Influence of Hydrophilic Surfactants on the W1–W2 Coalescence in Double Emulsion Systems Investigated by Single Droplet Experiments

Abstract

Double emulsions are a promising formulation for encapsulation and targeted release in pharmaceutics, cosmetics and food. An inner water phase is dispersed in an oil phase, which is again emulsified in a second water phase. The encapsulated inner water phase can be released via diffusion or via coalescence, neither of which is desired during storage but might be intended during application. The two interfaces in a double emulsion are stabilized by a hydrophilic and a lipophilic surfactant, to prevent the coalescence of the outer and the inner emulsion, respectively. This study focuses on the influence of the hydrophilic surfactant on the release of inner water or actives encapsulated therein via coalescence of the inner water droplet with the outer O–W2 interface. Since coalescence and diffusion are difficult to distinguish in double emulsions, single-droplet experiments were used to quantify differences in the stability of inner droplets. Different lipophilic (PGPH and PEG-30 dipolyhydroxylstearate) and hydrophilic surfactants (ethoxylates, SDS and polymeric) were used and resulted in huge differences in stability. A drastic decrease in stability was found for some combinations, while other combinations resulted in inner droplets that could withstand coalescence longer. The destabilization effect of some hydrophilic surfactants depended on their concentration, but was still present at very low concentrations. A huge spread of the coalescence time for multiple determinations was observed for all formulations and the necessary statistical analysis is discussed in this work. The measured stabilities of single droplets are in good accordance with the stability of double emulsions for similar surfactant combinations found in literature. Therefore, single droplet experiments are suggested for a fast evaluation of potentially suitable surfactant combinations for future studies on double-emulsion stability.

Contact-Mediated Nucleation of Subcooled Droplets in Melt Emulsions: A Microfluidic Approach

“…Taboada et al [25] measured coalescence times ranging from 10 s up to more than 30 min for different emulsifiers in single droplet experiments with water as the continuous phase. Regarding nonionic surfactants, Leister et al [64] obtained coalescence times between 5 and around 100 s. The measurements from Taboada et al [25] and Leister et al [64] are within the same range as our calculated theoretical coalescence times. We expect induction times to be within the mentioned range, providing that the surfactant is dissolved in water, which is in good agreement with our experimental data (Figure 13).…”

Oil Droplet Coalescence in W/O/W Double Emulsions Examined in Models from Micrometer- to Millimeter-Sized Droplets

“…For the interfacial tension measurements, microfluidics and single-droplet experiments, the MCT oil was purified according to the method described by Dopierala et al [36] before mixing with surfactants, since the impurities in the unpurified oil could have a great influence due to the small interfacial area compared to the sample volume [7].…”

“…Concerning the surfactant interactions, an often-discussed topic is whether the hydrophilic surfactant has a positive or a negative influence on the encapsulation efficiency. The general observation for PGPR-stabilized water droplets is that smaller hydrophilic surfactants are more likely to increase release than high-molecular-weight polymers [7,13,14]. Surprisingly, the outer interface, and thus O-O coalescence, is less commonly discussed in terms of the interactions between hydrophilic and lipophilic surfactants.…”

“…The stability of the oil droplets is especially in focus when lower concentrations of hydrophilic surfactants are suggested to enhance the encapsulation efficiency [8,13,17]. Since some hydrophilic surfactants are known to decrease the inner water droplets' stability with higher concentrations [7,18], one way to produce stable double emulsion formulations could lie in the drastic decrease in the hydrophilic surfactants' concentration. According to previous studies, this could help to keep the inner water droplets stable, but the oil droplets might then be more prone to coalescence.…”

“…To keep the inner water droplets emulsified within the oil, most research is performed on the oil-soluble surfactants polyglycerol polyricinoleate (PGPR) and Span 80 [5]. For stabilizing the oil droplets, a variety of different water-soluble surfactants were applied and compared in different studies [6,7]. These ranged from shortchained ionic [8] and non-ionic [9] surfactants over synthetic polymers [10] to biopolymers, such as pectins and proteins [11].…”