pH-Dependent Gelation of a Stiff Anionic Polysaccharide in the Presence of Metal Ions

Abstract

Cross-linking of polysaccharides by metal ions provides polymer gels highly required by industrial applications. In this article, we study the rheological properties and microstructure of solutions of a stiff anionic polysaccharide xanthan cross-linked by chromium (III) ions, and we demonstrate that their properties are highly sensitive to the preparation pH. Stable gels are obtained in a wide range of pH from 2.4 to 7.8. The maximum elastic modulus is observed for the gels made at pH 6.3, and by freeze-fracture transmission electron microscopy it is shown that they are characterized by the most dense network structure. However, out of this pH interval, no gelation is observed. At low pH (< 2.4) it is due to high protonation of carboxylic groups of xanthan preventing their interaction with chromium ions, and to the disappearance of oligomeric ions, which are effective in cross-linking. At high pH (> 7.8) the absence of gelation is caused by the transformation of reactive chromium ions into insoluble chromium hydroxide. At the same time, for the gels initially formed at pH 6.3, subsequent change of pH to strongly acidic (1.4) or basic (8.9) medium does not affect appreciably their properties, meaning that chromium cross-links are stable once they are formed. These observations open a reliable route to produce polysaccharide gels with required mechanical properties in a wide pH range where they initially cannot be prepared. It is also shown that the increase of pH to 6.3 of the initially ungelled solution prepared at pH 1.5 results in gelation. This effect offers a facile way for delayed gelation of polysaccharides, which is especially required by oil industry.

Dual Transient Networks of Polymer and Micellar Chains: Structure and Viscoelastic Synergy

“…The samples were equilibrated for 10–30 min in the measurement cell prior to investigation. The details of the measurements are described elsewhere [ 48 , 49 , 50 ].…”

Fabrication and Characterization of Polysaccharide Metallohydrogel Obtained from Succinoglycan and Trivalent Chromium

“…Given that the Cr 3+ monomer is small, it cannot be effectively co-ordinated with the carboxyl group (pK a ~3.8) of succinoglycan, where protonation of the carboxylic group can also impede their interactions with Cr 3+ [ 63 ]. However, a higher pH range from 3 to 5 can lead to olate forms of Cr 3+ (dimeric, trimeric, and tetrameric oligomers), which can lead to successful co-ordination with succinoglycan [ 43 ]. In neutral or alkaline conditions with higher pH at which gel formation occurs, chromium ions changed to green in aqueous solution and were converted to water-insoluble hydroxide Cr(OH) 3 [ 64 ].…”

“…However, the sample in which pH changed from 5 to a strong acid (pH 1) or strong base (pH 9) remained a gel ( Figure 11 c,d). A slight increase was observed in the final pH 1 gel because carboxyl groups that were not involved in the crosslinking with Cr 3+ became uncharged, thereby decreasing counter ion pressure and the swelling degree of the gel [ 43 ]. The results indicate that Cr 3+ and succinoglycan are strongly crosslinked, and thus, the already crosslinked metallohydrogel is less affected by pH.…”

“…A salecan-based hydrogel co-ordinated with Cr 3+ as a crosslinking agent affects the expansion capacity, pore size, and mechanical strength of the hydrogel by changing the Cr 3+ concentration [ 34 ]. In the study of xanthan/Cr 3+ systems, the gelation time changed based on the change in pH [ 43 ], where the properties of these crosslinks, formed primarily through the formation of chemical bonds between the Cr 3+ and acidic carboxylic (–COO) groups of xanthan, were relatively strong [ 44 ]. Therefore, this pH-controllable gelation in xanthan-based metallohydrogels suggests a potential application to selectively block water during oil recovery operations [ 45 ].…”

Printable Alginate Hydrogels with Embedded Network of Halloysite Nanotubes: Effect of Polymer Cross-Linking on Rheological Properties and Microstructure

“…Prior to each measurement, the samples were equilibrated for 10 min after loading in the cell. The details of the measurements are described elsewhere [ 33 , 34 , 35 ].…”