Physicochemical and rheological characterization of diesel fuel nanoemulsions at different water and surfactant contents

Journal of Molecular Liquids 231 (2017) 440–450

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Physicochemical and rheological characterization of diesel fuel nanoemulsions at different water and surfactant contents M.R. Noor El-Din ⁎, D.I. Osman, Ahmed M. Rashad, Marwa R. Mishrif, E.A. El-Sharaky Egyptian Petroleum Research Institute (EPRI), 1 Ahmed El- Zomor St., Nasr City, Cairo 11727, Egypt

a r t i c l e

i n f o

Article history: Received 13 October 2016 Accepted 31 January 2017 Available online 3 February 2017 Keywords: Nanoemulsion Ostwald ripening Rheology behivour Calorific value

a b s t r a c t This work aiming to produce a diesel fuel nanoemulsions with high performance exploited a new preparation technique of low energy emulsification method namely batch addition. This technique helps in production of diesel fuel nanoemulsions has a small water droplet sizes, highly transparent appearance and performance from points of view of physicochemical and rheology behaviors over a period of time as 3 months. To achieve this aim, twenty nanoemulsions were prepared at conditions of water and surfactant contents ranging from [5 to 10] to [4 to 10] wt/wt, of total weight of nanoemulsion, respectively at ambient temperature. The prepared nanoemulsions were evaluated for the rheological behaviors at interval time as 0, 1, 2 and 3 months. The results indicated that the rheological properties of the prepared emulsions behave as non-Newtonian flow in the range of shear rate from 132 to 191 s−1 with a yield value (τ) ranged from 2.14 to 5.11 D/cm2 at 6 wt% water content, 30 °C and 2 months time laps followed by Newtonian regime. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Generally, emulsions possess a droplets size range from 10 to 100 μm. Because of the unfavorable contact between the water and the oil droplets in the emulsified system, emulsion is unstable thermodynamically system, therefore, the collapse of the emulsion system with time is toke place [1]. Nanoemulsions are a category of emulsions that have small droplets size dimensions of 50 to 200 nm [1]. Recently, nanoemulsions are utilized in several industrial sectors as agriculture, food, textiles and fuel emulsions [1,2]. In comparison with other emulsions types, nanoemulsion is kinetically stable for long time. It keeps the water droplet diameter predominantly smaller than others emulsions over time [3]. The stabilization of nanoemulsions is governed by two mechanisms namely; coalescence and Oswald ripening [4]. The most important theory that utilized to calculate the Oswald ripening is Lifshitz–Slyozov and Wagner theory [5,6]. The outcomes of the droplet emulsion stabilization depend mainly on the concentration of dispersed phase and surfactant, and the methods used in the emulsion formation. In several industries, nanoemulsions are formed using two emulsification methods namely: low- [7] and/or high-emulsification methods [8]. These methods create violent demolished forces that mechanically dismantle the oil into very teeny droplets [9]. Although high-energy emulsification methods allow a great control of the droplet size and a large choice of composition, low-energy emulsification methods are interesting because they take advantage of the energy stored in the ⁎ Corresponding author. E-mail address: [email protected] (M.R. Noor El-Din).

http://dx.doi.org/10.1016/j.molliq.2017.01.104 0167-7322/© 2017 Elsevier B.V. All rights reserved.

system to promote the formation of small droplets [10]. High energy method as high pressure homogenization (HPH) and ultrasonication consume significant energy (ranging from108 to1010 W kg−1) to perform tiny droplets. In contrast, for low energy method, the consuming energy utilized to form small droplets by the formation of internal chemical energy of the system is (≈103 W kg−1) [10]. In the last two decades, direct emulsification systems (i.e. membranes and microfluidic devices) have been introduced for producing monodisperse emulsions at low energy consumption. Unlike in homogenization, in these systems droplets are made at their final size without further refinement. Microfluidic emulsification devices can be divided into two categories based on the droplet formation mechanism: shear-based and spontaneous or interfacial tension driven. In shear-based systems (e.g. T-, and Yjunctions) the flow of both phases influences the droplet size, while in spontaneous systems (e.g. microchannels and EDGE devices) only the dispersed phase does so [11]. Nanoemulsion fuel is a finely dispersed mixture of water in diesel fuel as continuous phase without visible separation. Some known advantages of nanoemulsion fuel are improved the combustion efficiency and reduced the polluted emissions that are released from the diesel engine as nitrous oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO) and carbon dioxide (CO2) [12]. The most factors affecting the stability and engineering performance of nominations are its theological properties. It is known fact that the rheology of emulsion necessary for using it in some industrial applications as fuel processing [13]. However, the slightly change of the water droplet formulations and/or Ostwald repining process causes the rheological behavior of the nanoemulsion to be modified. The increase in water content in animation causes its behavior to be pseudo-plastic and to

M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450

be time-dependent at higher shear rate [14]. As a fact, two factors have a great effect on the rheological properties of emulsions system as the viscosity of continuous phase and the interfacial rheology between the droplets [15]. Three different forces play a key note in the rheology of nanoemulsion described by [16,17]. These forces are 1) Brownian Diffusion, 2) hydrodynamic interaction and 3) surface forces (repulsive or attractive). This work aims to prepare highly kinetically stable water-indiesel fuel nanoemulsions using a modified low energy emulsification technique, which used at previously described work [17]. The performance of the prepared nanoemulsions was evaluated from points of view of physicochemical and rheology characterizations over a long period of time as 90 days. To achieve this objective, twenty water-in-diesel fuel nanoemulsions were prepared and evaluated for the stability at optimum HLB during different period of time laps as 0, 15, 30, 60 and 90 days. The rheological characteristics of the formed nanoemulsions were studied as a function of water loading, surfactant concentration, temperature and storage time.

utilized for forming stable nanoemulsion [18]. The mixed HLBMTS value was calculated as follows: HLBMTS ¼ ðHLBT5  ðT5Þ%Þ þ ðHLBS8  ðS8Þ%Þ

2.1. Diesel fuel A local commercial sample of diesel fuel (Grade 2) was supplied by Misr petroleum company, Assiut, Egypt and has the physicochemical properties of the used diesel fuel as illustrated in Table 1. Before emulsion preparation, diesel fuel sample was filtrated several times to fulfillment its purity. 2.2. Surfactants Two different analytical grades of non-ionic emulsifiers namely: polyoxyethylene 20 sorbitan trioleate (HLB = 11) and sorbitan monooleate (HLB = 4.3) donated as T5 and S8, respectively as emulsion were purchased from Fluka Chemie GmbH, Germany. The water used in all experiments was bi-distils.

2.3.2. Water droplet size measurement The water droplet radii (Zavg) of the prepared diesel fuel nanoemulsions were measured by dynamic light scattering (Malvern Zetasizer HT-ZS, Worcestershire, United Kingdom) at scattering angle 173o with an argon-laser (λ = 488 nm) and working temperature 25 °C. The mean hydrodynamic diameter (Dh) was calculated by the Stokes-Einstein Eq. (2) [19]:

2.3.1. Nanoemulsions formation (batch addition) Twenty samples of water-in-diesel fuel nanoemulsions were prepared by one-step low energy method namely batch addition method described by [17]. In this step, 5, 6, 7, 8, 9 and 10% (wt/wt) of bi-distilled water was added to a mixture of diesel fuel and blend emulsifiers of (T5) and (S8). The concentration of the blend emulsifiers (MTS) is 4, 6, 8 10% (wt/wt) from the total weight of emulsion. Water was dosed in constant rate (0.2 ml/2 min) and continuous stirring rotated at 1500 rpm (Scheme 1). The optimum HLB value of 10 (HLBMTS) is

Table 1 Physical properties of pure diesel fuel.

Item o

Color at 40 C Flash point, o C Kinematic viscosity at 40 oC Minimum Maximum Specific gravity 60/60 oF Minimum Maximum Pour point at winter seasons, oC (maximum 3) Calorific value, MJ/kg Ash content, mass % Sulfur content, mass % Water content , %

ASTM standard D1500–07 D93–11

Standard Limit value* Minimum, 4 Minimum, 55

D445–12

1.6 7

1.8

D792 – 13

0.820 0.870

0.830

>–3

–3

Minimum, 44.3 Maximum, 0.01 Maximum, 1 Nil

44.6 0.01 1 Nil

D97–11 D4868–11 D482–11 D2622–11 D4006–11

kT 3πηDh

Results 5 59

ð2Þ

where; D is the diffusion coefficient, k is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the medium. The particle size and size distribution were determined using Contin analysis mode. The refractive index (rd) for pure diesel fuel and water were determined by ABBE refractometers (DR-A1, ATAGO Co., LTD, Japan) and were found to be 1.46 and 1.33, respectively. 2.3.3. Calorific value, CA, (heat of combustion) The calorific value of the pure diesel fuel and the prepared nanoemulsions was measured by Anton Parr 6200 isoperibol calorimeter, Parr instrument company, USA according to ASTM D4809. CA is determined using the following equation: Calorific value ¼ gross heat of combustion ðHgÞ  1:8

ð3Þ

where Hg is calculated as follows: Hg ¼

2.3. Formation of water-in-diesel fuel nanoemulsions

ð1Þ

where; HLBT5, HLBS5 and HLBMTS are the HLB values of T5 (11.0), S8 (4.3) and the mixed surfactants (MTS), and T5% and S8% are the mass percentages of T5 and S8 in the mixed surfactants, respectively [18].

D¼ 2. Material and methods

441

Temperature rise  energy equivelent of benzoic acid weight of the sample:

2.3.4. Rheological behavior of water-in-diesel fuel nanoemulsion The rheological behavior (dynamic viscosity) of pure diesel fuel, water and the prepared diesel fuel nanoemulsions were measured using a Brookfield Rheometer type (Brookfield Engineering Laboratories, Inc., Middleboro, MA 02346–1031, USA). The cone spindle was of type LV(SC4-18), BOB/STATOR (PVS-B1-D-HC). The cup type was Hastelloy C. the dynamic viscosity was measured at temperature and shear rates ranging from 20 to 60 °C and 1–331 s−1, respectively. The influence of time variation (aging time) as 0, 1, 2 and 3 months on the rheological behavior for all the prepared diesel fuel nanoemulsion were conducted at the same previously conditions of temperature and shear rates. 3. Results and discussion 3.1. Determination of the optimum hydrophilic-lipophilic balance value (HLB) One of the most factors affect the preparation of highly stable nanoemulsion is HLB value of the used emulsifiers [18]. For measuring the optimum HLBMTS, different HLBMTS values of the blend emulsifiers (MTS) as 9.6, 9.8, 10, 10.2 and 10.4 wt/wt, water content (ϕ) of 5 wt/wt% and MTS concentration of 10 wt/wt% was utilized. 14 days is the minimum time required and recommended for diesel fuel nanoemulsion stability to be satisfied. The smallest water droplet radii (Zavg) of 26.23 ± 1.1without phase separation was exhibited by HLBMTS

442

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Scheme 1. Schematic flow diagram batch addition method to prepare diesel fuel nanoemulsions.

of 10 as shown in Table 2. Accordingly, the optimal HLBMTS utilized for the following experiments is 10. 3.2. Nanoemulsion stability The water droplet growth ratio of diesel fuel nanoemulsion was measured to define its stability. However, the breakdown processes of the nanoemulsions depend on two different mechanisms namely; coalescence and/or Ostwald ripening [20]. Generally, the idea of Ostwald ripening (ω) is based on the solubility of different size of the droplets. It is known that Ostwald ripening (ω) was defined and explained mathematically by Lifshitz and Wagner [5,6] and it is calculated as follows: ω ¼ dr 3 =dt ¼ 8=9½ðC ∞ γ V m D=dRT Þ

ð4Þ

where r is the mean average radius of the droplet, t is the storage time, C∞ is the solubility of the dispersed phase in the continuous phase, γ is the interfacial tension between the dispersed phase and the medium, Vm is the molar volume of the dispersed phase, D is the diffusion coefficient of the dispersed phase in the continuous phase, d is the density of the internal phase, R is the universal gas constant and T is the absolute temperature. Based on this equation, cubic of the water droplet radius (r3) and storage time (t) for different prepared diesel fuel nanoemulsions are shown in Fig. 1. Ostwald ripening rate of each prepared diesel fuel nanoemulsion is defined as the slope of its own straight line. The start and final water droplets radii, the Ostwald ripening rate of the diesel fuel nanoemulsions prepared are illustrated in Table 3. Results in Table 3 illustrate that the most stable diesel fuel nanoemulsion at zero time (starting point) that has the lowest ω value of 8 × 1024 m3 s−1 was obtained in case of using 10% wt/wt MTS concentration, 5% wt/wt water content, initial water droplet of 26.23 nm and working temperature 20 °C. Furthermore, the mean average water droplets size increases Table 2 HLBMTS values-water droplet size (zavg.) relation at 5 wt% water content, 10 wt% MTS and 20 °C. HLBMTS

Water droplet, nm

9.6 9.8 10 10.2 10.4

124.7 30.62 26.23 36.00 133.6

from 26.23 to 31.52 nm and ω value decreases from 8 × 1024 to 0.6 × 1024 m3 s−1 after 360 h (14 days as standard measuring time). This may be explained based on ω that the smallest water droplet size is attracted to the nearest bigger one by time. So, the final size enlarged to 31.52 nm [21]. The slight increase in the water droplet size (instability of nanoemulsion) may regards to the influence of Brownian diffusion that inhibit increase of water droplet size under Oswald ripening effect [22]. 3.3. Factors affecting the nanoemulsion stability 3.3.1. Surfactant concentration (MTS) To understand the effect of emulsifier concentrations on the stability of diesel fuel nanoemulsions over 14 days, two factors namely; Ostwald ripening rate (ω) and water droplet size will be studied. By studying the impact of MTS concentration on Ostwald ripening rates (ω), it is clear that ω diminishes from 9 to 0.9 × 10−24 m3 s−1 with increases MTS concentration from 4 to 10 wt% constant conditions of 5% wt/wt water content, optimum HLBMTS of 10 and working temperature of 20 °C (Table 3). This fact is considered a backfired to most theories previously reported in the literature [15]. This may be regarded to the following factors [23]: 1) increasing of MTS concentration result in increases the total surfactants covered the water droplet and decreases the interfacial tension of nanoemulsion system, 2) increasing of the emulsifiers concentration to certain point result in increase the number of micelles in the bulk solution. Hence, the flux (J) factor increases. J = D (ϭC/ϭx), where D and (ϭC/ϭx) are the molecular diffusion coefficient and the concentration gradient given by Fick's first law of the diesel fuel, respectively, 3) Due to the presence of more than one type of surfactant (T5) and (S8), the absorption of them on the water droplet interface leads to form a rigid interfacial membrane, 4) Due to the Brownian motion is generally affected MTS concentration and water droplet size, it notice that by increases the water droplet size, the Brownian motion increases and ω decreases and 5) The increase in the MTS concentration cause the aggregations of emulsifiers and Gibbs elasticity to increase, hence, ω increases. On the other hand, the effect of MTS concentration on the growth of water droplet size is illustrated in Table 3. For all surfactant concentrations, lowering of the MTS concentration from 10 to 4% (wt/ wt), it causes water droplet size to rise. This may attribute to increase the surfactants film intensity that prevents the water droplet from aggregation [24]. Concerning linearity of Ostwald ripening, it is notice that there is no linear relationship between Ostwald ripening and

M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450

443

Fig. 1. Nanoemulsion r3 as a function of time in a system of water/MTS/diesel fuel at 6 wt% MTS, HLBMTS = 10, different water contents, 20 °C and 360 h.

surfactant (Fig. 1). This could be interpreted that the adsorption of each S8 and T5 on the water surface during the formation of nanoemulsion system occurs physically (depends on the solubility of each on the continuous phase) more than mathematically [25].

diesel fuel interface. Consequently, the existence of a limited amount of surfactant leads to increase the interface film thinning, thus, the opportunity of water aggregation increases and ω decreases [23].

3.3.2. Water loading One of the more interesting phenomena, which take place during the formation of water-in-diesel fuel nanoemulsion, is the quantity of water added to the nanoemulsion system. The effect of water content as 5, 6, 7, 8, 9 and 10% wt/wt on the stability of nanoemulsion system at 6% (wt/wt) of MTS and HLBMTS of 10 was investigated. Table 3 indicates that the water droplet size growth from 30.91 to 188 nm with increasing of the water content from 5 to 10% (wt/wt) after 14 days, respectively. It is also noticeable that the value of Ostwald ripening (ω) decreases from 3 to 0.006 × 1024 m3 s−1 at the same tested water content. The main reason for this phenomenon may be explained as follows: at any emulsion system, the stability of emulsion system mainly depends on the concentration of surfactant that adsorbed on water/

3.3.3. Storage stability In this section, the variation of the droplet size growth of the formed nanoemulsions as a function of time was assessed for interval time of 0, 1, 2 and 3 months. At constant surfactant concentration, as the water content increases from 5 to 10% (wt/wt), the mean droplet size of the dispersed water increases with increasing the storage time up to 3 months (Table 4). On the other hand, as the surfactant concentration of the prepared diesel fuel nanoemulsion increases, at a fixed quantity of water, the size of the water droplets diminished for the same storage period. Generally, it is notice that there is an increase in Ostwald ripening (ω) value with time from 29.60 to 75.70 nm at 8% MTS concentration, 5% water content and interval time 0 to 90 days, respectively. This explained by enlargement in average particle size. So, the mean cause of instability or phase separation (eg. 10 water content and 4% surfactant (wt/wt)) occurrence. As mentioned before, Ostwald ripening (ω) is considered the main responsible factor to determine the stability of the nanoemulsions process as a function of time. As Ostwald ripening (ω) value increases, the chance of formation of water droplets in the biggest size is high. This may causes instability of the prepared nanoemulsion. In some cases, phase separation may be occurred in high and low percentages of water and surfactant as 10 and 4% (wt/ wt), respectively. Fig. 2 show that, for nanoemulsion contain 5 wt% water content, there is a monomodal particle size distribution with a slightly increasing in the average water droplet size (Zavg) from 26.23 to 47.48 nm in the interval of time ranging from 0 to 60 days. A bimodal distribution with (Zavg = 59.06 nm) appears in the period of 60– 90 days, whereas, one large peak (donated as (a)) concentrated around 51.23 nm (intensity percentage 74.6%), and another broad band of 3538 nm (donated as (b) and intensity percentage of 24.2%) was observed. The appearance of the last band means that the increase of water droplet size by coalescence process was started. So, the most stable water-in-diesel fuel emulsion among those nanoemulsions could only be stored for 90 days.

Table 3 Effect of water content and MTS concentrations on the initial radius (ro) and Ostwald ripening rate (ώ) for water-in-diesel oil nanoemulsion at 25 °C.

Water content, wt% 5

6

7

8

9

10

MTS concentration, % 4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10 4 6 8

ro, nm 31.58 30.91 29.00 26.23 34.81 31.47 29.85 28.00 77.05 34.50 33.55 30.51 124.7 35.96 34.08 33.14 182.8 43.31 37.93 35.26 220.5 188.0 50.13 48.08

–1

,m 3s × 10 24 0.9 3 5 9 0.4 0.1 0.15 8 0.08 0.5 0.6 7 0.06 0.02 0.03 6 0.004 0.01 0.02 0.3 0.003 0.006 0.007 0.08

3.4. Physical properties of the prepared nanoemulsions The physical properties of the prepared nanoemulsions as kinematic viscosity, density, calorific value will be detected as follows:

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Table 4 Storage stability of the prepared nanoemulsion at different water content and surfactant concentration after 0, 1, 2 and 3 months and 20 °C.

Water content, % Surfactant concentration, % Storage stability, months** 0 1 2 3

5 4

6

6 8

10

4

6

7 8

10

4

8

6

8

10

4

6

9

10

8

10

4

6

8

10

4

6

8

10

Water dropl et size *, % 31.58

30.91

29.00

26.23

34.81

31.47

29.85

28.00

77.05

32.55

33.55

30.51

124.7

35.96

34.08

33.14

182.8

43.31

37.93

35.26

220.5

188

60.80

48.08

49.57

40.23

37.84

33.53

56.10

43.90

40.42

38.29

90.87

47.59

46.68

43.63

150.3

55.07

50.55

48.08

194.7

69.69

68.97

54.53

230.3

199.8

75.70

63.80

78.21

59.10

52.43

47.48

88.94

66.79

61.96

50.38

135.3

78.10

70.02

54.68

190.4

83.29

72.67

65.29

220.9

153.7

95.75

81.27

250.1

202.7

160.5

95.74

98.89

79.29

75.70

59.06

101.7

87.62

79.63

61.93

176.8

133.6

117.9

64.91

218.4

146.8

134.8

74.45

266.8

166.1

156.2

97.65

277.1

244.4

184.5

165.3

*: The average water droplet size (nm) was measured for three times. **: Stability of the emulsion after 3 months: : Stable emulsion with undesired droplet size (> 200 nm). : Unstable emulsion (completely phase separation).

3.4.1. Kinematic viscosity of nanoemulsions The kinematic viscosity of both pure and all prepared diesel nanoemulsion was realized at different temperatures ranging from 20 to 60 °C and storage times 0, 1, 2 and 3 months as illustrated in Table 5. Generally, the kinematic viscosity of the prepared nanoemulsion is affected by three factors as water content, surfactant concentration and applied temperature as follows: 1. At constant water content (5% wt/wt) and working temperature (40 °C), the decreases of surfactant concentration from 10 to 4% causes the kinematic viscosity to diminish. This may regrade to insufficient surfactant molecules that adsorbed on the water/diesel fuel interface 2. At constant surfactant concentration (8% wt/wt) and working temperature (40 °C), the kinematic viscosity was increased with increases the amount of water. This may attribute to increase the rate of coalescence between each two neighbors droplets [26,27]. 3. As expected, the kinematic viscosity rate reduces non-linearly with increase in the applied temperature. This means that the prepared nanoemulsions depict temperature-independent behavior [26]. 3.4.2. Density of nanoemulsions According to the results obtained in Fig. 3, the density of the prepared nanoemulsion is depended on two factors as volume of dispersed phase (water content) and temperature as follows: 1) at constant working temperature of 30 °C, the density of the prepared nanoemulsions increases significantly with the increase of water content. This may regard to the action of London–van der Waals forces. With increasing the London–van der Waals forces between two droplets, the distances between

Fig. 2. Effect of time retardation on the droplet size distribution in a system of water/MTS/ diesel fuel at 5 wt% water content, HLBMTS = 10, 10 wt% MTS concentrations and ambient temperature after 0, 30, 60 and 90 days.

two water droplets are decreases. As a consequence, the density of system decreases. 2) At variable temperature ranging from 20 to 60 °C, it is notice that the behavior of density for both pure diesel fuel and the prepared nanoemulsions is temperature-dependent behavior. 3.4.3. Calorific value (CA) The calorific value of the pure diesel fuel and the prepared emulsion at different water content ranging from 5 to 9% wt/wt, 10 wt% MTS concentration and after 3 months are illustrated in Table 6. From the results obtained, it is clear that the calorific value of the prepared emulsion reduces with the increasing of the water content. This may be explained by the following reasons: 1) Lowering of the net volume of the diesel fuel in the emulsions burning. 2) Here, the diesel fuel emulsion system needs more energy to bring it up to a complete combustion. Thus, some part of the heating value was consumed during the water vaporization process [28]. This explains why the relationship between diesel fuel emulsion and calorific value is non-linear. From Table 6, it notice that the calorific value decreases significantly with the increase of emulsifier concentration. This may attribute to two reasons as follows: 1) the adsorption mechanism of each emulsifier of MTS on the water-in-diesel fuel interface and 2) the strength of the film formed of blend emulsifiers Table 5 Kinematic viscosity (cSt) of diesel fuel and the prepared nanoemulsions at different water contents, different MTS concentration, temperature range 20–60 °C and 3 months time lapse.

Kinematic Viscosity in (cSt) Surfactant concentration, % 20 °C 30 °C 40 °C 50 °C 0 3.59 2.85 2.32 1.97 2.88 2.43 4 4.77 3.58 6 4.81 3.69 2.98 2.51 5 8 5.06 4.18 3.08 2.59 10 5.83 4.36 3.43 2.81 4 5.24 3.74 3.06 2.54 6 5.68 4.19 3.21 2.62 6 8 5.81 4.74 3.30 2.85 10 6.17 5.01 3.51 2.89 4 5.67 4.49 3.23 2.75 6 5.85 4.67 3.44 2.79 7 8 6.01 5.34 4.15 2.91 10 6.34 5.56 4.98 3.01 4 – – – – 6 6.09 4.98 3.99 3.49 8 8 6.14 5.84 5.02 3.74 10 6.66 6.21 5.98 4.06 4 – – – – 6 6.19 5.15 4.55 4.12 9 8 6.35 6.19 5.46 4.34 10 6.94 6.51 6.01 4.97 4 – – – – 6 – – – – 10 8 6.87 6.37 5.56 5.45 10 7.14 6.82 6.11 5.65 :Out of desired droplet size (desired droplet size < 200 nm). Water–in–diesel fuel content, % Diesel fuel

60 °C 1.71 2.05 2.13 2.26 2.41 2.24 2.31 2.45 2.47 2.37 2.42 2.56 2.68 – 2.54 2.66 3.01 – 3.09 3.28 3.41 – – 4.21 4.91

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445

Fig. 3. Density of diesel fuel and the prepared nanoemulsions of water content 5–10 wt% at 7% surfactant concentration and temperatures 10–70 °C after 3 months time lapse.

around the water droplet. According to this, the energy needed to vaporize the water in formed nanoemulsions increases with increasing strength of the film formed on the water droplet. Thence, the calorific value depresses 3.4.4. Rheological measurement Most adsorbed surfactants and polymers coil at the water-in-oil interface show a non-Newtonian rheological behavior [28]. The rheological behavior (shear rate-shear stress) and (viscosity-shear rate) relationships curves for the water, pure diesel fuel and the prepared water-in-diesel fuel nanoemulsions at conditions of water content 5, 6, 7, 8, 9 and 10 wt%, 4 to10 wt% of MTS surfactant concentration and working temperatures range from 20 to 60 °C after 0, 1, 2 and 3 months time laps in the range of shear rates 132–310 s−1 are shown in Figs.4 and 5. Generally, all sets of nanoemulsions exhibit non-Newtonian flow behavior in the shear rate range (132–310 s−1). Concerning the viscosity-shear rate relationship, it is obvious from Fig. 4 that for the nanoemulsion systems exhibits a non-Newtonian flow character of low-shear viscosity in the range of 132 up to 191 s−1. When the shear rate is increased beyond 191 s−1, the emulsions become pseudoplastic (non-Newtonian behavior) over the entire range of water loading (ϕ). At low shear rate, the increase in viscosity and shear-thinning with increasing the volume fraction of dispersed phase may be attributed to the increase in the water droplet-droplet interactions, which causes the aggregation/coalescence of the water droplets [29]. As the volume fraction of the dispersed phase increases, the water droplets size increases and the water droplets density (number of droplets per unit volume of emulsions at a given dispersed phase volume fraction) increases. This facilitates the droplet–droplet collisions and coalescence of emulsion droplets. At this point, London–van der Ẅaals forces between the neighbors water droplets are dominant factor. With increasing the shear rate, the formed coalescent water releasing some amount of the continuous phase and resulting in a slight increase in viscosity [30]. In this regime, the most effective factor is Brownian diffusion, which may

Table 6 Effect of diesel fuel content and MTS concentrations on the calorific value for the pure diesel fuel and the prepared nanoemulsions after 3 months time lapse. Calorific value, MJ/kg Blank 46.6

MTS concentration, % 4 6 8 10 46.2 45.2 44.6 41.3

Diesel fuel content, % 95 94 93 46.2 41.35 36.74

92 33.13

91 29.44

90 27.17

prevent any increase in droplet size under the effect of Oswald ripening. Therefore, the increase of water diameter affects its flow properties inside the system. This phenomenon becomes considerable when approaching to the upper boundary of the domain of intermediate concentrations. Thus, non-Newtonian viscous flow for all the set nanoemulsion (low share rate) is replaced by a yield valuepseudoplastic system at high shear rate. Furthermore, it is clear that the viscosities of the nanoemulsions are of higher values than that for either water or diesel fuel per se [31]. Some factors affecting the emulsion rheology will be discussed in the following subsections:3.4.5. Factors affecting the rheological properties a. Water droplet size From Table 7 and Fig. 5, it is obvious that the mean water droplet size has a significant effect on viscosity of the nanoemulsion system. With the increase of the water loading in the nanoemulsion system, the mean water droplet size increases which lead consequently to increase the viscosity of the nanoemulsion. The explanation of this phenomenon may be attributed to the water droplet movement inside the diesel fuel [32]. A movement of liquid droplets inside a fluid medium under isothermal conditions can occur due to two reasons: 1) Brownian molecular fluctuations and 2) action of dynamic forces in the fluid flow. The ratio of these factors is determined by the dimensionless factor, the Peclet number, Pe, which is expressed as follows: Pe ¼

ηδ kb T=R3

ð5Þ

where η is the viscosity, σ is the shear rate, kb is the Boltzmann constant, Т is the absolute temperature, and R is the radius of a water droplet. The Peclet number is evidently the relationship between characteristic stresses, provided by dynamic (ηδ) and diffusional (kbT/R3) displacements. If Pe ≫ 1, the diffusion (or Brownian) movement can be neglected and the influential factor is the fluid dynamic force. As one can see, Pe depends strongly on the particle size (proportional to R3). So, molecular movements become noticeable for rather small particles only. They should be taken into consideration in the transition to the nano-size drops. An increase in water drop size (Pe ≪ 1) the most controlled power is the dynamic force than Brownian motion. The effect of water content on the viscosity flow curves for the water-in-diesel fuel nanoemulsion at water loading from 5 to 10 wt% at working condition of 8 wt% MTS surfactant, 40 °C and in the range of shear stress from

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M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450

Fig. 4. Rheological behavior (viscosity-shear rate relationship) of water, diesel fuel and the prepared nanoemulsions with water content 5, 6, 7, 8, 9 and 10 wt%, 8 wt% MTS concentration, 30 °C and zero time laps.

132 to 310 s−1 after 0 and 3 months interval time are shown in Table 8 and illustrated in Fig. 5, from which it is obvious that with increasing the water content from 5 to 9 wt%, the plastic viscosity of the set nanoemulsions increases significantly (with a Newtonian flow character of low-shear viscosity). This may be attributed to the following reasons: 1. As the dispersed water phase in the prepared nanoemulsion increases, the system tended to be unstable due to the limited amount of surfactants, which adsorbed on the water/diesel fuel interface. Thus, at a constant surfactant concentration and constant mixing rate, the surfactant molecules were adsorbed and the water droplet surface covered completely. With the increases of the water content, the number of surfactants that adsorbed on the w/o interface decreases (redistribution of the surfactant on the water surface). Accordingly, the chance of the water droplets agglomeration to form large size increases.

2. Since the solubility of the dispersed phase in the bulk phase is dependent upon the radius of curvature of the droplets, i.e. the solubility is increased with decreasing the radius, and thus will be decreased with increasing the water droplet size, this will lead in turn to large interfacial surface area (high interfacial tension) and in turn to more viscous water–diesel fuel interface monolayers stabilized by surfactant as previously mentioned and so viscosity increases with the increase of mean water droplet size [33,34] On the other hand, by increasing the water content up to 10 wt% at the same said conditions, a non-linearity behavior of the viscosity flow curve (the data do not fall on one line) was observed. This behavior was apparently related to the action of dynamic forces in fluid flow (Pe Number is 2.060 and 2.586 after 0 and 3 months, respectively). Another explanation of this phenomenon was mentioned by [35]. Because of the coalescence and the morphology of water droplet is affected by two forces as: 1) the driving forces of water droplet deformation

Fig. 5. Effect of water on the nanoemulsions viscosity at different water contents, 10 wt% MTS concentration, 60 °C and 3 months time lap.

M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450 Table 7 Plastic viscosity (cP) of diesel fuel and the prepared nanoemulsions at different water contents, different MTS concentration, temperature range 20–60 °C and 3 months time lapse.

Surfactant concentration %

Water content, %

20 °C

Diesel fuel (Blank)

0 5 6 7 8 9 10 5 6 7 8 9 10 4 6 7 8 9 10 4 6 7 8 9 10

4.32 5.64 6.16 6.62 – – – 5.59 6.64 6.83 7.28 7.36 – 6.07 6.71 6.95 7.02 7.19 7.58 6.83 7.23 7.41 7.71 8.01 8.24

4

6

8

10

Plastic viscosity in (cP) 30 °C 40 °C 50 °C 3.39 4.19 4.37 5.24 – – – 4.33 4.90 5.46 6.93 6.98 – 4.58 5.18 5.59 5.96 6.45 7.09 5.14 5.85 5.71 6.11 6.98 7.42

2.81 3.37 3.56 3.76 – – – 3.49 3.76 4.02 4.60 5.25 – 3.62 3.86 4.24 4.41 5.28 5.82 4.03 4.09 4.28 4.61 5.99 6.21

curve flow with up and down viscosity flow curve was noticed (unstable emulsion observed) (see Fig. 5). b. Surfactant concentration

60 °C

2.38 2.85 2.97 3.21 – – – 2.92 3.10 3.23 4.04 4.76 – 3.02 3.31 3.38 3.55 3.69 4.01 3.30 3.39 3.46 3.66 3.78 4.12

2.06 2.41 2.61 2.76 – – – 2.51 2.59 2.67 3.01 3.58 – 2.63 2.71 2.74 2.88 3.21 3.56 2.82 2.89 2.95 3.09 3.41 3.71

(shear stresses) and 2) interfacial tension between the water droplet and the diesel fuel (resistance force supporting the shape of a drop). Thus, the realized morphology of a drop is determined by the ratio of these forces expressed through the Capillary number Са:

Ca ¼

447

ησ γ=R

ð6Þ

One of the most important parameter which affects the rheological behavior of water-in-oil nanoemulsion is the surfactant concentration. To examine the effect of the surfactant concentrations on the plastic viscosity of the prepared nanoemulsion, the plastic viscosity (cP) of the prepared nanoemulsion were plotted against different MTS concentration as 4, 6, 8 and 10 wt% of the total emulsion weight, HLBMTS of 10, water content of 6 and 7 wt, 30 and 40 °C and aging storage time of 2 and 3 months are shown in Table 7, respectively. Results show that the plastic viscosity significantly increases with the increasing of MTS surfactant. This rise of viscosity of the nanoemulsion systems at higher surfactant concentration may be accounted for the act of the MTS surfactant (emulsifier) that build a viscous interface between water and diesel fuel which is regarded as a 2D entity, independent of the surrounding 3D fluid. This interface is considered to correspond to a highly viscous insoluble monolayer that leads to the slight increase of viscosity with the increase of surfactants concentration in the nanoemulsion due to more formed viscous water–diesel fuel interface monolayers stabilized by surfactant [36,37]. Accordingly, if the thickness of this layer is of the same order as the size of a drop, the apparent diameter (ηapp) appears larger than the real diameter of the drop itself. So, the viscosity of the emulsion increases. In addition, the diffusion coefficient of micelles decreases as their concentration increases, because crowding effects could increase the viscosity. From Fig. 6, it is noticed that the rheological data of the tested emulsions exhibited appreciable amount of yield stress. The yield stress may be attributed to the formation of interconnected network structure between the coalescence of emulsion water droplets. The yield stress depends upon the number of the dispersed droplets and the degree of coalescence of these droplets, and increases with an increase in the degree of coalescence and the number of droplets [38,39]. c. Temperature

where η is the viscosity of diesel fuel, σ is the shear rate, kb is the Boltzmann constant, γ is the interfacial tension, and R is the radius of a water droplet. Accordingly, the viscosity of the nanoemulsion increases with water droplet size and decreasing of γ. Whereas, at high shear rate and high water content, the nanoemulsion system tended to be unstable under influence of the share stress and the probability of coalescence small water droplet to form large one may be occurred. At the same time, the reorientation of surfactant adsorbed on the water/diesel fuel interface may be occurred. This behavior may lead to increase the aggregation of water droplet and structural breakdown of emulsions during shearing. Accordingly, the chance the water droplets agglomerate to form different size of water droplet (unequal structural breakdown of emulsions during shearing) increases [30]. Therefore, a non-linearity of the viscosity

The effect of temperature on the rheological behavior of the prepared water-in-diesel fuel nanoemulsion of 5 wt% water content, 10 wt% MTS concentration and 3 months storage time was studied at 20, 30, 40, 50 and 60 °C (Fig. 7). From the rheogram, it can be seen that the viscosity of emulsions decreases gradually with the increase of temperature from 20 to 60 °C. Also, the Arrhenius model was used to study the effect of temperature on apparent viscosity, which is given by the following equation:   −Ea η ¼ A exp RT

ð7Þ

On plotting data of the viscosity–temperature relationship on a semi-log scale as illustrated in Fig. 8, the obtained results fitted well

Table 8 Effect of water droplet size on dynamic viscosity of nanoemulsion system at 8% surfactant concentration, 40 °C and different water loading for 0, 1, 2 and 3 months time lapse.

Time laps, month Water content ,% 5 6 7 8 9 10

0

1

2

Water droplet size, nm

Pe

Plastic viscosity , cP

Water droplet size, nm

Pe

Plastic viscosity , cP

Water droplet size, nm

29.00 29.85 33.55 34.08 37.93 60.80

0.025 0.034 0.118 0.301 0.638 2.060

3.47 3.82 3.93 4.14 4.71 5.10

37.84 40.42 46.68 50.55 68.97 75.70

0.025 0.034 0.123 0.317 0.692 2.117

3.50 3.84 4.10 4.35 5.11 5.24

52.43 61.96 70.02 72.67 95.75 160.5

3 Pe

Plastic viscosity , cP

Water droplet size, nm

Pe

Plastic viscosity , cP

0.025 0.034 0.132 0.336 0.734 2.372

3.52 3.86 4.24 4.62 5.42 5.87

75.70 79.63 117.9 134.8 156.2 184.5

0.026 0.034 0.145 0.423 0.851 2.586

3.62 3.85 4.28 5.81 6.28 6.40

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Fig. 6. The variation of the shear stress with shear rate of water-in-diesel fuel nanoemulsion at different MTS concentration, 6 wt% water content, 30 °C and 2 months time lap.

with Arrhenius equation which is applied for Newtonian fluids with a correlation coefficient R2 that approximates unit value. This confirms the Newtonian flow behavior of our nanoemulsion system at all test temperatures [40,41].

effect of storage stability of all the prepared nanoemulsions on the rheological behavior was investigated from points of view of surfactant and water contents at different period of time ranging from 0 to 90 days. The results are illustrated in Tables 4 and 5, and the following points are detected:

d. Aging and storage time It is well known that emulsions and foams are subjected to coarsening phenomena like coalescence and Ostwald ripening by time. Also, the aggregation stability of the water droplets is determined mainly by the nature and concentration of a surfactant in the system creating and stabilizing the emulsion [36]. However, Ostwald ripening is almost impossible to stop and makes it therefore of significant importance for the long-term stability of foams and emulsions. Ostwald ripening mechanism could be interpreted also in the light of the solubility (and consequently the chemical potential) of the disperse phase droplet in the bulk phase which is dependent upon the radius of curvature of that droplet, with the solubility increasing with decreasing radius. However, the

1. At the same concentration of surfactant (for example in case 8 wt% of MTS), It is noticed that by increasing water content from 5 to 10 wt%, the droplet size and the plastic viscosity increases in the case of increasing the period of investigation up to 90 days (Table 8). 2. By increasing the surfactant concentration up to 10 wt% at the same water content, the average water droplet size and the plastic viscosity decreases with the increase in the storage period up to 90 days (Table 9). The aging of emulsions leads to changes of their rheological properties with time. Initially, droplets coagulate and aggregates appear with liquid of the continuous phase immobilized inside. This process results in an increase in viscosity at low shear rates. Coalescence leads to a

Fig. 7. Effect of temperatures on the rheological behavior of water-in-diesel fuel nanoemulsions at 5 wt% water loading, 10 wt% MTS concentration and 3 months time lap.

M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450

449

Fig. 8. Log viscosity vs. 1/T relationship according to Arrhenius equation for diesel fuel and (5, 6, 7, 8 and 9 wt%) water–diesel fuel nanoemulsion systems at 8 wt% MTS concentration and 2 months time lap.

decrease of the number of droplets per unit volume and this inevitably results in an evolution of the rheological properties of the emulsion due to aging. Accordingly, the rheological behavior of the nanoemulsion system is considered to be time-dependent character during the time of aging for three months [34].

Acknowledgment This project was supported financially by the Science and Technology Development Fund (STDF), Egypt, grant no. 5350. References

4. Conclusion In this paper, twenty stable nanoemulsions of water-in-diesel fuel type were prepared using batch addition technique. This technique causes the prepared nanoemulsions to have a highly physicochemical and rheology behaviors over a long time up to 90 days as compared with nanoemulsions prepared by other conventional methods. It is concluded that the prepared emulsions exhibited a low viscosity Newtonian character up to 10 wt% water loading. Also, the plastic viscosity of the prepared nanoemulsions increases slightly with increasing the surfactant concentrations. The viscosity–temperature relationship of the water–fuel nanoemulsion coincided Arrhenius equation that achieve the Newtonian character of such fluid systems for temperatures ranging from 20 to 60 °C. Also, it was found that, after shelf storage for 3 months, the viscosity of the prepared emulsified diesel fuel decreases slightly with time due to the increase of water droplets size. Lastly, the effect of water content and surfactant concentrations, as reported by most of the researchers, on the water droplet size of the nanoemulsions obtained have similarly expect rheological behavior over a long period of time.

Table 9 Dynamic viscosity of water, diesel fuel, and their nanoemulsions at 7 wt% water content, 40 °C and time lapse 0, 1, and 3 months. Time lapse, month

0 1 2 3

Plastic viscosity, cP Water

Diesel fuel

1

2.81

Surfactant concentration, wt% 4

6

8

10

3.54 3.61 3.69 3.76

3.78 3.87 4.95 4.02

3.93 4.10 4.24 4.28

4.21 4.25 4.26 4.28

[1] M.R. Noor El-Din, I.M. El-Gamal, S.H. El-Hamouly, H.M. Mohamed, M.R. Mishrif, A.M. Ragab, Rheological behavior of water-in-diesel fuel nanoemulsions stabilized by mixed surfactants, Colloids Surf. A Physicochem. Eng. Asp. 436 (2011) 318–324. [2] Jiajia Rao, David Julian Mcclements, Formation of flavor oil microemulsions, nanoemulsions and emulsions: influence of composition and preparation method, J. Agric. Food Chem. 59 (9) (2011) 5026–5035. [3] Fu Zhisheng, Min Liu, Junting Xu, Qi Wang, Zhiqiang Fan, Stabilization of water-inoctane nano-emulsion. Part I: Stabilized by mixed surfactant systems, Fuel 89 (2010) 2838–2843. [4] A.S. Kabalnov, A.V. Pertzov, E.D. Shchukin, Ostwald ripening in twocomponentdisperse phase systems: application to emulsion stability, Colloids Surf. A Physicochem. Eng. Asp. 24 (1987) 19–32. [5] I.M. Lifshitz, V.V. Slyozov, The kinetics of precipitation from supersaturated solid solutions, J. Phys. Chem. Solids 19 (1961) 35–50. [6] C.Z. Wagner, Z. Elektrochem, Theory of precipitate change by redissolution, Elektrochem 65 (1961) 581–591. [7] A. Forgiarini, J. Esquena, C. Gonzalez, C. Solans, Formation of nano-emulsions by low-energy emulsification methods at constant temperature, Longmuir 17 (2001) 2076–2083. [8] M.R. Noor El-Din, A.M. Al Sabagh, R. Miller, Preparation of water-in-jet fuel Nanoemulsions using a high-energy method, Int. J. Green Nanotechnol. Phys. Chem. 2 (2010) 20–29. [9] W. Liu, D. Sun, C. Li, Q. Liu, J. Xu, Formation and stability of paraffin oil-in-water nano-emulsions prepared by the emulsion inversion point method, J. Colloid Interface Sci. 303 (2006) 557–563. [10] P. Izquierdo, J. Esquena, T.F. Tadros, Phase behavior and nano-emulsion formation by the phase inversion temperature method, Langmuir 20 (2004) 6594–6598. [11] S. Sahin, O. Bliznyuk, A.R. Cordova, K. Schroën, Microfluidic EDGE emulsification: the importance of interface interactions on droplet formation and pressure stability, Sci. Rep. 6 (2016) 1–7. [12] A.M. Al-Sabagh, M.E. Mostafa, M.R. Noor El-Din, W.R. Aly, Water-in-diesel fuel nanoemulsions prepared by high energy: emulsion drop size and stability, and emission characteristics, J. Surfactant Deterg. 15 (2012) 139–145. [13] Sunsanee Udomratia, Shinya Ikedac, Shoichi Gohtani, Rheological properties and stability of oil-in-water emulsions containing tapioca maltodextrin in the aqueous phase, J. Food Eng. 116 (1) (2013) 170–175. [14] C.Y. Lin, K.H. Wang, The fuel properties of three-phase emulsions as an alternative fuel for diesel engines, Fuel 82 (2003) 1367–1375. [15] F. Barnaud, P. Schmelzle, P. Schulz, AQUAZOLETM: An original emulsified water-diesel fuel for heavy-duty applications, SAE Technical Paper, 2000 01-1861. [16] M.R. Noor El-Din, S.H. El-Hamouly, H.M. Mohamed, M.R. Mishrif, A.M. Ragab, Investigating factors affecting water-in-diesel fuel nanoemulsions, J. Surfactant Deterg. 17 (2014) 819–831. [17] M.R. Noor El-Din, Marwa R. Mishrif, R.E. Morsi, E.A. El-Sharaky, M.E. Haseeb, Rania T.M. Ghanem, A new modified low energy emulsification method for preparation

450

[18]

[19] [20] [21]

[22] [23]

[24] [25]

[26] [27]

[28]

M.R. Noor El-Din et al. / Journal of Molecular Liquids 231 (2017) 440–450 of water-in-diesel fuel nanoemulsion for use as alternative fuel, J. Dispers. Sci. Technol. 38 (2) (2017) 248–255. M. Porras, C. Solans, C. Gonzalez, J.M. Gutierrez, Properties of water-in-oil (W/O) nanoemulsions prepared by a low-energy emulsification method, Colloids Surf. A Physicochem. Eng. Asp. 324 (2008) 181–188. P.C. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel Dekker Inc, New York and Basel, 1977. P. Taylor, Ostwald ripening in emulsions, Adv. Colloid Interf. Sci. 75 (1998) 107–163. B.P. Binks, J.H. Clint, P.D.I. Fletcher, S. Rippon, S.D. Lubetkin, P.J. Mulqueen, Kinetics of swelling of oil-in-water emulsions stabilized by different surfactants, Langmuir 15 (1999) 4495–4501. I. Capek, Degradation of kinetically-stable o/w emulsions, Adv. Colloid Interf. Sci. 107 (2004) 125–155. G.W.J. Lee, T.F. Tadros, Formation and stability of emulsions produced by dilution of emulsifiable concentrates. Part I. An investigation of the dispersion on dilution of emulsifiable concentrates containing cationic and non-ionic surfactants, Colloids Surf. A Physicochem. Eng. Asp. 5 (1982) 105–115. K. Landfester, M. Willert, M. Antonietti, Preparation of polymer particles in nonaqueous direct and inverse miniemulsions, Macromolecules 33 (2000) 2370–2376. M.R. Noor El-Din, S.H. El-Hamouly, H.M. Mohamed, M.R. Mishrif, A.M. Ragab, Formation and stability of water-in-diesel fuel nanoemulsions prepared by high-energy method, J. Dispers. Sci. Technol. 34 (2013) 575–581. M.R. Noor El-Din, Sabrnal H. El-Hamouly, H.M. Mohamed, Marwa R. Mishrif, Ahmad M. Ragab, Egypt. J. Pet. 22 (2013) 517–530. Seid Mahdi Jafari, Elham Assadpoor, Yinghe He, Bhesh Bhandari, Re-coalescence of emulsion droplets during high-energy emulsification, Food Hydrocoll. 22 (2008) 1191–1202. M.R. Noor El-Din, Study on the stability of water-in-kerosene nano-emulsions and their dynamic surface properties, Colloids Surf. A Physicochem. Eng. Asp. 390 (2011) 189–198.

[29] T.F. Tadros, Rheology of Suspensions, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, UK, 2010. [30] P. Schramm, P. Becher (Eds.), Encyclopedia of Emulsion Technology, Marcel Dekker, Inc., NewYork 1983, pp. 421–422. [31] P. Kundu, Vimal Kumar, Indra Mani Mishra, Modeling the steady-shear rheological behavior of dilute to highly concentrated oil-in-water (o/w) emulsions: effect of temperature, oil volume fraction and anionic surfactant concentration, J. Pet. Sci. Eng. 129 (2015) 189–204. [32] M. Chiesa, J. Garg, Y.T. Kang, G. Chen, Thermal conductivity and viscosity of waterin-oil nanoemulsions, Colloids Surf. A Physicochem. Eng. Asp. 326 (2008) 67–72. [33] S.R. Derkach, Rheology of emulsions, Adv. Colloid Interf. Sci. 151 (2009) 1–23. [34] J. Einstein, Investigations on the Theory of the Brownian Movement, Dover, New York, 1956. [35] Y. Otsubo, R.K. Prud'homme, Flow behavior of oil-in-water emulsions, J. Soc. Rheol. Japan 20 (1992) 125–131. [36] M.A. Farah, R.C. Oliveira, J.N. Caldas, K. Rajagopal, Viscosity of water-in-oil emulsions: variation with temperature and water volume fraction, J. Pet. Sci. Eng. 48 (2005) 169–184. [37] S.R. Derkach, R. Miller, J. Krägel, Methods of measuring rheological properties of interfacial layers (experimental methods of 2D rheology), Colloid J. 71 (2009) 1–17. [38] Y. Otsubo, R.K. Prud'homme, Rheology of oil-in-water emulsions, Rheol. Acta 33 (1994) 29–37. [39] R. Pal, E. Rhodes, A novel viscosity correlation for non-newtonian concentrated emulsions, J. Colloid Interface Sci. 107 (1985) 301–307. [40] T.F. Tadros, Fundamental principles of emulsion rheology and their applications, Colloids Surf. A Physicochem. Eng. Asp. 91 (1994) 39–55. [41] H.A. Barnes, A Handbook of Elementary Rheology, The University of Wales Institute of Non-Newtonian Fluid Mechanics, Department of Mathematics, Penglais, Aberystwyth, 2000.