FOAM FORMATION AND FOAM STABILITY in Q and R is due to the of .liquid raised by the frame as a result of the thickness of its horizontal wire.' Along RS the surface of the is extended under a constant i}•i!Pull and the work of extending is ?/:•-S where ¾ is the force recorded •'•i•?'along RS and •-S the increase in (area. This demonstrates that the •.•?'force recorded along RS is the sur- ii!'iace tension of the liquid. ' ii!ii•Figure 4. Lamina formationß (See foot of i•:•?::!:: .: previous page.) •i. ." :., Using the slow traction speed (1 cm. per 10 min.) it has been possible to compare the mechanical stabilities of lamina• originated from Na oleate and Na dodecylsulphate systems. Curves have been prepared giving the results for these two systems at various dilutions. Lamina formation is first observed inNaoleatebetween M and M -- ß 5000 2000 The extension of the lamina increases M steadily till a concentration of 200 is reached, after which it decreases again. It is interesting to note that, although the surface tension remains constant for concentration above M the traction curve shows that 800, the lamina extension along RS varies greatly. On the other hand, the results obtained for Na dodecylsulphate show that in no case is the lamina extension marked. Only very small lamina• are observed at concentra- tions equal or greater than 00. Whereas with the Na oleate it,is possible to follow the slow drainage of the interlamellar liquid by observ- ing the interference bands, with the Na laurylsulphate the draining is much more rapid. This leads to a .very rapid thinning and a breakage of the small laminas. The fundamental difference in behaviour between the Na oleate and the Na laurylsulphate is easily found in the structure of the adsorbed layer. In the oleate system 221

JOURNAL OF TIlE SOCIETY OF COSMETIC CHEMISTS the adsorbed layer is formed by the fatty acid soap, which tends to give a liquid condensed structure, due to ion-dipole interaction between the fatty ion and the fatty acid resulting from the hydrolysis of the oleate. In the Na laurylsulphate no hydrolysis is possible and the adsorbed layer is formed by mutually repelling alkylsulphate ions. It was foundsø that the Na laurylsulphate could lead to traction curves similar to those of the oleate system when complexing agents such as octyl or nonyl alcohol were present in the aqueous phase. Under these conditions a mixed structure is obtained in the surface layer, where the alkylsulphate chains are separ- ated by the long chain alcohols. This results in an ion-dipole inter- action which stabilises the surface structure. Non-ionic surface-active agents, such as those obtained by the 'reaction of ethylene oxide on long chain alcohols, were studied in collaboration with Cohen. • These materials lead to prolonged laminae stability for a particular hydro- phobic/hydrophilic balance in the molecule. For an alcohol of n carbon atoms, these derivatives with n- 3 ethylene oxide lead to an extended RS portion of the traction curve. For a longer polyoxyethylene chain, the adsorbed layer is too hydrophilic and the lamina stability is markedly decreased. 2. High Traction Speeds For traction speeds of 1 cm. per 10 seconds or greater, it becomes 222 ' Figure 5. (See below.) possible to characterise the mechani- cal stabilities of .relatively unstable lamina• such as those obtained from ": Na laurylsulphate aqueous solution. In addition to the traction curves already described, a fourth type :: may be now observed where the .: extension of the lamin.a along RS occurs under increasing tension (see Figure 5). This is due to a lowering of the concentration of the adsorbed . molecules as the surface layer is:! extended along RS.- Type IIa obtained with relatively dilute solu-111111 tions of alkylsulphate, alkylsulphon- if( ! ate, alkylarylsulphonate, etc. suggests that the adsorption of the.!•.•}•'• molecules from the bulk solution insufficient to compensate for increase in surface area. Systems':•:• leading to Type IIa exhibit relatively smaller foaming volume:•ii as compared with those showing Type III traction curve. The increase in tension along RS}j• may reach values as high as 5 The author has previously published•:i• a whole series of traction curveS'i'i• obtained with Na oleate and laurylsulphate respectively.' Witl•?i •I Na oleate a lamina is first observed:i}: