Abstract: 【Objective】The efficiency of foliar-applied agrochemicals is closely related to the complexity of physicochemical properties of plant leaf surfaces. For better understanding the interfacial interaction between agrochemical spray liquids and plant leaf surfaces, the internal relation would be seek from the thermodynamic point based on pepper leaf surfaces. This will provide a basis for efficient use of pesticides on plants.【Method】Three test liquids were water (W), glycerol (G) and diiodomethane (DM). Their static contact angles of a single droplet on the pepper leaf surfaces of different varieties were determined by contact angle meter. Then the total surface free energy (SFE) and its components were evaluated by Harmonic mean (HM) method, Owens-Wendt-Rabel- Kaelble (OWRK) method, Van-Oss-Chaudhury-Good (OCG) method and ZDY method. Accordingly, solubility parameter (δ) of pepper leaf surfaces was calculated. 【Result】The leaves of Suzi-1 and GR pepper were wettable (θ＜90°) for W and Sujiao-13 pepper leaves were unwettable (θ＞90°) for W. Among the 4 methods, more physical characteristic information was obtained by the OCG method with 3 test liquids (W-G-DM) and calculated percentages of non-polar components of the SFE of pepper leaf surfaces (＞85%) were higher than the polar components (＜15%). The HM and OWRK methods based on 2 test liquids. When the 2 test liquids were polar (i.e. W-G), the percentages of non-polar or polar component of the SFE of the pepper leaf surface varied greatly, or even the opposite to the percentage obtained in the OCG method. When the 2 test liquids were polar and non-polar combination (i.e. W-DM or G-DM), the deviation of the SFE values calculated by OWRK method was lower than that by HM method based on the OCG method. The SFE values calculated by ZDY method with 1 test liquid were much higher than that of the other three methods. Compared with OCG method, the deviation of the SFE of pepper leaf surfaces was ＞100%. Based on the deviation from the OCG method within 10% of the SFE, SFE of Suzi-1 leaf surface was 37.72-43.11 mJ·m^-2 and solubility parameter was 18.89-22.77 mJ^1/2·m^-3/2. SFE of GR leaf surface was 37.53-40.95 mJ·m^-2 and solubility parameter was 18.81-20.09 mJ^1/2·m^-3/2. SFE of Sujiao-13 leaf surface was 33.21-36.92 mJ·m^-2 and solubility parameter was 17.17-18.58 mJ^1/2·m^-3/2.【Conclusion】Using water (W), glycerol (G) and diiodomethane (DM) as the test liquid, ZDY method is not suitable for calculating the SFE of pepper leaf surfaces and the rest methods (HM, OWRK, OCG) can be used to calculate the SFE. Among them, the HM or OWRK method should be paid more attention to the polarity of selecting test liquid combination. Meanwhile, the percentage of non-polar component of the SFE of 3 kinds of pepper is higher than that of the polar component.

Key words: pepper leaf surface, static contact angle, surface free energy, surface free energy components, solubility parameter