Branched PEI is a random polymer that is available as an aqueous solution. Using aqueous PEI solution (700,000 average molecular weight), modified PEI were prepared with different alkyl chain length and alkylation degree (Figure ). Owing to the hydrophobicity of the obtained polymers, alkylation degree of PEI samples were precisely determined by 1 H NMR analysis (Figure ), showing slight difference between intended and observed alkylation degree particularly for higher alkylation degree (Figure ), which can be ascribed to the steric congestion occurring at high alkylation degree and longer alkyl chain substitution. Nonetheless, we selected C16 alkylation as a standard method in this study owing to its high reproducibility and optimal performance in our CO2-mediated desalination. Alkylation of PEI with C16-Br yielded powdery PEI-C16, thus enabling its facile characterization and analysis. Moreover, it is insoluble in polar solvents (water and ethanol) in wide (2-14) pH ranges (Scheme S1D). The powder form of PEI-C16 indicated crystalline behavior with 20-33% alkylation degrees (calculated based on 1 H NMR spectroscopy). The DSC analysis showed no glass transition temperature for these polymers within 25 -200 °C (Figure ). Melting occurred at 123 and 133 °C for PEI-C16-33% and PEI-C16-20%, respectively, ascribable to the additional hexadecyl side chains on the PEI backbone (See Supporting Information TGA and DSC analysis).