Influence of non-ionic surfactant addition on the stability and rheology of particle-stabilized emulsions

Journal Pre-proof Influence of Non-Ionic Surfactant Addition on the Stability and Rheology of Particle-Stabilized Emulsions Ashwin Kumar Yegya Raman, Clint P. Aichele

PII:

S0927-7757(19)31074-X

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124084

Reference:

COLSUA 124084

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

23 May 2019

Revised Date:

1 October 2019

Accepted Date:

6 October 2019

Please cite this article as: Yegya Raman AK, Aichele CP, Influence of Non-Ionic Surfactant Addition on the Stability and Rheology of Particle-Stabilized Emulsions, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124084

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Influence of Non-Ionic Surfactant Addition on the Stability and Rheology of Particle-Stabilized Emulsions

Ashwin Kumar Yegya Raman a, Clint P. Aichele a,*

School of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma

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74078, United states

*Corresponding Author

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Clint P. Aichele, 420 Engineering North, School of Chemical Engineering, Oklahoma State

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University, Stillwater, Oklahoma 74078, United states Email address: [email protected]

First Author:

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Tel. No: 405-744-9110

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Ashwin Kumar Yegya Raman, 420 Engineering North, School of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, United states Email address: [email protected]

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Tel. No: 405-780-5432

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Graphical abstract

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Abstract Hypothesis

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Nanoparticle wettability is expected to influence the surfactant adsorption onto the oil-

emulsions. Experiments

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water interface, which in turn would affect the structure and rheology of Pickering

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We investigated the influence of non-ionic surfactant (Span 80) addition to Pickering emulsions stabilized by either partially hydrophobic (Aerosil R711) or hydrophobic (Aerosil R812S) fumed silica particles. Findings

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The addition of a non-ionic surfactant to emulsions stabilized by hydrophobic fumed silica particles decreased the droplet size of emulsions unlike emulsions stabilized by partially hydrophobic fumed silica particles. Even under low shear conditions, the addition of a nonionic surfactant displaced the hydrophobic fumed silica particles from the oil-water interface unlike partially hydrophobic fumed silica particles. Our results suggested that the

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displacement of hydrophobic fumed silica particles by the addition of a non-ionic surfactant was not due to the change in nanoparticle wettability by surfactant adsorption onto the surface of fumed silica particles. Viscosity of emulsions stabilized by partially hydrophobic fumed silica particles decreased upon the addition of a non-ionic surfactant. In contrast, the viscosity of emulsions stabilized by hydrophobic fumed silica particles

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increased upon the addition of a non-ionic surfactant. Depending on the nanoparticle wettability, the addition of a non-ionic surfactant influenced the elastic and viscous

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properties of Pickering emulsions.

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silica fumed silica particles, emulsion stability

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Keywords: Particle-stabilized emulsions, surfactant-nanoparticle interactions, rheology,

1. Introduction

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Emulsions are ubiquitously encountered in many industrial applications such as those in paints, cosmetics, food, energy, and pharmaceuticals [1-3]. In addition to surfactants,

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solid particles can also stabilize emulsions [4-12]. Colloidal particle size and concentration [9, 13-20], morphology [21-24], wettability[6, 25-28], and density [18, 20] play a critical role in determining the emulsion stability. Once adsorbed to the oil-water interface, solid particles of appropriate wettability, size, and shape are very difficult to

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desorb from the interface [26].

In industrial applications, solid particles are usually present along with the surfactants [29]. Stabilization of emulsions in the presence of both surfactants and solid particles has been investigated [13, 18, 27, 30-36]. The extent of particle attachment to the oil-water interface

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is determined by its wettability [26, 37]. Binks et al. [38] and Lan et al. [34] elucidated the influence of surfactants on particle attachment to the oil-water interface. They observed that surfactant adsorption onto the particles leads to a change in particle hydrophobicity [13, 20]. Lucassen et al. [39], Binks et al. [38, 40] and Hassander et al. [30] investigated the enhanced stabilization of oil-in-water emulsions in the presence of mixed surfactants

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and particles. They attributed the enhanced stability of emulsions to particle flocculation in the presence of surfactants. However, few studies have shown that the addition of fumed

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silica particles to surfactant-stabilized emulsions led to destabilization of emulsions [41, 42]. Katepalli et al.[43] investigated the influence of surfactant addition to Pickering

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emulsions.

Fewer investigations were carried out to elucidate the influence of surfactant addition to an

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existing Pickering emulsion [29, 40, 44-47]. Vlichez et al. [48] investigated the influence of addition of nonionic hydrophobic surfactants to Pickering emulsions stabilized by

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magnetite nanoparticles. Alargova et al.[49] investigated the influence of surfactant addition to foams stabilized using hydrophobic polymer microrods. They observed that adsorption of Sodium Dodecyl Sulphate (SDS) onto the hydrophobic polymer microrods led to a change in wettability of polymer microrods, which in turn led to foam

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destabilization. Subramaniam et al.[50] investigated the stability of particle-stabilized bubbles upon exposure to surfactants. They observed that upon surfactant addition, the surfactants adsorbed onto the particle surfaces, which presumably caused a change in particle wettability. This led to particle detachment from the air-liquid interface. Whitby et al. [29] investigated the influence of addition of an anionic surfactant to oil-in-water

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emulsions stabilized using silica fumed silica particles (Aerosil R816). They concluded that dilution of oil-in-water emulsions using solutions of anionic surfactant did not have appreciable effect on the droplet size. However, at high surfactant concentrations, enhanced rate of creaming and flocculation were observed. Vashisth et al.[44] investigated the influence of addition of an anionic surfactant to oil-in-water emulsions stabilized using

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silica fumed silica particles (Aerosil R816). They concluded that re-emulsification of particle-stabilized emulsions using an anionic surfactant (at high shear rates) led to particle

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displacement from the oil-water interface.

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Our work here highlights, instead, the effect of non-ionic surfactant addition to water-in-

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oil emulsions stabilized using fumed silica particles with different wettability (either partially hydrophobic or hydrophobic silica fumed silica particles) under low shear

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conditions. This work will help us elucidate the effect of nanoparticle wettability on the stability and rheology of Pickering emulsions upon the addition of a non-ionic surfactant

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under low shear conditions. We showed that depending on the nanoparticle wettability, the addition of a non-ionic surfactant to particle-stabilized emulsion affected the emulsion morphology and rheology.

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2. Materials and methods

Water-in-oil emulsions were prepared using 1-Bromohexadecane as the continuous phase. 1-Bromohexanedecane (ρ = 0.999 g/ml) was purchased from Fischer Scientific (Assay Percent Range = 97 %). De-ionized (DI) water was used as the dispersed phase. Two types of fumed silica fumed silica particles – partially hydrophobic (Aerosil R711 (R711)), and

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hydrophobic (Aerosil R812S (R812S)) – were used. Fumed silica fumed silica particles were provided by Evonik Corporation. The specific surface areas of R711 and R812S are ~150 ± 25 m2/g and 220 ± 25 m2/g respectively (as reported by the manufacturer). TEM images of fumed silica particles can be found elsewhere [42]. Sorbitan monooleate (Span

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80, HLB = 4.3 ± 1 (as stated by the vendor)) was used as the surfactant.

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2.1 Preparation of emulsions

Ultra-Turrax digital homogenizer was used for the preparation of water-in-oil emulsions.

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Silica fumed silica particles (1.5wt.%) were dispersed in 1-Bromohexadecane (68.3 wt.%)

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and ultrasonicated at 75 % amplitude for 1 minute in order to completely disperse the fumed silica particles. DI water (30.15 wt.%) was added in a drop wise manner to the

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mixture containing 1-Bromohexadecane and silica fumed silica particles. The mixture was sheared at 10,000 rpm for 10 minutes. The resulting solid-stabilized emulsion was diluted

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by adding 1-Bromohexadecane. The diluted emulsion had 70.7 wt.% 1-Bromohexadecane, 27.9 wt.% water, and 1.4 wt.% fumed silica particles. The chosen concentration of fumed silica particles to stabilize the water-in-oil emulsions was based on bottle tests.

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Surfactant was added to the solid-stabilized emulsion at two different concentrations: a) 26 wt.% of the nanoparticle concentration (Span 80 (S1)) b) same amount as the fumed silica particles (Span 80 (S2)). Surfactant was added to the solid-stabilized emulsion as follows: required amount of surfactant was dispersed in 1-Bromohexadecane (that was used for

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dilution). Then, 1-Bromohexadecane along with Span 80 was added to the solid-stabilized emulsion and shaken (using hand) to mix the oil with the emulsion.

Bottle tests were carried out to test the bulk stability of water-in-oil emulsions over the experimental time. The emulsions were observed before and after the experiment under an

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Olympus BX53 cross-polarized optical microscope equipped with a high-speed camera in order to ensure structural stability over the experimental time. Image J was used to quantify

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the droplet size of water-in-oil emulsions [51].

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2.2. Rheology of emulsions

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Discovery Hybrid Rheometer (DHR-3),[52] which was purchased from TA instruments, was used to measure the rheological behavior of water-in-oil emulsions. In order to avoid

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the sedimentation of water droplets, 1-Bromohexadecane, which has a density similar to that of water, was used as the continuous phase. The parallel plate geometry was used to

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measure the viscosity of water-in-oil emulsions. The shear rate was varied from 1 to 200 s. A solvent trap was used to minimize the loss of sample due to evaporation. The

viscoelastic behavior of solid-stabilized water-in-oil emulsions was investigated by

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performing oscillatory strain measurements at 1 Hz.

2.3 Confocal microscopy Confocal microscopy experiments (Leica TCS SP2 confocal microscope) were carried out to understand the spatial distribution of hydrophobic fumed silica particles upon surfactant

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addition. The procedure to fluorescently label hydrophobic silica fumed silica particles can be found elsewhere [42, 53].

3 Results and Discussion 3.1 Characterization of emulsions Figure 1 shows the optical microphotographs of water-in-oil emulsions stabilized using

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partially hydrophobic fumed silica particles at the 0th hour and after 24 hours. Figure S1

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shows the corresponding droplet size distribution. It was evident from Figures 1 and S1 that there was no significant change in the droplet size of emulsions. Also, the bottle tests

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showed no bulk phase separation of water and 1-Bromohexadecane. Similar tests were

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carried out for other systems that were investigated. From the above results, it was concluded that the emulsions under investigation were stable over the experimental time

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both at the macroscopic and microscopic level.

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3.2 Influence of surfactant addition on droplet size Figure 2 shows the microphotographs of water-in-oil emulsions stabilized by partially hydrophobic fumed silica particles in both the presence and absence of a surfactant. Figure S2 shows the corresponding droplet size distribution. It was evident that there was no

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appreciable change in the droplet size of water-in-oil emulsions upon the addition of a surfactant.

Figure 3 shows the microphotographs of water-in-oil emulsions stabilized by hydrophobic fumed silica particles in both the presence and absence of a surfactant. Figure S3 shows

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the corresponding droplet size distribution. By comparing Figures 2 and 3, it can be seen that there was a significant effect on the droplet size upon the addition of a surfactant to water-in-oil emulsions stabilized by hydrophobic fumed silica particles unlike water-in-oil emulsions stabilized by partially hydrophobic fumed silica particles. The droplets became finer upon the addition of a surfactant to water-in-oil emulsions stabilized by hydrophobic

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fumed silica particles. Under simple shear flow, tip streaming is observed upon surfactant accumulation at the tips of drops [54]. Tip streaming is usually observed when the ratio of

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the viscosities of dispersed and continuous phase is < 0.1. In our work, the ratio of viscosities of the dispersed phase to continuous phase was 0.15. Hence, in congruence with

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De Bruijn [54], we observed the formation of satellite drops upon the addition of a

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surfactant (from Figures 3a and 3b). Also, from Figures 2a and 3a, it was observed that water-in-oil emulsions stabilized by only hydrophobic fumed silica particles had a larger

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droplet size than water-in-oil emulsions stabilized by only partially hydrophobic fumed silica particles. This can be attributed to the lower energy of adsorption of hydrophobic

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fumed silica particles to the liquid-liquid interface than partially hydrophobic fumed silica particles.[6] By comparing Figure 3 with the droplet size distribution for water-in-oil emulsions stabilized by only a surfactant (Figure S4), we observed that upon increasing the addition of a surfactant to water-in-oil emulsions stabilized by hydrophobic fumed silica

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particles, the droplet size is predominantly dictated by surfactants [55] than hydrophobic fumed silica particles presumably due to the adsorption of surfactants onto the oil-water interface.

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In order to further investigate the above-mentioned observations, confocal microscopy experiments were performed. Figure 4 shows the confocal microscopy images of water-inoil emulsions stabilized by fumed silica particles upon the addition of a surfactant. Figures S5 and S6 show the confocal microscopy image of emulsions stabilized by only fumed silica particles. In the absence of a surfactant, the hydrophobic particles were located at the

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oil-water interface (Figure S5). In contrast, upon the addition of a surfactant, the hydrophobic fumed silica particles tend to be partially dispersed in the continuous phase

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(figure 4a). Thus, the addition of a non-ionic surfactant to water-in-oil emulsions stabilized using hydrophobic fumed silica particles aided in partial displacement of hydrophobic

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fumed silica particles (Figure 4a). The surfactant adsorption onto the liquid-liquid interface

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aided in decreasing the droplet size of emulsions (Figure 3). Raman et al. [42] observed that Span 80 did not adsorb strongly onto R812S (hydrophobic) fumed silica particles.

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Therefore, our results indicated that particle detachment from the oil-water interface appeared not to be linked with the change in hydrophobicity of nanoparticle upon surfactant

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adsorption, which was in accordance with the observations of Vashisth et al. [44]. On the contrary, from Figure 4b, it appeared that the addition of a non-ionic surfactant to waterin-oil emulsions stabilized using partially hydrophobic fumed silica particles did not aid in

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appreciable displacement of partially hydrophobic fumed silica particles.

3.3 Rheology of suspensions and emulsions 3.3.1 Suspensions Figure 5 shows the viscosity of 1-Bromohexadecane upon the addition of fumed silica particles and surfactants. 1-Bromohexadecane exhibited a Newtonian behavior. Even upon

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the addition of 1.5 wt.% R812S (hydrophobic) fumed silica particles, and 1.5 wt.% R812S with Span 80 (S2), 1-Bromohexadecane exhibited a Newtonian behavior. Table 1 shows the viscosity of suspensions that exhibited Newtonian behavior. Based on our results, it was evident that hydrophobic fumed silica particles did not show a stronger tendency for agglomeration [56] in both the presence and absence of Span 80. On the contrary, 1-

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Bromohexadecane exhibited a shear-thinning behavior upon the addition of partially hydrophobic fumed silica particles. Also, the addition of partially hydrophobic fumed silica

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particles significantly increased the viscosity of 1-Bromohexadecane unlike hydrophobic fumed silica particles. The number of –OH groups on the surface of hydrophobic fumed

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silica particles are lower than partially hydrophobic fumed silica particles. Therefore, the

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interactions between partially hydrophobic fumed silica particles (via hydrogen bonding) are higher than hydrophobic fumed silica particles [57]. Thus, our results indicated that the

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presence of partially hydrophobic fumed silica particles led to formation of a stronger 3D network [58] in 1-Bromohexanedecane than hydrophobic fumed silica particles.

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Interestingly, upon the addition of a surfactant, the viscosity of 1-Bromohexadecane containing partially hydrophobic fumed silica particles reduced significantly and exhibited a Newtonian behavior. Raman et al. [42] observed that sorbitan monooleate strongly adsorbs onto the surface of partially hydrophobic silica fumed silica particles than

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hydrophobic fumed silica particles. Thus, we postulate that the presence of a surfactant breaks the interactions between partially hydrophobic fumed silica particles by adsorbing onto the surface of fumed silica particles, thereby leading to a decrease in viscosity, which was in accordance with the literature [44].

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3.3.2 Emulsions Figure 6 shows the viscosity of water-in-oil emulsions stabilized using fumed silica particles in both the presence and absence of a surfactant. Water-in-oil emulsions stabilized by either partially hydrophobic or hydrophobic fumed silica particles showed a shear thinning behavior (Figure 6). Even upon the addition of a surfactant, water-in-oil emulsions

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stabilized using partially hydrophobic fumed silica particles showed a shear thinning behavior (Figure 6a). Upon the addition of a surfactant, there was a decrease in the viscosity

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of water-in-oil emulsions stabilized using partially hydrophobic fumed silica particles. In contrast, the viscosity of emulsions stabilized by hydrophobic fumed silica particles

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increased upon surfactant addition. This can be attributed to the change in the droplet size.

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Upon the addition of a surfactant, the droplets became finer that in turn led to an increase in viscosity. Overall, the emulsions that were investigated showed a pseudoplastic behavior

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[59],[60]. The shear thinning behavior was profound for water-in-oil emulsions stabilized using partially hydrophobic fumed silica particles than hydrophobic fumed silica particles.

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Water-in-oil emulsions stabilized using partially hydrophobic fumed silica particles have higher viscosity than water-in-oil emulsions stabilized using hydrophobic fumed silica particles. This can be attributed to the formation of a stronger 3D network in the continuous

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phase by partially hydrophobic fumed silica particles (as discussed in section 3.3.1).

3.4 Oscillatory Measurements The strength of the network structure in water-in-oil emulsions can be characterized from the storage (G’) and loss (G”) moduli [61]. Figure 7a shows the storage and loss moduli of water-in-oil emulsions stabilized by partially hydrophobic fumed silica particles upon the

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addition of a surfactant. Linear viscoelastic region was observed at low strain ranges where the storage and loss moduli were independent of the applied strain [62]. For water-in-oil emulsions stabilized by partially hydrophobic fumed silica particles, both in the presence and absence of a surfactant, viscoelastic solid like behavior was observed (G’ > G”). The storage modulus was highest for water-in-oil emulsions stabilized by only partially

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hydrophobic fumed silica particles. Upon increasing the surfactant addition, the storage modulus, which indicated the elastic properties of emulsions, decreased. This result was in

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congruence with Figure 6a; wherein, the addition of a surfactant increased the fluidity of

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emulsions stabilized using partially hydrophobic fumed silica particles.

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Figure 7b shows the storage and loss moduli of water-in-emulsions stabilized by hydrophobic fumed silica particles upon the addition of a surfactant. Similar to water-in-

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oil emulsions stabilized by partially hydrophobic fumed silica particles, linear viscoelastic region was observed at low strain ranges for emulsions stabilized by hydrophobic fumed

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silica particles (in both the presence and absence of a surfactant). Viscoelastic solid like behavior (G’ > G”) was observed for emulsions stabilized by only hydrophobic fumed silica particles and emulsions stabilized by hydrophobic fumed silica particles upon the

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addition of Span 80 (S2).

The storage modulus was decreased upon the addition of Span 80 (S1) to water-in-oil emulsions stabilized by hydrophobic fumed silica particles. On the other hand, the storage modulus values were comparable for water-in-oil emulsions stabilized by only hydrophobic fumed silica particles, and water-in-oil emulsions stabilized using

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hydrophobic fumed silica particles upon the addition of Span 80 (S2). This can be attributed to the change in the droplet size of water-in-oil emulsions upon the addition of a surfactant. From Figure 3, it can be observed that water-in-oil emulsions stabilized using only hydrophobic fumed silica particles resulted in coarse emulsions. On the contrary, the addition of Span 80 (S2) resulted in fine emulsions. The addition of Span 80 (S1) resulted

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in emulsions containing a mixture of coarse and fine emulsion droplets. Pal et al. [63] investigated the effect of droplet size on the rheology of emulsions. They observed that the

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storage modulus of coarse emulsions were higher than the storage modulus of emulsions containing a mixture of coarse and fine emulsion droplets. Also, they observed that fine

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emulsions had higher storage modulus than coarse emulsions. In congruence with results

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of Pal et al.[63], we observed that the addition of Span 80 (S1) to water-in-oil emulsions stabilized using hydrophobic fumed silica particles lowered the storage modulus, since it

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resulted in a mixture of fine and coarse emulsion droplets. Also, from the work of Pal et al.,[63] we would expect that upon the addition of Span 80 (S2) to water-in-oil emulsions

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stabilized by hydrophobic fumed silica particles (which resulted in fine emulsions), the storage modulus would be higher than water-in-oil emulsions stabilized by only hydrophobic fumed silica particles (coarse emulsions). However, we observed that the storage modulus values were comparable. We postulate that the surfactants can act as

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plasticizing agent in the presence of fumed silica particles, which can lead to lower storage modulus values than emulsions stabilized by only fumed silica particles [64]. Although the water-in-oil emulsions stabilized by only hydrophobic fumed silica particles had coarse droplets, the presence of a 3D network in the continuous phase presumably resulted in storage modulus values comparable to that of water-in-oil emulsions stabilized using

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hydrophobic fumed silica particles upon the addition of Span 80 (S2) (which resulted in fine droplets).

4. Conclusions We investigated the effect of surfactant addition to water-in-oil emulsions stabilized by

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either partially hydrophobic or hydrophobic silica fumed silica particles under low shear conditions. Depending upon the nanoparticle wettability, the addition of a non-ionic

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surfactant to a particle-stabilized emulsion affected the emulsion morphology and rheology. Our results showed the addition of a surfactant to a Pickering emulsion stabilized

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by partially hydrophobic nanoparticle did not affect the droplet size of emulsions [29, 45].

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On the contrary, we found that surfactant addition to emulsions stabilized by hydrophobic fumed silica particles resulted in the transition of a coarse emulsion to a fine emulsion,

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which in turn affected the strength of the network structure of the emulsions. At the investigated surfactant concentrations, even under low shear conditions, surfactant addition

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to emulsions stabilized by hydrophobic fumed silica particles resulted in the displacement of fumed silica particles from the oil-water interface unlike emulsions stabilized by partially hydrophobic fumed silica particles. Our results showed that the displacement of hydrophobic fumed silica particles from the oil-water interface was not due to the change

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in nanoparticle wettability upon surfactant adsorption, which was in accordance with Vashisth et al.[44]. Suspensions containing partially hydrophobic fumed silica particles showed a shear thinning behavior unlike suspensions with hydrophobic fumed silica particles. However, upon the addition of a surfactant, the suspension with partially hydrophobic fumed silica particles displayed Newtonian behavior. Emulsions stabilized

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using either hydrophobic or partially hydrophobic fumed silica particles showed a shear thinning behavior in both the presence and absence of a surfactant.

Our results suggest that, depending on the nanoparticle wettability, the morphology and rheology of Pickering emulsions can be modified by non-ionic surfactant addition, even

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under low shear conditions. These results have implications in drug delivery, recycling of particles that are used as encapsulates, and separation of particles from the oil-water

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interface [29, 44, 65, 66].

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Acknowledgment

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We thank Dr. Victor Lifton of Evonik for donating the silica fumed silica particles and providing consultation. We thank Brent Johnson of Oklahoma State University for helping

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us with confocal microscopy. This research did not receive any specific grant from funding

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agencies in the public, commercial, or not-for-profit sectors.

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Declarations of interest: none

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Figure Titles

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Figure 1: Microphotographs of water-in-oil emulsions stabilized using partially

lP

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hydrophobic nanoparticles (R711) a) 0th hour b) after 24 hours

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Figure 2: Microphotographs of water-in-oil emulsions stabilized using partially hydrophobic nanoparticles (R711) in the a) absence of a surfactant b) upon the addition

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of Span 80 (S1) c) upon the addition of Span 80 (S2)

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Figure 3: Microphotographs of water-in-oil emulsions stabilized using hydrophobic nanoparticles (R812S) in the a) absence of a surfactant b) upon the addition of Span 80

lP

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-p

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of

(S1) c) upon the addition of Span 80 (S2)

Figure 4: Confocal microscopy images of water-in-oil emulsions upon the addition of

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Span 80 (S2) to emulsions stabilized using a) hydrophobic nanoparticles (R812S) b)

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partially hydrophobic nanoparticles (R711) Scale bar: 50 μm

26

of ro -p

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lP

nanoparticles and surfactants

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Figure 5: Viscosity versus shear rate for 1-Bromohexadecane upon the addition of

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Figure 6: Viscosity versus shear rate for Pickering emulsions stabilized using either a) partially hydrophobic nanoparticles (in both the presence and absence of a surfactant) or b) hydrophobic nanoparticles (in both the presence and absence of a surfactant)

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Figure 7: Storage (G’) and loss moduli (G”) of water-in-oil emulsions stabilized by either partially hydrophobic nanoparticles (a) or hydrophobic nanoparticles (b) in the presence

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and absence of a surfactant

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Table 1: Viscosity of 1-Bromohexane in the presence of fumed silica particles and

lP

suspensions

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System

Newtonian Viscosity (Pa s) 0.0074

1-Bromohexadecane + 1.5wt.% R812S

0.011

1-Bromohexadecane + 1.5wt.% R812S + Span 80 (S2)

0.0106

1-Bromohexadecane + 1.5wt.% R711 + Span 80 (S2)

0.0142

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1-Bromohexadecane

28