Polythiophenes bearing glycolated side chains have rapidly surged as the highest performing materials for organic electrochemical transistors (OECTs) because of their ability to conjugate volumetric ion penetration with high hole mobility and charge density. Among them, p(g2T-TT) has one of the highest figures of merit. Our work provides an atomistic picture of the p(g2T-TT)-electrolyte interface in the "off"state of an OECT, expected to be dominated by cation-polymer interactions. Using a combination of molecular dynamics simulations and X-ray fluorescence, we show how different anions effectively tune the coordination and chelation of cations by p(g2T-TT). At the same time, softer and hydrophobic anions such as TFSI- and ClO4- are found to preferentially interact with the p(g2T-TT) phase, further enhancing the polymer-cation chelation. We highlight how the stronger hydrophobic nature of TFSI- causes its preferential accumulation at the polymer interface, further enhancing the anion-enabled cation-polymer chelation. Besides opening the way for a full study of electrolyte doping mechanisms in operating devices, our results suggest that tailoring the electrolyte for different applications and materials might be a viable strategy to tune the performance of mixed conducting devices.