- Email: [email protected]
α-pinene-in-water emulsions as influenced by surfactant concentration
Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Physical stability of N,N-dimethyldecanamide/␣-pinene-in-water emulsions as influenced by surfactant concentration ˜ L.A. Trujillo-Cayado, M.C. Alfaro ∗ , M.C. García, J. Munoz Reología Aplicada, Tecnología de Coloides, Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla c/P. García González, 1, E41012, Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 26 July 2016 Received in revised form 21 September 2016 Accepted 27 September 2016 Available online 11 October 2016 Keywords: Depletion flocculation Eco-friendly surfactant Emulsion Green solvent Microfluidization
a b s t r a c t In recent years, interest in submicron emulsions has increased due to their high stability and potential applications in the encapsulation and release of active ingredients in many industrial fields, such as the food industry, pharmaceuticals or agrochemicals. Furthermore, the social demand for eco-friendly solutions to replace hazardous solvents in many dispersion formulations has steadily risen. In this study, the influence of surfactant concentration on the formation and physical stability of submicron oil-in-water emulsions using a high-pressure dual-channel homogenizer (microfluidizer) has been investigated. The formulation involved the use of a blend of two green solvents (N,N-dimethyldecanamide and ␣-pinene) as dispersed phase and a nonionic polyoxyethylene glycerol ester derived from coconut oil as emulsifier (Levenol® C-201), which enjoys a European eco-label. Therefore, these emulsions may find applications as matrices for agrochemicals. Physical stability and rheological properties of the emulsions studied showed an important dependence on the eco-friendly surfactant concentration. The lowest surfactant concentration (1 wt%) yielded the onset of a creaming process after a short aging time and was not enough to avoid recoalescence during emulsification. On the other hand, the higher surfactant concentrations (4–5 wt%) resulted in depletion flocculation, which in turn triggered emulsion destabilization by coalescence. The optimum physical stability was exhibited by emulsions containing intermediate surfactant concentrations (2–3 wt%) since coalescence was hardly significant and the onset of a weak creaming destabilization process was substantially delayed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, the use of agrochemical products is associated with environmental pollution and the presence of toxic residues in the final products. An alternative is the use of low-risk pesticides and herbicides based on green solvents. These green solvents must be obtained from renewable materials and exhibit high biodegradability [1]. Emulsifiable concentrates (ECs) are among the most widely used agrochemical systems due to their versatility. However, in recent years there has been a strong trend to replace them with concentrated oil-in-water emulsions (EWs). The partial removal of oil results in less phytotoxicity and skin irritation to the customer. In addition, the sizes of droplets can be adjusted to an optimum distribution, which is important to increase biological efficacy [2]. Agrochemical products with small droplets and narrow droplet size distributions have been shown to improve biological activity [2].
∗ Corresponding author. E-mail address: [email protected] (M.C. Alfaro). http://dx.doi.org/10.1016/j.colsurfb.2016.09.043 0927-7765/© 2016 Elsevier B.V. All rights reserved.
Fatty acid dimethylamides are among the green solvents that can find applications in agrochemicals [3]. N,N-dimethyldecanamide (AMD-10TM ) is considered a safe biosolvent, according to the Environmental Protection Agency. Another biosolvent considered to be an interesting alternative to traditional solvents is ␣-pinene. It represents the major constituent of turpentine oils from the wood of most conifers and from leaf oils obtained from some herbs, such as juniper, rosemary or parsley [4]. For these reasons, the formulation of blends of these solvents in oil-in-water (O/W) concentrated emulsions has been studied in this work. The selection of the emulsifier used to stabilize the emulsion is crucial since it must fulfill several requirements; in addition to being able to stabilize an emulsion exhibiting suitable physicochemical properties, the surfactant must be biodegradable and non-hazardous for agricultural applications. Polyoxyethylene glycerol esters derived from coconut oil are non-ionic surfactants obtained from a renewable source. Their excellent wetting, interfacial and emulsifying properties are well documented [5–7]. These surfactants are fully innocuous to human
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
skin and hair and their properties are appropriate for the design of eco-friendly products [8–11]. Interestingly, these surfactants fulfill the environmental and toxicological requirements permitting their use as eco-label materials in Europe. One of these green surfactants, Levenol® C-201 (glycereth-17 cocoate) possesses an eco-label (DID list: 2133). One of the main challenges in the development of these O/W emulsions is the control of their physical stability in order to achieve an adequate shelf-life. Once emulsions are prepared, it is vital to detect at an early stage the onset of any destabilization process to shorten aging tests. In addition, determining the destabilization mechanisms provides outstanding feedback on formulation and processing variables [12]. Droplet size distributions (DSD) are perhaps the most important factor in determining properties like biological efficacy, rheology or shelf-life stability of emulsions [13]. On the whole, emulsions with smaller droplets and narrower DSDs result in longer stability. Double channel homogenization using a Microfluidizer (Microfluidics) is the methodology of choice if fluid-like emulsions with submicron mean diameters and narrow droplet size distributions are the targets, since they can reach extremely high shear rates [14]. Surfactant concentration is one of the key variables when dealing with emulsion formulation; hence it must be carefully controlled. On the one hand, a minimum emulsifier concentration is required to fully cover the available interfacial area created during the emulsification process. On the other hand, an excess of surfactant in solution will lead to the formation of a great number of micelles. It is well known that non-adsorbed surfactant micelles may induce the flocculation of the emulsion droplets due to a depletion mechanism [15,16]. This phenomenon, known as depletion flocculation, in turn, may trigger irreversible destabilization processes such as creaming and/or coalescence [17,18]. The main objective of this work was to study the influence of surfactant concentration on the physical stability of slightly concentrated O/W emulsions formulated with these eco-friendly solvents. A further goal was to prepare stable fine emulsions developed by double channel microfluidization, which may be find applications as matrices for the incorporation of active agrochemical ingredients. These emulsions not only contain an eco-friendly surfactant but also a blend of two green solvents, which contributes to the increasing demand for the development of more sustainable chemical dispersions.
155
2.2. Emulsification procedure The continuous phases were prepared by dissolving Levenol® C-201 surfactant in ultrapure water cleaned using a Milli-Q water purification system. Emulsions were prepared in two steps. First of all, 250 g of a coarse emulsion was prepared using an Ultraturrax T50 rotor-stator homogenizer with a toothed S50-G45F dispersion unit working at 4000 rpm for 120 s. Oil phase was slowly added for 30 s to the continuous phase and the temperature was fixed at 20 ◦ C using a semibatch set up assisted by a circulator to keep the temperature at the set point. Subsequently, secondary homogenization was carried out with a Microfluidizer (model M110P with an F12Y interaction chamber, Microfluidics, USA) at 15000 psi (103.42 MPa) in order to obtain lower droplet diameters and narrower droplet size distributions. Samples kept under storage at 20 ◦ C. 2.3. Emulsion droplet size analysis Size distributions of oil droplets were determined by laser diffraction using a Mastersizer X (Malvern, United Kingdom). All measurements were done three times for each emulsion 24 h after being prepared and results are reported as the mean and standard deviation. These measurements were carried out during 60 days of aging time to monitor the kinetics of emulsion destabilization by either coalescence or Ostwald ripening. The mean droplet diameter was expressed as Sauter mean diameter (D3,2 ) and volumetric mean diameter (D4,3 ):
N D3,2 =
i=1 N
n d2 i i=1 i
N D4,3 =
ni d3 i
i=1 N
ni d4 i
n d3 i i=1 i
(1)
(2)
where di is the droplet diameter, N is the total number of droplets and ni is the number of droplets having a diameter di. To determine the distribution width of droplet sizes, “span” was used, calculating from the following formula: span =
D(v, 0.9) − D(v, 0.5) D(v, 0.1)
(3)
Where D(v,0.9), D(v,0.5), D(v,0.1) are diameters at 90%, 50% and 10% of cumulative volume, respectively. 2.4. Physical stability 2. Materials and methods 2.1. Materials According to the composition of emulsions previously studied, 30 wt% O/W emulsions formulated with a non-ionic surfactant (glycereth-17 cocoate) at different concentrations were prepared using a mixture of green solvents, as dispersed phase. N,N-dimethyl decanamide (AMD-10TM ) and ␣-pinene were utilized with a 75/25 mass ratio [19]. Agnique AMD-10TM (density: 0.88 g/mL at 25 ◦ C) was kindly provided by BASF. ␣-Pinene (density: 0.84 g/mL at 25 ◦ C) was supplied by Sigma Chemical Company. The emulsifier used was a non-ionic surfactant derived from coconut oil. Namely, a polyoxyethylene glycerol fatty acid ester, glycereth-17 cocoate, received as a gift from KAO, was selected on account of its HLB number, 13. Its trade name is Levenol® C-201. The influence of Levenol® C-201 concentration on the stability and physicochemical properties of the emulsions was studied in the 1 to 5 wt% range.
Multiple light scattering measurements with a Turbiscan Lab Expert (Formulaction, France) were used in order to complete the study on emulsion destabilization. Measurements were carried out until 40 days at 20 ◦ C to determine the predominant mechanism of destabilization in each emulsion as well as the kinetics of the destabilization process. The Turbiscan Stability Index (TSI) is a parameter that can be used to estimate the physical stability of dispersions like suspensions or emulsions. This parameter is a statistical factor and its value is obtained as the sum of all processes taking place in the studied probe. An increase in TSI value means that the overall physical stability of the dispersion under study decreases. TSI values were calculated using the equation [11]: TSI = |scanref (hj ) − scani (hj )| (4)where scanref and scani are j the initial backscattering value and the backscattering value at a given time, respectively, hj is a given height in the measuring cell and TSI is the sum of all the scan differences from the bottom to the top of the vial.
156
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
2.5. Viscosity of continuous phases The viscosities of continuous phase solutions (from 1.43 wt% to 7.14 wt%) were measured with an Ubbelohde glass capillary viscometer. A volume of solution was pipetted into the capillary viscometer, which was equilibrated at 20 ◦ C in a water bath for 30 min prior to measurements. All the measurements were performed at 20 ◦ C ± 0.1 ◦ C and the result is the average of five measurements. Viscosity is obtained from the following equation: =C ××t
(5) (m2 /s2 )
Where (Pa s) is the viscosity of the continuous phase, C is a constant that depends on the glass capillary, (kg/m3 ) is the density of the continuous phase and t is the time. 2.6. Rheology of emulsions The rheological characterization involved flow curves and stress and frequency sweeps in small-amplitude oscillatory shear experiments (SAOS). Rheological experiments were conducted with a MARS controlled-stress rheometer (Haake, Germany) using a Z20 coaxial cylinder geometry (Ri = 1 cm, Re /Ri = 1.085) for the curves and a double-cone geometry (angle: 0.017 rad; diameter: 60 mm) for SAOS tests. A sandblasted surface treatment was used for the sensor systems to prevent wall-depletion (slip effects) from being significant under non-linear flow behaviour. Special surface sensor systems must be used when carrying out rheological tests with different types of dispersions as reviewed by Buscall (2010) [20]. Flow curve tests were run by stepped shear rate ramps determined in controlled-stress mode (0.05–2 Pa). An approximation to steady state response of 0.01% for each step was used, fixing a maximum time per point of 180 s as cut off criterion. Stress sweeps at three different frequencies (0.1, 1 and 3 Hz) were carried out in order to determine the linear viscoelastic range. Stress sweeps were performed in a range of 0.05 to 5 Pa. Subsequently, frequency sweeps were conducted at a stress lower than the critical stress that was previously determined by the aforementioned stress sweeps. Frequency sweep tests (from 20 to 0.02 rad s−1 ) were performed selecting a stress well within the linear viscoelastic range. To follow the effect of aging time, all rheological measurements were carried out 1, 3, 13, 21 and 40 days after preparation. These measurements were performed at 25 ◦ C ± 0.1 ◦ C, using a C5P Phoenix circulator (Thermo-Scientific, USA) for sample temperature control. Equilibration time prior to rheological tests was 180 s. All measurements were repeated 3 times for each emulsion. Sampling from the top part of the container in contact with air was avoided, so samples were taken at about 2 cm below the upper part of the container. Results represent the mean of three measurements. 2.7. Statistical analysis Resulting data from laser diffraction, multiple light scattering and rheological measurements have been analyzed using oneway analysis of variance (ANOVA) using StatPlus® :mac. Differences between means were determined using the Tukey test. All statistical calculations were conducted at a significance level of p = 0.05. 3. Results and discussion 3.1. Droplet size distributions Fig. 1A shows the droplet size distribution of emulsions with different surfactant concentration aged for one day. First of all, all emulsions exhibited bimodal distributions and an overwhelming number of droplets with submicron diameter. In all cases,
Fig. 1. (A) Droplet size distributions for emulsions containing different surfactants concentrations aged for 1 day and (B) Sauter (D3,2 ) and volumetric mean mean diameter (D4,3 ) of oil droplets or emulsions aged for 1 and 40 days aging time as a function of surfactant concentration. Standard deviation of the mean (three replicates) for D3,2 < 5% and D4,3 < 7%.
the main peak is below one micron and the second population is centred at about three microns. The occurrence of the second population of droplets is probably due to a re-coalescence phenomenon, induced by an excess of mechanical energy input during the emulsification process [21]. Moreover, the droplet size distributions markedly shifted to the left, i.e. to lower droplet diameters, when increasing Levenol® C-201 bulk concentration from 1 wt% to 2 wt% and roughly leveled off for surfactant concentrations above 2 wt%. However, a closer inspection of the results revealed that D3,2 values showed a steady fall with surfactant concentration in the (1 wt%–5 wt%) Levenol® C-201 range, which was significant according to the statistical analysis carried out (p < 0.05, p-value = 0.00). D3,2 values for the 3 and 4 wt% emulsions aged for one day are not significantly different from each other (Tukey test, p > 0.05). As far as the D4,3 values are concerned, a significant decrease was observed in the (1 wt%–5 wt%) Levenol® C-201 range as supported by standard deviation data and the ANOVA test (p< 0.05, p-value = 0.00). All D4,3 values for the emulsion aged for one day were significantly different from each other except for 2 and 3 wt% (Tukey test, p> 0.05). Fig. 1B shows the Sauter and volumetric mean diameters of emulsions with different Levenol® C-201 concentration aged for 1 and 40 days in order to check if destabilization by coalescence or
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
157
Table 1 Span, flow curves fitting parameters and fitting correlation coefficients for all emulsions studied aged for 24 h. Standard deviation of the mean (three replicates) for span < 5%, 1 < 8% and n < 5%. Surfactant (wt%)
Span
1 (mPa)
n
R2
1 2 3 4 5
1.58 ± 0.07 1.46 ± 0.06 1.37± 0.04 1.39 ± 0.04 1.40 ± 0.04
4.75 ± 0.40 (a) 17.40 ± 1.32 (a) 53.19 ± 4.31 (b) 162.68 ± 12.98 (c) 299.5 ± 24.11 (d)
0.99 ± 0.01 (a) 0.84 ± 0.01 (b) 0.68 ± 0.01 (c) 0.53 ± 0.01 (d) 0.44 ± 0.02 (e)
0.994 0.998 0.995 0.998 0.997
Ostwald ripening takes place. For all emulsions submicron Sauter and volumetric mean diameter values were obtained (see Table 1). An increase in surfactant concentration up to 5 wt% resulted in a marked drop in volume mean diameters after 24 hours aging time. It is well documented that emulsions with submicron diameters show high stabilities against creaming and coalescence [22]. Furthermore, the span values (droplet size polidispersity) of these emulsions ranged from 1.37 to 1.58, dropping to a minimum around 3 wt%. It has been reported that such an increase in polydispersity exerts a marked influence on the destabilization process of emulsions leading to a rise in the creaming rate due to higher values of the effective packing parameter [12], which in turn result in lower viscosity as predicted by the Krieger-Dougherty equation [23]. The time evolution of D3,2 and D4,3 after 40 days is consistent with the occurrence of coalescence or Ostwald ripening for some of the emulsions studied. ANOVA tests (p < 0.05) supported the occurrence of significant increments in the Sauter and volumetric mean diameters for the 1 wt% (D3,2 : p-value = 3.30 × 10−12 ; D4,3 : p-value = 1.47 × 10−5 ), 4 wt% (D3,2 : p-value = 1.05 × 10−8 ; D4,3 : pvalue = 2.13 × 10−13 ) and 5 wt% (D3,2 : p-value = 1.47 × 10−5 ; D3,3 : p-value = 9.65 × 10−18 ) Levenol® C-201 emulsions. By contrast, 2 wt% (D3,2 : p-value = 1.00; D4,3 : p-value = 5.53 × 10−2 ) and 3 wt% (D3,2 : p-value = 1.00; D4,3 : p-value = 1.74 × 10−1 ) did not show a significant increment with aging time for Sauter and volumetric mean diameters. It is noteworthy that stable submicron emulsions were obtained at Levenol® C-201 surfactant concentrations within 2 and 3 wt%. The increase in the D3,2 and D4,3 values of the emulsion with less surfactant (1 wt%) can be ascribed to the occurrence of coalescence, enhanced by creaming of the bigger droplets. The increase in the second peak with aging time usually points to the occurrence of a destabilization process by coalescence instead of an Ostwald ripening phenomenon. For Ostwald ripening the particle size distribution should exhibit a specific time-independent form that shifts to the right along the X-axis with time, whereas when coalescence is the dominant destabilization mechanism a bi-modal distribution is usually observed [24]. On the one hand, coalescence becomes the most important phenomenon at low surfactant concentration, when drops are not completely covered with surfactant [25] or the surfactant layers surrounding the oil droplets are not strong enough; hence they are partially destroyed during the emulsification process (recoalescence) or cannot withstand collisions among droplets after the emulsion preparation. On the other hand, a depletion flocculation process may trigger coalescence in emulsions containing the higher Levenol® C-201 surfactant concentrations (4–5 wt%). This mechanism is provoked by the exclusion of surfactant micelles between neighbouring oil droplets because of an osmotic pressure effect, which first results in flocculation and subsequently may cause the coalescence of emulsion droplets [26]. 3.2. Physical stability A decrease in backscattering at the bottom of the measuring cell was observed in samples with a Levenol® C-201 surfactant concen-
Fig. 2. (A) Increase of droplet diameter from the diameter at time zero and (B) creaming index (CI) as a function of the aging time for emulsions containing a total surfactant concentration ranged for 1–3 wt%. Samples kept under storage at 20 ◦ C.
tration of 1, 2 and 3 wt%, which indicated the onset of a creaming process after a given aging time. In the case of emulsions containing 4 and 5 wt% Levenol® C-201, an overall decrease in backscattering with aging time from the bottom to the top of the measuring cell was observed. This was especially noticeable in the middle part of the sample. On the whole, this evolution can be interpreted either as a flocculation process or as an increase in the emulsion droplet size. Given that laser diffraction measurements demonstrated that coalescence did take place after emulsions containing 1 wt%, 4 wt% and 5 wt% surfactant aged for 40 days, the evolution of backscattering may be attributed to coalescence rather than flocculation of oil droplets. Fig. 2A shows the increase in droplet diameter, taking the initial value as reference (D), as a function of aging time for all emulsions. This plot makes it possible to monitor the kinetics of the coalescence process. The D value has been calculated using the following equation: D =
Dt − D0 D0
(6)
where, D0 and Dt are the initial diameter value and the diameter value at a given time, respectively. A high increase in droplet size over time was detected for emulsions with 1, 4 and 5 wt% of surfactant concentration. Interestingly, the shortest delay time for the onset of coalescence and the greatest increase in mean droplet diameter were shown by the emulsion containing the highest surfactant concentration. In contrast, the 2 wt% and 3 wt% surfactant emulsions exhibited the longer (above 30 days) delay times for the onset of coalescence and the smaller oil droplet diameter increase
158
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
Fig. 3. Turbiscan Stability Index (TSI) as a function of surfactant concentration in the bottom, middle and global zones of the measurement cell. Standard deviation of the mean (three replicates) for TSI < 8%.
with aging time. The best results were exhibited by the 3 wt% emulsion. These results are consistent with the variation of Sauter and volumetric mean diameters found for these emulsions by laser diffraction. The creaming process was analyzed by using the creaming index concept (CI) [12]: CI =
HS × 100 HE
(7)
where HE is the total height of the emulsion and HS is the height of the serum layer. The calculated creaming indexes for emulsions with surfactant concentrations ranging from 1 wt% to 3 wt% are plotted in Fig. 2B as a function of aging time.The onset of creaming can be detected by a decrease in backscattering at the bottom of the measuring cell. This phenomenon could not be detected for the 4–5 wt% emulsions. The initial slope of the plot of CI versus aging time is related to the creaming rate (, mm/day): ω=
HE d(CI) × 100 dt
(8)
Fig. 2B illustrates the decrease in CI and and the higher delay times for the onset of creaming (see the inset of Fig. 2B) with increasing surfactant concentration from 1 wt% to 3 wt%. The latter emulsion exhibited the best results not only for the creaming index but also for stability against coalescence (Fig. 2A). Emulsions in the (1–3) wt% Levenol® C-201 concentration range became more stable against creaming as surfactant concentration increased due to the combination of a reduction in droplet sizes, lower polidispersity and the increase in emulsion viscosity. An overall analysis of the multiple light scattering results reveals that although emulsions containing 4 wt% and 5 wt% Levenol® C201 showed the best results against destabilization by creaming, they underwent immediate destabilization by flocculation and/or coalescence. However, it is important to make clear that the multiple light scattering technique itself does not make it possible to distinguish between flocculation and coalescence since both mechanisms provoke a variation in backscattering in the middle zone of the measuring cell. Laser diffraction demonstrated that oil flocs are broken down during the measurement test. In contrast, multiple light scattering is quite suitable to detect the onset of creaming, as it would involve a decrease in backscattering at the lower zone of the measuring cell. Nevertheless, this fact was not experimentally observed and on top of this, destabilisation by creaming could not be observed by the naked eye in emulsions aged for a long time. Fig. 3 illustrates that the emulsion with 1 wt% surfactant exhibited the highest value of TSI at the bottom of the sample placed
Fig. 4. Flow curves of emulsions aged for 24 h as a function of surfactant concentration. Continuous lines correspond to the power-law fitting equation. Standard deviation of the mean (three replicates) for 1 < 8%. Temperature = 20 ◦ C.
in a Turbiscan measuring cell. This may be ascribed in this case to a creaming phenomenon, which, in turn, must stem from the fact that this emulsion showed the highest D32 , D43 and span values and the lowest viscosity (Table 1). As far as the emulsions containing (2–3) wt% Levenol® C-201 are concerned, the fact that TSI remained nearly constant in the intermediate zone suggests that destabilization mechanisms like flocculation and coalescence were not significant. Conversely, the higher TSI values in the intermediate zone for emulsions formulated with 4 or 5 wt% surfactant confirmed that they underwent depletion flocculation, as described above, and coalescence, as experimentally checked by laser diffraction (Fig. 1B). Taking into account the Global TSI values, results of the ANOVA test demonstrated that there are significant differences in the physical stability of the emulsions studied (p< 0.05, p-value = 0.00). The emulsions containing 2 wt% and 3 wt% Levenol C-201 are significantly different from each other (Tukey test, p> 0.05), being the 3 wt% emulsion the most stable. 3.3. Rheology of emulsions Fig. 4 shows the flow properties of emulsions aged for 24 h as a function of surfactant concentration. The emulsion containing 1 wt% surfactant exhibited Newtonian behaviour ( = 5.02 mPa·s) at 20 ◦ C. On the other hand, all emulsions in the (2–5) wt% concentration range exhibited shear-thinning behaviour. It must be noted that the rheological behaviour of emulsions aged for 24 h qualitatively shifted from Newtonian to non-Newtonian when increasing the Levenol® C-201 concentration by just 1 wt% (from 1 wt% to 2 wt%). This is consistent with a marked drop in D4,3 , which must result in a substantial increase in the concentration of droplets (number of droplets per volume) and also in D3,2 values, which yields greater values of droplet surface and more friction under shear flow. In addition the non-Newtonian response is a clear indication of the formation of oil flocs, due probably to a depletion flocculation mechanism induced by the presence of surfactant micelles in the continuous phase [26]. The fluid-like behaviour of these emulsions at 20 ◦ C was supported by their relatively low apparent viscosity values. By way of example, they increased at the maximum shear stress tested (2 Pa), from ca. 5 mPa s for the Newtonian emulsions containing 1 wt% Levenol® C-201 to just ca. 30 mPa s when the surfactant concentration was 5 wt%. Results for emulsions in the (2–5) wt% concentration range are consistent with the formation of slightly
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
159
Table 2 Continuous phases density and viscosity values at 20 ◦ C. Note: These surfactant concentrations in continuous phases correspond to 1, 2, 3, 4 and 5 wt% in emulsions respectively. Standard deviation of the mean (five replicates) for 20 ◦ C < 1% and 20 ◦ C < 1%.
Fig. 5. (A) Shear stress at 1 s−1 and (B) power-law index for all emulsions studied as a function of aging time and surfactant concentration. Standard deviation of the mean (three replicates) for 1 < 8% and n < 5%. Temperature = 20 ◦ C.
concentrated emulsions which are prone to form a weakly flocculated microstructure [2]. Their shear-thinning behaviour can be attributed to a shear-induced deflocculation process. Flow curves fitted the power-law model (R2 > 0.99):
. n
=K×
(9)
Where (Pa) is the shear stress, K (Pa·sn ) is nowadays called “consistency”, n is the power law index and ˙ (s−1 ) is the shear rate. Pa·sn are the units of parameter consistency, which depend on the power law index values. This prevents the making of a direct comparison of this parameter for samples with different values of the power law index. To overcome this problem, we used a modified power law equation [11]:
= 1 ×
.
1s−1
n (10)
where 1 stands for the shear stress at 1 s−1 . For shear thinning materials, 0 < n < 1. A solid material would show n = 0, while a Newtonian liquid would show n = 1. The values of fitting parameters corresponding to Eq. (10) are shown in Table 1 for emulsions aged for 1 day. In addition, these values are shown in Fig. 5A and B as a function of surfactant concentration and aging time. Table 1 illustrates the significant (according to the ANOVA test; p < 0.05) effects of both 1 (p-
Surfactant (wt%)
20 ◦ C (kg/m3 )
20 ◦ C (mPa s)
1.43 2.86 4.29 5.71 7.14
1.0001 ± 0.0001 1.0017 ± 0.0001 1.0027 ± 0.0001 1.0036 ± 0.0001 1.0046 ± 0.0001
1.52 ± < 0.01 2.84 ± 0.01 5.58 ± 0.05 17.93 ± 0.08 35.85 ± 0.14
value = 2.28 × 10−10 ) and n (p-value = 7.86 × 10−13 ) on emulsions aged for 24 h containing different surfactant concentrations. Means with the same letter are not significantly different from each other (Tukey test, p > 0.05). The increase in 1 , and consequently in apparent viscosity, as well as the drop in power law index values with surfactant concentration for emulsions aged for 1 day may be due to the formation of droplets with progressively lower mean diameters [27] as well as to the increasingly greater number of micelles in the continuous phase, which yields a steady increase in viscosity of the continuous phase (see Tables 1 and 2) and also to the formation of increasingly flocculated emulsions due to depletion flocculation induced by surfactant micelles in the continuous phase [28,29]. As far as the role of the continuous phase is concerned, Table 2 shows the viscosity of continuous phases prior to emulsification. Increasing the surfactant concentration from 1.43 wt% to 7.14 wt%, which correspond to 1 wt% and 5 wt% in the final emulsions, respectively, yielded a rise in viscosity from 1.52 mPa s to 35.85 mPa s. Nonetheless, the increase in apparent viscosity at 1s−1 and 1 of emulsions was about two orders of magnitude (compare Tables 1 and 2). This increment indicates the important contribution of oil droplets to the emulsion viscosity at a relatively low shear rate. With regard to the effects of aging on the flow properties of the emulsions studied, it must be emphasized that sampling was made from the upper part of the container in order to detect at an early stage the expected results for destabilization by creaming or coalescence. The flow curves of the emulsion containing 1 wt% Levenol® C-201 surfactant exhibited slightly increasing values of 1 and also a weak fall of power law indexes for 21 days. Interestingly, the evolution of the rheological parameters for this emulsion involved a shift from Newtonian to shear thinning behavior. This was consistent with the fast creaming rate detected by multiple light scattering, even though it could not be observed by the naked eye (Fig. 2B). Flow curve results were sensitive to the formation of the soft network formed by loosely packed flocs of oil droplets as also pointed out by Trujillo-Cayado et al. (2016) for AMD10/␣pinene emulsions [19]. At low shear stresses and low shear rates the oil network is hardly disrupted by the shear field. This yields greater values of 1 for slightly creamed non Newtonian emulsions if compared with the Newtonian emulsion, whose microstructure must essentially consist of randomly located droplets throughout the emulsion. The slight drop in power law indexes (i.e. the fact that emulsions became more shear thinning with aging time), was likely due to the increasing relative concentration of oil droplets in the upper part of the sample. A straightforward interpretation of the shear thinning behavior involves firstly a network breakdown and subsequently the shear-induced orientation of droplets in the flow direction. Subsequently, coalescence started competing with creaming as supported by the significant increase in mean droplet diameter detected by multiple light scattering from day 21 onwards (Fig. 2A). Coalescence was confirmed by laser diffraction (Fig. 1B). The occurrence of simultaneous destabilization mechanisms hin-
160
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
ders the microstructural interpretation of rheological results [30]. Creaming and coalescence exert opposite effects. In the previous paragraph the increasing viscosity promoted by creaming has comprehensively been explained. Conversely, coalescence results in a decrease in viscosity on account of both the lower interfacial surface available for friction and in the decrease of oil droplet concentration due to the higher volumetric mean diameters. Both effects must counterbalance each other such that 1 and n levelled off from 21 days onwards (see Figs. 1B and 2B). The clear drop in the shear stress at 1 s−1 and the increase in power law index values for emulsions containing the higher surfactant concentrations studied (4–5) wt% (Fig. 5A and B) highlighted a dramatic change in the effect of aging time on the rheology and microstructure of the emulsions studied. Both rheological parameters indicate that coalescence was the dominant destabilization mechanism. These results were in agreement with those obtained by laser diffraction and multiple light scattering (Figs. 1B, 2A and B). Interestingly, the emulsions containing 2 wt% and 3 wt% Levenol® C-201 exhibited stable rheological parameters for the whole observation time selected to monitor possible aging effects. The emulsion containing 3 wt% Levenol® C-201 showed higher value of 1 and more marked shear thinning properties than the emulsion with 2 wt% Levenol® C-201, which supports the statement that the optimum Levenol® C-201 concentration to enhance the physical stability of 30 wt% N,N-dimethyl decanamide (AMD10TM /␣-pinene, 75/25) in water emulsions was around 3 wt% A further task was to assess the suitability of linear dynamic viscoelastic tests to monitor the physical stability of the emulsions studied. Small amplitude oscillatory shear tests are widely used to determine the linear viscoelastic behaviour of a fascinating variety of materials on account of their sensitivity to changes of microstructure. In order to ensure that the test is carried out under non-intrusive conditions, i.e. it does not provoke structural destruction in the sample, they must be performed at stress and strain amplitudes within the linear viscoelastic range (LVR). Therefore, oscillatory torque sweep tests at three different frequencies (0.1, 1 and 3 Hz) were previously conducted in order to estimate the maximum amplitude value of the sinusoidal shear stress function and the corresponding maximum strain amplitude that guaranteed linear viscoelastic behaviour. Small amplitude oscillatory shear (SAOS) measurements are sensitive to detect slight structural changes as a consequence of droplet flocculation. Only the emulsion containing 5 wt% Levenol® C-201 exhibits both measurable dynamic viscoelastic functions due to a higher grade of flocculation (induced by a depletion flocculation mechanism) and an increase of droplet-droplet interactions that produced a stronger structure than those obtained for 1–4 wt% emulsions. Fig. 6 shows the angular frequency dependence of the storage modulus (G’) and the loss modulus G” (i.e. the mechanical spectra) as a function of aging time. The former viscoelastic function is related to the elastic response of the sample, while the latter is related to the viscous response. The mechanical spectra corresponding to aging times of 1, 3 and 10 days are typical of semi-dilute dispersions since G” is dominant over G” at lower frequencies, while the reverse takes place at higher frequencies. Therefore, G’ values cross over those of G” at an intermediate angular frequency. The crossover frequency allows the onset of the so-called terminal relaxation zone to be estimated as the reciprocal of such an angular frequency. In this way, the shorter the crossover frequency is, the longer is the terminal relaxation zone; which means that the material under study will have more marked solid-like properties. The sensitivity of the crossover frequency and of the corresponding terminal relaxation has been found useful when checking (a) the decay of “viscoelasticity”, or rather of the elastic (solid-like) properties of suspoemulsions upon increasing temperature [31], (b) the relatively similar viscoelastic properties exhibited by galac-
Fig. 6. Influence of aging time on the mechanical spectra for the emulsion formulated whit a surfactant concentration of 5 wt%. Shear stress amplitude: 0.1 Pa.
tomannan solutions regardless of their raw materials [32] and (c) the influence of chitosan concentration and temperature on the viscoelasticity of its aqueous solutions [33]. Interestingly, mechanical spectra were reported to be sensitive to creaming percentage for more concentrated emulsions since the plateau relaxation zone became wider with increasing creaming [34]. The results obtained in this study show that aging from 1 to 10 days resulted in greater values of the crossover angular frequency (*), i.e. lower terminal relaxation times. They dropped from 10.8 s after aging for 1 day to 0.5 s after 10 days. This means that this emulsion became more fluid-like with aging time. In addition, G’ and G” values clearly decreased with aging time. All these results are consistent with the occurrence of predominant coalescence, induced by a previous depletion flocculation process as demonstrated by laser diffraction and multiple light scattering and indicated by the evolution of flow curves with aging time. Enhanced coalescence involves lower interfacial surface available for friction under shear and a decrease in the effective droplet concentration; i.e. lower number of droplets per volume. In other words, extensive coalescence tends to mean that viscoelastic responses become hardly measurable. 4. Conclusions O/W emulsions containing 30 wt% admixtures of the green solvents N,N-dimethyl decanamide and ␣-pinene with 75/25 mass ratio and formulated with a polyoxyethylene glycerol fatty acid ester (HLB 13) as emulsifier can be prepared such that submicron mean Sauter and volumetric mean diameters can be achieved. The processing conditions consist of a primary emulsification step consisting of a semibatch rotor-stator equipped with a toothed rotor working at 4000 rpm for 120 s, followed by a secondary homogenization step carried out at 103.42 MPa using a Microfluidizer with a double channel interaction chamber. Surfactant concentration exerted a marked influence on the droplet size distribution, rheological properties and physical stability of emulsions. The cooperative information provided by laser diffraction, multiple light scattering and rheology allows the physical destabilization of emulsions to be monitored and the dominant destabilization mechanism to be identified. The emulsion containing 1 wt% of the nonionic Levenol® C-201 surfactant undergoes fast creaming after a rather short aging time, although coalescence later becomes the dominant destabilization mechanism as demonstrated by the cooperative information pro-
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
vided by multiple light scattering, shear flow curves and laser diffraction. Depletion flocculation triggers coalescence, which becomes the main destabilization mechanism for emulsions formulated with the higher surfactant concentrations (4 wt% and 5 wt%). Emulsions containing 2 wt% and 3 wt% nonionic Levenol® C-201 surfactant present the best results. The emulsion with 3 wt% surfactant exhibits the longest physical stability since it shows the longest delayed times for the onset of the creaming and coalescence mechanisms. Acknowledgments The financial support received (Project CTQ2015-70700) from the Spanish Ministerio de Economía 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. References [1] M.J. Hernáiz, A.R. Alcántara, J.I. García, Applied biotransformations in green solvents, Chemistry 16 (31) (2010) 9422–9437. [2] Th.F. Tadros, Rheology of Dispersions. Principles and Applications, Wiley-VCH, 2009. [3] R. Höfer, J. Bigorra, Green chemistry—a sustainable solution for industrial specialties applications, Green Chem. 9 (3) (2007) 203–212. [4] S. Bertouche, V. Tomao, K. Ruiz, A. Hellal, C. Boutekedjiret, F. Chemat, First approach on moisture determination in food products using alpha-pinene as an alternative solvent for Dean–Stark distillation, Food Chem. 134 (1) (2012) 602–605. [5] E. Jurado, J.M. Vicaria, J.F. García-Martín, M. García-Román, Wettability of aqueous solutions of eco-friendly surfactants (ethoxylated alcohols and polyoxyethylene glycerin esters), J. Surfactants Detergents 15 (3) (2012) 251–258. ˜ [6] L.A. Trujillo-Cayado, P. Ramírez, L.M. Pérez-Mosqueda, M.C. Alfaro, J. Munoz, Surface and foaming properties of polyoxyethylene glycerol ester surfactants, Colloids Surf. A 458 (2014) 195–202. ˜ [7] L.A. Trujillo-Cayado, P. Ramírez, M.C. Alfaro, M. Ruíz, J. Munoz, Adsorption at the biocompatible ␣-pinene–water interface and emulsifying properties of two eco-friendly surfactants, Colloids Surf. B 122 (2014) 623–629. ˜ [8] J. Santos, L.A. Trujillo-Cayado, N. Calero, J. Munoz, Physical characterization of eco-friendly O/W emulsions developed through a strategy based on product engineering principles, AIChE J. 60 (7) (2014) 2644–2653. [9] J. Santos, L.A. Trujillo-Cayado, N. Calero, M.C. Alfaro, J. Munoz, Development of eco-friendly emulsions produced by microfluidization technique, J. Ind. Eng. Chem. 36 (2016) 90–95. ˜ [10] L.A. Trujillo-Cayado, M.C. Alfaro, A. Raymundo, I. Sousa, J. Munoz, Rheological behavior of aqueous dispersions containing blends of rhamsan and welan polysaccharides with an eco-friendly surfactant, Colloids Surf. B 145 (2016) 430–437. ˜ [11] L.A. Trujillo-Cayado, M.C. Alfaro, J. Munoz, A. Raymundo, I. Sousa, Development and rheological properties of ecological emulsions formulated with a biosolvent and two microbial polysaccharides, Colloids Surf. B 141 (2016) 53–58. [12] D.J. McClements, Critical review of techniques and methodologies for characterization of emulsion stability, Crit. Rev. Food Sci. Nutr. 47 (7) (2007) 611–649.
161
[13] D.J. McClements, Food Emulsions: Principles, Practices, and Techniques, CRC Press, Boca Ratón, USA, 2015. [14] S.Y. Tang, P. Shridharan, M. Sivakumar, Impact of process parameters in the generation of novel aspirin nanoemulsions-comparative studies between ultrasound cavitation and microfluidizer, Ultrason. Sonochem. 20 (1) (2013) 485–497. [15] J. Bibette, Depletion interactions and fractionated crystallization for polydisperse emulsion purification, J. Colloid Interface Sci. 147 (2) (1991) 474–478. [16] D.J. McClements, Ultrasonic determination of depletion flocculation in oil-in-water emulsions containing a non-ionic surfactant, Colloids Surf. A 90 (1) (1994) 25–35. [17] E. Dickinson, C. Ritzoulis, Creaming and rheology of oil-in-water emulsions containing sodium dodecyl sulfate and sodium caseinate, J. Colloid Interface Sci. 224 (1) (2000) 148–154. [18] S.J. Radford, E. Dickinson, Depletion flocculation of caseinate-stabilised emulsions: what is the optimum size of the non-adsorbed protein nano-particles? Colloids Surf. A 238 (1) (2004) 71–81. ˜ [19] L.A. Trujillo-Cayado, J. Santos, M.C. Alfaro, N. Calero, J. Munoz, A further step in the development of oil-in-water emulsions formulated with a mixture of green solvents, Ind. Eng. Chem. Res. 55 (27) (2016) 7259–7266. [20] R. Buscall, Letter to the Editor: wall slip in dispersion rheometry, J. Rheol. (1978-present) 54 (6) (2010) 1177–1183. [21] S.M. Jafari, Y. He, B. Bhandari, Production of sub-micron emulsions by ultrasound and microfluidization techniques, J. Food Eng. 82 (4) (2007) 478–488. [22] P. Izquierdo, J. Esquena, T.F. Tadros, C. Dederen, M.J. Garcia, N. Azemar, C. Solans, Formation and stability of nano-emulsions prepared using the phase inversion temperature method, Langmuir 18 (1) (2002) 26–30. [23] H.A. Barnes, A Handbook of Elementary Rheology. Aberystwwyth, The University of Wales Institute of Non-Newtonian Fluid Mechanics., United Kingdom, 2000. [24] E. Nazarzadeh, T. Anthonypillai, S. Sajjadi, On the growth mechanisms of nanoemulsions, J. Colloid Interface Sci. 397 (2013) 154–162. [25] B.P. Binks, Emulsions—Recent advances in understanding, in: Binks (Ed.), Modern Aspects of Emulsion Science, Royal Society of Chemistry, Cambridge, United Kingdom, 1998. [26] P. Manoj, A.D. Watson, D.J. Hibberd, A.J. Fillery-Travis, M.M. Robins, II. Steady-state rheological investigations, J. Colloid Interface Sci. 207 (2) (1998) 294–302. [27] H.A. Barnes, Rheology of emulsions—a review, Colloids Surf. A 91 (1994) 89–95. [28] R. Pal, Rheology of simple and multiple emulsions, Curr. Opin. Colloid Interface Sci. 16 (1) (2011) 41–60. [29] G.G. Palazolo, D.A. Sorgentini, J.R. Wagner, Coalescence and flocculation in o/w emulsions of native and denatured whey soy proteins in comparison with soy protein isolates, Food Hydrocolloids 19 (3) (2005) 595–604. [30] Th.F. Tadros, Application of rheology for assessment and prediction of the long-term physical stability of emulsions, Adv. Colloid Interface Sci. 108–109 (2004) 227–258. [31] J. Santos, L.A. Trujillo, N. Calero, M.C. Alfaro, J. Munoz, Physical Characterization of a commercial suspoemulsion as a reference for the development of suspoemulsions, Chem. Eng. Technol. 36 (11) (2013) 1883–1890. ˜ [32] F. Rincón, J. Munoz, P. Ramírez, H. Galán, M.C. Alfaro, Physicochemical and rheological characterization of Prosopis juliflora seed gum aqueous dispersions, Food Hydrocolloids 35 (2014) 348–357. ˜ [33] N. Calero, J. Munoz, P. Ramírez, A. Guerrero, Flow behaviour: linear viscoelasticity and surface properties of chitosan aqueous solutions, Food Hydrocolloids 24 (2010) 659–666. ˜ [34] N. Calero, J. Munoz, P. Cox, A. Heuer, A. Guerrero, Influence of chitosan concentration on the stability: microstructure and rheological properties of O/W emulsions formulated with high-oleic sunflower oil and potato protein, Food Hydrocolloids 30 (2013) 152–162.
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Physical stability of N,N-dimethyldecanamide/␣-pinene-in-water emulsions as influenced by surfactant concentration ˜ L.A. Trujillo-Cayado, M.C. Alfaro ∗ , M.C. García, J. Munoz Reología Aplicada, Tecnología de Coloides, Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla c/P. García González, 1, E41012, Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 26 July 2016 Received in revised form 21 September 2016 Accepted 27 September 2016 Available online 11 October 2016 Keywords: Depletion flocculation Eco-friendly surfactant Emulsion Green solvent Microfluidization
a b s t r a c t In recent years, interest in submicron emulsions has increased due to their high stability and potential applications in the encapsulation and release of active ingredients in many industrial fields, such as the food industry, pharmaceuticals or agrochemicals. Furthermore, the social demand for eco-friendly solutions to replace hazardous solvents in many dispersion formulations has steadily risen. In this study, the influence of surfactant concentration on the formation and physical stability of submicron oil-in-water emulsions using a high-pressure dual-channel homogenizer (microfluidizer) has been investigated. The formulation involved the use of a blend of two green solvents (N,N-dimethyldecanamide and ␣-pinene) as dispersed phase and a nonionic polyoxyethylene glycerol ester derived from coconut oil as emulsifier (Levenol® C-201), which enjoys a European eco-label. Therefore, these emulsions may find applications as matrices for agrochemicals. Physical stability and rheological properties of the emulsions studied showed an important dependence on the eco-friendly surfactant concentration. The lowest surfactant concentration (1 wt%) yielded the onset of a creaming process after a short aging time and was not enough to avoid recoalescence during emulsification. On the other hand, the higher surfactant concentrations (4–5 wt%) resulted in depletion flocculation, which in turn triggered emulsion destabilization by coalescence. The optimum physical stability was exhibited by emulsions containing intermediate surfactant concentrations (2–3 wt%) since coalescence was hardly significant and the onset of a weak creaming destabilization process was substantially delayed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, the use of agrochemical products is associated with environmental pollution and the presence of toxic residues in the final products. An alternative is the use of low-risk pesticides and herbicides based on green solvents. These green solvents must be obtained from renewable materials and exhibit high biodegradability [1]. Emulsifiable concentrates (ECs) are among the most widely used agrochemical systems due to their versatility. However, in recent years there has been a strong trend to replace them with concentrated oil-in-water emulsions (EWs). The partial removal of oil results in less phytotoxicity and skin irritation to the customer. In addition, the sizes of droplets can be adjusted to an optimum distribution, which is important to increase biological efficacy [2]. Agrochemical products with small droplets and narrow droplet size distributions have been shown to improve biological activity [2].
∗ Corresponding author. E-mail address: [email protected] (M.C. Alfaro). http://dx.doi.org/10.1016/j.colsurfb.2016.09.043 0927-7765/© 2016 Elsevier B.V. All rights reserved.
Fatty acid dimethylamides are among the green solvents that can find applications in agrochemicals [3]. N,N-dimethyldecanamide (AMD-10TM ) is considered a safe biosolvent, according to the Environmental Protection Agency. Another biosolvent considered to be an interesting alternative to traditional solvents is ␣-pinene. It represents the major constituent of turpentine oils from the wood of most conifers and from leaf oils obtained from some herbs, such as juniper, rosemary or parsley [4]. For these reasons, the formulation of blends of these solvents in oil-in-water (O/W) concentrated emulsions has been studied in this work. The selection of the emulsifier used to stabilize the emulsion is crucial since it must fulfill several requirements; in addition to being able to stabilize an emulsion exhibiting suitable physicochemical properties, the surfactant must be biodegradable and non-hazardous for agricultural applications. Polyoxyethylene glycerol esters derived from coconut oil are non-ionic surfactants obtained from a renewable source. Their excellent wetting, interfacial and emulsifying properties are well documented [5–7]. These surfactants are fully innocuous to human
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
skin and hair and their properties are appropriate for the design of eco-friendly products [8–11]. Interestingly, these surfactants fulfill the environmental and toxicological requirements permitting their use as eco-label materials in Europe. One of these green surfactants, Levenol® C-201 (glycereth-17 cocoate) possesses an eco-label (DID list: 2133). One of the main challenges in the development of these O/W emulsions is the control of their physical stability in order to achieve an adequate shelf-life. Once emulsions are prepared, it is vital to detect at an early stage the onset of any destabilization process to shorten aging tests. In addition, determining the destabilization mechanisms provides outstanding feedback on formulation and processing variables [12]. Droplet size distributions (DSD) are perhaps the most important factor in determining properties like biological efficacy, rheology or shelf-life stability of emulsions [13]. On the whole, emulsions with smaller droplets and narrower DSDs result in longer stability. Double channel homogenization using a Microfluidizer (Microfluidics) is the methodology of choice if fluid-like emulsions with submicron mean diameters and narrow droplet size distributions are the targets, since they can reach extremely high shear rates [14]. Surfactant concentration is one of the key variables when dealing with emulsion formulation; hence it must be carefully controlled. On the one hand, a minimum emulsifier concentration is required to fully cover the available interfacial area created during the emulsification process. On the other hand, an excess of surfactant in solution will lead to the formation of a great number of micelles. It is well known that non-adsorbed surfactant micelles may induce the flocculation of the emulsion droplets due to a depletion mechanism [15,16]. This phenomenon, known as depletion flocculation, in turn, may trigger irreversible destabilization processes such as creaming and/or coalescence [17,18]. The main objective of this work was to study the influence of surfactant concentration on the physical stability of slightly concentrated O/W emulsions formulated with these eco-friendly solvents. A further goal was to prepare stable fine emulsions developed by double channel microfluidization, which may be find applications as matrices for the incorporation of active agrochemical ingredients. These emulsions not only contain an eco-friendly surfactant but also a blend of two green solvents, which contributes to the increasing demand for the development of more sustainable chemical dispersions.
155
2.2. Emulsification procedure The continuous phases were prepared by dissolving Levenol® C-201 surfactant in ultrapure water cleaned using a Milli-Q water purification system. Emulsions were prepared in two steps. First of all, 250 g of a coarse emulsion was prepared using an Ultraturrax T50 rotor-stator homogenizer with a toothed S50-G45F dispersion unit working at 4000 rpm for 120 s. Oil phase was slowly added for 30 s to the continuous phase and the temperature was fixed at 20 ◦ C using a semibatch set up assisted by a circulator to keep the temperature at the set point. Subsequently, secondary homogenization was carried out with a Microfluidizer (model M110P with an F12Y interaction chamber, Microfluidics, USA) at 15000 psi (103.42 MPa) in order to obtain lower droplet diameters and narrower droplet size distributions. Samples kept under storage at 20 ◦ C. 2.3. Emulsion droplet size analysis Size distributions of oil droplets were determined by laser diffraction using a Mastersizer X (Malvern, United Kingdom). All measurements were done three times for each emulsion 24 h after being prepared and results are reported as the mean and standard deviation. These measurements were carried out during 60 days of aging time to monitor the kinetics of emulsion destabilization by either coalescence or Ostwald ripening. The mean droplet diameter was expressed as Sauter mean diameter (D3,2 ) and volumetric mean diameter (D4,3 ):
N D3,2 =
i=1 N
n d2 i i=1 i
N D4,3 =
ni d3 i
i=1 N
ni d4 i
n d3 i i=1 i
(1)
(2)
where di is the droplet diameter, N is the total number of droplets and ni is the number of droplets having a diameter di. To determine the distribution width of droplet sizes, “span” was used, calculating from the following formula: span =
D(v, 0.9) − D(v, 0.5) D(v, 0.1)
(3)
Where D(v,0.9), D(v,0.5), D(v,0.1) are diameters at 90%, 50% and 10% of cumulative volume, respectively. 2.4. Physical stability 2. Materials and methods 2.1. Materials According to the composition of emulsions previously studied, 30 wt% O/W emulsions formulated with a non-ionic surfactant (glycereth-17 cocoate) at different concentrations were prepared using a mixture of green solvents, as dispersed phase. N,N-dimethyl decanamide (AMD-10TM ) and ␣-pinene were utilized with a 75/25 mass ratio [19]. Agnique AMD-10TM (density: 0.88 g/mL at 25 ◦ C) was kindly provided by BASF. ␣-Pinene (density: 0.84 g/mL at 25 ◦ C) was supplied by Sigma Chemical Company. The emulsifier used was a non-ionic surfactant derived from coconut oil. Namely, a polyoxyethylene glycerol fatty acid ester, glycereth-17 cocoate, received as a gift from KAO, was selected on account of its HLB number, 13. Its trade name is Levenol® C-201. The influence of Levenol® C-201 concentration on the stability and physicochemical properties of the emulsions was studied in the 1 to 5 wt% range.
Multiple light scattering measurements with a Turbiscan Lab Expert (Formulaction, France) were used in order to complete the study on emulsion destabilization. Measurements were carried out until 40 days at 20 ◦ C to determine the predominant mechanism of destabilization in each emulsion as well as the kinetics of the destabilization process. The Turbiscan Stability Index (TSI) is a parameter that can be used to estimate the physical stability of dispersions like suspensions or emulsions. This parameter is a statistical factor and its value is obtained as the sum of all processes taking place in the studied probe. An increase in TSI value means that the overall physical stability of the dispersion under study decreases. TSI values were calculated using the equation [11]: TSI = |scanref (hj ) − scani (hj )| (4)where scanref and scani are j the initial backscattering value and the backscattering value at a given time, respectively, hj is a given height in the measuring cell and TSI is the sum of all the scan differences from the bottom to the top of the vial.
156
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
2.5. Viscosity of continuous phases The viscosities of continuous phase solutions (from 1.43 wt% to 7.14 wt%) were measured with an Ubbelohde glass capillary viscometer. A volume of solution was pipetted into the capillary viscometer, which was equilibrated at 20 ◦ C in a water bath for 30 min prior to measurements. All the measurements were performed at 20 ◦ C ± 0.1 ◦ C and the result is the average of five measurements. Viscosity is obtained from the following equation: =C ××t
(5) (m2 /s2 )
Where (Pa s) is the viscosity of the continuous phase, C is a constant that depends on the glass capillary, (kg/m3 ) is the density of the continuous phase and t is the time. 2.6. Rheology of emulsions The rheological characterization involved flow curves and stress and frequency sweeps in small-amplitude oscillatory shear experiments (SAOS). Rheological experiments were conducted with a MARS controlled-stress rheometer (Haake, Germany) using a Z20 coaxial cylinder geometry (Ri = 1 cm, Re /Ri = 1.085) for the curves and a double-cone geometry (angle: 0.017 rad; diameter: 60 mm) for SAOS tests. A sandblasted surface treatment was used for the sensor systems to prevent wall-depletion (slip effects) from being significant under non-linear flow behaviour. Special surface sensor systems must be used when carrying out rheological tests with different types of dispersions as reviewed by Buscall (2010) [20]. Flow curve tests were run by stepped shear rate ramps determined in controlled-stress mode (0.05–2 Pa). An approximation to steady state response of 0.01% for each step was used, fixing a maximum time per point of 180 s as cut off criterion. Stress sweeps at three different frequencies (0.1, 1 and 3 Hz) were carried out in order to determine the linear viscoelastic range. Stress sweeps were performed in a range of 0.05 to 5 Pa. Subsequently, frequency sweeps were conducted at a stress lower than the critical stress that was previously determined by the aforementioned stress sweeps. Frequency sweep tests (from 20 to 0.02 rad s−1 ) were performed selecting a stress well within the linear viscoelastic range. To follow the effect of aging time, all rheological measurements were carried out 1, 3, 13, 21 and 40 days after preparation. These measurements were performed at 25 ◦ C ± 0.1 ◦ C, using a C5P Phoenix circulator (Thermo-Scientific, USA) for sample temperature control. Equilibration time prior to rheological tests was 180 s. All measurements were repeated 3 times for each emulsion. Sampling from the top part of the container in contact with air was avoided, so samples were taken at about 2 cm below the upper part of the container. Results represent the mean of three measurements. 2.7. Statistical analysis Resulting data from laser diffraction, multiple light scattering and rheological measurements have been analyzed using oneway analysis of variance (ANOVA) using StatPlus® :mac. Differences between means were determined using the Tukey test. All statistical calculations were conducted at a significance level of p = 0.05. 3. Results and discussion 3.1. Droplet size distributions Fig. 1A shows the droplet size distribution of emulsions with different surfactant concentration aged for one day. First of all, all emulsions exhibited bimodal distributions and an overwhelming number of droplets with submicron diameter. In all cases,
Fig. 1. (A) Droplet size distributions for emulsions containing different surfactants concentrations aged for 1 day and (B) Sauter (D3,2 ) and volumetric mean mean diameter (D4,3 ) of oil droplets or emulsions aged for 1 and 40 days aging time as a function of surfactant concentration. Standard deviation of the mean (three replicates) for D3,2 < 5% and D4,3 < 7%.
the main peak is below one micron and the second population is centred at about three microns. The occurrence of the second population of droplets is probably due to a re-coalescence phenomenon, induced by an excess of mechanical energy input during the emulsification process [21]. Moreover, the droplet size distributions markedly shifted to the left, i.e. to lower droplet diameters, when increasing Levenol® C-201 bulk concentration from 1 wt% to 2 wt% and roughly leveled off for surfactant concentrations above 2 wt%. However, a closer inspection of the results revealed that D3,2 values showed a steady fall with surfactant concentration in the (1 wt%–5 wt%) Levenol® C-201 range, which was significant according to the statistical analysis carried out (p < 0.05, p-value = 0.00). D3,2 values for the 3 and 4 wt% emulsions aged for one day are not significantly different from each other (Tukey test, p > 0.05). As far as the D4,3 values are concerned, a significant decrease was observed in the (1 wt%–5 wt%) Levenol® C-201 range as supported by standard deviation data and the ANOVA test (p< 0.05, p-value = 0.00). All D4,3 values for the emulsion aged for one day were significantly different from each other except for 2 and 3 wt% (Tukey test, p> 0.05). Fig. 1B shows the Sauter and volumetric mean diameters of emulsions with different Levenol® C-201 concentration aged for 1 and 40 days in order to check if destabilization by coalescence or
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
157
Table 1 Span, flow curves fitting parameters and fitting correlation coefficients for all emulsions studied aged for 24 h. Standard deviation of the mean (three replicates) for span < 5%, 1 < 8% and n < 5%. Surfactant (wt%)
Span
1 (mPa)
n
R2
1 2 3 4 5
1.58 ± 0.07 1.46 ± 0.06 1.37± 0.04 1.39 ± 0.04 1.40 ± 0.04
4.75 ± 0.40 (a) 17.40 ± 1.32 (a) 53.19 ± 4.31 (b) 162.68 ± 12.98 (c) 299.5 ± 24.11 (d)
0.99 ± 0.01 (a) 0.84 ± 0.01 (b) 0.68 ± 0.01 (c) 0.53 ± 0.01 (d) 0.44 ± 0.02 (e)
0.994 0.998 0.995 0.998 0.997
Ostwald ripening takes place. For all emulsions submicron Sauter and volumetric mean diameter values were obtained (see Table 1). An increase in surfactant concentration up to 5 wt% resulted in a marked drop in volume mean diameters after 24 hours aging time. It is well documented that emulsions with submicron diameters show high stabilities against creaming and coalescence [22]. Furthermore, the span values (droplet size polidispersity) of these emulsions ranged from 1.37 to 1.58, dropping to a minimum around 3 wt%. It has been reported that such an increase in polydispersity exerts a marked influence on the destabilization process of emulsions leading to a rise in the creaming rate due to higher values of the effective packing parameter [12], which in turn result in lower viscosity as predicted by the Krieger-Dougherty equation [23]. The time evolution of D3,2 and D4,3 after 40 days is consistent with the occurrence of coalescence or Ostwald ripening for some of the emulsions studied. ANOVA tests (p < 0.05) supported the occurrence of significant increments in the Sauter and volumetric mean diameters for the 1 wt% (D3,2 : p-value = 3.30 × 10−12 ; D4,3 : p-value = 1.47 × 10−5 ), 4 wt% (D3,2 : p-value = 1.05 × 10−8 ; D4,3 : pvalue = 2.13 × 10−13 ) and 5 wt% (D3,2 : p-value = 1.47 × 10−5 ; D3,3 : p-value = 9.65 × 10−18 ) Levenol® C-201 emulsions. By contrast, 2 wt% (D3,2 : p-value = 1.00; D4,3 : p-value = 5.53 × 10−2 ) and 3 wt% (D3,2 : p-value = 1.00; D4,3 : p-value = 1.74 × 10−1 ) did not show a significant increment with aging time for Sauter and volumetric mean diameters. It is noteworthy that stable submicron emulsions were obtained at Levenol® C-201 surfactant concentrations within 2 and 3 wt%. The increase in the D3,2 and D4,3 values of the emulsion with less surfactant (1 wt%) can be ascribed to the occurrence of coalescence, enhanced by creaming of the bigger droplets. The increase in the second peak with aging time usually points to the occurrence of a destabilization process by coalescence instead of an Ostwald ripening phenomenon. For Ostwald ripening the particle size distribution should exhibit a specific time-independent form that shifts to the right along the X-axis with time, whereas when coalescence is the dominant destabilization mechanism a bi-modal distribution is usually observed [24]. On the one hand, coalescence becomes the most important phenomenon at low surfactant concentration, when drops are not completely covered with surfactant [25] or the surfactant layers surrounding the oil droplets are not strong enough; hence they are partially destroyed during the emulsification process (recoalescence) or cannot withstand collisions among droplets after the emulsion preparation. On the other hand, a depletion flocculation process may trigger coalescence in emulsions containing the higher Levenol® C-201 surfactant concentrations (4–5 wt%). This mechanism is provoked by the exclusion of surfactant micelles between neighbouring oil droplets because of an osmotic pressure effect, which first results in flocculation and subsequently may cause the coalescence of emulsion droplets [26]. 3.2. Physical stability A decrease in backscattering at the bottom of the measuring cell was observed in samples with a Levenol® C-201 surfactant concen-
Fig. 2. (A) Increase of droplet diameter from the diameter at time zero and (B) creaming index (CI) as a function of the aging time for emulsions containing a total surfactant concentration ranged for 1–3 wt%. Samples kept under storage at 20 ◦ C.
tration of 1, 2 and 3 wt%, which indicated the onset of a creaming process after a given aging time. In the case of emulsions containing 4 and 5 wt% Levenol® C-201, an overall decrease in backscattering with aging time from the bottom to the top of the measuring cell was observed. This was especially noticeable in the middle part of the sample. On the whole, this evolution can be interpreted either as a flocculation process or as an increase in the emulsion droplet size. Given that laser diffraction measurements demonstrated that coalescence did take place after emulsions containing 1 wt%, 4 wt% and 5 wt% surfactant aged for 40 days, the evolution of backscattering may be attributed to coalescence rather than flocculation of oil droplets. Fig. 2A shows the increase in droplet diameter, taking the initial value as reference (D), as a function of aging time for all emulsions. This plot makes it possible to monitor the kinetics of the coalescence process. The D value has been calculated using the following equation: D =
Dt − D0 D0
(6)
where, D0 and Dt are the initial diameter value and the diameter value at a given time, respectively. A high increase in droplet size over time was detected for emulsions with 1, 4 and 5 wt% of surfactant concentration. Interestingly, the shortest delay time for the onset of coalescence and the greatest increase in mean droplet diameter were shown by the emulsion containing the highest surfactant concentration. In contrast, the 2 wt% and 3 wt% surfactant emulsions exhibited the longer (above 30 days) delay times for the onset of coalescence and the smaller oil droplet diameter increase
158
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
Fig. 3. Turbiscan Stability Index (TSI) as a function of surfactant concentration in the bottom, middle and global zones of the measurement cell. Standard deviation of the mean (three replicates) for TSI < 8%.
with aging time. The best results were exhibited by the 3 wt% emulsion. These results are consistent with the variation of Sauter and volumetric mean diameters found for these emulsions by laser diffraction. The creaming process was analyzed by using the creaming index concept (CI) [12]: CI =
HS × 100 HE
(7)
where HE is the total height of the emulsion and HS is the height of the serum layer. The calculated creaming indexes for emulsions with surfactant concentrations ranging from 1 wt% to 3 wt% are plotted in Fig. 2B as a function of aging time.The onset of creaming can be detected by a decrease in backscattering at the bottom of the measuring cell. This phenomenon could not be detected for the 4–5 wt% emulsions. The initial slope of the plot of CI versus aging time is related to the creaming rate (, mm/day): ω=
HE d(CI) × 100 dt
(8)
Fig. 2B illustrates the decrease in CI and and the higher delay times for the onset of creaming (see the inset of Fig. 2B) with increasing surfactant concentration from 1 wt% to 3 wt%. The latter emulsion exhibited the best results not only for the creaming index but also for stability against coalescence (Fig. 2A). Emulsions in the (1–3) wt% Levenol® C-201 concentration range became more stable against creaming as surfactant concentration increased due to the combination of a reduction in droplet sizes, lower polidispersity and the increase in emulsion viscosity. An overall analysis of the multiple light scattering results reveals that although emulsions containing 4 wt% and 5 wt% Levenol® C201 showed the best results against destabilization by creaming, they underwent immediate destabilization by flocculation and/or coalescence. However, it is important to make clear that the multiple light scattering technique itself does not make it possible to distinguish between flocculation and coalescence since both mechanisms provoke a variation in backscattering in the middle zone of the measuring cell. Laser diffraction demonstrated that oil flocs are broken down during the measurement test. In contrast, multiple light scattering is quite suitable to detect the onset of creaming, as it would involve a decrease in backscattering at the lower zone of the measuring cell. Nevertheless, this fact was not experimentally observed and on top of this, destabilisation by creaming could not be observed by the naked eye in emulsions aged for a long time. Fig. 3 illustrates that the emulsion with 1 wt% surfactant exhibited the highest value of TSI at the bottom of the sample placed
Fig. 4. Flow curves of emulsions aged for 24 h as a function of surfactant concentration. Continuous lines correspond to the power-law fitting equation. Standard deviation of the mean (three replicates) for 1 < 8%. Temperature = 20 ◦ C.
in a Turbiscan measuring cell. This may be ascribed in this case to a creaming phenomenon, which, in turn, must stem from the fact that this emulsion showed the highest D32 , D43 and span values and the lowest viscosity (Table 1). As far as the emulsions containing (2–3) wt% Levenol® C-201 are concerned, the fact that TSI remained nearly constant in the intermediate zone suggests that destabilization mechanisms like flocculation and coalescence were not significant. Conversely, the higher TSI values in the intermediate zone for emulsions formulated with 4 or 5 wt% surfactant confirmed that they underwent depletion flocculation, as described above, and coalescence, as experimentally checked by laser diffraction (Fig. 1B). Taking into account the Global TSI values, results of the ANOVA test demonstrated that there are significant differences in the physical stability of the emulsions studied (p< 0.05, p-value = 0.00). The emulsions containing 2 wt% and 3 wt% Levenol C-201 are significantly different from each other (Tukey test, p> 0.05), being the 3 wt% emulsion the most stable. 3.3. Rheology of emulsions Fig. 4 shows the flow properties of emulsions aged for 24 h as a function of surfactant concentration. The emulsion containing 1 wt% surfactant exhibited Newtonian behaviour ( = 5.02 mPa·s) at 20 ◦ C. On the other hand, all emulsions in the (2–5) wt% concentration range exhibited shear-thinning behaviour. It must be noted that the rheological behaviour of emulsions aged for 24 h qualitatively shifted from Newtonian to non-Newtonian when increasing the Levenol® C-201 concentration by just 1 wt% (from 1 wt% to 2 wt%). This is consistent with a marked drop in D4,3 , which must result in a substantial increase in the concentration of droplets (number of droplets per volume) and also in D3,2 values, which yields greater values of droplet surface and more friction under shear flow. In addition the non-Newtonian response is a clear indication of the formation of oil flocs, due probably to a depletion flocculation mechanism induced by the presence of surfactant micelles in the continuous phase [26]. The fluid-like behaviour of these emulsions at 20 ◦ C was supported by their relatively low apparent viscosity values. By way of example, they increased at the maximum shear stress tested (2 Pa), from ca. 5 mPa s for the Newtonian emulsions containing 1 wt% Levenol® C-201 to just ca. 30 mPa s when the surfactant concentration was 5 wt%. Results for emulsions in the (2–5) wt% concentration range are consistent with the formation of slightly
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
159
Table 2 Continuous phases density and viscosity values at 20 ◦ C. Note: These surfactant concentrations in continuous phases correspond to 1, 2, 3, 4 and 5 wt% in emulsions respectively. Standard deviation of the mean (five replicates) for 20 ◦ C < 1% and 20 ◦ C < 1%.
Fig. 5. (A) Shear stress at 1 s−1 and (B) power-law index for all emulsions studied as a function of aging time and surfactant concentration. Standard deviation of the mean (three replicates) for 1 < 8% and n < 5%. Temperature = 20 ◦ C.
concentrated emulsions which are prone to form a weakly flocculated microstructure [2]. Their shear-thinning behaviour can be attributed to a shear-induced deflocculation process. Flow curves fitted the power-law model (R2 > 0.99):
. n
=K×
(9)
Where (Pa) is the shear stress, K (Pa·sn ) is nowadays called “consistency”, n is the power law index and ˙ (s−1 ) is the shear rate. Pa·sn are the units of parameter consistency, which depend on the power law index values. This prevents the making of a direct comparison of this parameter for samples with different values of the power law index. To overcome this problem, we used a modified power law equation [11]:
= 1 ×
.
1s−1
n (10)
where 1 stands for the shear stress at 1 s−1 . For shear thinning materials, 0 < n < 1. A solid material would show n = 0, while a Newtonian liquid would show n = 1. The values of fitting parameters corresponding to Eq. (10) are shown in Table 1 for emulsions aged for 1 day. In addition, these values are shown in Fig. 5A and B as a function of surfactant concentration and aging time. Table 1 illustrates the significant (according to the ANOVA test; p < 0.05) effects of both 1 (p-
Surfactant (wt%)
20 ◦ C (kg/m3 )
20 ◦ C (mPa s)
1.43 2.86 4.29 5.71 7.14
1.0001 ± 0.0001 1.0017 ± 0.0001 1.0027 ± 0.0001 1.0036 ± 0.0001 1.0046 ± 0.0001
1.52 ± < 0.01 2.84 ± 0.01 5.58 ± 0.05 17.93 ± 0.08 35.85 ± 0.14
value = 2.28 × 10−10 ) and n (p-value = 7.86 × 10−13 ) on emulsions aged for 24 h containing different surfactant concentrations. Means with the same letter are not significantly different from each other (Tukey test, p > 0.05). The increase in 1 , and consequently in apparent viscosity, as well as the drop in power law index values with surfactant concentration for emulsions aged for 1 day may be due to the formation of droplets with progressively lower mean diameters [27] as well as to the increasingly greater number of micelles in the continuous phase, which yields a steady increase in viscosity of the continuous phase (see Tables 1 and 2) and also to the formation of increasingly flocculated emulsions due to depletion flocculation induced by surfactant micelles in the continuous phase [28,29]. As far as the role of the continuous phase is concerned, Table 2 shows the viscosity of continuous phases prior to emulsification. Increasing the surfactant concentration from 1.43 wt% to 7.14 wt%, which correspond to 1 wt% and 5 wt% in the final emulsions, respectively, yielded a rise in viscosity from 1.52 mPa s to 35.85 mPa s. Nonetheless, the increase in apparent viscosity at 1s−1 and 1 of emulsions was about two orders of magnitude (compare Tables 1 and 2). This increment indicates the important contribution of oil droplets to the emulsion viscosity at a relatively low shear rate. With regard to the effects of aging on the flow properties of the emulsions studied, it must be emphasized that sampling was made from the upper part of the container in order to detect at an early stage the expected results for destabilization by creaming or coalescence. The flow curves of the emulsion containing 1 wt% Levenol® C-201 surfactant exhibited slightly increasing values of 1 and also a weak fall of power law indexes for 21 days. Interestingly, the evolution of the rheological parameters for this emulsion involved a shift from Newtonian to shear thinning behavior. This was consistent with the fast creaming rate detected by multiple light scattering, even though it could not be observed by the naked eye (Fig. 2B). Flow curve results were sensitive to the formation of the soft network formed by loosely packed flocs of oil droplets as also pointed out by Trujillo-Cayado et al. (2016) for AMD10/␣pinene emulsions [19]. At low shear stresses and low shear rates the oil network is hardly disrupted by the shear field. This yields greater values of 1 for slightly creamed non Newtonian emulsions if compared with the Newtonian emulsion, whose microstructure must essentially consist of randomly located droplets throughout the emulsion. The slight drop in power law indexes (i.e. the fact that emulsions became more shear thinning with aging time), was likely due to the increasing relative concentration of oil droplets in the upper part of the sample. A straightforward interpretation of the shear thinning behavior involves firstly a network breakdown and subsequently the shear-induced orientation of droplets in the flow direction. Subsequently, coalescence started competing with creaming as supported by the significant increase in mean droplet diameter detected by multiple light scattering from day 21 onwards (Fig. 2A). Coalescence was confirmed by laser diffraction (Fig. 1B). The occurrence of simultaneous destabilization mechanisms hin-
160
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
ders the microstructural interpretation of rheological results [30]. Creaming and coalescence exert opposite effects. In the previous paragraph the increasing viscosity promoted by creaming has comprehensively been explained. Conversely, coalescence results in a decrease in viscosity on account of both the lower interfacial surface available for friction and in the decrease of oil droplet concentration due to the higher volumetric mean diameters. Both effects must counterbalance each other such that 1 and n levelled off from 21 days onwards (see Figs. 1B and 2B). The clear drop in the shear stress at 1 s−1 and the increase in power law index values for emulsions containing the higher surfactant concentrations studied (4–5) wt% (Fig. 5A and B) highlighted a dramatic change in the effect of aging time on the rheology and microstructure of the emulsions studied. Both rheological parameters indicate that coalescence was the dominant destabilization mechanism. These results were in agreement with those obtained by laser diffraction and multiple light scattering (Figs. 1B, 2A and B). Interestingly, the emulsions containing 2 wt% and 3 wt% Levenol® C-201 exhibited stable rheological parameters for the whole observation time selected to monitor possible aging effects. The emulsion containing 3 wt% Levenol® C-201 showed higher value of 1 and more marked shear thinning properties than the emulsion with 2 wt% Levenol® C-201, which supports the statement that the optimum Levenol® C-201 concentration to enhance the physical stability of 30 wt% N,N-dimethyl decanamide (AMD10TM /␣-pinene, 75/25) in water emulsions was around 3 wt% A further task was to assess the suitability of linear dynamic viscoelastic tests to monitor the physical stability of the emulsions studied. Small amplitude oscillatory shear tests are widely used to determine the linear viscoelastic behaviour of a fascinating variety of materials on account of their sensitivity to changes of microstructure. In order to ensure that the test is carried out under non-intrusive conditions, i.e. it does not provoke structural destruction in the sample, they must be performed at stress and strain amplitudes within the linear viscoelastic range (LVR). Therefore, oscillatory torque sweep tests at three different frequencies (0.1, 1 and 3 Hz) were previously conducted in order to estimate the maximum amplitude value of the sinusoidal shear stress function and the corresponding maximum strain amplitude that guaranteed linear viscoelastic behaviour. Small amplitude oscillatory shear (SAOS) measurements are sensitive to detect slight structural changes as a consequence of droplet flocculation. Only the emulsion containing 5 wt% Levenol® C-201 exhibits both measurable dynamic viscoelastic functions due to a higher grade of flocculation (induced by a depletion flocculation mechanism) and an increase of droplet-droplet interactions that produced a stronger structure than those obtained for 1–4 wt% emulsions. Fig. 6 shows the angular frequency dependence of the storage modulus (G’) and the loss modulus G” (i.e. the mechanical spectra) as a function of aging time. The former viscoelastic function is related to the elastic response of the sample, while the latter is related to the viscous response. The mechanical spectra corresponding to aging times of 1, 3 and 10 days are typical of semi-dilute dispersions since G” is dominant over G” at lower frequencies, while the reverse takes place at higher frequencies. Therefore, G’ values cross over those of G” at an intermediate angular frequency. The crossover frequency allows the onset of the so-called terminal relaxation zone to be estimated as the reciprocal of such an angular frequency. In this way, the shorter the crossover frequency is, the longer is the terminal relaxation zone; which means that the material under study will have more marked solid-like properties. The sensitivity of the crossover frequency and of the corresponding terminal relaxation has been found useful when checking (a) the decay of “viscoelasticity”, or rather of the elastic (solid-like) properties of suspoemulsions upon increasing temperature [31], (b) the relatively similar viscoelastic properties exhibited by galac-
Fig. 6. Influence of aging time on the mechanical spectra for the emulsion formulated whit a surfactant concentration of 5 wt%. Shear stress amplitude: 0.1 Pa.
tomannan solutions regardless of their raw materials [32] and (c) the influence of chitosan concentration and temperature on the viscoelasticity of its aqueous solutions [33]. Interestingly, mechanical spectra were reported to be sensitive to creaming percentage for more concentrated emulsions since the plateau relaxation zone became wider with increasing creaming [34]. The results obtained in this study show that aging from 1 to 10 days resulted in greater values of the crossover angular frequency (*), i.e. lower terminal relaxation times. They dropped from 10.8 s after aging for 1 day to 0.5 s after 10 days. This means that this emulsion became more fluid-like with aging time. In addition, G’ and G” values clearly decreased with aging time. All these results are consistent with the occurrence of predominant coalescence, induced by a previous depletion flocculation process as demonstrated by laser diffraction and multiple light scattering and indicated by the evolution of flow curves with aging time. Enhanced coalescence involves lower interfacial surface available for friction under shear and a decrease in the effective droplet concentration; i.e. lower number of droplets per volume. In other words, extensive coalescence tends to mean that viscoelastic responses become hardly measurable. 4. Conclusions O/W emulsions containing 30 wt% admixtures of the green solvents N,N-dimethyl decanamide and ␣-pinene with 75/25 mass ratio and formulated with a polyoxyethylene glycerol fatty acid ester (HLB 13) as emulsifier can be prepared such that submicron mean Sauter and volumetric mean diameters can be achieved. The processing conditions consist of a primary emulsification step consisting of a semibatch rotor-stator equipped with a toothed rotor working at 4000 rpm for 120 s, followed by a secondary homogenization step carried out at 103.42 MPa using a Microfluidizer with a double channel interaction chamber. Surfactant concentration exerted a marked influence on the droplet size distribution, rheological properties and physical stability of emulsions. The cooperative information provided by laser diffraction, multiple light scattering and rheology allows the physical destabilization of emulsions to be monitored and the dominant destabilization mechanism to be identified. The emulsion containing 1 wt% of the nonionic Levenol® C-201 surfactant undergoes fast creaming after a rather short aging time, although coalescence later becomes the dominant destabilization mechanism as demonstrated by the cooperative information pro-
L.A. Trujillo-Cayado et al. / Colloids and Surfaces B: Biointerfaces 149 (2017) 154–161
vided by multiple light scattering, shear flow curves and laser diffraction. Depletion flocculation triggers coalescence, which becomes the main destabilization mechanism for emulsions formulated with the higher surfactant concentrations (4 wt% and 5 wt%). Emulsions containing 2 wt% and 3 wt% nonionic Levenol® C-201 surfactant present the best results. The emulsion with 3 wt% surfactant exhibits the longest physical stability since it shows the longest delayed times for the onset of the creaming and coalescence mechanisms. Acknowledgments The financial support received (Project CTQ2015-70700) from the Spanish Ministerio de Economía 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. References [1] M.J. Hernáiz, A.R. Alcántara, J.I. García, Applied biotransformations in green solvents, Chemistry 16 (31) (2010) 9422–9437. [2] Th.F. Tadros, Rheology of Dispersions. Principles and Applications, Wiley-VCH, 2009. [3] R. Höfer, J. Bigorra, Green chemistry—a sustainable solution for industrial specialties applications, Green Chem. 9 (3) (2007) 203–212. [4] S. Bertouche, V. Tomao, K. Ruiz, A. Hellal, C. Boutekedjiret, F. Chemat, First approach on moisture determination in food products using alpha-pinene as an alternative solvent for Dean–Stark distillation, Food Chem. 134 (1) (2012) 602–605. [5] E. Jurado, J.M. Vicaria, J.F. García-Martín, M. García-Román, Wettability of aqueous solutions of eco-friendly surfactants (ethoxylated alcohols and polyoxyethylene glycerin esters), J. Surfactants Detergents 15 (3) (2012) 251–258. ˜ [6] L.A. Trujillo-Cayado, P. Ramírez, L.M. Pérez-Mosqueda, M.C. Alfaro, J. Munoz, Surface and foaming properties of polyoxyethylene glycerol ester surfactants, Colloids Surf. A 458 (2014) 195–202. ˜ [7] L.A. Trujillo-Cayado, P. Ramírez, M.C. Alfaro, M. Ruíz, J. Munoz, Adsorption at the biocompatible ␣-pinene–water interface and emulsifying properties of two eco-friendly surfactants, Colloids Surf. B 122 (2014) 623–629. ˜ [8] J. Santos, L.A. Trujillo-Cayado, N. Calero, J. Munoz, Physical characterization of eco-friendly O/W emulsions developed through a strategy based on product engineering principles, AIChE J. 60 (7) (2014) 2644–2653. [9] J. Santos, L.A. Trujillo-Cayado, N. Calero, M.C. Alfaro, J. Munoz, Development of eco-friendly emulsions produced by microfluidization technique, J. Ind. Eng. Chem. 36 (2016) 90–95. ˜ [10] L.A. Trujillo-Cayado, M.C. Alfaro, A. Raymundo, I. Sousa, J. Munoz, Rheological behavior of aqueous dispersions containing blends of rhamsan and welan polysaccharides with an eco-friendly surfactant, Colloids Surf. B 145 (2016) 430–437. ˜ [11] L.A. Trujillo-Cayado, M.C. Alfaro, J. Munoz, A. Raymundo, I. Sousa, Development and rheological properties of ecological emulsions formulated with a biosolvent and two microbial polysaccharides, Colloids Surf. B 141 (2016) 53–58. [12] D.J. McClements, Critical review of techniques and methodologies for characterization of emulsion stability, Crit. Rev. Food Sci. Nutr. 47 (7) (2007) 611–649.
161
[13] D.J. McClements, Food Emulsions: Principles, Practices, and Techniques, CRC Press, Boca Ratón, USA, 2015. [14] S.Y. Tang, P. Shridharan, M. Sivakumar, Impact of process parameters in the generation of novel aspirin nanoemulsions-comparative studies between ultrasound cavitation and microfluidizer, Ultrason. Sonochem. 20 (1) (2013) 485–497. [15] J. Bibette, Depletion interactions and fractionated crystallization for polydisperse emulsion purification, J. Colloid Interface Sci. 147 (2) (1991) 474–478. [16] D.J. McClements, Ultrasonic determination of depletion flocculation in oil-in-water emulsions containing a non-ionic surfactant, Colloids Surf. A 90 (1) (1994) 25–35. [17] E. Dickinson, C. Ritzoulis, Creaming and rheology of oil-in-water emulsions containing sodium dodecyl sulfate and sodium caseinate, J. Colloid Interface Sci. 224 (1) (2000) 148–154. [18] S.J. Radford, E. Dickinson, Depletion flocculation of caseinate-stabilised emulsions: what is the optimum size of the non-adsorbed protein nano-particles? Colloids Surf. A 238 (1) (2004) 71–81. ˜ [19] L.A. Trujillo-Cayado, J. Santos, M.C. Alfaro, N. Calero, J. Munoz, A further step in the development of oil-in-water emulsions formulated with a mixture of green solvents, Ind. Eng. Chem. Res. 55 (27) (2016) 7259–7266. [20] R. Buscall, Letter to the Editor: wall slip in dispersion rheometry, J. Rheol. (1978-present) 54 (6) (2010) 1177–1183. [21] S.M. Jafari, Y. He, B. Bhandari, Production of sub-micron emulsions by ultrasound and microfluidization techniques, J. Food Eng. 82 (4) (2007) 478–488. [22] P. Izquierdo, J. Esquena, T.F. Tadros, C. Dederen, M.J. Garcia, N. Azemar, C. Solans, Formation and stability of nano-emulsions prepared using the phase inversion temperature method, Langmuir 18 (1) (2002) 26–30. [23] H.A. Barnes, A Handbook of Elementary Rheology. Aberystwwyth, The University of Wales Institute of Non-Newtonian Fluid Mechanics., United Kingdom, 2000. [24] E. Nazarzadeh, T. Anthonypillai, S. Sajjadi, On the growth mechanisms of nanoemulsions, J. Colloid Interface Sci. 397 (2013) 154–162. [25] B.P. Binks, Emulsions—Recent advances in understanding, in: Binks (Ed.), Modern Aspects of Emulsion Science, Royal Society of Chemistry, Cambridge, United Kingdom, 1998. [26] P. Manoj, A.D. Watson, D.J. Hibberd, A.J. Fillery-Travis, M.M. Robins, II. Steady-state rheological investigations, J. Colloid Interface Sci. 207 (2) (1998) 294–302. [27] H.A. Barnes, Rheology of emulsions—a review, Colloids Surf. A 91 (1994) 89–95. [28] R. Pal, Rheology of simple and multiple emulsions, Curr. Opin. Colloid Interface Sci. 16 (1) (2011) 41–60. [29] G.G. Palazolo, D.A. Sorgentini, J.R. Wagner, Coalescence and flocculation in o/w emulsions of native and denatured whey soy proteins in comparison with soy protein isolates, Food Hydrocolloids 19 (3) (2005) 595–604. [30] Th.F. Tadros, Application of rheology for assessment and prediction of the long-term physical stability of emulsions, Adv. Colloid Interface Sci. 108–109 (2004) 227–258. [31] J. Santos, L.A. Trujillo, N. Calero, M.C. Alfaro, J. Munoz, Physical Characterization of a commercial suspoemulsion as a reference for the development of suspoemulsions, Chem. Eng. Technol. 36 (11) (2013) 1883–1890. ˜ [32] F. Rincón, J. Munoz, P. Ramírez, H. Galán, M.C. Alfaro, Physicochemical and rheological characterization of Prosopis juliflora seed gum aqueous dispersions, Food Hydrocolloids 35 (2014) 348–357. ˜ [33] N. Calero, J. Munoz, P. Ramírez, A. Guerrero, Flow behaviour: linear viscoelasticity and surface properties of chitosan aqueous solutions, Food Hydrocolloids 24 (2010) 659–666. ˜ [34] N. Calero, J. Munoz, P. Cox, A. Heuer, A. Guerrero, Influence of chitosan concentration on the stability: microstructure and rheological properties of O/W emulsions formulated with high-oleic sunflower oil and potato protein, Food Hydrocolloids 30 (2013) 152–162.