Solution/air interfaces I. An oscillating jet relative method for determining dynamic surface tensions

The oscillating jet method has been investigated for the determination of the surface tension of water using horizontal jets from elliptical orifices in bell-shaped and uniform-channel tubes. Improved techniques have been developed for measuring the wave parameters, the flow rate and for extending the range of investigations to include the initial 80–90 msec of jet surface age.The surface tension values, calculated using the Bohr equation from measurements on successive waves of the water jets, were dependent on the characteristics of the orifice, its position, the flow rate and the wave serial number, but were within ±2 mN/m of the equilibrium value if the initial wave values were disregarded. An extension of the Bohr equation developed for vertical jets was found to be invalid for horizontal jets.Calculated surface tension versus surface age relationships for surfactant solutions also varied with the experimental conditions, but by fixing the position of the orifice tube, and standardizing with water, a relative method was developed for determining dynamic tensions that were independent of the tube used and of the flow rate. The validity of the method was illustrated by results obtained with two surfactant solutions using seven tubes (bell-shaped and uniform-channel) over an age range from 0.6 to 75 msec. The surface tensions of deionized water samples have been determined by the relative method and compared with those obtained by a static method.The true surface age along the jet surface is concluded to be close to the value derived from the mean axial velocity.Evidence is given indicating that, within the millisecond age range, water does not have a dynamic tension above the equilibrium value.     

Transient Surface Tension of an Expanding Liquid Sheet

We address the problem of dynamic surface tension using measurements of sheet diameters that results from the impact of a liquid jet of diameter d(0) on a small disk of diameter d(i) (d(i)/d(0) approximately 4). At low velocities, the sheet diameter D is related to d(0) by the Weber number We, constructed with the liquid density rho, the jet velocity u(0), and the surface tension sigma at the rim: D/d(0)=18 We=18 [rho u(0)(2)/(sigma/d(0))]. This relation expresses the equilibrium between inertial forces and surface tension forces at the sheet rim. When a surfactant has been dissolved in the bulk of the liquid prior to the formation of the initial jet, the rim surface tension, and therefore the sheet diameter, depends on the amount of surfactant adsorbed at the rim. This amount is fixed by a competition between surface formation induced by radial extension and repopulation of the liquid interface in surfactant. The experimental setup proposed here provides a method to measure dynamic surface tension from sheet diameter measurements and symmetrically to monitor the adsorption of a surfactant on a liquid surface. The available adsorption time ranges from 10 to 100 ms. Experimental data obtained with two surfactants are in agreement with a model of a diffusion-controlled adsorption at the interface. Copyright 2000 Academic Press.     

An absolute differential maximum bubble pressure surface tensiometer employing displaced capillaries

A modification of the differential maximum bubble pressure method for determining surface tensions is described. In this method, surface tension is calculated from the difference between maximum bubble pressures reached at capillaries of differing internal radii, vertically displaced by an amount calculated from the theory of Cuny and Wolf (1956) Ann Physik 17:57). The density dependence of the technique is eliminated and surface tension becomes a truly linear function of the differential maximum bubble pressure, which is easily measured. The absolute measuring technique is illustrated for equilibrium and dynamic surface tensions of a series of pure liquids and aqueous solutions.For dynamic measurements on surfactant solutions some important experimental considerations and limitations are described. In particular, a previously unrecognized source of error in estimating bubble surface ages is identified. It was found that the maximum bubble pressure for a large capillary does not immediately precede the detachment of the bubble, but occurs at one-third the overall bubble period. Thus, for large capillaries, subsequent to attaining the maximum bubble pressure, there exists a significant decay time in addition to the dead time. In general, surface ages corresponding to maximum pressure at small and large capillaries bubbling with the same period are not equal. This can lead to a large error in dynamic and equilibrium surface tensions of surfactant solutions. With suitable correction the technique is capable of measuring absolute surface tension, even for quite slowly equilibrating surfactant solutions.     

A Review of:“Lipid and Biopolymer Monolayers at Liquid Interfaces.” K,S. Birdi. Plenum,New York &London, 1989. pp. x + 325. $59.50

A procedure for using a standard image analysis system to assess the dynamic surface tension at air/liquid interface using the oscillating jet method was developed. The application is demonstrated by experimental data obtained from milk and from three different concentrations of commercial nonionic surfactants solutions (Tween 20 and Tween 80) of surface ages from 2 to 9 milliseconds.     

Effect of anionic surfactant and short-chain alcohol mixtures on adsorption at quartz/water and water/air interfaces and the wettability of quartz

Measurements of the advancing contact angles for aqueous solutions of sodium dodecyl sulfate (SDDS) or sodium hexadecyl sulfonate (SHS) in mixtures with methanol, ethanol, or propanol on a quartz surface were carried out. On the basis of the obtained results and Young and Gibbs equations the critical surface tension of quartz wetting, the composition of the surface layer at the quartz-water interface, and the activity coefficients of the anionic surfactants and alcohols in this layer as well as the work of adhesion of aqueous solutions of anionic surfactant and alcohol mixtures to the quartz surface were determined. The analysis of the contact angle data showed that the wettability of quartz changed visibly only in the range of alcohol and anionic surfactant concentration at which these surface-active agents were present in the solution in the monomeric form. The analysis also showed that there was a linear dependence between the adhesion and the surface tension of aqueous solutions of anionic surfactant and alcohol mixtures. This dependence can be described by linear equations for which the constants depend on the anionic surfactant and alcohol concentrations. The slope of all linear dependence between adhesion and surface tension was positive. The critical surface tension of quartz wetting determined from this dependence by extrapolating the adhesion tension to the value equal to the surface tension (for contact angle equal zero) depends on the assumption whether the concentration of anionic surfactant or alcohol was constant. Its average value is equal to 29.95mN/m and it is considerably lower than the quartz surface tension. The positive slope of the adhesion-surface tension curves was explained by the possibility of the presence of liquid vapor film beyond the solution drop which settled on the quartz surface and the adsorption of surface-active agents at the quartz/monolayer water film-water interface. This conclusion was confirmed by the work of adhesion of aqueous solutions of anionic surfactants and short-chain alcohol mixtures to the quartz surface determined on the basis of the contact angle data and molar fraction of anionic surfactants and alcohols and their activity coefficient in the surface layer.     

Dynamic Behavior of Surfactants in Solution

Adsorption of various surfactants at the gas liquid interface is studied with equilibrium and dynamic surface tension measurements. The Wilhelmey plate method and maximum bubble pressure method are used for this study. Dynamic surface tension of solutions of different surfactants, sodium lauryl sulfate (SLS), polyoxyethylene glycol 4‐tert‐octyl phenyl ether (Triton X 100), poly‐oxyethylene(20) cetyl ether (Brij 58), and tetraethylene glycol mono‐n‐dodecyl ether (Brij 30), is measured at different concentrations. Adsorption of different surfactants is compared on the basis of equilibrium and dynamic behavior. Effectiveness and efficiency of different surfactants is found from equilibrium surface tension measurement. A new parameter is defined to quantify the dynamic behavior of adsorption, which gives the concentration of surfactant needed to reduce surface tension to half of its maximum reduction within a defined time available for adsorption. The dynamics of surfactant solution is quantified by using this parameter.     

Influence of 1-decanol on the surface tension and wetting power of a new anionic surfactant derived from sugar

We studied the adsorption behaviour at the liquid/air and liquid/solid interface of a new anionic surfactant derived from sugar, the sodium decyl galacturonate. The surface tension of aqueous solutions, measured in equilibrium and as a function of time, is particularly affected by the presence of decanol, synthesis residue, which amount ranges between about 0 and 13%. The surface tension lowering is accelerated in presence of decanol, owing to its rapid diffusion to the interface or/and because it affects the mobility and adsorption process of the anionic surfactant molecules. The wetting power of surfactant solutions were also investigated in relation with textile treatment applications. We measured the kinetics of absorption of surfactant solutions in a piece of standard cotton and compared it to the absorption of pure decanol, a completely wetting liquid and to the absorption of an alkylpolyglucoside. The time at which the fabric piece is saturated appears to be related to the adsorption of surface-active molecules on the fibers at the advancing liquid front/fabric contact line. Decanol was found to promote absorption and micellar life-time seem to reflect the differences observed at high concentration. This study shows the importance of controlling the amount of surface-active residues which may alter the kinetics of surfactant adsorption, particularly in industrial processes where equilibrium conditions are not reached.     

Kinetics of the surface tension of micellar solutions: Comparison of different experimental techniques

The kinetics of the surface tension of micellar solutions of nonionic surfactant Triton X-100 is measured experimentally by means of three different techniques: oscillating jet, maximum bubble pressure and inclined plate. They allow to study the micellization kinetics at various time scales (from a few milliseconds to a few seconds) in fairly large concentration region up to 50 times CMC. The experimental data are satisfactorily explained by a theoretical model accounting for the kinetics of micellization, diffusion of surfactant species and expansion of the bubble interface. By this model are computed the characteristic times of diffusion and micellization, which are of comparable magnitude (about 5 to 200 ms), and the Gibbs' elasticity. The micellization time constant corresponds to the slow relaxation process known to coincide with the disintegration of micelles. Comparing our data with other data from literature one can conclude that more realistic information for the micellization kinetics is obtained by the maximum bubble pressure and the oscillating jet method. The inclined plate seems too slow to measure the relaxation processes in micellar solutions of this surfactant.     

Silicon-modified carbohydrate surfactants. VII: Impact of different silicon substructures on the wetting behaviour of carbohydrate surfactants on low-energy surfaces — distance decay of donor–acceptor forces

The wetting behaviour of carbohydrate surfactants bearing siloxane, carbosilane, polysilane or silane moieties has been investigated. By static surface tension (γ_lv, σ) and wetting tension (γ_sv−γ_sl, α) measurements on a non-polar perfluorinated surface (FEP®), the contact angles of aqueous surfactant solutions above the critical micelle formation concentration (cmc) were determined. Surface tension and wetting tension react independently on defined changes in the chemical structure of the surfactant molecules. Siloxane surfactants reduce the surface tension most effectively, whereas for a neopentyl-substituted silane derivative the lowest solid/liquid interfacial tension was found. The data for isomeric siloxanes, carbosilanes and silanes suggest that donor–acceptor forces at solid interfaces have a maximum range of about 4.5 Å. © 1998 John Wiley & Sons, Ltd.     

Surface potential and surface tension of aqueous 2-halogenoethanol-ethanol solutions

The surface properties of aqueous 2-fluoro-, 2-chloro-, 2-bromoethanol-ethanol mixtures were studied by surface tension measurements, applying the drop weight method, and by surface potential using the flowing jet method. The addition of ethanol to 2-halogenoethanol solutions causes a synergetic effect on surface tension. In the case of surface potential the synergetic influence exists only in the mixtures of ethanol-2-fluoroethanol. The studies on the surface interactions of the adsorbed molecules of particular components in the mixed film were carried out by the Rosen and Hua method. The weak interaction between adsorbed molecules was observed.     

Adsorption of multiple ammonium salts at the air/solution interface

The interfacial behavior of aqueous solutions of newly synthesized bis- and tris-ammonium salts (i.e., bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]alkylamine dichlorides and bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]dialkylammmonium trichlorides, respectively) was analyzed, both experimentally and theoretically. The dynamic and equilibrium surface tension of multiple ammonium salt solutions was measured by using a pendant drop shape analysis method. The determined surface tension isotherms indicated the lack of significant differences in surface activity between bis- and tris-ammonium salts, contrary to the expectations for divalent and trivalent surfactant ions. That effect was explained by assuming the formation of multiple surfactant ion-counterion associates. Taking into account the association process, a good correlation between experimental data and theoretical predictions was obtained by means of the "surface quasi two-dimensional electrolyte" (STDE) model of ionic surfactant adsorption. The degree of association necessary to explain the lack of difference in surface activity between bis- and tris-ammonium salts was in quantitative agreement with the results of measurements of the concentration of free chloride anions in the surfactant solution.     

A new technique for the measurement of the dynamic evolution of surface tension

We develop a novel technique which deduces the surface tension in air of a fluid as a function of surface age, beginning at age zero. The technique utilizes pointwise measurements of perpendicular free surface profiles of a steady oscillating jet corresponding to a discretization interval on the order of 0.1 ms. We implement the technique on constant-surface-tension test fluids (100% ethanol and 15% ethanol/85% water by volume) to demonstrate the extent to which the technique can qualitatively capture that the surface tensions of these fluids are constant in time, and quantitatively produce values of these constants consistent with static measurements. We then implement the technique on jets of two agricultural surfactant mixtures, Triton X-405 and Triton X-100, and quantitatively deduce the decay of surface tension as a function of surfactant concentration.     

A Study of Dynamic Interfacial Properties as Related to Foaming Properties of Sodium 2,5-dialkyl Benzene Sulfonates

In this article, foaming properties and dynamic interfacial properties of a series of sodium 2,5-dialkyl benzene sulfonates in aqueous solutions were carried out to elucidate the relationship between foaming properties and dynamic interfacial properties. The properties of foams generated from bubbling air through different surfactant solutions were measured using a modified Bikerman device. The dynamic surface tension and surface dilational elasticity were obtained from an image analysis technique based on the oscillating bubble method. The surfactants molecular adsorption at the air/water interface was introduced with Rosen empirical equation and the rate of adsorption was determined from measurements of the dynamic surface tension. The surfactant with the longest alkyl chain shows the lowest dynamic surface activity, which lead to the lowest foam volume. The short ortho straight alkyl chain has little effect on the arrangement of molecules at the interface and the foam stability changes a little with the changing of the ortho alkyl chain length. The foam stability is correlated with both the higher surface dilational elasticity and the larger surface monolayer strength.     

Micelle formation by N-alkyl-N-methylpyrrolidinium bromide in aqueous solution

The micellization of the ionic liquid N-alkyl-N-methylpyrrolidinium bromide (C(n)MPB, n = 12, 14 and 16) in aqueous solutions was investigated by surface tension measurements, electrical conductivity and static luminescence quenching. The effectiveness of the surface tension reduction (Π(cmc)), maximum surface excess concentration (Γ(max)) and the minimum area (A(min)) occupied per surfactant molecule at the air/water interface can be obtained from the surface tension measurements at 25 °C. The critical micelle concentration (cmc) at different temperatures and a series of thermodynamic parameters (ΔG, ΔH and ΔS) of micellization were evaluated from electrical conductivity measurements in the temperature range of 25-45 °C. The thermodynamic parameters show that the micelle formation is entropy-driven at low temperature and enthalpy-driven at high temperature. Furthermore, the micelle aggregation number (N(agg)) of C(n)MPB was calculated according to the Turro-Yekta method through static luminescence quenching and found that N(agg) (49, 55, and 59) increased with the hydrophobic chain length of C(n)MPB.     

A simplified method for predicting the dynamic surface tension of concentrated surfactant solutions

A simplified method for predicting the dynamic surface tension of concentrated surfactant solutions is proposed. It is implemented using the framework of the Henry's Law analytical solution to the Ward and Tordai equation for diffusion-controlled adsorption, with the necessary parameters being deduced from the measured equilibrium surface tension equation and a value for the surfactant monomer diffusivity. The method is tested by calculating the dynamic surface tension relaxations of aqueous C10E6 and C10E8 solutions over concentration ranges from well below to well above their critical micelle concentrations (cmc). Results are compared with measured relaxations over 0.001-50 s, and semiquantitative agreement is found, with the best results obtained for concentrations near the cmc. The predictive method may prove useful in such applications as the screening of candidate surfactants for inks used in inkjet printing.     

Interfacial tension of gelatin/sodium dodecylsulphate solutions against air,toluene, and diethylphthalate

The adsorption isotherms between aqueous solutions of sodium dodecylsulphate and gelatin against air, toluene, or diethylphthalate were determined using the spinning drop method. The results qualitatively and quantitatively agreed with those found by surface tension measurements on sodium dodecylsulphate/gelatin solutions using the ring method in the version of Du Noüy. Interaction between gelatin and the surfactant will yield complexes which are more interfacially active than the components by themselves. The saturation of the interfaces occurs at lower concentrations than in solutions of the single components.     

Novel calibration of a dynamic surface tension detector: flow injection analysis of kinetically-hindered surface active analytes

First, a novel technique for calibration of a dynamic surface tension detector (DSTD) is described. The DSTD measures the differential pressure as a function of time across the liquid-air interface of growing drops that repeatedly form and detach at the end of a capillary tip. The calibration technique utilizes the ratio of pressure signals acquired from the drop growth of two separate solutions, i.e. a standard solution and a corresponding mobile phase, such as water, both of which have a known surface tension. Once calibrated, the dynamic surface tension of an analyte is obtained from the ratio of the pressure signals from the analyte solution to that of the mobile phase solution. Thus, this calibration technique eliminates the need to optically image the radius of the expanding drop of liquid. Accurate dynamic surface tension determinations were achieved for aqueous sodium dodecyl sulfate (SDS) solutions over a concentration range of 0.5-5.4 mM. The measured surface tensions for these SDS solutions range from 70.3 to 46.8 dyne/cm and were in excellent agreement with the literature. A precision of 0.2 dyne/cm (1 S.D.) was routinely obtained. Second, the DSTD with this calibration technique was combined with flow injection analysis (FIA) for the study of model protein solutions and polymer solutions. The kinetic surface tension behavior of aqueous bovine serum albumin (BSA) solutions as a function of concentration and flow rate is presented. Evaluation of the dynamic surface tension data illustrates that a protein such as BSA initially exhibits kinetically-hindered surface tension lowering, i.e. a time dependence, as BSA interacts with the liquid-air interface of an expanding drop. FIA/DSTD is then shown to be an effective tool for the rapid study of kinetically-hindered surfactant mixtures. It was found that mixtures of SDS and the polymeric surfactant Brij(R)-35 (lauryl polyoxyethylene ether with an average molecular weight of 1200 g/mol) result in essentially an additive lowering of the surface tension. Mixtures of polyethylene glycol (PEG), with an average molecular weight of 1470 g/mol, and Brij(R)-35, however, result in a competitive (non-additive) surface tension with the Brij(R)-35 dominating the response.     

A Growing Drop Technique for Measuring Dynamic Interfacial Tension

A novel, growing drop technique is described for measuring dynamic interfacial tension due to sorption of surface-active solutes. The proposed method relates the instantaneous pressure and size of expanding liquid drops to the interfacial tension and is useful for measuring both liquid/gas and liquid/liquid tensions over a wide range of time scales, currently from 10 ms to several hours. Growing drop measurements on surfactant-free water/ air and water/octanol interfaces yield constant tensions equal to their known literature values. For surfactant-laden, liquid drops, the growing drop technique captures the actual transient tension evolution of a single interface, rather than interval times as with the classic maximum-drop-pressure and drop-volume tension measurements. Dynamic tensions measured for 0.25 mM aqueous 1-decanol solution/air and 0.02 kg/m³ aqueous Triton X-100 solution/dodecane interfaces show nonmonotonic behavior, indicating slow surfactant transport relative to the imposed rates of interfacial dilatation. The dynamic tension of a purified and fresh 6 mM aqueous sodium dodecyl sulfate (SDS) solution/air interface shows only a monotonic decrease, indicating rapid surfactant transport relative to the imposed rates of dilatation. Conversely, an aged SDS solution, naturally containing trace dodecanol impurities, exhibits dynamic tensions which reflect a superposition of the rapidly equilibrating SDS and the slowly adsorbing dodecanol.     

Dynamic surface properties of polyelectrolyte/surfactant adsorption films at the air/water interface: poly(diallyldimethylammonium chloride) and sodium dodecylsulfate

The dynamic surface elasticity, dynamic surface tension, and ellipsometric angles of mixed aqueous poly(diallyldimethylammonium chloride)/sodium dodecylsulfate solutions (PDAC/SDS) have been measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nanoscale. The oscillating barrier and oscillating drop methods sometimes led to different results. The surface viscoelasticity of mixed PDAC/SDS solutions are very close to those of mixed solutions of sodium polystyrenesulfonate and dodecyltrimethylammonium bromide but different from the results for some other polyelectrolyte/surfactant mixtures. The abrupt drop in surface elasticity when the surfactant molar concentration approaches the concentration of charged polyelectrolyte monomers is caused by the formation of microparticles in the adsorption layer. Aggregate formation in the solution bulk does not influence the surface properties significantly, except for a narrow concentration range where the aggregates form macroscopic flocks. The mechanism of the observed relaxation process is controlled by the mass exchange between the surface layer and the flocks attached to the liquid surface.     

Nonequilibrium surface tension for mixed adsorption kinetics

Experimental nonequilibrium surface tension measurements of 1–9 nonanediol solutions obtained by the oscillating-jet method have been interpreted in terms of our theoretical predictions derived for a mixed-controlled adsorption kinetics of the surfactant. The surface tension values have been calculated from the Szyszkowski equation using the Langmuir model of surfactant adsorption. Our theoretical results, obtained by a numerical solution of the adsorption equations, agree well with experimental data giving a value of the kinetics Szyszkowski constant very similar to the thermodynamic equilibrium value determined from experimental measurements of the static surface tension of 1–9 nonanediol solutions of various concentration. The approximate kinetic equation derived by P. Joos, G. Bleys, and G. Petre (J. Chim. Phys.79, 387 (1982)) for purely barrier-controlled adsorption proved to be less accurate.