exist, the theoretical description of a dilute solution given by the Rouse/Zimm theory is expected to be valid. At higher concentrations the solution becomes semi-dilute and polymers get eventually entangled depending on the degree of overlap of adjacent coils and their molar mass (Clasen et al., 2006). With heavy metals, three-valent cations and the majority of polycations CMC may form complexes (Wandrey et al., 2009). The average chain length of the cellulose backbone and the average degree of substitution (in terms of carboxymethyl units) strongly influence the properties of each individual CMC type; the more-hydrophobic lower substituted CMCs are thixotropic, but the more-extended higher substituted CMCs are pseudoplastic. At low pH, CMC may cross-link through lactonization between carboxylic acid and free hydroxyl groups (Emeje, Kunle, & Ofoefule, 2006). It is fairly stable over a pH range of 5.0–10.0, but acidification below a pH value of 5.0 reduces its viscosity and its long term stability. The polymer chains of CMC are most extended at low concentrations, low ionic strengths, and high values of pH (Kästner, Hoffmann, Dönges, & Hilbig, 1997). Despite CMC's immense importance the methods for its physico-chemical descriptions are still evolving (Shakun et al., 2013a, Shakun et al., 2013b).

It is extremely difficult to investigate the supramolecular structure of CMC in detail, as it spans different length- and time-scales. High-resolution techniques of crystallography and NMR are not applicable in the case of CMC due to the lack of a regular lattice in the structure and high molecular mass, respectively. There have been several attempts reported in the literature to characterize the macroscopic properties of CMC by its microscopic structure (Adel et al., 2010, Benchabane and Bekkour, 2008, Kästner et al., 1997, Kulicke et al., 1996). Most notably, the thickening properties of a polymer solution are related to the rigidity of the backbone, which is in turn characterized by the polymer persistence length. The persistence length of the polymer backbone can be obtained from intrinsic viscosity measurements (Yamakawa & Fujii, 1974). Light and X-ray scattering techniques, on the other hand, are well-suited to characterize molecular weight distribution, conformation or supramolecular structure of polymers in solution (Berne and Pecora, 2000, Brown and Mortensen, 2000, Sharma and Bohidar, 2000), including carbohydrate polymers (Dogsa et al., 2008, Doutch and Gilbert, 2013, Doutch et al., 2012, Josef and Bianco-Peled, 2012, Orehek et al., 2013, Roblin et al., 2013, Wang et al., 2012). Small-angle neutron scattering has revealed that CMC behaves as a Gaussian distribution of rigid segments and that its persistence length increases with dilution and charge density (Moan & Wolff, 1975). In addition, the small-angle X-ray scattering (SAXS) technique offers the possibility of analysing polymer sample in terms of the distribution of molecular dimensions, represented by the radius of gyration, the repulsion interaction correlation length, the persistence length and the size of heterogeneous regions. The persistence length of CMC in aqueous solutions estimated by SAXS was in the range of 2–4 nm (Muroga, Yamada, Noda, & Nagasawa, 1987). In another study the persistence length determined by Size Exclusion Chromatography – Multi-Angle Laser Light Scattering (SEC-MALLS) and potentiometric titration was estimated to be around 5.6 nm, whereas electrostatic persistence length was 1.6 nm at low salt and 0.3 nm at high salt concentration (Hoogendam et al., 1998). When complexed with alkaline earth metal ions, the radius of gyration of CMC as determined by SAXS significantly increased (Matsumoto & Zenkoh, 1992). Recent developments in the data analysis of SAXS experiments (Dogsa et al., 2005, Dogsa et al., 2008, Orehek et al., 2013), which enable one to obtain the structural and interaction parameters of polymers, provide a very promising new tool for probing the supramolecular organization of carbohydrate polymers in solution and were further explored and developed in this work.

In this study carboxymethyl cellulose with a molecular mass of 90 kDa and a 0.7 degree of substitution was used. Samples prepared at different pH values were investigated by SAXS, static and dynamic light scattering (SLS, DLS) and viscosimetry. As CMC is a polyelectrolyte, the repulsion between charged segments, the radius of gyration, repulsion interaction correlation length, persistence length, and the size of heterogeneous regions were modelled. The string-of-beads model (Dogsa et al., 2008, Orehek et al., 2013) was improved to account for Debye–Bueche inhomogeneities and was used to analyse CMC SAXS data. In the second step a new approach to estimate and visualize the macromolecular structure of CMC was developed. For this purpose various molecular shape parameters representing single helices with different pitch values, number of monomers per pitch, cross-sectional radius and different random coil content were used to describe the secondary structure elements of CMC. The distribution of local volume fraction of CMC polymers in supramolecular structure was calculated on the basis of the results obtained from SAXS data. The CMC supramolecular structure was represented graphically as a collection of subspaces at different pH values. The combined rheological, SLS, DLS and SAXS parameters were used to discuss structural and dynamic behaviour of CMC supramolecular structure. We believe that the improved approach could be of broader physico-chemical interest in polymer and particular carbohydrate polymer community.