Salicylate Isomer-Specific Effect on the Micellization of

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Salicylate Isomer-Specific Effect on the Micellization of Dodecyltrimethylammonium Chloride: Large Effects from Small Changes Bojan Šarac,*,† Guillaume Mériguet,*,‡ Bernard Ancian,‡ and Marija Bešter-Rogač† †

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, Ljubljana 1000, Slovenia UPMC Univ Paris 06, CNRS, ESPCI, UMR 7195 PECSA, F-75005 Paris, France

‡

S Supporting Information *

ABSTRACT: Specific effects of the sodium salts of m- and phydroxybenzoates (m-HB and p-HB) on the aggregation process of dodecyltrimethylammonium chloride have been investigated by isothermal titration calorimetry, electrical conductivity, and 1H NMR and compared with already reported data for the sodium salt of o-hydroxybenzoate (oHB). For p-HB, it has been found that the aggregate is only formed by spherical micelles at all p-HB concentrations. On the other side, the situation is more complex for o-HB, where two distinct states of aggregation can be involved, depending on the concentration of o-HB. At high salt concentration, rodlike micelles are formed, whereas at lower concentration spherical aggregates are predominant. The transition from the cylinder to the sphere increases the mobility of the surfactant because the core of the rodlike micelles is more closely packed due to the expulsion of water from the interior of the aggregate. m-HB exhibits an intermediate behavior between these two extreme situations. The effect of the position of hydrophilic substituents on the aromatic ring on the insertion of the hydroxybenzoate anion in the micellar aggregate has been discussed.

1. INTRODUCTION The ability of surfactant molecules to solubilize hydrophobic active substances is one of their most valuable and most used properties. As a result, the knowledge of the locus of solubilization and the distribution of the active substances in the amphiphilic entity is of primary importance to predict or to govern the behavior of the active substances. Salicylate, a salt of salicylic or 2-hydroxybenzoic acid (also o-hydroxybenzoic acid, where the position of the hydroxyl group is ortho to the carboxyl group), is one of these surface-active substances used as a nonsteroidal anti-inflammatory drug (NSAID). Moreover, it is also one of the most investigated among hydrotropic molecules, which are known to favor the dissolution of organic molecules in water and also to have a significant effect on the microemulsion stability and structure.1 The position of the hydrophilic groups on the aromatic ring and therefore the overall hydrophobic character of the surfactant or the counterion molecules have a dramatic effect on the selforganization of cationic surfactants, which turns out to be strongly dependent on the substitution pattern of the aromatic ring.2−7 Such large effects on the formation of micellar aggregates are then expected to occur when the overall balance between hydrophobic and hydrophilic moieties (carboxyl and hydroxyl) of the hydroxybenzoate ion is slightly shifted by a change of position of the substituents. Thanks to modern calorimetric, spectroscopic, conductivity, and other techniques, © 2013 American Chemical Society

many physicochemical properties of such systems can be measured, and there exists a vast amount of experimental data on micellization of surfactants in the presence of hydrotropes, but sometimes a lack of systematic research may lead to misleading conclusions. In a previous paper,8 the systematic investigation of sodium salicylate on the micellization of dodecyltrimethylammonium chloride (DTAC) was done by calorimetric, conductivity, and NMR measurements. It was found that the micellization of dilute DTAC in the presence of salicylate takes place at considerably lower cmc than with NaCl.9 It turns out that the process is exothermic at all temperatures, indicating the presence of additional strong interactions apart from the hydrophobic effect. Moreover, structural transformations, strongly dependent on the ratio of the DTAC and salicylate concentrations, were observed. It was concluded that salicylate ions not only interact with the headgroups but also insert further into the micelle core. However, some observations remain unclear, and the interpretation of some effects is still an open question. Recently, a conductivity study on diluted aqueous solutions of sodium 2-hydroxybenzoate (o-HB), 3-hydroxybenzoate (mReceived: January 14, 2013 Revised: March 6, 2013 Published: March 12, 2013 4460

dx.doi.org/10.1021/la400161n | Langmuir 2013, 29, 4460−4469

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HB), and 4-hydroxybenzoate (p-HB) has been carried out.10 It has been found that the mobility of the investigated anions is strongly dependent on the relative position of the carboxyl and hydroxyl groups (Figure 1) and is ranked in the order o-HB >

the syringe were approximately 20 times higher than its cmc; at higher concentrations of surfactant, solutions of about 0.2 M were used. The experimental data were analyzed using the Origin 7.0 software provided by MicroCal. A detailed description of the procedure was given in our previous work.9 2.2.2. Conductimetry. Conductivity was recorded with a PCinterfaced Agilent 4284 A LCR meter connected to a three-electrode measuring cell described elsewhere.12 The cell, with a cell constant B = 2.2130 ± 0.0003 cm−1, was calibrated with dilute potassium chloride solutions13 and immersed in the high-precision thermostat described previously.14 The water bath was set to each temperature of a temperature program with reproducibility within 0.005 K. The temperature was additionally checked with a calibrated Pt100 resistance thermometer (MPMI 1004/300 Merz) connected to an HP 3458 A multimeter. After the resistance, R, of 0.01 M sodium hydroxybenzoate solution was measured at a set temperature, successive aliquots of a stock solution of DTAC in the same sodium hydroxybenzoate solution were added by a programmable syringe pump (model 1250, J-KEM Scientific, St. Louis, MO), and the resistance of the solution was measured. As in the ITC experiment, to cover the whole concentration range, two separate sets of experiments with different concentrations of DTAC stock solutions were carried out at 298.15 K. A home-developed software package was used for temperature control and acquisition of conductance data. The measuring procedure, including corrections and extrapolation of the sample resistance to infinite frequency, has been described previously.14 The corresponding conductivities κ were obtained as κ = B/R, the temperature dependence of the cell being taken into account.13 Taking into account the sources of error (calibration, titration, measurements, impurities), the conductivities are accurate to within 0.5%. The sodium hydroxybenzoate solutions were treated as solvents; therefore, the corresponding electrical conductivity, κNaSal, was subtracted from the overall measured electrical conductivities, κoverall. The contribution of the surfactant to the conductivity was calculated as κ = κoverall − κNaSal. The specific conductivities of 0.01 M solutions of o-HB, m-HB, and p-HB were 0.0775, 0.0734, and 0.0730 S m−1, respectively. 2.2.3. Nuclear Magnetic Resonance Spectroscopy. The NMR measurements were carried out for constant hydroxybenzoate concentration (10 mM) and variable DTAC amount. All spectra were recorded by using a 1H/X BBO broad-band probe on a Bruker Avance DRX 500 NMR spectrometer operating at 499.76 MHz for 1H. The temperature was set to 298.0 ± 0.1 K thanks to a BCU unit and a BVT controller. The 1H nominal 90° pulse was systematically measured on the actual sample, varying from 12.5 to 13.5 μs, depending on the concentration ratio hydroxybenzoate/DTAC. All chemical shifts are measured from the internal residual proton of HOD (semiheavy water) in D2O at 4.71 ppm in all hydroxybenzoate/ DTAC solutions. Diffusion NMR experiments were also carried out on the same spectrometer and with the same probe equipped with a 57.0 G cm−1 gradient coil. The BPP−STE−LED sequence,15 stimulated echo (STE) with bipolar gradient pulses (BPPs) and longitudinal eddy current delay (LED), was used. The value of the gradient has been calibrated with the value of the self-diffusion coefficient of HOD in D2O (1.902 × 10−9 m2 s−1).16,17 For each sample, a series of 32 NMR diffusion experiments were recorded by a quadratic increase of the amplitude of the gradient and the data were analyzed by using the NMRpipe processing software package18 or iNMR.19 Rectangular gradients of constant duration (3 ms) were chosen for encoding and decoding, whereas the spoil gradient (2 ms) was a sine-shaped one and the LED was kept equal to a value of 10 ms in all the experiments. Each diffusion coefficient has been obtained by using two distinct diffusion times, Δ, 40 and 100 ms, and the reliability of the measurement was checked in each case on the diffusion coefficient of HOD in D2O. The standard deviation on each measurement is below 3%, while the statistical reproducibility is around 4%. Longitudinal relaxation times T1 were measured by the standard inversion−recovery experiment with delay times varying from 0.01 to 25 s. The T1 value was extracted with a nonlinear three-parameter

Figure 1. Structures of the o-, m-, and p-hydroxybenzoate anions.

m-HB > p-HB, which was ascribed to the possible hydration of the anions. The study of the other two isomers on the micellization of DTAC might help to understand the mechanism at play in the incorporation of salicylate molecules. Thus, the main purpose of this study is to investigate the influence of m-HB and p-HB on the thermodynamics of micellization of DTAC solutions at moderate concentrations of hydroxybenzoate (0.01 mol L−1) by isothermal titration calorimetry (ITC) and electrical conductivity measurements over a broad temperature range between 278.15 and 318.15 K. In addition, the interactions between the surfactant and aromatic counterions are investigated by 1H NMR spectroscopy and diffusion NMR experiments at 298.15 K. The results will be compared to those obtained for the micellization process of DTAC in o-HB aqueous solutions and discussed in terms of the obtained thermodynamic and transport parameters.

2. EXPERIMENTAL SECTION 2.1. Materials. DTAC (>98%) was purchased from Anatrace, Inc. (Maumee, OH). The compound was stored at a temperature below 278.15 K in a refrigerator according to the producer’s instructions and was used as received. Sodium m-hydroxybenzoate (3-hydroxybenzoic acid sodium salt, HOC6H4COONa, m-HB; puriss. p.a., ≥99.0%; TCI Europe nv, Belgium) and sodium p-hydroxybenzoate (4-hydroxybenzoic acid sodium salt, HOC6H4COONa, p-HB; puriss. p.a., ≥96.0%; Fluka, Germany) were stored in a desiccator over P2O5 and used without further purification. Stock solutions were prepared by weighing the compounds and water. Demineralized water distilled in a quartz bidistillation apparatus (DESTAMAT Bi18E, Heraeus; specific conductivity m-HB > p-HB. For o-HB the cmc, determined from conductivity, increases with temperature, whereas for m-HB and p-HB the mimimum at T ≈ 288 K was found. Values of β are much higher than values reported for DTAC in water (0.424 at 298.15 K),33 indicating a type of association of surfactant molecules different from that in water. Moreover, β

Figure 4. Thermodynamic parameters of micellization (ΔHM°, ΔGM°, TΔSM°) as a function of temperature obtained from the fitting of the model to the experimental data for all three isomers. 4463


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Table 2. Critical Micelle Concentration, cmc, aggregation number, nagg, and Thermodynamic Parameters, Obtained by the Fitting Procedure Described in the Text at 298.15 K with Corresponding Errors for All Three Isomers o-HB m-HB p-HB

cmc (mM)

nagg

ΔHM° (kJ mol−1)

ΔGM° (kJ mol−1)

TΔSM° (kJ mol−1)

Δcp,M° (J mol−1 K−1)

0.90 ± 0.02 4.5 ± 0.4 9.7 ± 0.8

20.3 ± 0.2 17.9 ± 0.8 17 ± 1

−11.9 ± 0.1 −5.5 ± 0.1 −2.5 ± 0.1

−34.75 ± 0.02 −26.91 ± 0.08 −25.4 ± 0.2

22.9 ± 0.1 21.4 ± 0.1 22.8 ± 0.2

−489 ± 8 −314 ± 9 −285 ± 8

Figure 5. 1H NMR spectrum of (bottom) 10 mM m-HB and 4 mM DTAC and (top) 10 mM m-HB and 30 mM DTAC. The assignments are described in the text and in Figure 6.

slope corresponds to the heat capacity of micellization, Δcp,M°, which is negative for all systems as is usual for the selfassociation of amphiphiles.35,36 As we can note from Table 2 the values for m-HB and p-HB are similar (∼−300 J mol−1 K−1), but much smaller (in the absolute sense) than for the ortho isomer (∼−490 J mol−1 K−1). For DTAC in different salt environments the values of Δcp,M° typically lie around the latter value, indicating that the whole micelle interior (e.g., nonpolar chains) is dehydrated upon micellization.37 On the contrary, in the presence of m-HB and p-HB, the interior of the micelles is only partially dehydrated, leading to the formation of more loosely packed micelles.38,39 Table 2 shows, besides the cmc, the values of the global fitting parameters at 298.15 K, whereas the corresponding model curves were already shown in Figure 2a. The values of ΔSM° were calculated according to the Gibbs relation ΔGM° = ΔHM° − TΔSM°. As noted, the fitting procedure gives information about the aggregation number around the cmc (nagg), which slightly decreases from o-HB to p-HB. At this point we must emphasize that so-obtained aggregation numbers are determined around the cmc and are much lower than the values from the literature. At first glance it appears rather strange that the value of nagg for o-HB is so low, which is unlike the fact of its ability to promote a transition from spherical to long rodlike micelle. We believe that smaller micelles are formed around the cmc, whereas a transition appears at a subsequent increase of the surfactant concentration, which eventually leads to the formation of complex aggregates, giving rise to high-viscosity solutions.8,40

ΔHM° is negative over the whole temperature regime with a slight temperature dependence (Figure 4), which can be ascribed to the well-known enthalpy−entropy compensation effect. Large, negative values reflect strongly favorable micellization process governed by its entropic component more distinctly in the case of m-HB and p-HB. This entropic contributions appear to be somehow similar around ambient temperature for the three isomers, which only differ in the enthalpic contribution. This shows the role of the interactions between the hydroxybenzoate and the surfactant in the change of micellization features, which we will examine from a more local perspective using NMR spectroscopy. 3.2. NMR Spectroscopy. NMR spectroscopy can give access to three complementary parameters of the systems: the chemical shifts, the relaxation times, and the diffusion coefficients. The chemical shift gives insight into the environment of the nuclei and hence the localization of the hydroxybenzoate molecules inside the micellar aggregate. The relaxation time is related to the local rotational mobility of the nucleus under study, whereas the diffusion coefficient gives information on the translational dynamics on a much larger scale than the aggregate size and can then be compared to the conductivity measurement, for example. 3.2.1. Chemical Shift Evolution. Apart from the signal at 4.7 ppm, assigned to the HOD resonance of the remaining proton of the solvent, the spectrum can be divided into two main parts (see Figure 5). Between 6 and 8 ppm lie the aromatic protons of the hydroxybenzoate molecules. They are numbered H2 to H6 according to their position on the aromatic ring (see Figure 1), the carbon number 1 corresponding to the carbon bearing 4464


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Figure 6. Evolution of the chemical shifts with the concentration of DTAC for the hydroxybenzoate (a) and the surfactant (b). Data for o-HB are taken from ref 8.

the formation of rodlike micelles for longer chain ammonium surfactants.39 The amphiphilic property of o-HB leads to a stronger binding with the surfactant, which makes possible a denser packing of surfactant molecules in aggregate and favors rod-shaped instead of spherical micelles.3 The absolute minimum value of the chemical shift indicates a much more apolar medium for all the protons of the molecule, which is compatible with the formation of rodlike micelles, which are denser than spherical micelles and from which water is expelled.42 This observation is consistent with the large value of Δcp,M° observed by ITC, which confirms that for this isomer the dehydration of the inner part of the micelle is more important than for the other hydroxybenzoates. For the other isomers, which are not as dissymmetric as o-HB, there must be an energetic compromise when they are inserted into the micellar aggregate since a hydrophilic group, the hydroxyl, should be partly inserted into the hydrophobic core, which leads to a weaker insertion. The evolutions of the chemical shifts of the surfactant molecules are reported in Figure 6b. The change of the chemical shift in the micellar aggregate due to the ring current of aromatic solutes in the vicinity of the surfactant has for long been a way to probe the micellar structure and to determine the locus of solubilization of this kind of solute.44,45 This effect, related to the aromatic solvent induced shift (ASIS), leads to a marked decrease of the chemical shift of the surfactant for the protons close to the aromatic molecules. The evolution of the spectra of the surfactant for the different isomers exhibits the same generic features. The ASIS decrease is observed for protons close to the polar head, e.g., Hα, Hβ, and HN, while an increase is seen for the methyl protons at the end of the hydrocarbon chain, Hω. This specific observation confirms that the locus of the solubilization of the hydroxybenzoate is close to the polar head of the surfactant, where the ASIS is stronger, while the tail of the surfactant, situated farther from the aromatic ring, experiences the opposite effect. This is not the case for apolar aromatic solutes such as benzene, where all the protons of the surfactant chain experience the same kind of chemical shift evolution.44 As for the hydroxybenzoate chemical shifts, the weakest effect is observed for p-HB while o-HB exhibits the strongest, which supports the highest interaction of the surfactant with o-HB. The evolution of the chemical shifts of DTA+ can be separated

the carboxylate group. Below the HOD resonance, the protons belong to the DTA+ molecule. Around 3.0 ppm the resonance of the methyl group protons (HN) attached to the nitrogen can be found together with the α methylene protons (Hα). Around 1.5 ppm, the peaks of the β methylene protons (Hβ) are observed, and in the vicinity of 0.8 ppm, the ω (terminal) methyl protons (Hω) are seen. Between these two peaks, the spectrum exhibits one or two broad peaks resulting from the remaining protons in the surfactant chain. The evolution of the chemical shifts of the hydroxybenzoate molecules are reported in Figure 6 (left). Whatever the isomer, two antagonistic trends are observed depending on the proton position with respect to the carboxylate group in the global change between the two limiting cases: dilute aqueous hydroxybenzoate and the hydroxybenzoate solubilized in the DTA+ micelle. For the protons close to the carboxylate moiety (ortho position, H2 and H6) an overall increase of the chemical shift (deshielding) is observed, whereas for the remote protons (meta and para positions, H3, H4, and H5) the trend is opposite (shielding). This phenomenon can be explained by the change of the environment of the protons during the micellization. The hydroxybenzoate molecules go from the polar solvating medium of the aqueous solution to an apolar micelle core, which should cause a shielding of the protons as observed for the H3, H4, and H5 protons. On the contrary, since H2 and H6 protons are deshielded, they should lie in a polar medium in the micelle, which suggests they are in the vicinity of the charged head of the surfactant with which the carboxylate group interacts strongly. This conclusion of hydroxybenzoate molecules with the carboxylate group pointing outward from the micelle is supported by molecular modeling studies41,42 and suggested in refs 5 and 4. The weakest effect is observed for p-HB, while o-HB exhibits the strongest change and m-HB lies between the two. As a result, this hierarchy can be ascribed to the increasing polarity of the isomer, which drives the interaction with the surfactant molecules. In addition, for o-HB the evolution is nonmonotonic as compared to that of the other isomers, and all the chemical shifts show a minimum around a concentration of DTA+ of 10−15 mM. Such behavior has already been observed for cetyltrimethylammonium bromide−sodium salicylate mixtures.43 This suggests the existence of an intermediate form, which is likely a rodlike micelle as o-HB is known to promote 4465


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Figure 7. Diffusion coefficients of the hydroxybenzoate (a) and the surfactant (b). Inset in (b): molar conductivity of the corresponding solutions. Data for o-HB are taken from ref 8.

into two regimes: a first one at a low concentration of DTA+, where the association is limited by the DTA+ needed to form the aggregates (the ASIS increases), and a second one, once the majority of the hydroxybenzoate molecules is inserted within the DTA+ micelles, where the hydroxybenzoate is diluted in the micelles (the ASIS decreases on average). 3.2.2. Dynamics. 3.2.2.1. Longitudinal Relaxation Time T1 Evolution. The longitudinal relaxation times T1 were measured for the same solutions for the hydroxybenzoate and surfactant (Figures S3 and S4 in the Supporting Information). For the hydroxybenzoates, an overall decrease of the relaxation time with an increase of the concentration of DTA+ is observed whatever the proton, which corresponds to a decrease of the mobility of the hydroxybenzoates after the insertion into the micellar aggregates. Note that the relaxation time for the H2 proton of m-HB is longer than that of the other because it has no close protons to see a significant NOE contribution, which would enhance the relaxation. For the surfactant, the relaxation time confirms the observations made on the chemical shift variation. The effect of o-HB is stronger than that of the other two, which will be discussed in detail below. p-HB is the least affected of the three; the relaxation time of the end of the tail only shows a weak decrease, and apart from a maximum around 5 mM DTA+, that of the ammonium proton decreases slowly. For comparison with the micelles of neat DTA+, the longitudinal relaxation times were measured for 30 mM solutions (above the cmc): T1(Hω) = 1.74 s and T1(HN) = 0.84 s. The relaxation times of the surfactant, for the micelles containing p-HB, are then very close to that of the pure DTA+ micelles. In addition, as the DTA+ concentration reaches 50 mM, all T1 values of the surfactant tend to the same value whatever the nature of the isomer. This limiting state corresponds to a high dilution of the hydroxybenzoate in the DTA+ micelle. The addition of o-HB, as for the chemical shift, causes a steep variation of the relaxation time for both the end of the tail and the ammonium proton corresponding to a brutal change of mobility. Then as the concentration of DTA+ increases the relaxation time increases as the spherical micelles form. The quantitative treatment of the longitudinal relaxation time for protons is not straightforward for several reasons. One of them

is that the motion of the surfactant molecules is a convolution of several moves that cannot be resolved easily: local motions, lateral diffusion within the micelle, and overall tumbling.46−48 The qualitative conclusion that can be drawn is that since large aggregates are forming the overall motion should be slowed and the relaxation time should decrease accordingly. In addition, the other motions should also be impeded by the more densely packed rodlike micelle. 3.2.2.2. Diffusion. The diffusion coefficients of the surfactant and the hydroxybenzoate isomers are plotted in Figure 7. For the former, the values compare very well with the molar conductivity (inset in Figure 7). The diffusion coefficients of the dilute hydroxybenzoate isomers (measured for 1 mM solutions and corrected for the viscosity of D2O) are listed in Table 3 and agree fairly with the values deduced from the Table 3. Diffusion Coefficient of Hydroxybenzoates Determined by NMR and Conductimetry

NMR conductimetryb a

Do‑HB × 1010 (m2 s−1)

Dm‑HB × 1010 (m2 s−1)

Dp‑HB × 1010 (m2 s−1)

9.3 ± 0.4a 9.18 ± 0.01

8.6 ± 0.3 7.90 ± 0.01

7.7 ± 0.4 7.17 ± 0.01

Reference 8. bReference 10.

limiting molar conductivities.10 Values for o-HB are also in reasonable agreement with the reported value of 9.59 × 10−10 m2 s−1 from the literature.49 The difference between the diffusion coefficients can be ascribed to the difference in solvation among the three isomers. As for the o-HB isomer, the carboxylate and the hydroxyl groups are close and an intramolecular hydrogen bond forms. The total number of solvating water molecules is then reduced by this phenomenon, which on the other hand enhances it diffusivity. This difference in solvation has been examined by Nagy et al.50 for the corresponding hydroxybenzoic acids. They show that o-hydroxybenzoic acid molecules form only 2−4 Hbonds with the surrounding water molecules while phydroxybenzoic acid molecules form 4−5 bonds, which supports our observation. This observation is also in agreement with the smaller molar volume of the salicylic acid compared to 4466


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the other isomers.51 On the contrary, for the m-HB and p-HB isomers, the two hydrophilic groups are separated and are solvated by a high number of water molecules. The para isomer, which exhibits a prolate form with a C2 axis, experiences a higher friction.52,53 The diffusion coefficient of this isomer is then expected to be the lowest. The evolution of the diffusion coefficient of DTA+ with the concentration of DTAC for the three hydroxybenzoates is fitted with a two-state model (monomer ⇆ micelle): D̅ = Dmon

⎛ cmc cmc ⎞⎟ + Dmic⎜1 − ⎝ C C ⎠

interaction between the aromatic ring and the tail of the surfactant on the other hand. What is exactly occurring between the large aggregate formation and the spherical micelles is still unclear. The existence of a diffusion maximum for the surfactant between 20 and 30 mM is confirmed by independent conductivity measurements (see the inset of Figure 7b). However, the diffusion of o-HB shows a monotonic decrease, which suggests that the addition of surfactant does not release it significantly from the large aggregate, where it should be more favorably inserted than in the more mobile spherical micelles. Between 20 and 30 mM, the surfactant diffuses even faster than in the pure DTAC micelle. The only explanation would be that the added surfactant in this region accumulates neither in the long aggregate nor in spherical micelles, but stays rather free in solution. Beyond this intermediate region, it forms spherical micelles whatever the nature of the hydroxybenzoate, as evidenced by the convergence of the diffusion and conductivity curves.

(2)

The fit parameters are constrained to give the same values for D mic and D mon whatever the isomer. The parameters corresponding to a two-state model (eq 2) are reported in Table 4. For meta and para, since a monotonic decrease is Table 4. Parameters Extracted from the Fit of the Diffusion Coefficient by a Two-State Model a

o-HB m-HB p-HB

cmc (mM)

Dmic × 1010 (m2 s−1)

Dmon × 1010 (m2 s−1)

0.6 ± 0.1 3.8 ± 0.4 7.7 ± 0.8

0.7 ± 0.3 1.3 ± 0.2 1.3 ± 0.2

4.8 ± 0.2 4.8 ± 0.2 4.8 ± 0.2

4. CONCLUSION Small changes, such as the position of the substituent on an aromatic ring, in the structure of counterions considerably influence the micellization processes. Whereas p-hydroxybenzoate only slightly decreases the cmc and incorporates weakly in the dodecyltrimethylammonium micelles, the meta and ortho isomers exhibit stronger effects. As for the meta isomer, the micellization enthalpy is stronger and the evolution of the diffusion coefficient of the surfactant shows that the structure of the aggregate is modified. This trend is confirmed by the ortho isomer (salicylate), for which an intermediate rodlike micelle appears due to the expulsion of water from the micelle interior and the simultaneous insertion of salicylate molecules in the vicinity of the polar head of the surfactant. The dissymmetry of the salicylate molecule makes its interaction with both the hydrophilic and hydrophobic parts of the surfactant more favorable than for the two other isomers. All these observations confirm recent simulation studies, which have shown the specific interaction of the salicylate molecule with ammonium surfactants41,42 and that the micellization can be significantly altered by a small change in the counterion structure.

a For o-HB the fit is limited to DTAC concentrations below 10 mM; see the text.

observed for the two isomers, the fit is performed for the whole concentration range, whereas for o-HB the fit is limited to the region below 10 mM, where the first aggregation process takes place. In the latter case, only a guide to the eye is drawn in Figure 7 since at least three species (monomer, rodlike micelle, and spherical micelle) might be present, as discussed before. In addition, we can note that the minimum observed for the diffusion of the surfactant is for the 1:1 proportion, which should be the composition where the fraction of rodlike micelles is maximal as seen for the other systems. As can be seen in Figure 7, a rather reasonable agreement of the fit with the experimental data is observed for p-HB, but the agreement is poorer for m-HB, which shows that the two-state model is not the best suited for this isomer. The shape of the micelle is then expected to vary with the addition of m-HB. This observation is consistent with the other parameters discussed above: chemical shifts and longitudinal relaxation times. It confirms that the effect of the hydroxybenzoate on the micellization of DTAC is stronger with m-HB than p-HB. cmc values are in reasonable agreement with those obtained by ITC and conductivity measurements (Tables 1 and 2), which, in comparison to NMR, are more reliable techniques for cmc determination. The evolution of the diffusion coefficient of the hydroxybenzoate with the concentration of DTA+ confirms the other observations (Figure 7a). Obviously, o-HB is the most incorporated one since this is the one that exhibits the steepest decrease of the diffusion coefficient during the addition of the surfactant, while p-HB is the least affected isomer. This trend is also supported by the diffusion coefficient value for [DTAC] = 50 mM, where o-HB is the slowest of the three, in agreement with a greater inclusion in the micellar aggregate. This higher inclusion can be explained not only by the compacity and lesser solvation of o-HB but also by the vicinity of the two polar groups of the molecule, which maximizes the interaction with the polar head of the surfactant on the one hand and the

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1, enthalpograms obtained by ITC and normalized enthalpograms used for fitting for o-HB, m-HB, and p-HB with corresponding fits between 278.15 and 318.15 K, Figure S2, conductivity curves for o-HB, m-HB, and p-HB between 278.15 and 318.15 K, Table S1, standard enthalpy of micellization, ΔHM°, standard Gibbs free energy, ΔGM°, obtained by a fitting procedure, calculated standard entropy, ΔSM°, and cmc between 278.15 and 318.15 K, Figure S3, evolution of the longitudinal relaxation time, T1, of hydroxybenzoate protons with the concentration of DTAC, and Figure S4, longitudinal relaxation time, T1, evolution of the surfactant protons with the concentration of DTAC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +386 1 2419 418 (B.Š.); +33 1 44273109 (G.M.). Fax: +386 1 2419 425 (B.Š.); +33 144273228 (G.M.). E-mail: bojan. 4467


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[email protected] (B.Š .); [email protected] (G.M.).

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Isabelle Correia (UPMC, Université Paris 6) for the NMR facilities and for helpful discussions. Financial support by the Slovenian Research Agency (Grant P1-0201) is gratefully acknowledged. This work was partially supported by bilateral program BI-FR/09-10-PROTEUS-012 and by COST Action CM1101.

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