The effect of surface-active additives on bubble coalescence in aqueous media

Abstract An experimental study has been aimed at examining the coalescence phenomena in aqueous solutions of aliphatic alcohols and electrolytes and at determining the effect of these surface-active compounds on bubble coalescence in viscous aqueous media with Newtonian and non-Newtonian behaviour, represented by aqueous solutions of 50 and 60 wt%, saccharose, and 0.2 and 0.5 wt%, xanthan, respectively. Bubble coalescence in aqueous solutions of alcohols and electrolytes was significantly hindered by increasing solute concentration and the coalescence percentage ψ exhibited sharp transition from 100% of bubble pairs coalescing to very low values ( ψ⩽10% ) over a narrow concentration range. Transition concentrations of respective surface-active additives, characterising suppression of coalescence in their aqueous solutions, were successfully correlated as a function of the term [σ n (∂σ/∂c) −2 ] , with exponent values n= 1 3 and 1 4 (or 1 6 ) for solutions of alcohols and electrolytes, respectively. The inhibitory effect of alcohols on bubble coalescence increased with the increasing length of the carbon chain and their transition concentrations were successfully correlated as a function of the number of carbon atoms in the alcohol molecule, c t =31n −7.2 C . The addition of aliphatic alcohols to fully coalescent viscous solutions of saccharose and xanthan modified significantly coalescence behaviour in these viscous media. Transition concentrations of individual alcohols in 50 wt% saccharose solution and 0.2 wt% xanthan solution were very close to those observed in aqueous alcohol solutions, further increase of medium viscosity resulted, however, in an increase of transition concentrations up to an order of magnitude. The character of their dependence on the number of carbon atoms in the alcohol molecule ( c t ∼n b C ) remained, however, unchanged for all viscous media studied and values of exponent varied only slightly between −7.1 and −7.7. The effect of electrolytes on bubble coalescence in viscous media was less significant, as compared with the alcohol additives, and differed with the nature of medium.