Research paper
Investigations on the influence of osmotic active substances on the structure of water in oil emulsions for the application as inner phase in double emulsions

https://doi.org/10.1016/j.colsurfa.2017.10.073Get rights and content

Abstract

Water in oil in water (WOW) double emulsions have a great potential as encapsulation systems in different industrial sections. Nevertheless, there are only few double emulsion based products available on market. An important limitation, regarding the product launch, is the insufficient stability of these products. Many instability mechanisms have to be taken into account: coalescence between inner water droplets, coalescence between outer oil droplets, coalescence between inner and outer water phase, and diffusion between the dispersed water droplets and the continuous outer water phase. In the present study, we focus on structural changes and stabilization mechanisms of inner water in oil (WO) emulsions. Osmotic active substances (OAS) have to be used in the inner water droplets to limit the diffusion processes between the water phases in double emulsions. We therefore investigated the influence of different OAS on the structural changes in WO emulsions. We focused on three subprocesses influencing the emulsification result: droplet breakup, stabilization directly after breakup and diffusional processes after droplet stabilization. For this, we prepared WO emulsions using sodium chloride, calcium chloride and glucose as OAS and polyglycerine-polyricinoleate as lipophilic emulsifier. The droplet size distribution was determined to evaluate structural changes. Furthermore, we measured interfacial properties and emulsion viscosities to evaluate stabilizing effects. Kinetic stabilization effects were evaluated by applying high pressure homogenization, known as a process with high requirements on the stabilization efficiency. The osmolality of the inner water phases was determined as parameter for the likelihood of diffusional processes. This characteristic value strongly depends on OAS type and concentration. Adsorption kinetics and diffusional processes turned out as the main factors influencing inner emulsion structure.

Introduction

Water in oil in water (WOW) double emulsions consists of an inner water in oil (WO) emulsion which is again dispersed in an outer water phase. Due to this multicomponent structure, WOW systems are proposed as encapsulation systems for hydrophilic substances. However, their implementation is still challenging, due to the thermodynamical instability [1], [2], [3], which results from different mechanisms. Like in single emulsions, droplets of both, the inner water phase and the outer oil phase, can coalesce. Coalescence leads to an increase in mean droplet diameter and at worst emulsion breakage. Furthermore, inner water droplets can coalesce with the outer water phase. This directly leads to a loss of inner water phase and thus in a loss of the typical double emulsion structure. Water molecules may also diffuse from the inner to the outer water phase and vice versa. This effect is based on differences in the Laplace pressure [4], [5].

These instability mechanisms raise many scientific issues that have to be investigated individually. Due to the complexity considering the instabilities in double emulsions, the stability of the outer emulsion and the loss of inner water phase in double emulsions were investigated systematically in previous studies [6], [7], [8]. In the present study, we focus on the stability and structure of the inner WO emulsion. The mean droplet diameter of the dispersed water phase in the inner WO emulsion has to be small enough (<1 mm) to fit into oil droplets of typical size found in double emulsions [9], [10]. Small droplets lead to a homogenous distribution of the inner water phase inside the oil droplet increasing the stability against coalescence with the outer water phase [11], [12]. Furthermore, the emulsion structure has to be stable under addition of osmotic active substances (OAS). The use of OAS in WO emulsions is necessary for their application in double emulsion systems, in order to balance differences in Laplace pressure [4], [5]. Thus, the influence of these substances on the emulsion structure has to be investigated in detail. The impact of different osmotic active substances and different amounts of inner water on diffusional processes between the water phases in double emulsions were investigated previously by e.g. Herzi et al. [13], [14].

Furthermore it is also reported, that OAS might influence the structure of the inner emulsion. For example, sodium chloride (NaCl) as OAS lead to a decrease in mean droplet diameter of the inner water droplets at constant concentration of the lipophilic emulsifier polyglycerine-polyricinoleate (PGPR) [9], [15], [16], [17], [18]. Scherze et al. [15] reported no change of the interfacial tension by adding NaCl in contrast to the sole use of PGPR. In contrast, Pawlik et al. [18] and Nikolowski et al. [17] reported a slight decrease in interfacial tension by adding NaCl. They assume that the addition of salt leads to strengthened interactions between PGPR and salt. This assumption was explained by an increase in dilational interfacial elasticity and viscosity [18]. Other studies focused on the impact of calcium salts on the mean droplet size in WO emulsions [19], [20], [21]. The same tendencies as for the use of NaCl were found: the droplet size decreased by adding calcium chloride (CaCl2) to the inner water phase. Marquez et al. [20] assumed, that a slight decrease in interfacial tension caused the decrease in droplet size. In addition, the authors reported that the interfacial dilational rheology parameters changed by the addition of CaCl2. Furthermore, a change in attractive forces between the droplets by adding electrolytes is discussed as another reason for decreasing mean droplet diameters [20], [22]. Diffusion of water molecules between water droplets of different sizes, called Ostwald ripening, is discussed as an influencing parameter on the droplet size in water in oil emulsions considering long-term stability over days and weeks [23], [24].

These studies imply that electrolytes added to the inner water phase change the attractive forces and thus decrease the water droplet diameter. To examine this hypothesis, we investigate the influence of ionic and anionic substances on the inner emulsion structure. For this, we added different concentrations of NaCl and CaCl2 as ionic substances, and glucose as anionic substance to the inner water droplets and measured the droplet size distribution after emulsification at constant processing conditions.

As changes in interfacial properties might affect the emulsification results by changing droplet breakup behavior, we measured those properties. Droplets being broken in emulsification have to be stabilized by emulsifier molecules. This is affected by the adsorption kinetics of the lipophilic emulsifier, as for all emulsification processes [25], [26]. We therefore investigated the influence of OAS on droplet breakup as well as subsequent droplet stabilization.

Interfacial tension and emulsion viscosity were measured to evaluate their influences on droplet breakup. Changes in the adsorption kinetics of the emulsifier were evaluated, to estimate the influence of OAS on the stabilization efficiency. For this, interfacial rheological characteristics were determined and emulsions were prepared by processes known for different breakup kinetics [27]. As we assume that diffusion processes start immediately after droplet breakup and stabilization, we included osmolality analytics.

Section snippets

Materials

Emulsions were prepared using demineralized water and rapeseed oil (Bernhard Schell GmbH Lichtenau, Germany). The OAS NaCl, CaCl2 and glucose monohydrate (glucose) with a purity >99.5 were purchased from Carl Roth (Karlsruhe, Germany). Paalsgard (Juelsminde, Denmark) kindly supplied Polyglycerine-polyricinoleate (PGPR). For measurements of interfacial properties, ultrapure water was used. Rapeseed oil was purified for these investigations, using Florisil® (Carl Roth Karlsruhe, Germany) [28].

Preparation of water in oil emulsions

For

Influence of osmotic active substances on the resulting water droplet size, using gentle breakup conditions

The emulsions were prepared under gentle droplet breakup conditions (GR). The mean droplet size d50,3 in dependency of the OAS concentration is depicted in Fig. 1. The resulting mean droplet size without addition of OAS was 2.65 μm (star). With addition of OAS mean droplet sizes decreased, as reported in [9], [10], [17], [18], [19], [20], [21], [22]. The biggest effect is seen with NaCl: At a NaCl concentration of 0.017 mol/l, the mean diameter decreases to a value of 0.88 μm (squares).

Conclusions

For successful implementation of water in oil emulsions as inner phase in double emulsions, the inner emulsion structure has to fulfil different requirements. For example, a monomodal droplet size distribution with a mean droplet diameter below 1 μm is usually required.

Besides a lipophilic emulsifier, OAS are added to the water phase, in order to limit water diffusion processes in double emulsions. Several authors report several effects of OAS on inner DSD, and the effect was explained by

Funding

This research project was supported by the German Ministry of Economics and Energy (via Arbeitsgemeinschaft industrieller Forschungsvereinigungen, Otto von Guericke“ e.V.) and GVT (Forschungs-Gesellschaft Verfahrens-Technik e.V., Frankfurt/Main) in the scope of project AiF 19443 N.

Acknowledgments

The authors would like to express their thanks to Lydia Schütz for supporting the experiments and Marc Wittner for proof reading the article.

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