Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle

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Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle Transitions by Single-Tailed Pyrrolidone Surfactants Zan Jiang, Kangle Jia, Xiong Liu, Jinfeng Dong, and Xuefeng Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02312 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015

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Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle Transitions by Single-Tailed Pyrrolidone Surfactants Zan Jiang, Kangle Jia, Xiong Liu, Jinfeng Dong, Xuefeng Li * College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P.R. China.

*corresponding author, Email: [email protected]

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Abstract：： We report a new family of multiple responsive fluids based on the single-tailed pyrrolidone surfactants, N-ethyl-2-pyrrolidone N-alkyl amine (CmNP, m = 10, 12, 14, 16, 18). These surfactants are highly sensitive to solution pH due to the presence of N-amino group in the molecules. Equilibrium surface tension results indicate that both the surface activity and micellization ability of CmNPs decrease with the increase of protonation degree, i.e. they exhibit higher critical micelle concentration (cmc) and higher surface tension at cmc (γcmc) at the acidic conditions than those at the basic conditions. The cmc values of CmNPs follow the well-known Klevens equation, which decrease linearly with the increase of the hydrocarbon chain length m at a given pH. More importantly, the self-assemblies of CmNPs are highly sensitive to pH, CO2 and CuCl2 as identified by turbidity and viscosity. The transitions between vesicles and wormlike micelles are further confirmed by rheology, static and dynamic light scattering (SLS and DLS), cryogenic transmission electron microscopy (cryo-TEM), and nuclear magnetic resonance (NMR) techniques systematically. Although the aggregate transitions induced by different factors are similar, however, the mechanisms are different. The pH- and CO2-induced transitions are attributed to variation in the protonation degree of N-amino group, however, CuCl2-induced transitions are a result of the formation of CmNP and CuCl2 coordination complexes as revealed by 2D Noesy NMR and UV-Vis spectra.

Keywords: critical micelle concentration, wormlike micelle to vesicle transition, pH response, CO2 response, coordination response.

2


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1. Introduction Fluids that are responsive to stimuli such as pH light

[9, 10]

and magnetic field

[1, 2]

, temperature

[3, 4]

, CO2

[11, 12]

, especially those with multiple responses

[5, 6]

, redox

[7, 8]

,

[13, 14]

, have recently

attracted much attention to scientists and engineers, owing to their potential applications in industry, smart materials development, drug and gene delivery, crude oil recovery enhancement, and so on [5, 6, 15]. For example, photo responsive fluids were applied in the district heating/cooling system as smart drag reduction successfully

[16]

. Stimuli-responsive fluids that are based on

surfactants are of particular interest because remarkable variations in rheological responses could be realized simply by aggregate morphology transitions, such as the transitions between wormlike micelles and spherical micelles or vesicles

[1~10, 13]

. Generally speaking, stimuli-responsive

surfactant systems can be built through two different routes. One is based on the surfactant/hydrotrope binary systems [17], in which additives are non-covalently incorporated with surfactants and the stimuli response might come from either additives or surfactants. Up to now, systems that respond to light

[7, 16, 18~20]

, temperature

[13, 21]

, CO2

[5, 6]

and pH

[22, 23]

have been

developed accordingly. Another is based on the synthetic chemistry that special stimuli-responsive moieties are covalently combined to surfactant molecules

[2, 8, 9, 14]

. Although this method is more

complex than the former one, however, it enriches the diversity of surfactants greatly. Beyond the stimuli response, the development of efficient and environmentally compatible surfactants is also a challenge [24, 25]. N-methyl pyrrolidone is a well-known biocompatible solvent with low vapor pressure and low toxicity. It has been used for various applications such as skin permeation enhancers, flocculation of particles, shampoo and cosmetics

[26]

. N-alkyl pyrrolidones

(CmPs), which are homologues of N-methyl pyrrolidone, are also a category of surface active 3


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chemicals [27], but the low consolute temperatures prevent them to form self-organized assemblies in aqueous solutions because phase separation occurs before micellization, thereby limiting their applications greatly. The development of pyrrolidone headgroup based surfactants

[28]

, by

chemically incorporating some special moieties into CmPs, might overcome this disadvantages and enhance their applications. We have recently reported a series of pyrrolidone headgroups based dimeric surfactants (Di-CmPs), which showed excellent surface activities and strong micellization abilities [29, 30]. pHand concentration-induced spherical micelle to vesicle transitions were also observed in Di-CmPs, endowing them potential applications in developing stimuli-responsive materials especially in gene and drug delivery vehicles. More importantly, these Gemini surfactants are environmentally friendly

[31]

, indicating Di-CmPs maintain the advantages of CmPs. However, these dimeric

surfactants still suffer from the low solubility especially when m is above 12 [29]. We have noticed that the single-tailed diamino-surfactants, N-alkyl-1, 2-ethylenediamines (CmN2N)

[32]

, exhibited

good solubility, excellent surface activity and rich self-assembly behavior. Thus, the development of pyrrolidone headgroups based single-tailed surfactants might be a good way to improve the physicochemical properties of CmPs, which could be realized simply through substituting one amino group of CmN2N with the pyrrolidone group. Based on this principle, we have developed a new family of pyrrolidone headgroup based single-tailed surfactants namely N-ethyl-2-pyrrolidone N-alkyl amine (CmNP, m = 10, 12, 14, 16, 18, Scheme 1), which can be considered as the monomeric form of Di-CmP. The major motivation of the present work is to clarify the surface and bulk phase properties of CmNPs, and establish the structure-property relationship of them. In this work, the surface and micellization properties are 4


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studied by employing equilibrium surface tension. Moreover, the interesting multiple stimuli-responsive transitions between wormlike micelles and vesicles, in terms of pH, CO2 and CuCl2, are also studied systematically. Various techniques such as rheology, static and dynamic light scattering (SLS and DLS), and cryogenic transmission electronic microscopy (cryo-TEM) are employed to reveal the macro-properties and micro-structures. Furthermore, the microenvironment changes of surfactants during transitions are also studied by NMR, which is of fundamental importance in understanding surfactant properties at the molecular level. The improved pyrrolidone headgroup based surfactants CmNPs might show some potential applications in nano-materials fabrication and gene delivery vehicles. (Scheme 1)

2. Experimental Methods 2.1 Materials 2-pyrrolidone and N-alkyl amine (CmH2m+1NH2, m = 10, 12, 14, 16 and 18; 98%) were obtained from TCI. Sodium hydride, dichloromethane, methanol, acetonitrile, ethyl acetate, DMF, NaOH, HCl, KI, K2CO3, anhydrous MgSO4, and NaCl were all analytical grade and purchased from Chinese Medical Co. (Shanghai). All reagents were used as received without further purification except 2-pyrrolidone and DMF, which were dried and distilled before use. CmNPs were synthesized according to the procedure mentioned in the Supporting Information Scheme S1. Water was Millipore Milli-Q grade. 2.2 pH Titration

5


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The protonation constants (pKa) of CmNPs were measured using the Red model PHSJ-5 digital pH meter (Leici, China). Generally, 0.1 mol/L hydrochloric acid aqueous solutions was added to 10 mL 0.03 mol/L CmNP aqueous solution until pH was about 2 and then subsequently titrated with 0.1 mol/L sodium hydroxide aqueous solution until pH was 12. Each titration point was recorded as the potential drift was stable. 2.3 Samples preparation Dispersions of CmNPs in water were prepared by direct sonication of samples in water, and the dispersions were stable and homogeneous. Samples with the required pH were prepared by titrating with HCl or NaOH aqueous solution directly and measured by the Rex model PHSJ-5 digital pH meter (Leici, China) with a temperature senor using an E-201D combination pH electrode. All samples were equilibrated overnight before measurements. 2.4 Surface tension measurements The equilibrium surface tensions of CmNP solutions were measured by the du Noüy ring method (Krüss K100, Germany). The adsorption amount was calculated according to the Gibbs adsorption equation (1) [33]:

Γ max = −

1 dγ ⋅ 2.303nRT d l o g c

(1)

where Γmax is the saturated adsorption amount in µmol·m-2, γ is the surface tension in mN·m-1, R is the gas constant, T is the absolute temperature, and c is the surfactant concentration. (dγ/dlog c) is the slope of the γ-log c curve when surfactant concentration is below the cmc. The value of n is depended on the specific circumstances, which is calculated by the following equation according to Rosen [33]. n = 1 + cs/(cs + cCl− ) 6


(2)

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where cs is the cmc of CmNP and cCl− is the concentration of chloride ion in aqueous solution. The minimum surface area of surfactant molecule Amin can be obtained from the saturated equation (3) [33],

Amin =

1 ×1024 N AΓ max

(3)

in which NA is Avogadro’s number and Amin is in nm2. 2.5 Static and Dynamic light scattering measurements Static and dynamic light scattering (SLS and DLS) measurements were performed on the Zetasizer instrument ZEN3600 (Malvern, UK) with a 173° back scattering angle and He-Ne laser (λ = 633 nm). CmNP samples at different pH were filtered with a 0.2 µm filter of mixed cellulose acetate to remove any interfering dust particles. The pH-dependent light scattering intensities were measured with the samples equilibrated overnight. The experimental data were normalized to make a better presentation. To obtain the size distributions, the autocorrelation functions were analyzed using CONTIN. 2.6 Rheological measurements Rheological measurements were performed on the RS 600 stress-controlled rheometer (TA Instruments, Germany) using the Couette geometry DG 41 with a gap of 5.1 mm and a measuring cell volume of 6.3 mL, which was equipped with a peltier-based temperature control. A solvent trap was used to minimize the sample evaporation. 2.7 Cryogenic transmission electron microscopy measurements Samples for cryo-TEM were prepared as following: 3-5 µL of sample dispersion was deposited on the surface of a TEM copper grid covered by a holey carbon film. After blotting away the excess dispersion to form a thin liquid film, the grid was immediately plunged into liquid 7


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ethane. The specimens were maintained at approximately –173 °C (l-N2) and imaged in a transmission electron microscopy (JEOL 1400 at an accelerating voltage of 200 kV or FEI Tecnai Spirit at an accelerating voltage of 120 kV) under low dose conditions. 2.8 UV-Vis spectra measurements UV-Vis spectra were obtained on the UV–vis Tu-1901 spectrophotometer (Pgeneral, China) using water as blank. 2.9 NMR measurements 2D Noesy NMR measurements for C16NP aqueous solution and the mixture of C16NP and CuCl2 were performed on the Bruker AVENCE-500 NMR spectrometer with a proton frequency of 500.13 MHz at 35 °C. A 90° pulse width of 8.2 µs, a mixing time of 100, 300 or 500 ms, a relaxation delay of 2 s, and an acquisition time of 205 ms were used. The experimental data were collected 2048 complex points, and processed with a Lorentz-to-Gauss window function and zero filling in both dimensions to display data on a 2048 × 2048 2D-matrix. The peaks are referenced with respect to the DOH in D2O.

3. Results and Discussion 3.1 Surface Adsorption behaviors Similar to their dimeric form Di-CmPs, CmNPs are also pH-sensitive due to the presence of N-amino group in the molecules. Because all the measured pKa values of CmNPs are around 7.5 (Supporting Information Figure S1 and Table S1), therefore, CmNPs should mainly be 1:1 type cationic surfactants and nonionic type surfactants at pH 3.5 and 10.5, respectively. The corresponding surface activity and micellization ability of CmNP at pH 3.5 and 10.5 are studied by 8


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equilibrium surface tension as shown in Figure 1a and 1b, respectively. It should be mentioned that all experiments in this work for C10NP, C12NP and C14NP are performed at 25 oC, whereas those for C16NP and C18NP are at 35 oC to insure above their Krafft temperatures at the acidic condition (Supporting Information Table S1). (Figure 1) The abilities to reduce the surface tension of water and form self-organized aggregates in the bulk phase are two inherent characteristics of surfactants. Obviously, CmNPs have the strong tendency to reduce the surface tension of water regardless of their protonation states. The surface adsorption efficiency of surfactant is often represented by pC20 (pC20 = – log10 c20), in which c20 is the required surfactant concentration to lower the surface tension of water by 20 mN·m-1

[33]

.

Generally speaking, the larger pC20 the surfactant is, the higher adsorption efficiency it has. The clear breakpoint in the γ-log c curve at a certain concentration for each CmNP corresponds to its cmc, which indicates the onset of micellization. The adsorption behaviors of CmNPs are analyzed in detail using the Gibbs equation (1), and some important physicochemical parameters such as cmc, γcmc, Γmax, pC20, Amin are summarized in Table 1. (Table 1) Figure 2a shows that the γcmc value of CmNP is depended on the hydrophobic chain length m strongly. For one thing, the γcmc value of CmNP decreases with the increase of m for a given pH, i.e. at pH 3.5, because CmNP with the longer alkyl chain can be packed closer at the air/water surface due to the stronger Van der Walls interactions. Another, the γcmc value at pH 3.5 is far larger than that at pH 10.5 for each CmNP. This is because CmNP can be packed closer in the latter condition owing to the reduction of electrostatic interactions between hydrophilic headgroups [32], 9


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which also results in a smaller minimum surface area Amin and a larger pC20 (Table 1). Similar phenomena are also observed in Di-CmPs

[29]

, however, the surface activity of dimeric surfactant

Di-CmP is much superior to that of CmNP with the same m, which is generally observed and is reasonable. This is because the single-tailed surfactants covalently connected by a spacer group, or dimeric surfactants, can both increase the hydrophobic force and decrease the electrostatic repulsion between two headgroups within one molecule [34]. Therefore, dimeric surfactants can be packed closer than the corresponding single-tailed ones at the air/water surface, resulting in a lower γcmc. Although the surface activities of both CmNP and Di-CmP can be enhanced upon increasing m, whereas the lowest γcmc value of CmNP series is far lower than that of Di-CmPs because of the solubility limitation of Di-CmP with a larger m. For example, the lowest γcmc values are 25.2 mN/m for C18NP at pH 10.5 and 27 mN/m for Di-C10NP at pH 11.0 in CmNP and Di-CmP homologues, respectively. (Figure 2) For surfactant homologues, cmc values often follow the Stauff-Klevens relationship as log cmc = A – B × m

[35]

, in which A and B are empirical constants that represent the nature of the

hydrophilic group and the contribution of each additional methylene unit on cmc, respectively. Figure 2b shows the obvious linear relationships between cmc and m for CmNPs, and two linear equations of log cmcpH=3.5 = 2.97 – 0.212 × m and log cmcpH=10.5 = 2.36 – 0.255 × m are obtained at pH 3.5 and 10.5, respectively. It is noticed that the value of A at pH 3.5 is much larger than that at pH 10.5, indicating the increase of protonation state strengthens the hydrophilicity but weakens its micellization ability due to the increase of electrostatic interactions between hydrophilic headgroups. The slightly smaller value of B at pH 3.5 indicates that the contribution of each 10


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additional methylene unit in micellization is also weakened. In other words, the hydrophobic interactions are strengthened when CmNP appears to be nonionic type surfactant at pH 10.5. Similar linear relationships are also observed in Di-CmPs at pH 2.5 and 11.0 [29], where Di-CmP is with the full protonation and without protonation, respectively. Compared CmNP with the corresponding Di-CmP, the cmc value of CmNP is far larger than that of Di-CmP. For example, the cmc values are 7.52 mmol/L and 0.0679 mmol/L for C10NP at pH 2.5 and Di-C10P at pH 3.5, respectively. Similar phenomena are commonly observed by comparing single-tailed surfactants with the corresponding dimeric ones [34].

3.2 Multiple stimuli-responses in the bulk phases 3.2.1 Schematic diagram of multiple stimuli-responses The representative multiple stimuli-responsive transitions, in terms of pH, CO2 and CuCl2, in CmNP aqueous solutions are shown in Scheme 2. The presence of N-amino group in CmNPs endows them pH-sensitivity in the self-assembly behaviors

[29, 30, 32]

, in specifically, the

pH-induced reversible vesicle to spherical micelle transitions via rodlike/wormlike micelle. In addition, vesicles can also transform into rodlike/wormlike micelles upon bubbling CO2, and then reversibly return to the original states once CO2 is removed through heating, indicating the CO2-induced reversible vesicle to rodlike/wormlike micelle transitions. It is also noticed that the rodlike/wormlike micelles formed in the special pH regions, which strongly relates to the hydrophobic chain length m, can further transform into vesicles upon adding CuCl2, suggesting the CuCl2-induced rodlike/wormlike micelle to vesicle transitions. (Scheme 2) 11


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3.2.2 pH-induced micelle to vesicle transitions According to the significant difference in cmc values of CmNPs, the concentrations of 0.12, 0.08, 0.06, 0.04, 0.02 mol/L are employed for C10NP, C12NP, C14NP, C16NP and C18NP, respectively, which are far larger than their corresponding cmc values and guarantee self-organized assemblies formed in the investigated pH regions. Macroscopically, all CmNP aqueous solutions can transform from initially bluish to optically transparent when the solutions are adjusted from basic to acidic conditions as monitored by SLS (Figure 3a). It is clear that the light scattering intensity is dropped remarkably when the solution pH is below a special value for each CmNP, and then remains nearly constant, indicating the pH-induced vesicle to spherical micelle transitions. Overall, the pH values where the smallest intensities are arrived for CmNPs shift toward the lower pH region when the hydrophobic chain length m is increased, which were also observed in similar CmN2N homologues [32]. For example, the values are about 6.6 and 4.5 for C10NP and C18NP, respectively. It is also noticed that the optically transparent solution becomes viscous in a special narrow pH region for each CmNP, indicating the rodlike/wormlike micelles formation, which can be observed from the pH-dependent zero-shear viscosity results clearly (Figure 3b). Steady state rheology results of 0.04 mol/L C16NP (Supporting Information Figure S2) show that fluid viscosities become very large at about pH 6.5 and evident shear-thinning behaviors are observed, suggesting the formation of wormlike micelles. The pH-dependent aggregate size distributions (Supporting Information Figure S3) also support the pH-induced vesicle to spherical micelle transitions via rodlike/wormlike micelle in CmNP homologues. (Figure 3) Figure 3b shows the pH-dependent zero-shear viscosity of C16NP, which can be roughly 12


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divided into four separated regions. In the optically transparent region I (pH < 4.5), the solution viscosity is as low as that of water (~0.001 Pa·s) and keeps nearly constant, indicating the spherical micelles formation region. In the optically transparent region II, the viscosity increases significantly from pH 4.5 to 6.5 and a maximum viscosity is arrived at about pH 6.5, suggesting the remarkable growth of micelles upon increasing solution pH and the formation of wormlike micelles. It should be mentioned that the achieved maximum viscosity of CmNP aqueous solution depends on the hydrophobic chain length m strongly. Generally, the larger the m, the higher the viscosity is. In the region III, the viscosity is decreased about 103 times by slightly increasing solution pH for about one pH unit, and the solution turbidity is also increased remarkably simultaneously (Figure 3a), indicating the wormlike micelles and vesicles coexisting region. Further increasing pH would result in a fully turbid solution with a very low viscosity (region IV), suggesting the vesicles formation region. Since viscoelastic fluids could be formed at the particular pH region for each CmNP, we have also noticed that the concentration of CmNP affects the viscosity significantly, and even transparent elastic fluid might be obtained upon increasing surfactant concentration (Supporting Information Figure S4). The presence of rodlike/wormlike micelles and vesicles in CmNP aqueous solutions is further confirmed by cryo-TEM observation. C10NP and C18NP that have the shortest and longest hydrophobic chain length, respectively, are typically selected. Figure 4a and 4b show the cryo-TEM images of optically transparent and viscous samples containing 0.12 mol/L C10NP at pH 8.0 and 0.02 mol/L C18NP at pH 6.3, respectively. Clearly, only short rodlike micelles with a length about tens of nanometers are observed in the C10NP sample. Whereas, very long and entangled wormlike micelles are observed in C18NP. The cryo-TEM images of 0.12 mol/L C10NP 13


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at pH 9.2 (Figure 4c) and 0.02 mol/L C18NP (Figure 4d) at pH 6.9 confirm vesicles formation well. (Figure 4) Similar pH-induced reversible vesicle to micelle transitions via rodlike/wormlike micelle were also observed in other N-amino groups containing surfactants

[2, 30, 32]

. In those systems, the

increase of electrostatic repulsion between headgroups were primary owing to the increase of protonation degree on N-amino groups, and aggregate transitions could be explained by the molecular packing parameter (P) well

[36]

. Herein, the limit molecular adsorption area (Amin) for

each CmNP is increased significantly from the basic to acidic condition (Table 1). For example, the values of C10NP are 0.40 and 0.67 nm2 per molecule at pH 10.5 and 3.5, respectively, resulting in the P values of 0.50 and 0.31, which support the formation of vesicles and spherical micelles, respectively. Since the effect of hydrophobic chain length m on Amin is limited for CmNP homologues at the same protonation state (Table 1), whereas the occupied volume of hydrophobic tail is increased with the increase of m. Therefore, the increase of hydrophobic chain length m is beneficial to the micellar growth. As a result, pH-induced vesicle to micelle transitions for CmNPs shift toward the lower pH value upon increasing m as observed in Figure 3. 3.2.2 CO2-induced vesicle to micelle transitions Interestingly, all the bluish CmNP aqueous solutions could also transform into optically transparent and viscoelastic fluids upon bubbling CO2, and then return to their initial states through heating. Figure 5 shows the photographs of typical transition induced by CO2 in 0.075 mol/L C16NP aqueous solution. Initially, the solution is bluish with its pH 7.5. It becomes optically transparent with its pH 6.1 after bubbling CO2 at the fixed flow rate of 0.1 L/min for 5 min, which 14


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will return reversibly through heating at 75 °C for about 1 h. The variation in solution pH upon bubbling or removing CO2 indicates that CO2 should play the same function as that of HCl partly. Cryo-TEM images show the presence of vesicles and wormlike micelles before (Figure 5a) and after (Figure 5b) bubbling CO2, respectively, confirming the CO2-induced vesicle to wormlike micelle transition well. (Figure 5) The CO2-responses in CmNP homologues are generally observed as monitored by viscosity measurements (Supporting Information Figure S5), and all fluids show shear-thinning behaviors upon bubbling CO2, in which the employed concentration for each CmNP is kept at 0.075 mol/L to make a better understand on the influence of hydrophobic chain length m. Figure 6a shows the zero-shear viscosities of 0.075 mol/L CmNPs in the absence and presence of CO2. Clearly, all the zero shear viscosities (η0) of original CmNP solutions are very low (about 0.001 Pa·s) because of vesicles formation, which are independent of m. However, η0 is increased significantly in those systems through bubbling CO2, suggesting the formation of rodlike/wormlike micelles. Also, η0 of CmNP solution is highly depended on m after bubbling CO2. On the whole, the larger m is, the higher the viscosity be. In other words, the larger m is, the longer the length of micelles be. For example, the values of η0 are 0.003 Pa·s and 3.5 Pa·s for C10NP and C18NP after bubbling CO2, respectively. In addition, such CO2-induced vesicle to rodlike/wormlike micelle transitions are completely reversible as monitored by viscosity measurements (Supporting Information Figure S6). Figure 6b shows the reversible transition of 0.04 mol/L C16NP solution, in which solution viscosities switch between about 0.001 Pa·s and 0.1 Pa·s perfectly. (Figure 6) 15


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During the reversible experiments (Figure 6b), the solution pH values are also reversibly changed between 7.45 ± 0.01 and 6.10 ± 0.04 through bubbling or removing CO2. CO2 is a weak acid, which mainly hydrolyzes into HCO3– in water and provides H+ simultaneously. Thus, the effect of CO2 on CmNP should be similar to that of HCl in some ways [37]. The results suggest that CO2-induced protonation increase on CmNP should be the major cause during the vesicle to rodlike/wormlike micelle transitions. Moreover, the CO2-induced transitions are rigidly reversible by comparison with those by pH regulation using HCl or NaOH as mentioned in Section 3.2.2, because additionally irremovable Cl– and Na+ would remain in the latter conditions. It should be mentioned that the final morphologies of aggregates induced by CO2 in CmNP homologues are mainly restricted to rodlike/wormlike micelles. This is because the limiting pH of saturated CO2 aqueous solution is about 5.6, which results in the solution pH of CmNP mainly locating in the rodlike/wormlike micelles formation pH region (Figure 3a) irrespective of the difference of counter ions. 3.2.3 CuCl2-induced micelle to vesicle transitions It is well-known that N-amino groups can coordinate with transition metal ions, such as Cu2+, Co2+, Ni2+, Ag+, and so on, to form metal complexes owing to the ions pair electrons of N atoms, thereby resulting in some novel physicochemical properties [38~41]. Figure 7 shows the photographs of typical transition induced by CuCl2 in C16NP aqueous solution, in which the optically transparent solution of 0.04 mol/L C16NP at pH 6.1 transforms into a turbid one upon adding equivoluminal 0.02 mol/L CuCl2 aqueous solution, which are generally observed in other CmNPs (Supporting Information Figure S7). Cryo-TEM results confirm the CuCl2-induced rodlike micelle (Figure 7a) to vesicle (Figure 7b) transition, and the size distributions measured by DLS also 16


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support the transitions (Supporting Information Figure S8a). (Figure 7) During the CuCl2-induced rodlike micelle to vesicle transition in 0.02 mol/L C16NP aqueous solution, the viscosity is reduced (Supporting Information Figure S8b). However, the addition of NaCl with the same ion strength as that of 0.01 mol/L CuCl2results in the increase in viscosity because of the electrostatic shielding effect (Supporting Information Figure S8b), indicating some special interactions such as coordination might be happened between C16NP and CuCl2. UV-Vis spectra (Supporting Information Figure S9) show that a new broad peak spanned the wavelength of 200~400 nm is appeared in the mixture of CuCl2 and C16NP, which is significantly different from the spiky peaks at about 200 nm of themselves, suggesting coordination complexes of C16NP and CuCl2 (Cu(C16NP)2Cl2) should be formed. To make a better understand on the coordination induced configuration and interaction variation of surfactant, 2D Noesy NMR is employed. Figure 8 shows the typical 2D Noesy NMR spectra of 0.02 mol/L C16NP in the absence (Figure 8a) and presence (Figure 8b) of 0.01 mol/L CuCl2. (Figure 8) The cross-peaks between Hb and Hg, Hh, Hf,i (marked in Figure 8a) in the pure C16NP aqueous solution suggest that the location of pyrrolidone group is very close to the hydrophobic chain. In other words, the pyrrolidone group might adopt a confirmation toward the nonpolar micellar core rather than in the polar water, and similar confirmation was also observed in other heterocyclic headgroup containing surfactant systems

[42]

. Moreover, some new cross-peaks

between Ha and Hf,i, Hd, He, Hg, Hh (marked in Figure 8b) are appeared in the presence of CuCl2, indicating the terminal group -CH3 in the hydrophobic chain should be very close to both the 17


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N-amino group and pyrrolidone group. Therefore, C16NP must adopt the interdigitated packing mode in vesicle bilayers. Since both the N-amino group and O atom in pyrrolidone group can coordinate with Cu2+ [43], therefore, the conformation of the coordination complexes of C16NP and CuCl2 (Cu(C16NP)2Cl2) in vesicle bilayers might adopt somewhat like the illustration as shown in Figure 9. (Figure 9)

4. Conclusions In summary, we have reported a new family of pyrrolidone based single-tailed surfactants N-ethyl-2-pyrrolidone N-alkyl amine (CmNP, m = 10, 12, 14, 16, 18) with excellent surface activities and multiple stimuli-responsive assembly behaviors. For one thing, the surface activities of CmNPs are highly depended on their protonation states, which could be controlled by solution pH simply. Another, the self-organized assemblies of CmNPs are multiple responses. pH-induced vesicle to micelle transition via rodlike/wormlike micelle is a general feature due to the efficient headgroups area increase, and the hydrophobic chain length has a little effect on the self-assembly behaviors. Similarity, CO2-induced reversible vesicle to rodlike/wormlike micelle transitions are also attributed to the protonation variation in CmNPs. In addition, CuCl2-induced rodlike/wormlike micelle to vesicle transitions are caused by the coordination complexes formation between CmNP and CuCl2. Distinguished from the surface active CmPs

[27]

, the improved chemicals CmNPs retain the

original advantages of CmPs, however, CmNPs are classic surfactants with the abilities to both reduce the surface tension of water and form self-organized assemblies in the bulk phase. 18


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Although the surface activity and micellization ability of CmNP are slightly inferior to its dimeric form Di-CmP

[29]

, however, CmNPs have the advantages of excellent solubility and the richness in

self-assembly behaviors with multiple responses. According to the multiple stimuli-responsive fluid properties of CmNPs aqueous solutions, they might have some potential applications in district heating/cooling systems as drag reduction [44]. Because of their excellent surface activities and the richness in self-assembly behaviors, CmNPs could develop the application fields in those of industrial technologies

[45~47]

such as stimuli-responsive emulsification, pesticide formulation,

waste water treatment and nano-material development.

Supporting Information. Synthesis and characterization of CmNP, pKa and Krafft temperature of CmNP, pH-dependent steady state rheology of C16NP, pH-dependent DLS of CmNP, concentration-dependent zero-shear viscosity and dynamic rheology of C16NP, steady state rheology of CmNP in the absence and presence of CO2, photographs of CmNP in the absence and presence of CuCl2, DLS, steady state rheology and UV-Vis absorbance spectra of C16NP in the absence and presence of CuCl2. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC 20973129 and 21273165).

References: 19


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[10] Song, B. L.; Hu, Y. F.; Zhao, J. X. A Single Component Photo Responsive Fluid Based on the Gemini Surfactant with Azobenzene Spacer. J. Colloid Interface Sci. 2009, 333, 820−822. [11] Pletneva, V. A.; Molchanov, V. S.; Philippova, O. E. Viscoelasticity of Smart Fluids Based on Wormlike Surfactant Micelles and Oppositely Charged Magnetic Particles. Langmuir 2015, 31, 110−119. [12] Brown, P.; Bushmelev, A.; Butts, C. P.; Cheng, J.; Eastoe, J.; Heenan, R. K.; Schmidt, A. M. Magnetic Control over Liquid Surface Properties with Responsive Surfactants. Angew. Chem. Int. Ed. 2012, 124, 2464−2466. [13] Jia, K. L.; Cheng, Y. M.; Liu, X.; Li, X. F.; Dong, J. F. Thermal, Light and pH Triple Stimulated Changes in Self-assembly of a Novel Small Molecular Weight Amphiphile Binary System. RSC Adv. 2015, 5, 640−642. [14] Yang, R.; Peng, S.; Hughes, T. C. Multistimuli Responsive Organogels Based on a Reactive Azobenzene Gelator. Soft Matter 2014, 10, 2188–2196. [15] Brown, P.; Butts, C. P.; Eastoe, J. Stimuli-Responsive Surfactants. Soft Matter 2013, 9, 2365−2374 [16] Shi, H.; Ge, W.; Oh, H.; Pattison, S. M.; Huggins, J. T.; Talmon, Y.; Hart, D. J.; Raghavan, S. R.; Zakin, J. L. Photoreversible Micellar Solution as a Smart Drag-Reducing Fluid for Use in District Heating/Cooling Systems. Langmuir 2013, 29, 102−109. [17] Hodgdon, T. K.; Kaler, E. W. Hydrotropic Solutions. Curr. Opin. Colloid Interface Sci. 2007, 12, 121–128 [18] Wang, D.; Dong, R. H.; Long, P. F.; Hao, J. C. Photo-Induced Phase Transition from Multilamellar Vesicles to Wormlike Micelles. Soft Matter 2011, 7, 10713–10719. 21


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[28] Login, R. B. Pyrrolidone-Based Surfactants (A Literature Review). J. Am. Oil Chem. Soc. 1995, 72, 759–771. [29] Jiang, Z.; Li, X. F.; Yang, G. F.; Cheng, L.; Cai, B.; Yang, Y.; Dong, J. F. pH-Responsive Surface Activity and Solubilization Pyrrolidone-Based Gemini Surfactants. Langmuir 2012, 28, 7174–7181. [30] Jiang, Z.; Liu, J.; Sun, K.; Dong, J. F.; Li, X. F.; Mao, S. Z.; Du, Y. R.; Liu, M. L. pH- and Concentration-Induced Micelle-to-Vesicle Transitions in Pyrrolidone-Based Gemini Surfactants. Colloid Polym. Sci. 2014, 292, 739–747. [31] Zhou, X.; Zhang, H. B.; Zhang, G. Y. Synthesis of Pyrrolidone Compounds as Surfactants. CN. Patent 101200445, 2008; Chem. Abstr. 2008, 149, 128728. [32] Yang, Y.; Dong, J. F.; Cai, B.; Jiang, Z.; Cheng, L.; Li, X. F. Environmentally Responsive Adsorption and Assembly Behaviors from N-alkyl-1,2-ethylenediamines. Soft Matter 2013, 9, 1458–1467. [33] Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley &Sons: New York, 1989. [34] Zana, R. Dimeric and Oligomeric Surfactants. Behavior at Interfaces and in Aqueous Solution: A Review. Adv. Colloid Interface Sci. 2002, 97, 203–253. [35] Klevens, H. B. Structure and Aggregation in Dilate Solution of Surface Active Agents. J. Am. Oil Chem. Soc. 1953, 30, 74–80. [36] Israelachvili, J. N. Intermolecular and Surface Force, 2nd ed.; Academic Press: New York, 1991. [37] Zhang, Y. M.; Feng, Y. J.; Wang, J. Y.; He, S.; Guo, Z. R.; Chu, Z. L.; Dreiss, C. A. 23


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[45] Zarzar, L. D.; Sresht, V.; Sletten, E. M.; Kalow, J. A.; Blankschtein, D.; Swager, T. M. Dynamically Reconfigurable Complex Emulsions via Tunable Interfacial Tensions. Nature 2015, 518, 520–524. [46] Cheng, L.; Li, X. F.; Dong, J. F. Size-Controlled Preparation of Gold Nanoparticles with Novel pH Responsive Gemini Amphiphiles. J. Mater. Chem. C 2015, 3, 6334–6340. [47] Zheng, H. Q.; Gao, C. B.; Peng, B. W.; Shu, M. H.; Che, S. A. pH-Responsive Drug Delivery System Based on Coordination Bonding in a Mesostructured Surfactant/Silica Hybrid. J. Phys. Chem. C 2011, 115, 7230–7237.

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Captions

Scheme 1. Molecular structure of CmNP. Scheme 2. Representative schematic diagram of pH-, CO2- and CuCl2-induced transitions between wormlike micelles and vesicles. Table 1. The physicochemical parameters of CmNPs. Figure 1. The γ-log c plots of CmNP in aqueous solutions at pH 3.5 (a) and 10.5 (b). Figure 2. The hydrophobic chain length m dependent γcmc (a) and cmc (b) of CmNP and Di-CmP [29]

.

Figure 3. (a) The scattering light intensity of CmNP as a function of pH, the insert photographs are 0.08 mol/L C12NP at pH 7.6 and 8.2, respectively. (b) The zero shear viscosity of C16NP as a function of pH. Figure 4. cryo-TEM images of 0.12 mol/L C10NP at pH 8.0 (a), 0.02 mol/L C18NP at pH 6.3 (b), 0.12 mol/L C10NP at pH 9.2 and 0.02 mol/L C18NP at pH 6.9 (d). Figure 5. Photographs of CO2-induced transition in 0.075 mol/L C16NP aqueous solution, and cryo-TEM images of 0.075 mol/L C16NP before (a) and after (b) bubbling CO2, respectively. Figure 6. (a) The zero shear viscosity of 0.075 mol/L CmNP solutions before and after bubbling CO2. (b) The zero shear viscosity of 0.04 mol/L C16NP solution after bubbling CO2 and heating repeated three times. Figure 7. Photographs of CuCl2-induced transition in 0.04 mol/L C16NP aqueous solution, and cryo-TEM images of 0.02 mol/L C16NP in the absence (a) and presence (b) of 0.01 mol/L CuCl2, respectively. 26


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Figure 8. 2D Noesy NMR spectra of 0.02 mol/L C16NP (a) and the mixture of 0.02 mol/L C16NP and 0.01 mol/L CuCl2 (b) in D2O. Figure 9. Proposed configuration and bilayer structures for C16NP and Cu2+ coordination complexes.

27


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Scheme 1. Molecular structure of CmNP.

28


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Scheme 2. Representative schematic diagram of pH-, CO2- and CuCl2-induced transitions between wormlike micelles and vesicles.

29


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Page 30 of 40

Table 1. The physicochemical parameters of CmNPs.

Surfactant

γcmc

cmc

Γmax

(mN/m)

(mol/L)

(µmol/m2)

3.5

32.1

7.52×10-3

1.47

3.4

0.67

10.5

29.0

4.76×10-4

3.19

4.5

0.40

3.5

31.3

2.73×10-3

1.43

3.8

0.68

10.5

27.8

2.13×10-4

3.96

4.9

0.42

3.5

30.8

8.15×10-4

1.49

4.4

0.66

10.5

26.4

6.27×10-5

4.03

5.5

0.41

3.5

30.6

4.26×10-4

1.53

4.5

0.65

10.5

25.6

1.60×10-5

4.04

6.2

0.41

3.5

30.3

1.44×10-4

1.62

5.1

0.64

10.5

25.2

5.09×10-6

4.22

6.4

0.39

pH

Amin pC20 (nm2)

C10NP

C12NP

C14NP

C16NP

C18NP

30


Page 31 of 40

C10NP

70

C10NP

70

C12NP

C12NP

C14NP

60

C14NP

60

C18NP

50 40 30 20

C16NP γ (mN/m)

C16NP γ (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30

a -7

C18NP

50

-6

-5

-4 -3 log c (mol/L)

-2

20

-1

b -8

-7

-6

-5 -4 log c (mol/L)

Figure 1. The γ-log c plots of CmNP in aqueous solutions at pH 3.5 (a) and 10.5 (b).

31


-3

-2

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CmNP, pH=3.5

34

a

CmNP, pH=3.5

CmNP, pH=10.5

CmNP, pH=10.5

b

1

10 cmc (mmol/L)

32 γcmc (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

30 28

0

10

-1

10

-2

10

26 Di-CmP, pH=2.5 24 6

8

10

Di-CmP, pH=2.5

-3

10

Di-CmP, pH=11.0 12 m

14

16

18

Di-CmP, pH=11.0 6

8

10

12 m

14

16

18

Figure 2. The hydrophobic chain length m dependent γcmc (a) and cmc (b) of CmNP and Di-CmP [29]

.

32


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1.0

a 0.8 C12NP

C12NP

pH 7.6

pH 8.2

C10NP C12NP C14NP

0.4

C16NP

3

4

-1

5

6 pH

7

8

9

III

II

Region: I

-2

10

C18NP 2

10

10

0.6

0.2

b

0

Zero shear viscosity (Pa⋅s)

Relative scattering intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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IV

-3

10

10

2

3

4

5

6

7

8

pH

Figure 3. (a) The scattering light intensity of CmNP as a function of pH, the insert photographs are 0.08 mol/L C12NP at pH 7.6 and 8.2, respectively. (b) The zero shear viscosity of C16NP as a function of pH.

33


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a

Page 34 of 40

b

100 nm

100 nm

c

d

100 nm

100 nm

Figure 4. cryo-TEM images of 0.12 mol/L C10NP at pH 8.0 (a), 0.02 mol/L C18NP at pH 6.3 (b), 0.12 mol/L C10NP at pH 9.2 and 0.02 mol/L C18NP at pH 6.9 (d).

34


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a

b C16NP pH 7.5

bubbling CO2

C16NP pH 6.1

heating

100 nm

100 nm

Figure 5. Photographs of CO2-induced transition in 0.075 mol/L C16NP aqueous solution, and cryo-TEM images of 0.075 mol/L C16NP before (a) and after (b) bubbling CO2, respectively.

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Before bubbling CO2 After bubbling CO2

0

-1

10

10

b Removing CO2

-1

η0 (Pa⋅ s)

η0 (Pa⋅s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

-2

10

Bubbling CO2

-2

10

a

-3

10 -3

10

10

12

14 m

16

1

18

2 Cycle

3

Figure 6. (a) The zero shear viscosity of 0.075 mol/L CmNP solutions before and after bubbling CO2. (b) The zero shear viscosity of 0.04 mol/L C16NP solution after bubbling CO2 and heating repeated three times.

36


b

a Mixture Mixing

100 nm

200 nm

Figure 7. Photographs of CuCl2-induced transition in 0.04 mol/L C16NP aqueous solution, and cryo-TEM images of 0.02 mol/L C16NP in the absence (a) and presence (b) of 0.01 mol/L CuCl2 , respectively.

37

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Page 38 of 40

Hb Hf,i

Hd,e

Hg Hh Hc

Hb

Ha Hf,i He Hd Hg Hh

Hc

Ha and Hf,i,Hd,He,Hg,Hh

a

Ha

b

Hb and Hg,Hh

Hb and Hf,i

Figure 8. 2D Noesy NMR spectra of 0.02 mol/L C16NP (a) and the mixture of 0.02 mol/L C16NP and 0.01 mol/L CuCl2 (b) in D2O.

38


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H N

H N

Cu2+ O O N

N

Figure 9. Proposed configuration and bilayer structures for C16NP and Cu2+ coordination complexes.

39


Langmuir

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Graphic Abstract 674x532mm (96 x 96 DPI)


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Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle

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