Estimating molecular weight cut off of porous membranes via “Positioning-Searching-Determining” triple-step method

Abstract

Although the molecular weight cut-off (MWCO) method is commonly used to assess the pore size of porous membranes, it requires a large quantity of standard substances. As such, a hybrid approach characterized by a “Positioning-Searching-Determining” three-step method was proposed to acquire a precise MWCO value for porous membranes more efficiently and cost-effectively. Firstly, the ratio of the convective flow permeability coefficient (L_d) to pure water permeability (A) was introduced to evaluate the surface defect of porous membranes. The mathematical modeling process depicting the approximate correlation among L_d/A, solute rejection (R_t), and MWCO showed that LnR_t could be expressed by L_d/A, and R_t could also be expressed by lnMWCO in the form of polynomial functions. Subsequently, by measuring the values of L_d/A, R_t, and MWCO for commercial ultrafiltration (CUF) membranes with different pore sizes, the governing equations along with the corresponding fitting parameters were obtained. Then, the obtained governing equations were used to assess the MWCO of homemade porous composite membranes (PCMs). Comparative analysis revealed that the currently established mathematical equations among R_t, L_d/A, and MWCO caused a more significant divergence between the predicted and measured values for PCMs when L_d/A or R_t was outside the range of the selected CUF membranes, indicating that simply using the governing equations was more likely to provide not an exact but an approximate MWCO value. In view of this situation, the “Positioning-Searching-Determining” hybrid strategy was constructed by using the approximate equations to obtain the approximate MWCO value and then obtaining the precise MWCO value through solute rejection experiments for the selected two standard substances. Experimental results showed that utilization of the hybrid stratagem to acquire MWCO could constrict the error ambit to a narrower range (−5% to +14 %) and minimize the quantum of chemical reagents. Therefore, the proffered hybrid modus operandi coalesces the excellences of the sheer predictive mode and the sheer measurement mode, thereby enhancing the dependability and reducing the measurement outlay. Moreover, the proposed novel hybrid tactic has the ability to transmute the MWCO determining tactic from blind and simplistic seeking to sagacious positioning and purposeful seeking.