158 



ATOMS, IONS, SALTS, AND SURFACES 



Air 



energy which the molecules themselves possess. Thus, if a person extends a surface by 

 doing work upon it, the liquid will also contribute its share to the total energy. The 

 Clapeyron equation tells us that the temperature, i.e., the wealth of the molecules in 

 kinetic energy, is an important factor in determining the extent of this contribution. 



If a surface is to be formed on a definite liquid at a definite temperature, a definite 

 amount of energy must be contributed and converted into potential energy. 



THE ORIENTATION OF MOLECULES IN SURFACES 



The ordinary observation of large-scale objects, such as logs or ships, as they lie 

 upon the surface of a body of water, indicates that these objects exhibit a character- 

 istic orientation with respect to the surface. Thus logs, when not too closely crowded 



together, lie flat upon the water, i.e., the longitudinal 

 axis is parallel to the surface. However, if one end of 

 each log is loaded with a mass of iron or brass of the 

 proper weight, it floats upon the surface and the longi- 

 tudinal axis becomes vertical. If there is just a suffi- 

 cient number of logs, the surface becomes covered with 

 a single layer of vertical logs with their sides more or 

 less in contact, while with any greater number, bunches 

 of logs are found raised above the common level in 

 certain places. If the number is smaller, a part of the 

 surface remains uncovered. These phenomena may be 

 illustrated by the use of a large number of cylindrical 

 sticks of wood 3 mm. in diameter and 14 cm. long, 

 weighted by a small cylinder of brass placed at one end. 

 These are thrown upon the surface of the water in a 

 large glass cylinder. This is represented in a diagram- 

 matic way in Figure 9. If one of the vertical sticks is 

 taken from the water, the brass weight removed, the 

 stick dropped upon a vacant space upon a water sur- 

 face, it at once assumes a horizontal position, thus 

 exhibiting another type of orientation. 



It is well known that the molecules or ions which make up a crystalline solid are 

 arranged in an orderly way. A certain type of orderly array of very long and highly 

 symmetrical molecules is found also in certain liquids, which are said to contain 

 liquid crystals. Ordinary liquids are often supposed to be characterized by a complete 

 disorder in the arrangement of their molecules, but it is probable that this disorder has 

 been overemphasized. For example, in organic liquids of the type of acetic acid, the 

 molecule of which consists of the polar carboxyl group, and the "non-polar" methyl 

 group, there is some evidence of molecular association, presumably a type of orienta- 

 tion in which two or more polar groups come close together in the pure liquid as they 

 do in benzene' (Fig. 11, upper part). The theory that the molecules in the surface of a 



' The number of molecules which unite in this manner must be large in some groups, as they give 

 definite X-ray patterns. The grouping may be more analogous to that given by the bristles of two 

 brushes which are set with bristles together; this is called the "cybotactic state." 



-- j^ Water r^ 



Fig. 9. — Orientation of mole- 

 cules of an alcohol (or an organic 

 acid) at the surface of its aqueous 

 solution. 



