Emil D. Manev

Critical thickness of thin liquid films: Theory and experiment

Abstract A new theory of the critical state of the thin liquid film, at which it either ruptures or forms a black spot, is propounded. The concept of the spontaneous growth of the surface fluctuational waves at the critical state, proposed by A. Scheludko [ Proc. K. Ned. Akad. Wet. Ser. B 65 , 76 and 87 (1962)], is employed. The consistent application of the Brownian motion theory infers that the rate-determining process is that of attaining the state of instability and not, as assumed by A. Vrij [ Disc. Faraday Soc. 42 , 23 (1966)], the consequent evolution of the instability. The theory is confirmed by new experimental data on the critical thickness of formation of black spots in films from aqueous solutions of sodium dodecyl sulfate + NaCl. In contrast to the previous studies [E. Manev, A. Scheludko, and D. Exerowa, Colloid Polym. Soc. 252 , 586 (1974); A. Scheludko, loc cit. ] where the mean critical thickness of the films is determined, in the present study the thickness in the thinnest part of the critical film is measured. In the same manner the rate of thinning at the critical state is obtained. The agreement of experiment with theory confirms the estimate of the van der Waals-Hamaker function according to Lifshitzs theory.

EFFECT OF THICKNESS NON-HOMOGENEITY ON THE KINETIC BEHAVIOUR OF MICROSCOPIC FOAM FILM

Abstract The effect of thickness non-homogeneity on the drainage of microscopic horizontal foam films was studied experimentally and theoretically. Quasi-static and asymmetrical distribution of thickness irregularities over the film surface was established in the experiment. This type of dimpling was modelled theoretically and employed to derive a new equation for the film thinning. It differs from the theoretical expressions known from the literature and is in better agreement with the experimentally determined functional dependence of thinning rate on film size.

Effect of surfactant concentration on the critical thicknesses of liquid films

SummaryIt is shown that an increase in surfactant concentration at first causes the critical thicknesses of rupture on microscopic films to decrease but gradually a nearly constant value is reached. With films of aqueous solutions of fatty acids (valeric, caproic, caprylic and capric acids), this dependence correlates well with the effect of the surfactant concentration on the damping of capillary waves. With surfactants of the detergent type (OPE-7, OPE-20) in addition to the change of the critical thickness, the transition from rupture to formation of black spots is described, as the surfactant concentration exceedsCbl. The remarkable fact in the latter case is the independence of the critical thickness of the final state, be it rupture or formation of first or second black films.On the basis of the experimental data the assumption is put forward that the critical thickness of rupture or the critical thickness of formation of black spots is substantially affected by macroscopic non-uniformities in the film thickness. Thus the conclusion is reached that the critical thickness of an ideally plane parallel film which is the object of the theory, must be obtained by extrapolation of the measured value toward extremely small radii.ZusammenfassungEs wird gezeigt, daß die kritischen Zerreißdicken mikroskopischer Schaumfilme, die mit der Zunahme der Tensidkonzentration zunächst abnehmen, dann aber ungefähr konstante Werte erreichen. Diese Abhängigkeit ist in guter Übereinstimmungmitdem Einfluß des Tensides auf die Kapillarwellendämpfung in wäßrigen Lösungen von vier Fettsäuren. Bei Tensiden, die gute Schäumer sind (OPE-7, OPE-2) zerreißt der Film mit der Zunahme der Konzentration überCbl nicht, sondern es bilden sich schwarze Flecke. Es ist sehr interessant, daß die kritische Dicke unabhängig von dem Endzustand (Zerreißen oder Bildung schwarzer Flecke) ist.Aufgrund dieser Experimente wird angenommen, daß die kritische Dicke bei dem Zerreißen oder bei der schwarzen Fleckebildung mikroskopischer Schaumfilme wesentlich von den makroskopischen Ungleichheiten der Filmdicke beeinflußt wird. Daraus wird gefolgert, daß die kritische Dicke eines idealen planparallelen Films durcheine Extrapolation der Meßwerte aufeinen unendlich dünnen Film ermittelt werden kann.

Mixtures of n-dodecyl-β-d-maltoside and hexaoxyethylene dodecyl ether — Surface properties, bulk properties, foam films, and foams

Mixtures of the two non-ionic surfactants hexaoxyethylene dodecyl ether (C(12)E(6)) and n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were studied with regard to surface properties, bulk properties, foam films, and foams. The reason for studying a mixture of an ethylene oxide (C(i)E(j)) and a sugar (C(n)G(m)) based surfactant is that despite being non-ionic, these two surfactants behave quite differently. Firstly, the physico-chemical properties of aqueous solutions of C(n)G(m) surfactants are less temperature-sensitive than those of C(i)E(j) solutions. Secondly, the surface charge density q(0) of foam films stabilized by C(n)G(m) surfactants is pH insensitive down to the so-called isoelectric point, while that of foam films stabilized by C(i)E(j) surfactants changes linearly with the pH. The third difference is related to interaction forces between solid surfaces. Under equilibrium conditions very high forces are needed to expel beta-C(12)G(2) from between thiolated gold surfaces, while for C(12)E(6) low loads are sufficient. Fourthly, the adsorption of C(12)E(6) and beta-C(12)G(2) on hydrophilic silica and titania, respectively, is inverted. While the surface excess of C(12)E(6) is large on silica and negligible on titania, beta-C(12)G(2) adsorbs very little on silica but has a large surface excess on titania. What is the reason for this different behaviour? Under similar conditions and for comparable head group sizes, it was found that the hydration of C(i)E(j) surfactants is one order of magnitude higher but on average much weaker than that of C(n)G(m) surfactants. Moreover, C(n)G(m) surfactants possess a rigid maltoside unit, while C(i)E(j) surfactants have a very flexible hydrophilic part. Indeed, most of the different properties mentioned above can be explained by the different hydration and the head group flexibilities. The intriguing question of how mixtures of C(i)E(j) and C(n)G(m) surfactants would behave arises organically. Thus various properties of C(12)E(6)+beta-C(12)G(2) mixtures in aqueous solution have been studied with a focus on the 1:1 mixture. The results are compared with those of the single surfactants and are discussed accordingly.

Dodecylamine collector — pH effect on mica flotation and correlation with thin aqueous foam film and surface force measurements

Abstract This paper correlates data collected from several different fundamental measuring techniques in order to develop an improved understanding of the surface chemical nature of the flotation of mica in dodecylamine collector. Free equilibrium foam film experiments enabled the stability, thickness and interfacial potentials at the air/dodecylamine solution interface to be determined. Also, the batch flotation response of mica in dodecylamine solution was measured in a Clausson Flotation cell. These results were compared with surface force data, as reported in an earlier publication by Rutland et al., in which the interaction and hydrophobic adhesion (pull-off force) between molecularly smooth mica sheets in the amine collector solution was determined. Also the surface potential and thickness of the adsorbed collector layer were reported, together with the equilibrium contact angles on the smooth mica surface. All these data covered a range of pH values. It was found that maximum flotation occurred at pH 8 but this did not correspond to the minimum in the double-layer repulsive interaction between the mica and the bubble. However, it was shown that this critical pH value could be equated to a tightly packed organized collector monolayer which gave maximum hydrophobicity to the mica surface. In addition, it was demonstrated that both the bubble and mica had about the same magnitude of positive charge (+65 mV) at this optimum pH. Finally, from the extended DLVO theory, it was shown that heterocoagulation between the bubble and the mica could only occur, providing there eas a long range hydrophobic interaction force to counterbalance the repulsive Van der Waals and electrostatic forces. A possible explanation for this long range attraction was spontaneous cavity formation or micro-bubbles caused by dissolved air.

FOAM FILMS AND SURFACE FORCE STUDIES OF AQUEOUS SOLUTONS OF OCTYL-β-GLUCOSIDE

ABSTRACT The interactions at the air/liquid and the liquid/solid interface have been studied for octyl-β-glucoside. With the thin film balance the thickness of the equilibrium liquid films is determined as a function of surfactant concentration, inert salt concentration and pH. At high surfactant concentration and/or at low pH black films, about 4.6 nm thick, are formed. The stabilizing forces in the black films are related to the interactions within and between the layers. The forces acting between octyl-β-glucoside surfactant layers adsorbed on hydrophobized mica were investigated with the interferometric type surface force apparatus. The forces measured between the sugar head-groups were similar to those acting between other small nonionic groups like dimethylamine oxide and monoglycerides. However, considerably more long-range repulsions are observed between surfactants with oligoethylene oxide head-groups.

Effect of surface forces and surfactant adsorption on the thinning and critical thickness of foam films

Abstract The new experimental results on thinning and rupture of microscopic foam films, obtained by the dynamic method of Scheludko and Exerowa, are analyzed with respect to the observed discrepancies with the theoretical predictions. The possible reasons for the deviations from the frequently used Reynolds’ equation, describing the thinning rate of planar circular horizontal liquid films with non-deformable and tangentially immobile surfaces, are discussed. The applicability of the theoretical description of the thinning process (accelerated drainage), advanced in a previous study, is tested here experimentally. The effect of surface tension and surface mobility on the thinning rate and critical thickness of rupture is established. The relative effects of different factors are estimated through specially designed experiments. They comprehend: bulk and surface diffusion of the surfactant; evaporation of liquid from the film; variation of the film radius during the process of drainage; inhomogeneity of the film thickness, etc. It is concluded that the influence of the surface forces and the surfactant concentration is significant for the thinning and rupture, and respectively for the life-time of the microscopic films, while the influence of evaporation is negligible under the conditions of the experiment.

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.

Frother/collector interactions in thin froth films and flotation

Abstract Aqueous thin film studies and surface tension measurements on a mixed surfactant system consisting of poly(ethylene oxide) (PEO), which was chosen as a model flotation frother, and potassium ethyl xanthate, which was chosen as a model flotation collector, enable the interaction between the two surfactants at the air/solution interface to be elucidated. For the film containing the non-ionic frother, the interface was charged and addition of low concentrations of xanthate acted as a common electrolyte and reduced the thickness of the film, inducing rupture. However, at high xanthate collector concentrations, the negatively charged xanthate was found to interact with the non-ionic PEO causing an accumulation of negative charge at the air/solution interface. Higher frother concentrations were necessary to produce non-rupturing thin films upon increasing the xanthate concentration.

Effect of the driving pressure on the rate of thinning of microscopic foam films

Abstract The effect of the driving pressure on the rate of thinning of microscopic foam films from aqueous solutions of nonionic [C 12 (EO) 8 ] and anionic [DDBS] surfactants was studied experimentally. At the ionic strengths of the solutions used (0.024 or 0.1 mol l −1 ) the driving pressure was mainly governed by the capillary pressure, while the disjoining pressure was negligible, especially at a large film thickness. The studied films were obtained in glass cells of different radii ( R c ) of the holder in which the film was formed. The values of the drainage constants ratios ( α 2 / α 1 ), experimentally obtained for films under different experimental conditions were compared to the theoretically calculated ratios of the thinning rates ( V 2 / V 1 ). The deviations from the classical Reynolds law, obtained experimentally, correspond to the dependence of the rate of thinning on driving pressure required by the theoretical approach of Manev–Tsekov–Radoev. The results confirm the conclusions of the approach, accounting for the effect of the film thickness non-homogeneity on the rate of thinning.