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Formation of Unique Unilamellar Vesicles from Multilamellar Vesicles under High Pressure Shear Flow Yuwen Shen, and Heinz Hoffmann J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b04646 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Formation of Unique Unilamellar Vesicles from Multilamellar Vesicles under High Pressure Shear Flow

Yuwen Shena,b,c* and Heinz Hoffmannd

a

Shandong Academy of Agricultural Sciences, Institute of Agricultural Resources and

Environment; Key Laboratory of Wastes Matrix Utilization, Ministry of Agriculture; Shandong Provincial Engineering Research Center of Environmental Protection Fertilizers, Jinan 250100, P. R. China b

State Key Laboratory of Nutrition Resources Integrated Utilization, Linshu 276700,

P. R. China c

Kingenta Ecological Engineering Group Co., Ltd. Linshu 276700, P. R. China

d

University of Bayreuth, BZKG, Gottlieb-Keim-Str. 60, 95448 Bayreuth, Germany

*

Corresponding author:

Yuwen Shen, Shandong Academy of Agricultural Sciences, Institute of Agricultural Resources and Environment, 202 Gongyebei Road, Jinan 250100, P.R. China Fax: (+) 86-0531-66655361 E-Mail: [email protected]

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Abstract: Vesicles in surfactant system are influenced by shear field. The high shear flow generated by a homogenizer is expected to affect the size of vesicles. Hence, it should be possible to control the size and dispersion of vesicles by tuning the shear. In this study, the influence of shear on the vesicle phase was studied by measuring the rheology and conductivity of a solution made of the nonionic surfactant Trideceth-5 (IT5), a polyethylene glycol ether of tridecyl alcohol with an average number of ethylene oxide of 5, and the anionic surfactant sodium dodecylsulfate (SDS). It was found that when shear was applied by a homogenizer, the bilayers of the multilamellar vesicles were stripped off and became unilamellar vesicles, which decreased the viscoelasticity of the system. However, because of the pressure provided by the homogenizer, the newly formed unilamellar vesicles were small and the relative distance between them was large. As a result, the vesicles were no longer crowded and could easily pass each other under shear. This is why the unilamellar vesicles generated by the homogenizer had low viscoelasticity and flow birefringence. Additionally, it took a long time for the unilamellar vesicles to relax back to the original state.

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1. Introduction Vesicles are very important because they have a wide range of applications in exceptionally selective membranes, drug delivery, microreactors, etc.1-7 Their surfactant lamellar phase and thermotropic smetic phase show fascinating phase transition under shear. In many systems, the surfactant bilayer structure is found unstable under shear flow. The relationship between flow and phase transition has received much attention, because the organization of the complex fluids may be affected by the flow field. Shear on planar lamellar phases may result in the formation of mutilamellar vesicle phase8-17 or the orientation transition of layers.10,17-22 Whereas shear-thinning can be achieved simply by aligning the anisotropic aggregates in the flow direction, shear-thickening usually involves the formation of new structures. Roux et al.23-26 reported that vesicles could be handled and transformed at varying shear rates. Defective lamellar structures were found at low shear rates and defect-free lamellar structures were found at high shear rates. As the low shear rate increased to an intermediate level, the defective lamellar structures transformed into vesicles whose size decreased with rising shear rate. Richtering et al.27-33 reported on reversible shear induced transitions, although the formed vesicles needed a long time to relax back to the original state. Mendes et al.34 investigated a transition from vesicles to wormlike micelles. Gentile et al.35 found that multilamellar vesicles could be formed from a planar lamellar phase under shear flow. Gradzielski et al.36,37 reported on the shear-induced transition of an ionic lamellar system from the lamellar phase to the vesicle phase. In the absence of shear, a stacked 3



lamellar phase could be formed when an L3 phase became charged by protonation upon the hydrolysis of methyl formate, but when shear was applied during the preparation a vesicle phase was then formed. In this system, the L3 phase could also be charged by adding the cationic surfactant tetradecyltrimethylammonium bromide (TTABr) to replace tetradecyldimethylamine oxide (TDMAO)36. Planar lamellae were preferentially formed at low TTABr content, and multilamellar vesicles were formed at higher TTABr content. It was found that the shear firstly transformed the planar lamellae into vesicles. Once the multilamellar vesicles were present, further increase of the shear rate stripped off the vesicle shells and eventually generated unilamellar vesicles at high shear rates. The elasticity and the yield stress of the unilamellar vesicles were both higher. The current work studies a totally different multilamellar vesicles system, i.e., the aqueous solution of IT5 and SDS. The solutions were subjected to high pressure up to 400 bar in a high pressure homogenizer, which generated unilamellar vesicles of much smaller size. The vesicles became so small that they lost their viscoelasticity, their yield stess and static birefringence, which rarely happened in other published systems. Previous studies reported that the multilamellar vesicles can be transformed into unilamellar vesicles sheared by a rhometer at 2000 s-1; however, the viscoelasticity of the system increased. Therefore, we wanted to know if similar unique unilamellar vesicles can be formed in the emulsified sample at high shear rate by rheology. In addition, the phase behavior at high shear rate was also studied by a rheometer. 4


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2. Experimental Section 2.1. Materials IT5 was purchased from SASOL GmbH (Germany). The fluid of IT5 separates into two phases at room temperature because it contains a small amount of IT3 and IT8. The anionic surfactant SDS was purchased from SERVA (Germany) and used without purification. Doubly distilled deionized water was used in all experiments. 2.2. Methods 2.2.1. Phase behavior Addition of 0.05 wt% SDS into 5 wt% IT5 aqueous solution resulted in an aqueous solution containing multilamellar vesicles with a yield stress. The sample was then stirred in double gap rheometer at different shear rates and put in a homogenizer subjected to different pressures. 2.2.2. Rheological measurements Rheological properties were measured on a Haake RS300 rheometer. A double gap sensor was used to avoid evaporation. The highest shear rate can be set to 11415 s-1. The gap size from R4-R3 was 0.3 mm and the gap size from R2-R1 was 0.25 mm. The lowest possible stress value amounted to 3 mPa. Viscoelastic properties were determined by oscillatory measurements from 0.01 to 10 Hz, firstly with constant strain amplitude and then with constant stress amplitude. 2.2.3. Emulsification measurements The aqueous solution of 5 wt% IT5/0.05 wt% SDS containing multilamellar 5



vesicles were handled in a homogenizer (APV-2000, Alvertslund, Denmark) to get unilamellar vesicles with smaller size. The highest pressure was 400 bar. 2.2.4. Conductivity Conductivity was tested using an LF2000 microprocessor conductivity meter (WTW) at room temperature with a glass electrode. 2.2.5. Freeze-fracture transmission electron microscopy (FF-TEM) A small amount of sample was placed on a 0.1 mm thick copper disk covered with a second copper disk. This sandwich was frozen by plunging into liquid propane that had been cooled by liquid nitrogen. Fracturing and replication were carried out at −140 °C, and Pt/C was deposited at 45°. The replicas were examined using a CEM 902 electron microscope (Zeiss, Germany). 2.2.6. Cryogenic transmission electron microscopy (cryo-TEM) Vitrified specimens were prepared in an environmental chamber and plunged into liquid ethane at its freezing point, then kept below −178 °C. The prepared specimens were examined on an Oxford CT-3500 cryo-holder system using a Philips CM120 microscope operated at 120 kV. Images were recorded digitally by a Gatan 791 Multiscan CCD camera in the minimal electron dose mode using the Digital Micrograph software package.

3. Results and Discussion 3.1. Phase behavior at different shear rates and pressures Figure 1 shows the pictures of the sheared and emulsified samples. Sample 1 was 6


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without shear and showed static birefringence with polarizers, and it contained densely packed multilamellar and unilamellar vesicles. Samples 2–4 were sheared at 500 s−1, 2000 s−1, and 8000 s−1 respectively for 10 min by a rheometer, and they all exhibited static birefringence with polarizers. Samples 5–11 were emulsified at different pressures (10–400 bar), and they were more transparent than Samples 1–4 without polarizers. The transparence of the sample increased with rising pressure. However, Samples 5–11 lost the static birefringence (Figure 1b), indicating that the size of aggregates in the solution was reduced significantly. Despite so, they still possessed flow birefringence, and the samples emulsified at lower pressures showed flow birefringence more easily (Figure 1c). The results suggested that the size of aggregates reduced with rising pressures. After standing for a week, Sample 6 and Sample 7 showed a minor level of static birefringence, and Samples 8–11 showed flow birefringence more easily than before. That is, the aggregates could return to their original state but only very slowly.

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Figure 1. Pictures of aqueous solutions containing 5 wt% IT5 and 0.05 wt% SDS. (A) Samples freshly sheared and emulsified without polarizers. (B) Samples freshly sheared and emulsified with polarizers. (C) Flow birefringence of samples emulsified at 10 bar and 100 bar. (D) Samples at rest for 7 days with polarizers.

3.2. Conductivity measurements Figure 2 shows the conductivity of the sheared and emulsified samples. Clearly, the conductivity increased with rising shear rate and pressure of emulsification, both of which stripped off the shells of the multilamellar vesicles and uncovered the 8


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charges to increase the conductivity. Emulsification caused more significant increase of conductivity than shearing, indicating that emulsification exposed much more covered charges than shearing. Figure 3 illustrates the change of conductivity with time for the sample emulsified at 200 bar. Upon emulsification, the conductivity increased immediately from about 70.0 µS·cm−1 to about 280.0 µS·cm−1. Afterwards, the conductivity gradually decreased and stabilized at about 190.0 µS·cm−1 after several days. Hence, the vesicle solution did not relax back to its original state.

Figure 2. Conductivity of the (a) sheared and (b) emulsified aqueous solutions containing 5 wt% IT5 and 0.05 wt% SDS.

Figure 3. Conductivity of the aqueous solution containing 5 wt% IT5 and 0.05 wt% SDS emulsified at a 200 bar. 9



3.3. Rheological properties Figure 4 shows the storage modulus (G') of the viscoelastic aqueous solution containing 5 wt% IT5 and 0.05 wt% SDS. The samples were sheared in a double gap rheometer at 0 s−1, 500 s−1, and 8000 s−1 respectively for at least 20 min. It was found that the storage modulus increased with rising shear rate, indicating that the shells of the multilamellar vesicle were stripped off by shear to form smaller unilamellar vesicles. As a result, the number of vesicles increased and the whole phase became more densely packed, and the higher density of vesicles consequently increased the storage modulus.

Figure 4. Storage modulus (G') of the aqueous solution of 5 wt% IT5 and 0.05 wt% SDS subjected to different shear rates.

Figure 5a shows the rheogram of 5 wt% IT5/0.05 wt% SDS without shear. The sample showed the properties of Bingham fluid, indicating that densely packed vesicles existed in this system.36,38,39 However, after emulsification at 200 bar, the sample lost viscoelasticity (Figure 5b) and no longer showed any properties of 10


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Bingham fluid. Specifically, after emulsification, the complex viscosity of the sample at the same frequency reduced by one order of magnitude. That is, after emulsification, the newly formed vesicles had a small diameter such that the distance between vesicles relatively increased. The small vesicles were no longer crowded and could easily pass each other under shear. The solutions thus lost the high viscoelasticity and the static birefringence.

Figure 5. (a) Oscillatory rheogram the aqueous solution of 5 wt% IT5 and 0.05 wt% SDS at a constant stress of 0.1 Pa. (b) Rheogram of the emulsified (200 bar) aqueous solution of 5 wt% IT5 and 0.05 wt% SDS at a constant stress 0.01 Pa.

3.4. Microstructures of vesicle transition under different shear rate and pressure Cryogenic transmission electron microscopy (cryo-TEM) and Freeze-fracture transmission electron microscopy (FF-TEM) was used to examine the phase transition induced by emulsification. Figure 6a shows the cryo-TEM micrograph of the aqueous solution of 5 wt% IT5 and 0.05 wt% SDS not subjected to shear. This sample was a viscoelastic solution with obvious birefringence, and its micrograph (Figure 6a) showed densely packed multilamellar and unilamellar vesicles with diameters around 11



300–500 nm. The FF-TEM micrograph of the aqueous solution was taken again two days after it was emulsified at 200 bar (Figure 6b), and showed that a large number of unilamellar vesicles existed in the system but were not densely packed. The emulsified sample was not viscous and had only minor flow birefringence. The vesicles in Figure 6b had a diameter around 75–85 nm and were almost monodisperse, and the moderate distance between them accounted for the low viscosity of the solution. The micrograph of the emulsified sample was examined again after nine days (Figure 6c, d), at which point the vesicles became more crowded and their size increased (100–200 nm in diameter). That is, the vesicles in the emulsified solution were gradually returning to their original state.

Figure 6. Cryogenic and Freeze-fracture transmission electron microscopy (FF-TEM) of the aqueous solution of 5 wt% IT5 and 0.05 wt% SDS. (a) Sample not subjected to shear, (b) 2 days after the sample was emulsified at 200 bar, (c, d) 9 days after the 12


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sample was emulsified at 200 bar.

Cryogenic transmission electron microscopy (cryo-TEM) was also used to examine the same IT5/SDS solution. The solution was firstly emulsified at 200 bar for 3 times in 5 h before cryo-TEM observation. Figure 7 shows that after emulsification, the multilamellar vesicles transformed into unilamellar vesicles. The diameter of most unilamellar vesicles was within 50–100 nm, and a few larger vesicles with diameter about 200 nm. The shape of the unilamellar vesicles appeared like potatoes rather than spheres. It is likely that after emulsification at high pressure, the newly formed unilamellar vesicles were not in their steady state and tended to relax, which was why they did not show up as spherical vesicles that had the lowest energy in their steady state.

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Figure 7. Cryogenic transmission electron microscopy (Cryo-TEM) of the aqueous solution of 5 wt% IT5 and 0.05 wt% SDS after emulsification at 200 bar after 5 hours. (a) photograph with 0.5 µm scale, (b, c) photographs with 200 nm scale and (d) photograph with 100 nm scale.

3.5. Theoretical model for controlling structure by pressure It can be assumed that when high pressure was applied by the homogenizer, the multilamellar vesicles in the solution were transformed into unilamellar vesicles. The average diameter d of the unilamellar vesicles can be calculated as follows:

φ=

D (1) D+d

where φ is the volume fraction of the surfactant, and D is the interlamellar spacing of 14


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the bilayers. A plausible value of D can be assumed to be 3.06 nm for the current system based on the dimensions of the surfactant molecules. Hence, according to Equation (1), d is approximately 58.13 nm. The cryo-TEM pictures of the sample emulsified at 200 bar (Figure 7) showed that the unilamellar vesicles were not completely monodisperse. The diameter of the vesicle ranged in 20–100 nm, but almost two thirds of the vesicles had a diameter around 50 nm. Figure 7 contains one vesicle with two bilayers from which the intra bilayer distance can be calculated about 45 nm. Since the cryo-TEM images were taken after the sample had been emulsified for 5 hours, the vesicles became a little larger comparing with that after a short time of emulsification. As a result, larger vesicles with diameter around 100 nm can be obtained. The calculated vesicle diameter with the equation was consistent with the experimentally determined

diameter.

To determine the number density C of the vesicles, assume that the unilamellar vesicles have the same outside and inside radius, both equal to d/2. The number density can then be calculated as follows: C = A/av (2) where A is the total area in an equimolar Lα phase (A = l2·l/d, l is the dimension of a cube containing the Lα phase), and av is the area of the unilamellar vesicle. The number density is calculated to be around 1.62×10^12 mL−1. From the number density, the mean distance x between the vesicles can be calculated by assuming a primitive packing: C = 1/x3 (3) 15



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The calculated x is around 85.12 nm. However, the distance observed from cryo-TEM is around 40–70 nm, and some even smaller than 40 nm. That is, the calculated distance is larger than the distance derived from the experiment. In order to find out the relation between pressure and vesicle diameter, assume that the conductivity of the sample is proportional to the volume outside the vesicles (x3, assuming a primitive packing as a cube outside of the vesicles) and inversely proportional to the volume of the vesicles: K (conductivity ) ∝

K (conductivity ) =

x3 d 4 / 3π ⋅ ( ) 3 2

or

C ⋅ 2 3⋅ ⋅ 3 4π ( x / d ) 3

(4)

The values for x and d of the samples emulsified at pressure 200 bar has been calculated as described above. By measuring the conductivity of the solution, it is straightforward to find that C = 4.73. By assuming that the constant C remains the same in other solutions under different pressures, the ratios of x/d and finally the values of x and d of the samples at different pressures can then be obtained. Figure 8 shows the semilog plot of d against pressure. Because the diameter of vesicle is dependent on the time, all data were measured at the same time under different homogenizer pressures. It was found that the functionality of d can be determined by the pressure in the plot. This plot indicates that vesicles of different sizes can be obtained by controlling the pressure of the emulsifier. The vesicle shells were stripped off by shear at high pressure，which led to the formation of new and smaller vesicles. Thus, the number density of vesicles was increased at the cost of the 16


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average number of shells per vesicle. Normally this increase of the number density of vesicles results in a higher storage modulus (Figure 4). However, the increased number density of the sample emulsified at high pressure showed decreased yield stress (in Figure S3). It is assumed that the smaller unilamellar vesicles can easily pass each other while the larger vesicles were trapped and cannot pass each other. The newly formed smaller unilamellar vesicles were not perfectly round (Figure 7). It is likely that the unilamellar vesicles had not reached their ideal curvature and were thus prone to further change. Therefore, a temporary change was imposed on the system by high pressure. Applying pressure to the vesicle solution is therefore a proper tool to control the size of vesicles and reduce the polydispersity in a short period.

Figure 8. Diameter of the vesicles plotted against the homogenizer pressure.

4. Conclusions In conclusion, we have demonstrated that the size of vesicles in IT5/SDS system can be controlled by a homogenizer. Smaller vesicles were formed when the shells of the multilamellar vesicles were stripped off by shear from rheometer. The newly 17



formed vesicles were evenly distributed and more densely packed. The number density of the vesicles in the solution increased, so did the viscoelasticity of the system. However, the size of the vesicles readily decreased at high pressure in a homogenizer. The solution lost its high viscosity and birefringence, and the small vesicles could easily pass each other. On the basis of previous studies,23-26 various phases could be induced by increasing the shear rate by rheometer. At low shear rates, multilamellar vesicles formed from planar lamellae but the high shear rates caused the multilamellar vesicles breakdown. The structural transition of the lamellar phase was driven by the mechanical balance between the applied viscous force and the internal relaxation mode of the lamellae.22 In our studies, the homogenizer provided the high pressure to the multilamellar vesicles. The number density of vesicles increased, but the viscoelasticity of the system decreased because the small unilamellar vesicles can easily pass each other while the multilamellar vesicles are trapped and cannot pass each other. The diagram described the transition observed at such high shear field qualitatively and quantitatively. The size and dispersion of vesicles could be easily controlled by tuning the pressure of a homogenizer.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.jpcb.xxxxxxx. Conductivity measurements and Yield stress and graphical model.

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Figure S1. Rotation rheogram of the aqueous solution containing 5 wt% IT5 and 0.05 wt% SDS without shear. Figure S2. Rotation rheogram of the aqueous solution containing 5 wt% IT5 and 0.05 wt% SDS after sheared at 8000 S-1 shear rate by a rhometer. Figure S3. Rotation rheogram of the aqueous solution containing 5 wt% IT5 and 0.05 wt% SDS after emulsified at 200 bar by a homogenizer. Figure S4. The bilayer of multilamellar vesicles could be stripped off by shear and high pressure. The system sheared by rheometer still exhibited static birefringence and high viscoelasticity. However the system emulsified at high pressure exhibited only flow birefringence and low viscoelasticity. It took a long time for the emulsified system to relax back to the original state.

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Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province (ZR2016BQ30 and ZR2016DB28), the National Key Research and Development

Plan

(2017YFD0800602),

the

Shandong

Development Plan (2017CXGC0304 and 2016ZDJS08A02). Conflict of Interest The authors declare no conflict of interest.

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Key

Research

and

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Formation of Unique Unilamellar Vesicles from ... - ACS Publications

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