SOAPS 



107 



OIL 



■tSaCl 



+ Ca CI2 



+ NaCL 

 + CaCl, 



soaps of the lower fatty acids to the colloidal ^els of the higher 

 honiologues. This is largely due to the steady increase in water- 

 holding capacity with increase in the length of the carbon chain. 

 The sodium soaps of the acetic series show this gradation in imbibi- 

 tion very well. For instance, a gram-molecule of the sodium soap 

 of caprylic acid can hold 200 c.c. of water while that of arachidic 

 acid is capable of imbibing 37,000 c.c. 



Still more important physiologically is the effect of altering the 

 cation. Sodium, potassium, ammonium, calcium and magnesium 

 soaps are found in physiological analyses and these differ from one 

 another, especially in their power to hold loater. Ammonium and 

 potassium soaps are so hydrophilic that they do not solidify but 

 form jellies (soft soap). 

 Sodium soaps also hold 

 a considerable amount 

 of water, but only about 

 I of that held by "soft" 

 soaps. So little water is 

 held by the soaps of 

 calcium and magnesium 

 that they do not form a 

 sol to any appreciable 

 extent. The addition 

 of alkali to a sodium 

 soap greatly increases its 

 hydrophilic properties. 



Sodium soaps, used as 

 emulsifying agents, pro- 

 duce oil-in-water sys- 

 tems (secretions ?), while 

 the calcium and magnesium soaps favour the water-in-oil type. 

 Therefore, in a mixture of these soaps there wdll be a com- 

 petition for the surface of the oil drops. If the lime salt pre- 

 dominates interfacial tension will become greater, while it will 

 be markedly decreased w'hen a superabvmdancc of sodium is 

 present. This is clearly illustrated in Fig. 23. In tube («) 

 the oil is allowed to drop from a capillary tube (stalagmometer, 

 Part II.) into very dilute sodium hydrate (N/1,000). This may be 

 taken as the standard = 5 drops per unit time. The second tube 

 demonstrates unequivocally what happens when, under the same 

 standard conditions, sodium chloride (N/6) is added to the soda. 

 There are now 30 drops per unit time — ^the surface tension has been 

 lowered 3,650 times. The substitution of the chloride of calcium 

 for that of the monovalent sodium (N/1,000) cuts down the drops 



Fig. 23. — To demonstrate the effect on the number and size 

 of drops formed from a ijiven vohime of olive oil dropping from 

 a standard tip tlirough solutions of O'ODl >', >'a<)H alone 

 (tube 1). and plus ()-15 N. XaCl (tube 2). plus 0-0007 X, Cal 1., 

 (tube 3), and in tube 4 plus both Nail and CaCl,. (After 

 (i. H. A. Clowes). 



