Effect on Foamability of Mesophase Precipitation in Aqueous Surfactant Solutions

3 EFFECT ON FOAMABILITY OF MESOPHASE PRECIPITATION IN AQUEOUS SURFACTANT SOLUTIONS 

In the case of two-chain surfactant molecules, the alkyl chain length is short for a given alkyl chain volume, v. The molecular packing parameter is therefore more 

FIGURE 4.3 Foamabilities (O) and turbidities ( ) of 3.8 mM AOT solutions in aqueous NaCl solutions. (Reprinted from Adv. Colloid Interface Sci., 48, Aveyard, R., Binks, B.P., Fletcher, P.D.I., Peck, T.G., Rutherford, C.E., 93. Copyright 1994, with permission from Elsevier.) 

Similar behavior is apparent with other two-chain anionic surfactants. Thus, for example, addition of NaCl at concentrations 0.025 M to aqueous solutions of sodium 6-phenyltridecanesulfonate produces vesicles of -500 nm diameter at surfactant concentrations 0.2 x 10- M [29], The latter compares with the CMC in the absence of salt of 5 x 10 M [29], Addition of increasing amounts of salt at surfactant concentrations higher than the CAC therefore produces a progressively increasing turbidity accompanied by a declining foamability much as observed with AOT [26]. Measurements of the breakdown times of vesicles are in the time range 0.1-10 s, which compares with 1 ms in the case of micelles [29]. This is clearly consistent with a contribution of slow vesicle breakdown rates to slow overall transport of surfactant to air-water surfaces, and therefore to low foamability. 

Addition of divalent metal ions, such as Ca to solutions of these two-chain sodium alkylbenzene sulfonates (NaLAS) can also result in the formation of a lamellar phase (La) liquid crystalline precipitate of approximate composition Ca(LAS)2. Onset of such precipitation produces a marked reduction in foamability as shown 

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