A Novel Algorithm for the Estimation of the Surfactant Surface Excess at Emulsion Interfaces (original) (raw)
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Molecular Dynamics Study of Nanoparticles and Non-Ionic Surfactant at an OilWater Interface
2020
Nanoparticles (NPs) and surfactants can spontaneously concentrate at the interface between two immiscible liquids, such as oil and water. Systems of high oil-water interfacial area, such as emulsions, are the basis of many industries and consumer products. Although NPs and surfactants are currently incorporated into many of these applications, their mutual interfacial behavior is not completely understood. Here we present molecular dynamics simulations of NPs and non-ionic surfactant in the vicinity of an oil-water interface. It was found that in low concentration the surfactants and NPs show cooperative behavior in lowering the oil-water interfacial tension, while at higher surfactant concentration this synergy is attenuated. It was also found that binding of surfactants to the NP surface decreases the surfactant efficiency in lowering the interfacial tension, while concurrently creating a barrier to NP aggregation.
A quantification of immersion of the adsorbed ionic surfactants at liquid|fluid interfaces
The electrical-double-layer (EDL)-based adsorption models for ionic surfactants propose all surfactant heads ideally aligned in the Stern layer, creating a single surface potential which controls the distribution of counterions within the diffuse layer. Despite their successful application to many surfactant systems, these models exhibit a noticeable shortcoming when it comes to explaining the larger surface excess of ionic surfactants at air|water interface than oil|water interface. Experiments and computation simulations have already shown that some surfactant head groups tend to immerse into the deeper layers of interfacial water at high surface coverage. Such an immersion alters the surface potential distribution and characteristics of EDL. However, a theoretical study of this phenomenon is not available yet. This paper presents a useful modeling approach to quantification of the surfactant immersion. We combined the ionic surfactant adsorption models with the theory of equilibrium adsorption constant. We modified several non-ionic surfactant adsorption models for describing the ionic surfactant adsorption by accounting for their immersion. The adsorption parameters were determined by fitting to experimental adsorption data and were then used along with the theory of equilibrium adsorption constants to successfully explain the reported difference in the surface excess of sodium dodecyl sulphate (SDS) at air|water and oil|water interfaces. The surfactant immersion directly affected the intermolecular interaction parameter which was closely related with the adsorption energy of surfactants at the interfaces. The collaborative effect of these two parameters was also found to be responsible for the immersion of surfactants and shaping A.A. Shahir et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 279–292 their different concentration profiles across various liquid|fluid interfaces. The immersion of surfactants was found to enhance their surface adsorption effectively. Also, our model successfully elucidated why SDS adsorption at the oil|water interface unexpectedly decreases as the hydrophobicity of the oil phase increases.
Effect of ionic surfactants on drainage and equilibrium thickness of emulsion films
Journal of Colloid and Interface Science, 2008
This paper presents new theoretical and experimental results that quantify the role of surfactant adsorption and the related interfacial tension changes and interfacial forces in the emulsion film drainage and equilibrium. The experimental results were obtained with plane-parallel microscopic films from aqueous sodium dodecyl sulphate solutions formed between two toluene droplets using an improved micro-interferometric technique. The comparison between the theory and the experimental data show that the emulsion film drainage and equilibrium are controlled by the DLVO interfacial forces. The effect of interfacial viscosity and interfacial tension gradient (the Marangoni number) on the film drainage is also significant.
2021
Fundamental insights into the interplay and self-assembly of nanoparticles and surface-active agents at the liquid-liquid interface play a pivotal role in understanding the ubiquitous colloidal systems present in our natural surroundings, including foods and aquatic life, and in the industry for emulsion stabilization, drug delivery, or enhanced oil recovery. Moreover, well-controlled model systems for mixed interfacial adsorption of nanoparticles and surfactants allow unprecedented insights into nonideal or contaminated particle-stabilized emulsions. Here, we investigate such a model system composed of hydrophilic, negatively, and positively charged silica nanoparticles and the oil-soluble cationic lipid octadecyl amine with in situ synchrotron-based X-ray reflectometry, which is analyzed and discussed jointly with dynamic interfacial tensiometry. Our results indicate that negatively charged silica nanoparticles only adsorb if the oil-water interface is covered with the positively ...
Journal of Colloid and Interface Science, 1991
The effect of sodium dodecyl sulfate (SDS) on ~-potential of oil-in-water emulsions stabilized with a nonionic surfactant (Tween 80) at two different concentrations, one above and one below its CMC, has been studied. Adsorption of SDS causes an increase in f-potential. An estimate of the free energy of adsorption can be made from the f-potential versus log concentration SDS plot. The demulsification rates of the emulsions are evaluated by counting the droplet number with time hemocytometrically. Flocculation is considered the main factor of instability. Electrostatic forces, as well as steric stabilization factors, are considered to be responsible for the stability in some cases, depending on Tween 80 concentration and the time of SDS addition. The adsorption behavior of SDS in the presence of Tween 80 is also discussed and the amount of SDS adsorbed per unit area of the interface is calculated with the help of the Gibbs equation.
Journal of Petroleum Science and Engineering, 2020
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1993
A comparison of experimental data with self-consistent field lattice calculations and molecular dynamics simulations has shown that the latter two approaches are able to predict in a qualitative sense the relation between the structure of a surfactant and its interfacial tension at an oil/water interface. Micelles can also be observed in the simulations and in the self-consistent field calculations. Advantages and disadvantages of the simulations and the self-consistent field calculations are discussed and it is concluded that current theoretical models provide reasonable descriptions of complex colloidal systems.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2005
In this work we describe a simple thermodynamic method for determination of the adsorption (amount per unit area) of ionic surfactant. The latter is obtained from the interfacial tension isotherm measured in the presence of arbitrary (and fixed) concentration of inorganic electrolyte. Polynomial fit of the interfacial tension versus the logarithm of the mean ionic activity is combined with the Gibbs adsorption equation written in a form suitable for arbitrary content of salt. This procedure is an extension of the approach of Rehfeld [J. Phys. Chem. 71 (1967) 738]. We have performed measurements with sodium dodecyl sulfate (SDS) on water/air and water/oil (n-hexadecane and Soybean Oil) boundaries, at different salt concentrations (10 and 150 mM NaCl). Wilhelmy plate method was used for the water/air measurements; for water/oil we applied drop shape analysis with pendant drops. The obtained isotherms, together with literature data, are processed and the adsorption is determined. The results are compared and discussed in view of the role of the salt and the type of the hydrophobic phase. On oil/water boundaries the adsorption is always lower than that on air/water surface; addition of inert electrolyte increases the adsorption. We analyze theoretically the asymptotic behavior of the adsorption as a function of the solute concentrations in the limit of high surface coverage. The treatment is based on models existing in the literature (Langmuir isotherm with account for the counterion binding, as formulated by Kalinin and Radke [Colloids Surf. A 114 (1996) 337]; the activity coefficients were taken into consideration in the frames of the Debye-Hückel theory). The obtained asymptotic functional dependence of the adsorption is used for fitting of data. The agreement is always good, in the concentration region below and near the critical micellization concentration (CMC). From the fits we determine the limiting adsorption at maximum coverage (i.e., at saturation); therefrom, the degree of coverage of the interface with surfactant is estimated. It turns out that at the CMC the coverage is lower than about 90%. Thus, we confirm literature results for absence of saturation with ionic surfactants at the CMC. The dependence of the surface coverage upon the mean ionic activity is rather insensitive toward the type of the fluid interface (air/water, oil/water with different hydrocarbons), and the salt concentration.
Journal of Dispersion Science and Technology, 2002
Surfactant-oil-water systems with a phase behavior insensitive to temperature and composition can be achieved by anionic-nonionic mixing. By using of a linear mixing rule and a linear temperature dependency, it is possible to interpret most of the features exhibited by the experimental phase behavior data obtained with sulfonate and ethoxylated alkylphenol mixtures. Deviation from the theoretical model are probably due to anionic and nonionic groups association which reduces the overall hydrophilic character.