Two variants of a sample holder were constructed for SAXS (small angle X-ray
scattering) and freeze fracture studies of colloid systems during shear
stress. The detailed construction of both variants is described. The SAXS
investigations give information about the structural changes in the range of
nm´s, and the freeze fracture technique makes possible the imaging of the
\mum-scale morphology. Combining these methods the origin of certain
macroscopical features, e.g. shear induced structural formations (like
tixotropy) can be examined in a wide size range of the colloid dimensions

Bóta, Attila

Fetter, György

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PERIODICA POLYTECHNICA SER. CHEM. ENG. VOL. 48, NO. 1, PP. 3–12 (2004)CONSTRUCTION OF A SHEAR CELL FOR SAXS AND FREEZEFRACTURE STUDIESGyörgy FETTER and Attila BÓTADepartment of Physical ChemistryandHAS–BUTE Laboratory of Soft MattersBudapest University of Technology and EconomicsH–1521 Budapest, HungaryPhone: (36 1) 463 2473e-mail: gfetter@mail.bme.hu, abota@mail.bme.huReceived: April 2, 2004AbstractTwo variants of a sample holder were constructed for SAXS (small angle X-ray scattering) and freezefracture studies of colloid systems during shear stress. The detailed construction of both variants isdescribed. The SAXS investigations give information about the structural changes in the range ofnm’s, and the freeze fracture technique makes possible the imaging of the µm-scale morphology.Combining these methods the origin of certain macroscopical features, e.g. shear induced structuralformations (like tixotropy) can be examined in a wide size range of the colloid dimensions.Keywords: SAXS, freeze fracture, shear cell, layer structure, domain structure.1. IntroductionThe investigation and interpretation of the shear-induced structural formations havea great interest in the description of the rheological behaviour of number of systemsexisting in different fields of industry, like production of foods, detergents, pharma-ceuticals [1]. Simultaneous scattering and rheological measurements are consideredas powerful methods [2, 3], but the mutual correspondence of the results obtainedby the two different kinds of methods is not unambiguous. The interpretation ofthe experimental data requires different assumptions expressed in different struc-tural models. The rheological features are explained with shear induced structuralchanges occurring in the range of nm’sizes, while the structural formations in therange of the µm’s are generally negligible. Therefore we intended to investigatethe size range of µm by using the method of freeze fracture.Our previous works revealed that the structure of a typical industrial surfactant-water system is drastically affected by shear [4, 5]. To obtain structural and visualinformation about the shear induced domain formation a shear cell was contstructedin two variants. The monitored size range extends from nm- up to µm-scales whichcan be detected by using small angle X-ray scattering (SAXS) and freeze fracturemethods, respectively. Corresponding to this kinds of measurements, one variant4 GY. FETTER and A. BÓTAof the cell was built for the SAXS studies and another one for freeze-fracture [6, 7].By this construction of the cell we had given up the measurements of the rheologicaldata obtained by the conventional rotational- or oscilloviscosimetry, but a numberof the shear parameters (shear rate, frequency, temperature of shear, thickness ofsample) can be set and the shear process itself can be reproduced very precisely.However, the conditions of the shear process occurring in the constructed cell canbe mimicked by an oscilloviscosimeter [7].In this paper the technical description of both variants of the constructed cellis reported. To demonstrate the advantages of the shear cell we present some typicalresults obtained on the non-ionic surfactant/water (Synperonic A7-water) system.In this system the lamellar phase exists in the concentration range from about 50up to 80 w/w %, between 10 and 60 ◦C. The system was already investigated veryintensively with various methods [4]–[10]. It was found that its viscosity decreasedextremely after some minutes of shear. The system exhibited tixotropy as the flow(shear stress as the function of the shear rate) and the viscosity curves were open:the value of the shear stress and the viscosity differed from their starting values.Using the two variants of the constructed cell we followed the changes occurredunder shear and we could give an explanation for the tixotropy of the A7/watersystem.2. The Construction of the Shear CellThe shear cell constructed for SAXS is a sandwich-like one. This kind of cellgeometry is already used by other research groups, too [11]. It consists of tworectangular bulks made of stainless steal with quadratic apertures in the centre ofthem for the transmission of the X-ray beam as it is shown in its technical scheme andphoto in Fig. 1a and 1b, respectively. The bulks are fastened together with screws.The sample is placed between two special, thin Plexiglas (PMMA) windows havingminor SAXS. One of the windows is fixed; the other one is movable by the outermover unit. The fixed window is set with a stainless steel frame in one of the bulks.The windows are set in the apertures of the bulks which have a height of 6 mmand a width of 16 mm. The thickness of the sample is adjustable with a convenientframe and its maximum value is 1 mm. The moving window is between the bulksand its free moving is assured by a Teflon (PTFE) sheet around it, which keeps thedistance between the bulks.The variation for freeze fracture uses one of the bulks with the fixed window,but instead of the other bulk there is a unit to press the alternating window to thesame position that was placed in the other SAXS variant as it can be observed inFigs. 2a,b. This unit consists of two spring-pushed arms that keep the movingwindow in its working position through two rollers. Against the springs arms canbe lifted up and in this position fast sampling is possible for the freeze fracturemethod. Incubation is carried out by water flow in both cases, through inner boresof the bulks.CONSTRUCTION OF A SHEAR CELL FOR SAXS 5Fig. 1. The sample holder for SAXSThe electromechanical mover includes two solenoids with iron cores, whichare alternately passed through by current and they draw a soft steel plate up anddown. The construction of this unit is demonstrated in Figs. 3a,b. This alternatingmotion is forwarded by a track to the moving window. Changing the distancebetween the solenoids, increasing or decreasing of the shear amplitude can beachieved between 0 and 5 mm, the resolution is 0.05 mm.The electronic unit driving the electromechanical mover is a double timerbased on a crystal oscillator, switching voltage to the solenoids one after the other.6 GY. FETTER and A. BÓTAFig. 2. The sample holder for freeze fractureThese time periods can be set separately by BCD (binary coded decimal) switches,the range is 0.001–100 s and the resolution is 0.001 s. The electronic unit permitstriggered periodic movements (‘up’ and ‘down’) to occur with constant velocitiesand at the end positions the mechanic mover (and the window, too) is waiting forthe time defined by the adjusted frequency. The path-time function of the movingwindow is a quadrangular sign. A drastic deformation of this sign function occurs athigher frequency. At the frequency of about 10 Hz, the mowing window is alwaysmoving ‘up’ and ‘down’ (there is no more waiting at the end positions) and itspath-time function becomes to a nearly sinusoidal form. That is the reason why thecondition of the shear cell by can be mimicked an oscilloviscosimeter. At higherCONSTRUCTION OF A SHEAR CELL FOR SAXS 7 moveable platform of the solenoid  soft steel sheet driving rod window fixing   control unit solenoid spring amplitude adjusting screw connecting platform to the sample holder moving window of the sample holder Fig. 3. The electromechanic moverfrequency (about 20–30 Hz) the mechanical unit is not able to follow the digitalunit and non-periodic movement occurs. The shear rate was calculated from theamplitude of the movement (adjusted with an error range of about 2%), the periodictime (measured electronically) and the width of the sample. The shear rate wasadjusted for a constant maximal value (at crossing the time axe, about 8 1/s) duringboth of the two kinds of measurements, e.e. SAXS and freeze-fracture.3. Experimental3.1. MaterialsThe Synperonic A7 is an ethoxylated fatty alcohol with C13 and C15 alkyl chainsand with an average ethoxylation number of 7 (but this number is between 0 and 15in the product of ICI Surfactants, Brussels, B). The tenside was used as supplied, itwas mixed with 20% distilled water, heated up to 60◦C and stirred for 30 min. thenultracentrifuged to yield a bubble-free material and finally left at 20◦C for 1 week.4. MethodsA modified compact Kratky camera (Anton Paar, Graz, Austria) supplied with alinear, one dimensional position-sensitive device (Mbraun, Garching, Germany)8 GY. FETTER and A. BÓTAFig. 4. The modified Kratky camera with the shear cellwas used to record the SAXS patterns which allowed relatively short expositiontimes (1–2 minutes). The source was a Ni-filtered Cu Kα radiation (λ = 1.542 Å).The Kratky camera had a unified inner space because of the vacuum needed forthe measurements, thus it had to be modified to be able to accommodate the shearcell. An open section was formed in the middle of the camera for the shear cell.The complete measurement apparatus, the SAXS camera with the shear cell andthe electronic unit can be seen in the Fig. 4.The freeze-fracture technique was used as usual. The gold specimen holderswere preincubated at the same temperatures as the block of the shear cell. Samplesin small volume (of about 1–2 µl) were taken out under the flexible window of theshear cell by using a fine palette-knife and were placed onto the gold holders, whichwere then immediately plunged into partially solidified Freon for 20 s freezing andthan placed and stored in liquid nitrogen. Fracturing was carried out at −100◦Cin a Balzers Freeze-Fracture Device (Balzers AG, Liechtenstein). The fracturedfaces were etched for 30 s at −110 ◦C. The replicas, prepared by platinum-carbonshadowing, were cleaned with a solution of hypochlorous acid and washed withdistilled water. From pure water, the replicas were picked up on 200 mesh coppergrids. The electron micrographs were taken with an electron microscope (JeolJEM-100 CX II, Japan).5. ResultsThe Synperonic A7-water system (80 w/w%, 20 ◦C) has a characteristic SAXSpattern having two Bragg reflections. The first Bragg reflection is relatively sharpCONSTRUCTION OF A SHEAR CELL FOR SAXS 9corresponding to a well ordered layer structure, while the second one is very small.The displacement of the Bragg peak informs about the mean periodic distance by asingle relation, namely d = 1/smax , where smax is the value of the scattering variable(s, defined as s = (2/λ) sin , where  is the half of the scattering angle), which isat the maximum of the Bragg reflection. Within the curvatures only the shapes ofthe Bragg peaks were influenced by shear, therefore only the Bragg profiles weredetected as a function of the shear time, as shown in Fig. 5. The shapes as a functionof the Bragg peak did not change significantly with shear. After 2 hours of shearthe layer distance was shifted from the starting value of 48.3 Å to 46.8 Å. After3 hours the layer distance decreased slightly to 46.5 Å while the intensity of thereflection was increased to about 1.5 times higher than it had been in the startingstate. The latter change indicated that the lamellar arrangement was more typicalfor the whole sample after shear than in the starting steady state. After the end ofshear the profile of the peak was broadened corresponding to a less ordered layerstructure. In the shear cell the changes of the thermotropic behaviour can also befollowed as presented in the case of the same A7-water system detected at 60◦C(Fig. 6). At 60 ◦C in steady state only very broadened Bragg reflection appearedindicating that the lamellar arrangement was destroyed drastically due to the higherthermal fluctuations. After 10 minutes of shear the peak was decreased significantlyand after 1 hour shear it practically disappeared.0,01 0,015 0,02 0,025 0,03 0,035 0,04scattering variable [1/A]rel. int. [a.u.]steady statesteady state after 2 h shear2 h shear1 h shear3 h shear10' shearFig. 5. SAXS patterns during shear at 20 ◦CFreeze fracture images exhibited the evidence of a well ordered layer arrange-ment of the steady state in full agreement with the SAXS measurement. The typicalpictures showed closely packed parallel layers with smooth surface as it can be ob-served in Fig. 7. However, unregular stacks and other defects appeared occasionally.It was obvious, that the stacks of the parallel layers were distributed randomly andno macroscopic orientation existed, therefore we can conclude that the profile of10 GY. FETTER and A. BÓTA0,01 0,015 0,02 0,025 0,03 0,035 0,04scattering variable [1/A]rel. int. [a.u.]steady state10' shear1 h shearFig. 6. 6 SAXS patterns during shear at 60 ◦Cthe Bragg peak is a consequence of a not oriented, ‘powder sample’ structure. Af-ter 5 minutes of shear the sample became dissected, the large sheets of layers areconsiderably broken, but a significant part of the sample was still unchanged asdemonstrated in Fig. 8. After 2 hours of shear the sample was totally dissected,large sheets did not exist as shown in Fig. 9. The homogeneous parts of the sam-ple were broken down into smaller domains with a maximal extension of about0.5 µm. The pictures of the freeze-fracture revealed that aggregates with largersizes (up to several µm) also existed. It must be stressed out that the elementarylamellar arrangement was always present, but the number of sheets and their lateralextension were reduced extremely. However, this domain size of the lateral exten-sion proved to be enough to exhibit significant Bragg reflection. Consequently, wecan suppose higher layer correlations inside of the stacks after the shear than beforeshear.6. ConclusionThe simultaneous SAXS and freeze fracture studies revealed that the nm-scale layerstructure and the µm-scale domain structure of the Synperonic A7-water system(80 w/w %) were influenced by shear. Applying shear stress, considerable changesoccurred in the domain structure in a short time, while the layer structure was nearlyconstant during a long period of shear. It means that the short-term alteration ofthe rheological behaviour during shear was caused mostly by the changes of thedomain structure and not of the layer structure.The two variants of the shear cells constructed proved to be a powerful methodto reveal the structural and morphological changes induced by shear.CONSTRUCTION OF A SHEAR CELL FOR SAXS 11 Fig. 7. Freeze fracture image in steady state at 20 ◦C (The width of the picture representsa bar of 9 µm.)Fig. 8. Freeze fracture image after 5 minutes of shear at 20 ◦C (The width of the picturerepresents a bar of 9 µm.)AcknowledgementsWe thank to Ms. T. Kiss (Budapest,H) for the technical assistance at the freeze-fracturetechnique. This work was supported by the Hungarian Scientific Funds OTKA (Bota, T21781, T 43055) and a bilateral German-Hungarian Program TÉT (D-42/1998).12 GY. FETTER and A. BÓTAFig. 9. Freeze fracture image after 2 hours of shear at 20 ◦C (The width of the picturerepresents a bar of 9 µm.)References[1] MCHICK, M. J. Ed., Nonionic Surfactants, Physical Chemistry: Surfactant Science Series;Marcel Dekker: New York, 1987.[2] PENFOLD, J. – STAPLES, E. – KHAN LODHI, A. – TUCKER, I. – TIDDY, G. J. T., Phys.Chem. B. 101 (1997), p. 66.[3] MANG, J. T. – KUMAR, S. – HAMMOUNDA, B., Euro Phys. Lett. 28 (1994), p. 489.[4] NÉMETH, ZS. – HALÁSZ, L. – PÁLINKÁS, J. – BÓTA, A. – HORÁNYI, T., Colloid andSurfaces A. 145 (1998), p. 7.[5] HALÁSZ, L. – NÉMETH, ZS. – PÁLINKÁS, J. – HORÁNYI, T. – BÓTA, A., Progr. ColloidPolym. Sci., 117 (2001), p. 159.[6] FETTER, GY. – HORÁNYI, T. – BÓTA, A., Progr. Colloid Polym. Sci., 117 (2001), p. 217.[7] BÓTA, A. – FETTER, GY., Langmuir (2004) in press.[8] DIMITROVA, G. T. – TADROS, TH. F. – LUCKHAM, P. F. – KIPPS, M. R., Langmuir 5 (1996),p. 315.[9] TADROS, TH. F. – DIMITROVA, G. T. – LUCKHAM, P. – FTAELMAN, M. C. – LOLL, P.,Euro Cosmetics 3 (1995), p. 17.[10] DIMITROVA, G. T. – TADROS, TH. F. – LUCKHAM, P., Langmuir 11 (1995), p. 1101.[11] HAMLEY, I. W. – POPLE, J. A. – GLEESON, A. J. – KOMANSCHEK, B. U. – TOWNS-ANDREWS, E., J. Appl. Cryst., 31 (1998), pp. 881–889.

CONSTRUCTION OF A SHEAR CELL FOR SAXS AND FREEZE FRACTURE STUDIES

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CONSTRUCTION OF A SHEAR CELL FOR SAXS AND FREEZE FRACTURE STUDIES

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