Dispersing carbomers, mixing technology matters!

Abstract

Mixing dry carbomer powder with water using magneto-hydrodynamic mixing yielded carbomer dispersions with higher viscosity and increased storage modulus as compared to conventional high shear mixing. ¹H NMR spectroscopy demonstrated this to be induced by a different water distribution, accompanied by lower ionization and higher degradation of the polymer in case of high shear mixing. This investigation reveals ¹H MAS NMR to provide suitable sensitivity and resolution to detect structural changes induced in organic polymers during their hydration.

Fig. 1. Schematic representation of carbomer polymer folding in different conditions. Hydration of the polymer leads to unfolding of the tightly coiled chains. Deprotonation of the carboxylic acid functional groups results in the development of a pH dependent negative charge. With increasing pH, increased intra-molecular electrostatic repulsion amplifies polymer chain unfolding, yielding a viscous hydrogel. Adapted from Shafiei et al. (2018).

Fig. 2. Left: viscosity measurement as function of applied shear stress for aqueous carbomer dispersions (1.22 wt% Carbopol®) mixed for 3 minutes with HSM and MHD. Right: strain sweep test results, plotted as storage (G′) and loss (G′′) modulus vs. applied shear strain (γ).

Fig. 3. (a) ¹H NMR spectra of the different carbomer gels prepared under HSM and MHD mixing conditions using different mixing times. (b) Enlarged view of the ¹H NMR spectra between 1 and 3 ppm, highlighting the ¹H resonances originating from the PAA backbone of the carbomer. The different ¹H signals have been assigned using color coding. (c) ¹H–¹H exchange spectroscopy (EXSY) spectra of the carbomer gel prepared with HSM after 1 minute and (d) after 3 minutes of mixing.