Surface Tension Measurements with the Drop Profile Analysis Tensiometry—Consideration of the Surfactant Mass Balance in a Single Drop

Surfactant Adsorption Layers: Experiments and Modeling

During recent years, great progress has been made in understanding the adsorption of surfactants at liquid interfaces. In addition to tensiometry, new efficient methodologies have been developed, in particular interfacial selective optical methods which allow direct access to the adsorbed amounts and interfacial layer compositions. In addition to these new experimental tools, the thermodynamic description by equations of state now allows one to provide a quantitative picture of surfactant interfacial layers. This is most notable for surfactant layers at water/oil interfaces. Additional knowledge about the structure of interfacial layers was gained through different types of molecular modeling. Improved interrelationships between these three aspects are the challenges for current and future work. Particular attention must be paid to dilational interfacial rheology studies, as these mechanical quantities are much more sensitive to small changes in the interfacial composition and structure.

Can small air bubbles probe very low frother concentration faster?

The existing literature on the rise velocities of air bubbles in aqueous surfactant solutions adsorbing at the water-air interface focuses mainly on large bubbles (D > 1.2 mm). In addition, due to the way the bubbles in rising bubble experiments are formed, their size is dependent on interfacial tension (the lower the interfacial tension the smaller the bubble). In this paper, smaller air bubbles (D < 505 ± 3 μm) are used to investigate the effect of the bubble size on the detection of two flotation frothers of different adsorption kinetics via bubble rise velocity measurements. We use an alternative method for bubble generation, allowing us to compare the rise velocity of bubbles of the same size in solutions of frothers of varying bulk concentration. The approach taken (ensuring consistent bubble size) ascertains that the buoyancy force component is kept constant when comparing the different solutions. As a consequence, any variations in the bubble rise velocity can be related to changes in the hydrodynamic drag force acting on a rising bubble. The interfacial behavior of frothers, i.e. the adsorption kinetics, interfacial activity and the maximum amount of molecules adsorbed at the interface, are determined from interfacial tension measurements and adsorption isotherms. The differences in the degree of tangential immobilisation caused by two different frothers are discussed in the context of differences in the structure of the dynamic adsorption layer, which is formed during the bubble rise.

Surfactant Adsorption to Different Fluid Interfaces

Surfactant adsorption to fluid interfaces is ubiquitous in biological systems, industrial applications, and scientific fields. Herein, we unravel the impact of the hydrophobic phase (air and oil) and the role of oil polarity on the adsorption of surfactants to fluid interfaces. We investigated the adsorption of anionic (sodium dodecyl sulfate), cationic (dodecyltrimethylammonium bromide), and non-ionic (polyoxyethylene-(23)-monododecyl ether) surfactants at different interfaces, including air and oils, with a wide range of polarities. The surfactant-induced interfacial tension decrease, called the interfacial pressure, correlates linearly with the initial interfacial tension of the clean oil-water interface and describes the experimental results of over 30 studies from the literature. The higher interfacial competition of surfactant and polar oil molecules caused the number of adsorbed molecules at the interface to drop. Further, we found that the critical micelle concentration of surfactants in water correlates to the solubility of the oil molecules in water. Hence, the nature of the oil affects the adsorption behavior and equilibrium state of the surfactant at fluid interfaces. These results broaden our understanding and enable better predictability of the interactions of surfactants with hydrophobic phases, which is essential for emulsion, foam, and capsule formation, pharmaceutical commodities, cosmetics, and many food products.

New view of the adsorption of surfactants at water/alkane interfaces - Competitive and cooperative effects of surfactant and alkane molecules

The theoretical description of the adsorption of surfactants at interfaces between aqueous solutions and oil was based over a very long time on models derived for the solution/air interface. Thus, most of the experimentally observed peculiarities could not be specifically considered but were merely interpreted in terms of a penetration of oil molecules into the alkyl chain layer of the adsorbed surfactant molecules. These penetrating oil molecules enhance the surfactant adsorption as compared to the water/air interface. Later on, for the special situations at water/oil interfaces a competitive adsorption of surfactant and oil molecules was postulated, allowing a much better description of experimental data. This picture, however, was unable to explain why the interfacial tension of the water/oil interface decreases very quickly when extremely small amounts of surfactants are added to the water. This effect cannot be of competitive nature, but a cooperativity of surfactant and oil molecules forming a mixed adsorption layer is required instead. This cooperative effect means that already few surfactant molecules adsorbed at the interface can induce a significant ordering of oil molecules in the interfacial layer. This new interfacial structure, in turn, attracts further surfactant molecules to adsorb. Improving the theoretical description of experimental data was finally achieved by applying suitable adsorption models for the two adsorbing compounds, i.e. a Frumkin adsorption model for the oil molecules and a Langmuir, Frumkin, or reorientation model for the adsorbing surfactant molecules. Here, the progress in modelling surfactant adsorption at water/oil interfaces is discussed mainly for the homologous series of the cationic surfactants C_nTAB, of the anionic surfactant SDS, and members of the homologous series of the non-ionic surfactants C_nDMPO at water/alkane interfaces.

Critical Synergistic Concentration of Binary Surfactant Mixtures

This paper presents a simple method for determination of synergism in binary surfactant mixtures. A homologous series of cationic alkyltrimethylammonium bromides (CnTAB, with n = 8, 12, 16, 18) mixed with three non-ionic surfactants (n-octanol, methyl isobutyl carbinol, tri(propylene glycol) butyl ether) was chosen as a model system. In addition to the cationic-non-ionic system, the mixture of anionic-non-ionic surfactants (sodium dodecyl sulphate and tri(propylene glycol) butyl ether) was investigated. The foam behavior of one-component solutions and binary mixtures was characterized as a function of surfactant concentration, number of carbons (n) in alkyl chain of CnTAB as well as type of surfactant. It was shown that synergism in foamability could be produced by the ionic-non-ionic systems, and the concentration below the synergism occurs, called the critical synergistic concentration (CSC), that can be easily predicted based on the surface tension data on individual components.

Particular Behavior of Surface Tension at the Interface between Aqueous Solution of Surfactant and Alkane

A two-component interfacial layer model was employed to describe the experimental results obtained for various surfactants. In contrast to the previous works, here it is shown that the adsorption activity of alkane depends on its interaction with the adsorbed surfactant and is proportional to the surface coverage by this surfactant. Also, it is assumed that this increase of the adsorption activity parameter is limited by some maximum value. This model provides a good description of the influence of hexane and dodecane, which results in the decrease of surface tension by 2-5 mN/m at very low surfactant concentrations. The adsorbed amounts of the surfactant and alkane molecules in this low surfactant concentration range have been calculated. The reorientation model of surfactant adsorption predicts a smaller number of alkane molecules per one surfactant molecule than that which follows from the Frumkin model.

Insights into the complex interaction between hydrophilic nanoparticles and ionic surfactants at the liquid/air interface

The effect of interaction between hydrophilic nanoparticles and ionic surfactants on the liquid/air interfacial properties has been investigated, and a possible mechanism has also been proposed.

Surface Tension and Adsorption Studies by Drop Profile Analysis Tensiometry

Surface tension and dilational viscoelasticity of solutions of various surfactants measured with bubble and drop profile analysis tensiometry are discussed. The study also includes experiments on the co-adsorption of surfactant molecules from a solution drop and alkane molecules from saturated alkane vapor phase. Using experimental data for 12 surfactants with different surface activities, it is shown that depletion due to adsorption of surfactant from the drop bulk can be significant. An algorithm is proposed quantitatively to take into consideration the depletion effect which is required for a correct description of the co-adsorption of alkanes on the solution drop surface and the correct analysis of experimental dynamic surface tension data to determine the adsorption mechanism. Bubble and drop profile analysis tensiometry is also the method of choice for measuring the dilational viscoelasticity of the adsorbed interfacial layer. The same elasticity moduli are obtained with the bubble and drop method only when the equilibrium surface pressures are sufficiently small (Π < 15 mN m-1). When the surface pressure for a surfactant solution is larger than this value, the viscoelasticity moduli determined from drop profile experiments become significantly larger than those obtained from bubble profile measurements.