Evaluation of in vitro cornea models for quantifying destructive effects of chemicals.

In vitro models are available as alternatives for the Draize eye irritation test. However, most of the alternative models are not quantitative nor designed to evaluate the effects of chemicals on the corneal barrier such as those encountered in pharmaceutical and cosmetic products. The objective of the present study was to investigate tissue electrical resistance to provide sensitive in vitro testing of tissue alteration caused by chemicals in pharmaceutical and cosmetic formulations as a potential eye irritation testing approach. The experimental protocols for effective tissue resistance measurements were examined using two in vitro eye models: porcine cornea and EpiCorneal. In these models, a test chemical was applied to the cornea or EpiCorneal tissue for 1 min, and tissue resistances/conductances were measured at 1-60 min after the application. The changes in conductance of the tissues after exposure to the chemicals were shown to provide quantitative evaluations to the influence of the chemicals. A correlation was found between the two in vitro models. The results suggest that these models can provide quantitative in vitro assessments of eye-irritating chemicals.

Role of pH in skin cleansing.

BACKGROUND: The importance of maintaining the acid-mantle of human stratum corneum to maintain its healthy barrier and skin's biological functions such as desquamation and lipid biosynthesis is well recognized in the literature. An outcome of this has been an increase in the number of skincare products formulated at or near the skin pH with an implication that a product formulated at skin pH will be good for skin. Such an assumption often does not take into account the specific interactions of ingredients in the product with the stratum corneum under skin pH conditions. OBJECTIVE: The objective of this research was to determine whether a skin cleansing product by virtue of its pH being same as "skin pH" is milder to skin. METHODS: A well established Forearm Controlled Application Test (FCAT) protocol was used in clinical studies to compare "skin pH" cleansing systems with neutral pH cleansing systems. Specifically, certain commercially available "skin pH" cleansing bars were compared with a neutral pH syndet bar in two separate FCAT studies. Since these bars differed in their surfactant composition, in a separate FCAT study, two identical prototype bar formulations differed only in their pH were compared. Additionally, two body wash liquid prototypes, identical in composition but differing only in their pH were also compared in another FCAT study. RESULTS: The results obtained showed that skin-cleansing systems formulated solely or predominantly with anionic surfactants under skin pH conditions can result in increased skin dryness and irritation compared to those under neutral pH conditions. The results are explained in terms of the increased electrostatic interaction of anionic surfactants with stratum corneum under low pH conditions compared to neutral pH conditions. CONCLUSION: Skin-cleansing systems formulated solely or predominantly with anionic surfactants under skin pH conditions can result in increased skin dryness and irritation compared to those under neutral pH conditions. Any skin cleansing product by virtue of its pH being same as that of "skin pH" does not guarantee that it will be good for skin. The mildness of a cleanser will be determined by the interactions of its surfactants and other ingredients with stratum corneum under its formulated pH conditions.

CONTEXTE: l'importance de la protection du manteau acide de la couche cornée humaine en vue de maintenir une barrière saine et les fonctions biologiques de la peau, telles que la desquamation et la biosynthèse lipidique, est bien reconnue dans la littérature médicale. Cela a eu pour résultat l'augmentation du nombre de produits cosmétiques formulés à un pH proche ou identique au pH de la peau, impliquant ainsi qu'un produit formulé au pH de la peau sera bon pour la peau. Cette hypothèse ne tient souvent pas compte des interactions spécifiques des ingrédients du produit avec la couche cornée dans des conditions de pH de la peau. OBJECTIF: l'objectif de cette recherche était de déterminer si un nettoyant pour la peau formulé à un pH identique au « pH de la peau ¼ est, pour cette raison, plus doux pour la peau. MÉTHODES: un protocole bien établi de test d'application contrôlée sur l'avant-bras (Forearm Controlled Application Test, FCAT) a été utilisé dans des études cliniques pour comparer les nettoyants à « pH de la peau ¼ et les nettoyants à pH neutre. Plus précisément, certains savons à « pH de la peau ¼ disponibles dans le commerce ont été comparés à un savon surgras à pH neutre dans deux études distinctes faisant appel au FCAT. La composition en termes d'agents de surface de ces savons étant différente, une étude distincte faisant appel au FCAT a comparé deux prototypes de savons de composition identique mais de pH différent. De plus, deux prototypes de savon liquide pour le corps, de composition identique mais de pH différent, ont également été comparés dans une autre étude faisant appel au FCAT. RÉSULTATS: les résultats obtenus ont montré que les nettoyants pour la peau formulés uniquement ou principalement avec des agents de surface anioniques dans des conditions de pH de la peau peuvent entraîner une augmentation de la sécheresse et de l'irritation cutanées, par rapport à ceux formulés dans des conditions de pH neutre. Les résultats s'expliquent par l'interaction électrostatique accrue des agents de surface anioniques avec la couche cornée dans des conditions de pH faible par rapport aux conditions de pH neutre. CONCLUSION: les nettoyants pour la peau formulés uniquement ou principalement avec des agents de surface anioniques dans des conditions de pH de la peau peuvent entraîner une augmentation de la sécheresse et de l'irritation cutanées, par rapport à ceux formulés dans des conditions de pH neutre. La formulation d'un nettoyant pour la peau à un pH identique au « pH de la peau ¼ ne garantit pas qu'il sera bon pour la peau. La douceur d'un nettoyant sera déterminée par les interactions de ses agents de surface et de ses autres ingrédients avec la couche cornée dans ses conditions de formulation en termes de pH.

Influence of skin furrows on tape stripping in characterizing the depth of skin penetration.

The stratum corneum (SC), the outermost layer of the skin and its major barrier for penetration, contains furrows of different depths on its surface. The presence of these furrows might lead to erroneous interpretation of the results in skin permeation studies using tape stripping, in which the material trapped in the furrows removed by the tapes representing different layers of the SC might be interpreted as material penetrating within these layers. The present objective was to investigate the effect of skin furrows on tape stripping results. Non-penetrating fluorescent materials were topically applied to split-thickness human and full-thickness porcine skin samples. Tape stripping was applied, and the tapes were assessed by fluorescence microscopy and quantitative analyses. The microscopy images were assessed visually to determine the presence of the applied material in the furrows. The penetration depth of the material was examined and the fluorescence content and pattern in each tape were analyzed. The results suggested that skin furrows could be important in the first 10 tapes, affecting the quantification of materials in the SC, particularly in permeation studies of materials with low penetration into the SC. Depending on the properties of the materials, skin rinsing could reduce the impact of furrows.

Anionic Surfactant-Induced Changes in Skin Permeability.

Anionic surfactants compromise skin's barrier function by damaging stratum corneum lipids and proteins. The objective of this study was to examine anionic surfactant-induced changes in the skin's polar and transcellular pathways and the resulting impact on surfactant penetration into the skin. Three anionic surfactant formulations and one control formulation were each applied to split-thickness human cadaver skin in vitro for 24 h. Electrical conductivity of the skin, determined using a four-terminal resistance method, and water permeation across the skin, determined using a radiolabeled water tracer, were simultaneously measured at several points over the experimental period. Surfactant permeation across the skin was similarly measured using a radiolabeled sodium dodecyl sulfate tracer. Anionic surfactants rapidly enhanced skin electrical conductivity and water permeability in the excised human skin, resulting in nonlinear enhancements in surfactant permeation across the skin over time. Surfactant penetration into the skin was found to increase linearly with increasing surfactant monomer concentration. Surfactant zeta potential was found to correlate well with skin conductivity, water permeation across the skin, and surfactant permeation across the skin, particularly with long surfactant exposures. Micelle charge is a significant predictor of anionic surfactant-induced damage to the human skin, with more highly charged surfactants inducing the most damage.

Composition and structural transitions of polyelectrolyte-surfactant complexes in the presence of fatty acid studied by NMR and cryo-SEM.

Insoluble complexes formed when a cationic polyelectrolyte is neutralized by the oppositely charged surfactant sodium dodecylethersulfate (SDES) in the presence and absence of lauric acid (LA) have been examined directly using NMR spectroscopy and cryo-SEM. Below the SDES critical micelle concentration (CMC') the insoluble complex contains about 10 times more water than just above CMC'. This is related to a structural transition of the complex, where water is contained mainly in larger compartments below CMC' and then mainly in narrower compartments above CMC'. The structure of the complex's solid matrix was monitored by recording two-dimensional T2-diffusion correlation spectra of the water proton resonance, which reveal the presence of several different water environments which correspond to different complex structures. Structural features in the micrometer range were confirmed using cryo-SEM. When LA is present, the larger water compartments seen below CMC' are to some extent present in the entire SDES concentration range, which is not the case in the absence of LA. Furthermore, the inclusion of LA into the SDES aggregates above CMC' leads to a lamellar sheetlike organization of the polyelectrolyte-stabilized surfactant phase. In the absence of LA, a stringy network of fibers is seen in cryo-SEM images, indicating a spherical or rodlike SDES phase. Consequently, the complex without LA holds about 1.7-1.9 times more water than the complex with LA above the SDES CMC'. T1 relaxation, (13)C chemical shifts, and (1)H resonance line widths of SDES in the system support the above observations. The combination of MAS NMR, T2-diffusion correlation, and cryo-SEM proved to be an effective method for studying structural transitions in the surfactant-polyelectrolyte(-LA) insoluble complexes.

Solubility limits and phase diagrams for fatty alcohols in anionic (SLES) and zwitterionic (CAPB) micellar surfactant solutions.

By analysis of experimental data, a quantitative theoretical interpretation of the solubility limit of medium- and long-chain fatty alcohols in micellar solutions of water-soluble surfactants is presented. A general picture of the phase behavior of the investigated systems is given in the form of phase diagrams. The limited solubility of the fatty alcohols in the micelles of conventional surfactants is explained with the precipitation of their monomers in the bulk, rather than with micelle phase separation. The long chain fatty alcohols (with n=14, 16 and 18 carbon atoms) exhibit an ideal mixing in the micelles of the anionic surfactant sodium laurylethersulfate (SLES) and the zwitterionic surfactant cocamidopropyl betaine (CAPB) at temperatures of 25, 30, 35 and 40 °C. Deviations from ideality are observed for the alcohols of shorter chain (n=10 and 12), which can be explained by a mismatch with the longer chains of the surfactant molecules. Using the determined thermodynamic parameters of the systems, their phase diagrams are constructed. Such a diagram consists of four domains, viz. mixed micelles; coexistent micelles and precipitate (dispersed crystallites or droplets); precipitate without micelles, and molecular solution. The four boundary lines intersect in a quadruple point, Q. For ionic surfactants (like SLES), a detailed theory for calculating the boundary lines of the phase diagrams is developed and verified against data for the positions of the kinks in surface tension isotherms. The theory takes into account the electrostatic interactions in the micellar solutions and the effect of counterion binding. The results can be useful for a quantitative interpretation and prediction of the phase behavior of mixed solutions of two (or more) surfactants, one of them being water soluble and forming micelles, whereas the other one has a limited water solubility, but readily forms mixed micelles with the former surfactant.

The nature of fatty acid interaction with a polyelectrolyte-surfactant pair revealed by NMR spectroscopy.

The interaction mechanisms of an oppositely charged polyelectrolyte-surfactant pair and dodecanoic (lauric) acid (LA) were experimentally investigated using a combination of nuclear magnetic resonance (NMR) techniques. It is observed that LA significantly affects the interaction between the anionic surfactant sodium dodecylethersulfate (SDES) and the cationic polymer guar modified with grafted hydroxypropyl trimethylammonium chloride (Jaguar C13 BF). Typically, oppositely charged polymers and surfactants interact electrostatically at a certain surfactant concentration known as the critical aggregation concentration (CAC). Once the polymer is neutralized by the surfactant, an insoluble complex (precipitate) is observed (phase separation), and, at concentrations beyond the surfactant critical micellar concentration (CMC'), the system returns to a one phase entity. In a system in which a mixture of SDES-LA is added to the polymer, NMR data show that below the neutralization onset, some of the polymer interacts with SDES, while some of the polymer is adsorbed at the surface of LA solid aggregates present in the system. Furthermore, SDES is found to aggregate in a lamellar-like structure at the polymer side chain prior to the SDES CMC'. Above the SDES (CMC'), LA is solubilized and incorporated at the palisade region of SDES micelles. Analysis of (1)H resonances provided estimated concentrations of all species in the system phases at all stages of interaction.

Three-dimensional chemical imaging of skin using stimulated Raman scattering microscopy.

Stimulated Raman scattering (SRS) microscopy is used to generate structural and chemical three-dimensional images of native skin. We employed SRS microscopy to investigate the microanatomical features of skin and penetration of topically applied materials. Image depth stacks are collected at distinct wavelengths corresponding to vibrational modes of proteins, lipids, and water in the skin. We observed that corneocytes in stratum corneum are grouped together in clusters, 100 to 250 µm in diameter, separated by 10- to 25-µm-wide microanatomical skin-folds called canyons. These canyons occasionally extend down to depths comparable to that of the dermal-epidermal junction below the flat surface regions in porcine and human skin. SRS imaging shows the distribution of chemical species within cell clusters and canyons. Water is predominately located within the cell clusters, and its concentration rapidly increases at the transition from stratum corneum to viable epidermis. Canyons do not contain detectable levels of water and are rich in lipid material. Oleic acid-d34 applied to the skin surface lines the canyons down to a depth of 50 µm below the surface of the skin. This observation could have implications on the evaluation of penetration profiles of bioactive materials measured using traditional methods, such as tape-stripping.

Micelle-monomer equilibria in solutions of ionic surfactants and in ionic-nonionic mixtures: a generalized phase separation model.

On the basis of a detailed physicochemical model, a complete system of equations is formulated that describes the equilibrium between micelles and monomers in solutions of ionic surfactants and their mixtures with nonionic surfactants. The equations of the system express mass balances, chemical and mechanical equilibria. Each nonionic surfactant is characterized by a single thermodynamic parameter--its micellization constant. Each ionic surfactant is characterized by three parameters, including the Stern constant that quantifies the counterion binding. In the case of mixed micelles, each pair of surfactants is characterized with an interaction parameter, ß, in terms of the regular solution theory. The comparison of the model with experimental data for surfactant binary mixtures shows that ß is constant--independent of the micelle composition and electrolyte concentration. The solution of the system of equations gives the concentrations of all monomeric species, the micelle composition, ionization degree, surface potential and mean area per head group. Upon additional assumptions for the micelle shape, the mean aggregation number can be also estimated. The model gives quantitative theoretical interpretation of the dependence of the critical micellization concentration (CMC) of ionic surfactants on the ionic strength; of the CMC of mixed surfactant solutions, and of the electrolytic conductivity of micellar solutions. It turns out, that in the absence of added salt the conductivity is completely dominated by the contribution of the small ions: monomers and counterions. The theoretical predictions are in good agreement with experimental data.

Disclike vs. cylindrical micelles: generalized model of micelle growth and data interpretation.

Here, we present a detailed theoretical model describing the growth of disclike surfactant micelles. The model is tested against light-scattering data for micellar solutions from mixed conventional surfactants and from fluorinated surfactants. Theoretical expressions are derived for the concentration dependencies of the number and mass average aggregation numbers. Central role in the theory is played by the difference between the chemical potentials of a surfactant molecule in cylindrical and discoidal micelles. This difference, scaled with the thermal energy kT, is denoted p. For p<0, the formation of cylindrical (rather than disclike) micelles is energetically favored. For p>0 disclike micelles are formed, but their growth is limited due to the rise of their positive peripheral energy. Because of that, disclike micelles can be observed in a relatively narrow interval, 0

Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films.

The stepwise thinning (stratification) of liquid films, which contain micelles of an ionic surfactant, depends on the micelle aggregation number, N(agg), and charge, Z. Vice versa, from the height of the step and the final film thickness one can determine N(agg), Z, and the degree of micelle ionization. The determination of N(agg) is based on the experimental fact that the step height is equal to the inverse cubic root of the micelle concentration. In addition, Z is determined from the final thickness of the film, which depends on the concentration of counterions dissociated from the micelles in the bulk. The method is applied to micellar solutions of six surfactants, both anionic and cationic: sodium dodecylsulfate (SDS), cetyl trimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), sodium laurylethersulfates with 1 and 3 ethylene oxide groups (SLES-1EO and SLES-3EO), and potassium myristate. The method has the following advantages: (i) N(agg) and Z are determined simultaneously, from the same set of experimental data; (ii) N(agg) and Z are determined for each given surfactant concentration (i.e. their concentration dependence is obtained), and (iii) N(agg) and Z can be determined even for turbid solutions, like those of carboxylates, where the micelles coexist with acid-soap crystallites, so that the application of other methods is difficult. The results indicate that the micelles of greater aggregation number have a lower degree of ionization, which can be explained with the effect of counterion binding. The proposed method is applicable to the concentration range, in which the films stratify and the micelles are spherical. This is satisfied for numerous systems representing scientific and practical interest.

Solubility limits and phase diagrams for fatty acids in anionic (SLES) and zwitterionic (CAPB) micellar surfactant solutions.

The limiting solubility of fatty acids in micellar solutions of the anionic surfactant sodium laurylethersulfate (SLES) and the zwitterionic surfactant cocamidopropyl betaine (CAPB) is experimentally determined. Saturated straight-chain fatty acids with n=10, 12, 14, 16, and 18 carbon atoms were investigated at working temperatures of 25, 30, 35, and 40°C. The rise of the fatty acid molar fraction in the micelles is accompanied by an increase in the equilibrium concentration of acid monomers in the aqueous phase. Theoretically, the solubility limit is explained with the precipitation of fatty acid crystallites when the monomer concentration reaches the solubility limit of the acid in pure water. In agreement with theory, the experiment shows that the solubility limit is proportional to the surfactant concentration. For ideal mixtures, the plot of the log of solubility limit vs. the chainlength, n, must be a straight line, which is fulfilled for n=14, 16, and 18. For the fatty acids of shorter chains, n=10 and 12, a deviation from linearity is observed, which is interpreted as non-ideal mixing due to a mismatch between the chainlengths of the surfactant and acid. The data analysis yields the solubilization energy and the interaction parameter for the fatty acid molecules in surfactant micelles. By using the determined parameter values, phase diagrams of the investigated mixed solutions are constructed. The four inter-domain boundary lines intersect in a quadruple point, whose coordinates have been determined. The results can be applied for the interpretation and prediction of the solubility, and phase behavior of medium- and long-chain fatty acids and other amphiphiles that are solubilizable in micellar surfactant solutions, as well as for determining the critical micellization concentration (CMC) of the respective mixed solution.

The metastable states of foam films containing electrically charged micelles or particles: experiment and quantitative interpretation.

The stepwise thinning (stratification) of liquid films containing electrically charged colloidal particles (in our case - surfactant micelles) is investigated. Most of the results are applicable also to films from nanoparticle suspensions. The aim is to achieve agreement between theory and experiment, and to better understand the physical reasons for this phenomenon. To test different theoretical approaches, we obtained experimental data for free foam films from micellar solutions of three ionic surfactants. The theoretical problem is reduced to the interpretation of the experimental concentration dependencies of the step height and of the final film thickness. The surface charges of films and micelles are calculated by means of the charge-regulation model, with a counterion-binding (Stern) constant determined from the fit of surface tension isotherms. The applicability of three models was tested: the Poisson-Boltzmann (PB) model; the jellium-approximation (JA), and the cell model (CM). The best agreement theory/experiment was obtained with the JA model without using any adjustable parameters. Two theoretical approaches are considered. First, in the energy approach the step height is identified with the effective diameter of the charged micelles, which represents an integral of the electrostatic-repulsion energy calculated by the JA model. Second, in the osmotic approach the step height is equal to the inverse cubic root of micelle number density in the bulk of solution. Both approaches are in good agreement with the experiment if the suspension of charged particles (micelles) represents a jellium, i.e. if the particle concentration is uniform despite the field of the electric double layers. The results lead to a convenient method for determining the aggregation number of ionic surfactant micelles from the experimental heights of the steps.

Stearic acid delivery to corneum from a mild and moisturizing cleanser.

OBJECTIVE: A mild moisturizing body wash with stearic acid, a key component of corneum lipids, and emollient soybean oil has been introduced in the market place. The objectives of this study are to determine the amount and the location of the stearic acid in the corneum after in vivo cleansing by the formulation. METHOD: Clinical cleansing studies for one and five consecutive days were carried out with the formulation containing soybean oil or petroleum jelly (PJ). The free stearic acid in it was replaced by the fully deuterated variant. The amounts of stearic acid in 10 consecutive corneum tape strips were measured by liquid chromatograph-mass spectroscopy. Separately, electron paramagnetic resonance (EPR) measurements were taken with a porcine skin after a wash by the soybean oil formulation with its free fatty acid replaced by its spin probe analogue, 5-doxyl stearic acid. RESULTS: Deuterated stearic acid was detected in all 10 consecutive layers of stratum corneum and the total amount after five washes with the soybean oil formulation was 0.33 µg/cm². The spin probe in cleanser-treated skin was incorporated in a partially ordered hydrophobic region similar to corneum lipids. The probe mobility increased in the temperature region where lipid disorder was expected. CONCLUSIONS: The estimated total fatty acid delivered to skin from cleansing is comparable to the amount of fatty acid in a corneum layer. The delivered fatty acid is most likely incorporated in the corneum lipid phase.

Method for analysis of the composition of acid soaps by electrolytic conductivity measurements.

Here, we propose a method for determining the stoichiometry of acid-soap crystallites. The method is based on dissolving the crystallites in water at an appropriate working temperature, followed by measurement of the electrolytic conductivity of the obtained solutions. The working temperature is chosen in such a way that the only precipitate in the solutions is that of carboxylic acid, whereas the carboxylate salt is dissociated, and its content in the dissolved crystals determines the solution's conductivity. In the theoretical model for data interpretation, we took into account the dependence of the molar conductance on the ionic strength. The method was applied for determining the stoichiometry of acid-soap crystals collected from solutions of potassium myristate (tetradecanoate) at 25 degrees C. The crystals were dissolved in water at working temperature of 40 degrees C, at which the conductivity was measured. The stoichiometry of all samples determined in the present study coincides with that independently obtained by another method that is based on in situ pH measurements.

Role of the counterions on the adsorption of ionic surfactants.

A theory accounting for the effect of the counterions on the adsorption constant, K, is proposed. The experimental values of K were determined by using surface and interfacial tension isotherms measured by us or available in the literature. By accounting for the adsorption energy u 0 of the counterion, a generalization of Gouy equation and a modified expression for the adsorption constant, K, are derived. The adsorption energy is calculated from the London equation, which involves the polarizabilities alpha 0i and the ionization potentials Ii of the respective components and the radius Rh of the hydrated ion. By careful analysis of the available experimental data for alpha 0i, Ii and Rh, coupled with some reasonable hypothesis, we succeeded to obtain linear dependences between the calculated values of u 0 and the experimental data for lnK with slopes rather close to the theoretical ones. The obtained results for u 0 were used to calculate the disjoining pressure isotherms of foam films stabilized by DTAF, DTACl and DTABr. It turned out that the type of the counterion has significant effect on the disjoining pressure.

Effect of the precipitation of neutral-soap, acid-soap, and alkanoic acid crystallites on the bulk pH and surface tension of soap solutions.

The natural pH of sodium dodecanoate (laurate), NaL, and sodium tetradecanoate (myristate), NaMy, solutions is measured as a function of the surfactant concentrations at 25 degrees C, and at several fixed NaCl concentrations. Surface tension isotherms are also obtained. Depending on the surfactant concentration, the investigated solutions contain precipitates of alkanoic acid, neutral soap, and acid soaps. The latter are complexes of alkanoic acid and neutral soap with a definite stoichiometry. A method for identification of the different precipitates from the experimental pH isotherms is developed. It is based on the analysis of precipitation diagrams, which represent plots of characteristic functions. This analysis reveals that for the NaL solutions there are three concentration regions with different precipitates, including lauric acid and 1:1 acid soap. In the case of NaMy solutions, we identified the existence of concentration regions with precipitates of myristic acid: 4:1, 3:2, and 1:1 acid soaps, and coexistence of two solid phases: 1:1 acid soap and neutral soap. The solubility products of the precipitates have been determined. Modeling the acid soaps of different stoichiometry as solid solutions of alkanoic acid and 1:1 acid soap, we derived a theoretical expression for their solubility products, which agrees well with the experiment. The kinks in the surface-tension isotherms of the investigated solutions correspond to some of the boundaries between the regions with different precipitates in the bulk. The theoretical analysis indicates that for the NaL solutions the adsorption layer is composed mostly of lauric acid, while for the NaMy solutions + 10 mM NaOH the adsorption layer is composed of nondissociated molecules of neutral soap. The developed approach could be applied to analyze the type of precipitates and the behavior of the surface tension for solutions of sodium and potassium alkanoates with different chain lengths at various temperatures and concentrations.

The colloid structural forces as a tool for particle characterization and control of dispersion stability.

Knowing the size and interactions of colloid particles, one can predict the stepwise thickness transitions and the contact angles of particle-containing liquid films. Here, we consider the inverse problem, viz. how to determine the particle properties by measurements with liquid films. We carried out experiments with films formed from aqueous solutions of two nonionic surfactants, Brij 35 and Tween 20, which contain spherical micelles of diameters in the range 7-9 nm. From the measured contact angles, we determined the micelle aggregation number and volume fraction. In addition, from the measured disjoining-pressure isotherms we determined the micelle diameter. In other words, the liquid-film measurements give information about the micelles, which is analogous to that obtainable by dynamic and static light scattering. Furthermore, we investigate the predictions of different quantitative criteria for stability-instability transitions, having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force is taken into account. Two different kinetic criteria are considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams are constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed.

Maximum bubble pressure method: Universal surface age and transport mechanisms in surfactant solutions.

Here, based on the theoretical analysis of results for two ionic surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (DTAB), we develop a new approach for quantitative interpretation of data from the maximum bubble pressure method. A given tensiometer is characterized by an apparatus function, A(t), and by an apparatus constant. The former represents the time dependence of the bubble surface area, whereas the latter is expressed through integrals of A(t). The experiment indicates that both of them are independent of the surfactant type and concentration. Moreover, if a certain criterion is satisfied, the experimental results depend on the surface dilatation only through the apparatus constant. This makes the data interpretation much easier. The knowledge of the apparatus constant gives a general time scale (universal surface age) that makes the results independent of the specific bubble-pressure setup and produces dynamic surface tension curves that are universal characteristics of the investigated solutions. A new equation for data processing is proposed, which provides excellent fits of the dynamic surface tension. In the case of micellar solutions, the data analysis enables one to identify the kinetic regime of adsorption (among four possible regimes). For the investigated surfactant solutions, the diffusion regime "BC" was identified, for which the fast micellar process is equilibrated, whereas the slow micellar process is negligible. Upgraded with the developed approach for quantitative data interpretation, the bubble-pressure tensiometry could be a useful tool for a detailed analysis of the adsorption processes in more complex systems.

Influence of electrolytes on the dynamic surface tension of ionic surfactant solutions: expanding and immobile interfaces.

Here, we derive analytical asymptotic expressions for the dynamic surface tension of ionic surfactant solutions in the general case of nonstationary interfacial expansion. Because the diffusion layer is much wider than the electric double layer, the equations contain a small parameter. The resulting perturbation problem is singular and it is solved by means of the method of matched asymptotic expansions. The derived general expression for the dynamic surface tension is simplified for the special case of immobile interface and for the maximum bubble pressure method (MBPM). The case of stationary interfacial expansion is also considered. The effective diffusivity of the ionic surfactant essentially depends on the concentrations of surfactant and nonamphiphilic salt. To test the theory, the derived equations are applied to calculate the surfactant adsorption from MBPM experimental data. The results excellently agree with the adsorption determined independently from equilibrium surface-tension isotherms. The derived theoretical expressions could find application for interpreting data obtained by MBPM and other experimental methods for investigating interfacial dynamics.