A Simple Molecular Model for the Spontaneous Curvature and the Bending Constants of Nonionic Surfactant Monolayers at the Oil/Water Interface † (original) (raw)
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Surface Tension and Viscoelasticity of Alkane Solubilized Surfactant Monolayers
Journal of Colloid and Interface Science, 1997
drodynamic coupling between the vertical motion of the sur-The behavior of solubilized alkanes at the surface of DoTAB face and the surface area changes is preserved. solutions for various surfactant bulk concentrations below and In the case of alkanes, the preparation of thin oil phases above the cmc is described. Several techniques such as tensiometry, is difficult, because in general, alkanes do not wet the surfacellipsometry, and Brewster angle microscopy, as well as an excited tant solutions completely. Alkane drops are in equilibrium capillary wave device are used for the study. Alkanes do not wet with a mixed oil-surfactant monolayer. Nevertheless, these the surface completely and form a mixed monolayer with the surmixed monolayers constitute a good model for a monolayer factant. Rather unusual viscoelastic behavior is observed for the at the oil-water interface. In the case of AOT surfactant, mixed monolayers. This suggests a new type of relaxation phenomthe alkane mixed monolayer properties are connected with enon, but its origin remains to be elucidated. ᭧ 1997 Academic Press the surfactant film bending elasticity and the phase behavior of the mixed system. Furthermore, a direct link was observed between the solubilization of alkanes into the monolayer and INTRODUCTION the wetting properties of the surfactant solutions with these Amphiphilic molecules form a rich variety of complex oils (4, 5). The mixed films obviously also have their own phases in oil-water mixtures. The interfaces between oil and practical interest. water microdomains are coated by surfactant monolayers. Very little is known about the viscoelastic behavior of Monolayer physical properties, microstructure, and interacmonolayers in the presence of solubilized oils. In this article tions are believed to govern phase behavior in these systems we present results on the solubilization of hydrocarbon oils (1). Similar monolayers are also present in metastable sysin monolayers of DoTAB (dodecyl trimethyl ammonium tems such as emulsions. In this case, monolayer properties bromide) cationic surfactant and on the surface viscoelasticare thought to control, to a large extent, emulsification and ity of the mixed layers. emulsion stability (2). In these processes, the monolayers are subjected to rapid area changes, and the relevant property EXPERIMENTAL appears to be the monolayer dilatational elasticity. Surface dilatational viscoelasticity can be investigated by Methods analyzing surface wave propagation. At oil-water interfaces, the propagation of longitudinal waves is sensitive to Surface tension is measured by the Wilhelmy plate method. Here we have used an open rectangular frame of dilatational parameters (3). Longitudinal waves can be excited mechanically. The frequency range available in this a platinum wire instead of a plate. This eliminates the problem of surfactant adsorption onto the plate and is well type of experiments is limited to 0.1 to 1 Hz. Capillary waves do not induce significant area changes at these interfaces. suited for surfactant solutions at air / water interfaces. The absolute accuracy on surface tension measurements is {0.2 However, at air-water interfaces, capillary wave properties are better coupled to area changes. In order to enlarge the dynes / cm. The relative accuracy is of the order of {0.1 dynes / cm. frequency range of the measurements, we have used a capillary wave device. By keeping the thickness of the oil phase Surface viscoelasticity e is deduced through surface wave properties. Although e affects the longitudinal surface wave smaller than the wavelength of the capillary waves, the hyproperties directly, at air/water interfaces there is maximum coupling between longitudinal waves and transverse
Phase behavior and interfacial curvature in water-oil-surfactant systems
Current Opinion in Colloid & Interface Science, 1996
Even during 1995, 60 papers appeared using cosurfacr ant in their formulations. If one does not have the choice of optimal surfacrants (there may be various commercial or scientific reasons), it may be helpful to use these add itives (e.g. in emulsion formulations, see below). Quantitative me asurements of structures and their interpretation is facilitated, however, if one restricts oneself to the three components really needed-a polar, a non-polar and an amphiphilic substance-and uses temperature to tune the phase behavior. Knowledge obtained by this procedure can then be used with success to evaluate other systems. reader is likely to survive only if he has some strategy as to how to proceed. All I can do is offer my personal current op inion. Microemulsions. Microcrnulsions are thermodynamically stable, macroscopically homogeneous mixtures of water, oil and surfactant. Microscopically, a surfactant film separates the two incompatible solvents. In this review, we recall the experimentally observed interfacial curvature of the film, as it expresses itself in a systematic metamorphosis of the microstructures and ph ase behavior, when a suitable tun ing parameter is varied [Zoo]. As such, in the past, temperature, pre ssure, fourth and fifth components have been applied. Ever since the pioneering work of Winsor [3], it has been known th at the general phase beha vior proceeds as shown schematically by the test tubes on the left-hand side of Figure 1. We discuss here, as model systems, non-ionic surfactants of the n-alkyl polyglycolether type. The right hand side of Figure 1 relates the interfacial tensions to the phase diagrams.
Simulating the effect of surfactant structure on bending moduli of monolayers
The Journal of Chemical Physics, 2004
We have used dissipative particle dynamics to simulate amphiphilic monolayers on the interface between oil and water. An ultralow interfacial tension is imposed by means of Monte Carlo to resemble the amphiphilic films that separate oil and water regions in microemulsions. We calculate the bending modulus by analyzing the undulation spectrum. By varying the surfactant chain length and topology we investigate the effect of surfactant structure and composition of the monolayer on the bending moduli. We find that increasing the thickness has a larger effect than increasing the density of the layer. This follows from the observations that at a given interfacial tension, the bending modulus increases with chain length and is larger for linear than branched surfactants. The increase with chain length is approximately linear, which is slower than the theoretical predictions at a fixed area. We also investigated a binary mixture of short and long surfactants compared to pure layers of the same average chain length. We find a roughly linear decrease in bending modulus with mole fraction of short surfactants. Furthermore, the mixed film has a lower bending modulus than the corresponding pure film for all mole fractions. Linking the bending moduli to the structure of the surfactants is an important step in predicting the stability of microemulsions.
Langmuir, 1995
The present study involves investigation of monolayer properties of sorbitan monooleate and diethanolamine derivatives of polyisobutylene succinic anhydride of three different molecular weights, viz. 500,700, and 1050, as single as well as binary mixtures at the oil-water interface. The average backbone carbon numbers of the polyisobutylene chains are 14,22, and 34, respectively. The monolayer studies at the oil-water interface were carried out with a specially designed Langmuir trough. The investigations were carried out at two different oil phases, viz. heptane and paraffin oil. The packing efficiency of these surfactants (single or in combination) are found to be greatly influenced by the oil chain length. Within the limits of experimental accuracy, the mixed surfactant systems comprising sorbitan monooleate and polyisobutylene surfactants of three different hydrocarbon chain lengths follow the rule of hydrocarbon chain length compatibility. The observed differences in packing features revealed the significance of oil penetration and cohesive interactions in the aliphatic layer of the surfactant film. The efficacy of these surfactants were further examined in water-in-oil emulsions and the results are consistent with the findings of monolayer properties. At the condition of chain length compatibility, the mixed surfactant system produced emulsions with maximum stability.
Physical Review E, 2006
The free energy of nonionic balanced microemulsions based on nonionic surfactants are analyzed using experimental data from ͑i͒ phase behavior, ͑ii͒ osmotic compressibility of the balanced microemulsion structure, which is obtained from small angle neutron scattering ͑SANS͒ experiments, and ͑iii͒ data on interfacial tensions obtained by T. Sottmann and R. Strey ͓J. Chem. Phys. 106 8606 ͑1997͔͒. The balanced microemulsion, where the spontaneous curvature vanishes at equal volumes of water and oil, has a finite swelling with the solvent with a minimum surfactant volume fraction, ⌽ S *. At higher surfactant concentrations the balanced microemulsion phase having the surfactant volume fraction ⌽ S1 coexists with a lamellar phase of volume fraction ⌽ S2. Under the constraint of ⌽ W = ⌽ O , where ⌽ W and ⌽ O are the water and oil volume fractions, respectively, the free energy density can be written as an expansion in the surfactant concentration. While the phase equilibria only depend on relative values of the expansion coefficients, absolute values can be obtained from compressibility and interfacial tension data. The osmotic compressibility of the surfactant film was measured by SANS through contrast matching water and oil. The phase behavior of nonionic surfactant-wateroil systems depends strongly on the chain length of the oil, when comparing a homologous series from octane to hexadecane using the same surfactant, here being pentaethylene oxide dodecyl ether ͑C 12 E 5 ͒. The three concentrations ⌽ S * , ⌽ S1 , and ⌽ S2 increase markedly as the chain length of the oil is increased. However, from the analysis of the surface tension data it is concluded that there are no major changes in the bending rigidities as the oil is changed. The data are analyzed within the model free energy densities G / V = a 3 ⌽ S 3 + a 5 ⌽ S 5 and G / V = a⌽ S 3 ͑1+b ln ⌽ S ͒. We find that within experimental accuracy, the first of these models provides a quantitatively consistent description of the data. For the second model there is a larger discrepancy between observed and calculated values.
The Journal of Chemical Physics, 2008
Mass transport across surfactant-covered oil-water interfaces of microemulsions plays an important role in numerous applications. In the current work, we use coarse-grained molecular dynamics simulations to investigate model systems containing flat hexadecane-water interfaces covered by monolayers of nonionic surfactants of various lengths. Several properties of the surfactant monolayers relevant to the mass transport are considered, including the monolayer microstructure, dynamics, and a free energy barrier to the solute transport. It is observed that the dominant contribution of a surfactant monolayer to the free energy barrier is a steric repulsion caused by a local density increase inside the monolayer. The local densities, and hence the free energy barriers, are larger for monolayers composed of longer surfactants. Since it is likely that the solute transport mechanism involves a sequence of jumps between short-lived pores within a monolayer, we perform a detailed analysis of s...
Journal of Dispersion Science and Technology, 2018
The molecular dynamics (MD) simulation method was used to simulate the aggregation of four types of surfactants at the oil/water interface. The effect of different cations such as Na þ , Mg 2þ and Ca 2þ was compared. The results show that the interaction between different types and concentrations of cationic and head group of surfactants causes the surfactant adsorption layer to bend and changes the aggregation morphology of the micelles. The radial distribution function (RDF) of the surfactant head groups with water molecules and cations is calculated. The calculated results show that it exists a chemical hydration layer and a physical hydration layer between the surfactant head groups and the water molecules. Cation has different degrees of impact, thus changes the original hydration structure. Nonionic and zwitterion surfactants have good salt resistance. Potential of Mean Force (PMF) was used to analyze the energy change when the cation interacted with the polar head. The nonionic and zwitterion surfactants are determined to have a good oil displacement effect even under the conditions of high calcium-magnesium mineralization. The present results could help in choosing surfactants used in oil/water with inorganic salts.
Excess of Solubilization and Curvature in Nonionic Microemulsions
Journal of Colloid and Interface Science, 1999
We measure separately the amount of solute dissolved in a surfactant monolayer and the average curvature of the relevant sample to establish a link between these two quantities. The model system chosen involves the common hydrophobic pesticide lindane (␥-C 6 H 6 Cl 6 ) in a nonionic surfactant solution of the ethylene oxide type. Excess solubilization, defined as the solubilization in the surfactant film by comparison with bulk oil, is quantified by the interfacial composition (molar ratio solute/surfactant) within the interfacial film. A linear relationship between the amount of solute adsorbed on the film and the induced variation in curvature of the surfactant film is deduced from the phase diagram, dosage, and small-angle scattering experiments in the case of micellar, Winsor I, and several Winsor III domains at equilibrium in the same ternary system. We discuss the linear relationship obtained with constraints set by molecular packing. FIG. 5. Variation of the reduced average curvature ⌬͗H͘l versus the interfacial composition . The dotted line corresponds to the relationship found with the wedge model. Experimental points (s) were obtained from phase diagrams and dosage from the variation of the Winsor III domain with the amount of lindane solubilized in the oil. (E) Corresponds to the Winsor I sample. (OE) Obtained in the micellar domain (2).
Numerical Analysis of Nonionic Surfactant Monolayers at Water/Air Interfaces
The Journal of Physical Chemistry B, 2004
The time evolution of a monododecyl pentaethylene glycol monolayer at the water-air interface is investigated using velocity rescaled NVT molecular dynamics. The model we consider consists of 40 surfactant molecules and 2350 water molecules enclosed in a periodic box. The time evolution of this system is ruled by the CFF91 force field that includes intra-and intermolecular degrees of freedom for both surfactant and water molecules. The interface we consider herein is hence described in all atomic detail. We discuss the initial condition problem and study the relaxation properties to stationarity. Transient regimes of self-assembly of the surfactant chains in entangled structures in both air and water are described. From the stationary configuration, we define the interface location, determine the mass distribution across the interface, and discuss the validity of the tilt angles notion when structural roughness is considered. Using pair distribution functions, we show that, besides typical tilt angles, monododecyl pentaethylene glycol molecules might also develop domains on the interface that suggest an in-plane orientational order.