表面羧基构型与润湿性之间的热力学关联

Thermodynamic Correlation Between Surface Carboxyl Configuration and Wettability

摘要

以聚丙烯酸（PAA）薄膜为模型体系， 通过调控分子量与固化温度， 制备了一系列具有不同水润 湿性的PAA薄膜. 借助衰减全反射傅里叶变换红外光谱（ATR-FTIR）解析， 将薄膜表面羧基（COOH）的分子 构型[自由羧基（\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}}）和氢键结合羧基（\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}）]与表面能的极性分量（{\gamma }^{\mathrm{s},\mathrm{p}}）直接关联. 通过将{\gamma }^{\mathrm{s},\mathrm{p}}分解为\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}}和\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}的贡献之和， 首次获得\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}的本征表面能极性分量{\gamma }_{\mathrm{H}\mathrm{B}}^{\mathrm{s},\mathrm{p}\mathrm{*}}为8.34 mN/m， 显著低于\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}} 的{\gamma }_{\mathrm{f}}^{\mathrm{s},\mathrm{p}\mathrm{*}}（34 mN/m）， 这一结果表明氢键作用显著降低了表面的{\gamma }^{\mathrm{s},\mathrm{p}}， 合理解释了PAA薄膜表面较大水接触角的差异， 并为通过润湿性反推表面\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}的比例提供了热力学基础. 进一步将该模型推广至羧基自组装单层（COOH-SAMs）体系时发现， 表面COOH密度（∑C_{OOH）对润湿行为具有关键调控作用： 当∑C OOH处于 4.30~5.25 nm⁻²时， COOH以自由态为主并可形成有效水合层， 最有利于超亲水表面的实现. 本研究不仅建立了表面 COOH 构型与宏观润湿性之间的定量热力学模型， 还通过推广至COOH-SAMs体系验证了该模型的普适性， 为多种COOH功能化表面亲水性的可控设计提供了统一理论框架.}

Abstract

In this study， polyacrylic acid（PAA） films were employed as a model system， and a series of PAA films with tunable water wettability was 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（\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}}） and hydrogen-bonded carboxyl（\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}）， were directly correlated with the polar component of surface energy（{\gamma }^{\mathrm{s},\mathrm{p}}）. By decomposing the {\gamma }^{\mathrm{s},\mathrm{p}} values of the PAA thin films as a sum of the contributions of \mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}} and \mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}， the intrinsic polar component of surface energy of \mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}}（{\gamma }_{\mathrm{H}\mathrm{B}}^{\mathrm{s},\mathrm{p}\mathrm{*}}） was quantified for the first time as 8.34 mN/m， significantly lower than that of \mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{f}}（{\gamma }_{\mathrm{f}}^{\mathrm{s},\mathrm{p}\mathrm{*}}=34 mN/m）. This result highlights that hydrogen bonding markedly reduces the {\gamma }^{\mathrm{s},\mathrm{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 \mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\mathrm{H}\mathrm{B}} groups（f_{\mathrm{H}\mathrm{B}}） from wettability measurements. Further extension of the model to carboxyl- terminated self-assembled monolayers（COOH-SAMs） revealed that surface COOH density（∑C^{OOH） critically regulates wetting behavior： when ∑C OOH 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.}

关键词

聚丙烯酸薄膜 / 氢键 / 衰减全反射傅里叶变换红外光谱 / 羧基构型定量 / 表面能极性分量

Key words

Polyacrylic acid film / Hydrogen bonding / Attenuated total reflectance Fourier-transform infrared spectroscopy / Quantitative carboxyl configuration / Polar component of surface energy