Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

NE-5C, 18B/2005P001

Structure Analysis of Base Oils by X-ray Diffraction MethodTomoko HIRAYAMA*1, Kazuaki NEMOTO1, Yuka MITSUNAGA1, Takashi MATSUOKA1
1
Doshisha Univ., Kyotanabe, Kyoto 610-0394, Japan
Introduction
Microscopic structure of lubricants has received a lot ofattention recently because of its contribution to friction
states. As is well known, general oil transforms its
structure to glass under high pressure, as realized incontinuously valuable transmission mechanics, forexample[1,2]. Past studies have focused mainly onmacro-characteristics of lubricants under high pressure,
such as their viscosity and bulk modulus[3,4]. Based onthese studies, some reports have shown that the traction
properties largely depend on the molecular structure of
the traction oil[5]. In addition, to investigate themolecular structure of the oil under high pressure, somesimulation studies with molecular dynamics techniques
have been done recently[6]. These studies indicate thegrowing attention being paid to the microscopic
behaviour of machine oils.
The X-ray diffraction method has been used as a
traditional method of getting information about the
microscopic structure of materials. In this method,
structure is estimated by analyzing obtained intensity
profiles of X-ray scattering. However, there has been noresearch directly investigating the microscopic structureof machine oils. We applied the X-ray diffraction method
to structure analysis of machine oils, focusing especiallyon their microscopic behavior under high pressure. Since
general machine oil consists of several base oils, wechose two base oils as samples, n-hexadecane anddicyclohexyl; the former is a normal alkane, and the latter
is a dual-cyclic alkane.
For this study, synchrotron radiation (SR) X-rays wereapplied to structure analysis because SR X-rays have acharacteristic high brightness and wide energy range. In
addition, the SR X-ray enables us to obtain accurate
intensity profiles in a short period compared to X-rays inlaboratories.
In summary, the final purpose of this study is toanalyze microscopic structure of base oils under highpressure using synchrotron radiation X-ray diffractionand to determine exactly the pressure at which the oiltransitions from liquid to solid. In this period, webasically investigated microscopic structure of the oils atnormal pressure with ‘Max80’ in the NE-5C beam line inPF-AR, as a first step.
Experimental Procedure
In this study, we used an AR-NE5C beam line with a
cubic multi-anvil type apparatus, called ‘Max80’, toanalyze microscopic structure.

Cubic Multi-Anvil Type Apparatus for High Pressure
The cubic multi-anvil type apparatus, which is a singlestage type press, uses six anvils made of cemented
carbide to create high pressure conditions. Each anvil has
a square edge face to press a sample specimen. Thesesizes are selected based on sample size. Two of these six
anvils are used as the upper and lower presses, and theother four anvils are used as side presses[7].
The optical alignment of this apparatus is shown in
Figure 1. The X-ray beam enters through two slits and thegap of anvils, and the sample is irradiated by the beam.
Finally, the diffracted X-ray from the sample is detectedby the one-dimensional Solid-State Detector (SSD).
Slit
Detector

Slit2θ

X-ray

SampleFigure 1: A diagram of optical alignment
Condition of Incident Angles
To obtain accurate information about the structure of
the sample oils, we measured the scattering intensity at 13
diffraction angles: 2θ = 2.5, 3.0, 4.0, 6.0, 8.0, 10.0, 12.0,14.0, 16.0, 18.0, 20.0, 22.0, and 25.0 degrees.

Sample Oil
Two base oils, n-hexadecane (CH3(CH2)14CH3)n anddicyclohexyl C6H11C6H11, were prepared as sample oils.
The former is a normal alkane, and the latter a dual-cyclicalkane. We chose these oils because their simple andcontrasting structures suited our basic study. The
molecular geometries of these oils are shown in Figure 2,
and their basic properties are listed in Table 1.

n(a) n-hexadecane(b) dicyclohexyl

Figure 2: Molecular geometry of sample oils
Table 1: Property of sample oils
Molecular weight
MP (centigrade)BP (centigrade)n-hexadecane
226.4
18287

dicyclohexyl
166.3
3-4
227

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Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

Scattering Intensity Profiles
Some examples of the obtained scattering X-ray
intensity profiles from dicyclohexyl are shown in Figure
3, where the vertical axis represents scattering intensity,and the horizontal axis represents energy E . It can beseen that the profiles are peculiar to typical liquid without
any sharp peaks from small bottle made by Teflon. In
addition, we confirmed that these intensity profiles did
not change over three measurements. This indicates that
X-ray irradiation did not degenerate the sample oil. We
used a similar process to obtain the intensity profiles of nhexadecane.
3500
25.0°
6.0 o

Intensity[count]

3000
2500
2000
1500

10.0

5.0 oo2θ =2.0°

16.0o

1000
500
00204060

80

Energy [keV]

Figure 3: Scattering intensity profiles of dicyclohexyl
Structure Factor
Figure 4 shows the structure factors of the sample oils
analyzed with the obtained intensity profiles. The verticalaxis represents the structure factor, S (Q) , while thehorizontal axis represents the scattering vector, Q . As canbe seen in the figure, both sample oils have several peaks
at almost the same Q values. This is due to thesimilarities of the structures of these samples. Both nhexadecane and dicyclohexyl consist of carbon andhydrogen, and the ratio of carbon to hydrogen is 0.32 and0.35, respectively. As a result, their structure factors arevery similar.

In Figure 5 and 6, it can be seen that there are several
peaks of the PDF and the RDF. The distance r having abroad peak is dependent on a bond distance and theheight of the profile is dependent on the relative amountof the bond. Some typical bond distances are listed inTable 2. The peaks around r = 1.10 Å and 1.54 Å
correspond to the C-H and C-C bonds of sample oils,respectively. As a whole, we were able to confirm thesuccess of the structure analysis by comparing it with thecommon bond distance at normal pressure.
Microscopic structures of the oils are much discussed
from the RDF in Figure 6. We can see that there areseveral peaks even at the range of r ≥ 2.0 Å in the figure.
These peaks seem to be caused by two atoms that are notdirectly connected each other but are in the samemolecule. To permit a clear discussion, we numbered all
the carbon atoms, as shown in Figure 7.
First, we focus on the results for n-hexadecane. As canbe seen from the RDF, there are two peaks, one aroundr = 2.5 Å and the other around 4.0 Å. The former is dueto two carbon atoms jumping over one carbon atom, likeC1-C3, and the latter is due to two carbons jumping overtwo carbon atoms, like C1-C4. The peak around r = 3.2 Åis thought to be from two hydrogen atoms connected toeach carbon atom, C1 and C3, for example.
The results for dicyclohexyl are similar. As can be seenfrom the RDF, the wide peak from about r = 2.5 to 3.0 Å
can be separated into two small peaks. One is due to two
carbon atoms jumping over one carbon atom, as with C1C3, and the other is due to two carbons jumping over twocarbon atoms in the same cyclohexane ring, as with C1C4.
Furthermore, at any r ≥ 4.0 Å, both RDFs are almost
smooth curves without any peaks. This indicates that
there is no regularity over a long distance because thesample oils are in a completely liquid phase.

g (r)Results and Discussion

5n-hexadecane

S(Q)4dicycrohexyl

3n-hexadecane

dicyclohexyl

0246810121416r [Å]2Figure 5: Pair distribution functions of sample oils

1

4000
2

4

6810

Q [Å-1]Figure 4: Structure factor of sample oils
PDF and RDF
Figures 5 and 6 show the PDF and RDF of the sample
oils obtained by the Fourier transform of the above S (Q )profiles. The vertical axis represents the PDF g (r ) or theRDF 4πr 2 g (r ) , while the horizontal axis represents adistance r from the referential atom.

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4πr 2g(r)0193-2

1.6
1.4
1.2
10.8
0.6
0.4
0.2
0300
200

dicyclohexyl
n-hexadecane

10000

123r [Å]45Figure 6: Radial distribution functions of sample oils

Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

Table 2: Distance of common bonds in alkane
BondC-C
C-H
C=C

Bond distance [Å]
1.54
1.10
1.33
226481014123164113579111315n(a) n-hexadecane

65891211710(b) dicyclohexyl

Figure 7: Structural formula with numbering carbons
Conclusion
The microscopic structure of base oils was analyzed
using a synchrotron radiation X-ray diffraction method.
Conclusions are summarized as: The PDF and RDF of then-hexadecane and dicyclohexyl were clearly obtained by
X-ray diffraction method without any degeneration of theoils. It was confirmed that they correspond to theirmicroscopic structures at normal pressure.
Acknowledgement
We wish to express sincere gratitude to Prof. K. Tsuji(Keio University), and Dr. T. Kikegawa (KEK), Dr. T.
Hattori and Dr. Y. Katayama (Japan Atomic EnergyAgency) for their sincere cooperation. This work wassponsored by the Ministry of Education, Science andCulture of Japan (No. 18760114).
References[1] N. Ohno, J. of Japanese society of Tribologists, 46(5),349 (2000), in Japanese.
[2] I. Fujishiro, Proc. of International Seminar on
Microstructures and Mechanical Properties of New
Engineering Materials (IMMM’93), 13(1), 275 (1993).
[3] Y. Nakamura, Transactions of the ASME, Journal ofTribology, 117(3), 519 (1995).
[4] Y. Nakamura, J. of Japanese society of Tribologists,46(5), 368 (2000), in Japanese.
[5] H. Tamura et al., J. of Japanese society of Tribologists,45(9), 65 (2000), in Japanese.
[6] Y. Tamai, J. of Japan Petrol. Institute, 25(3), 127(1982).
[7] W. Utsumi et al., J. of the Crystallography Society ofJapan, 42, 59 (2000), in Japanese.
* thirayam@mail.doshisha.ac.jp

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