Effects of Calcium Ions on the Solubility and Rheological Behavior of a

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Effects of Calcium Ions on Solubility and Rheological Behavior of a C22-tailed Hydroxyl Sulfobetaine Surfactant in Aqueous Solution Xiuling Ji, Maozhang Tian, Desheng Ma, Youyi Zhu, Zhao-Hui Zhou, Qun Zhang, and Yilin Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03614 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Effects of Calcium Ions on Solubility and Rheological Behavior of a C22-tailed Hydroxyl Sulfobetaine Surfactant in Aqueous Solution Xiuling Ji,† Maozhang Tian,§ Desheng Ma,§ Youyi Zhu,§ Zhao-Hui Zhou,§ Qun Zhang§,* and Yilin Wang*,†,‡ †

Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Beijing National Laboratory for

Molecular Sciences (BNLMS), CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

§

State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of Petroleum Exploration &

Development, Beijing 100083, People’s Republic of China

ABSTRACT Effects of calcium ions (Ca2+) on the solubility, aggregate structure and rheological behavior of a C22-tailed zwitterionic surfactant, erucyl dimethyl amidopropyl hydroxyl sulfobetaine (EHSB), have been investigated in aqueous solution. In comparison with sodium ions (Na+), Ca2+ ions exhibit much higher efficiency in decreasing the Krafft temperature (TK) of EHSB. Specifically, contrary to Na+ ions which have no obvious effect on the rheological properties of the EHSB solution, Ca2+ ions increase the viscosity of the EHSB solution at lower EHSB concentration, and enhance its elasticity at higher EHSB concentration. Moreover, Ca2+ ions raise the temperature needed for the elastic-to-viscous transition of the EHSB solution at higher concentration. At lower EHSB concentration, the hydrophobic interaction between the ultra-long hydrocarbon chains induces a tighter packing of the hydrophobic chains by forming more stretched configuration, while at higher EHSB concentration, the electrostatic attraction between Ca2+ ions and the sulfonate groups of EHSB 1

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induces a tighter packing of the headgroups by forming Ca2+-mediated bridges among the EHSB headgroups. Besides, the above interactions may strengthen the hydrogen bonding of OH groups and/or of C=O amide groups, which in turn facilitates the compact packing of the surfactant molecules in aggregates and promotes the growth and entanglement of wormlike micelles. Thus the EHSB solution shows Ca2+-dependent rheological behaviors. The solubility and rheological properties of the ultra-long chain surfactant solution can be simultaneously improved with the addition of divalent Ca2+ ions.

INTRODUCTION Zwitterionic surfactants show strong self-assembling ability due to the coexistence of both positive and negative charges in the same molecule. For the same reason, zwitterionic surfactants display complicated pattern in interacting with ions. Despite the fact that the net charges of the surfactants are neutral, the presence of opposing charges leads to large dipole moments in headgroups, which gives rise to interactions with both cations and anions.1,2 There are unimpeachable evidences from studies of both physical and kinetic properties that zwitterionic micelles preferentially bind anions, generating negative zeta potential.3-9 Moreover, their negative potential is correlated with the binding affinity of anions with zwitterionic micelles, which follows the Hofmeister series or the Pearson hard-soft classification.8,10 Thus zwitterionic micelles become anionoid due to anion binding, and then bind with cations.10-13 The cation binding in turn reduces the negative potential of the anion-bound zwitterionic micelles. However, the changing extent is different between monovalent cations and divalent cations. In general, the effect of monovalent cations is weaker than that of divalent cations.11 Meanwhile, the nature of cations also affects their binding with zwitterionic 2


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micelles.14 Huang et al.15 studied the effects of cations on critical micelle concentration (CMC), mean

hydrodynamic

radius

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separation

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phosphatidylcholine, and found that Ca2+ ions show stronger binding affinity with the phosphate group than Mg2+ ions and thus show more significant effects on the properties of the micelles. Moreover the binding situation of cations with zwitterionic micelles is strongly dependent on the nature of the ions of zwitterionic surfactants. It was found that both zwitterionic sulfobetaine micelles and phosphorylcholine micelles preferentially incorporate anions, but the subsequent binding situations of the micelles with cations are different.16,17 For sulfobetaine liposomes, the zeta potential still keeps negative even with higher concentration of Ca2+ ions, but the negative potential values become smaller. As to phosphorylcholine liposomes, the zeta potential gradually becomes positive as the Ca2+ concentration increases. As a result, the different binding situations of ions will significantly influence the aggregation behaviors and phase behaviors of zwitterionic surfactants. In recent years, zwitterionic surfactants with a ultra-long chain (longer than C18) have drawn increasing attention in academic and industrial research because they have very low CMC,18,19 and readily self-assemble into extremely long and flexible wormlike micelles at lower concentrations, thus exhibiting remarkable viscoelastic or even gel-like rheology.20,21 Such strong thickening ability and elastic behavior suggest their potential application in enhanced oil recovery (EOR). To ensure good solubility of zwitterionic surfactants, most rheological studies are often performed in the presence of NaCl. Kumar et al.20 reported that the rheology of the highly viscoelastic fluids of the C22-tailed zwitterionic surfactant erucyl dimethyl amidopropyl betaine (EDAB) was not affected by NaCl. They showed that the neutrality of EDAB headgroups gave rise to the weak electrostatic interaction with the added NaCl and thus the insensitivity to the NaCl concentration. Chu et al.19 3


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observed that the steady shear viscosity was constant in solution of 3-(N-erucamidopropyl-N, N-dimethyl ammonium) propane sulfonate (EDAS) with the addition of NaCl, which was thought to be caused by the weak electrostatic repulsion between EDAS headgroups and the negligible electrostatic screening effect of NaCl. Although zwitterionic surfactants have been found insensitive to monovalent salt NaCl, the effect of multivalent cations on this kind of surfactants has not yet been understood in literature. The multivalent cations, especially divalent Ca2+ ions, generally exist in tap water or other water sources. So to investigate the influence of Ca2+ ions on the aggregation and properties of ultra-long chain zwitterionic surfactants in aqueous solution is an important prerequisite to apply this kind of surfactant with high efficiency and unique properties. This work has studied the effects of calcium chloride (CaCl2) on the micellar systems of a C22-tailed zwitterionic surfactant, erucyl dimethyl amidopropyl hydroxyl sulfobetaine (EHSB). The corresponding molecular structure of this surfactant is shown in Scheme 1. At first, the solubility dependence of the EHSB solution on Ca2+ ions was studied, and the Ca2+ concentration range to ensure good solubility of the EHSB solution was obtained by establishing a relationship between Krafft temperature (TK) and the Ca2+ concentration. Then the effect of Ca2+ concentration on the rheological properties of the EHSB micellar solutions was studied. Very interesting rheological responses have been found, and the related mechanism has been discussed.

Scheme 1. Chemical structure and abbreviation of the surfactant used.

EXPERIMENTAL SECTION Materials. N-(3-dimethylaminopropyl)erucamide, used for the synthesis of the EHSB surfactant, 4


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was of 99.7% purity and was supplied by PetroChina Research Institute of Petroleum Exploration & Development. The EHSB surfactant was prepared according to the synthetic procedure previously reported.22 Afterwards, the raw product was recrystallized several times from the mixture of acetone-ethanol (2:1). The structure of the obtained EHSB was confirmed by electrospray ionization mass spectrometry (ESI-MS), and 1H nuclear magnetic resonance (1H NMR). ESI-MS: Calc., 561.43 (M+H+); found: m/z = 561.5. 1H NMR (400 MHz, CDCl3, δ in ppm): δ = 0.88 (t, J = 6.80 Hz, 3H, CH3), 1.25-1.26 (m, 28H, CH2), 1.57 (s, 2H, CH2), 2.00-2.01 (m, 6H, CH2), 2.19-2.21 (m, 2H, CH2), 3.06 (s, 2H, CH2), 3.13-3.30 (m, 6H, N+(CH3)2), 3.31 (s, 2H, CH2), 3.54 (s, 4H, CH2), 3.82-3.85 (m, 1H, CH), 4.65 (s, 1H, OH), 5.33-5.35 (m, 2H, -CH=CH-). Its purity was characterized by elemental analysis. Anal. Calcd for C30H60N2O5S (EHSB): C, 64.24; H, 10.78; N, 4.99. Found: C, 64.24; H, 10.63; N, 5.08. Calcium chloride dehydrate (CaCl2⋅2H2O) was purchased from Alfa Aesar with 99% min purity and used as received. The water used in all experiments is Milli-Q water (18.2 MΩ·cm). Surfactant solutions were prepared by adding designed amount of EHSB directly in a vial. Then the desired amount of concentrated CaCl2 solutions was added to the vial with a micropipet. After sealing, the samples were vortex mixed and equilibrated at 40 oC for a few hours to ensure complete dissolution. The resulting homogeneous samples were stored at 25 oC in a thermostatted bath for 3 days prior to measurements. Determination of Krafft temperature. The TK of zwitterionic surfactants is normally determined by spectrophotometry23,24 and/or visual observation.25 Because spectrophotometric data is unsuitable for the solutions with solid precipitate, visual observation was used to determine the TK of EHSB in this work. The TK values were determined by heating 5 mL of EHSB surfactant solution in a sealed test tube until the solution suddenly cleared and the reproducibility of three times for the same 5


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sample was ± 0.1 oC. Rheology Measurements. Steady and dynamic rheological experiments were performed on a DHR rotational rheometer (TA Instruments, Newark, DE). The EHSB/Ca2+ samples were run on a parallel geometry plate (40 mm or 60 mm diameter). The temperature was controlled to 0.001 oC of the desired value. Samples were equilibrated at the desired temperature for at least 5 min prior to experimentation. A solvent trap was used to minimize water evaporation. Dynamic frequency spectra were conducted in the linear viscoelastic regime of the samples, as determined from dynamic strain sweep measurements. Dynamic Light Scattering (DLS). The aggregate sizes of the EHSB/Ca2+ solution 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 was analyzed via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the EHSB/Ca2+ aggregates, and then the apparent equivalent hydrodynamic radius (Rh) was determined by 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. Cryogenic Transmission Electron Microscopy (Cryo-TEM). The EHSB/Ca2+ samples were prepared in a controlled environment vitrification system (CEVS) at 22 oC，and the relative humidity was kept to 100% to prevent evaporation from the sample during preparation. A micropipet was used to load 2.9 µL EHSB/Ca2+ solution onto carbon-coated holey film supported by a copper grid, which 6


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was held by tweezers. The excess solution was gently blotted with a piece of filter paper to obtain a thin liquid film (20-400 nm) suspended on the mesh holes. After waiting for about 10s to relax any stresses induced during the blotting, the samples were rapidly plunged into a reservoir of liquid ethane cooled by the nitrogen at -180 oC 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 50000. 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. Scanning Electron Microscopy (SEM). The morphologies of the EHSB/Ca2+ aggregates were also imaged by a field-emission scanning electron microscope (SEM, Hitachi S-4800). All the EHSB solutions had the same Ca2+ concentration of 500 mM. In order to maintain the original structures and morphologies of the formed aggregates, the samples were prepared by quickly freezing a small drop of the EHSB/Ca2+ solution on a clean silica wafer with liquid nitrogen. Immediately afterward, the frozen samples were lyophilized under vacuum at about −50 °C. Finally, a 1-2 nm Pt coating completed the sample preparation. Fourier Transform-Infrared Spectroscopy (FT-IR). Spectra were obtained with a Bruker Optics TENSOR-27 FT-IR spectrophotometer. The EHSB/Ca2+ samples were prepared by quickly freezing the solution in liquid nitrogen and subsequently lyophilized under vacuum before FT-IR measurements. To study the hydrogen-bonding states of the amide moiety, the selected amide I band contour was subjected to the second derivative calculation following the Savitsky-Golay method and Gaussian curve-fitting by using OMINIC ver. 9.0 (Thermo Fisher Scientific). 7


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RESULTS AND DISCUSSION Effect of Calcium Ion on Solubility. Prior to discussing the effect of calcium ions (Ca2+) on the rheological properties of EHSB solution, we begin by investigating its dissolution behavior as a function of Ca2+ concentration. Because EHSB is a zwitterionic surfactant with a 2-hydroxyl propylene spacer between the opposing charges, inner salts are readily formed either within the same headgroup or between neighboring headgroups by electrostatic attraction between ammonium and sulfonate groups. Accordingly, its headgroup is weakly hydrated. Aside from this aspect, EHSB bears an ultra-long erucyl (C22) chain, which always results in poor solubility. These two factors result that the Krafft temperature (TK) of EHSB in pure water is extremely high. In view of the salting-in phenomenon of zwitterionic surfactants upon addition of salts,23 herein Ca2+ ions are introduced to enhance the dissolution ability and decrease the TK of EHSB. The effect of the Ca2+ concentration on the TK of EHSB solution is shown in Figure 1. The effect of Na+ is also present for comparison.

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Ca2+ (mM) Figure 1. Variations in the Krafft temperature (TK) of EHSB solution against Ca2+ concentration (black) and Na+ concentration (blue). As can be seen, the TK of EHSB in pure water is extremely high. Nevertheless, when a small amount of Ca2+ ions or Na+ ions is added, the TK of EHSB begins to decrease sharply and linearly. 8


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For Ca2+ ions, the TK of EHSB solution is decreased to 52.0 oC at the Ca2+ concentration of 50 mM, and further reduced to 25.5 oC with increasing the Ca2+ concentration to 68 mM. Fitting the experimental points to a linear relationship, and the slope of the fitted line is shown to 1.47. Such a linear decrease in TK of the EHSB solution is also observed in the case of Na+ ions. Chu et al.26 once reported that the TK of the long-chain amidosulfobetaines, n-DAS (n = 18, 20, 22, and 24) sharply decreases when the NaCl concentration increases from 0 to 100 mM. However, the slope of the fitted line for Na+ addition is much smaller than that for Ca2+ addition. This indicates that the ability of Ca2+ ions in depressing TK is higher than that of Na+ ions. Why do Ca2+ ions decrease the TK of the EHSB solution more sharply than Na+ ions? As mentioned above, adding inorganic salt could disrupt the inner salts by interacting with the oppositely charged moieties in the headgroups of zwitterionic surfactants. Herein, adsorption of Cl- ions (from CaCl2) close to the quaternary ammonium groups decreases the positive charge density of EHSB, which in turn causes the sulfonate groups to be less attracted by the ammonium groups, and thus causes the spacer between positive and negative charges to extend. Concomitantly, the sulfonate groups bind with Ca2+ ions. As a result, the ion binding enhances the hydration, i.e., water solubility of EHSB. Divalent Ca2+ ions carry more charges than Na+ ions, so Ca2+ ions show much stronger ability in reducing the TK of EHSB than Na+ ions. This result also implies the hydration free energy cations or water structure may also have an important effect on decreasing TK. This linear relationship of TK against the Ca2+ concentration serves as the basis to determine the Ca2+ concentration range for the subsequent investigations. Meanwhile, the temperature is chosen at 40 oC in view of enhancing the homogeneity of the EHSB solution in the Ca2+ concentration range studied. Exceptions can be found in cases where temperature effect is considered. 9


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Effect of Calcium Ion on Rheological Properties and Aggregate Structures. As noted above, the TK of EHSB solution in the absence of Ca2+ ions is extremely high owing to the ultra-long hydrophobic chain and the formation of inner salts in the headgroups. By the same token, the critical micelle concentration (CMC) of EHSB is very low, around 10-3 mM, and the binding of ions on zwitterionic micelles usually starts above the CMC and increases rapidly with concentration.4 Three different EHSB concentrations (0.10, 1.0 and 10.0 mM) in the dilute region are selected to investigate the influence of Ca2+ ions on the rheological properties of the EHSB solution. Figure 2 presents the elastic modulus G' and the viscous modulus G'' plotted against frequency ω, and the shear viscosity η against shear stress σ for the EHSB solutions at the EHSB concentration (CEHSB) of 0.1, 1.0 and 10.0 mM and at 40 oC. Figure 3 shows the shear stress σ of the EHSB solution against shear rate ߛሶ at different Ca2+ concentrations for CEHSB of 1.0 and 10.0 mM. These rheological measurements were performed over the Ca2+ concentration range of 80 ~ 500 mM. Clearly, the situation at lower CEHSB is different from that at higher CEHSB. At CEHSB = 0.10 mM, the viscous modulus G'' of the EHSB solution is larger than the corresponding elastic modulus G' (Figure 2a), and correspondingly the shear viscosity η increases with increasing the Ca2+ concentration at lower σ (Figure 2a'), then abruptly drops to very low values beyond a critical σ. However, at CEHSB = 1.0 mM and 10.0 mM, the elastic modulus G' is larger than the viscous modulus G'' (Figure 2b and 2c), which is further confirmed by the increase of the stress plateau values in the curves of the shear stress (σ) versus shear rate (ߛሶ ) (Figure 3), while the curves of shear viscosity η against shear stress σ (Figure 2b' and c') show a “Z” shape. Moreover, all the curves move up to higher shear viscosity with the increase of CEHSB. In order to understand the rheological results above, the size distribution and morphology of the aggregates in these EHSB solutions at different Ca2+ concentrations have been 10


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studied by DLS, Cryo-TEM and SEM, and the representative results are presented in Figures 4 and 5. Obviously, the results demonstrate that the EHSB solution exhibits two different changing patterns at lower and higher EHSB concentrations, respectively. Next we will discuss the effects of Ca2+ on the rheological behavior of these EHSB solutions by combining all these results. -2

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viscosity η (right) of the EHAB solutions at the EHAB concentration (CEHSB) of 0.10 mM (a, a'), 1.0 mM (b, b'), and 10.0 mM (c, c') at 40 oC. The elastic modulus G' is shown as diamonds, and the viscous modulus G'' is shown as triangles. The signals ω and σ are frequency and shear stress, respectively. 100

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Figure 4. Size distribution (top) and scattering intensity (bottom) of the EHSB solution at 40 oC and at different Ca2+ concentrations for CEHSB = 0.1 mM (a, a'), 1.0 mM (b, b'), and 10.0 mM (c, c').

Figure 5. Cryo-TEM and SEM images of the EHSB aggregates. For CEHSB = 0.1 mM, CCa2+ = 100 mM (a) and 500 mM (a', A). For CEHSB = 1.0 mM, CCa2+ = 100 mM (b) and 500 mM (b', B). For CEHSB = 10.0 mM, CCa2+ = 100 mM (c) and 500 mM (c', C).

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Firstly look at the situation at lower EHSB concentration of 0.1 mM. The EHSB solution at this concentration exhibits a Ca2+-dependent viscous response as shown in Figure 2a and 2a'. Figure 2a shows that, at each Ca2+ concentration, the viscous modulus G'' exceeds the elastic modulus G' and both of them exhibit a linear increase with frequency. When the Ca2+ concentration is increased from 80 mM to 500 mM, the viscous modulus G'' shows an increase, whereas the elastic modulus G' keeps almost unchanged. Figure 2a' shows that the viscosity curves exhibit shear-thinning property, and the zero-shear viscosity grows more than three times with increasing the Ca2+ concentration. This indicates that the EHSB solution becomes more viscous at higher Ca2+ concentration. The DLS results in Figure 4a and 4a' indicates that the hydrodynamic radius of the EHSB aggregates grow from ~1 nm to ~200 nm and the scattering intensity becomes stronger when increasing Ca2+ concentration from 80 mM to 500 mM. The Cryo-TEM images in Figure 5a and 5a' show that EHSB exists as threadlike micelles at both Ca2+ concentration of 100 mM and 500 mM. Moreover, the SEM image in Figure 5A shows a densely packed structure for these threadlike micelles at 500 mM Ca2+. Taken together, increasing Ca2+ concentration induces the size increase of the threadlike micelles and thus reinforces the viscous properties of the EHSB solution at lower concentration of 0.1 mM. Then turn to the situation for the EHSB solution at higher EHSB concentration of 1.0 and 10.0 mM (Figure 2b, 2b', 2c and 2c'). Different from the rheological response at 0.1 mM, the rheological curves display elastic responses over the Ca2+ concentration range studied. For the larger concentration, the elastic modulus G' of the EHSB solution exceeds its viscous modulus G'' and both the moduli are independent of frequency ω at each Ca2+ concentration (Figure 2b and 2c). Moreover, the yield stress (σp) in the viscosity curves is observed instead of zero viscosity ηo at the EHSB concentration of 1.0 and 10.0 mM (Figure 2b' and 2c'). The yield stress (σp) is the critical shear stress, above which the 13


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shear viscosity first undergoes a rapid decrease, and then a slight decrease at higher shear stresses.27 Typically, the frequency-independent moduli and the yield stress σp suggest that there is no relaxation of stress even at long time scales, which is generally seen in gel samples. Meanwhile, the variation trend of the elastic modulus G' is similar to that of the yield stress σp with the variation of the Ca2+ concentration. With increasing the Ca2+ concentration, either the elastic modulus G' or the yield stress σp becomes larger. The maxima of the elastic modulus G' and the yield stress σp almost grow by 7 times (Figure 2b and 2b'). The DLS results (Figure 4b, 4b', 4c and 4 c') and the Cryo-TEM (Figure 5b, 5b', 5c and 5 c') and SEM (Figure 5B and 5C) images indicate that EHSB forms gel consisting of very long wormlike micelles, exhibiting Ca2+-enhanced elastic gel-like behavior. The DLS results show two size distributions with different scattering intensity at each Ca2+ concentration. With increasing the Ca2+ concentration from 80 mM to 500 mM, the size distribution with a smaller scattering intensity shifts from Rh of 1 nm to 10 nm, while the size distribution with a stronger scattering intensity keeps at a constant Rh of 50 nm. In particular, comparing with the neatly arranged wormlike micelles formed at the Ca2+ concentration of 100 mM (Figure 5b, 5c), the Cryo-TEM images from the samples prepared at 500 mM Ca2+ (Figure 5b', 5c') clearly shows the existence of the entangled wormlike micelles. Besides, the SEM images reveal the presence of non-uniformly distributed pores with the diameter ranging from 10 µm to 100 µm in the three dimensional network formed by these wormlike micelles (Figure 5B, 5C). All the above results suggest increasing the Ca2+ concentration induces the increase of the entanglement degree of the network of wormlike micelles, which in turn increases the elasticity of the EHSB solution at higher concentration. The occurrence of the Ca2+-dependent elastic gel-like property can be confirmed by shear banding theory. Shear band is a transition between a homogeneous and a nonhomogeneous state of steady flow, 14


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which bears different shear rates coexisting in the fluid. In most of the cases, shear band is associated with a stress plateau in the shear stress (σ) versus shear rate (ߛሶ ) curve. Herein, the rheological results in Figure 3 illustrate there is a stress plateau at intermediate shear rate for the 1.0 and 10.0 mM EHSB solution at each Ca2+ concentration. Moreover, increasing the Ca2+ concentration induces the increase of the stress plateau values, which is in accordance with the results of the elastic modulus G' and the yield stress σp. Nevertheless, it should be emphasized here that small stress overshoot at the starting points of the plateau region takes place for the 1.0 and 10.0 mM EHSB solution over the studied Ca2+ concentration. In particular, for the 1.0 mM EHSB solution, the stress plateau regions are not completely horizontal, exhibiting a slight slope. The reason may be that the enhanced stress induces the fluid to become unstable and the shear rate begins to increase rapidly.28 In short, the shear banding theory supports the conclusion that the increase of the Ca2+ concentration reinforces the elastic gel-like behavior of the EHSB solution at higher concentration. Effect of Calcium Ion on Intermolecular Interactions. As mentioned above, increasing the Ca2+ concentration induces a rheological enhancement in the EHSB solution: the viscosity increases at lower concentration of 0.1 mM, while the elasticity increases at higher concentration of 1.0 mM and 10.0 mM. In general, the viscosity/elasticity increase in surfactant solution is the result of intermolecular interactions, such as the hydrophobic interaction and hydrogen bonding. Therefore, in order to evaluate the role of different intermolecular interactions in the Ca2+-induced rheological enhancement, FT-IR measurements are performed for the 0.1 and 10.0 mM EHSB solutions, as shown in Figures 6 and 7. Figure 6 shows the OH stretching and CH2 stretching region in the FT-IR spectra for the EHSB solutions. For the 0.1 mM EHSB solution, a broad band with relatively low area is observed at 3427 15


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cm-1 for the Ca2+ concentration of 100 and 500 mM (Figure 6a). This band is assigned to the stretching vibration of bonded OH groups compared with the IR spectrum for the EHSB itself. Meanwhile, the fact that this OH stretching band appears at the same wavenumber for both Ca2+ concentrations indicates the strength of hydrogen bonding between the OH groups does not change with the Ca2+ concentration. However, in the CH2 stretching region, bands arising from the antisymmetric (νas, CH2) and symmetric (νs, CH2) stretching vibrations are observed at different wavenumbers for both the Ca2+ concentrations: higher wavenumbers of 2925 and 2854 cm-1 for 100 mM Ca2+, and lower wavenumbers of 2920 and 2851 cm-1 for 500 mM Ca2+ (Figure 6b). It is well known that the CH2 stretching frequency can be used as a monitor for the conformational disorder of chains.29-32 The appearance at lower wavenumbers for higher Ca2+ concentration suggests a tight packing of the hydrophobic chains in aggregates induced by strong hydrophobic interaction. In addition, the EHSB molecule has a cis unsaturated bond, which implies a kink in the hydrophobic chain. Thus the compact hydrophobic chains may be accompanied by a significant reduction in gauche defects. As to the 10.0 mM EHSB solution, different situations are found for the two groups. The OH stretching band with relatively high area shifts from 3428 cm-1 to 3424 cm-1 with the Ca2+ concentration increasing from 100 to 500 mM (Figure 6a'). Such IR shift to the lower wavenumber implies more OH groups are participating in hydrogen bonding. The bands corresponding to the antisymmetric (νas, CH2) and symmetric (νs, CH2) stretching vibrations are observed at the same wavenumbers for both the Ca2+ concentrations (Figure 6b'). That is to say, the increase in the Ca2+ concentration does not enhance the hydrophobic interaction between the EHSB molecules at the EHSB concentration of 10.0 mM.

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EHSB

EHSB

(a')

3700

(b)

0.1 mM EHSB/100 mM Ca2+ 0.1 mM EHSB/500 mM Ca2+

Absorbance

Absorbance

(a)


3550

3400

3250


(b')

3100


2940 2920 2900 2880 2860 2840 2820

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 6. FT-IR spetra in the OH stretching region (a, a') and CH stretching region (b, b') of the EHSB solutions at the EHSB concentration of 0.1 mM (top) and 10.0 mM (bottom). (a)

2+

100 mM Ca

2+

(b)

(a')

500 mM Ca

2+

100 mM Ca

(b')

2+

500 mM Ca

Absorbance

Absorbance

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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1680 1660 1640 1620 1600 1680 1660 1640 1620 1600

1680 1660 1640 1620 1600 1680 1660 1640 1620 1600

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 7. Second-derivative spectra (upper) and curve fitting results (lower) of amide I band. At CEHSB = 0.1 mM, CCa2+ = 100 mM (a) and 500 mM (b). At CEHSB = 10.0 mM, CCa2+ = 100 mM (a') and 500 mM (b').

Table 1 Summary of curve fitting results for amide I band of 0.1 and 10.0 mM EHSB with 100 and 500 mM Ca2+. Wave-number and the band area are represented by υ% and A. EHSB + Ca2+ (mM)

0.1

100 500

10.0

100 500

Band of ordered bonded C=O amide groups υ% (cm-1)

1626 1634 1622 1634 1627 1636 1624 1635

Band of disordered bonded C=O amide groups

Aob

Aob/At

υ% (cm-1)

1.470

0.624

1.789

Band of ordered free C=O amide groups

Adb

Adb/At

υ% (cm-1)

1647

0.596

0.253

0.655

1647

0.705

5.398

0.653

1647

5.198

0.692

1647

Band of disordered free C=O amide groups

Aof

Aof/At

υ% (cm-1)

1662

0.208

0.088

0.258

1664

0.169

1.854

0.225

1661

1.611

0.214

1663

17


Adf

Adf/At

1671

0.083

0.035

0.062

1677

0.066

0.024

0.866

0.105

1676

0.143

0.017

0.578

0.077

1677

0.127

0.017

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Since amide I band is sensitive to the hydrogen bonds between C=O groups, curve fitting calculation is used to compute particular peaks for the EHSB samples at the Ca2+ concentration of 100 and 500 mM (Figure 7). The curve fitting results have been summarized in Table 1. As can be seen, for all the studied EHSB samples, the curve fitting calculation shows five bands: at about 1624, 1634 cm-1 and 1647 cm-1 characteristic of bonded C=O amide groups for ordered and disordered form, and at 1662 cm-1 and 1676 cm-1 characteristic of free C=O amide groups for ordered and disordered form. Further inspection of Table 1 reveals that, for either 0.1 mM or 10.0 mM EHSB solution, with decreasing the Ca2+ concentration from 500 to 100 mM, the relative areas of the bands corresponding to the vibrations of bonded C=O amide groups become larger, while those of the bands corresponding to the vibrations of free C=O amide groups become smaller. This confirms that at higher Ca2+ concentration, more C=O amide groups are engaged in hydrogen bonding. In brief, the addition of Ca2+ ions induces the enhancement in the hydrophobic interaction and hydrogen bonding of C=O amide groups at lower EHSB concentration, while induces the enhancement in the hydrogen bonding of OH groups and C=O amide groups at higher EHSB concentration. Normally, Ca2+ ions affect the solution properties of zwitterionic surfactants in aqueous environments through tuning the electrostatic interaction and hydrophobic interaction. Either interaction plays a dominant role depending on the relative concentration range of Ca2+ ions and surfactants. At lower surfactant concentration, the addition of Ca2+ ions increases their binding to the oppositely charged surfactant headgroups and hence the electrostatic interaction is decisive. While at higher surfactant concentration, Ca2+ ions induce a decreased solubility of the hydrophobic chains in water, and thus the hydrophobic interaction becomes predominant. For the present surfactant system, the Ca2+ effects on hydrophobic interaction and hydrogen bonding revealed by IR spectra can 18


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provide an understanding to the origin of Ca2+-induced rheological enhancement. For the 0.1 mM EHSB solution, the Ca2+ ions are in great excess, and hence the salt-effect induced hydrophobic interaction is stronger. Moreover, because of the relatively small surfactant concentration, only sparsely dispersed threadlike micelles are observed in the solution, which thus explains the poorly viscous properties of the EHSB solution. As the Ca2+ concentration increases, the salt-effect becomes increasingly prominent, which further decreases the solubility of the hydrophobic chains in water. The process in turn strengthens the hydrophobic interaction, leading to the compact packing of the EHSB hydrophobic chains accompanied by a significant reduction in gauche defects. Ca2+-induced structural rearrangement of hydrophobic chains has been previously reported by several research groups.33-36 In addition, the enhanced hydrophobic interaction promotes the hydrogen bonding between the C=O amide groups. Both enhancements in molecular interactions make the EHSB molecules pack much more closely in the aggregates. Thus, the viscosity increase of the EHSB solution at 0.1 mM EHSB occurs upon increasing the Ca2+ concentration. As to the 1.0 and 10.0 mM EHSB solutions, the surfactant concentration becomes large and the wormlike micelles are densely and neatly packed, which is responsible for the elasticity of the EHSB solution at higher concentration. Under this condition, the Ca2+-induced electrostatic interaction is decisive. As the Ca2+ concentration increases, the electrostatic binding of Ca2+ ions to sulfonate groups becomes stronger, thus the electrostatic repulsion between the sulfonate headgroups becomes weaker. Meanwhile, Ca2+ ions with two positive charges can simultaneously bind more than one sulfonate groups, forming salt bridges among adjacent headgroups. Salt bridges are previously proposed to describe the gelation of vimentin-Ca2+ binding.37 The presence of these relatively stable salt bridges not only reduces the effective headgroup area of the EHSB micelles but also helps stabilize the more densely packed C22 19


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chains. Meanwhile, the Ca2+ effects on electrostatic interaction also strengthens the hydrogen bonding of OH groups and C=O amide groups. Thus, the addition of Ca2+ ions makes the EHSB headgroups pack much more closely and increases the entanglement degree of the network of wormlike micelles, enhancing the elasticity of the EHSB solution at higher concentration of 1.0 mM and 10.0 mM. In conclusion, Ca2+ ions induce the enhancement in the viscosity or elasticity of the EHSB solution by facilitating a tighter packing of the headgroups and/or the ultra-long hydrocarbon chains. Effect of Calcium Ion on Thermal Transition. On the basis of Ca2+-induced elasticity enhancement of the EHSB solution at the concentration of 1.0 and 10.0 mM, the effect of Ca2+ ions on the thermal transitions of these samples are studied as follows. Here, the Ca2+ concentration is chosen at 100 and 500 mM, where the transition temperature of both solutions is studied by means of dynamic rheological measurements as shown in Figures 8 and 9.

(a)

10-2

10-3

10-4

(b)

G', G" (Pa)

10-2

10-1

G', G" (Pa)

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 oC 50 oC 55 oC 70 oC

10-2

10-1

10-3

10-4

100

40 oC 50 oC 55 oC 70 oC

10-2

ω (rad/s)

10-1

100

ω (rad/s)

Figure 8. The elastic modulus G' and the viscous modulus G'' of the 1.0 mM EHSB solution plotted against frequency ω with Ca2+ concentration of (a) 100 mM and (b) 500 mM at 40 oC, 50 oC, 55 oC and 70 oC. The elastic modulus G' is shown as diamonds, and the viscous modulus G'' is shown as triangles.

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10-2

10-1

100

10-2

100

o

(a)

40 C 55 oC

10-1

100 40 oC o 55 C

(b)

10-1

G" (Pa)

(a')

0.12 0.08 0.04

70 oC 85 oC

-1

10-2

(b')

0.16

G" (Pa)

10-2

G', G" (Pa)

10

G', G" (Pa)

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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G' (Pa)

10

-1

70 oC 85 oC

10

-1

10

0

10

85 oC

0.12 0.18 0.24 0.30 0.36

G' (Pa)

10-1

-2

0.08 0.04

0.00 0.07 0.14 0.21 0.28

10

0.12

1

70 oC 85 oC

10

-2

10

-1

ω (rad/s)

10

0

10

1

ω (rad/s)

Figure 9. The elastic modulus G' and the viscous modulus G'' of the 10.0 mM EHSB solution plotted against frequency ω. The Ca2+ concentration is 100 mM (a, a') and 500 mM (b, b') and the temperatures used are shown in the plots. The elastic modulus G' is shown as diamonds, and the viscous modulus G'' is shown as triangles. Solid lines and insets in (a') and (b') correspond to a Maxwell model fitting and Cole-Cole plots, respectively. Figure 8 shows the Ca2+ effect on the thermal transitions of 1.0 mM EHSB solution. As can be seen, for the Ca2+ concentration of both 100 and 500 mM, the initial EHSB solution at 40 oC exhibits gel-like behavior with the elastic modulus G' exceeding the viscous modulus G'' over the investigated frequency range. However, when the temperature is increased to 50 oC, different situations are observed for the Ca2+ concentration between 100 and 500 mM. At 100 mM Ca2+, the EHSB solution transforms into a viscous fashion with the viscous modulus G'' being larger than the elastic modulus G'. This indicates that a thermal transition from an elastic gel to a viscous fluid takes place at this temperature. Whereas at 500 mM Ca2+, the EHSB solution still exhibits gel-like behavior, and such a transition from elastic gel to viscous fluid occurs only when the temperature increases to 55 oC. That is to say, the transition temperature of the EHSB solution at 500 mM Ca2+ is higher than that at 100 mM Ca2+. While further increasing the temperature to 70 oC, both the viscous modulus G'' and the 21


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elastic modulus G' decrease at these Ca2+ concentrations. In brief, the thermal transition from elastic gel to viscous fluid is readily to take place in the 1.0 mM EHSB solution at lower Ca2+ concentration. Figure 9 presents the Ca2+ effects on the thermal transitions of the 10.0 mM EHSB solution. The situation is similar to that at 1.0 mM EHSB. The 10.0 mM EHSB solution exhibits gel-like response accompanied by a temperature-increased elastic modulus G' between 40 oC to 55 oC (Figure 9a, 9b), and then switches to viscoelastic response at higher temperatures (Figure 9a', 9b'). Moreover, the transition temperature is still higher at larger Ca2+ concentration. However, the EHSB solutions at both the Ca2+ concentrations are the Maxwell fluids confirmed by Cole-Cole plots (inset in Figure 9a', 9b'), where the elastic modulus G' and the viscous modulus G'' cross over at a critical frequency value ωc above the transition temperatures. Above the transition temperatures, the critical value ωc at 500 mM Ca2+ is smaller than that at 100 mM Ca2+. This indicates that the EHSB solution at higher Ca2+ concentration takes longer time to relax than it does at lower Ca2+ concentration. Besides, increasing temperature shifts the critical value ωc to higher frequency ω (Figure 9a'). That is to say, the EHSB micellar solution at higher temperature needs a shorter relaxation time. The thermal transitions above indicate that the Ca2+ concentration adjusts the transition temperature of the EHSB solution, but does not affect its changing pattern. With increasing temperature, either at low or high Ca2+ concentration, the EHSB solution undergoes three processes: an enhanced elasticity of gels, a transition from elastic gels to viscous liquids, and a decreased viscosity of liquids. These thermal transition processes are related to the molecular structure of the surfactant and its intermolecular interactions. Typically, on one hand, increasing temperature enhances the molecular motion and solubility of EHSB, and thus causes a breakup and/or transition to the corresponding aggregates. On the other hand, salt-out effect becomes prominent at higher 22


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temperature, which strengthens the hydrophobic interaction between the long chains of EHSB, and thus facilitates the formation and/or growth of aggregates. Thus, it is proposed that two factors are responsible for the thermal transition. In the lower temperature range, the increase of the elastic modulus G' by heating suggests that the enhanced hydrophobic interaction assists in the growth of wormlike micelles or the formation of entangled networks. In other words, the initial temperature increase contributes to the network elasticity of the EHSB solution by increasing the number density of wormlike micelles incorporated into the entanglement network. When the temperature is increased to transition points, the EHSB gels transfer into viscous or viscoelastic liquids, and the transition temperature is higher at larger Ca2+ concentration. The thermal transition from elastic gel to viscous liquid is generally accompanied with a reduction in the micellar length, which typically occurs in wormlike micellar solutions. This means that the higher temperature partly breaks up the micellar structures, and results in the formation of the viscous or viscoelastic fluid. Nevertheless, the fact that the presence of higher Ca2+ concentration shifts the transition point to higher temperature indicates the Ca2+-enhanced hydrophobic interaction weakens the breakup effect induced by temperature. Thus, for larger Ca2+ concentration, the thermal transition of the EHSB solution takes place at higher temperature, suggesting that the addition of Ca2+ ions reinforces the stability of the wormlike micelles and the formation of entangled networks. In addition, it should be emphasized here that the elastic modulus G' of the 10.0 mM EHSB solution exhibits only a slight drop when the temperature is increased above the transition point. The reason is that the ultra-long chain of the EHSB surfactant contributes to a particularly high energy cost for micellar breakup. Raghavan et al.20 once pointed out that the surfactant, erucyl dimethyl amidopropyl betaine, favors the formation of very long wormlike micelles. Despite the fact that the temperature increase leads to the decrease of micellar contour 23


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length, most wormlike micelles in the EHSB solution are presumably still long enough to entangle. Consequently, either the number density of entangled wormlike chains in the network or the elastic modulus G' only slightly decreases. Due to the same reason, further increasing the temperature further reduces the micellar contour length, and thereby reduces the viscosity in these viscous or viscoelastic fluids. In summary, upon the increase of Ca2+ concentration, both the solubility and rheological properties (viscosity or elasticity) of the EHSB solution are improved. Moreover the larger Ca2+ concentration helps to stabilize the gel-like structure of the EHSB solution at higher concentration. In order to clearly follow the Ca2+ effect on the solubility and rheological behaviors of the EHSB solution, the aforementioned results and mechanism are summarized by a schematic illustration in Figure 10.

Figure 10. Simplified model of the prevailing structural evolution of the EHSB solution with the increase of the Ca2+ concentration: (a) In the absence of Ca2+ ions, inner salts are formed within the same (or between neighboring) EHSB headgroups; (b) When an appropriate amount of Ca2+ ions is added to the EHSB solution, the inner salts are disrupted due to the fact that Ca2+ and Cl- ions weaken the electrostatic attraction between ammonium and sulfonate groups; (c) Further addition of Ca2+ ions induces more Ca2+ ions to bind with sulfonate groups or makes the hydrophobic chains rearrange. 24


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CONCLUSION The present work has systematically studied the Ca2+ effect on the solubility, aggregate structure and rheological behaviors of a C22-tailed zwitterionic surfactant EHSB in aqueous solution. Firstly, the solubility dependence of the EHSB solution on Ca2+ ions is studied in detail, and good solubility is achieved by adding a small amount of Ca2+ ions. Because each Ca2+ ion carries two charges, electrostatic binding between Ca2+ ions and the sulfonate groups of the EHSB surfactant is very strong. The strong electrostatic binding significantly disrupts the inner salts formed in the EHSB headgroups, increases their ionic hydration, and thus is the main controlling factor for the solubility enhancement. Next, upon increasing the Ca2+ concentration, the EHSB solution experiences a rheological enhancement: the viscosity increases at lower EHSB concentration and the elasticity increases at higher EHSB concentration. Moreover, increasing the Ca2+ concentration can raise the transition temperature from elastic gels to viscous liquids in the EHSB solution of higher concentration. These Ca2+-induced rheological changes can be elucidated on the basis of the Ca2+-induced enhancements in the molecular interactions: enhanced hydrophobic interaction between the long and stretched hydrocarbon chains as well as enhanced hydrogen bonding of C=O amide groups are responsible for the viscosity increase, while enhanced electrostatic interaction between Ca2+ ions and the sulfonate groups, and strengthened hydrogen bonding of OH groups and C=O amide groups lead to the elasticity increase. All these interactions promote the compact packing of the surfactant molecules in aggregates, and in turn promote the growth and entanglement of wormlike micelles. Thus the EHSB solution shows Ca2+-dependent rheological behaviors, forming elastic gels and viscous liquids. This investigation provides an example that the solubility and rheological properties of the ultra-long chain surfactant solution can be simultaneously improved 25


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with the addition of divalent Ca2+ ions. This implies that divalent cations can serve as a good trigger for these ultra-long chain surfactants with desired rheological characteristics in many practical applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. Dissolution Results for EHSB with Different Concentrations of Na+ and Ca2+ Ions.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.L.W.); [email protected] (Q.Z.). Author Contributions Xiuling Ji and Maozhang Tian contributed equally to this manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by Beijing National Laboratory for Molecular Sciences and National Natural Science Foundation of China (21633002). REFERENCE (1) Baptista, M. S.; Politi, M. J. Dipole Oriented Anion Binding and Exchange in Zwitterionic Micelles. J. Phys. Chem. 1991, 95, 5936-5942. (2) Cuccovia, I. M.; Romsted, L. S.; Chaimovich, H. Determination of Halide Concentrations at the 26


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Interface of Zwitterionic Micelles by Chemical Trapping: Influence of the Orientation of the Dipole and the Nature of the Cation. J. Colloid Interface Sci. 1999, 220, 96-102. (3) Baptista, M. S.; Cuccovia, I.; Chaimovich, H.; Politi, M. Electrostatic Properties of Zwitterionlc Micelles. J. Phys. Chem. 1992, 96, 6442-6449. (4) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Aqueous Solutions of Zwitterionic Surfactants with Varying Carbon Number of the Intercharge Group. 2. Ion Binding by the Micelles. Langmuir 1995, 11, 4234-4240. (5) Marte, L.; Beber, R. C.; Farrukh, M. A.; Micke, G. A.; Costa, A. C. O.; Gillitt, N. D.; Bunton, C. A.; Profio, P. D.; Savelli, G.; Nome, F. Specific Anion Binding to Sulfobetaine Micelles and Kinetics of Nucleophilic Reactions. J. Phys. Chem. B 2007, 111, 9762-9769. (6) Hu, W. Z. Studies on Behaviors of Interactions between Zwitterionic Surfactants and Inorganic Ions by Using an Ion Chromatographic Technique. Langmuir 1999, 15, 7168-7171. (7) Masudo, T.; Okada, T. Potentiometric and Chromatographic Evaluation of Ion Uptake by Zwitterionic Micelles. Phys. Chem. Chem. Phys. 1999, 1, 3577-3582. (8) Iso, K.; Okada, T. Evaluation of Electrostatic Potential Induced by Anion-Dominated Partition into Zwitterionic Micelles and Origin of Selectivity in Anion Uptake. Langmuir 2000, 16, 9199-9204. (9) Priebe, J. P.; Souza, B. S.; Micke, G. A.; Costa, A. C.; Fiedler, H. D.; Bunton, C. A.; Nome, F. Anion-Specific Binding to N-Hexadecyl Phosphorylcholine Micelles. Langmuir 2010, 26, 1008-1012. (10) Tondo, D. W.; Priebe, J. M.; Souza, B. S.; Priebe, J. P.; Bunton, C. A.; Nome, F. The Chameleon-Like Nature of Zwitterionic Micelles. Control of Anion and Cation Binding in Sulfobetaine Micelles. Effects on Acid Equilibria and Rates. J. Phys. Chem. B 2007, 111, 11867-11869. 27


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Effects of Calcium Ions on the Solubility and Rheological Behavior of a

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