The Role of Processing Temperature in Flocculated Emulsions

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THE ROLE OF PROCESSING TEMPERATURE IN FLOCCULATED EMULSIONS Jenifer Santos, Nuria Calero, Luis A. Trujillo-Cayado, María Carmen Alfaro, and José Muñoz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04389 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on January 8, 2018

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Industrial & Engineering Chemistry Research

1

THE ROLE OF PROCESSING TEMPERATURE IN FLOCCULATED

2

EMULSIONS

3

J. Santos1, N. Calero*1, L.A. Trujillo-Cayado1, M.C. Alfaro1 and J. Muñoz1.

4

1

5

Química. Facultad de Química. Universidad de Sevilla c/ P. García González, 1,

6

E41012, Sevilla Spain.

7

* Corresponding author. Tel.: +34 954 557179; fax: +34 954 556447. E-mail

8

address: [email protected]

Reología Aplicada. Tecnología de Coloides. Departamento de Ingeniería

9 10

Abstract

11

Ecofriendly emulsions can suffer flocculation induced by processing. This

12

destabilization mechanism could lead to coalescence phenomenon with aging

13

time. This work is based on tuning the preparation temperature of ecofriendly

14

emulsions to reduce the flocculation. Bigger droplet sizes were shown for

15

emulsions processed above 35ºC. Emulsions prepared up to 15 ºC showed a

16

cross-over point in the mechanical spectra while a gel-type behavior was shown

17

for emulsions processed above this temperature. This fact pointed out two

18

different grades of flocculation. Emulsions with high flocculation degree showed

19

an increase of droplet size with aging time, which is related to coalescence. A

20

combined analysis of complex viscosity, volumetric diameter and backscattering

21

with aging time demonstrated that the most stable emulsion was prepared at 5

22

ºC since a reduction of both collision frequency and flocculation during the

23

preparation occurred. This work demonstrated the direct relation between

24

flocculation and processing temperature for these emulsions.

25

Keywords: Ecofriendly emulsions, flocculated emulsions, processing variables,

26

physical stability, applied rheology.

27

1. INTRODUCTION

28

Emulsions are a kind of disperse system consisting of two immiscible liquids.

29

The liquid droplets (the disperse phase) are dispersed in a liquid medium (the

30

continuous phase) 1. They have applications in several fields such as coatings, 1 ACS Paragon Plus Environment

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food, agrochemicals as well as cosmetics. Long-term stability is a pre-requisite

32

for their applications. Many destabilization processes can take place in

33

emulsions such as creaming, flocculation, coalescence and Ostwald ripening.

34

The flocculation of emulsions could be a double-edged sword since it can

35

provoke an increase of viscosity, which could enhance stability against

36

creaming, but coalescence could take place after a period of time for a

37

flocculated emulsion 2,3.

38

Flocculation occurs when repulsion is not sufficient to keep the droplets apart to

39

distances where the van der Waals attraction is weak 1. The repulsion forces

40

can be ionic or steric. Steric repulsion is a consequence of using nonionic

41

surfactants or polymers, for example, alcohol ethoxylates, or A-B-A block

42

copolymers. Not only the surfactant nature and concentration but also the

43

processing parameters (emulsification temperature, speed and time) are crucial

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in the formation of emulsions and in their physical stability

45

influence of processing temperature on the physical stability, flocculation and

46

rheology of emulsions has not been sufficiently studied yet. Furthermore,

47

changes in solubility of polyoxyethylene-type non-ionic surfactants with

48

temperature can be produced

49

temperatures but becomes lipophilic with increasing temperature due to

50

dehydration of the polyoxyethylene chains 9.

51

Organic solvents have played a vital role in the development of agrochemical

52

products without considering the consequences of the release of these harmful

53

chemicals in the land and sea. However, during the last decade an increase of

54

environmental awareness has led to the development and use of green solvents

55

and surfactants

56

dimethyldecanamide and D-Limonene)

57

(polyoxyethylene glycerol fatty acid ester, Glycereth-17 Cocoate) possessing

58

the

59

emulsions. This green surfactant possesses good interfacial properties at the

60

air/water and α-pinene/water interface

61

flocculated emulsions and gum-based emulsions 17,18 .

7,8

4,5,6

. However, the

. The surfactant is hydrophilic at low

10,11,12

. In this work, a mixture of ecofriendly solvents (N,N13,14

and an ecologic surfactant

ecolabel (DID list: 2133) were used to prepare concentrated green 15,16

. In addition, it has been used in

2 ACS Paragon Plus Environment

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These concentrated ecofriendly emulsions have been studied previously at

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room temperature

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method to reduce this destabilization process may be tune the emulsification

65

temperature. Hence, a systematic study of the influence of processing

66

temperature has been carried out in order to enhance the stability of these

67

ecological emulsions.

19

and they showed important flocculation problems. One

68 69

2. Materials and methods

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2.1. Materials

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N,N-Dimethyl Decanamide (Agnique AMD-10TM) was provided by BASF as a

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gift. D-Limonene was supplied by Sigma Chemical Company. Glycereth-17

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,Cocoate (Levenol C-201TM), was used as non-ionic surfactant. It was provided

74

by KAO Chemicals. In order to reduce the foam, an antifoaming agent was

75

used. Its trade name was RD antifoam emulsion, supplied by Dow Corning. The

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continuous phase was prepared with deionized water.

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2.2. Emulsion preparation

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Deionized water, 0.1 wt% antifoam emulsion and 4 wt% of Levenol C-201TM

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composed the continuous phase. The emulsions contains 40 wt% of two green

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solvents: 75 wt% AMD-10 and 25 wt% D-Limonene. The influence of the ratio

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of solvents was previously studied 20.

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Emulsions were prepared mixing both phases using a rotor-stator homogenizer

83

(Silverson L5M), at 8000 rpm during 60 seconds in a thermostatically-controlled

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water bath at 5, 15, 25, 35 or 45 ºC. Dispersed and continuous phase were

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previously tempered in the same bath.

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2.3 Droplet size distribution measurements.

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Laser diffraction technique was used to measure droplet size distributions

88

(Mastersizer X, Malvern, Worcestershire, United Kingdom) in triplicate for each

89

sample. In addition, the influence of aging time on droplet size distributions was

90

studied at 1, 7, and 28 days after preparation.



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Sauter diameter (D3,2) and volume mean diameter (D4,3) were used as reference

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mean diameters: N

D3, 2 = ∑ ni d i

93

N 3

i

i =1

i =1

2

Eq. (1)

3

Eq. (2)

i

i =1

N

D4,3 = ∑ ni d i

94

∑n d N

4

∑n d i

i

i =1

95

where di is the droplet diameter, N is the total number of droplets and ni is the

96

number of droplets having a diameter di.

97 98

2.4. Rheological measurements.

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All rheological measurements were carried out using a Haake MARS controlled-

100

stress rheometer (Thermo-Scientific, Germany). All tests were performed using

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a sandblasted double-cone geometry with an angle of 0.017 rad and a diameter

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of 60 mm. Small Amplitude Oscillatory Shear (SAOS) tests were conducted

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from 20 rad/s to 0.05 rad/s using a stress within the linear viscoelastic range. In

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addition, flow curves (0.05- 5 Pa) were performed using a multi-step protocol.

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2.5. Multiple light scattering

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Turbiscan Lab Expert was used in order to measure the backscattering profiles

107

of the samples for 30 days at 20 ºC Multiple light scattering technique allows

108

destabilization processes to be detected and quantified in different samples

109

such as emulsions, suspoemulsions and suspensions 21,22.

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2.6. Cryo-scanning electronic microscopy (cryo-SEM)

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Samples were placed on a sample holder and plunged into nitrogen slush.

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Frozen samples were etched and coated with gold and subsequently were kept

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at -120ºC for observation.

114

2.7. Statistical analysis.

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Laser diffraction and rheological tests were conducted three times each sample.

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These resulting data were analysed using one-way analysis of variance

117

(ANOVA). This was carried out using Microsoft excel 2013 with a significance

118

level of p= 0.05.

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3. Results and discussion 4 ACS Paragon Plus Environment

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Figure 1 shows Droplet Size Distribution (DSD) for emulsions studied as a

121

function of processing temperature at one day of aging time. All emulsions

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showed bimodal distributions, even trimodal distribution for the emulsion

123

processed at 15 ºC. This polidispersion is characteristic of this type of system

124

containing these eco-friendly solvents

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DSD in emulsions processed up to 35 ºC. However, there is a shift towards

126

bigger droplet sizes from 35 ºC to 45 ºC. This fact could be related to a

127

recoalescence process, which might be attributed to the increased molecular

128

movement and the enhanced collision probability between droplets at higher

129

emulsification temperatures

130

submicron mean diameters (Table 1).

131

Figure 2A illustrates the mechanical spectra at 20 ºC for emulsions processed

132

at different temperatures. There are two different behaviours: while emulsions

133

processed above 15 ºC show G’ higher than G’’ in all the frequency range

134

studied, emulsions processed at 5 ºC and 15 ºC show a crossover point. The

135

latter is typical of weakly structured materials. In this sense, G′ is lower than G″

136

in the lower frequency regime up to the crossover point (ω*), and G′ is higher

137

than G″ in the higher frequency regime above ω*. This crossover frequency

138

determines the onset of the terminal relaxation zone. The terminal relaxation

139

time (tr) was calculated as the inverse of ω* and decreased from 5 to 15 ºC

140

processing temperature. This fact indicates a reduction in the elastic nature of

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the system, which is related to a weaker structure. Shorter relaxation times lead

142

to relatively fast rearrangements and correlate well with the instability of

143

emulsions against creaming. Conversely, longer relaxation times point out that

144

the droplet-droplet interactions are stronger. This is currently correlated with

145

greater

146

suspoemulsions

147

structured than those processed at 15 ºC. It is important to highlight that the

148

emulsion processed at 15 ºC showed a third population of droplets in DSD.

149

Hence, this fact could be the cause of the weaker structure

150

increase of processing temperature above 15 ºC provokes an increase of both

151

viscoelastic parameters (G’ and G’’) that is not related to droplet-size effect.

152

This fact could be due to an increase of the flocculation since the collision

macroscopic 24,22

5

20,23

. There are no significant changes in

. In spite of this fact, all emulsions showed

stability

against

creaming

in

emulsions

and

. Therefore, the emulsion processed at 5 ºC is more


25

. However, the


153

frequency is higher with temperature. Hence, it could be related to a significant

154

increase of flocculation from 15 ºC to 25 ºC considering the jump in viscoelastic

155

parameter observed although DSD did not show significant differences.

156

Nevertheless, emulsions processed at 45 ºC exhibited lower viscoelastic

157

functions than those prepared at 35 and 25 ºC. This is due to the higher droplet

158

size that this system showed. Furthermore, 25 and 35 ºC emulsions presented

159

a minimum of G’’ at the characteristic frequency (ωc), which denotes a typical

160

weak gel-like behaviour. The plateau modulus associated to ωc are 79.25 and

161

74.03 Pa, respectively. This parameter has been previously used to distinguish

162

between grades of flocculation by Santos et al, 2016

163

emulsion could be more flocculated than its counterpart processed at 35 ºC.

164

Figure 2B shows flow curves for emulsions processed at different temperatures.

165

All emulsions exhibited a clear dependency of the viscosity with shear rate,

166

namely shear thinning behaviour. All curves were fitted fairly well to power-law

167

model (R2> 0.998) (Equation 3). = ·

168

19

. In this case, 25 ºC

Eq. (3)

169

Where is the viscosity, K is the consistency index, is the shear rate and n

170

the so-called “flow index”. Interestingly, the units of K depends on the flow index

171

values. This provokes that a direct comparison of this parameter with different

172

flow index is not possible. In order to avoid this, a modified power law equation

173

can be used 26,27:

= · (. 4) 1

174 175 176

where η1 is the viscosity value at 1s−1. The fitting parameters are shown in table 2.

177 178

Results of the ANOVA test demonstrated that there are significant differences in

179

η1 of the emulsions studied. The same trend can be found in both mechanical

180

spectra and η1 with processing temperature. η1

181

and 15 ºC are much lower than those processed at higher temperatures. This

182

fact is related to the flocculation grade aforementioned. In addition, there is a

of emulsions processed at 5


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decrease in η1 from 25 ºC to 35 ºC emulsions, which suggests a reduction of

184

flocculation grade. Furthermore, it is important to highlight the differences

185

between flow index (n) for the emulsions studied. Flow index for emulsions

186

processed above 15 ºC are much lower than those processed below this

187

temperature. These low values have been shown previously in flocculated

188

emulsions

189

emulsions were more flocculated than those prepared at lower temperature.

190

Figures 3A and 3B show cryo-SEM micrographs of the emulsions processed at

191

5oC and 35oC, respectively. Both figures reveal a continuous network-like matrix

192

formed by interconnected droplets chains. However, it is easier to detect

193

independent droplets in emulsion processed at 5 ºC. This fact is associated to

194

the lower flocculation of this emulsion in relation to that processed at 35 ºC.

195

Furthermore, these micrographs support laser diffraction results about droplet

196

size.

197

Figure 4 shows the influence of aging time on volumetric diameter for emulsions

198

processed at different temperatures. Results of the ANOVA test demonstrated

199

significant differences in volumetric diameter of day 1 in related to day 28 for all

200

emulsions studied. An increase of volumetric diameter in both different grade

201

and different aging times can be observed for all emulsions studied. Emulsions

202

processed above 15 ºC showed an increase of volumetric diameter from day 7

203

of aging time. However, all emulsions prepared up to 15 ºC did not present this

204

increase until day 28. On top of that, the emulsion processed at 25 ºC showed

205

the greatest coalescence. Interestingly, this emulsion seemed to be the most

206

flocculated taking into account the rheological behaviour. Hence, this points out

207

that the droplets merged after a period of flocculation. On the top of that, the

208

lowest increase is underwent by the least emulsion flocculated (5 ºC emulsion).

209

Therefore, these results supports the hypothesis about the different grades of

210

flocculation. However, the emulsion prepared at 15ºC , which seemed to be

211

less flocculated than the 5ºC emulsion, showed a higher increase of volumetric

212

diameter at 28 days of aging time. This is probably due to the occurrence of the

213

third population of droplets just after preparation. This multimodal distribution

214

could be the main drawback of this emulsion.

19,28

. Hence, it supports the SAOS results that pointed out these



215

Figure 5 shows the influence of aging time on complex viscosity as a function of

216

processing temperature. 25, 35 and 45 ºC emulsions presented a decrease of

217

complex viscosity with aging time, which is related to an increase of droplet

218

size. However, 15 ºC emulsion showed a slight increase of this parameter. This

219

fact indicates a flocculation and/or creaming process. Hence, 15 ºC emulsion

220

not only underwent coalescence but also flocculation and/or creaming.

221

Furthermore, 5 ºC emulsion did not show significant changes of complex

222

viscosity with aging time (ANOVA test). This may be a cause of antagonistic

223

mechanisms occurring simultaneously. In order to clarify this point, a Multiple

224

Light Scattering study has been carried out.

225

Figures 6A and 6B show the variation of Backscattering (BS) as a function of

226

measuring cell height with aging time for emulsions prepared at 5 ºC and 15 ºC,

227

respectively. Figure 6B has been chosen as a way of example for the emulsions

228

prepared at 15 ºC and above since the BS curves for 25ºC, 35 ºC and 45 ºC

229

emulsions presented the same trends. Figure 6A does not show a clear trend in

230

the first 24 hours of aging time, probably due to structural reorganization. After

231

the first 24 hours, an increase of BS in the low part of the measuring cell is

232

detected. This fact points out a flocculation mechanism in the bottom of the

233

measuring cell. Furthermore, this flocculation provokes an oiling off process in

234

the upper zone of the measuring cell. No variation in BS in the middle part was

235

detected. It is important to note that the variations of BS in the middle part of the

236

measuring cell are related to a process of flocculation and/or coalescence. This

237

methodology does not permit us to differentiate between these two

238

mechanisms. Hence, the coalescence/flocculation is not extensive in all the

239

measuring cell. Coalescence took place just in the top of the measuring cell.

240

Therefore, a flocculation/coalescence and oiling off mechanism occurred.

241

Figure 6B shows a decrease of BS in the low zone of the measuring cell until 16

242

days of aging time. After this time, an increase of BS in this part can be

243

observed. However, this increase is due to the extensive coalescence and/or

244

flocculation that takes place in all the measuring cell. Thus, that creaming could

245

be covered up by flocculation and/or coalescence after day 16. Furthermore, a

246

decrease of BS in the upper part is observed, which is related to oiling off

247

process. A similar behaviour was shown for emulsions prepared at 25 ºC, 35 ºC 8 ACS Paragon Plus Environment

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248

and 45 ºC. It is important to highlight that only the destabilization of emulsions

249

prepared above 25 ºC were detected by naked-eye in the study time.

250

Only the increase of BS in the middle part of the measuring cell has been

251

analysed since the creaming process in the emulsions studied could be covered

252

up by flocculation/coalescence. Figure 7 shows the BS variation in the middle

253

zone of the measuring cell with aging time for emulsions prepared at different

254

processing temperatures. The variation of BS in the middle zone is directly

255

related to the increase of droplet size or floc size. This method can not

256

distinguish between a floc or a droplet. No variation in BS was presented for

257

emulsion prepared at 5 ºC, found to be the most stable emulsion studied.

258

Conversely, a clear increase of BS for the other emulsions studied in different

259

grades was detected. Flocculated emulsions (those prepared above 15 ºC)

260

exhibited higher increases of BS than that prepared at 15 ºC. Finally, MLS

261

results supports laser diffraction and rheology results.

262

Conclusions

263

Processing temperature did not show a big influence on DSD below 35 ºC.

264

Interestingly, emulsions prepared at 5 ºC and 15 ºC exhibited viscoelastic

265

properties, which are typical of weakly structured materials. Emulsions prepared

266

at 25 ºC, 35 ºC and 45 ºC showed a weak-gel behaviour. 25 ºC and 35 ºC

267

emulsions showed very similar behaviour but with different values of Plateau

268

modulus. Those differences of this parameter pointed out different grades of

269

flocculation of these emulsions since no droplet-size effect was detected. Flow

270

curves showed shear-thinning behaviour for all emulsions studied. Emulsions

271

prepared above 15 ºC exhibited very low values of flow index, which is

272

consistent with flocculated emulsions. All emulsions presented coalescence but

273

in different grades being 5 ºC the emulsion that showed the least increase in

274

droplet size with aging time. Laser diffraction and rheological results as a

275

function of aging time supported the hypothesis about different grades of

276

flocculation induced by processing temperature. Hence, a strict control of the

277

preparation temperature is necessary in order to tune flocculation grade and

278

slow

279

demonstrated to be a powerful tool to show the slight structural differences

280

between emulsions with similar DSD but with different stability. Multiple Light

down

the

destabilization

process.

Furthermore,


rheology

has


281

Scattering has been an important method to clarify the destabilization

282

mechanisms that were taking place simultaneously in these emulsions. On top

283

of that, emulsion prepared at 5 ºC showed the best overall stability, which make

284

it attractive for a potential application as a possible matrix for agrochemical

285

products. In addition, this emulsion possesses a more fluid behaviour making

286

easy its handling and performance.

287

Acknowledgements

288

This article is based upon work from COST Action MP1305, supported by

289

COST (European Cooperation in Science and Technology). In addition, the

290

financial support received (Project CTQ2015-70700-P) from the Spanish

291

Ministerio de Economia y Competitividad and the European Commission

292

(FEDER Programme) as well as from V Plan Propio Universidad de Sevilla is

293

kindly acknowledged.

294 295

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Muñoz, J. Surface and Foaming Properties of Polyoxyethylene Glycerol

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Development and Rheological Properties of Ecological Emulsions

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Formulated with a Biosolvent and Two Microbial Polysaccharides.

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Colloids Surfaces B Biointerfaces 2016, 141, 53.

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Stability of N, N-Dimethyldecanamide/α-Pinene-in-Water Emulsions as

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Influenced by Surfactant Concentration. Colloids Surfaces B Biointerfaces

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(19) Santos, J.; Calero, N.; Munoz, J. Optimization of a Green Emulsion Stability by Tuning Homogenization Rate. RSC Adv. 2016, 62 (6), 57563. (20) Santos, J.; Trujillo‐Cayado, L. A.; Calero, N.; Muñoz, J. Physical

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Characterization of Eco‐friendly O/W Emulsions Developed through a

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and O/W/O Wax Emulsion Stability. Colloids Surfaces A Physicochem.

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391

Figure captions

392

Figure 1. Influence of processing temperature on droplet size distributions for

393

the emulsions studied.

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Figure 2A. Mechanical spectra for the emulsions studied as a function of

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processing temperature at 20 ºC.

396

Figure 2B. Flow curves as a function of processing temperature for the

397

emulsions studied at 20 ºC. Lines represented the fitted to power-law model.

398

Figure 3A. Cryo-SEM micrograph of the green emulsion processed at 5oC.

399

Figure 3B. Cryo-SEM micrograph of the green emulsion processed at 35oC.

400

Figure 4. Influence of processing temperature on volumetric diameter with aging

401

time.

402

Figure 5. Influence of aging time on complex viscosity at 0.1 rad/s as a function

403

of processing temperature.

404

Figure 6A. Variation of backscattering versus measuring cell height as a

405

function of time for the emulsion prepared at 5ºC.

406

Figure 6B. Variation of backscattering versus measuring cell height as a

407

function of time for the emulsion prepared at 15ºC.

408

Figure 7. Variation of backscattering in the middle zone of the measuring cell (5-

409

25 mm) as a function of processing temperature at room temperature.

410


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411

Table 1. Sauter diameter for the green emulsions studied as a function of

412

processing temperature.

413 T (oC) 5 15 25 35 45

D(3.2)(µm) 0.34 ± 0.03 0.34 ± 0.03 0.34 ± 0.03 0.33 ± 0.03 0.44 ± 0.05

414 415 416

Table 2. Flow curves fitting parameters for power-law model for the emulsions studied as a function of processing temperature. T (oC)

5 15 25 35 45

η1 (Pas) 0.53 0.65 3.26 1.8 1.8

417 418

Standard deviation of the mean (three replicates) < 8%

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Standard deviation of the mean (three replicates) < 10%

420


n 0.40 0.40 0.18 0.19 0.15


421

Graphical abstract

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Figure 1. Influence of processing temperature on droplet size distributions for the emulsions studied. 296x209mm (150 x 150 DPI)

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Figure 2A. Mechanical spectra for the emulsions studied as a function of processing temperature at 20 ºC. 296x209mm (150 x 150 DPI)


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Figure 2B. Flow curves as a function of processing temperature for the emulsions studied at 20 ºC. Lines represented the fitted to power-law model. 296x209mm (150 x 150 DPI)



Figure 3A. Cryo-SEM micrograph of the green emulsion processed at 5oC. 52019x39014mm (1 x 1 DPI)


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Figure 3B. Cryo-SEM micrograph of the green emulsion processed at 35oC. 52019x39014mm (1 x 1 DPI)



Figure 4. Influence of processing temperature on volumetric diameter with aging time 296x209mm (150 x 150 DPI)


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Figure 5. Influence of aging time on complex viscosity at 0.1 rad/s as a function of processing temperature. 296x209mm (150 x 150 DPI)



Figure 6A. Variation of backscattering versus measuring cell height as a function of time for the emulsion prepared at 5ºC 296x209mm (150 x 150 DPI)


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Figure 6B. Variation of backscattering versus measuring cell height as a function of time for the emulsion prepared at 15ºC. 296x209mm (150 x 150 DPI)



Figure 7. Variation of backscatering in the middle zone of the measuring cell (5-25 mm) as a function of processing temperature at room temperature. 296x209mm (150 x 150 DPI)


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The Role of Processing Temperature in Flocculated Emulsions

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