Liquid–liquid interfacial properties of mixed nanoparticle–surfactant systems

Abstract

In this study the effect of particles of nanometric dimensions on the interfacial properties of liquid–liquid systems is addressed.

The nanoparticle transfer and the consequent attachment to the fluid interface are mainly governed by their hydrophobic/lipophilic character. For this reason a model particle–surfactant system has been investigated, namely a nanometric colloidal silica dispersion plus CTAB, where the role of the cationic surfactant is varying the particle surface properties adsorbing on them.

The interfacial properties of the dispersion against hexane were determined as a function of the surfactant concentration, measuring the interfacial tension and the dilational viscoelasticity versus frequency. These results were then crossed with a study on the behaviour of micrometric oil droplets inside the same dispersions, as well as on the morphology of the respective nanoparticle stabilised emulsions.

By this multiple investigation it was possible to find out that diffusion transport of particles from the bulk to the interface plays an important role, as well as the reorganisation of the mixed particle-surfactant layer. Moreover from these results the irreversible aspect of the nanoparticle attachment to the fluid interface was also evidenced which, under particular conditions, can provide the formation of a solid-like layer.

Introduction

Solid particles of micro/nanometric dimension are used as stabilizers in emulsion and foam technology, often in association with surfactants. For this reason the effect of the surfactant/particle structure on fluid interface properties is an important scientific and technological topic, still presenting many open questions.

Depending on their wettability, nanoparticles tend to transfer from the bulk phase to the interface. Such surface accumulation provides in general a change of mechanical properties of the interfacial layers. For this reason nanoparticles can play a role in preventing coalescence of drops and bubbles and have a stabilizing effect for emulsions and foams.

A number of experimental studies can be found in literature concerning particle stabilised emulsions and foams [1], [2], [3]. They mainly refer to cases where the amount of nanoparticles is large enough to saturate the fluid interface and consequently bubble or drop coalescence is prevented essentially by steric effect. Relatively few studies are instead available about composite interfacial layers where nanoparticles interact each other and with the fluid interface under unsaturated conditions.

Aim of this work is advancing in understanding particle/fluid interface interactions and their effect on the macroscopic interfacial properties, with particular attention to the low particle surface coverage. These properties are in fact expected to be related to the characteristics of the disperse systems, like emulsions or foams, obtained with the same composition.

The affinity of nanoparticles with the water/oil interfaces is determined by the hydrophilic or lipophylic character of their surface. This is evident considering the expression of the free-energy change, ΔE_P, associated to the transfer of one spherical particle of radius R from the aqueous phase to a planar water/oil interface. Such energy variation reads [4]:

$$\Delta E_{\text{P}}=-\pi R^2\gamma {(1-\text{cos}\vartheta )}^2$$

where ϑ is the solid–water contact angle, and γ is the water/oil interfacial tension.

In the case of partial wetting of particles, at nanometric scale, the interfacial layers is a multiphase zone where three kinds of interfaces are present: a liquid–liquid, and two solid–liquid. Due to this non-homogeneity, the formal definition of the interfacial tension is a critical item. In spite of rigorous thermodynamic treatments are still unavailable, the macroscopic mechanical properties of the composite interfacial layer can be operationally described by an effective interfacial tension, defined as the quantity entering in the mechanical equilibrium conditions of a macroscopic interface. Owing to its definition this effective quantity can be unambiguously measured by tensiometric methods exploiting the Laplace equation, like bubble/drop shape and capillary pressure methods.

In the following, for sake of brevity, the terms “interfacial tension” is utilised, standing for the above-defined effective quantity.

The here studied system is a mixed silica nanoparticle–cationic surfactant dispersion where the solid surface of the particles is modified by the surfactant adsorption. In fact, while silica nanoparticles alone are completely hydrophilic, the adsorption of surfactant as individual ions confers to the surface a partially hydrophobic character, that increases with the surfactant concentration. Using the present system it is then possible to investigate the effect of nanoparticles as a function of their wettability, by changing the surfactant concentration.

In some recent works [5], [6] this kind of surfactant–nanoparticle mixture have been studied as stabilizer for oil in water emulsions. There, surfactants are also utilised to tune the particle wettability in order to investigate its influence on the emulsion rheology and stability.

In a previous paper [7], results obtained for the same system are presented, concerning the influence of surfactant and particle concentration on the equilibrium and dynamic interfacial tension and on the low frequency surface rheological properties. In that study a decrease of the equilibrium interfacial tension was observed due to the increasing of the nanoparticle concentration in the interfacial layer, both for dispersion/air and dispersion/oil interfaces. Moreover, dynamic interfacial tension was found to vary according to a multiple kinetic process. From that study we concluded that the transfer of nanoparticle into the interfacial layer modifies the equilibrium interfacial tension and different kinetic processes are involved in the equilibration of such composite system. In fact, beside the diffusion of particles and the attachment/accumulation to the liquid/fluid interface, other re-arrangement processes of the layer can occur, like a redistribution of surfactant between particle and liquid interface or a particle aggregation process.

Such conclusions were also corroborated by the investigation of low frequency surface dilational rheology, where the dependence on the surfactant concentration indicates the presence of an interfacial relaxation process.

In the present work the same composite system has been further investigated by measuring the dilational viscoelasticity at higher frequency in order to advance in the comprehension of the dynamic properties of the dispersion/hexane interfacial layer.

Dilational surface rheology is important for this kind of systems mainly because it is expected to play a role in the stability of films, foams and emulsions, being responsible of the capability of interfaces to dampen external disturbance. This correlation has been already shown for surfactant stabilised films [8] and emulsions [9]. The other important aspect of dilational rheology is that from viscoelasticity versus frequency data, provided a suitable theoretical model, it is possible to obtain information about the kinetics of the relaxation processes in the interfacial layer [10], [11], [12].

Although for this kind of composite systems a detailed model does not exist so far, the dilational rheology investigation can give information about the existence of kinetic mechanisms in the interfacial layer allowing its characteristic frequency (or time) to be evaluated.

In order to find out correlations between the properties of a single interface and the characteristics of the corresponding disperse systems, other kinds of experimental studies have been performed and presented in the following. Namely, the behaviour of micrometric hexane droplets in the dispersion has been observed by optical microscopy as well as the characteristics of the oil in water emulsions obtained with the same composition.