Effect of selected monovalent salts on surfactant stabilized foams.

Surfactant-stabilized foams have been at the centre of scientific research for over a century due to their ubiquitous applications in different industries. Many of these applications involve inorganic salts either due to their natural presence (e.g. use of seawater in froth floatation) or their addition (e.g. in cosmetics) to manipulate foam characteristics for the best outcomes. This paper provides a clear understanding of the effect of salts on surfactant-stabilized foams through a critical literature survey of this topic. Available literature shows a double effect of salts (LiCl, NaCl and KCl) on foam characteristics in the presence of surfactants. To elucidate the underlying mechanisms of the stabilizing effect of salts on foams, the effect of salts on surfactant-free thin liquid films is first discussed, followed by a discussion on the effect of salts on surfactant-stabilized foams with the focus on anionic surfactants. We discuss two distinctive salt concentrations, salt transition concentration in surfactant-free solutions and salt critical concentration in surfactant-laden systems to explain their effects. Using the available data in literature supported by dedicated experiments, we demonstrate the destabilizing effect of salts on foams at and above their critical concentrations in the presence of anionic surfactants. This effect is attributed to retarding the adsorption of the surfactant molecules at the interface due to the formation of nano and micro-scale aggregates.

On the stability of thin films of pure water.

The stability of water films has been the focus of many researchers in the recent decades. Unfortunately, there is no consensus on the stability of these foam films or on the mechanisms responsible for stabilizing water films. This paper examines the reported results on this matter and scrutinizes them based on speciation analysis of the dissolved species and the recent achievements in the adsorption of inorganic ions on the air/water interface. Our results confirm the key role of surface contamination, interface approach velocity and evaporation in the drainage and lifetime of these water films. It confirms the stabilizing effect of contamination and the destabilizing effect of air-water interface approach velocity. Moreover, the negative sign of the surface/zeta potential of the air/water interface and its dependence on the pH value were explained.

On the nature of the superspreaders.

This is a review article on the basic and the latest achievements on superspreading. The complete and fast spreading of droplets on many surfaces in the nature is a special phenomenon discovered in 1960-ies Intensive studies on this phenomenon have been conducted since that time, but the mechanism of superspreading remained in completely unveiled till nowadays. Here we scrutinized the basic literature on superspreading from the last 25 years and also present results related to superspreaders acquired in the present work. The literature in superspreading can be divided to the following groups: (i) works on the properties of the trisiloxane surfactants; (ii) works on the mechanisms of superspreading; (iii) MD simulations; (iv) works on the effect of the trisiloxane surfactants on thin liquid films. There is a number of review articles published in the last decade related to mainly works from groups (i) and (ii). The works on MD simulations (iii) and the effects on trisiloxane surfactants on thin liquid films (iv) are still few despite they are important from the scientific view point. We conducted our own study on the effect of the superspreaders on foam films in rectangular frame and confirmed that the superspreaders cause powerful Marangoni effect within the foam films. Such a strong Marangoni effect has been never observed with the ordinary surfactants. We scrutinized and discussed the basic works from the groups (i)-(iv) on the superspreading and added our own investigation on the distinguishable effects of superspreaders and non-superspreaders on thin foam films. The work could be useful to both beginners and specialists in the field of wetting/de-wetting and superspreading.

CMC prediction for ionic surfactants in pure water and aqueous salt solutions based solely on tabulated molecular parameters.

The critical micelle concentration (CMC) of various surfactants is difficult to predict accurately, yet often necessary to do in both industry and science. Hence, quantum-chemical software packages for precise calculation of CMC were developed, but they are expensive and time consuming. We show here an easy method for calculating CMC with a reasonable accuracy. Firstly, CMC0 (intrinsic CMC, absent added salt) was coupled with quantitative structure - property relationship (QSPR) with defined by us parameter "CMC predictor" f1. It can be easily calculated from a number of tabulated molecular parameters - the adsorption energy of surfactant's head, the adsorption energy of its methylene groups, its number of carbon atoms, the specific adsorption energy of its counter-ions, their valency and bare radius. We applied this method to determine CMC0 to a test set of 11 ionic surfactants, yielding 7.5% accuracy. Furthermore, we calculated CMC in the presence of added salts using the advanced version of Corrin-Harkins equation, which accounts for both the intrinsic and the added counter-ions. Our salt-saturation multiplier, accounts for both the type and concentration of the added counter-ions. We applied our theory to a test set containing 11 anionic/cationic surfactant+salt systems, achieving 8% accuracy.

Ion-specific effects in foams.

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 0.5mM 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.

Wetting properties of phospholipid dispersion on tunable hydrophobic SiO2-glass plates.

We study the wetting properties of very small droplets of salty aqueous suspensions of unilamellar liposomes of DMPC (dimyristoylphosphatidylcholine), situated on SiO2-glass surfaces with different levels of hydrophobicity. We evaluated two different measures of hydrophobicity of solid surfaces - receding contact angles and the thickness of wetting films trapped between an air bubble and the solid surface at different levels of hydrophobicity. We established a good correlation between methods which differ significantly in measurement difficulty and experimental setup. We also reveal details of the mechanism of wetting of different surfaces by the DMPC liposome suspension. Hydrophilic surfaces with water contact angles in the range of 0° to 35° are readily hydrophobized by the liposomes and only showed corresponding contact angles in the range 27°-43°. For same range of surface hydrophobicities, there was a clear reduction of the thickness of the wetting films between the surface and a bubble, reaching a minimum in the 35°-40° range. At higher levels of hydrophobicity both pure water and the liposome suspension show similar contact angles, and the thickness of wetting films between a bubble and those surfaces increases in parallel. Our analysis showed that the only force able to stabilize the film under these experimental conditions is steric repulsion. The latter suggests that nanobubbles adsorbed on hydrophobic parts of the surface, and coated with a DMPC layer, may be the cause of the 40-70 nm thickness of wetting films we observe.

Hydrodynamics of thin liquid films: Retrospective and perspectives.

This review presents a summary of the results in the domain of microscopic liquid film hydrodynamics for several decades of experimental and theoretical research. It mainly focuses on the validation, application and further development of the Stefan-Reynolds theory on the liquid drainage, based on the accumulated knowledge of surface forces, surface tension caused by the surfactant adsorption, and diffusion of surfactants. Liquid films are of primary significance for colloidal disperse systems, and diverse industrial processes. The transient stability of the froth phase and the froth drainage is a function of the drainage and rupture of liquid films between air bubbles. In flotation, the bubble-particle attachment is controlled by the thinning and rupture of the intervening liquid film between an air bubble and a mineral particle. Both the experimental and theoretical results are mostly related to the foam liquid films between two bubbles, but can be principally generalized for emulsion films, formed in another liquid, as well as wetting films between a bubble and a solid surface.

How to determine the adsorption energy of the surfactant's hydrophilic head? How to estimate easily the surface activity of every simple surfactant?

A definite way to determine the adsorption energy of the surfactant's hydrophilic head on the air water interface is presented. For this purpose, the Davies adsorption theory and the most advanced version of Helfand-Frish-Lebowitz adsorption theory were applied to the surface tension isotherms of homologous series of sodium alkyl sulfate (CnH2n+1SO4Na, n=7-12), thus deriving the equilibrium adsorption constant, the cross-sectional area of the surfactant molecule, the interaction coefficient and the cohesion constant versus the number of the carbon atoms into the alkyl sulfate molecule. Thus, the total adsorption energy of each particular homolog was calculated in line with the latest development of the adsorption theory, thus calculating the dimensionless adsorption energy of the hydrophilic head Ehead/kBT. In our particular case (SO4(-)) we calculated Ehead/kBT=-2.79, which indicates the strong propensity of the SO4(-) to be surrounded by water molecules. The procedure for calculation Ehead/kBT does not depend on the charge of the hydrophilic head. Similarly, we calculated Ehead/kBT of another six well known in the literature hydrophilic heads (COOH, OH, DMPO, DEPO, N(CH3)3(+), and NH3(+)), indicating that the adsorption energy of the CH2 group depends slightly on the type of the hydrophilic head, but it affects substantially the adsorption energy of the whole surfactant molecule. Finally, we defined and validated a parameter called adsorption capacity of surfactants with simple molecular structure, for easy estimation of their surface activity. Linear dependence between the CMC of ionic surfactants and their adsorption capacity was established.

Tribology of thin wetting films between bubble and moving solid surface.

This work shows a successful example of coupling of theory and experiment to study the tribology of bubble rubbing on solid surface. Such kind of investigation is reported for the first time in the literature. A theory about wetting film intercalated between bubble and moving solid surface was developed, thus deriving the non-linear evolution differential equation which accounted for the friction slip coefficient at the solid surface. The stationary 3D film thickness profile, which appears to be a solution of the differential equation, for each particular speed of motion of the solid surface was derived by means of special procedure and unique interferometric experimental setup. This allowed us to determine the 3D map of the lift pressure within the wetting film, the friction force per unit area and the friction coefficient of rubbing at different speeds of motion of the solid surface. Thus, we observed interesting tribological details about the rubbing of the bubble on the solid surface like for example: 1. A regime of mixed friction between dry and lubricated friction exists in the range of 6-170 µm/s, beyond which the rubbing between the bubble and solid becomes completely lubricated and passes through the maximum; 2. The friction coefficient of rubbing has high values at very small speeds of solid's motion and reduces substantially with the increase of the speed of the solid motion until reaching small values, which change insignificantly with the further increase of the speed of the solid. Despite the numerous studies on the motion of bubble/droplet in close proximity to solid wall in the literature, the present investigation appears to be a step ahead in this area as far as we were able to derive 3D maps of the bubble close to the solid surface, which makes the investigation more profound.

Bubble rubbing on solid surface: experimental study.

This is the first interferometric study in the literature on wetting film entrapped between bubble and moving solid substrate. Unique experimental setup was specially designed for monitoring the thickness profiles of wetting film, intercalated between the bubble and moving solid surface. For this reason, special procedure developed for this study was applied for determination of 3D film thickness profiles. This allowed us to determine 3D profiles of the disjoining, the lift pressures as well as the viscous stress tensor as a function of the velocity of the solid surface. Thus, one can see that a strong linear dependence between the average film thickness and the speed of motion of the solid surface exists until a certain critical speed of motion, beyond which the dependence becomes weaker but keeps its linear trend. Similar is the propensity with the average lift pressure. Moreover, one can observe how the inhomogeinity of the film surfaces changes upon increasing the speed of motion of the solid surface. The proposed technique reveals new possibilities for investigation of bubbles and solid surfaces on deeper level when they are in relative motion towards each other. Thus, one can conduct detailed tribological studies on bubbles moving in close proximity to solid surfaces.

On the growth of pneumatic foams.

This work reports on the behaviour of tenacious and transient pneumatic foams produced at large range of values of gas delivery rates (or superficial velocities) and surface tensions. Experimental data from the literature and produced in the course of this study were processed and analyzed. The tenacious foams were stabilized via Polyoxiethylene-2 sulfate (SDP(2)S) in presence of 0.024M NaCl and 0.003M AlCl(3) ( CMC = 1.83×10(-2) M) in the concentration range of 3.33×10(-3) M to 3.8×10(-2) M (0.18CMC -2.08CMC corresponding to values of the dynamic surface tension in the range of 42.7mN/m to 37.5mN/m. The range of gas delivery rates was from 20.5ml/min to 482.8ml/min. It was found out that the rate of foam generation coincides with the gas delivery rate until a certain critical value of the latter, beyond which the rate of foam growth exceeds the rate of gas delivery. The level of this exceeding depends on the dynamic surface tension. The lower the value of the dynamic surface tension the larger the level of this exceeding. This rule was found valid until a certain upper limit of the gas delivery rate, at which the dependence on the dynamic surface tension ceases to exist. The second set of experiments was conducted on transient foams. The latter were stabilized by three members of homologue surfactants series: sodium octylsulfate (SOS), sodium decylsulfate (SDeS), and sodium dodecylsulfate (SDS) in the concentration range of 0.01CMC -0.1CMC corresponding to 72.75mN/m to 68.18mN/m. The aqueous solutions of the three surfactant homologues had identical values of static surface tension at the same ratio C/CMC . Bikerman's "unit of foaminess" was measured for each particular case. It was shown that at identical equilibrium surface tensions both the foaminess and the rate of foam decay increase upon lengthening of the surfactant's hydrocarbon chain. It was indicated as well that the foaminess increases linearly upon raising the gas delivery rate until a certain critical value, above which substantial increase is observed. It was finally concluded that both the tenacious and the transient foams have completely different behaviour. For this reason they should be modeled separately but more experimental data are still needed.

Delta-comb potential in modeling three-phase contact line (TPCL) on periodically patterned surfaces.

This work is a study of wetting of small water droplets on smooth glass surfaces with periodic patterns in the form of imprinted net with hydrophilic cells and hydrophobic bars. Microcover slides consisted of soda lime glass were used. The imprinted images of the net were with cell sizes in the range 40-200 µm, which corresponds to a quite narrow scope of hydrophilic surface fractions f(1)(30-36%) due to the relative increase in the size of the hydrophobic bars. The receding contact angles θ(R) of small water droplets, positioned on the patterned surfaces, were measured. The experiment showed significantly lower receding contact angles as compared to the theoretical expectations by the Cassie formula, which accounts for the contribution to the contact angle of the surface fraction of the imprinted hydrophobic/hydrophilic net. For this reason, we developed new theory accounting for the periodicity of the surface and the contribution of the three-phase contact line on the contact angle. This new theory considered delta-comb potential energy Δ(x,y) of the surface, effective line tension κ, and the lattice parameter a. The restriction of theory was discussed as well. It was pointed out that the theory is not valid for very small and very large lattice parameters.

Foam production--ratio between foaminess and rate of foam decay.

Foams are usually characterized by the foaminess of their surfactant solutions and the rate of foam decay. These two properties have been described many times separately in the literature. There is a certain correlation between them, which can vary depending on the type and the concentration of the surfactants, the method of foam generation, etc. We suggest with this work a new parameter unifying foaminess and rate of foam decay. The foam production is a parameter, which is a ratio between foaminess and rate of foam decay. It was shown an example how foaminess, rate of foam decay and foam production depends on C/CMC (C - surfactant concentration, CMC - critical micelle concentration) of aqueous solutions of sodium octylsulfate (SOS). In addition, it has been stressed that a number of scientific problems on transient foams can be solved by means of the approach pointed out by this study. An example, for which the foam production depends on the way of foam generation, is given. A new criterion for assessing the ability of the surfactants to stabilize foams has been suggested. Thus, the stronger surfactants do not always produce more stable transient foams than the weaker ones, as usually is assumed.

Foams and antifoams.

Foams and antifoams are two entities with completely different natures. For example, the foams are structures of bubbles in contact, while the antifoams are emulsions containing hydrophobic particles. The interaction between them makes the foam decay faster and in the same time exhausts the antifoam. The mechanism of such an effect is complex of many phenomena taking place in the foam. Thus the antifoams are known as powerful foam suppressors. For these reasons, they are very important from fundamental and practical viewpoints. This paper summarizes the knowledge on antifoams since their very creation till nowadays. In this regard, the review discloses the scientific interpretations on antifoams in chronological order in accord with the literature. Thus, for example it begins with description of the first antifoams (oils) from the 1940s and the pioneering studies of S. Ross and his group. The first physical methods for studying antifoams were presented along with the concepts of spreading and entering coefficients of oils (W. Harkins, 1941, J. Robinson and W. Woods, 1948). The further development of the antifoams (oils+hydrophobic particles) was described by means of the works of R. Kulkarni et al., A. Dippenaar and P. Garrett in the late 1970s and the early 1980s. The theoretical models on the antifoam performance of R. Pelton and P. Garrett, developed in 1980s and 1990s, were presented and analyzed as well in regard with their limits of applicability. Substantial advance on the experimental techniques for studying antifoams has been achieved by introducing different variants of the film trapping technique (FTT) developed by D. Wasan et al., I. Ivanov et al. and T. Tamura et al. in the middle and the late 1990s. An assessment of these techniques was carried out in regard with their capacity for detailed studying the antifoam action within the thin liquid films. Finally, the latest knowledge on the antifoams was achieved due to N. Denkov and his group, who harnessed both the most successful type of FTT and the interferometric thin film setup of Scheludko to conduct innovative experiments on the antifoam's action in the foam films under different conditions. They derived new more detailed understanding on the antifoam's action. For this reason, we must acknowledge the series of works under the supervision of N. Denkov performed between 1996 and 2004 as the lately ones in the field. The present work contains in addition a subchapter devoted to describing alternative methods for design and control of the foam stability. As far as the foaminess and the rate of foam decay depends on the states of the surfactant adsorption layers situated on the bubble surfaces, both foaminess and foam durability can be designed by means of proper choices of surfactants, concentrations and methods of foam generation. Therefore, this paper scrutinized the very mechanism of foam generation whose product is initial foam. Afterwards it was pointed out that the elastic modulus of the foam bubbles is responsible for the further "life" of the already generated foam. A compilation between foaminess and average rate of foam decay named foam production was shown as more successful way to describe the foaming capacity of the frothers. In addition, the properties of tenacious famous under various conditions were exhibited as well. This subchapter does not give any formula for precise design of foams with entailed durability but rather outlines new ways to achieve such recipe.

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.

Wetting films on chemically patterned surfaces.

The behavior of thin wetting films on chemically patterned surfaces was investigated. The patterning was performed by means of imprinting of micro-grid on methylated glass surface with UV-light (λ=184.8 nm). Thus imprinted image of the grid contained hydrophilic cells and hydrophobic bars on the glass surface. For this aim three different patterns of grids were utilized with small, medium and large size of cells. The experiment showed that the drainage of the wetting aqueous films was not affected by the type of surface patterning. However, after film rupturing in the cases of small and medium cells of the patterned grid the liquid from the wetting film underwent fast self-organization in form of regularly ordered droplets covering completely the cells of the grid. The droplets reduced significantly their size upon time due to evaporation. In the cases of the largest cell grid, a wet spot on the place of the imprinted grid was formed after film rupturing. This wet spot disassembled slowly in time. In addition, formation of a periodical zigzag three-phase contact line (TPCL) was observed. This is a first study from the planned series of studies on this topic.

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.

Electro-Marangoni effect in thin liquid films.

The present paper reports a new drainage model accounting for the electro-Marangoni effect in thin liquid films stabilized by ionic surfactants. It was shown that the liquid outflow during the film drainage drifts charges from the diffuse part of the electrical double layer toward the film rim and thus generates a streaming potential along the film plane. This creates reverse fluxes near the film surfaces due to the requirement for zero electric current in the film. In a previous paper on this model (Tsekov et al. Langmuir, 2010, 26, 4703), the immobile surfaces were assumed. Here, the film surfaces were considered mobile, and surface velocity is controlled by an electro-Marangoni number. It was also shown that the motion of the charges makes the film surfaces more mobile, and they flow in reverse direction to the overall liquid outflow. The theory was validated by experimental data on drainage of planar foam films stabilized by cationic (tetrapentyl ammonium bromide) and anionic (sodium dodecyl sulfate) surfactants. A good agreement between the theoretical prediction and experimental data was found.

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.