Temperature-Induced Aggregate Transitions in Mixtures of Cationic

Jan 11, 2016 - Xiuling Ji†, Maozhang Tian‡, and Yilin Wang† .... Xuemin Liu , Jingwen Wang , Zhenggang Cui , Heping Yao , Xin Ge , Wen Chen , Fe...
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Temperature-Induced Aggregate Transitions in Mixtures of Cationic Ammonium Gemini Surfactant with Anionic Glutamic Acid Surfactant in Aqueous Solution Xiuling Ji,† Maozhang Tian,‡ and Yilin Wang*,† †

Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, People’s Republic of China S Supporting Information *

ABSTRACT: The aggregation behaviors of the mixtures of cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (C12C4(OH)2C12Br2) and anionic amino acid surfactant Ndodecanoylglutamic acid (C12Glu) in aqueous solution of pH = 10.0 have been studied. The mixture forms spherical micelles, vesicles, and wormlike micelles at 25 °C by changing mixing ratios and/or total surfactant concentration. Then these aggregates undergo a series of transitions upon increasing the temperature. Smaller spherical micelles transfer into larger vesicles, vesicles transfer into solid spherical aggregates and then into larger irregular aggregates, and entangled wormlike micelles transfer into branched wormlike micelles. Moreover, the larger irregular aggregates and branched micelles finally lead to precipitation and clouding phenomenon, respectively. All these transitions are thermally reversible, and the transition temperatures can be tuned by varying the mixing ratios and/or total concentration. These temperature-dependent aggregate transitions can be elucidated on the basis of the temperature-induced variations in the dehydration, electrostatic interaction, and hydrogen bonds of the headgroup area and in the hydrophobic interaction between the hydrocarbon chains. The results suggest that the surfactants carrying multiple binding sites will greatly improve the regulation ability and temperature sensitivity.

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mixtures is between micelles and vesicles. Davies et al.22 reported that the vesicles in the mixture of cetyltrimethylammonium bromide (CTAB) and 5-methylsalicylic acid (5mS) transform into long wormlike micelles when the temperature is raised above a critical value. They showed that higher temperature increases the affinity of 5mS with water and thus reduces the effective concentration of 5mS at the aggregate/ water interface. The desorption of 5mS from vesicles increases the effective headgroup area and reduces the volume of hydrocarbon chains, thereby driving the vesicles to long wormlike micelles with higher curvature. Manohar et al.23 observed that the transition from vesicles to giant and viscoelastic wormlike micelles in the mixture of CTAB and sodium 3-hydroxynaphthalene-2-carboxylate (SHNC) upon heating and thought that the solidlike headgroup region results in vesicles, while heating makes the solidlike region melt giving rise to wormlike micelles. Engberts et al.24 observed the transition from vesicles to wormlike micelles in the equimolar

INTRODUCTION Self-assembly of surfactants in aqueous solutions generates micelles, vesicles, liquid crystals, and other organized aggregates.1−3 Surfactant molecular structure, concentration, temperature, pH, salt, and so on can induce aggregate transitions and adjust the properties of the aggregates.4−13 Among them, temperature-induced aggregate formation and transition in aqueous solutions of surfactants exhibit wide potential applications in many product formulations and performing procedures,14−17 such as drug delivery, dye entrapment and release, and enhanced oil recovery. Mixing two oppositely charged surfactants is one of the most convenient and efficient approaches to construct surfactant systems with the feature of temperature response. Electrostatic interaction between oppositely charged headgroups, hydrophobic interaction between hydrocarbon chains, and/or hydrogen binding between some groups are the main driving forces to the formation of aggregates in oppositely charged surfactant mixtures. Changing temperature can influence these interactions and in turn induce aggregate transitions with different structures.18−21 Upon the change of temperature, one of the most often observed transitions in this kind of surfactant © 2016 American Chemical Society

Received: November 16, 2015 Revised: January 6, 2016 Published: January 11, 2016 972

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determine that the C12C4(OH)2C12Br2/C12Glu mixtures in aqueous solution should show strong aggregation ability and sensitive temperature responsibility. At first, the aggregation behaviors of the C12C4(OH)2C12Br2/C12Glu mixture at 25 °C were studied, and spherical micelle, wormlike micelles and vesicles were obtained by changing mixing ratio and total surfactant concentration. Then the temperature-induced transitions of these aggregates were studied. Very abundant aggregate transitions have been found, and the related mechanism has been discussed.

mixture of cetyltrimethylammonium with aromatic counterion 5-ethyl salicylate upon heating, which was thought to be caused by the decrease of the counterion binding upon increasing temperature. In particular, Huang et al.25 reported that a transition from cylindrical micelle to vesicle induced by increasing temperature in the mixture of sodium dodecyl sulfate (SDS)/dodecyltriethylammonium bromide (DTEAB). In addition, increasing temperature can also promote the growth of micelles,26,27 the aggregation of vesicles,28,29 and phase separation.30,31 Gemini surfactant molecule32−35 has two hydrocarbon chains and two headgroups connected by a spacer group. Compared with single-chain surfactants, gemini surfactants have more variations in chemical groups and much stronger synergetic interaction ability; thus they provide more options for forming mixed surfactant systems with multifarious temperature response. However, although surfactant mixtures containing gemini surfactant have been studied by many researchers, temperature response in this kind of mixtures has been rarely found in the literature. A previous work36 studied the variations of surface tension with temperature in a mixture of an anionic gemini surfactant containing a triazine ring with cetyltrimethylammonium bromide. The present work is focused on the aggregation behaviors upon temperature change in mixtures of a cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammonium bromide)2,3-butanediol (C12C4(OH)2C12Br2) and an anionic amino acid surfactant N-dodecanoylglutamic acid (C12Glu). The molecular structures of the surfactants are shown in Scheme 1.

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EXPERIMENTAL SECTION

Materials. The cationic gemini surfactant C12C4(OH)2C12Br2 was synthesized and purified as previously reported.33,34 The amino acid surfactant C12Glu was purchased from JC-Trading Company and purified in water three times. Milli-Q water (18.2 MΩ·cm) was used throughout. Turbidimetric Measurements. Turbidimetric titrations were carried out by adding C12Glu solution in a stepwise manner to C12C4(OH)2C12Br2 and vice versa under stirring and temperature control. Turbidity values expressed in absorbance were measured at 450 nm by using a Brinkmann PC920 probe colorimeter equipped with a 1 cm path length fiber-optics probe or a temperature-controlled Shimadzu UV−vis spectrophotometer (model UV-2450). Isothermal Titration Microcalorimetry (ITC). Calorimetric measurements were conducted by a TAM 2277-201 microcalorimetric system (Thermometric AB, Järfȧlla, Sweden) with a stainless-steel sample cell of 1 mL. The sample cell was initially loaded with 700 μL of water or surfactant solution of pH 10.0. Each aliquot of 10 μL of the oppositely charged surfactant solution at the same concentration was injected consecutively into the stirred sample cell via a 500 μL Hamilton syringe controlled by a 612 Thermometric Lund pump until the desired mixing ratio range had been covered. During the whole titration process, the system was stirred at 60 rpm with a gold propeller, and the interval between two injections was long enough for the signal to return to the baseline. The observed enthalpy (ΔHobs) was obtained by integrating the areas of the peaks in the plot of thermal power against time. The reproducibility of experiments was within ±4%. All the measurements were performed at 25.00 ± 0.01 °C. Differential Scanning Calorimetry (DSC). The DSC thermograms of the C12C4(OH)2C12Br2/C12Glu mixtures were obtained with a VP-DSC microcalorimetric system (MicroCal, USA). Data were collected between 10 and 110 °C at a scanning nominal rate of 30 °C/ h, and no dependence on scanning rate was observed for scans between 0 and 90 °C/h. The surfactant solution was degassed for 10 min before being introduced into a Tantaloy 61 TM alloy sample cell of 500 μL. The reference cell was filled with water at pH 10.0 for all the measurements. A water−water calibration scan was subtracted from each recorded differential scan produced by each surfactant solution. Data analysis and curve fitting were performed using the Origin software provided by MicroCal. Dynamic Light Scattering (DLS). The sizes of C12C4(OH)2C12Br2/C12Glu aggregates were measured with an LLS spectrometer (ALV/SP-125) equipped with a multi-τ digital time correlator (ALV-5000). A solid-state He−Ne laser with 22 mW output power at a wavelength of 632.8 nm was employed as a light source, and the scattering angle was 90°. The freshly prepared samples were introduced into a 7 mL glass bottle through a 0.45 μm membrane filter of hydrophilic PVDF prior to measurements. The correlation function of scattering data were analyzed via the CONTIN method to obtain the distribution of d iffu s i on coeffic ie n t s (D) of the C12C4(OH)2C12Br2/C12Glu aggregates, and then, the apparent equivalent hydrodynamic radius (Rh) was determined using the Stokes−Einstein equation Rh = kT/6πηD, where k is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of water. Rheology Measurements. The steady rheological properties of the C12C4(OH)2C12Br2/C12Glu surfactant solutions were investigated

Scheme 1. Chemical Structures of C12C4(OH)2C12Br2 and C12Glu

C12C4(OH)2C12Br2 has a four-carbon length spacer and two hydroxyl substitutions and self-assembles into wormlike micelles at high concentration in aqueous solution.37 Previous reports38−42 have indicated gemini surfactants with hydroxyl groups in spacer greatly promote their micellization in aqueous solution and exhibit strong aggregation capability because of the intermolecular hydrogen binding between the −OH groups, while C12Glu can only self-assemble into spherical micelles over a wide range of concentration.43 Nevertheless, the carboxylate groups of C12Glu at pH 10.0 are entirely deprotonated, and each molecule carries two negative charges; thus, stronger electrostatic interaction should exist between C12Glu and cationic charged C12C4(OH)2C12Br2. The two hydrocarbon chains of C12C4(OH)2C12Br2 should have strong hydrophobic interaction with the chain of C12Glu. Meanwhile, the carboxylate groups and amido groups of C12Glu may form intermolecular hydrogen bond with themselves and with the −OH groups of C12 C 4(OH) 2 C 12Br2 . All these factors 973

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Langmuir on a ThermoHaake RS300 rheometer (cone and plate geometry of 35 mm in diameter with the cone gap equal to 0.105 mm). A solvent trap was used to minimize water evaporation. For the solution with low viscosity, a double-gap cylindrical sensor system with an outside gap of 0.30 mm and an inside gap of 0.25 mm was employed. Cryogenic Transmission Electron Microscopy (Cryo-TEM). The C12C4(OH)2C12Br2/C12Glu samples were prepared in a controlled environment vitrification system (CEVS) at 22 °C, and the relative humidity was kept to 100% during preparation. A micropipet was used to load the samples onto carbon-coated holey film supported by copper grid. Excess solution was gently blotted with filter paper to obtain a thin liquid film (20−400 nm) suspended on the mesh holes. After waiting for about 10 s to relax any stresses induced in blotting, the samples were rapidly plunged into a reservoir of liquid ethane cooled by nitrogen at −180 °C and then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with an FEI Tecnai 20 electron microscope (LaB6) operated at 200 kV with the low dose mode (about 2000 e/ nm2) and the nominal magnification of 50 000. For each specimen area, the defocus was set to 1−2 μm. The images were recorded on a Gatan ultrascan 894 CCD with a scanning step 2000 dpi corresponding to 2.54 Å/pixel.

region. During the titration process, the original concentrations of C12C4(OH)2C12Br2 solution and C12Glu solution are the same, and thus CT of the resulting solution keeps constant, while Xg gradually changes with the titrations. As shown in Figure 1, the C12C4(OH)2C12Br2/C12Glu mixture exhibits several regions as marked by the dashed lines. The regions with shadow form precipitate, where the charge ratio of C12C4(OH)2C12Br2 to C12Glu is around 1:1, and they show very high turbidity, low viscosity, and fluctuated enthalpy changes. The other regions form soluble aggregates. The size distribution and morphology of these soluble aggregates were studied by DLS and cryo-TEM, and the representative results are presented in Figures 2 and 3. Next we will discuss the aggregation behaviors of the C12C4(OH)2C12Br2/C12Glu mixture by combining all these results.

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RESULTS AND DISCUSSION Aggregate Formation at 25 °C. Prior to investigating the influence of temperature on aggregate transitions in C12C4(OH)2C12Br2/C12Glu solutions, the aggregation behavior of the mixtures at 25 °C were studied by changing total surfactant concentration (C T ) and molar fraction of C12C4(OH)2C12Br2 (Xg). Figure 1 shows the turbidity ex-

Figure 2. Size distributions of the C12C4(OH)2C12Br2/C12Glu aggregates formed at different Xg for CT = 2.0 mM (a) and 20.0 mM (b).

The combination of all the results in Figures 1−3 demonstrates that the mixture exhibits several aggregate regions as marked by the dashed lines in Figure 1. Except the precipitation area with shadow, where the turbidity is very larger and the ΔHobs value changes in a significantly fluctuating mode, all other regions form soluble aggregates as described and discussed below. First look at the situations at CT = 2.0 mM. The aggregates show six different regions as shown in Figure 1a−c. Over the whole Xg, the viscosity of the C12C4(OH)2C12Br2/C12Glu solutions is very low and close to that of water (1.0 mPa·s). However, the turbidity and ΔHobs change with Xg. In region i (Xg = 0−0.28), the turbidity is very close to zero, and ΔHobs shows very small endothermic values; moreover, both of them almost do not change. The DLS result indicates that the hydrodynamic radius (Rh) of the aggregates in this region (Figure 2a, Xg = 0.15) is ∼8 nm, and the cryo-TEM image (Figure 3i) shows that the small aggregates are spherical micelles. In region ii (Xg = 0.28−0.35), the turbidity starts to increase gradually, and the solution becomes slightly bluish. The ΔHobs value becomes more endothermic. The DLS result (Figure 2a, Xg = 0.32) and the cryo-TEM image (Figure 3ii) show that the aggregates in this region are vesicles with a hydrodynamic radius of around 200 nm. Then the mixture goes into the precipitation region, i.e, region iii (Xg = 0.35−0.58), where the mixing charge ratio of C12C4(OH)2C12Br2/C12Glu gets close to 1:1. In this region, the turbidity shows a maximum, and afterward the precipitate starts to be redissolved. While further increasing Xg into region iv (Xg = 0.58−0.64), the precipitate has been completely redissolved, and the turbidity becomes smaller. Meanwhile, the ΔHobs value gradually changes from larger endothermic to very small value. The

Figure 1. (a, a′) Turbidity expressed by the absorbance at 450 nm, (b, b′) zero-shear viscosity η0, and (c, c′) the observed enthalpy ΔHobs of the C12C4(OH)2C12Br2/C12Glu solutions plotted against the molar fraction of C12C4(OH)2C12Br2 (Xg) for CT = 2.0 mM (left) and 20.0 mM (right) at 25 °C. The ΔHobs curves were obtained by titrating C12C4(OH)2C12Br2 into C12Glu at lower Xg or titrating C12Glu into C12C4(OH)2C12Br2 at higher Xg.

pressed by the absorbance at 450 nm, zero-shear viscosity η0, and the observed enthalpy ΔHobs for the C12C4(OH)2C12Br2/ C12Glu solutions plotted against Xg at CT of 2.0 and 20.0 mM. The ITC curves were performed by titrating C12C4(OH)2C12Br2 solution into C12Glu solution in the C 12 Glu-rich region or titrating C 12 Glu solution into C12C4(OH)2C12Br2 solution in the C12C4(OH)2C12Br2-rich 974

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Figure 3. Cryo-TEM images of the C12C4(OH)2C12Br2/C12Glu aggregates. For CT = 2.0 mM, Xg = 0.15 (i), 0.32 (ii), 0.62 (iv), 0.70 (v), and 0.80 (vi). For CT = 20.0 mM, Xg = 0.15 (I), 0.32 (II), 0.70 (IV), and 0.80 (V).

1.00; Xg = 0.80; Figure 3V). At this larger CT, wormlike micelles are observed instead of vesicles at CT = 2.0 mM. In the regions of wormlike micelles (II and IV), the maximum viscosity almost grows by 2 orders of magnitude, and the mixture becomes viscous and displays an ability to trap bubbles. Besides, the mixed solutions in these two regions exhibit shear thinning property (Figure S1), which is characteristic of non-Newtonian response. The aggregate transitions above have been summarized in Figure 4. The results indicate that both the mixing ratio and CT

soluble aggregates in this region are vesicles with the hydrodynamic radius of around 100 nm (Figure 2a, Xg = 0.62; Figure 3iv). The morphology of this region is the same as that in region ii, but the content of C12C4(OH)2C12Br2 in the mixture in this region is larger than that in region ii. So the charge properties of the vesicles in these two regions should be opposite. In region v (Xg = 0.64−0.74), although the turbidity is close to zero and does not change anymore, the ΔHobs gradually changes from very small values to larger endothermic values. The average aggregate size decreases to a smaller value of about 10 nm (Figure 2a, Xg = 0.70), and the morphology becomes wormlike micelles (Figure 3v). With C12C4(OH)2C12Br2 becomes the major component, i.e., in region vi (Xg = 0.74−1.00), the aggregates transfer into small spherical micelles with a hydrodynamic radius of about 2 nm (Figure 2a, Xg = 0.80; Figure 3vi). In brief, when the total concentration is 2.0 mM, with the increase of the molar fraction of C12C4(OH)2C12Br2, the C12C4(OH)2C12Br2/C12Glu mixture successively form spherical micelles, vesicles, precipitate, vesicles, wormlike micelles, and spherical micelles. Then turn to the situation at CT = 20.0 mM. Different from the aggregate transitions at CT = 2.0 mM, the aggregate transitions display five regions as presented in Figure 1a′−c′. The five regions are spherical micelles with Rh of about 8 nm (region I, Xg = 0−0.28; Figure 2b, Xg = 0.15; Figure 3I), fingerprint-like wormlike micelles (region II, Xg = 0.28−0.45; Figure 2b, Xg = 0.32; Figure 3II), precipitate (region III, Xg = 0.45−0.67), entangled wormlike micelles (region IV, Xg = 0.67−0.73; Figure 2b, Xg = 0.70; Figure 3IV), and small spherical micelles with Rh of about 3 nm (region V, Xg = 0.73−

F ig ur e 4 . S u m m a r y o f t h e a g g r e g a t e tr a n s i t i o n s i n C12C4(OH)2C12Br2/C12Glu mixtures at 25 °C by changing CT and mixing molar ratio.

adjust the structures of the C12C4(OH)2C12Br2/C12Glu aggregates. At both low and high CT, either C12Glu or C12C4(OH)2C12Br2 is in great excess, and the mixtures selfassemble into spherical micelles. Upon the addition of the oppositely charged surfactant, at low CT, the spherical micelles separately transform into vesicles in the C12Glu-rich region and wormlike micelles in the C12C4(OH)2C12Br2-rich region, while 975

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Figure 3i are formed at 25 °C, Xg = 0.15, and CT = 2.0 mM. Then the variation of the micelles with the temperatures is studied.

at high CT spherical micelles transform into entangled wormlike micelles, no matter in the C12Glu-rich region or in the C12C4(OH)2C12Br2-rich region. The strong electrostatic binding between oppositely charged C12Glu and C12C4(OH)2C12Br2 due to two charges on each of them significantly reduces the area of the headgroups, which is the main controlling factor for the aggregate transitions induced by changing the Xg at the same total surfactant concentration. As to the aggregate transitions induced by changing CT, the variations of electrostatic binding, the hydrophobic interaction between the hydrocarbon chains and dehydration, and hydrogen bonds of the headgroups are all important, and they cooperatively control the transitions. Temperature-Induced Aggregate Transitions. On the basis of the aggregates constructed above at 25 °C, aggregate transitions induced by raising temperature were studied as follows. First, the variation of the turbidity expressed by the absorbance at 450 nm against the molar fraction of C12C4(OH)2C12Br2 (Xg) was employed to characterize the changing tendency of the size of the C12C4(OH)2C12Br2/ C12Glu aggregates at 25, 35, 45, 55, and 65 °C. The results are shown in Figure 5. For each temperature, the turbidity curve

Figure 6. Temperature effect on spherical micelles of C12C4(OH)2C12Br2/C12Glu at a fixed Xg = 0.15 and CT = 2.0 mM: (a) turbidity, (b) DSC curve, and (c) size distribution between 25 and 90 °C; (d) cryo-TEM image from the sample at 70 °C; (e) DSC curves at CT = 2.0 mM and different Xg; (f) DSC curves at Xg = 0.15 and different CT.

Figure 6a shows that the turbidity of C12C4(OH)2C12Br2/ C12Glu mixture at Xg = 0.15 keeps at zero upon increasing temperature from 25 to 68 °C, and the solution is clear and colorless. Afterward, the turbidity starts to increase precipitously, and the mixture begins to scatter light strongly and shows a bluish hue. When the C12C4(OH)2C12Br2/C12Glu mixture is cooled from 85 to 25 °C, the bluish color disappears, and the mixture transforms to a clear, colorless solution again. The bluish hue is a manifestation of the Tyndall effect due to the presence of large scatters in solution, and it is generally seen in vesicle solutions.44 The DSC curve in Figure 6b shows an endothermic peak at about 68 °C, indicating that a phase transition takes place at this temperature. The size distribution from DLS in Figure 6c indicates that the hydrodynamic radius of the C12C4(OH)2C12Br2/C12Glu aggregates grow from ∼8 to ∼50 nm when the temperature increases from 25 to 70 °C. The cryo-TEM image in Figure 6d confirms that small spherical micelles have transferred into vesicles. Taken together, increasing temperature induces the transition from the spherical micelles to vesicles, and this process is thermally reversible. As mentioned above, the spherical micelles formed at Xg = 0.15 and CT = 2.0 mM are transferred into vesicles upon heating in the C12C4(OH)2C12Br2/C12Glu mixture. Then, the micelle-to-vesicle transition at different mixing molar fractions and different CT is studied by DSC. Figure 6e presents the DSC curves of the C12C4(OH)2C12Br2/C12Glu mixture with different Xg at CT = 2.0 mM. Xg changes in the range of 0.25−0.12, where the mixture forms spherical micelles at 25 °C. Clearly, all the DSC curves display a single endothermic peak ascribed to micelle-to-vesicle transition, and the transition peak shifts to higher temperatures with decreasing Xg. While lowering Xg from 0.25 to 0.12, the transition temperature increases from 61

Figure 5. Turbidity of the C12C4(OH)2C12Br2/C12Glu mixtures plotted against Xg for CT = 2.0 mM at 25, 35, 45, 55, and 65 °C.

contains two parts: one part at lower Xg and another at higher Xg. The precipitation region at each temperature was between the two parts and was not recorded because the turbidity values are very large and unstable. Obviously, the precipitation regions become wider with the increase of temperature. This indicates that the mixture is more easily to form precipitate at higher temperature because of strong dehydration. Meanwhile, the variation trends of the turbidity curves at higher temperatures are similar to that at 25 °C. With the increase of Xg, the turbidity curves all exhibit the same changing pattern, increasing at lower Xg region, and then decreasing at higher Xg region. Nevertheless, increasing temperature lowers the Xg values at which the turbidity starts to increase, while enhances the Xg values at which the turbidity stops to decrease. Between 45 and 50 °C, the shifts between the starting points and the end points are the most obvious. In short, temperature significantly alters the aggregation behavior of the C12C4(OH)2C12Br2/ C12Glu mixture in aqueous solution. In the following sections, spherical micelles, vesicles, and wormlike micelles formed at 25 °C are chosen as initial aggregates to investigate temperatureinduced aggregate transitions. Temperature-Induced Aggregate Transition from Spherical Micelles. All the results about the temperature effect on spherical micelles of the C12C4(OH)2C12Br2/C12Glu mixture are shown in Figure 6. The initial spherical micelles shown in 976

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Langmuir to 74 °C. In addition, with further decreasing Xg, the micelle-tovesicle transition peak upon heating cannot be observed. These results indicate that the micelle-to-vesicle transition in the mixture containing less C12C4(OH)2C12Br2 needs more energy. Figure 6f shows the DSC curves of the C12C4(OH)2C12Br2/ C12Glu mixture with different CT at Xg = 0.15. The DSC curve at Xg = 0.15 and CT = 1.0 mM does not show obvious peak; however, all other DSC curves at CT = 2.0−5.0 mM display an endothermic peak. Moreover, both the aggregate transition temperature and the peak area increase with increasing CT. That is more heat is needed to facilitate the micelle-to-vesicle transition at higher CT. The micelle-to-vesicle transition in the C12C4(OH)2C12Br2/ C12Glu mixture described above is related to the molecular structure of the surfactants and intermolecular interactions between them. Yin et al.25 reported heating-induced micelle-tovesicle transition in the SDS/DTEAB mixture, and they pointed out that the system exhibits pseudononionic features and the aggregation number increases with temperature just like nonionic surfactants do. Herein, strong electrostatic interaction exists between C12C4(OH)2C12Br2 and C12Glu because the cationic gemini surfactant C12C4(OH)2C12Br2 has two positive charges while two carboxyl groups of C12Glu are almost completely ionized at pH = 10.0. When Xg is below 0.28, C12Glu is in great excess, and hence the electrostatic interaction is not strong enough to induce the micelle-to-vesicle transition at room temperature. As temperature is raised, progressive dehydration of the ionic headgroups of the surfactants over micellar surface takes place.45 The dehydration process strengthens the electrostatic interaction between cationic and anionic headgroups, which in turn causes a reduction in effective headgroup area of surfactants and makes the surfactant molecules pack much closer. Concomitantly, the effective molecular packing parameter value is increased to match the requirement for vesicle formation. Thus, the aggregate transition from micelle to vesicle occurs upon heating. With a decrease in Xg from 0.28 to 0.12, the transition starts at higher temperatures because the electrostatic interaction between the ionic headgroups is weakened by the decreasing of Xg so that more heat is needed to break the hydration shell of the headgroup surface. Therefore, the micelle-to-vesicle transition temperature increases with decreasing Xg. On another hand, the micelle-to-vesicle transition temperature increases with the increase of CT, possibly because the similar reasons. When the amount of the micelles is larger, higher temperature is needed to break the hydration shell of the headgroup surface. In addition, the hydroxyl groups in the alkyl spacer chain of C12C4(OH)2C12Br2 could form hydrogen bonds with water more readily, thus screening the charges of the ionic headgroups. As the temperature is elevated, the hydration layer around the hydroxyl substitutions is broken with a concomitant weak screening effect, leading to an increase in electrostatic interaction between cationic and anionic headgroups. In brief, the aggregate transition from spherical micelles to vesicles with temperature in C12C4(OH)2C12Br2/C12Glu mixtures results from a combination of strong dehydration and the breakup of hydrogen bonds in the headgroup area. Temperature-Induced Aggregate Transitions from Vesicles. The spherical vesicles of the C12C4(OH)2C12Br2/ C12Glu mixture are constructed at 25 °C, Xg = 0.61, and CT = 2.0 mM. All the results about the temperature effect on the vesicles are shown in Figure 7.

Figure 7. Temperature effect on vesicular solution of C12C4(OH)2C12Br2/C12Glu at Xg = 0.61 and CT = 2.0 mM: (a) turbidity, (b) DSC curves, and (c) size distribution between 25 and 85 °C; (d, e) cryo-TEM images from the samples at 40 and 76 °C; (f) DSC curves at Xg = 0.61 and different CT.

Clearly, Figure 7a shows that the turbidity of C12C4(OH)2C12Br2/C12Glu mixture gradually increases with increasing the temperature from 25 to 85 °C, and the increase between 25 and 70 °C is more significant. Visually, the mixture changes from a clear and slightly bluish solution at 25 °C to turbid one at higher temperatures and precipitates at about 80 °C. The precipitate could be dissolved when cooled down, so that this process is thermally reversible. Figure 7b also shows a single exothermic peak with the onset of the peak temperature at ∼76 °C, which corresponds to the appearance of precipitates. In this process, the size and morphology also change significantly. The size distribution from DLS in Figure 7c indicates that the aggregate size increases from 109 to 630 nm in radius with temperature increasing from 25 to 80 °C. The cryo-TEM image in Figure 7d shows that the mixture at 40 °C does not exist as vesicles anymore and turns into solid spherical aggregates, and the cryo-TEM image in Figure 7e shows that the mixture at 76 °C transfer to larger irregular aggregates. Both of the cryo-TEM images indicate that these spherical aggregates tend to aggregate and fuse together. That is to say, upon heating, the vesicles aggregate into large solid spherical aggregates and then form precipitates. Figure 7f plots the DSC curves of C12C4(OH)2C12Br2/ C12Glu mixtures at Xg = 0.61 and different CT. All the DSC curves display a very similar exothermic peak, indicating that the mixture experiences a similar phase transition at different CT. In particular, the phase transition temperature increases with increasing CT. In other words, the precipitation takes place at higher temperature when CT is raised. Different from the micelle-to-vesicle transition with an endothermic peak in DSC curves, the temperature-induced transition from vesicles to solid spherical aggregate and irregular precipitates corresponds to an exothermic process in DSC curves. Normally, vesicle formation has already accompanied by intense dehydration of surfactant headgroups, and the solvation degree of individual surfactant molecule in vesicles is relatively low. This suggests that the present 977

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Langmuir transition from vesicles to spherical aggregates and larger irregular aggregates may originate from other controlling interaction instead of the dehydration with increasing temperature. Given that the pH used is 10.0 and the pKa of γ-COOH is ∼5.0, moreover the carboxyl group of C12Glu located at the α-position dissociates more easily than that at the γ-position with increasing pH; the C12Glu molecules in aqueous solution at pH 10.0 mainly have charged carboxylates α-COO− and γCOO−. Because in the two vesicle regions, the Xg values are 0.28−0.35 and 0.58−0.64. That is to say, the vesicles are only weakly negatively charged or weakly positively charged. With the increase of temperature, the electrostatic binding between oppositely changed C12C4(OH)2C12Br2 and C12Glu is further strengthened, which further lowers the net charge density of the headgroup area. Meanwhile, the increase of temperature enhances the molecule motion and in turn weakens the order of bilayer hydrophobic microdomain of vesicles. Besides, the increase of temperature may also weaken or destroy the hydrogen bonds of the hydroxyl groups in the spacer of C12C4(OH)2C12Br2 with water. All these factors promote the association and collision of the bilayer structure and furthermore induce that the vesicles transfer to large solid spherical aggregates and finally precipitate out. The temperature-induced vesicle transition above differs from that in other vesicular systems upon heating. For the case of vesicular systems formed by the mixtures of cationic surfactant and hydrophobic aromatic counterions (e.g., CTAB and 5-methylsalicylic acid),22 heating-induced vesicle-to-micelle transitions were reported and discussed by two possible mechanisms: desorption of aromatic counterions from vesicles due to their increased solubility in bulk water or a Coulombic solid−fluid transition at the aggregate surface. As for the mixture of SDS with didodecyldimethylammonium bromide (DDAB),46 vesicles were found to undergo a transition from gel-to-liquid crystalline phase upon heating. Whereas the vesicles in the mixture of SDS with dodecyltributylammonium bromide (DTBAB) aggregate with each other with increasing temperature,28,29 and an enhancement of the intervesicular hydrophobic interaction among the exposed butyl chains on the DTBAB headgoup was thought to be responsible for such a transition. Temperature-Induced Aggregate Transitions from Wormlike Micelles. As summarized in Figure 4, wormlike micelles can also be obtained at 25 °C with the C12C4(OH)2C12Br2/C12Glu mixtures at both low and high CT. In region v at CT = 2.0 mM, the mixture exists as wormlike micelles at 25 °C, while in region II and IV at CT = 20.0 mM, the mixture exists as entangled wormlike micelles. In view of the pronounced viscoelastic properties often presented by entangled wormlike micellar solution, wormlike micelles formed at CT = 20.0 mM and Xg = 0.43 are chosen as representative to investigate the aggregate transitions from wormlike micelles induced by changing temperature. All the results are presented in Figure 8. Figure 8a shows that the turbidity of the wormlike micellar solution keeps at zero at lower temperature and begins to rapidly increase since ∼70 °C. Afterward, the turbidity reaches a plateau at ∼80 °C. Accordingly, an obvious single endothermic peak appears in the DSC curve (Figure 8b), and the solution becomes cloudy. So the endothermic peak corresponds to the cloud point. The occurrence of cloud point upon heating is a phenomenon generally observed in nonionic surfactant systems or pseudononionic surfactant systems, which is normally caused

Figure 8. Temperature effect on wormlike micellar solutions at Xg = 0.43 and CT = 20.0 mM: (a) turbidity, (b) DSC curves, (c) size distribution between 25 and 80 °C, and (d) steady shear viscosity η between 25 and 60 °C; (e, f) cryo-TEM images from the samples at 25 and 80 °C; (g) DSC curves at CT = 20.0 mM and varying Xg; (h) DSC curves at Xg = 0.37 and different CT.

by attractive interactions between nonionic micelles or by the transition from linear to branched micelles. Herein, the size distributions from DLS in Figure 8c illustrate there are two peaks with different scattering intensity between 25 and 40 °C: the peaks with stronger scattering intensity are at Rh of 20 nm, while the peaks with smaller scattering intensity are at Rh of 700 nm. The appearance of the larger size may be attributed to the entanglement of wormlike micelles. With further increasing the temperature to 70 °C, the locations of the two size peaks are getting closer to each other, and the scattering intensity is varying in the opposite direction. When the temperature reaches 80 °C, there is only a single size peak at Rh of 40 nm. Corresponding changes are also reflected in the rheological response. Figure 8d shows that the zero-shear viscosity η0 almost keeps constant between 25 and 35 °C and then obviously decreases with the increase of the temperature. Moreover, the solutions exhibit a shear-thinning response. In particular, compared with the wormlike micelles formed at 25 °C (Figure 8e), the cryo-TEM image from the sample prepared at 80 °C (Figure 8f) clearly shows the existence of 3-fold junctions or branching points, being marked by arrows. In short, the increase of the temperature makes the linear wormlike micelles change into branched wormlike micelles accompanying the appearance of the cloud point. The origin of this temperature-induced aggregate transition can be understood from two respects. On one hand, as the temperature raises, the increased molecular motion and the destruction of intermolecular hydrogen bonds weaken the interactions between wormlike micelles, which causes a drop in viscosity. On the other hand, heating-induced dehydration of 978

DOI: 10.1021/acs.langmuir.5b04211 Langmuir 2016, 32, 972−981

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78 °C while increasing Xg from 0.35 to 0.43. This suggests that the closer the mixture to charge neutralization, the lower the cloud point. When the mixture is much closer to charge neutralization, the transition from linear to branched wormlike micelles will take place at lower temperature. Besides, the DSC curves of the C12C4(OH)2C12Br2/C12Glu mixture with Xg = 0.37 (Figure 8h) indicate that the cloud point temperature and the wormlike transition point increase by about 24° with increasing the total surfactant CT from 10.0 mM to 20.0 mM. In brief, both the decrease in Xg and the increase in CT strengthen the hydration degree of the headgroup area, and thus more heat is required to achieve dehydration for aggregate transitions. In summary, upon the increase of temperature, the aggregates of the C12C4(OH)2C12Br2/C12Glu mixtures undergo a series of significant transitions: smaller spherical micelles transfer into larger vesicles, larger vesicles transfer into solid spherical aggregates and then larger irregular precipitates, and entangled wormlike micelles transfer into branched wormlike micelles. Moreover, all these transitions are thermally reversible. In order to clearly follow the aggregate transitions, the aforementioned results are summarized in Figure 9 with a schematic illustration.

the headgroup area assists in the enhancement of the electrostatic interaction between C12C4(OH)2C12Br2 and C12Glu and thereby leads to a much closer packing of the surfactant molecules in the aggregates, which can facilitate the growth of the wormlike micelles. At low temperatures, the above two effects may reach a balance and thus result in unchanged viscosity and unchanged aggregate structure. But at high temperatures, the wormlike micelles grow into highly branched wormlike micelles, and meanwhile the destruction of intermolecular hydrogen bonds reduces the entangled extent among wormlike micelles. This is also supported by the decrease of the viscosity and the disappearance of the larger size peak 80 °C in the DLS data. In addition, the relationship between cloud point and rheology can be further rationalized by the microstructure changes from linear wormlike micelles to branched wormlike micelles. As the temperature increases, the entangled network of linear wormlike micelles with a higher viscosity transforms into connected branched wormlike micelles with a lower viscosity and in final cloud point appears accompanying the significant growth of the branched wormlike micelles. The micellar branching theory was previously proposed by Kaler and co-workers47 to explain cloud point in the mixture of cationic surfactant solutions with salts. The clouding behavior of the above wormlike micellar solutions induced by increasing temperature has been elucidated by the formation of branched micelles, but the molecular origin of micellar branching is still unclear. Because the analogy of the endothermic peaks in the DSC curves between spherical micellar solutions and wormlike micellar solutions, dehydration of the ionic headgroups is also proposed to be the main factor in the development of branched micelles. As the temperature is raised, strong dehydration strengthens the electrostatic interaction between oppositely charged surfactants, which is in favor of the reduction of the effective headgroup area and the concomitant increase of the packing parameter. Consequently, the branches are formed instead of highly curved end-caps. Besides, the breaking of hydrogen bonding of −COO− or −OH with water serves as a supplementary contribution to micellar branching through increasing the electrostatic interaction and/or weakening the hydration layer. In addition, temperature-induced micellar branching can also be explained by the difference between branch energy and end-cap energy of the micelles with temperature. Earlier, Drye and Cates48 theoretically reported that the free energy cost for branch formation is much higher than for end-cap formation. Later on, Bernheim-Groswasser49 pointed out that branch energy and end-cap energy increase linearly but inversely with temperature. In other words, forming branches will be energetically much more favorable than forming end-caps at a critical point. These theories also support the formation of branched micelles with increasing temperature in the present C12C4(OH)2C12Br2/C12Glu wormlike micellar solution. On the basis of the above understanding about the temperature effect on the wormlike micelles in the C12C4(OH)2C12Br2/C12Glu mixture at Xg = 0.43 and CT = 20.0 mM, DSC is used to further understand the effect at different mixing ratios and different CT. Figure 8g shows that all the DSC curves of the mixture at 20.0 mM display an endothermic peak ascribed to the cloud point and the transition point from entangled linear wormlike micelles to branched wormlike micelles, and the cloud point decreases from 101 to

Figure 9. Summary of the aggregate transitions in the C12C4(OH)2C12Br2/C12Glu mixture as a function of temperature.

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CONCLUSION The present work has systematically studied temperature effect on the aggregate transitions in a surfactant mixture of cationic ammonium gemini surfactant C12C4(OH)2C12Br2 with anionic amino acid monomeric surfactant C12Glu in aqueous solution. First, the aggregation behavior of the mixture at 25 °C is studied in detail, and smaller spherical micelles, larger vesicles, and entangled wormlike micelles are formed by controlling the mixing ratios and the total surfactant concentration. Because either C12C4(OH)2C12Br2 or C12Glu carries two charges, electrostatic binding between them is very strong. The strong electrostatic binding significantly reduces the area of the headgroups and thus is the main controlling factor for the aggregate transitions. Next, upon increasing the temperature, these aggregates experience different aggregate transitions. Smaller spherical micelles transfer into larger vesicles, larger vesicles transfer into solid spherical aggregates and then larger irregular aggregates, and entangled wormlike micelles transfer into branched wormlike micelles. The larger irregular aggregates and branched micelles finally lead to precipitation and clouding phenomenon, respectively. Moreover, all these transition processes are thermally reversible, and the transition 979

DOI: 10.1021/acs.langmuir.5b04211 Langmuir 2016, 32, 972−981

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Prepared by Different Methods. Chem. Phys. Lipids 2000, 105, 201− 213. (7) Yang, Y.; Dong, J.; Li, X. Micelle to Vesicle Transitions of NDodecyl-1, ω-Diaminoalkanes: Effects of pH, Temperature and Salt. J. Colloid Interface Sci. 2012, 380, 83−89. (8) Söderman, O.; Herrington, K. L.; Kaler, E. W. Transition from Micelles to Vesicles in Aqueous Mixtures of Anionic and Cationic Surfactants. Langmuir 1997, 13, 5531−5538. (9) Mendes, E.; Narayanan, J.; Oda, R.; Kern, F.; Candau, S. J. ShearInduced Vesicle to Wormlike Micelle Transition. J. Phys. Chem. B 1997, 101, 2256−2258. (10) Lin, Y.; Han, X.; Cheng, X.; Huang, J.; Liang, D.; Yu, C. pHRegulated Molecular Self-Assemblies in a Cationic−Anionic Surfactant System: From a “1−2” Surfactant Pair to a “1−1” Surfactant Pair. Langmuir 2008, 24, 13918−13924. (11) Johnsson, M; Wagenaar, A.; Stuart, M. C. A.; Engberts, J. B. F. N. Sugar-Based Gemini Surfactants with pH-Dependent Aggregation Behavior: Vesicle-to-Micelle Transition, Critical Micelle Concentration, and Vesicle Surface Charge Reversal. Langmuir 2003, 19, 4609− 4618. (12) Huang, X.; Cao, M.; Wang, J.; Wang, Y. L. Controllable Organization of a Carboxylic Acid Type Gemini Surfactant at Different Ph Values by Adding Copper(II) Ions. J. Phys. Chem. B 2006, 110, 19479−19486. (13) Fernandes, R. M. F.; Marques, E. F.; Silva, B. F. B.; Wang, Y. Micellization Behavior of a Catanionic Surfactant with High Solubility Mismatch: Composition, Temperature, and Salt Effects. J. Mol. Liq. 2010, 157, 113−118. (14) Yatvin, M. B.; Weinstein, J. N.; Dennis, W. H.; Blumenthal, R. Design of Liposomes for Enhanced Local Release of Drugs by Hyperthermia. Science 1978, 202, 1290−1293. (15) Haddou, B.; Canselier, J. P.; Gourdon, C. Cloud Point Extraction of Phenol and Benzyl Alcohol from Aqueous Stream. Sep. Purif. Technol. 2006, 50, 114−121. (16) Purkait, M. K.; Banerjee, S.; Mewara, S.; DasGupta, S.; De, S. Cloud Point Extraction of Toxic Eosin Dye Using Triton X-100 as Nonionic Surfactant. Water Res. 2005, 39, 3885−3890. (17) Quina, F. H.; Hinze, W. L. Surfactant-Mediated Cloud Point Extraction: An Environmentally Benign Alternative Separation Approach. Ind. Eng. Chem. Res. 1999, 38, 4150−4168. (18) Bott, R.; Wolff, T.; Zierold, K. Temperature-Induced Transitions from Rodlike to Globular Micellar Aggregates in Aqueous Cetyltrimethylammonium Bromide in the Presence of 9-Anthrylalkanols. Langmuir 2002, 18, 2004−2012. (19) Li, H.; Wieczorek, S. A.; Xin, X.; Kalwarczyk, T.; Ziebacz, N.; Szymborski, T.; Holyst, R.; Hao, J.; Gorecka, E.; Pociecha, D. Phase Transition in Salt-Free Catanionic Surfactant Mixtures Induced by Temperature. Langmuir 2010, 26, 34−40. (20) Majhi, P. R.; Blume, A. Temperature-Induced Micelle-Vesicle Transitions in DMPC−SDS and DMPC−DTAB Mixtures Studied by Calorimetry and Dynamic Light Scattering. J. Phys. Chem. B 2002, 106, 10753−10763. (21) Tsuchiya, K.; Nakanishi, H.; Sakai, H.; Abe, M. TemperatureDependent Vesicle Formation of Aqueous Solutions of Mixed Cationic and Anionic Surfactants. Langmuir 2004, 20, 2117−2122. (22) Davies, T. S.; Ketner, A. M.; Raghvan, S. R. Self-Assembly of Surfactant Vesicles That Transform into Viscoelastic Wormlike Micelles Upon Heating. J. Am. Chem. Soc. 2006, 128, 6669−6675. (23) Salkar, R. A.; Hassan, P. A.; Samant, S. D.; Valaulikar, B. S.; Kumar, V. V.; Kern, F.; Candau, S. J.; Manohar, C. A Thermally Reversible Vesicle to Micelle Transition Driven by a Surface SolidFluid Transition. Chem. Commun. 1996, 1223−1224. (24) Buwalda, R. T.; Stuart, M. C. A.; Engberts, J. B. F. N. Wormlike Micellar and Vesicular Phases in Aqueous Solutions of Single-Tailed Surfactants with Aromatic Counterions. Langmuir 2000, 16, 6780− 6786. (25) Yin, H.; Zhou, Z.; Huang, J.; Zheng, R.; Zhang, Y. TemperatureInduced Micelle to Vesicle Transition in the Sodium Dodecylsulfate/

temperatures can be tuned by varying the mixing ratios and/or the total surfactant concentration. These temperature-induced aggregate transitions can be elucidated on the basis of the temperature-induced variations in the dehydration, electrostatic interaction, and hydrogen bonds of the headgroup area and in the hydrophobic interaction between the hydrocarbon chains. This investigation provides an example that temperature can realize a series of aggregate transitions in the same surfactant mixture. The results suggest that the surfactants carrying multiple binding sites will greatly improve the regulation ability. Therefore, gemini surfactants and other surfactants with multiple headgroups and multiple chains are good choices to efficiently construct surfactant systems with temperature sensitivity, high regulation ability, and other desired characteristics. Such kinds of tunable aggregate transitions have potentials in many practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04211. Steady shear viscosity, temperature effect on dynamic rheological properties, and concentration effect on aggregate transitions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the State Key Laboratory of Polymer Physics and Chemistry for providing the DLS equipment, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education in Peking University for Rheological measurement, and the Center for Biological Imaging (CBI), Institute of Biophysics, Chinese Academy of Science for technical support in electron microscopy. In particular, we are grateful to Dr. Xiaojun Huang and Dr. Gang Ji for their help of preparing cryoTEM samples and taking TEM images. This work was financially supported by Beijing National Laboratory for Molecular Sciences and National Natural Science Foundation of China (Grants 21025313 and 21321063).

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REFERENCES

(1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of SelfAssembly of Hydrocarbon Amphiphiles into Micelles and Bilayers. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525−1568. (2) Zana, R. Surfactant Solutions: New Methods of Investigation; Marcel Dekker: New York, 1987. (3) Evans, D. F.; Wennerström, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet; Wiley-VCH: New York, 1999. (4) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (5) Silva, B. F. B.; Marques, E. F.; Olsson, U. Unusual Vesicle− Micelle Transitions in a Salt-Free Catanionic Surfactant Temperature and Concentration Effects. Langmuir 2008, 24, 10746−10754. (6) Feitosa, E.; Barreleiro, P. C. A.; Olofsson, G. Phase Transition in Dioctadecyldimethylammonium Bromide and Chloride Vesicles 980

DOI: 10.1021/acs.langmuir.5b04211 Langmuir 2016, 32, 972−981

Article

Langmuir Dodecyltriethylammonium Bromide System. Angew. Chem., Int. Ed. 2003, 42, 2188−2191. (26) Kalur, G. C.; Frounfelker, B. D.; Cipriano, B. H.; Norman, A. I.; Ragavan, S. R. Viscosity Increase with Temperature in Cationic Surfactant Solutions Due to the Growth of Wormlike Micelles. Langmuir 2005, 21, 10998−11004. (27) Yu, Z.; Xu, G. Physicochemical Properties of Aqueous Mixtures of Tetrabutylammonium Bromide and Anionic Surfactants. 1. Temperature-Induced Micellar Growth and Cloud Point Phenomenon. J. Phys. Chem. 1989, 93, 7441−7445. (28) Yin, H.; Huang, J.; Gao, Y.; Fu, H. Temperature-Controlled Vesicle Aggregation in the Mixed System of Sodium N-Dodecyl Sulfate/N-Dodecyltributylammonium Bromide. Langmuir 2005, 21, 2656−2659. (29) Yin, H.; Lin, Y.; Huang, J.; Ye, J. Temperature-Induced Vesicle Aggregation in Catanionic Surfactant Systems: The Effects of the Headgroup and Counterion. Langmuir 2007, 23, 4225−4230. (30) Kjellander, R. Phase Separation of Non-Ionic Surfactant Solutions. A Treatment of the Micellar Interaction and Form. J. Chem. Soc., Faraday Trans. 2 1982, 78, 2025−2042. (31) Mehreteab, A.; Loprest, F. J. Formation of Pseudo-Nonionic Complexes of Anionic and Cationic Surfactants. J. Colloid Interface Sci. 1988, 125, 602−609. (32) Menger, F. M.; Keiper, J. S. Gemini Surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906−1920. (33) Zana, R. Gemini (Dimeric) Surfactants. Curr. Opin. Colloid Interface Sci. 1996, 1, 566−571. (34) Zana, R.; Talmon, Y. Dependence of Aggregate Morphology on Structure of Dimeric Surfactants. Nature 1993, 362, 228−230. (35) Han, Y.; Wang, Y. L. Aggregation Behavior of Gemini Surfactants and Their Interaction with Macromolecules in Aqueous Solution. Phys. Chem. Chem. Phys. 2011, 13, 1939−1956. (36) Hu, Z.; Wang, L.; Guo, J.; Cao, D.; Chang, J.; Fan, L. Interaction of a Novel Anionic Gemini Surfactant Containing a Triazine Ring with Cetyltrimethylammonium Bromide in Aqueous Solution. J. Surfactants Deterg. 2015, 18, 17−24. (37) Pei, X.; Zhao, J.; Li, E. Micellar Growth: Role of Molecular Geometry and Intermolecular Hydrogen Bonding. Colloids Surf., A 2013, 420, 59−63. (38) Wang, X.; Wang, J.; Wang, Y. L.; Yan, H. Effect of the Nature of the Spacer on the Aggregation Properties of Gemini Surfactants in an Aqueous Solution. Langmuir 2004, 20, 53−56. (39) Pei, X.; Zhao, J.; Ye, Y.; You, Y.; Wei, X. Wormlike Micelles and Gels Reinforced by Hydrogen Bonding in Aqueous Cationic Gemini Surfactant Systems. Soft Matter 2011, 7, 2953−2960. (40) Rosen, M. J.; Mathias, J. H.; Davenport, L. Aberrant Aggregation Behavior in Cationic Gemini Surfactants Investigated by Surface Tension, Interfacial Tension, and Fluorescence Methods. Langmuir 1999, 15, 7340−7346. (41) Wettig, S. D.; Nowak, P.; Verrall, R. E. Thermodynamic and Aggregation Properties of Gemini Surfactants with Hydroxyl Substituted Spacers in Aqueous Solution. Langmuir 2002, 18, 5354− 5359. (42) Rosen, M. J.; Liu, L. Surface Activity and Premicellar Aggregation of Some Novel Diquaternary Gemini Surfactants. J. Am. Oil Chem. Soc. 1996, 73, 885−890. (43) Bordes, R.; Holmberg, K. Physical Chemical Characteristics of Dicarboxylic Amino Acid-Based Surfactants. Colloids Surf., A 2011, 391, 32−41. (44) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. Spontaneous Vesicle Formation in Aqueous Mixtures of Single-Tailed Surfactants. Science 1989, 245, 1371−1374. (45) Hayter, J. B.; Zulauf, M. Attractive Interactions in Critical Scattering from Non-Ionic Micelles. Colloid Polym. Sci. 1982, 260, 1023−1028. (46) Marques, E. F.; Khan, A.; Lindman, B. A Calorimetric Study of the Gel-to-Liquid Crystal Transition in Catanionic Surfactant Vesicles. Thermochim. Acta 2002, 394, 31−37.

(47) Raghavan, S. R.; Edlund, H.; Kaler, E. W. Cloud-Point Phenomena in Wormlike Micellar Systems Containing Cationic Surfactant and Salt. Langmuir 2002, 18, 1056−1064. (48) Drye, T. J.; Cates, M. E. Living Networks: The Role of CrossLinks in Entangled Surfactant Solutions. J. Chem. Phys. 1992, 96, 1367−1375. (49) Bernheim-Groswasser, A.; Wachtel, E.; Talmon, Y. Micellar Growth, Network Formation, and Criticality in Aqueous Solutions of the Nonionic Surfactant C12E5. Langmuir 2000, 16, 4131−4140.

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