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

The Surface Tension of Synthetic Blood used for ASTM F1670 Penetration Tests

The American Society for Testing and Material (ASTM International) F1670 test method was based on research involving transmission of bloodborne pathogens (Hepatitis B, Hepatitis C, and HIV) in the 1980s. The test method details the measurement of synthetic blood penetration through garments. A key parameter affecting penetration is synthetic blood surface tension which is measured via du Noüy ring tensiometer. However, little is known about the sources of variation impacting surface tension measurements. In this study, the synthetic blood used for ASTM F1670 was evaluated from within the ASTM F903 test apparatus and with two mixing treatments. Measurements were compared against two outside laboratories and with two alternate tensiometric methods (pendant drop and capillary rise). It was found that using the methods specified in the ASTM F1670 test method, surface tension of the synthetic blood was not 40-44 dynes/cm as was expected. The surface tension was initially above 50 dynes/cm and declined to below 40 dynes/cm after 60 minutes. The surface tension within the penetration cell was relatively constant over time, showing that the surface tension measurements outside the penetration cell are not indicative of the surface tension within the apparatus during the test. Shaking the synthetic blood, a mixing procedure detailed in the ASTM F1670 test method, increased the surface tension. The increase was greatest in a container having more airspace. Du Noüy ring measurements by NIOSH compared to external labs were within 15%. Testing with alternate methods showed that the "open-to-atmosphere" methods (ring and drop) began lower and declined rapidly when compared to the "closed-to-atmosphere" method (capillary). Results of this research will help amend the ASTM F1670 standard to better characterize the measurement and handling of synthetic blood used in the ASTM F1670 test and to provide a framework for consideration of test fluid used in future ASTM standards.

Visualization of an adsorption model for surfactant transport from micelle solutions to a clean air/water interface using fluorescence microscopy

This work pertains to visualizing a transport model for adsorption of surfactants from micelle solutions onto a clean air/water interface. Under the condition of surfactant adsorption from very dilute solutions, the time scale for diffusion of a surfactant monomer is much slower than the time scale for kinetic breakdown of the aggregates. A theoretical model predicts two regimes for the adsorption dynamics. We visualize these two regimes under the mechanism of solubilization using fluorescence microscopy, in which an insoluble fluorescent probe, NBD-HAD (4-(hexadecylamino)-7-nitrobenz-2-oxa-1,3-diazole), is used to illuminate the micelles. The dye fluoresces in the microenvironment of micelles but is quenched in the aqueous solution on laser excitation. The region containing micelles is illuminated, but the region which does not contain micelles appears dark. For surfactant solution of C(14)E(6) at concentration just above the critical micelle concentration (C(CMC)), C(CMC)=4.4 mg/L, a dark region between the bright image of the air/water interface and the micelle-containing zone is observed. This dark region becomes smaller with time and finally disappears once equilibrium is reached. For a surfactant solution of C(14)E(6) at the concentration of 4.74C(CMC), which is higher than a critical total surfactant concentration (C(T)(c)) of 4.25C(CMC), we observe bright images through surfactant solutions during the adsorption process. Fluorescence images validate the theoretical model.

Fluorescence visualization and modeling of a micelle-free zone formed at the interface between an oil and an aqueous micellar phase during interfacial surfactant transport

This study examines the transport of surfactant that occurs when an aqueous micellar phase is placed in contact with a clean oil phase in which the surfactant is soluble. Upon contact with oil, surfactant monomer on the aqueous side of the interface adsorbs onto the oil/water interface and subsequently desorbs into the oil and diffuses away from the surface. The depletion of aqueous monomer underneath the interface disturbs the monomer-micelle equilibrium, and aggregates break down to replenish the monomer concentration and accelerate the interfacial transport. The depletion of monomer and micelles drives the diffusive flux of these species toward the surface, and the combined effects of diffusion and aggregate kinetic disassembly, alongside kinetic adsorption and desorption at the interface and diffusion away from the interface into the oil, determine the interfacial transport rate. This interfacial transport is examined here in the quasi-static limit in which the diffusion of monomer and micelles in the aqueous phase is much slower than the time scale for micelle disassembly. In this limit, when the initial bulk concentration of micelles in the aqueous solution is small, the micelle diffusive flux to the surface cannot keep up with the micelle breakdown under the interface, and a micelle-free zone forms. This zone extends from the surface into the aqueous phase up to a boundary that demarcates the beginning of a zone, containing micelles, that extends further into the aqueous phase. Micelles diffuse from the micelle zone to the boundary, where they break down, causing the boundary to retreat. Released monomer diffuses through the micelle-free zone and partitions into the oil phase. The focus of this study is to verify this transport picture by visualizing the micelle-free zone and comparing the movement of the zone to predictions obtained from a transport model based on this two-zone picture. A small hydrophobic dye molecule (Nile red) is incorporated into the micelles; the dye fluoresces only in the hydrophobic environment of the micelles, providing visual contrast between the two zones. Through spatial mapping of the fluorescence using confocal microscopy, the movement of the micelle-free zone boundary can be measured and is shown to compare favorably with simulations of the transport model.

New methodology to determine the rate-limiting adsorption kinetics mechanism from experimental dynamic surface tension data

We present a new methodology to determine the rate-limiting adsorption kinetics mechanism (diffusion-controlled vs mixed diffusion-barrier controlled), including deducing the kinetics parameters (the diffusion coefficient, D, and the energy-barrier parameter, beta), from the experimental short-time dynamic surface tension (DST) data. The new methodology has the following advantages over the existing procedure used to analyze the experimental DST data: (a) it does not require using a model for the equilibrium adsorption isotherm, and (b) it only requires using the experimental short-time DST data measured at two initial surfactant bulk solution concentrations. We apply the new methodology to analyze the experimental short-time DST data of the following alkyl poly(ethylene oxide), CiEj, nonionic surfactants: C12E4, C12E6, C12E8, and C10E8 measured using the pendant-bubble apparatus. We find that for C12E4 and C12E6, the effect of the energy barrier on the overall rate of surfactant adsorption can be neglected for surfactant bulk solution concentrations below their respective critical micelle concentrations (CMCs), and therefore, that the rate-limiting adsorption kinetics mechanism for C12E4 and C12E6 is diffusion-controlled at any of their premicellar surfactant bulk solution concentrations. On the other hand, for C12E8 and C10E8, we find that their respective CMC values are large enough to observe a significant effect of the energy barrier on the overall rate of surfactant adsorption. In other words, for C12E8 and C10E8, the rate-limiting adsorption kinetics mechanism shifts from diffusion-controlled to mixed diffusion-barrier controlled as their premicellar surfactant bulk solution concentrations increase. We test the new methodology by predicting the short-time DST profiles at other initial surfactant bulk solution concentrations, and then comparing the predicted DST profiles with those measured experimentally. Very good agreement is obtained for the four CiEj nonionic surfactants considered. We also compare the results of implementing the new methodology with those of implementing the existing procedure, and conclude that using a model for the equilibrium adsorption isotherm can lead not only to different values of D and beta, but it can also lead to a completely different determination of the rate-limiting adsorption kinetics mechanism. Since the new methodology proposed here does not require using a model for the equilibrium adsorption isotherm, we conclude that it should provide a more reliable determination of the rate-limiting adsorption kinetics mechanism, including the deduced kinetics parameters, D and beta.

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.

Kinetics of adsorption from micellar solutions

Previous studies on surfactant adsorption mostly deal with dilute systems without aggregation in the bulk phase. At the same time, micellar solutions can be more important from the point of view of applications. If one attempts to estimate the equilibrium adsorption, neglecting the influence of micelles can lead to reasonable results. The situation differs for non-equilibrium systems when the adsorption rate can increase by an order of magnitude at the increase of the surfactant concentration beyond the CMC. A critical survey of various models describing the influence of micelles on adsorption kinetics at the liquid-gas interface is given and the theoretical results are compared with existing experimental data. The theories proposed for the case of large deviations from the equilibrium are usually based on some unjustifiable assumptions and can describe the kinetic dependencies of adsorption in only a limited number of situations. Consequently, only rough estimates of the kinetic coefficients of micellization can be obtained from experimental data on dynamic surface tension. More rigorous equations can be derived if the system only deviates slightly from equilibrium. In the latter case, the agreement between theoretical and experimental results is essentially better and measurements of the dynamic surface elasticity of micellar solutions allow us to study the micellization kinetics.