We present a critical review on ion-specific effects in foams in the presence of added salts. We show the theoretical basis developed for understanding experimental data in systems with ionic surfactants, as well as the nascent approaches to modeling the much more difficult systems with non-ionic surfactants, starting with the most recent models of the air-water interface. Even in the case of ionic surfactant systems, we show methods for improving the theoretical understanding and apply them for interpretation of surprising experimental results we have obtained on ion-specific effects in these systems. We report unexpectedly strong ion-specific effects of counter-ions on the stability and the rate of drainage of planar foam films from solutions of 0.5 mM sodium dodecyl sulfate (SDS) as a function of concentration of a series of inorganic salts (MCl, M = Li, Na, K). We found that the counter-ions can either stabilize the foam films (up to a critical concentration) or destabilize them beyond it. The ordering for destabilization is in the same order as the Hofmeister series, while for stabilization it is the reverse Therefore, the strongest foam stabilizer (K+), becomes the strongest foam destabilizer at and beyond its critical concentration, and vice versa. Though the critical concentration is different for different salts, calculating the critical surfactant adsorption level one could simplify the analysis, with all the critical concentrations occurring at the same surfactant adsorption level. Beyond this level, the foam lifetime decreases and films suddenly start draining faster, which may indicate salt-induced surfactant precipitation. Alternatively, formation of pre-micellar structures may result in slower equilibration and fewer surfactant molecules at the surface, thus leading to unstable foams and films.
Bibliographical noteFunding Information:
We thank for financial support the European research council (ERC) grant EMATTER ( #280078 ) to S.S., as well as COST action “ Smart and Green Interfaces ” ( MP1106 ) for STSM funding. Stoyan Karakashev thanks the Bulgarian National Science Fund Grant DDVU 02/54 and FP7 project Beyond Everest. We thank Prof. I.B. Ivanov for the kindly supplied data of the surface tension measurements and for fruitful discussions.
© 2015 Elsevier B.V. All rights reserved.
- Critical micelle concentration
- Foam film drainage
- Foam films
- Ion-specific effects
ASJC Scopus subject areas
- Surfaces and Interfaces
- Physical and Theoretical Chemistry
- Colloid and Surface Chemistry