Monolayers of Mixed Surfactants at the Oil-Water Interface, Hydrophobic Interactions, and Stability of Water-in-Oil Emulsions (original) (raw)

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

Interfacial Composition, Structural and Thermodynamic Parameters of Water/(Surfactant+ n Butanol)/ n Heptane Water-in-Oil Microemulsion Formation in Relation to the Surfactant Chain Length

Journal of Surfactants and Detergents, 2010

Interfacial behavior, structural and thermodynamic parameters of a water/(surfactant+n-butanol)/n-heptane water-in-oil (w/o) microemulsion have been investigated using the dilution technique at different temperatures, and [water]/[surfactant] mole ratios. The cationic surfactants used were alkyltrimethyl ammonium bromides (CnTAB, n = 10, 14 and 16) while the nonionic surfactants were polyoxyethylene (20) sorbitan monoalkanoates (polysorbate), viz., palmitate (PS 40), stearate (PS 60) and oleate (PS 80). The distribution of cosurfactant between the oil–water interface and the bulk oil at the threshold level of stability, and the thermodynamics of transfer of the cosurfactant from the bulk oil to the interface were evaluated. Structural parameters such as the dimensions, population density and effective water pool radius of the dispersed water droplets in the oil phase and the interfacial population of the surfactant and cosurfactant have been evaluated in terms of the surfactant chain length.

A Simple Molecular Model for the Spontaneous Curvature and the Bending Constants of Nonionic Surfactant Monolayers at the Oil/Water Interface †

Langmuir, 2000

A simple analytical model for the spontaneous curvature and the bending constants of nonionic surfactant monolayers at an oil/water interface is developed with the goal of allowing correlation and prediction of phase behavior. The surfactant molecules are treated as diblock copolymers grafted to the interface. The change in the free energy of the surfactant monolayer due to the bending of the interface is calculated as a sum of the contributions of the hydrophobic and the hydrophilic blocks. The equilibrium thickness of a spherical and cylindrical surfactant monolayer is found by minimizing the sum of the free energy of mixing and the stretching energy of the hydrophilic and the hydrophobic blocks. Comparison with the Helfrich model gives expressions for the spontaneous curvature H0, the bending elasticity κ, and the saddle splay modulus κ j, of the surfactant monolayer. An analytical expression is obtained for the phase inversion temperature, which correlates well with experimental data for a wide series of nonionic surfactants CiEj and oils (n-alkanes). The model gives the temperature dependence of the spontaneous curvature and the bending parameters of nonionic surfactant monolayers. Phase boundaries in C12E5/octane/water microemulsions are calculated without use of any adjustable parameters and are in a good agreement with experimental data.

Oil-in-water emulsion films stabilized by polymeric surfactants based on inulin with different degree of hydrophobic modification

Colloids and Surfaces A-physicochemical and Engineering Aspects, 2009

Oil-in-water emulsion films stabilized by hydrophobically modified inulin polymeric surfactants with different degree of substitution, DS, of alkyl chains on the inulin backbone, were studied. Four different graft co-polymers with increasing DS were used. Equivalent film thickness, h w , and disjoining pressure, , were measured at different NaCl concentrations and 2 × 10 −5 mol dm −3 concentration of polymers. At a constant capillary pressure of about 36 Pa h w decreases with increase of NaCl concentration till a critical concentration, C el,cr , of 5 × 10 −2 mol dm −3 is reached above which the thickness remains constant indicating a transition from electrostatic to steric stabilization. For the four polymeric surfactants this dependence was virtually the same except for the polymer with the highest DS where above C el,cr a jump in the thickness to a Newton Black Film (NBF) was observed. For a wide range of NaCl concentrations (up to 2 mol dm −3 ) the measured disjoining pressure isotherms,˘vs. h w , showed in all cases a jump to NBF at a critical film thickness and critical disjoining pressure, which again indicates a transition from electrostatic to steric stabilization. This transition occurred at lower film thickness and lower capillary pressure as the DS was increased. A reduction in disjoining pressure at the transition point occurs with increase of degree of substitution. The results were discussed in relation to loop-size induced changes in steric repulsion.

Comparison of oil-in-water emulsion films produced using ABA or ABn copolymers

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2009

Film thickness and disjoining pressure in emulsion films formed between two oil droplets in aqueous solutions of two ABA poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers of the Pluronic series, F108 and P104, were investigated at different NaCl concentrations. The microinterferometric thin film pressure balance experimental technique was used. The results obtained were compared with those previously reported for an AB n graft copolymer of hydrophobically modified inulin, namely INUTEC SP1. It was noted that there are marked differences in the behavior of the emulsion films stabilized with Pluronics ABA copolymers and those based on the AB n graft copolymer INUTEC SP1. This difference in behavior was used to throw some light on the difference in stability of practical emulsions when using these two types of polymeric surfactants. It is concluded that emulsions using INUTEC SP1 should be more stable than those using Pluronics ABA copolymers in particular at high electrolyte concentrations.

Molecular dynamics study of the effect of inorganic salts on the monolayer of four surfactants at the oil/water interface

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.

Interfacial rheology of Span 80 adsorbed layers at paraffin oil–water interface and correlation with the corresponding emulsion properties

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007

The correlation between the interfacial properties of the surfactant adsorbed layer at the water-oil interface and the emulsification and stability of the corresponding emulsion has been investigated. To this aim an oil soluble surfactant, sorbitane monooleate (Span 80), has been studied at the paraffin oil-water interface, together with the water in oil emulsions stabilized by the same surfactant at the same bulk concentrations, above the cmc. The equilibrium and dynamic interfacial tension and the interfacial rheological properties have been measured by coupling two different tensiometers: a capillary pressure tensiometer (CPT) and a drop shape tensiometer. By this way the dilational viscoelasticity has been determined in a frequency range from 0.01 to 20 Hz. To investigate the emulsion properties, besides the observation by optical microscope, a differential scanning calorimeter (DSC) has been used which allows for the evaluation of the drop size distribution. The results show a correlation between the surfactant concentration and the emulsion properties, which can be explained on the basis of the dilational properties. For very concentrated solutions however the stabilizing effect of inverse micelles must be accounted for. Moreover, dilational properties have been found to be also important to determine emulsification conditions.

Diblock copolymers adsorbed at a water-oil interface

European Physical Journal E, 2001

We probe the conformation of a diblock copolymer layer adsorbed at the surface of water-in-oil emulsion droplets at various concentration of a molecular surfactant. The diblock copolymer is made of a hydrophobic polybutadiene part linked to a hydrophilic polyethylene oxide one. The measure provides the equilibrium thickness of the polymer layer that is obtained with two different techniques, i.e. dynamic light scattering and force measurements. The structure of the layer is shown to change from a "mushroom" conformation in which the adsorbed chains form independent Gaussian coils to a conformation where they interact and extend in the continuous phase. The transition from one regime to the other is progressive as the ratio surfactant/polymer bulk concentration varies.

Rheology and stability of oil-in-water nanoemulsions stabilised by anionic surfactant and gelatin 1) addition of nonionic, cationic and ethoxylated-cationic co-surfactants

Advances in Colloid and Interface Science, 2008

The oil-in-water emulsions used in silver-halide photographic coatings are stabilised with anionic surfactants and made in the presence of excess gelatin, which acts as an electrosteric stabilising agent and continuous phase viscosifier. The oil droplet sizes are close to 100 nm but the adsorbed gelatin increases the effective volume of the droplets significantly. These nanoemulsions are manufactured and coated at temperatures in excess of 40°C, where gelatin adopts a random coil structure. At oil concentrations above 15% by volume, the emulsions are viscoelastic liquids with a high low-shear viscosity and strong shear-thinning. The viscosity and shear-thinning can be decreased by reducing the adsorption of gelatin, which can be achieved by addition of nonionic surfactants. This is a rheological study of the effects of adding novel, nonionic sugar-based surfactants on the rheology of photographic nanoemulsions, with additional measurements of static and dynamic surface tension. These surfactants have two sugar (gluconamide) heads and either one or two alkyl tails. Homologous series of each type of sugar surfactant were investigated over a wide range of alkyl tail length. The optimum surfactant choice for commercial applications depends not only on rheological effects but also on ease of synthesis, purification and dissolution, and of course, cost. The dynamic surface tension of the emulsion containing the anionic-nonionic surfactant mixture must also be compatible with the multilayer coating process.

Interfacial properties and stability of oil-in-water emulsions stabilized with binary mixtures of surfactants

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.