Study of phase separation and anomalous molecular behavior of Jojoba oil using dielectric spectroscopy

Accepted Manuscript Study of phase separation and anomalous molecular behavior of Jojoba oil using dielectric spectroscopy

G.M. Turky, R.A. El-Adly PII: DOI: Reference:

S0167-7322(17)31573-8 doi: 10.1016/j.molliq.2017.06.126 MOLLIQ 7572

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

12 April 2017 10 June 2017 29 June 2017

Please cite this article as: G.M. Turky, R.A. El-Adly , Study of phase separation and anomalous molecular behavior of Jojoba oil using dielectric spectroscopy, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.126

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ACCEPTED MANUSCRIPT Study of phase separation andanomalous molecular behaviorof Jojobaoil using Dielectric Spectroscopy G. M.Turky1 and R. A. El-Adly2 Department of Microwave Physics& Dielectrics, National Research Centre (NRC) 33 El Bohouthst.(former ElTarirst.)- Dokki, Giza, Egypt, P.O.12622 2 Process Development Division, Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt 1

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Abstract Jojoba oil is composed almost entirely of liquid wax ester. Because growing

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conditions, harvesting and storage treatment can affect the composition of these waxes. Broadband dielectric spectroscopy and differential scanning calorimetry are

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complementary combined to study the molecular dynamics at the phase transitions, and in between, over temperatures range from -100 up to 100 oC. Two different trends

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are foundup and downa non-equilibrium or meta-stable state region. In addition, The rheological behavior and physicochemical properties of jojoba oil was studied, which

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revealed that the jojoba oil has a Newtonian behavior at low shear rate but it has nonNewtonian at high shear rate; it has also effective in controlling the oxidative deterioration. The chemical identification of jojoba oil using gas chromatography

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technique indicates that the main components were eicosenic and docosenic acids;

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itwas concluded that the jojoba oil contained little or no glycerin and that most of its components fell in the chain-length range of C36-42.

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Keywords: Jojoba oil; BDS; TzeroTM Technology; Relaxation time; Dielectrics

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ACCEPTED MANUSCRIPT Introduction Jojoba is known in botanical literatures as Simmondsia chinenasis (Link) of the family Buxaceaa and as Simmondsia californica Nutall. The first name is the correct one, although it perpetuates a geographical misnomer. In late 1970 sperm whale was included by the US Government in the list of endangered species and imports of oil, meal and other products derived from whales were banned. At that time, sperm oil

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consumption in the United States was about 40-50 million pounds per year, with half that figure used in lubricant applications. No single natural or synthetic replacement

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with the unique qualities of sperm whale oil has yet been found, but enough experimental evidence has accumulated in the last years that jojoba oil is not only an

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excellent substitute of sperm oil but its potential industrial uses go beyond those of

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sperm oil [1]

The chemical composition of jojoba oil is unique in that it contains little or no

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glycerin and that most of its components fall in the chain-length range of C36-C42. Linearity and closerange composition are probably the two outstanding properties that give jojoba oil its unique characteristics. The oil is characterized of being a monoester

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of high molecular weight and straight chain fatty acids and fatty alcohols that has a

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double bond on each side of the ester. The molecular structure of the oil can be represented by the following general formula:

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CH3-(-CH2-) 7-CH=CH-(CH2-) m-COO-(-CH-) n-CH=CH-(CH2-) 7-CH3, where, m and n are between 8 to 12 [2]. The major constituents in Egyptian jojoba oil, as indicated by capillary gas liquid

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chromatography (GLC), were for wax esters about C38, C40, C42 and C44 with ratio 6.3%, 30.1%, 51.1% and 10.0 % respectively [3]. Many investigations have discussed the possibilities for economic development of jojoba oil and its chemical and industrial potential. It also may be useful in the manufacture of many products such as cosmetics, carbon paper, stencil and pharmaceuticals [4]. The oil has a high dielectric strength. This is an indication that the jojoba oil has the ability to resist electrical breakdown, which makes it suitable for electrical applications[5].In the lubricant application, in particular lubricating greases area, jojoba oil and its meal has developed research and shifting towards new 2

ACCEPTED MANUSCRIPT derivatives with potential application to newer areas of lubricant use [6-8]. Jojoba oil is unique as a source of C20 and C22 straight chain alcohols, which may prove valuable in the manufacture of detergents, wetting agents, dibasic acids, long chain ethers, hydroxyl ethers and sulfated products. since the jojoba oil, as a dielectric oil, must carry and dissipate heat, factors that significantly affect the relative ability of the oil to work as a dielectric coolant include viscosity, specific heat, thermal

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conductivity, and the coefficient of expansion. The values of these properties, particularly in the range of operating temperatures for the equipment at full rating, must be weighed in the selection of suitable dielectric oil for specific applications.

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Mineral oil has been used for many decades as dielectric oil in many applications.

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This isdue to its high stability during the transformation processes. However, it is not biodegradable. The use of mineral oil as a dielectric fluid in transformers typically

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results in the production of large quantities of non-biodegradable waste that are costly to dispose. Mineral oil is also relatively flammable and for this reason has regulatory restrictions imposed upon its use and containment.

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The dissipation factor of a dielectric fluid is a measure of the dielectric losses in the fluid; a low dissipation factor indicates low dielectric loss and a low concentration of

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soluble, polar contaminants. Mineral oil is also relatively flammable and for this reason has regulatory restrictions imposed upon its use and containment.

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Increasingly, however, there has been concern that the idea dielectric fluid should also be biodegradable, non-toxic and renewable to exhibit little or no detrimental impact on the environment. Mono-unsaturated fatty acid-containing oils may be obtained

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from renewable sourcesand thus are attractive candidates for substituting petroleumbased dielectric fluids.

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Advantageously, mono-unsaturated fatty acid-containing oils may have higher flash and fire point characteristics than mineral oils, which ensure better safety in operation, handling, storage and transportation of such oils and thus the operational safety of transformers using vegetable oil-based dielectric fluids. Dielectric Measurements of the jojoba oil is carried out using Broadband Dielectric Spectroscopy, BDS. This technique investigates the response of the material under the applying of electromagnetic field in the frequency range from micro to terahertz; see for instance [9-13]. The huge range of frequency is the main feature of BDS over all other spectroscopic techniques.On this vast frequency range, the dielectric dispersion and absorptionoriginatefrom different processes in the molecular and sub-molecular 3

ACCEPTED MANUSCRIPT scales. It became recently the main tool to determine dipolar fluctuations, Interfacial polarization and translational motion of charge carriers reflecting the conductivity contribution in the dielectric spectrum. According to its versatile industrial uses such as electric insulators, plasticizers and transformer oils in addition to its primary uses in the fields of cosmetics lubricants, adhesives and medicines pharmaceuticals, it is very interesting to study the electrical and dielectric properties of the jojoba oil on the

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broad range of frequency and temperatures.

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Experimental

Jojoba oil under investigationwas kindly supplied from Egyptian Natural Oil

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Company. The physico-chemical characteristics of the jojoba oil were carried out using ASTM and IP standard test methods as listed in Tables 1. The rheological

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measurements of the oil under investigation were performed on a Brookfield programmable Rheometer LV DV-III UITRA used in conjunction with Brookfield

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software, RHEOCALC V.2, through RHEOCALC all Rheometer function (rotational speed, instrument % torque scale time interval, set temperature) are controlled by a computer. The software also recorded the corresponding shear stress, shear rate and

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dynamic viscosity.Melting and purity of the Jojoba oil were carried out under nitrogen

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with heating rate 1oC min-1on SDT Q2000 V24.4 Build 116 with advanced TzeroTM Technology. Hermetic aluminum pan was used for the investigated jojoba oil. The sample weight was about 3.8 mg. The sample washermetically sealed to ensure there

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is no loss of volatiles. In order to determine the purity of the investigated oil, Van't Hoff considered the fact of reducing melting point according to the impurities as

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follows (show for instance [22, 23]):

)----------(1)

One can correlate the melted fraction of the material, F, to the partial aria Ap and full area Af of the melting peak on DSC thermogram as given by equation 2:

-------------------------------------(2), where: Tf: The melting temperature that follows the temperature of the liquid phase, To: The melting temperature of the material at 100% purity, R : The gas constant, 4

ACCEPTED MANUSCRIPT Hf: The molar-heat of fusion determined from the area of the melting peak : The impurity concentration in mole fraction, Timp: The impurity substance melting temperature, The oxidation stability of oil under investigation was determined using ASTM D94399. The jojoba oil and base was contacted with oxygen in the presence of water and a copper coil oxidation catalyst at 120°C. The total acid number was recorded every 24 hours for 96 hours. This test method is widely used for specification purposes and is

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considered of value in estimating the oxidation stability and the deteriorationof

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lubricants, especially those that are prone to water contamination.

The dielectric measurements are carried out using BDS which composed of a High

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Resolution Alpha Analyzer (Novocontrol GmbH) assisted by Quatro temperature controllers providing temperature stability better than 0.2 K was used. The sample cell

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consists of two gold coated brass circular electrodes of 20 mm diameter separated from each other by small Teflon strips of 120 m thickness arranged in a parallel-

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plate capacitor geometry. The emptysample capacitor is used as a reference, thereby eliminating the additional contributions ofthe cables and the measurement cell. We firstly corrected the sample thickness according to the capacity of the empty cell. This

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thickness is found tobe different from that measured by a digital micrometer. The

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jojoba oilis then filled into thecell by capillary forces.

Results and Discussion

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Physicochemical andrheological properties The physicochemical properties of jojoba oil are presented in Table1. This data show the oil is usually a low-acidity(total acid number @72hours is 0.89 mg KOH/gm and

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peroxide value is 0.9), light golden fluid. This reveal that the jojoba oil has been effective in controlling the oxidative deterioration and efficient in preventing the formation of oxidation compounds. It was attributed to the tocopherol isomers in jojoba oil which acted as natural antioxidant. The results obtained agree with those reported [3, 10]. In addition, it has high viscosity index (257), free from rancidity and high flash point. It is non-volatile (evaporation loss is zero after duration test ASTMD- 972), this reveal the oil is essentially unchanged under the condition of the evaporation test.

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ACCEPTED MANUSCRIPT Experimental data Obtained in Table 2, show that the identified fatty acids in jojoba oil are myristic, palmetic, palmitoleic, stearic, oleic, linoleic, linolenic, eicosenoic, docosenoic, tetracosenoic and Hexacosenoic with percentage, 0.03, 1.62, 0.6, 0.17, 10.11, 0.19, 0.25, 57.13, 11.36, 10.6 and 8.46 respectively. This indicates that the main components were eicosenic and docosenic acids. The results obtained agree with those reported by Miwa [14]. This proved that the chemical composition of jojoba oil

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was unique in as much as it contained little or no glycerin and that most of its components fell in the chain-length range of C36-42.

Linearity and close range composition are probably the two out-standing properties

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that gave jojoba oil its unique characteristics. In the meantime, jojoba oil molecules

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contain two double bonds separated by an ester bond. These three active centers had been proven to be the source of a very large number of intermediates.

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Table (1): physico-chemical properties of jojoba

0.863

D.1298

1.4652

D.1218

26 7.5

D.445 D.445

Viscosity index

257

D. 189

TAN, mg KOH/gm @72h

0.89

D-664

Average Molecular weight

640

GPC

Iodine value

82.1

D-5554

Oxidation stability index

51

Peroxide value

0.9

Jojoba oil

Density, g/ml @ 25/25, oC Refractive index, nD20

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Characteristics

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Kinematics viscosity, c St. at 40oC at 100oC

Pour point, oC 3 Flash point 310 0 Evaporation loss,wt%@125 C none Table (2): Fatty acid composition of jojoba oil Common name

Chemical structure

Myristic acid Palmitic acid Palmitoleic acid

C14:0 C16:0 C16:1

Test

Cd 12b-92 ISO 3960 D91 D-92 D- 972

Chemical name CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)5CH=CH(CH2)7COOH

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Mole% 0.03 1.62 0.6

ACCEPTED MANUSCRIPT C18:0

CH3(CH2)16COOH

0.17

Oleic acid Linoleic acid

C18:1 C18:2

10.11 0.19

α-Linolenic acid

C18:3

Reconolic acid

C18:1

Arachidic acid Eicosenoic acid Docosenoic acid Tetracosenoic acid Hexacosenoic acid

C20:0 C20:1 C22:1 C24:1

CH3(CH2)7CH=CH(CH2)7COOH CH3(CH2)4CH=CHCH2CH=CH(CH2)7COO H CH3CH2CH=CHCH2CH=CHCH2CH=CH(C H2)7COOH CH3(CH2)5 CH-OH-CH2CH=CH(CH2)7COOH CH3(CH2)18COOH CH3(CH2)8CH=CH(CH2)8 COOH CH3(CH2)7CH=CH-CH2-CH2(CH2)9COOH CH3(CH2)7CH=CH- CH2-CH2-CH2CH2(CH2)9COOH CH3(CH2)13CH=CH(CH2)9COOH

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Stearic acid

-----57.13 11.63 10.6 8.46

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C26:1

0.25

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Rheological behavior of the jojoba oil at different temperatures is graphically presented in Figure 1. In this figure, the shear rate was varied in the range 20-96 sec-1.

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Two regions are noticed here, first region (low shear rate) ranging from 20 up to 50 sec-1 and second region (high shear rate) ranging from 50 up to 96 sec-1. The first region show that the apparent viscosity decreases with increasing temperatures. The

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increase of temperature tends to increase molecular motions and reduce the attractive forces between lamellar molecules of jojoba oil. But in second region, viscosity

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leveling off is obtained at all temperatures can be attributed to by increasing the shear rate it modifies the arrangement of jojoba molecules in a way that effect of changes of

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viscosity would not be apparent and may be cis-isomer of jojoba molecules turn into trans-isomer. It was concluded that the influence of temperature on viscosity is more significant that the shear rate. In this respect, the jojoba oil under investigation shows

Viscosity.cP

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pseudo-plastic behavior.

35

at temp.30 temp.90

30

at temp.50 at temp.110

temp.70

25 20 15 10 5 0 0

20

40

60

80

100

Shear rate S-1 for jojoba oil @ different temptures

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0.16 0.14

jojoba…

0.12 0.1 0.08

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0.06 0.04 0.02 0 50

100 Time ,Min

150

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0

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Total Acid number mg of KOH

Fig. 1. Rheological properties of jojoba oil at different temperatures

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Fig.2. Effect of total acid number on time for jojoba oil and base mineral oil

The effects of the variation in the oxidation time on total acidity had been investigated

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from zero time to 120 hours. The analytical data obtained are presented in Figure 2. This figure presented an overview on the values of total acid number of jojoba and base oils which resulted from the oxidation reaction that happened during the tested

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time. The results showed that the total acid number values for base mineral oil were

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increased markedly after 25 hours. In contrast, in case of the jojoba oil the total acid number values were almost stable until 50 hours and slightly increased after 75 hours. This reveal that the jojoba oil has been more effective in controlling the oxidative

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deterioration and more efficient in preventing the formation of oxidation compounds. This was attributed to the tocopherol isomers in jojoba oil, which acted as natural

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antioxidant. The results obtained agree with those reported [3, 15]. DSC measurements It became well known that the transport processes and hence the electrical and dielectric properties are influenced by the thermal history and different phase transitions within the polymeric material[16,17]. We usually use the Standard Differential Scanning Calorimetry (SDSC) to determine different transitions in the material on the heating and cooling branches. In the literatures[18-21] the relationship between molecular dynamics and different phase transitions (melting and crystallization transitions) as well as the glass transition, which in fact is not a phase transition, is investigated and reported.In addition, it could beemployed to determine 8

ACCEPTED MANUSCRIPT the purity of many compounds. Unfortunately, its accuracy is usually not acceptable. Nowadays, the developed TzeroTM Technology is used to determine the purity of the investigated to high accuracy. Figure 3 shows that the melting point, defined by its peak position, is at about 13.25 o

C. If we consider the solid/liquid/solid sandwich like structure at the melting point,

then there will be a meta-stable phase during annealing. This makes the melting

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D

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using van't-Hoff- plot [eqns. 1&2] are added in the figure.

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process more complicated than it seems to be [22,23].The purity curves calculated

Fig. 3.DSC diagram and purity analysis of jojoba oil using Advanced Tzero TM Technology.

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This plot is defined as the partial area of the melting peak against the corresponding temperature of the partial area. This means that the corresponding points on the DSC

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curve are closely spaced in the region of the pre-melting tail.The very low purity determined here (61.72 mole %) does not reflecting unknown or even unwanted impurities.This supposed to be due to the different ratios of a lot number of components described in table (2). The main fatty acid component in the jojoba oil, Eicosenoic acid is about 57.13 mole %.This value is comparable with the purity value that determined by the DSC advanced TzeroTM Technology. The assignment of ratios of different fatty acids as components in jojoba oil and their relation to the high impurity ratio is out of the scope of the current work.

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Dielectric Measurements The dielectric behavior found for the jojoba oil is quitedifferent from that to be expected. Figure 4 shows the temperature dependence of dielectric loss, '', on the temperatures ranging from -100 up to +100 oC at three spot frequency points in the lower frequency range, namely 0.1, 1 and 10 Hz. Three distinguished regions are

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clearly noticed here. At higher temperatures, the dielectric loss decreases gradually with decreasing temperatures until about 10 oC. This is due to the slowing down of the

3

10

2

10

1

Freq /Hz 0.1 1.0 10.0

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10

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fluctuation of the polar moieties and the mobility of the free charge carriers.

-1

10

-2

-80

-40

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-120

D

10

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'' 100

0

40

80

120

Temp

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Fig. 4. The dielectric loss of the investigated jojoba oil against temperature at three points of frequency as indicated.

The second range, at descending temperature order, is a phase transition one orsolidification range (+10 to -10 oC). The dielectric lossdecreases abruptly with

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decreasingtemperature (about three decades for 20 oC range). This might be due to the non-equilibrium or meta-stable state at this range of solidification. The oil isdisordered at the molecular-scale and homogeneous at the macroscopic-scale, it may processes some amount oforder at an intermediate, so-called mesoscopic- scale due to a delicate balance of internal interactions and thermal effects. Further decrease of temperature leads to a range of dielectric dynamic relaxation. It is semi-circle-like peak which reflecting a super position of two or more dynamics. If its origin is one dynamic, it will be of a very broad distribution.This can be discussed in the framework of the dynamic glass transition and the fluctuation of the function 10

ACCEPTED MANUSCRIPT group carboxylate [shown in Fig. 5]. The heterogamous structure of the considered oil may also lead to the well-known interfacial polarization.

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I

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II

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Fig. 5. Jojoba structure:I- 3-D and II- 3D

In the following, we will discuss the three distinguished temperature regions in some

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details. Figure 6 depicted the complex dielectric function (* = ' - i") and the real part of conductivityfunction (σ* = σ' + iσ'') at lower temperatures ranging (from -50 up to - 20 oC) against frequency range1 mHz - MHz. Figure (6a-c) shows the

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frequency dependence of ', '' and 'at various lower temperatures as indicated. A

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very clear dispersion step  (= L - H) is noticed in figure 1a.The values of higher frequency permittivity, H, collapse together to be about 2.4 at all indicated

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temperatures.

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ACCEPTED MANUSCRIPT 4.2

1.4 1.3



3.6

1.1 1.0

a

' 3.0

-50

-40 -30 o Temp. C

-20

Joj-oil

2.4 10

1.2

0

-2

10

-3

10

-8

10

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10

c

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''

-1

PT

b 10

-10

Temp C

' 10-12 -14

10

-16

10

-1

10

1

-40 -25

-35 -20

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10

-50 -45 -30

10

3

10

5

10

7

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Freq. [Hz]

Fig. 6.The permittivity, ', dielectric loss, ", and the real conductivity,', against frequency at seven low temperatures. The inset shows the temperature dependence of  values.

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The indicated linear increase of the  with increasing temperature (inset of figure 6a)

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agrees well with the gradual increase of the intensity of the dynamic peak in 1b. This reflects the increase of polarity with increasing temperature according to theOnsager[24] relation between permanent dipole moment and dielectric dispersion

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step (). Well-pronounced peaks strongly shifting tolower frequencies with decreasing temperature dominate the dielectric loss spectra at various lower

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temperatures.This relaxation process or processescan beanalyzed by fitting the model function of Havriliak - Negami[25, 26] (HN-function) which reads:

----------------- (3)

where

is a characteristic relaxation time related to the frequency position of

maximal

loss

p

(relaxation

rate)



describes

the

value

of

the

real

part of permittivity at p.and  are fractional parameters (0 ≤1 and 0≤)

characterizing

the

shape

of

the

relaxation

time

spectra,

and

denotes the dielectric strength.This dynamic peak can beascribed to the α relaxation which corresponds to micro-Brownian motion of the backbone segments. 12

ACCEPTED MANUSCRIPT

3.5

4.0

4.5

5.0

5.5

6.0

1.0

1.0

0.8

0.8

0.6

0.6



a

0.4

PT

0.4 2

1.6

10

0

10

RI

1.2

-2



-4

10

-6

10

3.5

b 4.0

4.5

5.0

5.5

NU

-8

10

SC

 [sec]

10

0.8

0.4 6.0

MA

1000 K/T

Fig. 7. Temperature dependence of the a-fractional shape parameters and b- characteristic relaxation time and dielectric strength. The lines are just guides for eyes.

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The dynamic glass transition is related directly to the calorimetric glass transition Tg

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that, considered to be one of the unsolved problem in condensed matter physics. Figure 7 shows that the relaxation time increased linearly in the semi-log scale by about eight decades with decreasing temperature. It is also noticed that the symmetric , systematically decreased with decreasing temperature. The

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shape parameter,

dielectric relaxation strength,  , increases linearly with increase of temperature

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along the super cooled range until the solidification temperature. Further increase of temperature shows a abrupt decrease with raising temperature. This anomalous temperature range accompanied by uncertain behavior on the asymmetric parameter  between -10 and 10 C. Worthwhile,  equal one in the super cooled range meaning HN- equation turned to be Cole Davidson CD-equation [27-29] in this range.Figure 8 shows the DSC dependence of the heat flow and the dielectric loss on temperature ranging from -50 up to 150 oC. The figure shows that, the melting peak starts very gradually with a remarkable distortion. The temperature range of the melting peak build up is about 20 oC starting from -10 up to +10oC. It is exactly the range of

13

ACCEPTED MANUSCRIPT discontinuity of the all-dielectric properties or distortion range as shown in dielectric loss behavior as a representative example. 1

10

0

0 DSC

Jojoba oil

BDS

-2

-1

10

-2

10

-3

PT

-3

Heating Cooling

-40 -20

0

20

40

60 o

Temp [ C]

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-4

80 100 120 140

-5

SC

''

10

-1

Heat Flow (W/g)

10

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Fig. 8. Temperature dependence of the: upper- Heat flow and Lower- dielectric loss at 1 kHz

The fit using only one HN-function is not satisfactory enough. This is due, on one

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hand, to the highly deviation of the experimental points from the fit curve and onthe other hand to the unexpected inverse trend of the dielectric strengthof

- process in

super cooled liquids. [30-33].The best fit based by two Cole-Cole (CC-functions)

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[34]isshown in the inset of figure 9.In this scenario one has to treat thisbroad peak as

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a superposition of the Maxwell Wagner Siller(MWS- polarization) on higher frequency wing of the peak) in addition to dynamic glass transition. The perfect insulating polymeric materials usually show a linear dependence of the real

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conductivity on the frequency especially at lower temperatures. The clear deviation from linearity shown in figure 6c originates from the fluctuations of these both investigated dynamics.

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Figure 9 depicts the temperature dependence of the relaxation rate fp for the both determined processes. Both dynamic relaxations follow the Arrhenius relation, which reads:

---------------------(4)

14

ACCEPTED MANUSCRIPT

15

A

log (fp/Hz)

0.1 Eps''

12

0.01

9 3.8

4.0

4.2

4.4

1000 K/T

4.6 1.2

-1

1

10

10

3

5

10 Freq. [Hz]

10

0.9

6

0.6

3



0.3

1.0



B

Equation

y = a + b*x

W eight

No W eighting

Residual Sum of Squares

0.00547

Adj. R-Square

0.97545

0.00408 0.95021 Value

Intercept

Beta 1

Slope Intercept

Beta 2

-process MWS-procress

3.8

4.0

4.2

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0.4 3.6

0.14104 0.03445

2.05677

0.10041

-0.34281

0.02475

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0.6

Slope

Standard Error

3.39303 -0.65236

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0.8

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0.0

4.4

4.6

1000 K/T

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Fig. 9. Activation plot:A- The logarithm of relaxation rate fpand B- shape parameter , against 1000/T. The upper right inset on A is the fitted spectrum with two CC-functions and the lower left one is the temperature dependence of the dielectric strength. The lines are linear fit to the data.

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Where kB is the Boltzmann constant, EA the activation energy, and f∞ is the pre-

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exponential factor. The determined activation energy, EA, for -process is 107.67 and for the MWS- process is 86.14(kJmol-1). The effect of temperature on the dielectric strength of both relaxation dynamics is shown in the lower left inset of the figure.

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Once again, an anomalous behavior is remarkably shown in the figure. A mirror image for the two  parameters with an inflection points at about -35 oC. The CC-

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shape parameter, , shows a linear increase with decreasing temperatures. The rate of change of  is higher in case of MWS-dynamic. This process originates from the interfacial polarization due to the heterogeneous structure of the investigated jojoba oil.Thedielectric properties gave valuable information of the jojoba oil characteristics. One has to shed more light on that magic oil which has a very broad field of application. Its unique structure as a complex solution of long chain, un-branched esters ranging from 34 to 48 carbon atoms is a challenge and needs further investigations in order to understand and find out other fields of applications.

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ACCEPTED MANUSCRIPT Figure 10 illustrated graphically the real part of conductivity against frequency at four temperatures, two of them in the meta-stable state, namely -10 and zero degree

-8

10

-10

10

-12

10

-14

10

-16

10

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10

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-6

-20 C -10 C 0C +20 C

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10

-1

10

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Sig' [S/cm]

Celsius. The other two temperatures are chosen to be just up and down that range.

1

10

3

10

5

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Freq. [Hz]

Fig. 10.The real part of conductivity against frequency at the four indicated temperatures.

At 20 C the dependence of conductivity is characterized by a plateau on the low

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frequency side. It is well known that for conductive systems the value of the

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plateauyieldsdirectlythe dc-conductivity, DC, and the characteristic frequency, c,at which dispersion sets in and turns into a power law at higher

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frequencies. This behavior obeys the well-known Jonscher power law, which reads:

---------------(5)

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In which, the exponent s lies between 0 and 1,

characterizesthe onset of dc

conductivity. Further increase of temperature shows that the conductivity values increasewithtemperature in analogous to semiconductor behavior [35-37]. At the intermediate temperatures (the meta-stable state), the conductivity shows an unusual dependence at lower and at higher frequencies. However, it is still follow, to some extent, the Jonscher equation (eqn. 5). At lower temperatures, -20 oC as representative example, the conductivity decreases gradually with decreasing frequency and tends to be in the order of sub-femto Siemens per cm at 0.1 Hz. The dramatic non-linearity of the temperature dependence of conductivity shown here is by somehow a deviation from the ideal insulating characterization in the oil. 16

ACCEPTED MANUSCRIPT Applying equation 5 on the data, the determined main two fit parameters namely, c and DCare depicted against temperature in figure 11. 6

10

5

10

4

10

2

-11

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10

10

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3

DC-Sig

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c[Hz]

c

10

-10

DC [S/cm]

10

1

10 -20

20

A 40

60

80

-12

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0

10 100

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Fig. 11. The effect of temperature on the DC conductivity (right) and on the characteristic frequency (left) showing the discontinuity of the behavior.

Both fit parameters follow an Arrhenius type thermal activation. The discontinuity at

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the melting range is shown here in the temperature dependence of both parameters.

Conclusions

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Jojoba oil is unique and has a very broad range of applications. It is a liquid wax and differs from the known vegetable oils and animal fats in its composition mainly of linear wax esters. Its main components were found to be eicosenic and

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docosenic acids. Broadband Dielectric spectroscopy is used to study its electrical and dielectric properties at temperatures ranging from -100 up to 100 and on frequency

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range 0.1 up to 10MHz. Two distinguished characteristics are investigated at higher and lower temperatures that separated by a range of what we call meta-stable state.

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[6] R.A. El-Adly, E.A. Ismail, Chapter 8: Lubricating greases based on fatty byproducts and jojoba constituents, in: Chang-Hung Kuo, Tribology-lubricants andlubrication, ISBN 978-953-307-371-2, INTECH publisher, 2011, pp. 201-222

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ACCEPTED MANUSCRIPT Highlights:

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

Jojoba oil is characterized by its composition mainly of linear wax esters. BDS is combined with TzeroTM to study molecular dynamics in this oil. Two different trends are found up and down a non-equilibrium or meta-stable state region. The rheological behavior shows two different trends according to the shear rat.

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  

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