Influence of a mixed ionic/nonionic surfactant system and the emulsification process on the properties of paraffin emulsions

Abstract

Paraffin emulsions are important in technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels. Small particle size (about 1.0 μm) and low polydispersity are required to form stable paraffin emulsions for these applications. In this context, the main objective of the present work is to study the influence of the surfactant system and the emulsification process on the properties of paraffin emulsions. A high pressure homogenizer was used to prepare the emulsions and its characterization was made by means of optical microscopy, laser diffraction and electrophoretic mobility measurements. Emulsions were prepared as a function of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure. A simple theoretical model to predict the minimum particle size was used, assuming that surfactant is either at the oil–water interface or as monomer in the external phase. Experimental and theoretical data are on good agreement and the formation of stable emulsions is explained according to such model. This result could be of prime importance in order to formulate new paraffin emulsions.

Introduction

Paraffin emulsions are used in a wide range of technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels in the furniture industry. In order to obtain stable paraffin emulsions, for different applications, small particle size (about 1.0 μm) and low polydispersity are required. Recent works [1], [2] dealing with paraffin emulsions, stabilized with a mixed ionic/nonionic surfactant system, led to interesting results. At high ionic/nonionic surfactant ratios (<44/56) a significant change in emulsion properties was observed, increasing storage modulus (G′) and emulsion stability; while decreasing particle size [1]. Moreover, the phase behaviour of the surfactant system in water revealed that lamellar liquid crystalline aggregates were present at very low surfactant concentration (down to 0.2 wt% surfactant). Considering that the total surfactant concentration was about 2 wt%, the high stability could be related to the formation of a surfactant multilayer arrangement, surrounding the particles [2].

Emulsions are stabilized generally by surfactants that adsorb at interfaces, such as oil–water interface, and form normally a monolayer surrounding the droplets, which retards the destabilization processes. However, other kind of surfactant self-aggregation, such as liquid crystalline phases, can be present at the interface. Friberg et al. described the stabilization by multilayers around the droplets, increasing the viscosity of the system and enhancing emulsion stability [3], [4], [5], [6], [7], [8]. This stabilization mechanism was found in formulations with a 1/1 oil/water ratio and surfactant concentrations between 3 and 4 wt% [6]. Recent works [9], [10], [11] revealed that the surfactant concentration required to form multilayers surrounding the droplets is normally higher (between 10 and 15 wt%). It should be pointed out that the formation of such multilayers depends on many factors such as the nature of the surfactant, the interfacial area and the surfactant solubility in the continuous phase.

In this context, little is known about the role of the emulsification process in the emulsion properties of such systems. Moreover, the influence of the surfactants on paraffin emulsion and the possible self-assembled structures in the continuous phase have not been subject to systematic studies. Consequently, in this work the influence of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure on paraffin emulsion properties have been studied.