# Molecular-Level Insights into Meta-Phenylenediamine and Sulfonated Zinc Phthalocyanine Interactions for Enhanced Polyamide Membranes: A DFT and TD-DFT Study

## TL;DR

This study explores how adding sulfonated zinc phthalocyanine to polyamide membranes can improve their performance in water treatment.

## Contribution

The novel contribution is the computational investigation of MPD and Zn(SO2−)4Pc interactions to enhance membrane properties.

## Key findings

- MPD/Zn(SO2−)4Pc complexes show reduced HOMO-LUMO gaps and enhanced charge delocalization in water.
- Strong hydrogen bonding and π–π stacking interactions suggest Zn(SO2−)4Pc can improve membrane functionality.
- The 120° conformation of MPD/Zn(SO2−)4Pc is most stable due to π–π interactions and hydrogen bonding.

## Abstract

Access to clean water is a pressing global concern and membrane technologies play a vital role in addressing this challenge. Thin-film composite membranes prepared via interfacial polymerization (IPol) using meta-phenylenediamine (MPD) and trimesoyl chloride (TMC) exhibit excellent separation performance, but face limitations such as fouling and low hydrophilicity. This study investigated the interaction between MPD and sulfonated zinc phthalocyanine, Zn(SO2−)4Pc, as a potential strategy for enhancing membrane properties. Using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT), we analyzed the optimized geometries, electronic structures, UV–Vis absorption spectra, FT-IR vibrational spectra, and molecular electrostatic potentials of MPD, Zn(SO2−)4Pc, and their complexes. The results show that MPD/Zn(SO2−)4Pc exhibits reduced HOMO-LUMO energy gaps and enhanced charge delocalization, particularly in aqueous environments, indicating improved stability and reactivity. Spectroscopic features confirmed strong interactions via hydrogen bonding and π–π stacking, suggesting that Zn(SO2−)4Pc can act as a co-monomer or additive during IPol to improve polyamide membrane functionality. A conformational analysis of MPD/Zn(SO2−)4Pc was conducted using density functional theory (DFT) to evaluate the impact of dihedral rotation on molecular stability. The 120° conformation was identified as the most stable, due to favorable π–π interactions and intramolecular hydrogen bonding. These findings offer computational evidence for the design of high-performance membranes with enhanced antifouling, selectivity, and structural integrity for sustainable water treatment applications.