Ana Forgiarini

Studies of the relation between phase behavior and emulsification methods with nanoemulsion formation

The main aim of this work was to study the relationship between the type of phases present during the emulsification process, the order of addition of components and the droplet size of the resulting emulsions. In this study, a pseudoternary water/poly(oxyethylene) nonionic surfactant/decane system was chosen as a model system to form oil-in-water emulsions at 25 °C. The phase behavior of the model system was determined at constant temperature in order to know the equilibrium phases and also those involved in the emulsification process. The low-energy emulsification methods studied were A. Addition of oil to an aqueous surfactant dispersion. B. Addition of water to a surfactant solution in oil. C. Mixing preequilibrated samples of the components.

A new method to estimate the stability of short-life foams☆

Abstract In classical foam stability studies, foam height variation is monitored versus time. The decay pattern depends, however, upon the foam structure at the start of the decay; in many instances this structure changes significantly during the first few minutes, and it is difficult to select a proper “zero time” of decay. We have found that the decay behavior is very well defined when the original state of the foam is taken as the equilibrium state of the classical Bikermans experiment, i.e. when the foam formation by bubbling (at the bottom of the column) exactly compensates the foam collapse (at the top). It is found that under such starting conditions, short life foam decay exhibits a linear variation in the foam column height with the logarithm of the elapsed time. A dimensionless H vs. log t plot exhibits the same features for different systems; thus, both a characteristic height and a characteristic time can be extracted from the experimental data, the latter being readily related to the foam stability. These parameters are used to quantify the effect of additives on the decay of several foam systems containing nonionic and anionic surfactants.

Formation and stability of nano-emulsions in mixed nonionic surfactant systems

The formation of nano-emulsions has been studied in water/ mixed nonionic surfactant/oil systems using two emulsification methods. In one method, the composition was kept constant and the temperature was changed (phase-inversion temperature, PIT, method), while in the other method, water was added dropwise to a solution of the mixed surfactants in oil at constant temperature (method B). The droplet size and stability were determined as a function of surfactant mixing ratio, W1, at 25 °C. The droplet size of nano-emulsions obtained by the PIT method is practically independent of W1 and falls in the range 60-80 nm. In contrast, the droplet size of nano-emulsions prepared by method B, is highly dependent on W1 and varies between 60 and 300 nm. At W1 values where the PIT or the hydrophile—lipophile balance temperature (Thlb) of the system is close to 25 °C, the droplet sizes of the nano-emulsions are similar for both emulsification methods. There are three equilibrium phases of the latter compositions: an aqueous micellar solution or oil-in-water microemulsion (W m), a lamellar liquid-crystalline phase and an oil phase (O) in addition, these nano-emulsions showed higher kinetic stability than those with lower W 1 values (higher T hlb) and consisting of two liquid phases (W m + O).

The relation between phase behavior and formation of narrow size distribution W/O emulsions

ABSTRACT Water-in-oil (W/O) emulsions of the water/C12E5/isooctanol/isooctane system have been prepared at 25° C. Phase behavior studies of the system with constant (2.5 and 6 wt.%) isooctanol concentration showed that the surfactant becomes more lipophilic with the increase in the alkanol concentration. Emulsification was carried out using four low-energy emulsification methods using the slow addition of one or various components to the rest of them, with gentle agitation. Emulsions with low-polydis-persity were obtained when the emulsification process started with a single lamellar liquid crystalline phase. If in addition to a lamellar liquid crystalline phase, other phases, such as excess water phase, were initially present, emulsions with intermediate polydispersity were produced. When a lamellar liquid crystalline phase was not involved and the spontaneous natural curvature of the surfactant was not changed during emulsification, highly polydisperse emulsions were obtained.

Effect of Emulsifier Type on the Characterization of O/W Emulsions Using a Spectroscopy Technique

The droplet size distribution (DSD) of emulsions is the result of two competitive effects that take place during emulsification process, i.e., drop breakup and drop coalescence, and it is influenced by the formulation and composition variables, i.e., nature and amount of emulsifier, mixing characteristics, and emulsion preparation, all of which affect the emulsion stability. The aim of this study is to characterize oil-in-water (O/W) emulsions (droplet size and stability) in terms of surfactant concentration and surfactant composition (sodium dodecyl benzene sulphonate (SDBS)/Tween 80 mixture). Ultraviolet-visible (UV-vis) transmission spectroscopy has been applied to obtain droplet size and stability of the emulsions and the verification of emulsion stability with the relative cleared volume technique (time required for a certain amount of emulsion to separate as a cleared phase). It is demonstrated that the DSD of the emulsions is a function of the oil concentration and the surfactant composition with higher stability for emulsions prepared with higher SDBS ratio and lower relative cleared volume with the time. Results also show that smaller oil droplets are generated with increasing Tween 80 ratio and emulsifier concentration. GRAPHICAL ABSTRACT

Interfacial rheology of low interfacial tension systems using a new oscillating spinning drop method

When surfactants adsorb at liquid interfaces, they not only decrease the surface tension, they confer rheological properties to the interfaces. There are two types of rheological parameters associated to interfacial layers: compression and shear. The elastic response is described by a storage modulus and the dissipation by a loss modulus or equivalently a surface viscosity. Various types of instruments are available for the measurements of these coefficients, the most common being oscillating pendent drops instruments and rheometers equipped with bicones. These instruments are applicable to systems with large enough interfacial tensions, typically above a few mN/m. We use a new type of instrument based on spinning drop oscillations, allowing to extend the interfacial rheology studies to low and ultralow interfacial tension systems. We present examples of measurements with systems of high and low tension, discuss the possible artifacts and demonstrate the capability of this new technique. We emphasize that the data shown for low interfacial tensions are the first reported in the literature. The instrument is potentially interesting for instance in enhanced oil recovery or demulsification studies.

Rheological Changes of Parenteral Emulsions During Phase‐Inversion Emulsification

An efficient emulsification procedure for parenteral soybean oil‐in‐water, based on current know‐how on transitional inversion, was investigated. A fine droplet size lipid emulsion was produced using much lower mechanical energy than the typical industrial process. The aqueous phase was added gradually during mixing and various rates of water addition, as well as surfactant concentration, were evaluated. It was found that as addition rate and surfactant content increased, flow behavior changed significantly at intermediate water content, becoming highly viscoelastic. This behavior was related to the formation of a liquid crystalline phase that, at later mixing stages, turned into small droplets.

On the instability of emulsions made with surfactant-oil-water systems at optimum formulation with ultralow interfacial tensions

We have studied emulsions made with two- and three-phase oil-water-surfactant systems in which one of the phases is a microemulsion, the other phases being water or/and oil excess phases. Such systems have been extensively studied in the 1970-1980s for applications in enhanced oil recovery. It was found at that time that the emulsions became very unstable in the three-phase systems, but so far few explanations have been proposed. In the most complete one, Kabalnov and colleagues related the emulsion stability to the probability of hole nucleation in the liquid film separating two nearby emulsion drops and associated this probability to the curvature elastic energy of the surfactant layer covering drop surfaces. We propose a different explanation, linked to another type of interfacial elastic energy, associated with compression of the surfactant layers. As found long ago, the three-phase systems are found near optimum formulation (hydrophile lipophile difference, HLD = 0), where the interfacial tension exhibits a deep minimum. The determination of interfacial elastic properties in low interfacial tension systems is not straightforward. In our present work, we used a spinning drop tensiometer with an oscillating rotation velocity. We show that the interfacial compression elastic modulus and viscosity also exhibit a minimum at optimum formulation. We propose that this minimum is related to the acceleration of the surfactant exchanges between the interface, oil and water, near the optimum formulation. Furthermore, we find that the surfactant partitions close to equally between oil and water at the optimum, as in earlier studies. The interfacial tension gradients that slow the thinning of liquid films between drops are reduced by surfactant exchanges between drops and the interface, which are fast whatever the type of drop, oil or water; film thinning is therefore very rapid, and emulsions are almost as unstable as in the absence of surfactant.