Characterization of Counterion and Surface Influence on Micelle

Oct 7, 2006 - Mode Atomic Force Microscopy in Air ... UniVersity A Mira-Be´jaia, Algeria ... in aqueous solutions and in air, this new AFM imaging me...
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J. Phys. Chem. B 2006, 110, 21710-21718

Characterization of Counterion and Surface Influence on Micelle Formation Using Tapping Mode Atomic Force Microscopy in Air Lila Chaal,†,‡ Franc¸ oise Pillier,† Boualem Saidani,‡ Suzanne Joiret,† Alain Pailleret,*,† and Claude Deslouis† UPR 15 CNRS Interfaces et Syste` mes Electrochimiques, UniVersity P. et M. Curie, 4 Place Jussieu, 75252 Paris Ce´ dex 05, France, and Laboratory Electrochimie et Corrosion, Department Ge´ nie des Proce´ de´ s, UniVersity A Mira-Be´ jaia, Algeria ReceiVed: March 15, 2006; In Final Form: July 7, 2006

Cylindrical micelles prepared in aqueous solutions from cationic surfactants octadecyl trimethylammonium (OTA+) or cetyltrimethylammonium (CTA+) and parachlorobenzoate (PCB) counterion were successfully imaged after evaporation of water using tapping mode atomic force microscopy (TM-AFM) onto very smooth gold and glass substrates. With the help of the obtained topography AFM images, it was shown that the micellar structures are preserved on gold substrates after evaporation of the solvent despite the new set of stresses due mainly to capillary forces and dehydration. The influence of the substrate on the resulting micellar morphology observed in air was investigated for these two materials: cylindrical micelles were evidenced as loosely adherent on gold surface in the presence of parachlorobenzoate (PCB) and identical, geometrically speaking, to those known to exist in aqueous solutions. In this situation, topographic AFM images allowed us to determine accurately their geometrical characteristics such as diameter and length in the nanometer range. On the other hand, AFM images obtained in air on glass surfaces revealed micellar structures that are different from those existing in the bulk of the solution. Indeed, bilayer-type micelles with a thickness close to twice the surfactant monomer expected length were observed, indicating that the well-established and strong influence of glass on micelle geometry at the glass/solution interface is maintained after evaporation of water. These results have been analyzed on the basis of positive charge of gold deduced from electrochemical impedance spectroscopy (EIS) and Raman spectroscopy measurements on one hand and of the negative charge of glass on the other hand. Although these results appeal to new theoretical considerations dealing with dynamics of evaporation of micellar solution drops and/or with counterion contributions to macromolecular interactions in aqueous solutions and in air, this new AFM imaging method appears to be the more adequate one to image and measure the micelles formed in the presence of water.

Introduction Direct observation of micellar structures in the liquid phase or on surfaces is a challenging purpose for which only a few competitive techniques were proposed until now. To the best of our knowledge, two main techniques emerged from the literature over the last 10 years. The cryo-transmission electronic microscopy (cryo-TEM) technique is performed through the fastfreezing process of aqueous micellar solutions before analysis. This technique bears the disadvantage of subjecting the micellar samples to brutal although extremely short temperature treatments as well as to a blotting process that leads to very high shear rates.1-2 Comparing the respective drawbacks of cryo-TEM in liquids with the technique we introduce in this work, namely, AFM in air, both suffer from a change in the thermodynamic conditions that results from a fast sweep through wide regions of the phase diagram plotted in the surfactant concentration and solution temperature coordinates. The cryo-TEM method obviously meets the latter one, whereas AFM in air involves progressive increase of both the * Corresponding author. Phone: 33 1 44 27 41 69; fax: 33 1 44 27 40 74; e-mail: [email protected]. † University P. et M. Curie. ‡ University A Mira-Be ´ jaia.

surfactant and the counterion concentrations while evaporation proceeds. In addition, for gold, if specific micelle-substrate interactions are lessened, the evaporation process should just increase the concentration of cylindrical micelles instead of changing their morphology. On the other hand, in situ AFM appeared in the 1990s as a very powerful technique for the imaging of micelles adsorbed at the solid-liquid interface thanks to the pioneering work of Manne et al.3-4 It appears from this latter paper3,4 and the abundant bibliography citing it that most micellar structures based on alkyltrimethylammonium-type surfactants could be imaged on surfaces where the interactions of the surfactants with the surfaces were rather strong, leading to self-assembled layerlike structures in which each surfactant molecule undergoes strong interactions with other surfactant molecules and/or with the underlying surface depending on the surfactant concentration.5-13 The main types of surfaces used for such studies were mica, glass, gold, or graphite, for example. Consequently, it is now well-established that the surface micelle morphology observed on such substrates is frequently different from the micelle shape predicted or observed in the bulk solution, with the substrate thus playing a dominant role in determining the surface micelle structure. The strong interactions between the surface and the surfactants are thus also responsible for an

10.1021/jp061607q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006

Characterization of Micelle Formation using TM-AFM efficient immobilization of the micellar structures. This second role is tremendously important for an imaging purpose using AFM in micellar solutions as it keeps the micelles from floating in solution or from being destroyed by the AFM tip during the scanning. At this point, it must be emphasized that the AFM mode mainly used so far for imaging micelles in solution is the so-called noncontact repulsive mode in solution. When mentioned, the resulting AFM images shown in the literature appear to be mainly deflection images.5,7 These latter images display the particularity of preventing the authors from carrying out vertical measurements of the micellar structures, as admitted in the literature.6,7,12 In this paper, our goal was to demonstrate that an evaporation process leading to the complete absence of solvent, water in this work, has no significant influence on the morphological characteristics of micelles initially formed in aqueous solution. This main and possibly unexpected conclusion was drawn from AFM imaging experiments carried out in air on gold surfaces bearing micellar structures initially formed in aqueous solutions containing cationic surfactant/counterion mixtures. Interestingly, this original AFM in air-based imaging method and the topography images it provides can thus be used for the accurate determination of measures and morphological characteristics of micelles at the nanometer scale, unlike the two imaging methods introduced in the literature and briefly discussed previously. Beyond this AFM imaging step, our goal is to establish in the near future a strong correlation between morphological parameters of micelles and viscoelastic phenomena observed in the presence of these same micellar solutions. Preliminary results reported in this paper indeed confirm the feasibility of this objective at least from a qualitative point of view. Experimental Procedures Octadecyltrimethylammonium chloride (OTA+, Cl-) and cetyltrimethylammonium chloride (CTA+, Cl-) were both purchased from Akzo Nobel, France. In the mother solution, OTA+, Cl- was solubilized with the following content: surfactant 50%, 2-propanol 36%, and water 14% in weight. The PCB counterion was generated from insoluble para-chlorobenzoic acid (purchased from Sigma Aldrich) by neutralization with sodium hydroxide. Micellar solutions were prepared using adequate concentrations of these reactants and double-distilled water. Samples dedicated to AFM observations were prepared in a very simple manner by depositing several micellar solution drops using a micropipet either on very smooth glass substrates (whose roughness is below a few angstroms as determined by AFM) or on mica substrates coated with an approximately 65 nm thick gold film using the evaporation technique under ultrahigh vacuum. Both types of substrates were carefully rinsed with alcohol and double-distilled water before use. It must be emphasized that AFM and Raman experiments were carried out using the same conditions for the gold surface preparation. In particular, no annealing procedure was used for the gold substrates. AFM observations were carried out in air using a Molecular Imaging instrument in the acoustic resonant mode. This latter was composed of a Pico-LE base equipped with a micropositioning device aimed at the precise positioning of the AFM tip in the x-y plane of the sample, a large zone scanner (100 µm × 100 µm) bearing a photo detector, and the AFM nose adequate for TM-AFM experiments. A PicoScan 2100 controller connected to a computer was used to drive the scanner and to collect the data generated by the laser impact on the photo

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21711 detector. For this purpose, rectangular silicon cantilevers bearing conical silicon tips were used. Their resonance frequency was 280-365 kHz, and their spring constant was 25-50 N m-1. All AFM images shown hereafter underwent a plane correction process. Additional surface charge data for gold were obtained from electrochemical impedance spectroscopy (EIS) performed in the high-frequency (HF) range between 100 kHz and 100 Hz. Actually, in that range, the imaginary part of the impedance Im(Z) is such that

|Im(Z)| f

1 CHFω

when ω f ∞. The HF capacitance, CHF, is dependent on the dielectric permittivity in the double layer and is therefore sensitive to the adsorption of organic compounds. It was determined by extrapolating

1 ω|Im(Z)| to a zero value of 1/ω. The electrochemical setup for EIS consisted of a Frequency Response Analyzer FRA 1255 and of a potentiostat 1287 both from SOLARTRON. The gold electrode was mounted as a rotating disk (diameter 3 mm), and a classical three-electrode cell (250 cm3) was used. The gold electrode was rotated at 100 rpm with the AE apparatus during the acquisition of EIS diagrams. Raman spectra were recorded using a Labram from HoribaJobin-Yvon-Dilor with a notch filter as a primary stage to reject the laser light, a grating at 600 grooves mm-1 giving a 4 cm-1 resolution, and a CCD detector operating at 210 K. Radiation at λ ) 632.3 nm from a HeNe laser was used as excitation. The laser power was set at 1 mW using neutral density filters. Raman light was focused and collected in backscattering geometry with an Olympus confocal microscope, objective *50 ULWD. For recording Raman light on top of the metal, the confocal hole was set at 100 µm, while it was open to 1000 µm for collecting the spectrum of the solution. Results The AFM images shown hereafter were obtained on the condition that several aspects related to the drying step as well as to AFM imaging parameters be precisely optimized. Indeed, it must be first emphasized that the drying step appeared to be crucial for the ability of our different micellar samples to be imaged using AFM. An incomplete drying process led only to nonequilibrated unstable contacts between the AFM tip and the sample surface, making the AFM imaging impossible. This was possibly a situation resulting from antagonistic attractive and repulsive electrostatic forces, the first ones being assigned to the water layer covering the sample and/or micelle surface, while the latter could be assigned to the surface charge of the micelles. As a matter of fact, while the samples appeared to be visually dry within a few minutes or hours, clean imaging could not be obtained before several days or weeks in most cases. Both water and 2-propanol, used for solubilizing the surfactants, are quite volatile compounds, the latter having a vapor pressure (≈5.9 kPa at 25 °C) about twice the value for water (≈3.2 kPa) at the same temperature. This probably means that there remained for a longer time a shell of water and/or 2-propanol dipoles that disappeared more gradually14 (and maybe partially only). Let

21712 J. Phys. Chem. B, Vol. 110, No. 43, 2006 us emphasize that, except in a few cases15 where organic block copolymers were imaged in air from micellar assemblies formed in an organic solvent, the imaging method mainly reported so far for imaging micelles deposited on surfaces is AFM in the liquid phase. It relies on a repulsive noncontact method based on the repulsive forces existing between AFM tips and ionic micelle surfaces.3-13 In our experimental conditions, it was clearly evidenced that micellar structures could be imaged only on the condition that the amplitude set point is precisely chosen. Indeed, in the tapping mode that is the AFM imaging mode chosen in this work, the AFM tip-to-substrate distance was imposed through a servo-controlled oscillation amplitude. It appeared that imaging could be carried out only for the maximum amplitude value, allowing the intermittent contact to be maintained and thus corresponding to the minimum interaction between the AFM tip and the micellar sample. Such a condition corresponded to the longest distance between the nonexcited tip and the sample. When all these optimized conditions are fulfilled, AFM images such as those shown in Figure 1 were easily obtained. In Figure 1A,B showing, respectively, topography and phase images corresponding to a sample prepared with the micellar solution 1 (see its composition in Table 1), one can clearly observe rod-like structures deposited randomly at the top of a rather smooth surface. This latter one is a gold film known to be quite smooth when deposited on atomically flat mica surfaces using the evaporation technique in ultrahigh vacuum. Its AFM characterization reveals plateaus with a 1,1,1 crystallographic orientation separated by holes and/or crevices whose depth was a function of the thickness of the deposited gold film (image not shown). The initial overall RMS roughness of such films was found to be in the nanometer range in our experimental conditions. It must be nevertheless emphasized that those features could be only slightly observed in Figure 1. This might actually suggest that a material layer with an unknown thickness was formed or adsorbed on gold during the evaporation process of the micellar solution. Indeed, it is usually admitted that gold, which is well-known to be hydrophilic when freshly prepared under ultrahigh vacuum, promotes the adsorption of various organics (among which are aromatic ones) from aqueous solutions,16 which renders it permanently hydrophobic under ambient atmosphere. It is therefore conceivable that the counterion present in solution 1, namely, PCB, forms a first adsorbed layer on the freshly cleaned gold substrates. However, the amphiphilic nature of PCB and thus the possible interactions of a PCB-based adsorbed layer with the micellar structures existing in the bulk solution can hardly be predicted as it may depend on the orientation of the adsorbed PCB molecules. This latter issue was investigated using Raman spectroscopy. The Raman spectrum recorded from PCB solution and shown in Figure 2b (see Table 2 for the corresponding vibrational assignments) appeared in good agreement with previous studies.17 The assignment has been revisited on the basis of studies of benzoic acid (BA) and metal iodobenzoates from,18,19 in particular, the aromatic part of the molecule as band symmetry is to be used as a probe of adsorbate geometry.20 First, in solution (or adsorbed at the electrode as will be observed later on), parachlorobenzoate (PCB) is in the anionic form (and not in the para-chlorobenzoic acid (PCBA) form) as stated by the lack of νCdO vibration in the Raman spectra at 1630 cm-1 and the existence of νsCOO- corresponding to the anionic form of the molecule. As the solution pH is higher than PCBA’s pKa (≈4.2), the formation of the benzoate is obvious.

Chaal et al.

Figure 1. AFM images (2 µm × 2 µm) obtained from micellar solution 1 deposited on gold-coated mica. (A) topography, (B) phase, (C) vertical profile taken along the gray arrow drawn on A).

Second, the differences encountered in our experiments between the Raman spectrum of the adsorbed molecule (see Figure 2a) and the one in solution (see Figure 2b) are maximal for the bands corresponding to the carboxylate functional group (see Table 2). An opposite result was found in other studies

Characterization of Micelle Formation using TM-AFM

Figure 2. (a) In situ Raman spectrum of PCB on gold surface (inset: high-wavenumber range) and (b) Raman spectrum of a PCB aqueous solution: pH ) 8, [PCB] ) 7.5 mM, 1 mW laser power, 30 s acquisition time, 10 accumulations for spectrum a, 300 s acquisition time, 1 accumulation for spectra b. Spectra are shifted up for a need of clarity.

TABLE 1: Composition of the Micellar Solutions Used for AFM Imaging surfactant (S) (mM) solution

OTA+

1 2 3 4

4.25

counterion (C) (mM)

CTA+ 5

3.75 1.5

PCB

 ([C]/[S])

3.75 3.75 3.75 3.75

0.88 0.75 1 2.5

TABLE 2: Vibrational Assignment of Raman Wavenumbers (cm-1) for PCB in Solution and on Au Metal band wavelength (cm-1) PCB in solution 1640 (broad) 1595 1556 1491 1393 1177 1142 1096 1070 1016 845 779 728 631

PCB on Au

assignmenta

3077

2 βH2O 8a νas(COO-) 19a νs(COO-) ν(C-O) 9a 9b 18b 18a 12 βs(COO-) 11 γs(COO-) 6b

1591 1537 1375 1280 1165 1137 1092 1080 1017 842

a Denoted in Wilson notation.16 ν and ν are, respectively, symmetric s as and antisymmetric stretches. β stands for in-plane deformation, and γ stands for out-of-plane deformation.

dealing with the adsorption of BA21 or PCBA17 on silver substrates where the molecule was found to be flat on the metal substrate with obvious interactions between π-electrons from the ring and the metal. Values of the stretching modes are redshifted by 25 cm-1 for the symmetric and 19 cm-1 for the antisymmetric stretches, leading to a difference ∆ν between the two modes of 145 cm-1. The intensities of the bands corresponding to the carboxylate group are also modified, lowered for stretching, and null for out-of-plane deformation, while the intensity of in-plane deformation is kept quite constant upon adsorption. Bands corresponding to the ring vibration are measured at the same frequencies (or with (5 cm-1 resolution of the

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21713 apparatus) without any noticeable variation in their relative intensities or fwhm (full width at half-maximum). These two facts are an indication of an absence of interactions between the aromatic ring and the metal surface. The presence of the 3077 cm-1 band (on the side of the water stretching modes in the inset of Figure 2a) in the spectrum of the adsorbed molecule is also an indication of a molecule not lying flat on the gold surface. However, the benzene vibrations of the adsorbed molecule spectra show that the e2g bands are also present in the spectrum (bands: 18b and 9b). This precludes the possibility of a strictly perpendicular position. One question still remains unclear at this stage: what is the nature of the bond between gold and carboxylate group? Three configurations are known:22 bidendate chelation (with one metal ion) or bidendate bridging (with two metal ions) where a symmetric bonding between M and two oxygen atoms of the carboxylate group is formed or a monodendate coordination where only one oxygen interacts with the metal. According to ref 23, the relative intensities of βCOO- and νsCOO- are different in the three configurations. As our situation corresponds rather to βCOO- more intense than νsCOO-, it suggests a one-legged attachment with only one oxygen adsorbed on the surface. Moreover, the ∆ν value is also an indication of the complicated geometry in the case of carboxylates: when ∆νcomplex (162 cm-1 in our case) is higher than ∆νsodium salt (128 cm-1),24 the monodendate geometry is assumed, and finally, the existence of a weak but still measurable anti-symmetric stretching together with the rather weak feature at 1280 cm-1 attributed to a C-O vibration discard the possibility of symmetric adsorption in a bidendate or bridging geometry. To summarize the analysis of the Raman spectrum recorded on PCB adsorbed on a gold surface, a model can be proposed where the molecule, in its anionic form, is adsorbed on the metal surface by one of the oxygen atoms from the carboxylate functional group and takes a tilted orientation with respect to the surface plane. Such an orientation is thus likely to confer to the gold surface a rather pronounced hydrophobic character that may be sufficient to explain the absence of attractive interactions between the hydrophilic micelle surfaces and the substrate. This issue will be discussed and further evidenced later in this paper. In any case, Raman spectroscopy allowed the identification of the thin molecular layer suggested by the AFM image reported in Figure 1. In a further analysis of the rod-like structures observed in Figure 1, a vertical profile (see Figure 1C) of one isolated rod allowed us to measure 4 and 35 nm for its height and width, respectively. While the height reported here is in rather good agreement with the diameter expected for an OTA+ surfactantbased tightly packed cylindrical micelle, the 35 nm width suggested here can easily be linked to the same diameter of 4 nm on the basis of convolution considerations taking into account the tip radius defined by SEM observation of the tip used to produce Figure 1. Simple geometrical consideration yielded a tip radius at the apex of about 43 nm, a value only slightly higher than that of the AFM tip provider. Moreover, one can reasonably imagine that the micelles are trapped in salt base railways (or shells). It is indeed likely that the precipitation (or concentration) of salt (namely, sodium chloride) is favored locally during the evaporation process around the micellar structures by the hydrophilic character of the micelle surface. The dimensions given previously are nevertheless in very good agreement with those expected from the AFM image of a cylinder displaying a 4 nm diameter. For indication, the length of this cylinder can also be measured and was found to be about

21714 J. Phys. Chem. B, Vol. 110, No. 43, 2006 360 nm by taking into account convolution effects on each extremity of the micelle. In Figure 1A,B, micelles seem to display a random orientation that confirms a weak interaction with the underlying surface. Aggregation of these micelles does not seem to be systematic, as several almost completely isolated micelles can be observed. This may indicate that the interactions between the micelles are rather weak as well. On the other hand, the presence of cylindrical micelles is in very good agreement with the micellar morphology expected in the bulk phase of the micellar solution used to prepare this sample.1-2 While the cmc for OTA+ is not available to our knowledge, it is known from literature that the cmc decreases as the chain length of alkyltrimethylammonium-type surfactants increases. Knowing that the cmc for CTA+ is 1.3 mM from theory and 0.57 mM from experiments,25 one can reasonably expect that the lowest OTA+ concentration used in this work, namely, 1 mM, is above the cmc of OTA+, as it contains 18 carbon atoms while CTA+ contains only 16. Moreover, one knows from literature that the presence of an appropriate counterion bearing a hydrophobic moiety strongly promotes the formation of cylindrical micelles for much lower surfactant concentrations.26,27 This is actually confirmed from Figure 1. To confirm the presence of cylindrical micelles in such solution, one can also use the surfactant packing parameter. It was actually established that a value varying between 0.33 and 0.5 for this parameter leads to the formation of cylindrical micelles. It can be easily calculated on the condition that the area per surfactant headgroup be known. Using 65 Å2 for this area as proposed in the literature, the surfactant packing parameter was found to be 0.36 if the hydrophobic chain length and volume are, respectively, estimated as 2.44 and 0.57 nm3. This value, therefore, strongly corroborates the formation of cyclindrical micelles. Let us emphasize that the counterion PCB may lead to a decrease of the area per surfactant headgroup, which promotes even more the formation of cylindrical micelles. By comparison to Figure 1, a somewhat different situation is observed on Figure 3A (topography image) and Figure 3B (phase image) that were obtained from micellar solution 2 (see its composition in Table 1). In Figure 3, cylindrical micelles appear to be much more numerous than in Figure 1, leading thus to an almost fully covered gold surface. They appear as various superimposed aggregates oriented in random directions in which the cylindrical micelles are mostly perfectly parallel. As a consequence, the average length of micelles appearing in Figure 3 can hardly be determined, making the comparison with the micelle lengths obtained from Figure 1 impossible. A weak interaction of the micelles with the gold surface is again clearly evidenced. The difference in micelle population observed from the comparison between Figures 1 and 3 can hardly be attributed to the role of the counterion as other explanations dealing, for example, with the sample volume or the evaporation process may also be responsible for this difference. Let us emphasize moreover that our samples were found to be inhomogeneous in population as portions of the gold samples were completely naked while others were partially or almost fully covered with micelles. As discussed earlier for Figure 1, one can easily conclude that the micelles expected to form in solution 2 are cylindrical. The role of the different chemical compounds in the adsorption processes on gold was also assessed by HF EIS measurements. Using the procedure defined in the Experimental Procedures, CHF could be estimated from curves such as those plotted in Figure 4. Those curves are characterized by a quasihorizontal plateau at the lower or mid-frequencies (until about 1 kHz) and then a sharp decrease. This latter region is very

Chaal et al.

Figure 3. AFM images (2 µm × 2 µm) obtained from micellar solution 2 deposited on gold-coated mica. (A) Topography and (B) phase.

Figure 4. Variation of the high-frequency capacitance at a freshly polished gold electrode as a function of the 1/ω value for various applied potentials in the solution containing OTAC (Arquad S50, 2.5 mM) and PCB (3.75 mM)

often subject to artifacts induced by the reference electrode that sometimes shifts the experimental data in the first quadrant of the Nyquist plane (pseudo-inductive behavior). This might also be analyzed as a dispersive effect on the impedance due to a microroughness and therefore be sensitive only in the HF range.

Characterization of Micelle Formation using TM-AFM

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21715

Figure 5. Variation of the high-frequency capacitance at a freshly polished gold electrode as a function of the applied potential for various solutions. The plotted capacitance values correspond to 1/ω values approaching 0. In the inset are reported the concentrations of OTAC, PCB, and NaCl expressed in millimolar, millimolar, and molar, respectively.

The consequence is that instead of the classical capacitance, a constant phase element (CPE) must be considered with

|Im(Z)| f

-1 (jωC)n

where 0 e n e 1. The net result for the apparent capacitance is that it decreases when the frequency increases. This effect is visible in Figure 4. Finally, the values used for calculating the capacitances were those obtained from extrapolation to 1/ω zero values of the horizontal portions of the curves. The results for all conditions are reported in Figure 5. Examination of all the values indicates that, except for the higher cathodic potentials (i.e., from -1100 mV/SCE), CHF is systematically lower than the usual value (around 40 µF cm-2) of the double-layer capacitance, Cd. It must be also emphasized that both OTA+ and PCB contain lipophilic parts, and their eventual adsorption would therefore contribute to lower the local dielectric constant value and hence that of CHF. When PCB is the single present species (i.e., in the absence of NaCl and OTA+), CHF < Cd in the whole potential domain and decreases for increasing potentials. This clearly indicates that PCB adsorbs at all covered potentials and that it has a Coulombic origin due to the carboxylate group, which tends to increase as the metal charge becomes more positive. When OTA+ is added but without NaCl, one observes a slight decrease of CHF at -1100 mV/SCE and a slight increase for all other potentials. It can be assumed that one part of adsorbed PCB is desorbed to form micelles in the bulk with OTA+ and that at the higher cathodic potentials, free OTA+ can adsorb as the metal charge turns gradually to negative values. When NaCl is added, CHF increases at all potentials while keeping a minimum around -100 mV/SCE. This indicates a competitive adsorption between PCB and Cl-. From all EIS and Raman data, it can be concluded that (i) PCB adsorption is stronger than that of OTA+ and that the gold surface is mainly positive in the whole potential domain. (ii) the rest potential, at which the evaporation process for AFM observations was begun, lies within the potential range used for EIS measurements (Figure 5). (iii) PCB is adsorbed on gold by its hydrophilic part and extends its hydrophobic part toward solution.

Figure 6. AFM images (5 µm × 5 µm) obtained from micellar solution 1 deposited on glass. (A) Topography and (B) vertical profile taken along the gray arrow drawn in panel A.

The adsorbed PCB layer can thus be considered to a certain extent as a hydrophobic carpet on the gold substrate allowing the deposition of the cationic surfactant-based micellar structures, such as those observed in Figure 1, without any strong interactions between them. To confirm the chemical identity of the structures defined previously as cylindrical micelles, a micellar sample prepared from the deposition of several drops of solution 1 on a glass slide was prepared and then imaged using TM-AFM in air. The resulting image (see Figure 6A, topography image) shows a layer with perfectly well-defined contours at the top of a perfectly smooth surface. Let us emphasize for the need of comparison that a glass sample carefully cleaned, rinsed with the usual solvents (alcohol and double-distilled water), and then carefully dried revealed an extremely smooth surface when imaged in air with TM-AFM (image not shown) as no features were observed in our experimental conditions except noise. The layer structure is 4.4 nm high as measured from the vertical profile shown on Figure 6B, and it can therefore be attributed to the deposited surfactant solution. Indeed, this height can be reasonably identified to be twice the length of a C18 surfactant molecule (estimated to be 2.44 nm from literature7). This, therefore, implies that the layer observed in Figure 6A is actually a bilayer-type micelle. This type of structure has already been

21716 J. Phys. Chem. B, Vol. 110, No. 43, 2006

Chaal et al.

mentioned for this surfactant on mica.11 In such micelles, the hydrophilic glass surface, which is also anionic when immerged in aqueous solution, behaves as a huge poly-anionic entity able to form ion pairs with the cationic tetraalkylammonium-type surfactant molecules, leading thus to a surfactant monolayer in which the hydrophobic alkyl tails of the surfactant molecules are exposed to water. This thermodynamically unfavorable situation is compensated by the formation of a second surfactant layer based on hydrophobic-hydrophobic interactions of the alkyl tails of the surfactants. To the best of our knowledge, the exact role of counterions such as PCB (see Table 1) on the formation of such bilayers is poorly documented in the literature, but it seems rather weak. By comparison with Figures 1 and 3, the bilayer-type micelles observed in Figure 6 also confirm that the geometry of the micelles observed on a given type of surface is highly dependent on the strength and nature of the interactions between the surface and the surfactant molecules. Figures 7 and 8 both show topographic images of (OTA+ + PCB) micellar samples prepared, respectively, from micellar solutions 3 ( ) 1) and 4 ( ) 2.5) ( is the counterion to surfactant concentration ratio; see their composition in Table 1). One can easily observe that both images show cylindrical micelles lying randomly on a naked gold surface. The average length was found to be 86 and 247 nm, respectively, from the measurement of 16 and 23 micelles, respectively. It appears that for the same concentration in PCB (3.75 mM), the average micelle length increased as the OTA+ concentration decreased, corresponding to an increase of . Knowing that a higher value for  was shown to correlate satisfyingly with a better efficiency of drag reduction phenomena in these two micellar solutions, one can therefore establish at least from a qualitative point of view a correlation between the average micelle length and the drag reduction phenomenon.28 It appears that an AFM characterization of micelle morphology using our procedure is able to provide a precise determination of parameters that may help in the understanding and prevision of viscoelastic behavior of surfactant solutions in general and of drag reduction phenomena in particular. Further experiments are currently underway to better control the preparation step as well as the evaporation process of our micellar samples. Our next goal will be to carry out a systematic study of the micellar systems briefly investigated in this work to establish a closer and quantitative correlation introduced previously. Discussion At this stage, it is worth discussing other fundamental interrogations arising from the observations reported earlier in this paper. For example, the population density of micelles clearly varies from a sample to another as shown by our AFM images obtained on gold substrates. Besides many experimental parameters that can have a strong influence on this population density (surfactant concentration, initial volume of the evaporated drop), one can envisage the dynamics of evaporation. It appears in very recent literature that the dynamics of evaporation of liquid drops deposited on flat substrates is not yet fully understood and characterized nowadays, although it obviously implies among other parameters the viscosity of the evaporating solution that will depend on the micellar composition in our case and on the surface tension. For example, such an issue was investigated in the case of the completely wetting drops of pure liquids on smooth substrates, a system for which the theory is still being elaborated.29 In this very recent contribution, the authors carried out experimental investigations in an extended range of volatile alkane drop sizes evaporating on oxidized

Figure 7. AFM images (10 µm × 10 µm) obtained from micellar solution 3 deposited on gold-coated mica. (A) Topography, (B) phase, and (C) zoomed area inside the square zone represented in panel A.

silicon wafers under normal atmosphere to provide trends relevant enough to be integrated in a theoretical analysis. The

Characterization of Micelle Formation using TM-AFM

Figure 8. AFM images (5 µm × 5 µm) obtained from micellar solution 4 deposited on gold-coated mica. (A) Topography and (B) phase.

resulting theory probably takes into account most of the physical aspects of an evaporation process. It confirms the experimental power laws obtained for the radius of receding evaporating drops. In parallel, it provides a first attempt to predict the value of the contact angle in a dynamic situation as well as a generalization of Tanner’s law in the case of evaporating liquids. Nevertheless, the authors emphasize that this theory has to be improved. They propose to take into account the sensitivity of the solution to the hydrodynamic term by introducing in the equations a real profile of the drop instead of the slope at the edge. This theory dealing with dynamics of evaporation is potentially utilizable in our studies to predict the evolution of drop geometry and size with time. Unfortunately, it is only partially relevant for the interpretation of the evaporation process of our micellar solution as it concerns pure liquids only. Moreover, no information can be extracted from this theory concerning the consequences of the evaporation process for the micelles in terms of distribution and stability, for example. Much closer to our situation is the work published by Okubo et al. in which macroscopic and microscopic dissipative structural patterns were found to form and were characterized in the course of the drying of drops of aqueous solutions of

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21717 dodecyltrimethylammonium chloride deposited on a cover glass.30 At a macroscopic scale, broad ring patterns were found to form from the solidification of surfactant molecules around the outside edges of the film solution. It was demonstrated experimentally that the drying time T and the pattern area S, respectively, decreased and increased as the surfactant concentration increased. It was also established that T decreased significantly and S increased as the ethanol fraction increased in water/ethanol micellar solutions, whereas both T and S were found to decrease as the concentration of potassium chloride, calcium chloride, or lanthanum chloride increased. Those observations therefore confirm that 2-propanol, sodium chloride, and of course surfactant concentrations will strongly influence the formation of the patterns expected from the evaporation of our micellar solutions. It is also suggested that the convection of water and surfactants at different rates under gravity and the translational and rotational Brownian movement of the surfactant molecules control the macroscopic pattern formation. At a microscopic level, translational Brownian diffusion of the surfactant molecules as well as electrostatic and hydrophobic interactions of the surfactants between them and/or with the glass cover are likely to be determining factors in the formation of the observed patterns. Although deeply instructive, these studies suffer from a lack of relationships first with the process of micellization likely to occur in the course of the evaporation process as well as from a lack of morphological and chemical characterization, at the nanometer (or molecular) scale, of the various patterns observed. As a consequence, no information is provided concerning the chemical identity of the patterns observed and the evolution (or the stability) of the micelles during and after the evaporation process. At a molecular level, the micellization process is in itself the target of intense research effort. While the real composition of ionic micelles and micelle/solution interfaces is more and more characterized in detail thanks to powerful techniques,31,32 theoretical models allowing the composition and stability of micelles and micelle/solution interfaces to be justified or even predicted are still under progress.25,33 Nevertheless, a fully predictive, molecular-thermodynamic theory of micellization for ionic surfactant-electrolyte systems, in which counterion binding is accounted for explicitly, was proposed recently. More precisely, a thermodynamic framework describing the free energy of the micellar solution is combined with a detailed molecular model of micellization, which evaluates the various physicochemical contributions to the standard work of micelle formation (or free energy of micellization). By comparison with previous theories (see references cited in refs 25 and 33), one can note the following improvements. (i) The model of counterion binding assumes that the bound counterions adsorb at the same plane as the ionic surfactant heads within the Stern layer where they remain as independent entities. (ii) The theory is aimed at modeling the behavior of ionic surfactant systems containing multivalent counterions as well as lipophilic counterions. In the case of the latter, it is admitted that the lipophilic moiety of the counterion resides in the micelle hydrophobic core consisting of the surfactant tails and therefore contributes to the free energy changes associated with the formation of the micelle core. (iii) The free energy contributions associated with the formation of the micelle core have been evaluated in a more rigorous manner. On the basis of their own appreciation, the authors indicate that the level of detail with which these nonelectrostatic free energy contributions are modeled affects the actual quantitative agreement of the theoretical predictions with the experimental data, particularly in the case of micelle

21718 J. Phys. Chem. B, Vol. 110, No. 43, 2006 sizes. As such, this theory is likely to be easily applicable for the prediction of the morphological characteristics of the micelles existing in our micellar solutions before evaporation. Unfortunately, its use cannot be extended to the evaluation of the stability of the micelles observed in our AFM images after evaporation. Indeed, the molecular model on which this theory is built naturally includes considerations related to the presence of a diffuse layer (outside the Stern layer) that obviously disappears after complete evaporation of water. One can easily predict that the evaporation process will first lead to an increase of the ionic strength in this diffuse layer via a progressive thinning of this latter. The end of the evaporation process is likely to correspond to the situation where the diffusion layer has disappeared to give birth to a salt shell around the micelles. At this point, it must be mentioned that, although the drying time necessary to make the AFM imaging of the micelles possible in our experimental conditions is very long, only the softest interaction between the AFM tip and the micelles allowed the imaging procedure to be carried out successfully. This may indicate that the micellar structures are not in the solid state nor trapped in solid salt shells after complete evaporation but rather surrounded by a gel-like envelope whose dielectric constant is undetermined but possibly even lower than in the Stern layer defined in the aqueous phase, where it was supposed to be already very low due to the high degree of orientation of water molecules resulting from the very high concentration of salt in this region. Actually, the role, the amount, and the structure of water surrounding the micelles after complete evaporation are undetermined. In any case, this new environment is in consequence sufficient to make the theory reported previously inadequate to predict the stability of the micellar structures imaged in this work as long as it will not be chemically and electrostatically defined in a quantitative manner. In other words, the electrostatic free energy contribution to the overall free energy can hardly be defined for micelles in air on the basis of (i) the results reported in this paper and (2) the micellization theory summarized previously. Conclusion In this paper, TM-AFM experiments were carried out in air for imaging cylindrical micelles deposited randomly on gold surfaces in the presence of parachlorobenzoate counterion. The main result reported above is the ability of the ionic surfactantbased micellar structures encountered in this work to maintain their morphological characteristics after the complete evaporation of water. The AFM imaging method of micelles introduced here is therefore competitive by comparison with other imaging procedures of micelles reported in the literature as it appeared to be relevant to determine in air and with a nanometric resolution the morphological characteristics of stable micelles initially formed in aqueous solution without any significant perturbation coming from the evaporation process. Complementary information was provided concerning the micellar solution/gold interface using Electrochemical Impedance Spectroscopy and Raman spectroscopy. On such a substrate, PCB was shown to adsorb via one of the oxygen atom of its carboxylate group using Raman spectroscopy, leading thus to an upward orientation for the hydrophobic parachlorophenyl ring of PCB. This orientation of adsorbed PCB molecules was first ascribed to the positive sign of the surface charge of gold substrate demonstrated with electrochemical impedance spectroscopy measurements. The other observation to keep in mind is that the well-known and strong influence of glass substrate on the structural aspects

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