Stoyan I. Karakashev

Anomalous ion effects on rupture and lifetime of aqueous foam films formed from monovalent salt solutions up to saturation concentration

We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO 3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstable they thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO 3, the films were unstable at all radii and salt concentrations they thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.

Hofmeister effect on micellization, thin films and emulsion stability

The Hofmeister effect on the critical micelle concentration (CMC), the thin liquid film electrostatic disjoining pressure (Π(el)) and the critical coalescence pressure of emulsion drops (P(CR)) were investigated. For CMC literature data were used, but Π and P(CR) were measured by us. The essence of the theoretical approach was to modify existing theories of CMC and Π(el) by using generalized Gouy equation and dimensionless surface potential (Φ(S)), involving the counterion specific adsorption energy (u(0)). The computational procedure of u(0) does not involve any adjustable parameters. Linear dependences of ln(CMC), Φ(S) and P(CR) on u(0) were found in conformity with Hofmeister series. The experimental slopes of ln(CMC) and Φ(S) vs. -u(0)/k(B)T were negative and very close to the theoretical ones. A hypothesis was put forward for explanation of the positive slopes of P(CR) on u(0). The obtained results suggest that the counterion specific adsorption energy u(0) encompasses all major factors, involved in the Hofmeister effect for the studied phenomena. If this is confirmed by analysis of more phenomena, revealing Hofmeister effect, one could claim that u(0) is the factor controlling the Hofmeister effect and a powerful tool for its study.

Understanding the role of ion interactions in soluble salt flotation with alkylammonium and alkylsulfate collectors

There is anecdotal evidence for the significant effects of salt ions on the flotation separation of minerals using process water of high salt content. Examples include flotation of soluble salt minerals such as potash, trona and borax in brine solutions using alkylammonium and alkylsulfate collectors such as dodecylamine hydrochloride and sodium dodecylsulfate. Although some of the effects are expected, some do not seem to be encompassed by classical theories of colloid science. Several experimental and modeling techniques for determining solution viscosity, surface tension, bubble-particle attachment time, contact angle, and molecular dynamics simulation have been used to provide further information on air-solution and solid-solution interfacial phenomena, especially with respect to the interfacial water structure due to the presence of dissolved ions. In addition atomic force microscopy, and sum frequency generation vibrational spectroscopy have been used to provide further information on surface states. These studies indicate that the ion specificity effect is the most significant factor influencing flotation in brine solutions.

Tuneable Control of Interfacial Rheology and Emulsion Coalescence

Breaking point: Switchable peptide surfactants are used to demonstrate that the extent of cross-linking in an interfacial surfactant layer can control the rate of emulsion coalescence. Pictured is the rupture of an aqueous thin film where the peptide layer lacks sufficient strength to prevent hole formation, but nonetheless dramatically slows the rate of hole expansion.

Effect of Environmental Humidity on Static Foam Stability

The quality of foaming products (such as beer and shampoo) and the performance of industrial processes that harness foam (such as the froth flotation of minerals or the foam fractionation of proteins) depend upon foam stability. In this study, experiments are performed to study the effect of environmental humidity on the collapse of static foams. The dependency of the rate at which a foam collapses upon humidity is demonstrated, and we propose a hypothesis for bubble bursting due to Marangoni instability induced by nonuniform evaporation to help explain the dependency. This hypothesis is supported by direct experimental observations of the bursting process of isolated bubbles by high speed video recording and the thinning of isolated foam films under different values of humidity and temperature by microinterferometric methods.

Anomalous thickness variation of the foam films stabilized by weak non-ionic surfactants

The constant thickness (H) of metastable free films of various non-ionic surfactant solutions was measured at surfactant concentrations less than the critical micelle concentrations or solubility limits with fixed 5x10(-5) M sodium chloride (NaCl) serving as the background electrolyte. The surfactants include n-pentanol, n-octanol, methyl isobutyl carbinol (MIBC), polypropylene glycol (PPG-400), tetraethylene glycol monooctyl ether (C(8)E(4)), and tetraethylene glycol monodecyl ether (C(10)E(4)). H was interferometrically measured. For each surfactant in this study, the H-versus-surfactant-concentration curve finds a peak at a concentration around 5x10(-6)-1x10(-5) M and a valley at a higher concentration. The measured H values were compared to those predicted from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which considers solely the contribution from electrostatic double-layer repulsion with van der Waals attraction being neglected in the present work. In determining the double-layer repulsion, the ionic strength was determined from the electrolytic conductivity measurement of the film-forming solutions and the surface potential was estimated from the zeta-potential measurement of air bubbles. It was found that the DLVO theory failed to explain the thickness variance with surfactant concentration, implying that additional non-DLVO attractive forces might be required to explain the experimental results. Finally, the possible origins of these attractive forces were discussed.

An investigation of bubble coalescence and post-rupture oscillation in non-ionic surfactant solutions using high-speed cinematography

Most processes involving bubbling in a liquid require small bubbles to maximise mass/energy transfer. A common method to prevent bubbles from coalescing is by the addition of surfactants. In order to get an insight into the coalescence process, capillary bubbles were observed using a high speed cinematography. Experiments were performed in solutions of 1-pentanol, 4-methyl-2-pentanol, tri(propylene glycol) methyl ether, and poly(propylene glycol) for which information such as the coalescence time and the deformation of the resultant bubble upon coalescence was extracted. It is shown in this study that the coalescence time increases with surfactant concentration until the appearance of a plateau. The increase in coalescence time with surfactant concentration could not be attributed only to surface elasticity. The oscillation of the resultant bubble was characterised by the damping of the oscillation. The results suggested that a minimum elasticity is required to achieve an increased damping and considerable diffusion has a detrimental effect on the dynamic response of the bubble, thereby reducing the damping.

Equilibrium Adsorption of Surfactants at the Gas-Liquid Interface

Theories on equilibrium adsorption of surfactants at the gas-liquid interface have been reviewed andvalidated. For the adsorption of nonionic surfactants, the thermodynamic approach of Butler has been used,in conjunction with the Lucassen-Reynders dividing surface, to describe the adsorption layer state and adsorptionisotherm as a function of partial molar area. Applying the Butler–Lucassen–Reynders modelingapproach provides the generalized adsorption isotherm and equation of state, which is capable of describingthe effect of the surfactant orientational states and aggregation at the interface. For Langmuirian andFrumkinian surfactant adsorption, the Butler–Lucassen–Reynders modeling approach produces thesame predictions for surface tension as described by the well-known Langmuir and Frumkin adsorption isotherms.The adsorption of ionic surfactants and ionic–nonionic surfactant mixtures has been described followingthe traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapmanelectrical double layer electrostatics. The developed theories have been validated through comparison withthe experimental data on surface tension. Regression analysis by minimizing the reduced chi-square hasbeen used to best fit the models to the experimental data to obtain the model free parameters. For thesurfactant homologous series of octaethyleneglycol-n-alkyl ethers C n H2n+1O(CH2CH2)8H,the negative sign of the intermolecular interaction parameter obtained in the regression analysis of surfacetension has not been resolved by the model for the surfactant orientational state at the interface. Forthe surfactant series, the surface aggregation model gives physically consistent fitting and parameters.The models for adsorption of ionic surfactants have been validated using the surface tension of a seriesof sodium n-hexadecylsulfates with the sulfate group located at the differentpositions in the hydrocarbon chain, a homologue series of sodium alkyl sulfates, and a seriesof alkali dodecylsulfates. Improved adsorption models for ionic surfactants have been developed throughfundamental modeling of the adsorption processes and the molecular interactions in the adsorption layers.The improved predictions reduce the required number of free parameters and agree with the surface tensionand surface potential data better than the conventional models.

Thin liquid film drainage: Ionic vs. non-ionic surfactants

This paper compares the rate of drainage of thin liquid films (TLF) containing ionic surfactants to that of TLF containing non-ionic surfactants. In essence, the theory of drainage has been developed for films containing non-ionic surfactants, while the validation of the models, based on the theory in the literature, has been performed with experiments on TLF with both, non-ionic and ionic surfactants, usually in the presence of significant background concentrations of electrolyte. Due to the complexity of problem, the dynamic effects on the electrical double layer (EDL) during the film drainage have been ignored for many years in the literature. These effects were finally treated theoretically and the problem solved numerically in a recent work. The new theoretical development however has not yet been validated. In addition, the differences in the kinetics of thinning of TLF with ionic and non-ionic surfactants have not been exposed in the literature until present. This paper is dedicated to revealing these differences. Experiments on kinetics of thinning were conducted with microscopic planar TLF, containing two non-ionic surfactants (tetraethyleneglycol mono-octylether C(8)E(4) and n-dodecyl-D-maltoside C(12)G(2)) and two ionic surfactants (sodium dodecylsulfate SDS and tetrapentylammonium bromide TPeAB). The TLF with non-ionic surfactants drain according to the well-known theories of Scheludko or Radoev-Manev-Ivanov, which confirms their validity. On the contrary, TLFs with ionic surfactants drain in general at significantly slower rate, as compared to the TLF with non-ionic surfactants, when far from equilibrium. When they are close to the equilibrium conditions, the former drain according to the theory developed for TLF with non-ionic surfactants. An analysis of the experimental results, involving the latest achievements in the field is performed, indicating the complex behaviour of the electrical double layer under dynamic conditions.

Correlation in the properties of aqueous single films and foam containing a nonionic surfactant and organic/inorganic electrolytes.

Correlation of the behavior of foam from aqueous solutions of C(10)E(8) + TPeAB mixtures with the properties of the single foam films is sought through a theoretical analysis based on experimental data. The state in the adsorption layer on the air/solution interface has been determined from the surface tension experimental data by applying the Frumkin adsorption isotherm for mixed surfactants. Diverse parameters of the adsorption layer have been calculated. Values of the surface potential, estimated from the electrostatic disjoining pressure in the films, are found to be in good agreement with those calculated through the Grahame equation for the air/water interface. The energy of interaction (attraction) in the mixed adsorption layers is determined. The behavior of a foam body produced from solutions of the same composition, is also investigated in two independent ways using the stationary foam column and the foam life-time (decay rate) methods. Both techniques gave similar results with respect to foam stability.