Origin of Surface Forces in Liquids

A gas phase is converted to a liquid phase when appropriate pressure and temperature are present (which means that molecules become so close that a new phase, liquid, is formed). If temperature is lowered even further, then a solid phase is formed. Liquid water freezes at 0°C to form solid ice (at 1 atm). This kind of transition can be explained by analyzing molecular interactions in liquids, which are responsible for their physicochemical properties (such as boiling point, melting point, heat of vaporization, and surface tension). Since molecular structure is the basic parameter involved in these transitions, one needs to analyze the former characteristics. This analysis is the basis of the quantitative structure activity related (QSAR) (Barnes, 2011 Birdi, 2003b). QSARs have been used to predict physical properties of liquids in extensive detail (Birdi, 2003a Gotch, 1974 Livingstone, 1996). 

In solids, molecular structures are measured by using x-ray methods. However, one cannot estimate molecular structures of liquids with the same accuracy. Liquid structures have been estimated through indirect methods. In the following, based on simple principles, one can estimate the difference in molecular distances in liquid or gas as follows. In the case of water (for example), following data are known (at room temperature and pressure)  

Example Water Volume per mole liquid water = 

This illustrates that the approximate ratio of distance between molecules in gas phase and liquid phase will be about 10 (1000) (from simple geometrical considerations of volume [proportional to length ] and length). In other words, the surface chemistry is related to those molecules, which are situated in this transition region. Experiments have clearly shown that this transition region is of molecular dimension. The same is true for all liquids, with only minor differences. 

In the case of water, it is cohesive forces that maintain water, for example, in liquid state at room temperature and pressure (Franks, 1975 Fraxedas, 2014). It is nseful as an example to compare cohesive forces in two different molecnles, snch as H2O and H2S. At room temperature and pressure, H2O is liquid while H2S is a gas. This means that H2O molecnles interact with different forces, which are stronger and thus form a liquid phase. On the other hand, H2S molecules exhibit much lower interactions and thus are in a gas phase at room temperatnre and pressure. In other words, hydrogen bonds (between H and O) in water are stronger than hydrogen-snlfur bonds. 

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