Phase Structure Transition and Properties of Salt-Free Phosphoric

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Phase Structure Transition and Properties of SaltFree Phosphoric Acid/Nonionic Surfactants in Water Lihuan Wang, Wenrong Zhao, Renhao Dong, and Jingcheng Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01596 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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Phase Structure Transition and Properties of Salt-Free Phosphoric Acid/Nonionic Surfactants in Water Lihuan Wang, Wenrong Zhao, Renhao Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, Shandong, P. R. China.

* To whom correspondence should be addressed. E-mail: [email protected]; Tel: +86-531-88366074; Fax: +86-531-88564750.


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Abstract: Precise control of phase-structure transition for the synthesis of multi-dimensional soft materials is a fascinating target in amphiphilic molecule self-assembly. Here, we demonstrate a spontaneous formation of closely packed lamellar phase consisting of uni- and multi-vesicles through the incorporation of a small amount of an extractant, di-(2-ethylhexyl) phosphoric acid (DEHPA), into the highly swollen, planar lamellar phase of a nonionic tetraethylene glycol monododecyl ether (C12EO4) surfactant in water. It is figured out that the introduction of negative membrane charges results in the electrostatic repulsion among the lamellae, which suppresses the Helfrich undulation and induces a phase-structure transition from planar lamellae to closely packed vesicles. Our results provide an important insight into amphiphilic molecule self-assembly where additives and the pH can satisfy the opportunities for the precise-tuning of the lamellar structures, which makes a way for the development of lamellar soft materials.


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1. INTRODUCTION Amphiphilic molecular self-assembly, via non-covalent interactions, has been recognized as a fascinating, bottom-up approach to achieve stable, structurally well-defined soft materials at the nano- and microscale.1,2 A large number of self-assembled structures such as micelles, discs, lamellar structures, tubes, and fibers have been reported in aqueous solution of natural or synthetic amphiphiles.3-8 In particular, closely packed concentric lamellae (also called vesicles) are much of interest because of their use as simple models for biological membranes, biocompatible delivery systems of drug or cosmetics, microreactors, and soft templates for synthesizing materials. 9-13 However, the precise tune of the structure transition between planar lamellae and closed lamellae is still challenging.12 Here, using nonionic tetraethylene glycol monododecyl ether (C12EO4) and di-(2-ethylhexyl)phosphoric acid (DEHPA) mixture as model system, we demonstrate the phase transition from planar to bent and finally closed lamellae by charging the bilayers. Poly(oxyethylene) monoalkyl ether (CnEOm) is an environmentally friendly nonionic surfactant in widespread use in consumer products, industrial processes, and research laboratories.14 Binary systems of CnEOm in water generally behave highly swollen flat lamellar phase depending on the temperature and concentration.14 The spontaneous formation of closed lamellae is energetically unfavorable. In order to bend planar lamellae, one way is to introduce extra energy to drive the curvature transition, such as shearing force.15 Unfortunately, the shear-induced packed lamellae will finally recover into planar states after long-time molecule diffusion. Another strategy is to charge the membrane to produce electronic repulsion among the lamellae, resulting in spontaneous bending curvature, such as mixing CnEOm with ionic amphilphilic molecules. However, the preparation of closed


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lamellae by mixing anionic surfactant and CnEOm is still rare.14,16 The reported studies focused on the formation of mixed micelles by mixing C12EOm with single-tail anionic surfactant, such as sodium dodecyl sulfate (SDS).17 Electron spin echo modulation indicated that the SDS headgroups located inside the EO head layer. The mixed micelles grow at low SDS concentration, because that the addition of SDS with its smaller headgroup reduces steric repulsion between the relatively larger C12EO6 headgroups. At high SDS concentration, the micellar size decreased as a result of the increased electrostatic interaction between SDS headgroups.18,19 A mixture of C6SO3Na/C6EO5/H2O system with short hydrophobic tails also showed a lowering of the cmc compared with the C6EO5/H2O system due to the formation of mixed micelles.20,21 The diffusion coefficients in these mixtures indicated a strong coupling between the diffusing species. Guided by the mixed micelle models, it is believed that the anionic surfactant can penetrate into the palisade layer of the CnEOm planar lamellae, afford extra membrane charges, and tune their interfacial curvature.22,23 In this study, we studied the phase behavior of C12EO4 and DEHPA in water. The chemical structures of C12EO4 and DEHPA are presented in Figure S1. DEHPA is an organophosphorus, oil-soluble compound, which is usually used in the solvent extraction of uranium as well as the rare earth metals. It is also a traditional anionic surfactant, which can form reverse micelles in mixed solvents of isooctane and water.14 Using DEHPA and C12EO4, a salt-free anionic/nonionic surfactant complex was constructed without the influence of salts for the studies of membrane charges on the phase transition. Furthermore, DEHPA is very sensitive to the variation of pH but against with the presence of low amount of salts, 14 which is beneficial to the research of pH influence on the aggregate structures. Our results provide basic theoretical considerations of amphiphilic molecular self-assembly by membrane charging and


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make an avenue for the development of lamellar soft materials.

2. EXPERIMENTAL SECTION 2.1. Chemical and Materials. Tetraethylene glycol monododecyl ether (C12EO4) nonionic surfactant (purity > 99%) was purchased from ACROS Organics (USA) and di-(2-ethylhexyl)phosphoric acid (DEHPA) anionic surfactant (purity>98%) was purchased from ALDRICH Chemistry (USA). They were used as received for phase study and rheological measurement. 2.2. Phase Diagram. The phase behavior of the surfactant mixture was studied by visual inspection of 200 solution samples, including the assistance of crossed polarizers and rheology. The samples were obtained by dissolving various amounts of C12EO4 and DEHPA in aqueous solution. The solutions were stirred and then left at 25.0  0.1C for at least 4 weeks for equilibration. 2.3. Polarizing Light Microscopy (PLM) Observations. Polarized observations were performed by an AXIOSKOP 40/40 FL (ZEISS, Germany) microscope. Samples were prepared by dropping several drops of solution into a 1 mm thick trough, which was covered by another glass slide to avoid solvent evaporation. 2.4. Dynamic Light Scattering (DLS) Measurements. DLS was carried out at scattering angles of 90°, 60°, and 45°. A standard Brookhaven Commercial laser light scattering spectrometer equipped with a Coherent Radiation 200 mW diode pumped solid-state (DPPS) 488 nm laser and a Brookhaven Instruments Corporation (BI-9000AT) correlator were used for the measurements. The obtained data were analyzed by CONTIN.24 2.5. Small-Angle X-Ray Scattering (SAXS) Measurements. SAXS experiments were performed at 298 K at Beamline 1W2A using a SAXS apparatus at the Beijing


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Synchrotron Radiation Facility. The incident X-ray wavelength was 0.154 nm, and the imaging plate was a Mar CCD (165 mm) with a resolution factor of 2048  2048. The obtained two dimensional scattering patterns consisted of concentric circles, and the intensity (I) versus the scattering vector (q) profile was independent of the azimuthal angle. To extract the structural information, thus, the 2D SAXS patterns were converted to one-dimensional (1D) profiles by circularly averaging.25 The scattering vector range was chosen from 0.02 to 3 nm-1 (q = (4 sin ( /2))/ , where  and  are the scattering angle and wavelength, respectively). The distance from the sample to the detector was 1120 mm, and the data accumulation time was 120 s for each sample. 2.6. Freezing-Fracture

Transmission

Electron

Microscopy

(FF-TEM)

Observations. The aggregate structures in the L  -phase solution with high viscosity were determined by FF-TEM. A small amount of solution was mounted on a specimen holder. The sample was frozen by quickly plunging the specimen holder into liquid ethane cooled by liquid nitrogen. Fracturing and replication were carried out on a freeze-fracture apparatus (EM BAF 060, Leica, Germany) at a temperature of -150 C. Pt/C was deposited at an angle of 45 to shadow the replicas and C was deposited at an angle of 90 to consolidate the replicas. The replicas were transferred onto copper grid and then observed using a JEOL JEM-1400 TEM operated at 120 kV. 2.7. Cryogenic Transmission Electron Microscopy (cryo-TEM) Observations. The onion-like aggregates in the L



-phase solution with low viscosity were

determined by cryogenic transmission electron microscopy (cryo-TEM). The samples were prepared in a controlled-environment vitrification system (CryoplungeTM3, Gatan, USA) at 25C under 95% relative humidity. During the process, a micropipette was used to load 1  L of solution onto a copper grid coated with carbon film. The excess solution was blotted with two pieces of filter paper, resulting in the formation


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of very thin films suspended on the mesh holes. After 10 s, the samples were quickly plunged into liquid ethane cooled by liquid nitrogen. The vitrified samples were transferred to a cryogenic specimen holder (Gatan 626), and then examined with JEOL JEM 1400 TEM operated at 120 KV. 2.8. Rheological Measurements. Rheological measurements were carried out on a HAAKE RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti) for low viscosity samples and a cone-plate sensor system (C35/1 Ti L07116 at 25.0  0.1C. Ti, 35 mm in diameter; cone angle, 1  .) for samples with high viscosity. The diameters of the rotor and the shear cell are 41.420 mm and 43.400 mm, respectively. In steady shear measurements, the shearing rate increased from 0.01 to 100 s-1 in a stepwise mode. In the measurement of viscosities at each fixed shear rate, the experimental time was fixed as 6000 s to obtain a steady value. The viscoelastic properties of the samples were determined by oscillatory measurements with an amplitude sweep in the frequency range of 0.01 to 10 Hz. Prior to the frequency sweep, the linear viscoelastic regime was determined by stress sweep measurement. All samples for rheological measurements were prepared at least 4weeks before.

3. RESULTS AND DISCUSSION 3.1. Phase Behavior of Ternary DEHPA/C12EO4/H2O System. In order to study the influence of DEHPA on C12EO4 lamellar solution, a systematical study on the phase behavior of the ternary DEHPA/C12EO4/H2O system was carried out at 25.0  0.1C. The partial phase diagram after equilibration of solution samples for 4 weeks was given in Figure 1 together with typical photographs of solution samples within different regions. Rich phase behavior can be seen in the phase diagram: a slightly turbid emulsion region, a homogeneous viscously lamellar phase (L  ), a lamellar


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phase at the bottom equilibrating with a clear micellar phase on the top (L1/L  ), and an upper micellar phase coexisting with precipitates (L1/precipitates). The precipitates resulted from the over-saturated DEHPA. The optical microscopy showed a number of O/W emulsion droplets with the size from 8 to 85 µm in a sample solution of cC12EO4 = 1 wt% and cHDEHP = 3 wt% (Figure S2, SI).

Figure 1. Partial Phase diagram of DEHPA/C12EO4/H2O ternary system at 25.0  0.1  C. Lamellar phase (Lα) occurs in a narrow region. Insets are photos of sample solutions with (right sample) and without (left sample) polarizers. Crosses (×) represent the samples that were prepared for phase-behavior determination. 3.2. Phase Transition from Planar Lamellae to Densely Packed Onions. The solubility of DEHPA in water is low at room temperature. However, DEHPA can be dissolved in C12EO4 aqueous solution. Nonionic C12EO4 molecules can spontaneously form flexible bilayer membranes through the hydrophobic interaction and the Helfrich undulation in a wide concentration range in water.26,27 A phase structure transition from planar lamellae to closed uni- and multi-lamellar vesicles was initiated by adding a small amount of DEHPA into 17 wt% C12EO4 lamellar solution.


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Figure 2. FF-TEM images of different lamellar structures: (a) flat lamellae in 17 wt% C12EO4/H2O system, scale bar: 1 µm, (b) bent lamellae in ternary 1 wt% DEHPA/17 wt% C12EO4/H2O system (the arrows show small vesicles), scale bar: 500 nm, and (c) densely packed closed lamellae in ternary 3 wt% DEHPA/17 wt% C12EO4/H2O system (the arrows show onion-like vesicles), scale bar: 2 µm. (d) DLS CONTIN plots of 3 wt% DEHPA/17 wt% C12EO4/H2O sample performed at three scattering angles. FF-TEM was performed to confirm the phase-structure transition in solution. Figure 2a showed that there were highly swollen, planar lamellae in the 17 wt% C12EO4 aqueous solution. The thickness of the lamellae was estimated from the TEM images as around 35 ± 2 nm but the inter-layer distance varied tremendously accompanied with strong thermal undulations. After addition of 1 wt% DEHPA to 17


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wt% C12EO4 aqueous solution, large curved lamellae with smooth curvature were obtained with the coexistence of small closed lamellae (the arrows shown in Figure 2b). Those large curved lamellae closely packed with the bilayer thickness of 20 ± 2 nm. Further incorporation of 3 wt% DEHPA into 17 wt% C12EO4 lamellar phase resulted in the formation of uni- and multilaemllar onions with closely packed bilayers (the arrows shown in Figure 2c). The closely packed multilamellar onions are poly-dispersed size ranging from 200 nm to 4.5 m (also see Figure S3) and the size of small unilamellar vesicles ranges from 30 nm to 90 nm. Dynamic light scattering (DLS) measurements were carried out to check the aggregate size for the solution sample of 3 wt% DEHPA/17 wt% C12EO4 at scattering angles of 90°, 60°, and 45°. The hydrodynamic radius (Rh) were obtained by CONTIN analysis, and the Rh did not depend on the scattering angles, which also confirmed the presence of spherically symmetric aggregates (Figure 2d). The Rh had a wide size distribution ranging from several nanometers to 10 micrometers, suggesting the presence of multi-dispersed vesicles, which is in consistent with the FF-TEM images (Figure 2c, Figure S3). However, the subsequent addition of 8 wt% DEHPA into 17 wt% C12EO4 caused a phase separation, leading to the formation of a two-phase solution comprising a top micellar phase and a bottom lamellar phase. This revealed that excess DEHPA cannot be dissolved in palisade layer of the C12EO4 lamellae. The sample photographs of phase transition for 17 wt% C12EO4 with different amounts of DEHPA were shown in Figure S4. To further prove the phase transition, cryo-TEM was performed to study the influence of the addition of DEHPA on the planar lamellar structure. Figure 3a shows cryo-TEM images of planar lamellae of the 17 wt% C12EO4/H2O solution. After


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addition of 1 wt% DEPHA, a number of closed single uni- and multi-lamellar vesicles were formed with the size from 20 to 500 nm (Figure 3b and Figure S5). The unclosed lamellae can also be observed in the 1 wt% DEHPA/17 wt% C12EO4/H2O system. For 3 wt% DEHPA/17 wt% C12EO4/H2O sample solution, densely stacked multi-lamellar onions with dozens of layers were clearly found (Figure 3c and Figure S6). These closed lamellae have a poly-dispersed size in consistent of FF-TEM observations.

Figure 3. Cryo-TEM images of different lamellar structures: (a) flat lamellae in 17 wt% C12EO4/H2O system (the arrows show flat lamellae), scale bar = 500 nm, (b) vesicles coexisted with unclosed lamellae in 1 wt% DEHPA/17 wt% C12EO4/H2O system (the arrow shows the coexisted unclosed lamellae), scale bar = 200 nm, and (c) densely packed multi-lamellar onions of 3 wt% DEHPA/17 wt% C12EO4/H2O system,


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scale bar = 200 nm. Next, SAXS measurements were performed to provide overall mean information for the structure transitions. Figure 4 shows one-dimensional profiles of the lamellar phase of 17 wt% C12EO4 aqueous solution with the addition of DEHPA. Without DEHPA, though FF-TEM and cryo-TEM results confirmed the existence of flat lamellae, the SAXS curve did not exhibit any periodic peak due to the fluctuant bilayers. The combination of microfluidic techniques and SAXS allows a dynamic insight in the aggregate formation process, and thus scan the phase transition in real time, which can be applied for the further study of C12EO4 aqueous solution.28 At cDEHPA = 1 wt%, three obvious scattering peaks were detected, that is, q = 0.251, 0.595 and 0.850 nm-1, respectively, which provided the proof of lamellar structures. However, the relative peak positions q1 : q2 : q3, deviated from ideal 1:2:3. In conjunction with the results of FF-TEM (Figure 2b) and cryo-TEM (Figure 3b), the deviation is probably due to the coexistence of planar lamellae and vesicles with different sizes in this intermediate concentration region. The interlayer spacing d, meaning the thickness of bilayer membrane and water layer, are determined from d = 2  /q = ~25 nm. As the DEHPA concentration further increased to 3 wt% and 5 wt%, the interlayer spacing decreased to pass through a minimum value of 18.4 nm and then increased to be 20.5 nm, as shown in Figure 4. As is studied, the lamellar repeat distance is determined by the balance of four interactions, i.e., van der Waals interaction, an electrostatic interaction, Helfrich repulsion (due to the membrane fluctuation), and a short range repulsion.29 Here, the decrease of the inter-layer distance suggested that the addition of a small amount of DEHPA charged the lamellae and suppressed the Helfrich undulation, leading to more closely stacked lamellae.


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Figure 4. SAXS curves of the lamellar structures with the increase of DEHPA concentrations at cC12EO4 = 17 wt%. In conjunction with FF-TEM, DLS, cryo-TEM, and SAXS measurements, we demonstrate that, with the insertion of DEHPA into the planar lamellar phase of C12EO4, the phase transition was triggered from planar lamellae to densely packed uni- and multi-vesicles (Figure 5). We reason that this phase transition is owing to the following interactions: (I) the insertion of DEHPA molecules induced the partial segregation of C12EO4 molecules;30 (II) the phosphoric acid head-groups of the DEHPA molecules can negatively charge the EO layers exposed to water, leading to strong in-plane electrostatic repulsion between the head groups; (III) meanwhile, the out-plane electrostatic repulsion by the membrane charging was generated, which suppressed the undulation of lamellae. Therefore, the synergistic effect of partial segregation, electrostatic repulsion, and layer undulation drives the phase transition, leading to the formation of uni- and multi-lamellar vesicles.


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Figure 5. An illustration of the phase-structure transition from planar lamellae to packed uni- and multi-lamellar onions induced via incorporation of DEHPA into C12EO4 bilayers. 3.3. Rheological Behavior of the Structure Transition. Rheology has been credited as a powerful method to not only to show the macro-properties of surfactant systems but also to confirm the microstructures of complex fluids.31 The rheological properties of the lamellar solutions before and after adding DEHPA were studied by means of yield stress and oscillatory experiments. Without charges, the C12EO4/H2O solution had a low yield stress (  0) of about 2.3 Pa (Figure S7a) and had a very low viscoelasticity with G (~27 Pa) being a little larger than G (~5 Pa) (Figure 6a), due to the presence of flat, fluctuant lamellae.


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Figure 6. Typical oscillatory rheograms for lamellar solution in C12EO4/H2O and DEHPA/C12EO4/H2O systems at room temperature. (a) 17 wt% C12EO4, (b) 3 wt% DEHPA/17 wt% C12EO4, and (c) Elastic modus (G) versus frequency. cDEHPA = 0, 1, 3, and 5 wt%. (d) Yield stress (τ0) values for the 17 wt% C12EO4/H2O solution with the increase of DEHPA. After addition of 3 wt% DEHPA, the mixed DEHPA/C12EO4/H2O solution exhibited a typical rheological behavior of packed lamellar phase (Figure 7b, Figure S7b). Both storage modulus G  and loss modulus G   were almost frequency independent over the whole frequency range of 0.01-10 Hz. A large increase in the viscoelastic property was achieved with the G value of ~100 Pa and G value of ~10 Pa. The elasticity dominated the solution properties much over the viscosity, as G′ was almost more than a factor of 10 larger than the viscous modulus G′′. The complex viscosity    decreased over the whole frequency range with a slope of -1, and no zero-shear viscosity can be extrapolated. Moreover, the solution exhibited high yield stress, that is, a minimum stress was required to reach 8.3 Pa in order to make the sample flow (Figure S7c). Such a rheological behavior exhibited a typical feature of Bingham fluid, which was ascribed the stiffening of the bilayer by charging the planar, fluctuant lamellae by anionic DEHPA and the formation of densely packed vesicles. Figure 7c shows the trend of the storage modulus G of the lamellar solution with


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various DEHPA concentrations in the frequency range from 0.01 to 10 Hz by oscillatory rheological experiments. Compared with the C12EO4/H2O solution, one expects a large increase of the elastic properties by more than a factor of 10 upon the addition of DEHPA as cDEHPA = 1 wt%. However, further addition of DEHPA of 5 wt% induced the G to decrease to 50 Pa. The yield stress values also increased from 2.3 Pa to the maximum value of 8 Pa and then decreased to 2 Pa with the addition of DEHPA from 1 wt% to 5 wt% (Figure 7d). This indicates that the addition of anionic surfactant can achieve the maximum viscoelasticity properties of the vesicles at a saturation of the effective membrane charge density. 3.4. pH-Induced Reverse Transition from Vesicles to Planar Lamellae. According to the above structure transition mechanism from planar lamellae to vesicles, we further concentrated on an approach for the controlled reverse transition from vesicles to planar lamellae, providing an important understanding of precise tuning of lamellar structure for practical usage, such as controlled drug delivery and release. The DEHPA molecules have been proved to be inserted into the C12EO4 lamellae with the phosphoric acid head groups charging the layer surface. Moreover, DEHPA has a solubility response to pH in water. Therefore, the variation of pH of the solution is thought to tune the surface charges via changing the amount of DEHPA in the layers. Regarding to the 3 wt% DEHPA/17 wt% C12EO4/H2O solution consisting of vesicles, the pH was measured as 1.86. When the pH was increased to 3.78 by addition of NaOH, the solution turned into solid-state gel along with the dramatic increase of the viscosity. Even when the test tube was tilted, the solution did not flow (Figure 7a). Furthermore, the varicolored birefringence phenomenon became much more remarkable. FF-TEM observation indicated that the original concentric lamellar


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structures were transformed into dozens of bent lamellae (Figure 7b and Figure S8). Compared with the planar lamellae of the 17 wt% C12EO4 aqueous solution, these bent lamellae were densely packed with less Helfrich undulation. However, over pH = 3.78, the solid-like gels started to flow again when the test tube was tilted accompanied with the formation of opaque turbid liquid and the declination of the birefringence textures (Figure 7a). FF-TEM observations showed the presence of planar lamellae with larger layer distance (Figure 7c). It is predicted that the electrostatic repulsion between head groups was obviously increased at higher pH value, finally resulting in a loose lamellar structure.

Figure 7. Structure transition from densely closed to planar lamellae in 3 wt% DEHPA and 17 wt% C12EO4 aqueous solution. (a) The viscosity varieties with the increase of the pH at 25.0  0.1  C. Without polarizers (left) and with polarizers


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(right). From top to bottom, pH = 1.86, 3.78 and 4.32. (b) FF-TEM image of the densely stacked, bent lamellae at pH=3.78. Scale bar = 200 nm. (c) FF-TEM image of the swollen, planar lamellae at pH=4.32. Scale bar: 200 nm The SAXS measurements showed that, with the increase of pH value from 1.86 to 4.32, all of the DEHPA/C12EO4/H2O solution samples exhibited typical feature of lamellar phases (Figure 8a). The comparison of SAXS curves suggested that the packing density of the lamellar structures was increased with the variation of pH from 1.86 to 3.78, along with an obvious reduction of the interlayer spacing from 18.4 nm to 14.8 nm. However, further increase of pH to be 4.32 caused the widening of the interlayer distance, suggesting that the lamellae become packed loosely.

Figure 8. (a) SAXS curves of 3 wt% DEHPA and 17 wt% C12EO4 aqueous solution by increasing pH from 1.86 to 2.27, 3.78, and 4.32, respectively. (b) Storage and loss modulus, G  (filled symbols) and G   (open symbols), vs. frequency for 3 wt% DEHPA and 17 wt% C12EO4 aqueous solutions with increasing pH. Rheological experiments indicated that the shear viscosity (Figure S9a) and the yield stress (Figure S9b) of the sample solution passed through a maximum and then sharply declined with the addition of NaOH. The viscoelastic property measurement


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indicated that the storage modulus increased to reach 120 Pa at pH = 3.78. Especially, the solution always exhibited the characteristic of Bingham fluid, and was dominated by the elasticity resulting from the densely packed bilayers. However, with the rise of pH to be 4.82, the storage modulus drastically decreased to ~10 Pa (Figure 8b). The observations of FF-TEM, SAXS and rheological measurements provided a significant proof that the tuning of the pH achieved a facile structure control of the amphiphilic lamellae. Typically, a structure transition from stiff vesicles to swollen planar lamellae was observed via the addition of NaOH into the DEHPA/C12EO4/H2O solution. 4. CONCLUSIONS Our results clearly demonstrated that the addition of small amount of DEHPA in C12EO4-rich lamellar phase induced the structure transition from flexible planar bilayers to densely packed vesicles. The electrostatic repulsion coming from DEHPA molecules played the key role in the structure transition through suppressing the Helfrich undulation of nonionic lamellae. The obtained lamellar solution exhibited strong viscoelastic properties, high yield stress and great stability. Furthermore, increasing the pH of the vesicle solution can achieve a reverse transition from vesicles to flat lamellae. Our results provide an important insight into amphiphilic molecule self-assembly where the number of additives and the pH provide opportunities for the fine-tuning of the lamellar structure, which makes a way for the future development of lamellar soft materials.

Acknowledgements This work was financially supported by the NSFC (Grant No. 21420102006 & 21273134). L. Wang acknowledges the Postdoctoral Innovation Projects of Shandong


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Phase Structure Transition and Properties of Salt-Free Phosphoric

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