2 PRINCIPLES OF STRUCTURE 49 



which causes a rise in surface tension), the surface properties of the 

 water are almost unchanged. 



With increasing length of the paraffin chain, the hydrophobic 

 character of the alcohol molecules becomes more pronounced, and 

 finally their affinity for water is so small that they accumulate in 

 quantity at the surface. The same applies to the fatty acids. Their 

 aliphatic chains are so hydrophobic, that they float on the surface of 

 the water. These floating molecules tend to keep as far apart as possible, 

 in much the same way as the gas molecules in a given volume. They 

 spread over the whole available water surface. The expansive pressure 

 which brings about this spreading can be measured by means of a 

 movable barrier. In the apparatus, originally designed by Langmuir 

 in 1 91 7 (Langmuir tray), the spreading pressure is transferred from 

 a movable barrier to a torsion balance and measured in dyn/cm with 

 an accuracy of up to o.oi dynes per cm. 



The surface law found wich this measuring instrument is similar to 

 the gas law "volume X pressure = constant", in that the product of 

 surface per mole and surface pressure is constant. The floating mole- 

 cules therefore behave like a gas : the surface density can be increased 

 by reducing the surface. This "surface compression", however, cannot 

 be carried too far ; if the surface is reduced below a certain limit, the 

 surface pressure increase becomes steeper than that required by a 

 constant value of the product. At this Hmit the molecules, which 

 hitherto were freely movable, cluster into a close-packed monolayer 

 (monomolecular film), which has less compressibility. In these films 

 the polar molecules stand up, withdrawing their hydrophobic groups 

 from the water and dipping their hydrophilic groups into the water 

 (Fig. 43c). 



The thickness of the film can be calculated from the amount of 

 substance spread on the water and the size of the surface (Adam, 1930). 

 This thickness corresponds to the length 1 of the chain molecule 

 (Fig. 43c), and the values found in this way compare well with those 

 derived from the X-ray investigation of molecule lattices. From the 

 molecular weight of the substance under examination, i.e., from the 

 number of molecules packed in the surface layer, the distance between 

 the chain molecules can be computed; here again the values obtained 

 are similar to those found by X-ray analysis for the distance between 

 the chains in molecule lattices (order of magnitude: 4-5 A). 



