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

Synergistic effects between a non-ionic and an anionic surfactant on the micellization process and the adsorption at liquid/air surfaces

Predicting the behaviour of solutions with surfactants of significantly different critical micelle concentration (CMC) values remains a challenge. The study of the molecular interactions within micelles and interfaces in surfactant combinations used in everyday products is essential to understand these complex systems. In this work, the equilibrium and dynamic surface tension in the presence of mixed non-ionic (tristyrylphenol ethoxylates) and anionic (sodium benzene sulfonate with alkyl chain lengths of C10-C13) surfactants, commonly encountered as delivery systems in agrochemicals, were studied and their CMC values were determined. For the surfactant mixtures, four molar ratios were examined: nEOT/nNaDDBS = 0.01, 0.1, 1, 4 and two different cases were analysed, the premixed and the add one by one surfactant. The surface tension for single surfactants stabilised quickly, while the mixtures needed a long time to reach equilibrium; up to 15 h for the premixed mixtures and 40 min when surfactants were added one by one. The CMC values for the nEOT/nNaDDBS = 0.01, 0.1 premixed surfactant mixtures were found to be in between the CMC values of the single surfactants, but those for the nEOT/nNaDDBS = 1 and 4 mixtures were lower than the CMCs of both single surfactants. Calculations based on the regular solution theory suggested that there are attractive forces in the mixed micelles and at the interface layers, while the supramolecular assemblies in the bulk (i.e., micelles) and at interfaces (surfactant films) are preferentially enriched in EOT.

Interconnectivity and morphology control of poly-high internal phase emulsions under photo-polymerization

We here demonstrate that the interconnectivity and morphology of photo-polymerized HIPEs can be controlled by changing the type of initiators and stabilizers, and the intensity of light.

Comparison of surfactant mass transfer with drop formation times from dynamic interfacial tension measurements in microchannels

Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.

Microfluidic Dynamic Interfacial Tensiometry (μDIT)

We designed, developed and characterized a microfluidic method for the measurement of surfactant adsorption kinetics via interfacial tensiometry on a microfluidic chip. The principle of the measurement is based on the deformability of droplets as a response to hydrodynamic forcing through a series of microfluidic expansions. We focus our analysis on one perfluoro surfactant molecule of practical interest for droplet-based microfluidic applications. We show that although the adsorption kinetics is much faster than the kinetics of the corresponding pendant drop experiment, our droplet-based microfluidic system has a sufficient time resolution to obtain quantitative measurement at the sub-second time-scale on nanoliter droplet volumes, leading to both a gain by a factor of ∼10 in time resolution and a downscaling of the measurement volumes by a factor of ∼1000 compared to standard techniques. Our approach provides new insight into the adsorption of surfactant molecules at liquid-liquid interfaces in a confined environment, relevant to emulsification, encapsulation and foaming, and the ability to measure adsorption and desorption rate constants.

Surfactant adsorption onto interfaces: measuring the surface excess in time

We propose a direct method to measure the equilibrium and dynamic surface properties of surfactant solutions with very low critical micellar concentrations (CMC) using a pendant drop tensiometer. We studied solutions of the nonionic surfactant hexaethylene glycol monododecyl ether (C(12)E(6)) and of the ionic surfactant hexadecyl trimethyl ammonium bromide (CTAB) with concentrated sodium bromide (NaBr). The variation of the surface tension as a function of surface concentration is obtained easily without the need for complex models and compares well with the result obtained using the Gibbs adsorption equation. The time-dependent surface concentration of each surfactant was also measured, and the adsorption process was found to be diffusion-controlled. The diffusion coefficients of the two surfactants can be extracted from the data and were found in very good agreement with literature values, further validating the method.

New methodology to determine equilibrium surfactant adsorption properties from experimental dynamic surface tension data

In this paper, we explore a novel approach to predict equilibrium adsorption properties from experimental dynamic surface tension (DST) data and the known rate-limiting adsorption kinetics mechanism, an approach that has never been pursued in the DST literature. Specifically, we develop a new methodology to predict the equilibrium surface tension versus surfactant bulk solution concentration (ESTC) behavior of nonionic surfactants from experimental DST data when the adsorption kinetics rate-limiting mechanism is diffusion controlled. The new methodology requires the following three inputs: (1) experimental DST data measured at a single surfactant bulk solution concentration, Cb, (2) the diffusion coefficient of the surfactant molecule, D, and (3) a single equilibrium surface tension data point, to predict the entire ESTC curve applicable over a wide range of surfactant bulk solution concentrations which are less than, or equal to, Cb. We demonstrate the applicability of the new methodology by predicting the ESTC curves of the two alkyl poly (ethylene oxide) nonionic surfactants C12E4 and C12E6, and validate the results by comparing the predictions with (a) equilibrium surface tension measurements, (b) surface-expansion measurements, and (c) pendant-bubble dynamic surface tension measurements for t<approximately 100-200 s (when the assumption of diffusive transport of surfactant molecules in the bulk solution is valid). Very good agreement is obtained between the predictions and the measurements in (a), (b), and (c) for both C12E4 and C12E6. On the basis of these results, we conclude that the new methodology presented here represents an efficient method to predict reliable ESTC curves for nonionic surfactants.

Possible existence of convective currents in surfactant bulk solution in experimental pendant-bubble dynamic surface tension measurements

Traditionally, surfactant bulk solutions in which dynamic surface tension (DST) measurements are conducted using the pendant-bubble apparatus are assumed to be quiescent. Consequently, the transport of surfactant molecules in the bulk solution is often modeled as being purely diffusive when analyzing the experimental pendant-bubble DST data. In this Article, we analyze the experimental pendant-bubble DST data of the alkyl poly (ethylene oxide) nonionic surfactants, C12E4 and C12E6, and demonstrate that both surfactants exhibit "superdiffusive" adsorption kinetics behavior with characteristics that challenge the traditional assumption of a quiescent surfactant bulk solution. In other words, the observed superdiffusive adsorption behavior points to the possible existence of convection currents in the surfactant bulk solution. The analysis presented here involves the following steps: (1) constructing an adsorption kinetics model that corresponds to the fastest rate at which surfactant molecules adsorb onto the actual pendant-bubble surface from a quiescent solution, (2) predicting the DST behaviors of C12E4 and C12E6 at several surfactant bulk solution concentrations using the model constructed in step 1, and (3) comparing the predicted DST profiles with the experimental DST profiles. This comparison reveals systematic deviations for both C12E4 and C12E6 with the following characteristics: (a) the experimental DST profiles exhibit adsorption kinetics behavior, which is faster than the predicted fastest rate of surfactant adsorption from a quiescent surfactant bulk solution at time scales greater than 100 s, and (b) the experimental DST profiles and the predicted DST behaviors approach the same equilibrium surface tension values. Characteristic (b) indicates that the cause of the observed systematic deviations may be associated with the adsorption kinetics mechanism adopted in the model used rather than with the equilibrium behavior. Characteristic (a) indicates that the actual surfactant bulk solution in which the DST measurement was conducted, most likely, cannot be considered to be quiescent at time scales greater than 100 s. Accordingly, the observed superdiffusive adsorption behavior is interpreted as resulting from convection currents present in a nonquiescent surfactant bulk solution. Convection currents accelerate the surfactant adsorption process by increasing the rate of surfactant transport in the bulk solution. The systematic nature of the deviations observed between the predicted DST profiles and the experimental DST behavior for C12E4 and C12E6 suggests that the nonquiescent nature of the surfactant bulk solution may be intrinsic to the experimental pendant-bubble DST measurement approach. To validate this possibility, we identified generic features in the experimental DST data when DST measurements are conducted in a nonquiescent surfactant bulk solution, and the DST measurements are analyzed assuming that the surfactant bulk solution is quiescent. An examination of the DST literature reveals that these identified generic features are quite general and are observed in the experimental DST data of several other surfactants (decanol, nonanol, C10E8, C14E8, C12E8, and C10E4) measured using the pendant-bubble apparatus.