Progress in the Formulation of Concentrated Ecological Emulsions for

Research Article pubs.acs.org/journal/ascecg

Progress in the Formulation of Concentrated Ecological Emulsions for Agrochemical Application Based on Environmentally Friendly Ingredients Luis A. Trujillo-Cayado, María C. García, Jenifer Santos, José A. Carmona, and María C. Alfaro* Reología Aplicada, Tecnología de Coloides, Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, c/P. García González, 1, E41012, Sevilla, Spain ABSTRACT: Researchers are engaged in exploring new ecological solvents for different applications, in accordance with the current trends in green chemistry and green engineering. This study is focused on the development of stable emulsions using eco-friendly ingredients, such as two green solvents as dispersed phase (N,N-dimethyl decanamide and α-pinene) and a nonionic polyoxyethylene glycerol ester derived from coconut oil as emulsifier. α-Pinene is a renewable essential oil that could find numerous agrochemical applications. In this investigation, we study the influence of dispersed phase concentration on droplet size distribution, physical stability, and rheological properties of highly concentrated eco-friendly emulsions. The laser diffraction technique revealed submicron droplet sizes for all studied emulsions. A coalescence process was detected in the most concentrated emulsions, not only by laser diffraction measurements, but also by rheology. Increasing the dispersed phase concentration from 30 to 45 wt % yielded an increase in the zero shear viscosity and a decrease in the flow index. All samples above 35 wt % dispersed phase displayed clear viscoelastic properties with a predominance of the elastic over the viscous component in the frequency range studied. Flow curves and mechanical spectra were quite sensitive to destabilization by creaming and coalescence. Multiple light scattering demonstrated that the emulsions prepared with a dispersed phase concentration of 35 wt % exhibited the highest physical stability. These emulsions may find applications related to the design of biotechnological complex systems with different uses, such as matrices for agrochemical products or emulsion-based encapsulation and delivery systems. KEYWORDS: Eco-friendly surfactant, Emulsion, Green solvent, Multiple light scattering, Rheology

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INTRODUCTION Recently, the development of eco-friendly products and technologies has become more interesting as a result of an increasing concern for the environment and public health The introduction of stringent environmental and safety regulations affecting the agrochemical industry has spurred interest in the search for new ecological formulations. Furthermore, it has become increasingly more difficult to find compounds of high activity that are eco-friendly and cheap to manufacture. In the agrochemical industry, pesticides need solvents in order to dissolve solid actives. Chemists prefer the use of oil-in-water (O/W) emulsions for molecular solubilization of inorganic and organic ingredients in solvents due to the higher biodegradability and lower toxicity of these colloidal systems. Furthermore, the use of oil-in-water emulsions allows the dissolution of both nonpolar and polar ingredients simultaneously. The main advantage of these emulsions is that they are aqueous based formulations, thus making their application easy and reducing their environmental impact.1 An ecological emulsion may contain a variety of different green solvents as oil phase, such as essential oils, terpenes and fatty acid dimethylamides (FADs). FADs have been recognized as green solvents because they are obtained from renewable raw materials and in addition exhibit excellent environmental, © 2017 American Chemical Society

health, and safety (EHS) properties. N,N-Dimethyl decanamide is considered to be a green solvent, according to the Environmental Protection Agency in 2005, and has excellent solubilizing properties with respect to agrochemical actives. It is, therefore, a suitable solvent for agrochemical use.2 α-Pinene is an essential oil that may be obtained from oleoresins of plants and provides some activity as a biobased pesticide.3 This ecological biosolvent is not soluble in water but is miscible with other organic solvents.4 The emulsion formulation of essential oils for agrochemical use has been attempted earlier in a number of studies.5 In addition, environmentally friendly surfactants have attracted significant interest recently. Polyoxyethylene glycerol esters derived from coconut oil are nonionic surfactants obtained from a renewable source that fulfill the environmental and toxicological requirements to be used as eco-friendly foaming, wetting and/or emulsifying agents. Glycereth-17 cocoate was selected as emulsifier due to its excellent wetting, superficial, interfacial and emulsifying properties.6−8 This green surfactant possesses an eco-label (DID list: 2133). Received: January 11, 2017 Revised: March 20, 2017 Published: March 29, 2017 4127

DOI: 10.1021/acssuschemeng.7b00106 ACS Sustainable Chem. Eng. 2017, 5, 4127−4132

Research Article

ACS Sustainable Chemistry & Engineering N

All emulsion properties are determined by the choice of the formulation, the emulsification method and process variables. Many important properties of emulsions strongly depend on structural parameters such as the volume ratio of the phases, droplet size distribution and mean droplet size.9 Therefore, the correct selection of the emulsion composition and formulation is essential to improve the emulsification development.10 The oil phase of an emulsion is one of the focal points of current progress in innovative formulations. In particular, researchers are engaged in exploring the use of green solvents to replace traditional organic solvents. Fast biodegradability and low or null toxicity are requirements that these eco-friendly solvents must fulfill. In addition, green solvents must be obtained from renewable resources, as well as satisfying the customer requirements. The dispersed phase concentration in an O/W emulsion influences its rheological properties and physical stability.11 Dispersed phase concentration is usually characterized in terms of the oil phase mass fraction (ϕm): m ϕm(%) = 100 × oil mE (1)

D3,2 =

N ∑i = 1 nidi 2

(2)

N

D4,3 =

∑i = 1 nidi 4 N

∑i = 1 nidi 3

(3)

where ni is the number of droplets with diameter di. Physical Stability. Emulsion physical stability against creaming, coalescence, and flocculation was evaluated by multiple light scattering measurements. A Turbiscan Lab Expert (Formulaction, France) was used for 30 days at 20 °C. This technique makes it possible to determine the predominant mechanism of destabilization in each sample as well as the kinetics of the destabilization process. To characterize the creaming process, the creaming index was used (CI):13 CI(%) =

HS × 100 HE

(4)

Where, HE is the total height of the emulsion and HS is the height of the serum layer. The initial slope of the plot of CI versus aging time is related to the creaming rate (ω):

ω=

where ϕm is the mass of the oil phase (moil) divided by the total mass of emulsion (mE). The oil phase concentration plays a key role in O/W emulsion stability by different mechanisms, such as flocculation or depletion flocculation. In this sense, there is some evidence that increasing dispersed phase concentration will enhance O/ W emulsion physical stability via weak flocculation of the droplets. Several techniques such as multiple light scattering, laser diffraction and rheology may be used to evaluate the ability of an emulsion to resist destabilization phenomena caused by modification of the droplet size or droplet migration. The aim of this investigation is to study the influence of dispersed phase concentration (30−50 wt %) on the rheological properties, droplet size distribution, and physical stability of oil-in-water emulsions formulated with a mixture of two green solvents and an eco-friendly emulsifier.

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∑i = 1 nidi 3

H d(CI) × E dt 100

(5)

The Turbiscan stability index (TSI) is a parameter that takes into account every destabilization mechanism happening in a given sample. In order to calculate this statistical factor, the following equation can be used:12 TSI =

∑ |scan ref (hj) − scani(hj)| j

(6)

where scani is the initial value of the backscattering (BS), scanref is the BS at a given time, hj is a given height in the vial, and the Turbiscan stability index is the sum of all the scan differences in the measuring cell. Rheology of Emulsions. The rheological characterization involved flow curves and stress and frequency sweeps in small-amplitude oscillatory shear experiments (SAOS). Rheological characterization was performed with a Haake MARS rheometer (Germany) with a sandblasted Z20 coaxial cylinder geometry for the flow curves. SAOS experiments were measured using a sandblasted double-cone geometry (1°, Ri = 32 mm; Re = 32.35 mm). The frequency sweep tests were carried out from 20 to 0.05 rad/s by selecting a stress within the linear range. All rheological measurements were carried out after 1, 6, 15, 21, and 30 days aging time to follow the effect of aging time and performed at 20 °C ± 0.1 °C, using a C5P Phoenix circulator (Thermo-Scientific, USA) for sample temperature control. Equilibration time prior to rheological tests was 300 s. Samples were taken at about 2 cm from the upper part of the container. Statistical Analysis. Each emulsion was measured in triplicate for rheological and laser diffraction tests and the results expressed as mean value ± standard deviation. In order to perform a statistical analysis on the resulting data, a one-way analysis of variance (ANOVA) and a simple regression analysis were carried out using StatPlus:mac and a significance level of p = 0.05.

MATERIALS AND METHODS

Materials. Oil-in-water emulsions with different dispersed phase concentrations (from 30 to 50 wt %) were prepared using a mixture of green solvents, N,N-dimethyl decanamide (AMD-10), and α-pinene. Taking into account previously reported results, a mass ratio of 75/25 (AMD-10/α-pinene) was used.12 Agnique AMD-10 (0.88 g/mL at 25 °C) was obtained from BASF. α-Pinene (0.84 g/mL at 25 °C) was acquired from Sigma Chemical Company. The ratio of dispersed phase/surfactant was fixed with a value of 10/ 1. The emulsifier used was a nonionic surfactant derived from coconut oil (polyoxyethylene glycerol fatty acid ester, glycereth-17 cocoate). Its trade name is Levenol C-201, and it was received as a gift from KAO. Emulsification Procedure. The continuous phases were prepared by dissolving the emulsifier in ultrapure water cleaned using a Milli-Q water purification system. The samples (total amount of 250 g) were prepared in two steps. First, the mixture of solvents was added slowly at 4000 rpm for 30 s to the aqueous phase. Subsequently, secondary homogenization was carried out with a rotor-stator device equipped with a mesh screen (Silverson L5M) using a rotational speed of 7500 rpm over 90 s. Temperature was fixed at 20 °C. Emulsion Droplet Size Analysis. The size distributions, Sauter mean diameter (D3,2), and volume-weighted mean diameter (D4,3) of the emulsions droplets were studied for 30 days after preparation using a laser diffraction technique (Mastersizer X, Malvern Instruments, United Kingdom):

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RESULTS AND DISCUSSION Droplet Size Distributions. Regardless of the oil phase concentration, all emulsions studied (after aging for 24 h) showed two populations of droplets (see Figure 1A). In the range 30−45 wt %, the first peak is below 1 μm and the second population is centered at about 3 μm. An excess of mechanical energy input probably due to a recoalescence phenomenon is the cause of this second peak.14 Figure 1B shows the Sauter and volume mean diameters (D3,2 and D4,3, respectively) of emulsions with different dispersed phase concentrations aged 4128


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ACS Sustainable Chemistry & Engineering

flocculation takes place. Brownian motion, which is the dominant mechanism in emulsions with small droplet sizes, causes droplet collisions. This collision frequency increases with dispersed phase concentration in oil-in-water emulsions, so an increase in this concentration could provoke flocculation between oil droplets during the homogenization process.13 Physical Stability. The emulsions with 30, 35, and 50 wt % of oil phase concentration showed a decrease in backscattering (BS) in the lower zone of the measuring cell, which suggested the occurrence of a creaming destabilization process, which takes place because of the density difference between the oil phase and the aqueous phase.17 In Figure 2, the creaming index

Figure 2. Creaming index (CI) as a function of the aging time for emulsions with a dispersed phase concentration of 30 (squares), 35 (circles), and 50 wt % (diamonds). Samples kept under storage at 20 °C.

(CI) was plotted as a function of aging time for these emulsions. From this figure the kinetics of the creaming destabilization can be analyzed and quantified. As can be observed, after a delay period, a linear dependence of the CI with aging time occurred, the slope of which makes it possible to obtain the creaming rate (ω) (see Table 1). It was found that

Figure 1. (A) Droplet size distributions for emulsions aged for 24 h and (B) Sauter mean diameters and volumetric mean diameters for all emulsions aged for 1 and 30 days as a function of the oil phase concentration.

for 1 and 30 days. In general, for all emulsions, submicron mean diameters were achieved. A clear tendency for D3,2 to decrease from 320 to 280 μm with an increase in the oil concentration was observed in the range 30−45 wt %. By contrast, a minimum D4,3 value was achieved for a dispersed phase concentration of 35 wt %. Taking into account that the oil phase/emulsifier ratio was fixed, an increase in the oil phase concentration increased the viscosity of the aqueous phase (ηA). This fact could cause a change of the regime during the homogenization process from inertial to viscous, so ηA could influence the droplet mean diameters of the emulsions processed with rotor-stator devices.15,16 Nevertheless, emulsions aged for 24 h show a marked increase of D3,2 and D4,3 by increasing the concentration of the dispersed phase from 45 to 50 wt %. Taking into account the time evolution for 30 days of the mean diameters (D3,2 and D4,3) for the five emulsions, along with the ANOVA test results, the results obtained demonstrate that the emulsions in the range 30−40 wt % range did not undergo a significant coalescence process in 30 days. However, the results pointed to the occurrence of some coalescence for emulsions containing 45 and 50 wt % oil phase, which is favored by a previous flocculation mechanism, since coalescence occurs when the droplets are in contact for a long time. Flocculation between droplets tends to occur when the attractive interactions between droplets dominate the longrange repulsive interactions, but not the short-range repulsive interactions. The collision frequency between droplets and collision efficiency increases the rate at which droplet

Table 1. Creaming Rate (ω) and Turbiscan Stability Index (TSI) as a Function of Dispersed Phase Concentration oil phase concentration (wt %)

ω (%/day)

TSI40days

30 35 40 45 50

0.17 0.12

7.7 3.5 12.1 17.7 44.2

0.35

ω was very similar for 30−35 wt % systems although the emulsion with 35 wt % of oil phase exhibited the lowest value. An important change in this parameter was found for the 50 wt % emulsion, which exhibited the highest creaming rate value. In addition, this emulsion showed the highest CI after 30 days, which is consistent with the previous results since the creaming rate is influenced by the droplet size. Emulsions with 40 and 45 wt % of oil phase did not show destabilization by creaming. This result was in agreement with those obtained from both laser diffraction and rheological measurements (as can be seen later) due to the fact that these emulsions exhibited lower mean diameters and higher viscosities. According to McClements (2007), an emulsion with high viscosity and low mean diameters will be stable against creaming.13 The average value of the backscattering measurement in the central zone of the measuring cell (between 20 and 25 mm of 4129


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destabilization process by coalescence and by creaming. Some studies reveal that the influence of dispersed phase concentration on O/W emulsion physical stability is related to surfactant concentration. In this sense, if the surfactant concentration is not sufficient to cover all oil droplets, it may enhance instability via bridging flocculation.20 Considering all destabilization processes, the TSI parameter was obtained (see Table 1). The results revealed that the emulsion formulated with a total dispersed phase concentration of 35 wt % was the most stable. This result is in concordance with the previous analysis taking into account that TSI is a parameter that measures all mechanisms involved in the destabilization of these emulsions: creaming, coalescence, and flocculation. Rheological Properties. As can be seen in Figure 4, all studied emulsions exhibited (24 h after preparation) a

sample height) has been plotted as a function of aging time in Figure 3. In general, this evolution can be interpreted as an

Figure 3. Destabilization kinetics in the 20−25 mm zone monitored over some days for all studied emulsions as a function of the dispersed phase concentration.

increase in the emulsion droplet size by coalescence or flocculation. To illustrate this behavior the BS was fitted to an exponential equation (R2 > 0.99), typically used for a firstorder kinetic model:18 ΔBS = ΔBSE + (BS0 − ΔBSE)exp(−kt )

(7)

where ΔBS stands for the variation of the BS as a function of aging time, ΔBSE is the corresponding decrease in BS when equilibrium is reached, BS0 is the initial value of backscattering, and k is the first-order kinetic coefficient. This kinetic equation has proved useful to monitor the physical stability of oil-inwater emulsions.19 Table 2 lists the values of eq 7 parameters as a function of dispersed phase concentration. The first-order kinetic coef-

Figure 4. Flow curves of emulsions aged for 24 h as a function of the dispersed phase concentration. Continuous lines illustrate the data fitting to the Cross equation. Standard deviation of the mean (three replicates) for η < 8%. Temperature = 20 °C.

pseudoplastic behavior that fitted the Cross model fairly well (R2 > 0.99): η0 η= 1−n γ̇ 1 + γ̇

Table 2. Fitting Parameters of the First-Order Kinetic Equation for the BS in the 20−25 mm Zone of the Measuring Cell versus Aging Time as a Function of the Dispersed Phase Concentration oil phase concentration (wt %)

ΔBSE (%)

BS0 − ΔBSE (%)

30 35 40 45 50

−0.47 −1.75 −3.38 −15.16 −22.71

2.22 1.70 3.23 15.37 21.82

k (s−1) 2.2 2.6 2.7 3.3 9.2

× × × × ×

10−7 10−7 10−7 10−7 10−7

() c

(8)

η0 is the zero-shear viscosity, γ̇ is related to the critical shear rate for the onset of shear-thinning response, and (1−n) is a parameter related to the slope of the power-law region; n being the so-called power-law index. For emulsions aged for 24 h, an increase in the dispersed phase concentration from 30 to 40 wt % yielded an increase in the zero shear viscosity and a decrease in the flow index (see Figure 5A and B), which indicates that oil concentration increases emulsion structure via droplet interaction improvement.21 Zero-shear viscosity and power-law index values for the emulsions containing 40 and 45 wt % showed no significant differences. The decrease in η0 values for the 50 wt % emulsion aged for 1 day may be due to the aforementioned increment in the droplet size. Figure 5A and B also show zero shear viscosity and powerlaw index values at different aging times for all emulsions studied. The η0 increases slightly with aging time for the 30 wt % emulsion. The increase in η0 and the decrease in n with aging time indicate a higher concentration of the dispersed phase in the upper part of the sample.22 This fact is related to an incipient flocculation and/or incipient creaming.23 Emulsions with 35 and 40 wt % did not show any significant changes in η0

R2 0.96 0.97 0.98 0.99 0.98

ficient (k), the most relevant parameter, exhibited a tendency to increase with the rise in the relative concentration of the oil phase, indicating a higher rate of increase in droplet size. Nevertheless, the low values of this parameter should be noted for emulsions with smaller oil phase concentrations. Therefore, it can be stated that no significant changes in droplet size associated with a coalescence/flocculation phenomenon were detected. In contrast, the emulsions with a greater oil content (45 wt % and specially 50 wt %) exhibited higher values of k, indicating a significant increase in droplet size with aging time as a consequence of a destabilization process by coalescence, as was demonstrated by the evolution of the mean diameters obtained by laser diffraction. The emulsion with the highest dispersed phase concentration (50 wt %) underwent a 4130


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Figure 5. (A) Zero-shear viscosity and (B) power-law index for all emulsions studied as a function of aging time and dispersed phase concentration. Standard deviation of the mean (three replicates): η0 < 8% ; n < 5%. Temperature = 20 °C. Figure 6. (A) Influence of the dispersed phase concentration on the mechanical spectra for emulsions studied aged for 24 h. (B) Influence of aging time on the G′ and G″ moduli at a frequency of 1 Hz for emulsions studied. Temperature = 20 °C.

and n with aging time. On the contrary, the zero shear viscosity decreases with aging time for the emulsions of 45 and 50 wt %, which is an indication of a coalescence process. The 40 wt % emulsion exhibited a coalescence process in multiple light scattering and laser diffraction measurements whereas 50 wt % showed creaming and coalescence. In addition, n increases from day 1 to day 30 for the 40 and 50 wt % emulsions, which is an indication of a decrease in shear-thinning behavior due to the increment in droplet size. Figure 6A illustrates the mechanical spectra for those emulsions aged for 24 h that exhibited a measurable linear viscoelastic range. The results obtained corresponded to the socalled plateau relaxation zone. The emulsion containing 35 wt % of dispersed phase shows lower values of G′ than G″ in the lower frequency regime and G″ is greater than G″ in the higher frequency regime, as a consequence of a crossover point between G′ and G″. All samples above 35 wt % dispersed phase displayed clear viscoelastic properties with a predominance of the elastic over the viscous component in the frequency range studied. G′ turned out to be relatively independent of frequency, which is typical of gel-like materials. The addition of oil phase caused the formation of a weaker structure above 45 wt %, which is consistent with laser diffraction and flow curves results. Figure 6B shows G′ and G″ at 1 Hz for the emulsions that exhibited a measurable linear viscoelastic range as a function of time. Results of the ANOVA test demonstrated that there are

no significant differences in G′ and G″ values of the emulsions with aging time in the (35−40) wt % range. However, G′ and G″ values for 45 and 50 wt % experienced an important decrease with aging time, which could be associated with a coalescence phenomenon. From the results obtained, it can be stated that the rheological properties and physical stability of emulsions studied were strongly influenced by the oil phase concentration. All emulsions exhibited shear thinning behavior, which fitted the Cross model. An increase in the dispersed phase concentration from 30 to 45 wt % yielded an increase in the zero shear viscosity and a decrease in the flow index. All samples above 35 wt % dispersed phase showed viscoelastic properties with a predominance of the elastic over the viscous component in the frequency range studied. Both the flow curves and mechanical spectra were sensitive enough to detect these slight structural changes as a consequence of the destabilization processes. The laser diffraction technique revealed submicron droplet sizes for all emulsions studied, which tended to grow with aging time by coalescence in the most concentrated emulsions (45−50 wt %). Multiple light scattering demonstrated that the 30, 35, and 50 wt % emulsions were destabilized by creaming. Namely, emulsions with smaller 4131


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(14) Jafari, S. M.; Assadpoor, E.; He, Y.; Bhandari, B. Re-coalescence of emulsion droplets during high-energy emulsification. Food Hydrocolloids 2008, 22 (7), 1191−1202. (15) Santos, J.; Calero, N.; Muñoz, J. Optimization of a Green Emulsion Stability by Tuning Homogenization Rate. RSC Adv. 2016, 6, 57563−57568. (16) Vankova, N.; Tcholakova, S.; Denkov, N. D.; Ivanov, I. B.; Vulchev, V. D.; Danner, T. Emulsification in turbulent flow: 1. Mean and maximum drop diameters in inertial and viscous regimes. J. Colloid Interface Sci. 2007, 312 (2), 363−380. (17) Vanapalli, S. A.; Palanuwech, J.; Coupland, J. N. (2002). Stability of emulsions to dispersed phase crystallization: effect of oil type, dispersed phase volume fraction, and cooling rate. Colloids Surf., A 2002, 204 (1), 227−237. (18) Pal, R. Shear viscosity behavior of emulsions of two immiscible liquids. J. Colloid Interface Sci. 2000, 225 (2), 359−366. (19) García, M. C.; Alfaro, M. C.; Calero, N.; Muñoz, J. Influence of polysaccharides on the rheology and stabilization of α-pinene emulsions. Carbohydr. Polym. 2014, 105, 177−183. (20) Chuah, A. M.; Kuroiwa, T.; Kobayashi, I.; Nakajima, M. The influence of polysaccharide on the stability of protein stabilized oil-inwater emulsion prepared by microchannel emulsification technique. Colloids Surf., A 2014, 440, 136−144. (21) Dokić, L.; Krstonošić, V.; Nikolić, I. Physicochemical characteristics and stability of oil-in-water emulsions stabilized by OSA starch. Food Hydrocolloids 2012, 29 (1), 185−192. (22) Santos, J.; Trujillo-Cayado, L. A.; Calero, N.; Muñoz, J. Physical characterization of eco-friendly O/W emulsions developed through a strategy based on product engineering principles. AIChE J. 2014, 60 (7), 2644−2653. (23) Santos, J.; Calero, N.; Muñoz, J. Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions. Chem. Eng. Res. Des. 2015, 100, 261−267.

droplet sizes and higher viscosity exhibited enhanced stability against creaming. The most stable emulsion of all those obtained was that prepared with 35 wt % of oil phase.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 954 557180. Fax: +34 954 556447. E-mail address: [email protected]. ORCID

María C. Alfaro: 0000-0002-0110-2290 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support received (Project CTQ2015-70700) from the Spanish Ministerio de Economiá y Competitividad and from the European Commission (FEDER Programme) is kindly acknowledged. The authors are also grateful to BASF and KAO for providing materials for this research.

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REFERENCES

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Progress in the Formulation of Concentrated Ecological Emulsions for

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