pH-Induced Re-entrant Microstructural Transitions in Cationic

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pH-Induced Re-entrant Microstructural Transitions in Cationic Surfactant – Hydrotrope Mixtures Chinedu Dominic Umeasiegbu, Vemuri Balakotaiah, and Ramanan Krishnamoorti Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02211 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015

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pH-Induced Re-entrant Microstructural Transitions in Cationic Surfactant – Hydrotrope Mixtures Chinedu D. Umeasiegbu, Vemuri Balakotaiah, and Ramanan Krishnamoorti* Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004 Abstract The structural transitions occurring with change in pH for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal) were investigated at various temperatures using dynamic light scattering and smallangle neutron scattering. Direct structural studies show a transition from rigid cylindrical micelles at neutral pH to spherical micelles at ~ pH 2 upon protonation of salicylate molecules; however, an unanticipated reversion to flexible cylindrical micelles with further decrease in pH was observed. We also observed these microstructure transitions from cylinders at high pH to spherical micelles at intermediate pH to flexible cylindrical micelles at low pH were highly sensitive to temperature. Our results suggest that in addition to the well described electrostatic and hydrophobic interactions in cationic surfactant - hydrotrope mixtures, the pH-induced microstructural changes are potentially governed by complementary cation-π and hydrogen bonding interactions.

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Introduction Cationic surfactants, at modest concentrations above their critical micelle concentration, generally form spherical micelles in aqueous solution.1 These spherical micelles transform into wormlike micelles in the presence of inorganic (e.g., Cl- and Br-) or penetrating counterions called hydrotropes (e.g., salicylate or tosylate).1–8 Likewise microstructural change from spherical to cylindrical micelles can result from increasing the surfactant concentration or addition of oppositely charged surfactants.7,9,10 Hydrotropes, like surfactant molecules, consist of hydrophilic (either ionic or nonionic) and hydrophobic parts but the hydrophobic part is generally too small to cause spontaneous self-aggregation.11 Due to the hydrophobic part, hydrotropes bind more strongly to the headgroup region of the micelles compared to inorganic counterions, so that entangled wormlike micelles can form at low surfactant and counterion concentrations.6,7,12–15 Entangled wormlike micelles show a striking viscoelastic behavior characterized by increased viscosity and have found wide applications in drag reduction, enhanced oil recovery, personal care products and pharmaceuticals.4,7,16–19 However, the equilibrium microstructure in surfactant solutions is determined by a delicate balance of hydrophobic and electrostatic forces which causes the micellar structure to be a strong function of variables such as surfactant concentration, salinity, temperature or pH.7,17,20,21 The change in micellar structure and associated rheological behavior due to variation in pH has been of particular interest for the stimulation of heterogeneous carbonate reservoirs where injected acid preferentially flows into high permeability zones leaving lower permeability zones unstimulated.22,23 pH-sensitive selfassembled systems are thus employed to ensure total zonal coverage: as injected acid reacts 2 ACS Paragon Plus Environment

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with the carbonate resulting in pH increase, such systems induce gelation of the reacted acid diverting unreacted acid into lower permeability zones. The use of pH as a tool to control viscoelasticity has also been investigated in biological studies where pH control via low-toxicity amine-oxide surfactants is employed in controlling interactions with biomolecules.20 Brinchi et al.18 reported that viscoelastic wormlike micellar solutions of the amine-oxide surfactant p-dodecyloxybenzyldimethylamine oxide (pDoAO) reverted to a fluidlike solution when acid was added. However, most of these studies have involved the covalent binding of pH-responsive moieties such as amine or carboxyl functional group to a hydrocarbon backbone to render them pH-sensitive.24,25 These molecules are not only difficult to synthesize but offer little control for tailoring pH-sensitivity e.g., the pH at which gelation occurs is fixed upon synthesis. An alternative way of forming pH-responsive selfassembly systems is to exploit the electrostatic and hydrophobic interactions between cationic surfactants and hydrotropes.20,25 Cationic surfactants with a trimethylammonium head group form wormlike micelles at low concentrations in the presence of hydrotropes.3,12–15,20 Whereas inorganic counterions bind weakly to the micelle interface through electrostatic interactions occupying the double layer around the micelle, hydrotropes such as salicylate penetrate the micelle inserting its hydrophobic aromatic ring into the micelle core with its ionic group at the micelle interface and thus more effectively screening the electrostatic repulsion between head groups.12,13,26,27. With protonation of the ionic group with decrease in pH, the electrostatic screening is lost and thus resulting in a pH-responsive assembly.


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Although numerous studies have been undertaken to characterize the morphological and rheological changes in cationic surfactant - hydrotrope mixtures, little has been done with respect to pH.2,3,5,17,28 Even the well-studied cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal) assembly has no systematic studies on its pH-responsive behavior, especially as a function of temperature. Sakaiguchi et al.29 reported a qualitative change in viscoelasticity of aqueous solutions of CTAB and salicylic acid (SA) with pH. The changes in viscoelasticity were determined by observing decay and recoil of small bubbles in solution brought to rest after vigorous agitation. Lin et al.25 reported the formation of pH-responsive fluids when hydrotropes with a carboxyl functional group were introduced into a cationic surfactant solution. More recently, Rose et al.30 proposed pH-dependent changes in the micellar binding ability of phthalic acid as the factor regulating the reversible switching between liquid-like and gel-like states on adjusting the pH of cetylpyridinium chloride (CPC)/phthalic acid solutions observed in their work. However, none of these focused on understanding the microstructural changes and underlying molecular interactions that bring about the pH-responsive rheological behavior, a critical need to be able to predict and tailor pH-sensitivity for specific applications. Consequently, we investigated the pH-induced structural changes in aqueous CTAB/NaSal solutions through a combination of dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Our results show an unexpected microstructural behavior with changing pH and offers new insight into the molecular interactions controlling self-assembly in cationic surfactant/hydrotrope mixtures.


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Materials and Methods Materials. Cetyltrimethylammonium bromide (CTAB), deuterium oxide (D2O) with 99.9% isotopic purity and deuterium chloride (DCl) with 99% isotopic purity were obtained from Sigma-Aldrich; sodium salicylate (NaSal) was obtained from Alfa Aesar while hydrochloric acid (HCl) was obtained from Macron Chemicals. All chemicals were used without further purification. Sample preparation. Aqueous CTAB/NaSal solutions were prepared by appropriately adding DCl and HCl to D2O and deionized water, respectively in a 20ml disposable vial, which was repeatedly rinsed with deionized water and then oven dried, to adjust the solution pH† to the desired value. CTAB and NaSal were then added appropriately to obtain the desired CTAB concentration and NaSal-to-CTAB ratio. For DLS experiments, only 5mM CTAB solutions at a NaSal-to-CTAB ratio of 0.8 were prepared with D2O (and DCl). The solutions were allowed to equilibrate for an hour at 60 °C, then sonicated for 30 minutes in a low power ultrasonic bath at the same temperature to ensure complete dissolution and subsequently stored overnight at room temperature. Solutions for SANS experiments were prepared similarly but with D2O and DCl used for all samples to reduce incoherent scattering due to hydrogen. †

Note that pH is used interchangeably for both hydrogenated and deuterated solutions

to describe the hydrogen ion activity and the values reported were calculated from the negative logarithm of the acid concentration in the unit mol/L rather than measured. The calculated pH values were confirmed experimentally through measurements at room temperature (~ 25 °C) with a pH electrode. A pH of 5 is chosen for neutral solutions without added acid which was close to the measured values. Finally, for the sake of convenience and simplicity, we ignored the effect of temperature on pH as both the solution pH and salicylate pKa profiles with temperature 5 ACS Paragon Plus Environment

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are expected to be the same resulting in a constant degree of ionization of the salicylate molecules at the different temperatures.31 Dynamic light scattering (DLS). Dynamic light scattering measurements were performed on a Brookhaven Instruments goniometer (BI-200SM) equipped with a temperature controller, a digital correlator that calculates the photon intensity autocorrelation function and a Mini-L30 laser (wavelength λ=637.6 nm). The angle of scattering was fixed at 45° due to the very dilute samples and measurements were run for a duration of 1 minute and repeated 5 times to obtain an average intensity autocorrelation function, () = 〈( )( + )〉/〈( )〉 .32 To determine the relaxation time (τR) of the micelles from DLS measurements, we fitted the intensity autocorrelation function g2(τ) to the Siegert equation shown below and obtained the first-order autocorrelation function g1(τ).33 () = + () (1) We then obtained τR by assuming the first-order autocorrelation function could be described by the cumulant 2nd order expansion.33 () = exp (− ⁄ )1 + ( ⁄(2!)) (2) The second moment μ2 represents the polydispersity of the relaxation time distribution. τR is a characteristic decay time of the first-order autocorrelation function and correlates to the size of the scattering object. It is related to the hydrodynamic radius (RH) of the scattering object via the expression below: =

1

=

6 !"# , (3) $% &


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where q is the magnitude of the scattering vector defined as 4πn/λ(sinθ/2), n the refractive index of the solvent, λ=637.6 nm, θ = 45° the scattering angle, η the viscosity of the solvent, kB Boltzmann constant and T is the absolute temperature. Small-Angle Neutron Scattering (SANS). SANS measurements were made on the NG-7 30m beamline at NIST in Gaithersburg, Maryland.34 Samples were held in quartz cells with 4 mm path lengths and placed in a temperature-controlled chamber. Three sample-to-detector distances and incident neutron wavelengths (λ) of 6.0 Å, with a wavelength spread ∆λ/λ of 0.1, resulted in a q-range of 0.003-0.5 Å-1. Here, q is the magnitude of the scattering vector and is defined as 4π/λ(sinθ/2) where θ is the scattering angle. Complementary SANS measurements were performed on the NG-B 10m beamline (nSOFT) at two sample-to-detector distances and incident neutron wavelengths of 6.0 Å, with a wavelength spread ∆λ/λ of 0.13, to yield a qrange of 0.006-0.5 Å-1 The measured intensities were corrected for detector efficiency, empty cell scattering and dark background, and normalized to an absolute scale using the direct beam flux method documented by Barker et al.35 The two-dimensional scattering intensity data was then radially averaged to obtain one-dimensional I(q) vs q data sets and corrected for incoherent scattering background by subtracting out the constant scattering intensity due to the hydrogen in the sample. SANS Data Analysis: The coherent scattering intensity (Icoh(q)) for a solution of monodispersed interacting micelles can be expressed as36 )*+ () = ,- (.- − ./ ) 0- 1()2() (4)


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where φm denotes the volume fraction of micelles, ρm and ρs are the scattering-length densities of the micelle and the solvent respectively, and Vm is the volume of the micelle. P(q) is the single particle form factor, and S(q) is the inter-particle structure factor. For non-interacting micelles, S(q) ~ 1. The SANS intensity data can be separated into three principal regions each corresponding to a characteristic dimension of the scattering particle.37,38 The low-q (Guinier) region, reveals information on the overall size of the scattering particle and corresponds to the Guinier regime for both spherical micelles and cylindrical micelles and even larger structures such as vesicles, but special techniques such as USANS or SANS with lens are required to access it. In the intermediate-q region, the Guinier regime is generally captured for spherical micelles due to their smaller dimension, but not for cylindrical micelles except for very short cylinders. Thus, the intermediate-q region can be used to differentiate spherical and cylindrical micelles; I(q) scales as q0 for spherical particles, as q-1 for rigid cylinders while flexible cylinders scale as q-x, with 1 < x < 2.37,39 From this scaling, the particle shape can be determined in model-free fashion. Finally, the high-q region contains information regarding the interface between the assembled structure and the solvent. Model-dependent fitting of the intensity data can be done using functions which describe different structures.35,40,41 The form factor for non-interacting monodisperse spheres of radius R is given by (sin(") − " cos(")) 1/4+565 () = 73 > . (5) (")= For cylinders with radius r and length L, the form factor is given by


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1)ABCDE56 () = F

P⁄

Q

2H (I sin J) sinK(L cos J)⁄2M N sin J OJ , (6) G I sin J (L cos J)⁄2

where J1(x) is the Bessel function of order 1.

Results pH-induced structural transitions We begin by discussing the qualitative changes in the character of aqueous CTAB/NaSal solutions with changes in pH. We visually observed that the CTAB/NaSal solutions were initially (at neutral pH) viscoelastic but lost their viscoelasticity with addition of HCl or DCl. This suggests an underlying structural change with decrease in pH. To elucidate directly the changes in microstructure occurring with pH, we use DLS and SANS measurements. Due to the solubility at 25 °C in H2O (18mM) and D2O (10mM) of salicylic acid,42 formed on addition of HCl or DCl to the solution, we chose a CTAB concentration of 5mM to ensure complete dissolution. The pKa of salicylate molecules is 3.0 so that salicylate counterions increasingly exist as protonated salicylic acid molecules below this pH value. DLS studies were performed at 30 °C and the results are summarized in Figure 1 (intensity autocorrelation functions and the corresponding fits are shown in Supporting information, Figure S1), where the diffusional relaxation time (τR) of micelles in CTAB/NaSal solutions at 5mM CTAB concentration and a NaSal-to-CTAB ratio of 0.8 are shown for different pH values. It can be seen that there is a decrease in τR with reduction in pH as expected upon protonation of salicylate counterions. However, with further reduction in pH there is an unanticipated increase in τR.


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DLS studies at 50 °C (Figure 2) more clearly show this unusual microstructural behavior. τR decreases as pH is reduced and reaches a plateau at a pH value of ~ 2. The diffusional relaxation time (τR) is proportional to the hydrodynamic size of the diffusing micelle and thus the diffusional relaxation time of cylindrical micelles is expected to be significantly larger than that of spherical micelles.20,25 Since spherical micelles formed by surfactants have a constant radii equal to the extended length of the surfactant tail, we conjecture that the plateau corresponds to a transition from cylindrical micelles to spherical micelles. However, the unanticipated increase in τR of the micelles with further reduction in pH below 1 is significant and suggests a second structural change at lower pH. We verify both transitions from direct structural studies in later sections of this paper. The same trend was observed for deionized/hydrogenated water (H2O) solutions of CTAB/NaSal at different NaSal-to-CTAB ratios (CS/CD) and the results are summarized in Figure 3. For all three ratios of NaSal-to-CTAB, τR decreases as pH is reduced and reaches a plateau at a pH of ~ 1.5. It should be noted that with pH decrease, the plateau in τR at 50 °C for a NaSal-to-CTAB ratio (CS/CD) of 0.8 occurs at a slightly higher pH (~ 2.0) for the D2O solutions compared to the H2O solutions (~1.5). This is explained by the greater ionization constants of acids in H2O compared to D2O; the difference in pKa value, ΔpKa = pKa (in D2O) – pKa (in H2O), range from 0.2 < ΔpKa < 0.7.43 Thus protonation of the salicylate molecule occurs at a higher pH in D2O. The above results are particularly interesting as such microstructural behavior with pH is unique and challenges current understanding of pH-responsive surfactant self-assembly systems employed in reservoir stimulation.22,23


10

-3

10

-4

CS/CD = 0.8

T = 30 °C

0

1

2

3

4

5

pH Figure 1:

Diffusional relaxation time (τR) vs pH for aqueous 5mM CTAB solution in D2O at

CS/CD = 0.8 and at 30 °C. The broken lines are for better visualization.

Relaxation time, τR (s)

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-3

10

CS/CD=0.8

-4

10

T = 50 °C

0

1

2

3

4

5

pH Figure 2:

Diffusional relaxation time (τR) vs pH for aqueous 5mM CTAB solution in D2O at

CS/CD = 0.8 and at 50 °C. The broken lines are for better visualization.


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-3

10

1.2 1.0 CS/CD=0.8 -4

10

T = 50 oC

0

1

2

3

4

5

pH Figure 3:

Diffusional relaxation time (τR) vs pH for aqueous 5mM CTAB solution in

deionized water (H2O) at different NaSal-to-CTAB ratios (CS/CD) and at 50 °C. The broken lines are for better visualization. Direct structural studies of the micellar structures were performed through SANS measurements. Figure 4 shows the SANS intensity data for aqueous (D2O) 5mM CTAB solution at CS/CD = 0.8 with changing pH at 50 °C. The inset of Figure 4 provides the absolute coherent scattering data for four pH values and demonstrates qualitatively the transition from cylindrical to spherical micelles, indicated by the change from ~ q-1 to ~ q0 scaling at low q, as the pH reduces from 5 to 1. On further reduction in pH, the intensity at low q increases sharply and the q-dependence of the intensity is best captured as ~ q-1 indicating a reversion to cylindrical micelles. The coincidence of I(q) at high q for all pH values indicates a constant circular micellar cross-section.


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5

10

pH 5 pH 1 pH 0.3 pH 0

4 3

10

2

10

1

10

0

10

-1

10

-2

10

Icoh (q) (cm -1 )

-1

Icoh (q) (cm )

10

-3

10

1

10

0

10

-1

10

-2

10

0.01

-1

10

0.01

0.1

Å

q( )

-4

Figure 4:

pH 5 pH 1 pH 0.3 pH 0

-1

Å

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q(

0.1

-1

)

Rescaled SANS data for aqueous 5mM CTAB solution in D2O at CS/CD = 0.8 at 50

°C and at different pHs: (◇) 5, (□) 1, (○) 0.3 and (▽) 0. The spectra are shifted vertically by a factor of 20 with respect to the next below. The solid curves are fits to the data using the appropriate model. To quantify the structural data and the transitions between cylinders and spheres, we adopt two methods. First, we determined the slope of log I(q) vs log q over a q-range of 0.008 Å < q < 0.02 Å : () ~ ST

E(VWX Y)

J = − UE(VWX Z)[ for 0.008 Å < < 0.02Å (7)

This q-range in the data best captures the change in microstructure and thus modelindependent information regarding the micelle shape can be obtained. For quantification of the geometric parameters of the microstructure (Table 1), we turn to modeling the SANS data using appropriate form factor models. We neglect structure factor


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contributions i.e., we set S(q) = 1, as the concentration of CTAB used and the self-assembled structures were in the dilute regime. This assumption is validated by the absence of a peak in I(q) and an independence of the I(q) profile on CTAB concentration at pH values corresponding to dissociated salicylate (Supporting information, Figure S2). The model fits to the scattering data show a transition from cylindrical to spherical to flexible cylindrical micelles for aqueous 5mM CTAB solutions at CS/CD = 0.8 at 50 °C as the pH reduces from 5 to 0 (Figure 4). A radius of ~ 20 Å was obtained at all pH values corresponding to a constant micelle cross-section about equal to the extended length of the CTAB tail. The extracted radius polydispersity is similar to that obtained in previous SANS studies on CTAB solutions.44 The SANS data at the lowest pH values were best fit by flexible cylinders in agreement with the steeper than q-1 scaling. Due to the limited q-range of our SANS data, the cylinder length could not be accurately determined. However, the persistence length was obtained and our results show a decrease with pH indicating increased cylinder flexibility with decreasing pH. Effect of temperature on pH-induced structural transitions The structural transitions with pH described above based on SANS measurements are absent at 30 °C with cylindrical micelles present over the entire pH range. While the DLS results for CS/CD = 0.8 (Figure 1) suggests a transition to spherical micelles, direct structural SANS studies (Table 1 and Supporting information, Figures S3 and S5) exhibit no significant change in microstructure at 30 °C as pH is varied. We tried to fit the SANS data for the solution at pH 1 separately to models for spherical and cylindrical micelles but these fits were unsuccessful (Supporting information, Figure S4). The ellipsoid model which is an intermediate between a cylinder and sphere gave a better fit but with somewhat unphysical geometric parameters (e.g., 14 ACS Paragon Plus Environment

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radius polydispersity) and best serves to show that the transition between cylinder and sphere is continuous rather than first-order. Alternatively the data can be adequately modeled by a sum of sphere and rigid cylinder model which is supported by the visual observation of coexisting spherical and cylindrical micelles in cryogenic transmission microscopy (cryo-TEM) studies on cationic surfactant solutions.5,30,45 This coexistence can also be justified thermodynamically based on the Gibbs-Duhem phase rule for multicomponent systems and so is expected to occur both with changing pH and temperature.17 As such, the sum of sphere and rigid cylinder model was also employed at other temperatures for SANS intensity data with intermediate I(q) scaling (Supporting Information, Figure S6 and Table S1). In these fits of coexisting spherical and cylindrical micelles, we have kept the radius of the cylinder and spherical micelles constant in order to reduce the number of parameters to be fitted and obtain physically significant geometric parameters (volume fractions of sphere and rigid cylinders) reflecting the continuous transition between spherical and cylindrical micelles. 12,46 Figure 5 shows the I(q) scaling for aqueous 5mM CTAB solutions with a NaSal-to-CTAB ratio (CS/CD) of 0.8 at different temperatures. Cylindrical micelles are formed over the entire pH range at 30 °C. On the other hand, re-entrant microstructural transition with pH is clearly visible at 50 °C and 70 °C. Interestingly, at 80 °C we observe the formation of spherical micelles forming over the entire pH range. Cylindrical micelles formed in solutions with pH values corresponding to almost complete protonation of salicylate counterions, but which have a low solution ionic strength (HCl concentration) show a drastic transition with temperature increase becoming spherical micelles at 50 °C.


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α = −d(log I)/d(log q)

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T = 30 °C

1.0

60 °C

50 °C 0.5

70 °C

0.0

80 °C

0

1

2

3

4

5

pH Figure 5:

I(q) scaling vs pH for aqueous 5mM CTAB solution in D2O at CS/CD = 0.8 and

different temperatures. The broken lines are for better visualization.


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Table 1: Micellar parameters for aqueous 5mM CTAB solution in D2O at CS/CD = 0.8 at different pHs. Sphere/ Slope pH Model Radius, R (Å) Cylinder PD lp (Å) at low q fraction (%) Temperature = 30 °C 5

0.99 ± 0.01

Rigid cylinder

20.8 ± 0.1

0.17

2.7

1.01 ± 0.01

Rigid cylinder

19.9 ± 0.1

0.22

2.3

0.99 ± 0.01

Rigid cylinder

20.1 ± 0.1

0.22

2

1.01 ± 0.01

Rigid cylinder

20.4 ± 0.1

0.21

1

0.79 ± 0.01

Ellipsoid

1

0.79 ± 0.01

Sphere + Rigid cylinder

R1= 122.1 ± 2.3 R2= 23.8 ± 0.0 R3= 24 28.6 ± 0.7 R4= 20 71.4 ± 0.7

1.00 0.02 0.29 0.19

0.7

1.10 ± 0.01

Flexible cylinder 20.9 ± 0.1

0.13

409 ± 14

0.3

1.16 ± 0.01


0.15

357 ± 8

0

1.17 ± 0.01


0.15

309 ± 4

Temperature = 50 °C 5

0.98 ± 0.01

Rigid cylinder

20.3 ± 0.1

0.17

2.7

0.97 ± 0.01

Rigid cylinder

19.5 ± 0.1

0.20

2.3

-0.04 ± 0.01

Sphere

23.7 ± 0.1

0.24

2

0.02 ± 0.01

Sphere

21.2 ± 0.2

0.34

1

0.07 ± 0.02

Sphere

24.7 ± 0.1

0.17

0.7

0.16 ± 0.02

Sphere

24.4 ± 0.1

0.20

0.3

1.11 ± 0.01


0.12

326 ± 9

0

1.26 ± 0.01


0.12

239 ± 4

For ellipsoid model, R1 is the polar radius and R2 the equatorial radius. For the coexistence (sum of sphere and rigid cylinder) model, R3 is the sphere radius (fixed at 24 Å) and R4 is the rigid cylinder radius (fixed at 20 Å). Also for the coexistence model, the contour length of the rigid cylinder was fixed at 500 Å to yield physically significant volume fractions of sphere and rigid cylinders reflecting the continuous transition between spherical and cylindrical micelles. PD refers to the radius polydispersity and lp the persistence length of cylinder.


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Discussion Our structural studies on aqueous CTAB/NaSal solutions reveal that at intermediate temperatures cylindrical micelles are observed at neutral pH which transition to spherical micelles with decrease in pH but revert back to cylindrical micelles at very low pH. It is worth noting that there are a number of alternative and rather conflicting conjectures on the molecular interactions controlling the morphology of aqueous cationic surfactant solutions in the presence of hydrotropes.5,12,13,17,45,47–52 In what follows, we try to rationalize the observed microstructure changes with pH. We first consider the molecular interactions at play in NaSal/CTAB solutions in the absence of any added acid. It is well known that unlike inorganic counterions such as Cl- and Br-, that occupy the double layer around the micelle, the salicylate counterion (Sal-) locates at the micelle interface and effectively screens the electrostatic repulsion resulting in the formation of cylindrical micelles. As pH is reduced below its pKa, the salicylate counterions become protonated. At a pH of 2, only about 10% of salicylate molecules are ionized and the electrostatic screening is weak causing a transition to spherical micelle as observed for CS/CD = 0.8 at 50 °C. In the absence of salicylate counterions, stronger screening by Cl- seems a likely explanation for the reversion to cylindrical micelles with further decrease in pH of the solution. However, DLS measurements of aqueous 5mM CTAB solutions at low pH undermine this argument. At a pH of 0 corresponding to 1M HCl and at higher concentrations of HCl, in the absence of NaSal the DLS results suggest the formation of spherical micelles (Supporting information, Figure S7). This indicates that screening by Cl- is insufficient to explain the observation of cylindrical micelles for NaSal/CTAB solutions at pH 0.3 and 0. 18 ACS Paragon Plus Environment

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Similar re-entrant structural transitions observed for aqueous CTAB solutions in the presence of p-toluic acid, in which the micelle hydrodynamic diameter decreased with pH below the pKa of p-toluic acid (4.3) but increased below pH ~ 2, was explained in terms of increasing hydrophobicity of p-toluic acid molecules.48 We find this explanation unlikely as changing hydrophobicity of a hydrotrope has been shown to result in micelle growth in cationic surfactant solutions only when associated with stronger shielding of the electrostatic repulsion between headgroups such as in the case of decreased solubility and stronger binding of dibasic hydrotropes, with distinct pKa values, on switching from di-ionized to uni-ionized state with pH decrease.20,30 It has also been shown for solutions with large excess of anionic hydrotropes over cationic surfactant molecules resulting in a negatively charged micelle surface, in which growth of cylindrical micelles and a significant increase in viscosity occur with increasing temperature due to the increasing solubility of the hydrotrope in the bulk water and the consequent desorption of excess bound hydrotropes.53 We next consider the role of other intermolecular driving forces for surfactant selfassembly besides electrostatic and hydrophobic interactions such as cation-π interactions and hydrogen bonding.12,28 Cation-π interaction is the attraction between a cation and the electronrich π face of an aromatic ring and the electrostatic model is accepted as defining its fundamental nature. It has been shown to be quite significant and roughly on the same magnitude as cation-anion interactions in aqueous solutions, but is very sensitive to the geometry of the interaction; it is strongest when the cation is perpendicular to the π face of the aromatic ring.54–56 The cation-π interactions exist at all pH values probed here but become stronger with decreasing pH due to a change in the locus of solubilization of the salicylate 19 ACS Paragon Plus Environment

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molecules. We believe that ionized salicylate molecules are positioned in an inner location of the micelle-water interfacial region (palisade layer) for effective electrostatic interaction between the CTA+ headgroup and COO- group of the salicylate molecule. However, with decreasing pH and consequent protonation, salicylate molecules are positioned at an outer location at the micelle-water interface with the CTA+ headgroup perpendicular to their π face resulting in stronger cation-π interactions. This conjecture is supported by 1H NMR studies on aqueous CTAB/phenol solutions where it was observed that phenol locates in the outer micellewater interfacial region in neutral form, but occupies the palisade layer of the micelle at higher pH in its phenolate form.52 The constant micelle radius observed here with changing pH further supports this argument as it implies the salicylate molecule isn’t solubilized in the core but rather is positioned at the interface and thus capable of cation-π interaction. However, studies on the effect of neutral naphthols and methoxynaphthalene on CTAB micelles showed the formation of cylindrical micelles with the former but not the later points to the importance of the hydroxyl (OH) group of the salicylate molecules.50,51 1H NMR results indicate both naphthol and methoxynaphthalene insert into the CTAB micelle as expected due to their hydrophobic naphthalene rings, but UV absorption spectra measurements showed the protruded OH group of naphthol acts as a hydrogen bond donor in intermolecular hydrogen bonding with interfacial water unlike in methoxynaphthalene. Being a proton donor in the Hbond, electron density is released from the O-H bond towards the oxygen and thus towards the aromatic ring of naphthol. They hypothesized that this shift in electron density optimally orients the aromatic π-system of naphthol for stronger screening of the electrostatic repulsion between CTAB headgroup via cation-π interaction.50,51 This hypothesis seems plausible in 20 ACS Paragon Plus Environment

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explaining the temperature dependence of the pH-induced structural changes in CTAB/NaSal solutions observed here, which requires temperature-sensitive non-electrostatic intermolecular forces to rationalize. Hydrogen bonding is a highly sensitive function of temperature and has been shown to be inversely proportional to the square of the absolute temperature. On the other hand, cation-π interaction (primarily electrostatic in nature) is expected to be weakly temperature-dependent similar to electrostatic interactions.55,57 Thus, significant weakening of hydrogen bonding interactions results in a transition from cylindrical micelles at 30 °C to spherical micelles at 50 °C for solutions around pH 2 and 1 (9% and 1% ionization of salicylate molecules respectively and also at low ionic strength), where the microstructure depends greatly on non-electrostatic interactions. From Figure 5, we see that cylindrical micelles resulting from non-electrostatic interactions i.e at pH < pKa and low ionic strength (HCl concentration < 0.5M or pH > 0.3), show a drastic transition to spherical micelles with temperature increase. We have discussed the role of hydrogen bond strengthened cation-π interactions in the pH-induced structural transitions but this still does not completely explain the re-entrant transitions observed at intermediate temperatures (50 °C and 60 °C). The bulk aqueous concentration of Cl- increases significantly on reduction in pH; at 0.3 and 0, we have 0.5M Cland 1 M Cl- respectively. Although Cl- is much less effective at electrostatically screening CTAB headgroup repulsion and does not form cylindrical micelles even at 1M HCl (pH 0) in the absence of NaSal, we believe that in complement with hydrogen bonding strengthened cationπ interactions, it results to a reversal to cylindrical micelles at pH 0.3 (0.5M HCl) (Supporting information, Figure S7). Figure 6 summarizes the molecular forces resulting in the pH-induced 21 ACS Paragon Plus Environment

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transitions occurring in CTAB/NaSal system. A re-entrant region which represents the regime of spherical micelles is enveloped by cylindrical micelles existent at high pH and low pH. The reentrant transition though initially absent at 30 °C due to stronger hydrogen bonding interactions, appears at 50 °C with increasing temperature but spherical micelles are eventually formed in almost the entire pH range probed at high temperatures. Finally, SANS results revealed flexible cylindrical micelles in low pH solutions. Light scattering studies by Imae et al.1 on aqueous CTAB solutions in the presence of 0.5M NaBr showed the cylindrical micelles formed were considerably flexible with flexibility increasing slightly with NaBr concentration. This supports our observation of flexible cylindrical micelles at very low pH which corresponds to a high concentration of Cl-. On the basis of the surfactantpolyelectrolyte analogy, the persistence length of the cylindrical micelles can be described as the sum of an intrinsic and an electrostatic contribution: lp=lp0+lpe.58 The intrinsic persistence length lp0 is due to the micelle cross-section and is ~ 90 Å while the electrostatic persistence length lpe is dependent on the Debye length ĸ-1 of the system and thus the ionic strength I (the concentration of free counterions). The Odijk-Skolnick-Fixman (OSF) theory58 predicts this dependence as ~ (ĸ-1)2. The Debye length depends inversely on ionic strength as ~ (I)-0.5 so that lpe ~ (I)-1. Assuming the acid concentration as the ionic strength (I), the OSF theory underpredicts the persistence length of 5mM CTAB solution in D2O at CS/CD = 0.8, Temperature = 30 °C and pH = 0 (1M HCl) based on those for pH 0.7 and pH = 0.3 yielding ~150 Å and ~225 Å respectively compared to ~310 Å (Table 1). However, the theory qualitatively captures the increasing flexibility of cylindrical micelles with increasing ionic strength/decreasing pH at both 30 °C and 50 °C. Previous SANS studies on aqueous CTAB/NaSal solutions revealed dilute CTAB 22 ACS Paragon Plus Environment

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solutions formed rigid cylindrical micelles at 35 °C in the presence of sodium salicylate at concentrations as high as 300mM NaSal.4 This supports the absence of flexibility at higher/neutral pH observed here and points to the contrasting effects of nonpenetrating and penetrating counterions on micellar flexibility. It might also explain the under-predicted persistence length at pH 0, but a more detailed investigation perhaps at higher CTAB and NaSal concentrations would be needed to verify this.4,58

Figure 6:

Schematic representation of the microstructures formed in aqueous CTAB/NaSal

solutions with changing pH and the underlying molecular forces in play. Conclusions The microstructural changes with pH in aqueous CTAB/NaSal solutions were studied through a combination of DLS and SANS.

Direct structural studies through SANS at

intermediate temperatures confirmed the formation of cylindrical micelles in these solutions at neutral pH which transition to spherical micelles with decrease in pH as expected due to 23 ACS Paragon Plus Environment

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protonation of the salicylate counterion. However, we observed a re-entrant transition in which these spherical micelles became flexible cylindrical micelles with further reduction in pH. Interestingly at low temperatures we see only the formation of cylindrical micelles at all values of pH. On the other hand at the highest temperature, we observe only the formation of spherical micelles at all pH values. We rationalized this temperature dependence by a weakening of the hydrogen-bond strengthened cation-π interactions, which play an important role in cylindrical micelle formation in the absence of ionized salicylate molecules. Overall this work brings new fundamental insight into the driving molecular forces behind wormlike micelle formation in cationic surfactant/hydrotrope solutions, showing in addition to the hydrophobic effect and electrostatic interactions, the importance of hydrogen bonding and cation-π interactions. Further investigations might provide deeper insights into the molecular mechanisms through which hydrogen bonding and cation-π interactions contribute to the equilibrium microstructure and associated rheological behavior of cationic surfactant/hydrotrope mixtures. Author Information *Author to whom correspondence should be addressed. Email: [email protected]. Acknowledgements The authors thank Halliburton for providing financial support. This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0944772. We acknowledge the support of the National Institute of Standards and Technology, U.S.


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Department of Commerce, in providing the neutron research facilities used in this work. We are also grateful to Dr. Butler (NIST) for insightful discussions and Dr. Ashkar (NIST) for help in acquiring SANS data relevant to this work. Supporting Information Available: Intensity autocorrelation data at different pH values and their fits to models are presented. SANS intensity data validating our assumption of negligible intermicellar interactions and coexistence of sphere and rigid cylinder model, at a temperature T = 30 °C. Table of geometric parameters from model fits of SANS data at other temperatures. DLS data showing comparative microstructure in the absence and presence of sodium salicylate in CTAB solutions. This material is available free of charge via the Internet at http://pubs.acs.org. References

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pH-Induced Re-entrant Microstructural Transitions in Cationic

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