第三章表面张力及表面能.pptVIP

第三章表面张力及表面能 3.1表面能及表面张力 3.2弯曲液面下的附加压力 3.3表面过剩及吉布斯吸附公式 表面张力与表面自由能 表面张力? (surface tension) 表面自由能? (surface free energy) 思考题 表面张力与哪些因素有关？即表面张力受哪些因素影响？ 一些液体的表面张力 一些油水界面张力（20℃） 小结： 表面张力与表面自由能 表面张力（surface tension）是垂直作用于液体表面上任一单位长度、与液面相切的收缩表面的力。表面张力是液体的基本物理化学性质之一。单位：mN/m(毫牛/米) 表面自由能（surface free energy）：是恒温恒压下增加单位表面积时体系自由能的增量，称作比表面自由能，简称表面自由能。单位：mJ/m2（毫焦/米2） If a water film is formed on a frame with a slider length of 1 cm, then the film pulls on the slider with a force of 2 × 0.01 m × 0.072 Jm?2 = 1.44 × 10?3N That corresponds to a weight of 0.15 g. Without this attraction there would not be a condensed phase at all, there would only be a vapor phase. The sheer existence of a condensed phase is evidence for an attractive interaction between the molecules. At the surface, molecules are only partially surrounded by other molecules and the number of adjacent molecules is smaller than in the bulk(Fig. 2.3). This is energetically unfavorable. In order to bring a molecule from the bulk to the surface, work has to be done. With this view γ can be interpreted as the energy required to bring molecules from inside the liquid to the surface and to create new surface area. Therefore often the term “surface energy” is used for γ. Example:Estimate the surface tension of cyclohexane from the energy of vaporization ΔvapU = 30.5 kJ/mol at 25?C. The density of cyclohexane is ρ = 773 kg/m3, its molecular weight is M = 84.16 g/mol. For a rough estimate we picture the liquid as being arranged in a cubic structure. Each molecule is surrounded by 6 nearest neighbors. Thus each bond contributes roughly ΔvapU/6 = 5.08 kJ/mol. At the surface one neighbor and hence one bond is missing. Per mole we therefore estimate a surface tension of 5.08 kJ/mol. To estimate the surface tension we need to know the surface area occupied by one molecule. If the molecules form a cubic structure, the volume of one unit cell is a3, where a is the distance between nearest neighbors. This distance can be calcu