Molecular Variations in Aromatic Cosolutes: Critical Role in the

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Molecular Variations in Aromatic Cosolutes: Critical Role in the Rheology of Cationic Wormlike Micelles Thiago H. Ito,† Paulo C. M. L. Miranda,‡ Nelson H. Morgon,† Gabriel Heerdt,† Cécile A. Dreiss,§ and Edvaldo Sabadini*,† †

Department of Physical-Chemistry, Institute of Chemistry and ‡Department of Organic-Chemistry, Institute of Chemistry, University of CampinasUNICAMP P.O. Box 6154, 13084-862, Campinas, SP Brazil § Institute of Pharmaceutical Science, King’s College London, 150 Stamford Street, SE1 9NH London, U.K. S Supporting Information *

ABSTRACT: Wormlike micelles formed by the addition to cetyltrimethylammonium bromide (CTAB) of a range of aromatic cosolutes with small molecular variations in their structure were systematically studied. Phenol and derivatives of benzoate and cinnamate were used, and the resulting mixtures were studied by oscillatory, steady-shear rheology, and the microstructure was probed by small-angle neutron scattering. The lengthening of the micelles and their entanglement result in remarkable viscoelastic properties, making rheology a useful tool to assess the effect of structural variations of the cosolutes on wormlike micelle formation. For a fixed concentration of CTAB and cosolute (200 mmol L−1), the relaxation time decreases in the following order: phenol > cinnamate> o-hydroxycinnamate > salicylate > omethoxycinnamate > benzoate > o-methoxybenzoate. The variations in viscoelastic response are rationalized by using Mulliken population analysis to map out the electronic density of the cosolutes and quantify the barrier to rotation of specific groups on the aromatics. We find that the ability of the group attached to the aromatic ring to rotate is crucial in determining the packing of the cosolute at the micellar interface and thus critically impacts the micellar growth and, in turn, the rheological response. These results enable us for the first time to propose design rules for the selfassembly of the surfactants and cosolutes resulting in the formation of wormlike micelles with the cationic surfactant CTAB.

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INTRODUCTION Wormlike micelles (WLMs) are one of several possible selfassembled morphologies adopted by surfactants; their formation can be rationalized by the critical packing parameter (cpp),1 which considers geometrical features of the surfactants to determine the optimal shape of the aggregate into which they can pack. The elongation of aggregates into flexible cylindrical micelles can reach contour lengths on the order of micrometers2,3 Their remarkable, tunable viscoelastic characteristics has led to their exploitation from personal care products to oil exploration fields.3−7At low concentration, these giant structures can be used as a mechanical and nondegradable hydrodynamic drag-reducing agent.5,8−10 One of the most widely studied WLM system is based on the combination of the cationic surfactant cetyltrimethylammonium bromide (CTAB) with the aromatic cosolute o-sodium salicylate.3,11 The unidimensional elongation of the micelles can be obtained at high ionic strength with common ions, but at low concentration, the process is strongly favored by using aromatic ions such as salicylate, which is classified as a strongly binding counterion. This is because, besides the charge neutralization of the headgroups, which brings them closer together, the insertion of the aromatic anions into the palisade layer, driven by hydrophobicity, reduces the cpp and consequently increases micellar curvature.12 However, this © 2014 American Chemical Society

sphere-to-rod transition is highly sensitive to the substitution pattern of the aromatic anion. For example, the self-assembly structures formed by cationic surfactants are strongly affected when different isomers of salicylate13 and chlorobenzoate14 are used. This has been explained by considering the microenvironment of the counterions at the interface: in the case of salicylate, the hydroxyl group at the para position leads to an unfavorable environment, while this is improved with the meta substitution, the counterion being tilted; therefore, the most favorable orientation at the aqueous interface is obtained with the osalicylate.15 Another interesting packing scenario to mention is that of the responsive CTAB micelles formed by the addition of omethoxycinnamate (OMCA).7,11,16 The transition from transto cis-OMCA induced by UV light markedly affects the packing of the counterions at the micellar interface, leading to a shortening of the micelles and remarkable variations in viscosity.11,16 This system is an interesting example demonstrating the possibility of switching macroscopic properties via a simple trigger that controls the molecular detail of the constitutive units. Received: July 4, 2014 Revised: August 29, 2014 Published: September 15, 2014 11535

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Clearly, subtle variations in the architecture of the aromatic molecule affect the interaction between the surfactant and the aromatic salt and, thus, the packing of the amphiphiles within the aggregates and, as a result, the rheological properties. Therefore, rheology is a powerful technique to investigate the influence of systematic structural changes in the aromatic cosolutes on the propensity to form wormlike micelles and their properties. While the rheology of CTAB micelles with a range of counterions has been extensively studied,3 the precise role of the molecular structure of the aromatic cosolutes in dictating the packing at the interface is not well-understood. The objective of this work is therefore to rationalize the impact of the architecture of the aromatic cosolutes on the surfactant assemblage into wormlike micelles or otherwise and therefore their rheological response. More specifically, the following parameters are examined in benzoate and cinnamate derivatives and phenol: the distance between the aromatic ring and the carboxyl group, the substitution or otherwise of the hydroxyl group by a methoxy group or hydrogen, and the absence of charge in the aromatic cosolute. Their impact on micellar self-assembly is studied by linear rheology and small-angle neutron scattering (SANS) measurements, and the results are rationalized by mapping out the electronic density of the cosolutes and analyzing the barrier to rotation of specific groups on the aromatics using computational chemistry.

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Scheme 1. Molecular Structure of the Various Aromatic Cosolutes Studied, Together with Values of the Relaxation Time and Plateau Elastic Moduli Obtained from the Maxwell Model for 200 mmol L−1 Equimolar Mixtures of CTAB and Cosolutea

EXPERIMENTAL SECTION

Materials. The surfactant CTAB was obtained from Sigma-Aldrich. The aromatic cosolutes (Scheme 1) were obtained from Sigma-Aldrich [OMCA, cinnamic acid, o-methoxybenzoic acid (OMBA), and 3phenylpropanoic acid], Merck (sodium salicylate and phenol), and Synth (sodium benzoate). Sodium o-hydroxycinnamic acid (OHCA) was synthesized (as described in the Supporting Information). The salts of the cosolutes were prepared from their acid form by neutralizing the respective THF solutions with NaOH, followed by the lyophilization of the solvent. Phenol was purified by dissolving the samples in toluene and maintaining contact with CaSO4 for 8 h. The process was repeated three times and the phenol was then recrystallized at low temperature.17 Solutions of WLMs were prepared using ultrapure water (18 MΩ cm) and then heated at 75 °C for 1 h before being cooled gradually to room temperature. Rheological Measurements. Oscillatory experiments were conducted on a Haake RS1 rheometer equipped with a water bath and plate−plate sensor (35 mm diameter and 1 mm gap). The stress applied was 3 Pa (which was chosen well within the linear viscoelastic range). The temperature was maintained at 25 °C. In order to minimize evaporation, the measurements were conducted using a solvent trap. Small-Angle Neutron Scattering Measurements. SANS experiments were carried out on LOQ at ISIS (Rutherford Appleton Laboratory, Didcot, UK). The instrument uses incident wavelengths from 2.2 to 10 Å, sorted by time-of-flight, with a fixed sample-to-detector distance of 4.1 m, providing a q-range between ∼0.007 and 0.28 Å−1. Samples holders were 1 or 2 mm thick quartz cuvettes. All the systems contained an equimolar concentration (100 mmol L−1) of surfactant and cosolute in D2O. All scattering data were first normalized for sample transmission and then background-corrected using the pure solvent (D2O). The data were then converted to the differential scattering cross sections using the standard procedures at ISIS.18 SANS data were fitted with the SasView software,19 using the cylinder model. This model considers WLM as composed of a sequence of rigid cylinders (Figure 1), with cross-section radius r and length lp, which for long structures whose contour length falls outside the detection limit of neutrons can be assimilated to a persistence length (Figure 1). The scattering length density of the wormlike micelles was calculated as a volume-weighted average, assuming an equimolar mixing of the CTAB molecules and cosolutes.

a

The arrows highlight the comparative features under study: (1) The presence of an o-hydroxyl group on the aromatic ring; (2) the presence of additional carbons between the ring and carboxylate group; (3) the substitution of the o-hydroxyl group by a methoxy group; and (4) the presence or otherwise of a double-bond in the cinnamate derivative.

Figure 1. Schematic representation of wormlike micelles formed by smaller rigid cylinders with persistence length lp and radius r as seen by small-angle neutron scattering measurements.

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RESULTS AND DISCUSSION The vials in Figure 2 contain equimolar concentrations of CTAB and aromatic cosolutes (200 mmol L−1). The pale yellow color of the solutions formed with OHCA and OMCA is due to the weak light absorption of these two organic cosolutes in the blue region of the electromagnetic spectrum. The vials were inverted at the same time to show the variations in retardation of flow under 11536

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Figure 2. Gravitational effect on gels containing 200 mmol L−1 of CTAB and 200 mmol·L−1 of (1) phenol, (2) cinnamate, (3) OHCA, (4) salicylate, (5) OMCA, (6) benzoate, and (7) OMBA. The values (on the left) correspond to the time of observation after inversion of the vials.

gravitational force for the range of cosolutes studied. They are positioned in a sequence that follows their flow behavior, from more solidlike to more liquidlike (left to right): phenol > cinnamate > OHCA > salicylate > OMCA > benzoate > OMBA. Viscosity measurements (Supporting Information) performed on a range of equimolar solutions (30−200 mmol L−1) confirm wide variations in the rheological response, which follow the trend observed visually (Figure 2). Interestingly, we note that in the phenol/CTAB mixtures that show the strongest solidlike behavior (Figure 2), the viscosity drops very rapidly with dilution: the samples lose their viscoelastic characteristics at concentrations below 100 mmol L−1. In this respect, phenol departs from the behavior of the other cosolutes, a point which is discussed later on. These results clearly demonstrate that small variations in cosolute architecture lead to very different macroscopic responses in mixtures with CTAB. A notable observation is that a liquid solution is formed with OMBA (Figure 2), which differs from salicylate by a methoxy group substituting the ohydroxyl group or from OMCA by a closer proximity of the carboxylate group to the ring (Figure 2, Scheme 1). At the opposite extreme, the strong gel-like characteristics obtained with phenol are very surprising considering its lack of electric charge (the negative charge on all other cosolutes provides a drive for the insertion of CTA+ cations in the micelles). On the basis of these observations, the formation of wormlike micelles, and thus the viscoelastic properties resulting from their entanglement, is affected by the following features of the aromatics:

(I) The presence or otherwise of a negative charge. (II) The presence of a hydroxyl group or its methoxyl variant. (III) The presence of two additional carbons between the aromatic ring and the carboxylate group. Understanding how these features impact the molecular packing of the cosolutes, and thus impact the rheological response, is the objective of this study. For salicylate/CTAB2,20,21 and OMCA/CTAB11,16 systems, it is well-documented that the high viscoelasticity is associated with the entanglement of the WLM chains. Although WLMs can be compared with polymeric system, their rheology is unique because, in contrast to a covalent polymer, the assemblies formed by the surfactant and aromatic additive are constantly breaking and re-forming. Two excellent reviews on the structure and rheology of wormlike micelle solutions have been published by Cates and Fielding22 and Berret.23 The dynamics of the WLM chains is described by a combination of reptation and breaking processes, which are characterized by two relaxation times, τr and τb, respectively. Cates and Candau demonstrated that the overall relaxation time, τR, can be obtained by eq 2,24 if tr ≪ tb τR =

(1)

τrτb

(2)

In this situation, WLMs behave as a Maxwellian fluid, and the elastic modulus G′ and the viscous modulus G″ are given by eqs 3 and 4, respectively 11537

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Figure 3. Dynamic moduli as a function of oscillation frequency, at 25 °C, for equimolar mixtures of CTAB and the different aromatic cosolutes studied (200 mmol L−1): (A) derivatives of benzoate, (B) derivatives of cinnamate, and (C) phenol. The lines correspond to fits to the Maxwell model.

G′(ω) = G0

(ωτR )2 2

1 + (ωτR )

G″(ω) = G0

(3)

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(ωτR ) 1 + (ωτR )2

(4)

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where G0 is the plateau modulus (at high frequency) and τR the inverse of the frequency at which G′ and G″ crossover. The rheological behavior of the systems formed from equimolar solutions of CTAB and the aromatic cosolutes is shown in Figure 3. For ease of comparison, the graphs have been grouped according to their structure: derivatives of benzoate (A), derivatives of cinnamate (B), and phenol (C). The Maxwell model appropriately describes the behavior of the systems. The values of τR and G0 obtained from fitting the rheological curves at 25 °C are given in Scheme 1. These values correlate well with the observations of the vials’ inversion (Figure 3), with τR decreasing in the following order: phenol > cinnamate > OHCA > salicylate > OMCA > benzoate > OMBA. The value of τR for OMBA is too short to be measured. Under gravitational stress, the WLM chains lose their mechanical energy, mainly by reptation. In this case, the gravitational force is applied continuously and is equivalent to a frequency tending to zero. If the frequency of the stress, ω, is increased, as during a rheological experiment, some elastic energy is stored, since reptation becomes more restricted. Therefore, G′ progressively increases, crossing the G″ curve, and the behavior of both curves are described by the Maxwell model (eqs 3 and 4). Our results demonstrate that τR is very sensitive to the molecular structure of the aromatic cosolute. According to the reptation-reaction kinetics model of Cates and Candau,24 the reptation and breaking times depend on the average contour length of the micelle (L̅ ), and the corresponding relaxation times are given by the following scaling laws (see, for example, ref 23): τr ∼ L̅ 3c 3/2

τb ∼

c L̅

Figure 4. Mulliken electrostatic potential distribution for (A) salicylate and (B) OHCA. The hydrogen bond formed between the carboxyl and hydroxyl groups can be clearly observed in salicylate. (C) Barriers for the rotation (considering the dihedral angle) of the carboxyl groups of benzoate and salicylate.

(5)

Next, the effect of additional carbons between the ring and the carboxyl group is considered for all cosolutes (Scheme 1, comparative feature 2). This can be assessed by comparing the aromatic cosolutes above each other in rows 1 and 2 of Scheme 1. For instance, cinnamate has two additional carbons between the carboxyl and the aromatic ring compared to benzoate, and so does OHCA compared to salicylate, and OMCA compared to OMBA. In all cases, the two additional carbon atoms induce a higher relaxation time. Thus, in mixtures with CTAB (see also Figure 2), they must favor WLM growth by a better incorporation of the aromatic cosolute at the interface. Finally, if we inspect the effect of substituting the hydroxyl group by a methoxy group at the ortho position (comparison 3 of Scheme 1, salicylate vs OMBA, and OHCA vs OMCA), in both cases, this results in a drastic drop of the rheological properties; we note in particular that WLMs formed with OMBA are too short in comparison those formed with the other cosolutes (the samples remain liquidlike). In order to investigate the correlation between the carboxyl/ hydroxyl group distance and the mechanisms of wormlike micelle formation, we have calculated the Mulliken electrostatic potential distributions of the aromatic cosolutes (details of the method are given in the Supporting Information). This method, which is based on the electronic density division in individual atomic charges, gives a quantitative description of molecular properties, such as chemical reactivity and molecular interactions.27 Figure 4A shows that a hydrogen bond is formed between the carboxylate and hydroxyl group in salicylate, while in OHCA the distance between those two groups prevents the formation of such a bond. In salicylate, therefore, the presence of this intramolecular interaction hinders the rotation of the carboxyl

(6)

By combining with eq 2, τR ∼ L̅ . Therefore, for a fixed concentration of CTAB, τR can be directly correlated to the average contour length of the micellar systems containing CTAB and the aromatics. In other words, τR can be used as a criterion to discuss the interaction between CTAB and the cosolutes and their incorporation in the palisade layer, leading to the formation of longer WLMs. On the basis of the evolution of τR (in decreasing order), cinnamate > OHCA> salicylate > OMCA > benzoate, some governing rules based on the cosolute structure can be established (Scheme 1). The case of phenol is treated separately. First, the presence (or not) of a hydroxyl group at the ortho position (comparison 1 in Scheme 1) is considered. On the basis of the stronger viscoelastic response obtained for salicylate compared to benzoate (Figures 2 and 4, Scheme 1), it can be inferred that the presence of the hydroxyl group at the orthoposition promotes a better positioning of the cosolute at the micellar interface, favoring electrostatic interaction between the carboxyl and the cationic heads of the surfactants and, thus, driving the elongation of the wormlike micelles. This is consistent with the interpretation described previously for osalicylate.12 In aromatics bearing a hydroxyl group, the distance between the carboxyl and hydroxyl groups may be a key to rationalize these results, for instance, when comparing salicylate with OHCA (comparison 2, Scheme 1). The geometry of osalicylates favors the formation of a hydrogen bond between these two groups, avoiding the free rotation of the carboxyl group, thus halting the aromatic anion in the planar form and favoring its packing within the micelle palisade.25,26 11539

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group, thus ensuring a better packing of the salicylate in the micellar palisade. In benzoate, obviously, such a bond does not exist. Hence, if we refer back to comparative feature 1 in Scheme 1, the higher viscoelasticity of salicylate compared to benzoate can now be attributed to the existence of a hydrogen bond in salicylate between the carboxylate and hydroxyl groups. In benzoate, the steric hindrance caused by the rotation of the carboxyl group at the compacted surface of the WLM is likely to reduce the stabilization of the micelle, while in salicylate the existence of the hydrogen bond favors the incorporation of the salicylate inside the micelles, compared to benzoate. As shown in Figure 4C, the barriers for the free rotation of the carboxylate (dihedral angle) are low for benzoate (≈25 kJ mol−1) compared to salicylate (≈75 kJ mol−1). For OHCA, no hydrogen bond can be formed due to the larger distance between the hydroxyl and carboxylate groups (Figure 4B). However, the relaxation time for OHCA is longer in comparison with the system formed with salicylate. This means that another relevant structural effect in the aromatic explains the favorable incorporation of OHCA at the micelle palisade (this is discussed in the next paragraph). Finally, if the hydroxyl is substituted by a methoxyl group (feature 3 in Scheme 1, OMBA vs salicylate), the absence of a hydrogen bond enables the free rotation of the carboxylate group, thus justifying the absence of viscoelasticity in mixtures of CTAB and OMBA (Figures 2 and 3). In that case, the larger size of the methoxyl group is also likely to affect negatively the packing of this cosolute in the micellar palisade. The formation of a hydrogen bond explains the results in the benzoate family of compounds (Scheme 1, comparison of salicylate with both benzoate and OMBA). However, the consideration of feature 2the presence of extra carbons between the carboxylate group and the aromatic ringshows that all compounds in the cinnamate family induce larger relaxation times compared to their benzoate counterparts. Therefore, another structural aspect must be responsible for the more favorable formation of WLMs with cinnamate derivatives. Could the double bond also restrict the rotation of the group, thus favoring the incorporation of these cosolutes in the palisade layer? In order to investigate this aspect, we introduced the compound sodium 3-phenylpropanate, which has a structure quite similar to sodium cinnamate but bears a saturated carbon chain instead (comparison 4, Scheme 1). An equimolar system containing 200 mmol L−1 of CTAB and 3phenylpropanate was prepared and visually compared to the equivalent mixture with cinnamate (Figure 5). Clearly, the rheological characteristics of both samples differ completely. While the system containing cinnamate behaves as a viscoelastic solidlike sample (see also Figures 2 and 3), a purely liquidlike response is obtained with 3-phenylpropanate (Figure 5), pointing to the absence of wormlike micelle formation. This demonstrates the critical the role of the double bond in the structure of the cosolute. In order to further analyze this aspect, density functional theory (DFT) calculations were carried out within the hybrid generalized gradient approximation (Supporting Information). Optimized geometries for cinnamate (Figure 6, left) and 3-phenylpropanate (Figure 6, right) were obtained at the B3LYP/6-31G(2df,p) level of theory. The green grids represent the isosurface of the ground-state electron density as calculated with DFT. The molecular volume is appropriate to indicate the steric hindrance caused by the rotation of the groups of the two molecules. The calculations were performed at these optimized geometries, and the values for cinnamate (0.167 nm3 molecule−1) are around 25% lower than for 3-phenylpropanate

Figure 5. Gravitational effect on systems containing 200 mmol L−1 of CTAB and 200 mmol L−1 of cinnamate (left) and 3-phenylpropanate (right). The photo was obtained just after inversion of the vials.

Figure 6. Fully optimized equilibrium structures of cinnamate (A) and 3-phenylpropanate (B). The planarity of the cinnamate molecule is visualized by comparing the length of the side of the box (x, y, and z) that contains the two molecules. Clearly, x is narrower for cinnamate in comparison with 3-phenylpropanate.

(0.220 nm3 molecule−1). The more planar conformation and the higher internal rotation barrier of cinnamate compared to 3phenylpropanate (see Supporting Information) are due to the hyperconjugation of cinnamate and are relevant to explain the difference between the volumes of the two structures. This difference in volume affects the insertion of the aromatic molecule in the micelle palisade, explaining the more efficient packing of cinnamate and therefore the more favorable drive for WLM formation. OHCA and OMCA have a double bond between the carboxyl group and the aromatic ring, and for this reason, the hyperconjugation effect is also responsible for the high level of incorporation of these two cosolutes at the micellar palisade. In summary, while it is well-known that the incorporation of aromatic cosolutes is mainly driven by the hydrophobic effect and charge neutralization, our results clearly show that the free rotation of the anionic group creates a steric hindrance. This is in agreement with the rupture of the long WLMs formed by CTAB and trans-OMCA when irradiated by UV light.7,11,16 It is very plausible that cis-OMCA has a huge steric effect reducing its incorporation into the CTAB micelles. It is now quite clear that the meta- and para- isomers of salicylate are not able to produce wormlike micelles, since the larger distance between the hydroxyl and the carboxyl groups prevents the formation of a hydrogen 11540

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Figure 7. SANS patterns for wormlike micelles formed by CTAB and different cosolutes. The concentration of each component is 100 mmol L−1 in D2O at 25 °C. Solid lines represent fits to the cylinder model.

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bond, thus allowing the free rotation of the carboxyl group at the crowded surface of the micelle. The importance of steric effects was clearly demonstrated by comparing the system containing cinnamate and 3-phenylpropanate. Finally, we examine the surprising gel-like samples formed by the only noncharged cosolute studied: phenol. WLMs formed by CTAB and phenol have already been characterized by cryoTEM.28 The authors proposed that the repulsions between the CTAB headgroup are reduced by the introduction of phenol at the micelle palisade, leading to the formation of WLMs. Compared to the other cosolutes, phenol does not have a strong interaction with the cationic headgroup of CTA+, since it is not charged. Therefore, WLM formation is essentially driven by the hydrophobic effect, resulting from the incorporation of the aromatic ring in the palisade layer of the micelles. The absence of charge on the cosolute results in high micellar electrostatic density, creating (similarly to polyelectrolytes) strong charge repulsions between the WLM chains. Therefore, the chains must be more stretched, which could explain the longer relaxation time. In order to investigate the structure of the WLM, SANS measurements on CTAB (100 mmol L−1) and the various cosolutes (100 mmol L−1) were carried out. The results are shown in Figure 7. As can be seen, except for OMBA and phenol, all cosolutes exhibit a similar scattering pattern with the intensity smoothly increasing toward low q values, indicating the presence of elongated structures.3 For OMBA, the presence of a plateau at low q suggests a finite size of the aggregates compared to the other systems, i.e., shorter micelles. Fitting the curves with a rod model confirms the presence of elongated cylindrical structures for all samples, with shorter length for OMBA (ca. 49 Å). In all systems studied, the cross-section r remains fairly constant at ca. 21 ± 1 Å and in good agreement with the length of the alkyl chain of CTA in WLMs formed with this surfactant (r = 20.3 Å).29 The scattering pattern obtained in mixtures of CTAB and phenol with a strong structural peak markedly departs from those of the others and confirms the presence of very strong interactions. The result obtained for this system is compatible with high charge interactions and stiffer chains. By taking the value of 2π/qmax, a characteristic correlation distance can be estimated at ≈18 nm.

CONCLUSION

Small variations in the structure of aromatic cosolutes have been shown to significantly affect the rheology of WLM formed with the cationic surfactant CTAB. The formation of WLM can be examined by considering the mass action law, in which the length of the micelles depends on the extent of incorporation of the aromatic cosolute into the micellar palisade. The length of the WLMs and, as a result, their entanglement directly affect the viscoelastic response of the system. Therefore, rheological measurements can be used to evaluate the effect of molecular variations in aromatic cosolutes on wormlike micelle formation. For a fixed concentration of CTAB and cosolute, the following order of decreasing relaxation times was observed: phenol > cinnamate > OHCA > salicylate > OMCA > benzoate > OMBA. In the case of salicylate, a cosolute well-known to induce the elongation of CTAB micelles, our study reveals that the favorable incorporation of the aromatic molecule results from the formation of a hydrogen bond between the hydroxyl group (at the ortho position) and the carboxylate group, which prevents the free rotation of the single bond, thus reducing steric hindrance and allowing a better insertion of the aromatic molecule into the micelle. In comparison, the combination of CTAB with sodium benzoate induces a weaker viscoelastic response, due to the free rotation of the carboxylate group, resulting in poorer packing, while the addition OMBAwith a methoxy replacing the hydroxyl group in the ortho position prevents the formation of wormlike micelles (as inferred from the liquidlike response); this was attributed again to the free rotation of the carboxylate group and bulky methyl group, which hinders the favorable packing of the cosolute in the palisade layer. The addition of two carbons between the ring and the carboxylate in cinnamate derivatives substantially enhances the viscoelastic properties, reflecting a favorable incorporation of the cosolutes. The presence of the double bond was found to be critical. The double bond in cinnamate derivatives was shown to reduce the free rotation of the group, and additionally, the hyperconjugation ensures a better packing of the aromatic inside the micelle. This was observed by comparing the saturated counterpart of cinnamate, namely, 3-phenylpropanate, which did not produce wormlike micelles. Surprisingly, the addition of a 11541

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(12) Nettesheim, F.; Kaler, E. W. Phase behavior of systems with wormlike micelles. In Giant Micelles: Properties and Applications; Zana, R., Kaler, E. W., Eds.; CRC Press, Boca Raton, FL, 2007; Surfactant Science Series Vol. 140, pp 223−247. (13) Šarac, B.; Mériguet, G.; Ancian, B.; BeŠter-Rogac, M. Salicylate isomer-specific effect on the micellization of dodecyltrimethylammonium chloride: Large effects from small changes. Langmuir 2013, 29, 4460−4469. (14) Smith, B. C.; Chou, L.-C.; Lu, B.; Zakin, J. L. In Structure and Flow in Surfactant Solutions; ACS Symposium Series; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; p 578. (15) Bijma, K.; Engber, J. B. F. N. Effect of counterions on properties of micelles formed by alkylpyridinium surfactants. 1. Conductometry and 1H NMR chemical shifts. Langmuir 1997, 13, 4843−4849. (16) Ketner, A. M.; Kumar, R.; Davies, T. S.; Elder, P. W.; Raghavan, S. R. A Simple class of photorheological fluids: Surfactant solutions with viscosity tunable by light. J. Am. Chem. Soc. 2007, 129 (6), 1553−1559. (17) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals, 6th ed.; Butterworth-Heinemann: Oxford, UK, 2009. (18) Heenan, R. K.; King, S. M. Osborn, R.; Stanley, H. B. RAL Intl. Rep., 1989 (RAL-89-128). (19) SasView project, originally developed by the DANSE project under NSF award DMR-0520547; http://www.sasview.org/. (20) Shikata, T.; Sakaiguchi, Y.; Urakami, H.; Tamura, A.; Hirata, H. Enormously elongated cationic surfactant micelle formed in CTAB− aromatic additive systems. J. Colloid Interface Sci. 1987, 119, 291−293. (21) Shikata, T.; Hirata, H.; Kotaka, T. Micelle formation of detergent molecules in aqueous-mediaViscoelastic properties of aqueous cetyltrimethylammonium bromide Solutions. Langmuir 1987, 3, 1081−1086. (22) Cates, M. E.; Fielding, S. M. Rheology of giant micelles. Adv. Phys. 2006, 55, 799−879. (23) Berret, J. F. In Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2005. (24) Cates, M. E.; Candau, S. J. Statics and dynamics of worm-like surfactant micelles. J. Phys.: Condens. Mater. 1990, 2, 6869. (25) Rakitin, A. R.; Pack, G. R. Necessity of aromatic carboxylate anions to be planar to induce growth of cationic micelles. Langmuir 2005, 21, 837−840. (26) Liu, J.; Dong, B.; Sun, D.; Wei, X.; Wang, S.; Zheng, L. Enthalpy measurements for the formation of salt-induced wormlike micelles using isothermal titration microcalorimetry. Colloids Surf., A 2011, 380, 308− 313. (27) Jensen, F. Introduction to Computational Chemistry; John Wiley & Sons: Chichester, UK, 2003. (28) Agarwal, V.; Singh, M.; McPherson, G.; John, V.; Bose, A. Microstructure evolution in aqueous solutions of cetyltrimethylammonium bromide (CTAB) and phenol derivatives. Colloids Surf., A 2006, 281, 246−253. (29) Helgeson, M. E.; Hodgdon, T. K.; Kaler, E. W.; Wagner, N. J. A systematic study of equilibrium structure, thermodynamics, and rheology of aqueous CTAB/NaNO3 wormlike micelles. J. Colloid Interface Sci. 2010, 349, 1−12.

noncharged cosolute, phenol, leads to the formation of WLMs with the longest relaxation time, which was attributed to the electrostatic repulsions between the highly charged cationic micelles, leading to more rigid structures. Morphological investigations with SANS confirmed the formation of elongated structures in all cases and the occurrence of strong interactions in the CTAB/phenol systems, which corroborates the rheological results. Overall this work brings new fundamental insight into the molecular mechanisms that control wormlike micelle formation, showing, in addition to the hydrophobic effect and electrostatic interactions, the importance of intermolecular hydrogen bonds and the steric effect.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Preparation of OHCA, flow curves and the determination of zero shear viscosity in a range of CTAB and aromatics concentrations, computational analysis and electrostatic potential maps of the aromatic cosolutes, and calculated rotational barrier for cinnamate and 3-phenylpropanate. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Brazilian agencies CAPES, CNPq, FAPESP, and PETROBRAS for financial support and fellowships and the Center for Scientific Computing (NCC/GridUNESP) of the São Paulo State University (UNESP) for computing time. ISIS (Rutherford Appleton Laboratory, Didcot, UK) is acknowledged for the provision of beam time and Ann Terry is thanked for her help with the measurements.

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REFERENCES

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dx.doi.org/10.1021/la502649j | Langmuir 2014, 30, 11535−11542