Appl. Phys. A 74 [Suppl.], S534–S536 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201781

Applied Physics AMaterials
Science & Processing

Neutron spin echo studies of the effects of temperature and pressure
in a ternary microemulsion
Y. Kawabata1,∗∗ , M. Nagao2 , H. Seto3 , S. Komura4 , T. Takeda3,∗ , D. Schwahn5
1 Graduate School of Bio-Sphere Science, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
2 Institute for Solid State Physics, The University of Tokyo, Tokai 319-1106, Japan
3 Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
4 Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Tokyo 192-0397, Japan
5 Institut für Festkörperforschung des Forschungszentrums Jülich GmbH, 52425 Jülich, Germany

Received: 16 July 2001/Accepted: 11 December 2001 –  Springer-Verlag 2002

Abstract. In order to clarify the self-assembling mechanisms in complex fluids involving amphiphiles, we have investigated dynamic features of amphiphilic membranes anddroplets at high temperature and at high pressure in a ternary
microemulsion, consisting of AOT, water, and n-decane.
A high-pressure cell for neutron spin echo (NSE) experiments
has been improved, and the static and dynamic features ofdroplets are observed in detail by means of small angle neutron scattering and NSE. It is found that the size fluctuationand the diffusion of droplets are enhanced by increasing temperature, while they are suppressed by increasing pressure.
PACS: 61.12.Ex; 82.70.-y; 62.50.+p
Recently, physical aspects of microemulsion systems composed of oil, water, and surfactants have been studied intensively. Amphiphiles decrease interfacial tension of water andoil, and the mixture forms various mesoscopic structures depending on its composition, temperature, and pressure. It has
been suggested that the bending modulus κ of amphiphilicmonolayers plays an important role in self-assembling mechanisms [1, 2]. Neutron spin echo spectroscopy (NSE) is a decisive technique to clarify the behavior of κ [1–3].
In a semi-dilute water-in-oil droplet system consistingof an ionic surfactant AOT (dioctyl sulfosuccinate sodiumsalt), water, and n-decane, increasing temperature or pressureinduces a macroscopic phase separation to a dilute droplet
structure and a dense droplet structure [4–6]. In order toclarify the mechanisms of this phase transition, we have investigated dynamic features of monolayers and droplets bymeans of NSE [7, 8].
In this study, we investigated the temperature or pressure
dependences of the semi-dilute droplet system by small angle
neutron scattering (SANS) and NSE and compared the effects∗ Corresponding author.

(Fax: +81-824/240757, E-mail ttakeda@hiroshima-u.ac.jp)∗∗ Present address: Department of Chemistry, Faculty of Science, Tokyo
Metropolitan University, Tokyo 192-0397, Japanof temperature with those of pressure. The SANS result indicated that the size fluctuation of droplets is enhanced bytemperature. From the NSE measurements, a difference oftemperature and pressure effects on the membrane dynamics
is emphasized.
1 Experiments
A mixture of 5.4 vol. % of AOT (99% purity, purchasedfrom Sigma Inc.), 4.6 vol. % of D2 O (99.9 at. %-D, purchasedfrom Isotec Inc.), and 90 vol. % of C10 D22 (99.9 at. %-D, purchased from Isotec Inc.) was used. The composition leads to10 vol. % of droplets. Because the neutron-scattering lengthsof D2 O and C10 D22 are almost the same, only the AOT layers
could be observed (shell contrast).
The SANS experiments were carried out at the SANS-U
spectrometer at the C1-2 port in the cold neutron guide hallof JRR-3M JAERI, Tokai, Japan and at KWS1 of FRJ2 in
FZ-Jülich, Jülich, Germany.
1.1 Settings for SANS-U
The wavelength of the incident neutron beam was 7.0 Å witha spread of δλ/λ = 10%. The distances from the sample toa two-dimensional position-sensitive detector were 1 m and−1
4 m. The momentum transfer q ranged over 0.01 Å ≤ q ≤−10.14 Å . The temperature was changed from 23 ◦ C to 80 ◦ Cwith a step of about 3 ◦ C at ambient pressure. The pressure
was changed from 0.1 MPa to 66.5 MPa with a step of about
5 MPa at room temperature.
1.2 Settings for KWS1
The wavelength of the incident neutron beam was 7.0 Å witha spread of δλ/λ = 20%. The distances from the sample toa two-dimensional position-sensitive detector were 2 m and−18 m. The momentum transfer q ranged over 0.006 Å ≤S535−1◦q ≤ 0.14 Å . The temperature was changed from 13 C to25 ◦ C with a step of about 2 ◦ C with increasing pressure from0.1 MPa to 30 MPa with a step of about 10 MPa.
1.3 Settings for ISSP-NSE
The NSE experiments were performed at the ISSP-NSE spectrometer at the C2-2 port of JRR-3M JAERI, Tokai, Japan.
The wavelength of the incident neutron beam was 7.14 Å witha spread of δλ/λ = 18%. The measured momentum transfer−1
q ranged over 0.04 Å ≤ q ≤ 0.13 Å−1 and the Fourier time
t over 0.15 ns ≤ t ≤ 15 ns. The temperature was set at 19, 25,and 35 ◦ C at ambient pressure. The pressure was set at 0.1, 20,
and 40 MPa at room temperature.
So far, it was difficult to keep the pressure constant fora long time (about one week for each observation) with
our old high-pressure cell made of non-magnetic stainlesssteel [7, 8]. Therefore, a new high-pressure cell made ofnon-magnetic stainless steel and inconel was developed with
Hikari High-Pressure Machinery Co. Ltd. Because the cell
was magnetized slightly and neutron-spin depolarization occurred, we set some correction coils to increase the guiding
magnetic field. The improved cell made it possible to keep thepressure unchanged at least for one week and precise data athigh pressure could be obtained.
2 Results and discussion
2.1 SANS experiments
Typical SANS spectra I(q) are shown in Fig. 1. The forwardscattering intensity I(0) became larger with increasing temperature and pressure, because of the critical scattering ofa concentration fluctuation of droplets [4, 5]. Although thesample was not at the critical composition, I(0) increased
when approaching the phase boundary and decreased above
it. From such a behavior of I(0), we estimated that the transition temperature at ambient pressure was Tc = 59◦C andFig. 1. SANS profiles observed at three conditions. The SANS profile at
T = 25 ◦ C and P = 0.1 MPa is shown in absolute units. Two other curves
are shifted by a factor of 80 cm−1

the transition pressure at room temperature was Pc = 40 MPa.
Thus, the conditions of the NSE measurement were assured
to be in one phase region. In order to deduce the mean radius R0 = R and the polydispersity index of the dropletsp ( p2 = R2 /R2 − 1), we utilized the model proposed byGradzielski et al. [9, 10] as follows:(1)I(q) = NS(q) h(R)F(q,R) dR ,where N is the number density of the aggregates, R the radius of the droplets, h(R) a Gaussian distribution function for
the radii, and S(q) the structure factor, which is assumed tobe unity for systems with a low volume fraction of droplets inthis q-region. F(q,R) is the form factor of the monodisperse
spheres with the shell-contrast condition.
From the pressure-variation experiments, p = 0.14 and
R0 = 32 Å were obtained independent of pressure. On theother hand, it was found that R0 decreased from 32 Å to28 Å and p increased from 0.16 to 0.18 with increasing temperature. This means that the size fluctuation of droplets isenhanced by increasing temperature while it is not affected byincreasing pressure.
2.2 NSE experiments
Figure 2 presents the normalized intermediate correlation
functions I(q, t)/I(q, 0) at T = 43◦ C, observed by NSE experiments. They were analyzed using a theory proposed byMilner and Safran [11], which was successfully applied byHuang et al. [12] and by Farago et al. [13] to express thedynamics of microemulsion droplets. They could fit the obtained I(q, t) to a single exponential function as follows:
I(q, t)/I(q, 0) = exp −Deff (q)q 2 t ,(2)where Deff (q) is the effective diffusion coefficient. Taking theshape deformation of the droplets into account, Deff (q) consists of two terms:
Deff (q) = Dtr + Ddef (q) ,(3)Fig. 2. Intermediate correlation functions obtained from the NSE experiments at T = 43 ◦ C, P = 0.1 MPa for different q’s. Lines are the fitting
results according to (2)S536

where Dtr is the translational diffusion coefficient independent of q and Ddef (q) is the diffusion coefficient concerning
the shape deformation. The second term is given as [12, 13]:
Ddef (q) =5λ2 f 2 (qR0 )|a2 |2 ,
q 2 [4π [ j0(qR0 )]2 + 5 f 2 (qR0 )|a2 |2 ]

f 2 (qR0 ) = 5 [4 j2(qR0 ) − qR0 j3 (qR0 )]2 .

temperature on the dynamics of droplets are opposite to thoseof pressure, although the apparent phase separation induced
by temperature is similar to that induced by pressure.
3 Conclusion(4)Here, jn is the nth spherical Bessel function, λ2 the damping
frequency of the droplet deformation of the n = 2 mode, and|a2 |2  the mean-square amplitude of the n = 2 mode. Mainly,
the n = 2 mode can be observed in the time window of theISSP-NSE spectrometer, and the modes of n = 2 are out ofthe dynamic range of the spectrometer.
Figure 3 shows the effective diffusion coefficient Deff obtained from the fitting results according to (2). Deff becomes
smaller by decreasing temperature or increasing pressure.
Hence, the diffusion of droplets is enhanced by increasing
temperature while it is suppressed by increasing pressure.
The bending modulus κ could be deduced using Milner andSafran’s theory [11, 14]. It was also found that κ increased
with increasing pressure while it decreased with increasing
temperature. These results are consistent with our previous
results that amphiphilic monolayers become rigid with increasing pressure while they become flexible with increasing
temperature [7, 8, 15, 16]. It has been found that the effects ofWe performed SANS and NSE experiments on a ternary microemulsion droplet system in order to investigate its staticand dynamic features. By improving the high-pressure cellfor NSE experiments, we could obtain the pressure dependence of the droplet dynamics. It has been found that theeffects of temperature on the dynamics of the droplets are opposite to those of pressure, although the apparent structuresof the phases induced by temperature are similar to those induced by pressure. The size fluctuation and the diffusion ofthe droplets are suppressed by increasing pressure, while they
are enhanced by increasing temperature.
Acknowledgements. We appreciate Mr. M. Koizumi at Hikari High-Pressure
Machinery Co. Ltd. for developing the high-pressure cell for the NSE experiments. One of the authors (M.N.) was supported by a Grant-in-Aid for
Encouragement of Young Scientists (No. 10740187) and H.S. and T.T. werealso supported by Grants-in-Aid for Scientific Research (Nos. 12640377,09640466, and 13440119) from the Japanese Ministry of Education, Science, Sports, and Culture. Our SANS and NSE experiments at JAERI weredone under the approval of the Neutron Scattering Program Advisory Committee (Proposal Nos. 00-199 and 00-200).

References

Fig. 3. Deff obtained from NSE data at various conditions. Lines are fitting
results to (3) and (4). With increasing temperature, Deff becomes larger,
whereas it becomes smaller with increasing pressure

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