Abstract: In this study, polyacrylic acid (PAA) films were employed as a model system, and a series of PAA films with tunable water wettability were systematically prepared by varying molecular weight and curing temperature. Using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), the molecular configurations of surface carboxyl groups (COOH)—free carboxyl (〖COOH〗_f) and hydrogen-bonded carboxyl (〖COOH〗_HB)—were directly correlated with the polar component of surface energy (γ^(s,p)). By decomposing the γ^(s,p) values of the PAA thin films as a sum of the contributions of 〖COOH〗_f and 〖COOH〗_HB, the intrinsic polar component of surface energy of 〖COOH〗_HB (γ_HB^(s,p*)) was quantified for the first time as 8.34 mN/m, significantly lower than that of 〖COOH〗_f (γ_f^(s,p*) = 34 mN/m). This result highlights that hydrogen bonding markedly reduces the γ^(s,p), providing a rational explanation for the relatively large water contact angle observed on PAA thin films. Furthermore, it establishes a thermodynamic basis for estimating the fraction of surface 〖COOH〗_HB groups (f_HB) from wettability measurements. Further extension of the model to carboxyl-terminated self-assembled monolayers (COOH-SAMs) revealed that surface COOH density (∑COOH) critically regulates wetting behavior: when ∑COOH ranges from 4.30 to 5.25 nm?2, COOH groups predominantly exist in a free state and facilitate effective hydration layers, thereby promoting superhydrophilicity. Overall, this study not only establishes a unified thermodynamic framework linking surface COOH configurations to macroscopic wettability but also validates its universality by extending it to COOH-SAMs systems, thereby providing a unified theoretical framework for the controllable design of hydrophilicity in various COOH-functionalized surfaces.

Key words: Polyacrylic acid (PAA) films, hydrogen bonding, ATR-FTIR, quantitative carboxyl configuration, polar component of surface energy