Evaluating simple, low-cost techniques to estimate molecular weights and charge densities of polydisperse commercial polyacrylamides for soil and sediment applications

Nonionic and anionic polyacrylamides (PAMs) are being used more extensively in soil erosion prevention and sediment control applications. Of their various physicochemical properties, the molecular weight (MW) and charge density (CD) of PAMs have been reported as being the most important characteristics for soil stabilization and sediment flocculation. However, these properties rarely have been measured by researchers or practitioners in the fields of soil and environmental sciences, presumably due to the difficulty of the measurements or lack of access to proper equipment. The objective of this research was to demonstrate the applicability of two relatively simple, low-cost approaches for estimating CDs and MWs of polydisperse, commercially available PAMs. Values obtained with the simple measurement techniques (acid-base titration for CD, viscosity determination for MW) were compared to those measured using sophisticated state-of-the-science techniques (carbon-hydrogen-nitrogen (CHN) elemental analysis for CD, multi-angle laser light scattering for MW). Measured CD and MW values were also compared to the values provided by PAM vendors when available. Results from the simple titration method were in excellent agreement with the more complicated CHN analysis for CD values, with differences never more than ±8%. In contrast, values provided by vendors had deviations in CDs ranging from-30% to +17% from measured CHN values and were never closer than 7% absolute difference. Replicate measurements resulted in a coefficient of variation in CD values ranging from 2% to 15% using the CHN technique, but only α 0.2% to 3% coefficient of variation with the simple titration method. Molecular weights estimated from the simple viscosity technique were lower than those obtained from multi-angle laser light scattering, a factor that reflected the viscosity-versus weight-average MWs of our polydisperse commercial PAM samples. Because quantitative MW information often may not be provided by vendors, as well as the fact that common application practices may lead to shear forces that reduce the MW, we believe viscosity-based MW values should be satisfactory for most users of commercial PAM products. Additional simplifications and modifications that we suggest in this article will make both the titration and viscosity techniques useful for a wide variety of PAM users, particularly those who do not have access to sophisticated analytical laboratories but yet have the need to maintain adequate quality assurance and control in their soil erosion prevention and sediment control applications.