Production of more sustainable emulsions formulated with eco-friendly materials

Production of more sustainable emulsions formulated with eco-friendly materials 1 Mario Fresnedaa, Luis A. Trujillo-Cayadoa, M Carmen Garcíaa, Maria-Carmen Alfaro2 Rodrigueza* and José Muñoza. 3 a Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, C/ 4 P. García González, 1, E41012, Sevilla, Spain. 5 * Corresponding author: Maria-Carmen Alfaro-Rodriguez; Tel.: +34 954 557180; fax: 6 +34 954 556447; E-mail address: [email protected] 7 8 Abstract 9 Sustainable development involves the search for new products with a low environmental 10 impact. Hence, the aim of this work is to obtain stable and concentrated aqueous 11 emulsions containing bitter fennel oil and a biomass-derived emulsifier by studying 12 different processing variables and techniques. Firstly, the effects of the application of a 13 premix step previous to homogenization and of the geometry of the high-energy rotor14 stator device used (Silverson L5M or Ultraturrax T50) on the droplet size distribution 15 (DSD) and physical stability (PS) of emulsions were investigated. The use of a premix 16 worsens both the physical stability and the average droplet diameters of the emulsions, 17 the most stable emulsion being that obtained with the Silverson L5M alone. Secondly, 18 the stability of this emulsion was improved. To achieve this goal, this coarse emulsion 19 was microfluidized at different pressures (from 5000 to 25000 psi), reaching submicron 20 sizes and monomodal distributions, except for that subjected to 25000 psi which resulted 21 in clear overprocessing. Creaming and oiling off were the main destabilization processes. 22 The emulsion exhibiting the lowest droplet sizes and the best physical stability was 23 prepared at 20000 psi. This work contributes to the development of sustainable 24 agrochemical prototypes. 25 26 Keywords: Bitter fennel oil, Wheat derived surfactant, Emulsion, Premix in primary 27 homogenization, Microfluidizer, Physical stability. 28 Introduction 29 In order to reduce both human health and environmental risks, scientists have a great 30 effort to work out future strategies with respect to green chemistry (Durand et al., 2016). 31 One of them is the incorporation of eco-friendly materials in the design of new products. 32 For this reason, sustainability has become an important requirement for solvents, leading 33 to the need to develop new green solvents. The non-polar characteristic of some of these 34 organic solvents means that they cannot be dispersed directly in another aqueous phase 35 (Israelachvili, 2011). Hydrophobic compounds could be incorporated as functional or 36 active ingredients in a wide variety of colloidal systems with application in sectors such 37 as food or agrochemicals. An important application of this type of system in the field of 38 agrochemicals is the use of emulsions of essential oils, which can be used as a matrix for 39 pesticides, where the essential oil functions as a solvent friendly to the environment. In 40 previous works, emulsions containing essential oils such as alpha-pinene (García et al., 41 2014; García et al., 2015), limonene (Trujillo-Cayado et al., 2016) or thyme essensial oil 42 (Martin et al., 2018) were studied for this purpose. 43 Another important issue is that the fact of considering the use of the biodegradable raw 44 materials leads to obtaining both a final product with little impact on the environment and 45 waste which is less harmful to it (Bom et al., 2019). As indicated Saéz-Martinez et al 46 (2016) the future must be made up of green, sustainable and recyclable chemicals. In the 47 present work, in order to achieve this purpose, emulsions containing bitter fennel essential 48 oil as an ecological solvent were produced. The bitter fennel plant is a common perennial 49 hemicryptophyte from the Mediterranean basin whose main components are estragole 50 and trans-anethole in addition to the cyclic monoterpenes of fenchone and limonene 51 (Gross et al, 2002). 52 Frequently, essential oils have been utilized as natural preservatives and as fragrances in 53 cosmetic products, but as a result of their antimicrobial and antioxidant properties, new 54 applications are emerging for them in sectors such as food or agriculture (Rodríguez-Rojo 55 et al., 2012). Their hydrophobicity, as mentioned above, makes many of their applications 56 difficult, although it is their volatility that supposes a greater barrier (Llinares et al., 2018). 57 One way to reduce this problem is the formulation of stable emulsions that reduce 58 evaporation (Rodríguez-Rojo et al., 2012). Oil-in-water emulsions are one of the main 59 components of many commercial products. Currently, this type of emulsion can be found 60 in foods, vitamin supplements, drugs, cosmetics, personal care items and agrochemicals. 61 However, emulsions are, from a thermodynamic point of view, extremely unstable and 62 the two phases that compose them tend to separate (Borwankar et al., 1992). For this 63 reason, other components such as surfactants, proteins or thickeners are commonly used 64 to improve emulsion stability, with the use of the surfactant playing an important role in 65 this. The surfactant molecules tend to position themselves at the interface produced 66 between the dispersed oil droplets and the aqueous continuous phase (Dickinson et al., 67 1989). Thus, several processes involved in emulsion destabilization are controlled by the 68 presence of the surfactant, which influences electrostatic and steric repulsion and 69 consequently, the emulsion stability (McClements, 2007). For this purpose, Applyclean 70 6548 has been employed as emulsifier. It is a non-ionic surfactant derived from wheat 71 which belongs to the family of alkyl poly pentosides (APP). This surfactant has all the 72 necessary characteristics to be considered as an ecological surfactant, since it derives from 73 a renewable source and its production is carried out by means of a process that respects 74 the environment (Trujillo-Cayado et al., 2018). In fact, this surfactant, developed by the 75 French company Wheatoleo, has ECOCERT certification. Its main characteristics include 76 its low toxicity, its rapid biodegradation and its HLB, which is between 9.0 and 9.5. In 77 addition, it should be noted that the use of this surfactant, which is obtained from wheat 78 straw, avoids the waste accumulation in the environment and converts these wastes into 79 added-value materials. Furthermore, as Lin et al (2013) point out, its raw material does 80 not compete with food production. 81 Another point to take into account to increase of these emulsions sustainability is the use 82 of the most friendly production strategy. This is to say to perform a cleaner production in 83 which the environmental contamination is reduced as Krolczyk et al. purposed in a 84 previous work (Krolczyk et al., 2017). Recent works have carried out this aim centering 85 their studies in ecological procedures (Mia, et al., 2018). It should be noted that today it 86 is so important to provide solutions for environmental degradation that there are 87 numerous studies that are being developed related to sustainable production. Among the 88 most recent ones, the one carried out by Correa et al. (2019) for the biofuel production, 89 the study of Nidheesh and Kumar (2019) for the cement and steel production or that of 90 Mark et al. (2019) for the production of phenolic compounds. In this way, Bom et al., 91 (2019) in their review about sustainability in the cosmetic industry considered several 92 processing variables such as the use of the same equipment in process in which several 93 steps are involved, the production energy optimization, the reduction of the washing 94 water, etc. The same points can be used in other industries. In addition, it is worth making 95 an effort to find out the devices which produce less impact on the environment. As 96 Krolczyk et al (2019), reported, this fact can be the opener for a cleaner and friendly 97 production. For this purpose, the main objective of this work is to evaluate different 98 processing conditions to produce an environmentally friendly emulsion from a mixture 99 whose main components are bitter fennel essential oil and water, in order to select the 100 most sustainable processing protocol that provides greater emulsion stability. Firstly, the 101 influence of a premix stage in the processing of oil-of-fennel-in-water emulsions was 102 studied. For this, four experiments were carried out. On the one hand, two systems were 103 subjected to a premixing process with a low-energy rotor-stator equipment prior to the 104 emulsification with high-energy rotor-stator devices (two different geometries) and on 105 the other hand, two systems in which the emulsification was carried out directly, without 106 any type of premix. The importance of this stage of the study lies in determining the 107 number of steps of the most sustainable processing. Subsequently, the stability of the 108 emulsions obtained as a function of aging time was studied by means of laser diffraction 109 and multiple light scattering techniques. The most stable emulsion produced during this 110 stage was then improved with the microfluidization equipment (Microfluidizer M-110P). 111 The influence of pressure on its properties was analyzed. In this way, the optimal 112 processing conditions were established for the emulsification of a mixture whose main 113 elements are bitter fennel essential oil, wheat biomass-derived surfactant and water which 114 could be used as a prototype for sustainable agrochemicals. An analysis of both 115 processing variables led to the optimization of the processing protocol in order to achieve 116 the most respectful with the environment. 117 Materials and methods 118 Materials 119 Bitter fennel essential oil (density: 0.897 Kg/m3) kindly supplied by Destilaciones Bordas 120 Chinchurreta S.A was used as dispersed phase. Applyclean 6548 (D-xylofuranose, 121 oligomeric tetradecyl and octodecyl glycoside, C14, C18 alcohol) (Martín et al., 2018), 122 produced by Wheatoleo, was utilized as emulsifier. In order to preserve the emulsions, 123 sodium azide (Panreac) was included. MilliQ water was used to complete the 124 formulation. 125 Studied emulsions contained 40 wt% oil phase, 4 wt% surfactant, 0.1 wt% sodium azide 126 and MilliQ water as continuous phase. 127 Preparation of emulsions 128 The continuous phase was prepared by mixing the necessary amount of sodium azide in 129 Milli-Q water at room temperature by means of a magnetic stirring plate (SM 162, Stuart, 130 Scientific Laboratory Supplies). Subsequently, the dispersed phase was obtained by 131 dissolving the appropriate quantity of Applyclean 6548 in bitter fennel essential oil at 132 70ºC for 15 minutes. For this purpose, a Phoenix (Thermo-Scientific) bath was employed. 133 Once both dispersed and continuous phases were prepared, four different emulsions were 134 obtained using the following protocols: 135 1. IKA-UT50 emulsion. These emulsions were produced by adding the dispersed 136 phase to the continuous phase at 400 rpm for one minute using an IKA-VISC MR137 D1 low-energy rotor-stator homogenizer (IKA Labortechnik, Germany) and then 138 continuing for one additional minute at the same rate. This emulsion was 139 immediately homogenized with a high speed rotor-stator homogenizer 140 (Ultraturrax T50) at 2000 rpm for one minute and then 30 s at 6000 rpm. 141 2. IKA-SL5M emulsion. The process used in this case was similar to that of the 142 previous emulsion. In this protocol, a Silverson L5M rotor-stator homogenizer 143 was utilized instead of the Ultraturrax T50. 144 3. UT50 emulsion. The dispersed phase was added to the aqueous phase for 1 minute 145 at 2000rpm by means of the Ultraturrax T50 homogenizer. Lastly, 6000 rpm for 146 30 s was applied as a final homogenization. 147 4. SL5M emulsion. The processing protocol was similar to the UT50 emulsion, but 148 the equipment used was Silverson L5M. 149 In a further stage, the most stable emulsion produced during the first stage was submitted 150 to a second homogenization process by means of Microfluidizer M-110P (Microfluidics, 151 EEUU). In this equipment, batches of 200 g were processed at different pressures (5000 152 psi, 10000 psi, 15000 psi, 20000 psi and 25000 psi). 153 At least two samples were prepared with every protocol. 154 Emulsion characterization 155 Laser diffraction 156 This technique was employed to determine the droplet size distribution. For this purpose, 157 a Mastersizer 2000 (Malvern, Uk) was used. Milli-Q water was utilized as the dispersant 158 medium. The refraction index was 1.54 for the oil dispersed phase and the refraction and 159 adsorption indexes for the aqueous medium were 0.5 and 1.33, respectively. 160 In order to analyse the influence of the processing protocols on the mean droplet 161 diameters, Sauter diameter (D[3,2]) and volume mean diameter (D[4,3]) have been 162 employed. These parameters are defined as following: 163    == = N iii N iii dndnD 1 2 1 3 2,3 164    == = N iii N iii dndnD 1 3 1 4 3,4 165 where N is the total number of droplets, di is the droplet diameter and ni is the number of 166 droplets having a diameter di. 167 To find out the distribution width of droplet sizes distribution, the “span” was used, which 168 was determined as follows: 169 ]D[v]D[vvD=Span ,0.5),0.1],0.9[( − 170 where D[v, 0.9] and D[v, 0.1] stand for the 90th and 10th percentiles and D[v, 0.5] for the 171 median. 172 In order to study the emulsion stability, these measurements were performed at 1 day, 5 173 days and 14 days of aging time. 174 At least two replicates of each test were performed at room temperature. 175 Multiple light scattering 176 Turbiscan Lab (Formulation, France) was used to evaluate the physical stability of the 177 emulsions obtained. Tests were carried out at room temperature. Turbiscan determines 178 backscattering and transmission as a function of the length of the cell containing the 179 sample, but in this work only backscattering is presented as a consequence of the fact that 180 the transmission values recorded were null. 181 Results and discussion 182 Influence of the premix and the high-energy rotor-stator device used on the 183 microstructure and physical stability of emulsions 184 Figure 1 shows the influence of the processing protocols on the droplet size distribution 185 of the emulsions containing bitter fennel essential oil as dispersed phase after 24 hours of 186 aging. Monomodal distributions with a peak at around 2.5 m, were exhibited for all the 187 emulsions produced. As can be observed, emulsions obtained with a previous 188 homogenization step presented a droplet size distribution centred on higher droplet sizes. 189 This could be a result of a re-coalescence mechanism related to an excess of applied 190 energy which could have provoked the breaking of several droplets. This fact led to the 191 displacement of the curve towards higher diameters and, therefore, to emulsions with 192 larger droplets sizes (Jafari et al., 2008; García et al. 2016). In addition, the emulsions 193 obtained with the Silverson L5M equipment showed droplet size distributions displaced 194 towards smaller diameters than those prepared by means of Ultraturrax T50. This result 195 had already been observed in a previous work (Trujillo-Cayado et al., 2018). Although, 196 under the same operation conditions, the Ultraturrax T50 device applies higher energy 197 than Silverson L5M, the latter is more effective, probably due to the geometry of the rotor 198 used. This finding was supported by the mean diameter values and the standard deviations 199 exhibited in Table 1. Therefore, the best results were presented by emulsions obtained 200 only using Silverson 5M. From a sustainable production point of view this result is very 201 important since involves an energy reduction and higher production efficiency (Alayón 202 et al., 2017) 203 By way of example, Figure 2 illustrates the evolution of droplet size distribution with 204 aging time for the emulsion developed by IKA-SL5M processing. As can be observed, 205 there was no significant changes in the mean droplet sizes and, therefore, there was no 206 destabilization by coalescence or Ostwald ripening during the evaluation time. The same 207 result was obtained for the other emulsions investigated. This fact revealed that 208 Appyclean 6548 perfectly fulfills its role as an emulsifier since not only made it possible 209 to obtain concentrated emulsions, but that it was also able to satisfactorily cover the 210 oil/water interface protecting it against the rupture. 211 The emulsion stability was assessed by multiple light scattering monitoring for at least 27 212 days. All emulsions showed similar results with a decrease in the backscattering with the 213 aging time at the bottom and at the top of the vessel containing the sample, which is 214 coherent with the existence of a destabilization mechanism by creaming and oiling off, 215 respectively (McClements, 2015). There was no change in the backscattering in the 216 middle of the measuring cell which demonstrated droplets sizes remained invariable 217 (Mengual et al., 1999). This result is in agreement with that obtained from the laser 218 diffraction technique. In Figure 3, the backscattering evolution over the whole of the 219 container height as a function of the aging time is shown for one selected emulsion, IKA220 SL5M. Additionally, in order to compare the different emulsions studied and better 221 visualize the changes produced, in Figure 4 the backscattering in reference mode (BS) 222 of these emulsions at 27 days of aging time as function of the measuring cell length is 223 shown. This figure supports the analysis above, namely all emulsions suffered similar 224 effects, a marked decrease in the BS at the bottom as a consequence of a creaming 225 368 Literature cited 369 Alayón, C., Säfsten, K., Johansson, G. Conceptual sustainable production principles in 370 practice: Do they reflect what companies do? Journal of Cleaner Production 141 (2017) 371 693-701. https://doi.org/10.1016/j.jclepro.2016.09.079. 372 373 Borwankar, R.P., Lobo, L. A., Wasan, D. T. Emulsion stability-kinetics of flocculation 374 and coalescence, J. 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LWT - Food Science and Technology, 473 66 (2016), 590–597. https://doi.org/10.1016/j.lwt.2015.11.002 474 475 Figure captions 476 Figure 1. Influence of the processing protocols on the DSD of the primary emulsions at 1 477 day of aging time. Room temperature. 478 Figure 2. Backscattering in whole measuring cell length as a function of aging time for 479 IKA-SL5M emulsion. 480 Figure 3. Delta backscattering at 27 days of aging time as a function of the measuring cell 481 length for all studied systems. 482 Figure 4. Influence of the processing protocol on the creaming index (CI) as a function 483 of aging time. 484 Figure 5. Effect of aging time on the droplet size distributions for SL5M emulsion. Room 485 temperature. 486 Figure 6. Influence of the processing protocol on the Turbiscan Stability Index as a 487 function of aging time. 488 Figure 7. Influence of the pressure applied by M-110P microfluidizer homogenizer on the 489 droplet size distribution. Room temperature. 490 Figure 8. Influence of aging time on the droplet size distribution of the emulsion obtained 491 at 20000 psi. 492 Figure 9. Influence of the applied pressure on the BS as a function of the measuring cell 493 length at 55 days of aging time. 494 Figure 10. Influence of the applied pressure on the creaming index as a function of aging 495 time. 496 Figure 11. Influence of applied pressure on the TSI as a function of aging time. 497 498 Tables 499 Table 1. Influence of processing protocol on the mean diameters and span values. Room 500 temperature. 501 Table 2. Influence of processing protocol on the mean diameters and span values. Room 502 temperature. 503 Table 3. Influence of the applied pressure on the creaming rate () and the time of 504 creaming onset (t0,c). 505 506 Table 1. Influence of processing protocol on the mean diameters and span values. Room 507 temperature. 508 D3,2 (m) D4,3 (m) Span IKA - UT50 2.37 ± 0.07 2.91 ± 0.09 1.215 ± 0.068 UT50 2.04 ± 0.05 2.63 ± 0.07 1.079 ± 0.051 IKA - SL5M 1.98 ± 0.05 2.57 ± 0.07 0.988 ± 0.044 SL5M 1.82 ± 0.06 2.29 ± 0.05 1.164 ± 0.071 509 510 Table 2. Influence of processing protocol on the mean diameters and span values. Room 511 temperature. 512 Pressure (psi) D3,2 (m) D4,3 (m) Span SL5M 1.82 ± 0.06 2.29 ± 0.05 1.164 ± 0.071 5000 0.74 ± 0.04 1.06 ± 0.06 1.541 ± 0.088 10000 0.68 ± 0.03 1.17 ± 0.07 1.295 ± 0.065 15000 0.59 ± 0.02 0.91 ± 0.05 2.042 ± 0.101 20000 0.52 ± 0.02 1.53 ± 0.08 4.585 ± 0.327 25000 0.78 ± 0.03 1.70 ± 0.08 3.506 ± 0.287 513 Table 3. Influence of the applied pressure on the creaming rate () and the time of 514 creaming onset (t0,c). 515 Pressure (psi)  (%/day) t0,C (h) 5000 0.054 0 10000 0.053 125 15000 0.031 355 20000 0.030 436 25000 0.039 136 516 Figure captions Figure 1. Influence of the processing protocols on the DSD of the primary emulsions at 1 day of aging time. Room temperature Figure 2. Backscattering in whole measuring cell length as a funtion of the aging time for IKA-SL5M emulsion Figure 3. Delta backscaterring at 27 days of aging time as a function of the measuring cell length for all studied systems Figure 4. Influence of the processing protocol on the creaming index (CI) as a function of the aging time Figure 5. Effect of the aging time on the droplets sizes distributions for SL5M emulsion. Room temperature Figure 6. Influence of the processing protocol on the Turbiscan Stability Index as a function of the aging time Figure 7. Influence of the pressure applied by M-110P microfluidizer homogenizer on the droplets sizes distribution. Room temperature Figure 8. Influence of aging time on the droplet size distribution of the emulsion obtained at 20000psi Figure 9. Influence of the applied pressure on th BS as a function of the measuring cell length at 55 days of aging time Figure 10. Influence of the applied pressure on the creaming index as a function of the aging time Figure 11. Influence of applied pressure on the TSI as a function of the aging time 0.1 1 10 0 2 4 6 8 10 12 14 16 18 Volume (%) D/m IKA - UT50 UT50 IKA - SL5M SL5M Figure 1. Influence of the processing protocols on the DSD of the primary emulsions at 1 day of aging time. Room temperature. 010000 20000 30000 40000 50000 0 20 40 60 80 100 Backscattering (%) Length (mm) t = 0 4 dias 5 dias 11 dias 12 dias 18 dias 19 dias 27 dias Figure 2. Backscattering in whole measuring cell length as a funtion of the aging time for IKA-SL5M emulsion 010000 20000 30000 40000 50000 -100 -80 -60 -40 -20 0 BS (%) Length (mm) IKA - SL5M SL5M IKA - UT50 UT50 Figure 3. Delta backscaterring at 27 days of aging time as a function of the measuring cell length for all studied systems 0100 200 300 400 500 600 700 0 5 10 15 CI (mm) Aging time (h) IKA - SL5M SL5M IKA - UT50 UT50 Figure 4. Influence of the processing protocol on the creaming index (CI) as a function of the aging time. 0.1 1 10 100 0 2 4 6 8 10 12 14 Volume (%) D/ m Day 1 Day 5 Day14 Figure 5. Effect of the aging time on the droplets sizes distributions for SL5M emulsion. Room temperature 0100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 55 60 TSI Aging time (h) IKA - SL5M SL5M IKA - UT50 UT50 Figure 6. Influence of the processing protocol on the Turbiscan Stability Index as a function of the aging time. 1E-3 0.01 0.1 1 10 100 1000 10000 100000 0 2 4 6 8 10 12 14 Volume (%) D/m SL5M P=5000 psi P=10000 psi P=15000 psi P=20000 psi P=25000 psi Figure 7. Influence of the pressure applied by M-110P microfluidizer homogenizer on the droplets sizes distribution. Room temperature. 0.1 1 10 100 0 1 2 3 4 5 6 7 8 Volume (%) D/ m Day 1 Day 7 Day 14 Figure 8. Influence of aging time on the droplet size distribution of the emulsion obtained at 20000psi.