Photon Factory Activity Report 2016 #34 (2017)

BL-15A2, BL-10C/2015G555

Lamellar/Onion Intermediate Structure Formed in Low Shear Rate Region
in a Nonionic Surfactant (C14E5)/Water System
Kento Suzuki, Youhei Kawabata, and Tadashi Kato*
Department of Chemistry, Tokyo Metropolitan University
1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
1 Introduction
The lamellar-to-onion transition induced by shear flow
has been found more than 20 years ago by the Frenchgroup [1]. Although the onion formation has beenreported for many systems, conditions and mechanism forthe transition have been still unclear. A few years ago, wehave reported for the first time the re-entrant
lamellar/onion (lamellar-onion-lamellar) transition withvarying temperature under a constant shear rate in anonionic surfactant C14E5/water system (CnEm is anabbreviation of CnH2n+1(OC2H4)mOH) by using
simultaneous measurements of shear stress/small-angle
X-ray scattering (rheo-SAXS) [2]. We have investigatedthe change in the lamellar orientation in the lamellar-toonion and onion-to-lamellar transition processes near theupper and lower transition. The results suggest theexistence of intermediate structures depending on thetemperature alone at a constant shear rate. In the present
study, we have performed rheo-SAXS experiments as afunction of the shear rate at a constant temperature paying
attention to the intermediate region between lamellar andonion phases.

The enhancement of the orientation to the neutral
direction before formation of onions has also beenreported by Richtering and coworkers [5,6] inC10E3/water system by using small-angle neutron
scattering. First [5] they have attributed it to theformation of multi-lamellar cylinders as intermediate
structures between lamellae and onions and later [6] toeither multi-lamellar cylinders or a coherent stripe
buckling proposed by Zilman and Granek [7]. The
important results in the present study are the existence of
two shear thickening regions I and II, and small
difference in the diffraction intensity in the shear-thinning
region between I and II. These results suggest that themulti-lamellar cylinders are more plausible not astransient structure but as steady state determined by theshear rate.

2 Experiment
Rheo-SAXS measurements have been performed on
the beamline 15A2 and 10C by using a rheometer AR550(TA Instruments). Details of the shear cell have beenreported previously [3,4]. The scattered beam wasrecorded with the camera length of 2.6 m using thePILATUS3 2M. The approximate q range is from 0.1 to2.4 nm-1. The exposure time was 10 s.
3 Results and Discussion
Figure 1 shows shear-rate dependences of the 2-D
SAXS patterns (a, b), the viscosity (c), and the diffraction
peak intensities for the three principal directions (d) for
the C14E5/water system (50 wt%, 55C). The peakintensity has been obtained by averaging over theazimuthal angle 10. In the lower shear rate range
bellow 0.1 s-1, the most lamellae are oriented to thevelocity gradient direction. We have observed the shearthickening in the two regions, i.e., 0.1 - 0.2 s-1 (referred toas the region I) and 1-3 s-1 (the region II). In the region I,
the peak intensity for the neutral direction rapidlyincreases with increasing shear rate. The intensity for theflow direction increases above 0.3 s-1, but still much
lower than for these two directions. In the region II, theintensities for these three directions coincide,corresponding to the onion formation.

Fig. 1: shear-rate dependences of the 2-D SAXS patterns
in the radial (a) and tangential (b) configurations, theviscosity (c), and the diffraction peak intensities for thethree principal directions (d) for the C14E5/water system(50 wt%, 55C).
References[1] O. Diat et al., J. Phys. II France, 3, 1427 (1993).
[2] D. Sato et al., Langmuir, 29, 121 (2013).
[3] Y. Kosaka et al., Langmuir, 26, 3835 (2010).
[4] M. Ito et al., Langmuir, 27, 7400 (2011).
[5] J. Zipfel et al., Europhys. Lett., 53, 335 (2001).
[6] F. Nettesheim et al., Langmuir, 19, 3603 (2003).
[7] A. G. Zilman and R. Granek, Eur. Phys. J. B, 11, 593 (1999).
* kato-tadashi@tmu.ac.jp