868 



MA TURE 



[Decembkk 15, 1923 



The field of force surrounding an attracting molecule 

 may in reality l>e vtr\' " irrtgulur," and may be specially 

 localised around certain active or " polar " group. 

 Its region of sensible magnitude may be very variable 

 and relatively small comfwired with molecular dimen- 

 sions. The chemical constitution of the molecule is 

 now regarded as determining the var>'ing nature of 

 the field of force surrounding it, so that parts of the 

 molecule possessing high " residual chemical affinity " 

 give rise to six^cially powerful regions of force. In this 

 way the older " physical " theories of cohesion accord- 

 ing to central forces with uniform orientation have 

 been to some extent replaced, or at all events supple- 

 mented, by " chemical " theories according to which 

 the attractive force-fields are highly localised round 

 active chemical groups and atoms, arc relatively 

 minute in range, and can be saturated or " neutralised " 

 by the atoms or groups of neighbouring or juxtaposed 

 molecules. 



W. B. Hardy has been the chief pioneer in the 

 development of these newer theories, having been led 

 thereto by his researches on surface tension, surface 

 films, composite liquid surfaces, and static friction and 

 lubrication. If y^ be the surface tension of a liquid A, 

 7„ that of another practically immiscible liquid B, and 

 y^H the interfacial tension at the interface A/B, then 

 the quantity W = 7^ + 71,-7^8 represents the decrease 

 of free surface energ>', and therefore the maximum 

 work gained, when a surface of A is allowed to approach 

 normally and touch a surface of B at constant tempera- 

 ture. Comparing different liquids A with water as a 

 constant liquid B, Hardy has shown that the quantity 

 W is extremely dependent on the chemical constitution 

 of A, and is especially high when A contains the atomic 

 groups characteristic of alcohols, acids, and esters. 

 Thus, for such saturated substances as octane, cyclo- 

 hexane, CSg and CCI4, the values of W at ordinary 

 room temperature lie between 21 and 24. Compare 

 with these values the following : 



(a) Introduction of a hvdroxyl group : 



Octyl alcohol . .46 



Cyclohexanol . . .51-4 



(^) Introduction of a carboxyl group : 



n — Caprjiic acid . . . 464 

 Oleic acid . . .44-7 



The natural inference from results such as these is 

 that the cohesional forces are essentially chemical in 

 origin and that they depend in large measure on the 

 presence of " active " atoms or groups of atoms, 

 namely, those possessing strong fields of " residual 

 chemical affinity " ; in other words, powerful and 

 highly localised stray fields of electrical or electro- 

 magnetic force (or of both types). The existence of 

 such atoms or atomic groups is strong presumptive 

 evidence of the unsj-mmetrical fields of force postulated 

 by Hardy and therefore of the molecular orientation 

 at surfaces. 



This question of the orientation of molecules at the 

 surfaces of liquids has been greatly extended in recent 

 years by a detailed study of the extremely thin and 

 invisible films formed by the primary- spreading of 

 iiily substances on the surface of water. In a continua- 

 tion and development of the work of Miss Pockels, 

 the late Lord Rayleigh showed many years ago that 

 when olive oil forms one of these invisible films on 



NO. 2824, VOL. I 1 2] 



water there is no fall in surface tension until the suria < 

 concentration reaches a certain very small value. 11< 

 made the highly and important suggesii n 



that this concentr rks the point where ther» i^ 



formed a continuous layer just one mok( ule \.\\v k 

 In the case of olive oil, he found this critical thicknta^ 

 to be 10' cm., and concluded that this number repre- 

 sented the order of magnitude of the diameter of a 

 molecule of the oil. This method was greatly developed 

 by Devaux. 



Although these researches had firmly established the 

 theor>' of the formation of a unimolecular surface 

 layer and therefore of the existence* of a netc " txeo- 

 dimensional " phase 0/ matter, we owe it to I. I. 

 to have made a very important advance by c< : 

 this conception with the ideas of chemically active 

 groups and molecular orientation. Influenced, no 

 doubt, by the ideas of Hardy, Langmuir reasoned that 

 the formation of these primary unimolecular films 

 must be due to the presence of active groups in the 

 molecules, which are attracted inwards towards the 

 water and thus cause the long open chain molecules of 

 the fatty acids to be oriented on the water surface with 

 their long hydrocarbon axes vertical and side by side. 



Working by means of the method of Devaux, 

 Langmuir put these ideas to the test of experiment, 

 and determined the internal molecular dimensions of 

 a unimolecular layer. Calculation of the average 

 distance between two adjacent carbon atoms in the 

 three acids gave the value 1-4x10 ' cm. This 

 distance must be of the order of magnitude of the 

 distance between the centres of the carbon atoms 

 in the cr\-stal structure of a diamond, which is now 

 known to be 1-52 x 10* cm. 



These regularly oriented and unimolecular surface 

 films on water have been recently investigated in a 

 ver>' detailed and careful manner by N. K. Adam, 

 who has improved the method employed by Devaux 

 and Langmuir. P"rom a closer analysis of the relation- 

 ship between the force of surface compression and the 

 surface concentration (expressed as area occupied per 

 molecule), he has showTi that a distinction must be 

 made between the close packing of the polar or active 

 end groups (head groups) of the molecules and the 

 subsequent close packing of the hydrocarbon chains. 



Some interesting results have also been obtained 

 in Sir William Bragg's laboratory- by Dr. A. Muller. 

 In these experiments layers of crystallised fatty acids 

 on glass plates have been examined by an X-ray 

 photographic method. From these results it appears 

 that the unit cell is a long prism, the cross section of 

 which remains constant for the substances investigated, 

 whilst the length of the prism increases linearly with 

 the number of carbon atoms in the molecule. The 

 increase in length of the unit prism per carbon atom 

 in the molecule is found to be 2-0x10"* cm. Since 

 it appears likely that there are two molecules arranged 

 along the long axis of each unit cell (prism), it would 

 follow that the increase in the length of the molecule 

 per carbon atom added is 10 x 10"* cm. Comparing 

 this result with the value for the distance between 

 the carbon centres in the diamond lattice, it would 

 appear that the carbon atoms in the long hydrocarbon 

 chains of the higher saturated fatty acids are arranged 

 in a zig-zag, or more probably in a spiral or helix. 



