SOLEIL II - Come to SOLEIL
- Phonebook & CONTACTS
In the manufacturing processes for many surfactant-based products, mechanical mixing is an important control parameter because it homogenizes multi-component systems and enhances their crystallization kinetics. Indian and French scientists have just demonstrated a unique reversible crystallization effect induced by a mechanical shear force, in lamellar phases consisting of double layers of surfactants. The present finding, obtained on the SWING beamline, also has important implications for understanding the solidification of edible fats and oils which is very essential to improve the texture and taste of food products as in margarine, reduced fat spreads, icecreams, chocolate and honey or in personal/ homecare products as in the manufacturing of bar soaps, lotions, toothpastes and lubricant greases.
Cristal: Under certain conditions (temperature, pressure, mechanical stresses, etc.), the atoms of a material can assemble and order themselves to form a crystalline solid. In foodstuffs, this transition is important in the freezing stages, particularly with regard to improving the texture and flavour of products
As a general rule, shear is known to assist crystallization by lowering the energy barrier for nucleation. A crystalline phase can thus be induced by applying a flow, This phenomenon is not a dynamic phase transition because the induced crystalline phase is maintained if the shear is removed. But recently, researchers from the Indian Institute of Science, (Mr. Vikram Rathee and Prof. A.K. Sood), Jawaharlal Nehru Centre for Advanced Scientific Research (Dr. Rema Krishnaswamy) Raman Research Institute (Dr. Antara Pal and Prof. V. A. Raghunathan) and Laboratoire de Physique des Solides (LPS), Orsay, France (Dr. Marianne Impéror and Prof. Brigitte Pansu) have demonstrated a new shear reversible shear-induced crystallization phenomenon, which is the first example of shear-induced dynamic crystallization.
This remarkable result was obtained in a specific aqueous surfactant solution , which forms a lamellar fluid phase (phase Lα) at equilibrium. This type of phase is very common in aqueous surfactant systems, and consists of a periodic assembly of surfactant molecule bilayers separated by layers of water. The main result is that the application of a stationary shear force generates the appearance of a crystalline phase (phase LC), and this effect is reversible: when the shear is removed, the crystalline phase disappears and the system returns to its equilibrium state. After the initial studies carried out in Bangalore, new time-resolved x-ray scattering measurements under shear (Rheo-SAXS) performed on the SWING beamline at SOLEIL turned out to be crucial in demonstrating i) the reversibility under shear above the crystallisation temperature at equilibrium, and ii) the structure of the shear-induced crystalline phase (Lc).
The system studied has a very different behavior than a simple fluid of colloidal spheres [1]. Here, the structural transition induced by shear could be due to a rearrangement of the molecules in the bilayers. In fact, the system consists of a mixture of two species in water: an anionic one and a cationic one. A microphase separation between these two molecules inside the bilayers would make it possible to adjust the spontaneous curvature at the bilayer-water interface and explain the phenomenon.
A type of transition that has not yet been explored
Such a shear-induced non-equilibrium phase transition is a possibility that was completely unexplored till the present work. These findings will motivate further Rheological Small angle X-ray scattering investigations by Dr. Rema Krishnaswamy at JNCASR in collaboration with Prof. A. K. Sood at the Indian Institute of Science
Shear reversible crystallization in weakly swollen isotropic and lamellar phases of a cationic-anionic mixed surfactant system:
The studies were carried out inthe lamellar (Lα) and isotropic (Li)phases consisting of bilayers formed by a commercially relevant anionic surfactant sodium dodecylsulfate and a strong binding cationic counter-ion para-toluene hydrochloride in water. The isotropic Li phase is characterized byshort range translational correlations. It occurs when the quasi-long range positional order of the Lα phase is destroyed at higher temperatures or at higher concentration of counterions through the creation of defects in the bilayer [2]. The present study is different from the earlier studies on sponge phases [3].IThe role of steady shear (at Peclet No. << 1) in weakly swollen, bilayer forming concentrated phasesis to drive a non-equilibrium transition from Li→Lα→crystalline phase (Lc).The high resolution powder X-ray diffraction data obtained for the non-equilibrium Lc phase could be indexed to a triclinic lattice (see Fig 1, below).
Figure 1: A) The diffraction pattern of the steady state non-equilibrium Lc phase obtained in tangential geometry on shearing the sample for 316 s. Indexing scheme of the diffraction pattern indexed to a triclinic unit cell (B) is shown. (C) Schematic sketch of the proposed crystalline structure. Time resolved Rheo-SAXS data of the Li phase in the radial geometry showing the shear reversibility of the Lc phase. The experiment is carried out at T= 33oC,above the equilibrium solidification (Krafft) temperature of the Lc phase: A) Growth of the Lc phase with time, during the application of steady shear at 10 s-1; the times shown correspond to that from the start of shear. B) Melting of Lc phase on stopping the shear; the times shown correspond to that from the stoppage of shear.
By following the growth kinetics with optical microscopy it was shown that the crystalline Lc phase nucleates in a shear-induced Lα phase (Fig 2), with a higher concentration of the organic counterions as compared to the starting Li phase at rest. Consequently, the driving mechanism for the formation of Lc phase is believed to be the shear-induced redistribution of the hydrophobic counterions in the bilayers of the Lα phase. Understanding shear as a strategy to control the spontaneous curvature of bilayers of mixed amphiphilic systems is also expected to have significant applications in the design of novel drug delivery systems, controlling the permeability of bilayers in onion phases used for drugs encapsulation, besides tuning the viscosity during flow processing surfactant-based products in consumer industry.
Figure2: Polarizing optical micrographs at different times after the onset of shear at 10 s-1 at 33 oC, showing A) growth of shear-induced Lαidentified from oily streak texture in Li phase (indicated by the dark optically isotropic background) B) growth of birefringent crystallites (Lc as indicated by arrows) in the Lα phase and C) final non-equilibrium steady state of Li and Lc phases in coexistence.
References:
1] Ackerson, B.J., and Pusey, P. N.1988, Shear-Induced Order in Suspensions of Hard Spheres, Phys. Rev. Lett.61: 1033-36.
2] Pal, A., Pabst, G. and Raghunathan, V. A.2009 ,Defect-mediated lamellar-isotropic transition of amphiphilic bilayers, Soft Matter Comm.8 9069-9072.
3] Porcar, L., Hamilton, W. A. and Butler, P. D.2003, Scaling of structural response of L3 Sponge Phases in the "Sweetened" Cetylpyridinium/Hexanol/Dextrose/Brine system, Langmuir19 10779-10794 and references therein