On the determination of equilibrium surface tension values of surfactant solutions

By using values ofdσ/dt as criterion for the establishment of the adsorption equilibrium great errors can result for strong surface active substances (gG_x/a?10^?2 cm). A simple linear extrapolation from curves σ or (σ₀-σ)^?1 on 1/√t (results of dynamic surface tension measurements) can lead to greater errors, too. Conditions are given under which the application of such extrapolation procedures is only possible. To get accurate equilibrium surface tension values of solutions of very strong surfactants (Γ_x/a>10^?1 cm) an extrapolation-interpolation algorithm based on adsorption-desorption measurements is suggested.     

Equilibrium surface tension of aqueous surfactant solutions

Summary The applicability of the drop weight method for determining time dependent surface tension of surfactant solutions was experimentally proved.The presence of CO₂ and traces of lauryl alcohol and long chain homologs lower the surface tension of sodium dodecyl sulfate solutions in a measurable extent. The chemical purification of materials and recrystallizations are insatisfactory to obtain sufficient purity; the cleaning of the surface itself is needed by e.g. foaming. The equilibrium surface tension of pure surfactant solutions that is often required to interpret interfacial phenomena and to calculate theGibbs adsorption excess can be determined with reliable accuracy by linear extrapolation to zero time from measurements made in function of time up from 1 minute on moderately contaminated solutions.As a criterion of surface purity the time dependence of surface tension and the average life-time of thin liquid film are suggested.

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Zusammenfassung Die Tropfengewichtsmethode eignet sich auch zur Bestimmung der zeitabhängigen Oberflächenspannung wäßriger Tensidlösungen. Zur Entfernung von Verunreinigungen, die die Oberflächenspannung von Natriumdodecylsulfatlösungen herabsetzen, wie Spuren von Laurylalkohol und höheren Homologen, reichen weder chemische Methoden noch wiederholtes Umkristallisieren aus, sondern die Oberfläche selbst ist zu reinigen, z. B. durch Ausschäumen. Der Gleichgewichtswert der Oberflächenspannung läß sich mit hinreichender Genauigkeit berechnen, in dem man die an mäßig konzentrierten Lösungen über einer Minute Meßzeit gewonnenen Daten auft = 0 linear extrapoliert. Als praktisches Kriterium für die Reinheit der Oberfläche kann man sowohl die Zeitabhängigkeit der Oberflächenspannung, wie auch die mittlere Lebensdauer von dünnen Lösungsfilmen heranziehen.

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With 6 figures     

Dynamic surface tension behavior in a photoresponsive surfactant system

The surface properties of a nonionic photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail have been investigated. Cis-trans photoisomerization of this azobenzene group alters the ability of the surfactant to pack into adsorbed monolayers at an air/water interface or into aggregates in solution, thereby causing a significant variation in surface and bulk properties following a change in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains the trans isomer almost exclusively, whereas samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium in which significant amounts of both isomers are present. At concentrations well above the cmc but under different illumination conditions (dark, UV light, visible light), freshly formed surfaces exhibit profoundly different surface tension trajectories as they approach essentially identical equilibrium states. This common equilibrium state corresponds to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after the creation of a fresh interface, whereas the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and finally end with a second relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. The cis surfactant reaches the interface more quickly, leading to an initially cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Fluorescence studies are used for cmc determination in the samples, and the results suggest that the two isomers segregate into distinct aggregate phases. The critical concentration associated with the formation of cis-rich aggregates is much larger than that of the trans-rich aggregates, which accounts for the faster diffusion of the cis isomer to a fresh interface. Models of the diffusion and adsorption of surfactant are developed. These consider the role of aggregates in the adsorption process by examining the limiting behavior of three aggregate properties: dissolution rate, mobility, and ability to incorporate into the interface. These models are used to analyze the surface tension relaxation of dark and UV samples, and the predictions are found to be in agreement with the observed characteristic relaxation time scales for these samples, though the results are inconclusive regarding the specific role of aggregates. High-intensity illumination focused on a surface saturated with surfactant is used to drive photoisomerization of the adsorbed surfactant, and rapid, substantial changes in surface tension result. These changes are consistent with proposed conformations of the adsorbed surfactant and with monolayer studies performed with a Langmuir film balance.     

On relativistic surface tension

On the basis of special relativity and classical thermodynamics, the surface tension was shown to be a relativistic invariant.     

Mixed adsorption and surface tension prediction of nonideal ternary surfactant systems

To deal with the mixed adsorption of nonideal ternary surfactant systems, the regular solution approximation for nonideal binary surfactant systems is extended and a pseudo-binary system treatment is also proposed. With both treatments, the compositions of the mixed monolayer and the solution concentrations required to produce given surface tensions can be predicted based only on the -LogC curves of individual surfactants and the pair interaction parameters. Conversely, the surface tensions of solutions with different bulk compositions can be predicted by the surface tension equations for mixed surfactant systems. Two ternary systems: SDS/Hyamine 1622/AEO₇, composed of homogeneous surfactants, and AES/DPCl/AEO₉, composed of commercial surfactants, in the presence of excess NaCl, are examined for the applicability of the two treatments. The results show that, in general, the pseudo-binary system treatment gives better prediction than the extended regular solution approximation, and the applicability of the latter to typical anionic/cationic/nonionic nonideal ternary surfactant systems seems to depend on the combined interaction parameter, : the more it deviates from zero, the larger the prediction difference. If 0, good agreements between predicted and experimental results can be obtained and both treatments, though differently derived, are interrelated and tend to be equivalent.Poster presentation at the 14th SIS conference, June 2002, Barcelona     

Headgroup and hydrocarbon tail effects on the surface tension of sugar-based surfactant solutions

Measurements of surface tension isotherms were conducted for water solutions of pure and mixed n-decyl-beta-d-glucopyranoside (C(10)-Glu) and n-decyl-beta-d-maltopyranoside (C(10)-Mal) surfactants. By applying the Gibbs surface tension equation, the surface densities of Glu and Mal were derived for different compositions and concentrations. The surface fractions were compared with theoretically calculated values where the headgroups were modeled as hard disks. Satisfactory agreement was found for hard-disk sizes of 22.9 and 11.3 A(2) in the case of a 1:1 mixture. The results of the hard-disk calculations were employed to estimate the configurational free energy of the n-decyl-hydrocarbon chain. The results obtained agree well with previous calculations for the n-dodecyl chain. Comparison with n-dodecyl beta-d-maltopyranoside (C(12)-Mal) indicated a further contribution, with the longer hydrocarbon chain giving rise to a higher surface tension in good agreement with data for hydrocarbon liquids. Furthermore, the interpenetration of the headgroup into the hydrocarbon film was studied by means of comparing surface-tension data for n-decyl- and n-dodecyl-ethylene-oxide-based surfactants and n-decyl- and n-dodecyl-beta-d-thiomaltopyranosides (C(10)-S-Mal and C(12)-S-Mal, respectively) and -maltopyranosides. It was found that lengthening the tetra(etylene oxide) chain by one segment affects the surface tension only marginally, indicating little interpenetration of the additional ethylene-oxide group into the hydrocarbon film. For the thiomaltosides, however, the corresponding effect was found to be remarkably high.     

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.     

A model for simulating the dynamic surface tension behavior of aqueous surfactant dispersions

A dynamic adsorption model for surface-active materials at air/liquid interfaces with the consideration of aggregate dissolution effect was developed to investigate the dynamic surface tension behavior of aqueous surfactant dispersions. Two catanionic surfactants, cetylpyridinium dodecylsulfate (CP-DS) and dodecyltrimethylammonium dodecylsulfate (DTMA-DS), with low critical aggregation concentrations were chosen as model systems. Dynamic surface tensions of aqueous CP-DS and DTMA-DS systems were measured by a drop volume tensiometer. A model with diffusion-controlled or mixed-kinetic dynamic adsorption mechanisms considering the dissolution effect of dispersed aggregates was developed to simulate the dynamic surface tension data. An analysis by comparing the model predictions with experimental data demonstrated that the dynamic surface tension behavior of aqueous CP-DS and DTMA-DS dispersions could be described with a diffusion-controlled dynamic adsorption model taking the aggregate dissolution effect into account.     

Simultaneous study of interfacial tension,surface tension,and viscosity of few surfactant solutions with survismeter

Surfaces and interfaces are receptive valuable significant property of chemical molecules due to their potential to develop several phenomena in a self‐controlled mechanism. Science of surfaces is vast and is being used industrially since time immemorial. Their accurate and simultaneous estimation is necessary; therefore, the survismeter was used for measuring them along with viscosity. Individually tensiometers, X‐ray reflective microscope, and viscometers are used for surface tension, interfacial tension, and viscosity, respectively. These devices are sophisticated, expensive, and individually consume much time and resources with poor reproducibility in measurements. Survismeter is an alternative device for similar measurements together with higher accuracies and reproducibility. It works on a principle of capillary flow and pressure gradient (PG) inside liquid‐holding and air‐filled bulbs. Several liquids have been used for study with ± 0.01 mN/m, ± 0.01 mN/m and ± 1 × 10^?5 N s/m² accuracies in respective data. Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamic surface tension of polyelectrolyte/surfactant systems with opposite charges: two states for the surfactant at the interface

The molecular reorientation model of Fainerman et al. is conceptually adapted to explain the dynamic surface tension behavior in polyelectrolyte/surfactant systems with opposite charges. The equilibrium surface tension curves and the adsorption dynamics may be explained by assuming that there are two different states for surfactant molecules at the interface. One of these states corresponds to the adsorption of the surfactant as monomers, and the other to the formation of a mixed complex at the surface. The model also explains the plateaus that appear in the dynamic surface tension curves and gives a picture of the adsorption process.     

Ultralow interfacial tension for enhanced oil recovery at very low surfactant concentrations

The interfacial tension (IFT) between alkanes and several individual surfactants and their mixtures has been investigated, using three kinds of alkyl hydrocarbons: decane, dodecane, and tetradecane. For individual and mixed surfactant systems, critical micelle concentrations and areas per molecule at the hydrocarbon-aqueous solution interface were calculated; for the mixed surfactant systems, betasigma(LL), the molecular interaction parameter at the hydrocarbon-aqueous solution interface, and beta(M), the molecular interaction parameter in mixed micelle formation, were calculated. It was found that IFT in the 10(-3) mN/m (ultralow) range can be obtained at surfactant concentrations below 0.05 wt % and even at concentrations below 0.01 wt %, when mixtures of certain surfactants are used at the proper ratio. Surfactants with branched-chain alkyl groups show a much better IFT reduction effectiveness than those with straight-chain alkyl groups. Contrary to what has been observed at the air-aqueous solution surface, mixtures of two homologues with two hydrophobic groups show significant molecular interactions, with both betasigma(LL) and beta(M) having negative values in the 4-5 range in some cases, with the betasigma(LL) value more negative than beta(M). The relationship between micellar shape and ultralow IFT was investigated by calculating the critical packing parameter of the surfactants. It was found that ultralow IFT between the surfactant mixtures and the three hydrocarbons investigated could reach ultralow (<10(-2) mN/m) values when the critical packing parameter is very close to 1.     

On surface tension theory of salt melts

Density functional theory was developed to take into account the effect of charge density gradient on the surface tension of asymmetric ionic melts. The predicted values are compared with the experimental data on the surface tension of the majority of alkali metal halides.     

Temperature dependence of aggregation and dynamic surface tension in a photoresponsive surfactant system

The response of a nonionic photoresponsive surfactant system to changes in temperature is reported. This surfactant contains the light-sensitive azobenzene group, and when exposed to light, a solution of this surfactant contains a mixture of the cis and trans photoisomers of this group. The temperature of the surfactant solution has a strong impact on the time needed for the surfactant to diffuse and adsorb to a freshly formed interface. At surfactant concentrations that give rise to trans aggregates but not to cis aggregates, the transport of cis and of trans isomers to the surface of a pendant bubble have quite different temperature dependencies, owing largely to the difference in their aggregation states in bulk solution. Diffusion and adsorption of the cis isomer are described reasonably well by a simple diffusion model that accounts for the effect of temperature on the diffusion coefficient. The trans isomer, which was primarily bound in aggregates during these measurements, exhibits a stronger dependence of this adsorption time scale on the temperature of the solution. This temperature dependence of trans diffusion and adsorption is quantitatively consistent between samples containing only the trans isomer and samples containing a mixture of isomers. Fluorescence studies were done to determine the effect of temperature on the cmc of the surfactant. The critical concentration associated with the formation of cis-dominant aggregates increases modestly with increasing temperature. The cmc of the trans isomer also increases with increasing temperature, most significantly when the temperature exceeds about 35 degrees C. These trans cmc temperature-dependence data were incorporated into diffusion models that account for the potential roles of aggregates in the adsorption process. The observed temperature dependency of the trans adsorption time scale is consistent with a model that includes the effect of temperature on both the diffusivity and the supply of monomer via its effect on the cmc. Specifically, the results suggest that the dissolution of trans-dominant aggregates is important to the trans adsorption process. Further fluorescence studies were performed in which surfactant solutions containing aggregates were diluted rapidly, and the rate of dissolution of these aggregates was inferred from fluorescence decay. Aggregate breakup in colder trans samples is slower than in warmer samples, but these dissolution time scales are significantly shorter than those associated with the adsorption process. This is consistent with the assumption that aggregation kinetics do not contribute to the observed adsorption kinetics.     

Prediction of the surface tension of surfactant mixtures for detergent formulation using Design Expert software

Abstract  

The formulation of a detergent product is a complicated task which depends on many economical, ecological, and practical parameters. The goal is to achieve a product which will have optimal washing properties and significantly decrease the surface tension. Therefore, prediction of the minimal surface tension of a mixture of four different surfactants can be very beneficial for the industry. In this work this task was performed by using Stat-Ease Design Expert software. This program was successfully applied for prediction of the minimal surface tension and optimization of the composition of different surfactant mixtures. Predicted data were compared with experimental results, and very good correlation was achieved. For further application, more complex optimization procedures could be performed by adding additional parameters to the Design Expert software. Therefore, this program is extremely useful for the laundry industry and could be successfully applied in many other, more complex tasks.     

Alternating current tast-polarographic determination of surface activity of lung surfactant

It is demonstrated that lung surfactant, like DL-α-lecithin, acts as a typical surface active substance in phase-selected alternating current tast-polarography. It lowers both the electrical double-layer capacity and the surface tension on the mercury electrode/saline interface and gives rise to characteristic adsorption-desorption capacity peaks. Capacity decrease, peak height, and diminution of the supporting electrolyte hump are controlled by the quantity, i.e. surface activity, of lung surfactant. A linear relationship between the logarithm of capacity values and surface activity measured with the de Noüy ring method is derived. Based on these facts it is possible to express surface activity of lung surfactant in capacity terms and to use the phase-selected alternating current tast-polarography in surface activity studies of lung surfactant.     

Real-time investigation of lung surfactant respreading with surface vibrational spectroscopy

The respreading of a lung surfactant monolayer at the air-water interface is investigated with broad bandwidth sum frequency generation (BBSFG) spectroscopy. The lung surfactant mixture contains chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA), and KL4 (a 21-residue polypeptide analogue to the surfactant protein SP-B). DPPC-d62 serves as a probe molecule for the spectroscopic investigation. The BBSFG spectra of DPPC-d62 in the lung surfactant mixture are obtained in the C-D stretching region in real-time during film compression and expansion in a Langmuir trough. The BBSFG intensity of the CD3 stretch peak from DPPC-d62 terminal methyl groups is used as a measure of the interfacial density of DPPC-d62 after careful consideration of orientation effects. For the first time, the interfacial loss of DPPC in a complex lung surfactant mixture is quantified. Spectroscopic results reveal that there is an 18% DPPC-d62 interfacial loss during film respreading. However, the surface pressure-area isotherm measurements demonstrate that there is a rather large trough area reduction (37%) during film expansion. The relatively small interfacial loss of DPPC-d62 and the rather large trough area reduction indicate that the respreading of DPPC and non-DPPC components in the lung surfactant is not uniform and a surface refinement process exists during film compression and expansion. This refinement process results in a DPPC-enriched monolayer with a significant depletion of non-DPPC components after film respreading. Implication for replacement surfactant design from this work is discussed.     

On the characterization of host-guest complexes: surface tension, calorimetry, and molecular dynamics of cyclodextrins with a non-ionic surfactant

Three host-guest systems have been characterized using surface tension (sigma), calorimetry, and molecular dynamics simulations (MD). The hosts were three native cyclodextrins (CD) and the guest the non-ionic carbohydrate surfactant octyl-beta-d-glucopyranoside. It is shown that, for any host-guest system, a rough screening of the most probable complex stoichiometries can be obtained in a model free form, using only calorimetric data. The sigma data were analyzed using a model that includes a newly proposed adsorption isotherm. The equilibrium constants for several stoichiometries were simultaneously obtained through fitting the sigma data. For alpha- and beta-CD, the predominant species is 1:1 and to a lesser extent 2:1, disregarding the existence of the 1:2. For gamma-CD, the 1:2 species dominates, the other two being also present. In an attempt to confirm these results, 10 ns MD simulations for each CD were performed using seven different starting conformations. The MD stable conformations agree with the results found from the experimental data. In one case, the spontaneous dissociation-formation of a complex was observed. Analysis of the trajectories indicates that hydrophobic interactions are primarily responsible for the formation and stability of the inclusion complexes. For the 2:1 species, intermolecular H-bonds between CD molecules result in a tight packed structure where their original truncated cone shape is lost in favor of a cylindrical geometry. Together, the results clearly demonstrate that the often used assumption of considering only a 1:1 species is inappropriate.     

Enhanced surfactant determination by ion-pair formation using flow-injection analysis and dynamic surface tension detection

Chemical analysis of surface active species (surfactants) is of interest for many applications, such as in process monitoring, biomedical applications, environmental monitoring and surface science investigations. Recently, we reported a dynamic surface tension detector (DSTD) based upon optically probing the size of a repeating drop resulting from constant flow of an aqueous solvent out of the end of a capillary. Presence of a surfactant in a growing drop reduces the surface tension at the air-solvent interface, causing the drop to detach at a smaller volume, which is detected. The DSTD has a kinetic dependence, and with increasing flow rate the sensitivity decreases due to diffusional and adsorption effects. We report that for the sodium salt of dodecylsulfate (DS), the DSTD performs significantly better with a stainless steel (S.S.) capillary dropper than with a fused silica dropper because the S.S. dropper exhibits a smaller adsorption effect as a function of time. Flow-injection analysis with the DSTD of DS was found to enhance sensitivity 50-fold by in-situ reaction with the ion-pair reagent tetrabutylammonium hydroxide (TBA) in water, even though the TBA alone was not very surface active. The TBA-DS system serves as a model for a selective detection method in which surface activity is exploited and enhanced. The detection limit for DS, as TBA-DS, was 400 ppb. Additionally, weakly surface active species such as TBA could be analyzed "indirectly" by ion-pair formation with DS. The enhanced sensitivity is due to increased packing of the ion-pairs at the air-aqueous solvent interface. The flow rate dependence on the sensitivity of detecting the TBA-DS ion-pair was examined. Two limiting conditions were observed as a function of ion-pair concentration: sensitivity decreases linearly with inverse flow rate at high flow rates and approaches a steady state at slower flow rates.