Thermotropic Behavior of a Cationic Surfactant in the Adsorbed and

Mar 30, 2012 - Knowledge about how temperature affects the internal structure and dynamics of surfactant aggregates can lead to a better understanding...
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Thermotropic Behavior of a Cationic Surfactant in the Adsorbed and Micellar State: An NMR Study Christian Totland and Willy Nerdal* Department of Chemistry, University of Bergen Realfagbygget, Allégaten 41, N-5007 Bergen, Norway S Supporting Information *

ABSTRACT: Knowledge about how temperature affects the internal structure and dynamics of surfactant aggregates can lead to a better understanding of their behavior in complex environments and processes. 13 C chemical shifts of the cationic surfactant tetradecytrimethylammonium bromide (TTAB) in micellar solution and when adsorbed on silica particles are recorded in the temperature range 8−78 °C, and give information on the conformation of the alkyl chain carbons. Adsorbed TTAB has conformational disorder similar to free TTAB at about 70 °C, with an increase in the rate of conformational changes occurring above 50 °C. Furthermore, no significant change in TTAB adsorption density was observed in the temperature range studied, and the results indicate a bilayer arrangement of the adsorbed surfactants. The number of gauche conformers increases linearly with temperature for the alkyl chain carbons in TTAB micelles. However, the total increase in gauche conformers is significantly smaller for micellar than for adsorbed TTAB within the temperature range studied. The fraction of micellized TTAB molecules in solution is found to increase with temperature.

1. INTRODUCTION Surfactant phase transitions occurring in aqueous solution have been the focus of numerous previous studies.1−5 The main features of the phase behavior of ionic surfactants in solution are the critical micelle concentration (CMC) and Krafft temperature (TK), above which micelles form. However, further increase in temperature or concentration can cause other phase transitions to occur. Some surfactants form vesicles above a critical temeprature, and further heating causes the vesicular bilayer to transition from a gel to liquid crystalline state.3 Gel to liquid crystalline phase transitions have been extensively studied by several experimental techniques such as NMR,6−8 infrared spectroscopy,9 and theoretical simulations.10,11 From these studies it is found that throughout the gel phase the alkyl chain conformation is predominately trans, whereas the number of gauche conformers increases at higher temperatures, causing a liquid crystalline phase to occur. Similarly, self-assembly of ionic surfactants adjacent to solid surfaces is central in many processes such as detergency,12,13 flotation,13,14 as well as wetting and enhanced oil recovery (EOR).15−18 The majority of previous studies have been concerned with adsorption kinetics and the structure of the adsorbed surfactant aggregates above and below equilibrium concentration levels.19 However, the thermotropic behavior of adsorbed surfactants has received less attention, despite its potential importance. Some previous studies have focused on the effects of temperature on surfactant adsorption both in petroleum reservoir cores 18 and in laboratory model systems.20−23 The conclusions from such studies is in general that adsorption of both ionic and nonionic surfactants at low © 2012 American Chemical Society

concentrations decreases slightly with increasing temperature, whereas the opposite is true for nonionic surfactants at high concentrations. Some studies also indicate that adsorbed aggregates are more stable at higher temperatures when the surface cover is sufficiently high.22 However, understanding amphiphilic association requires detailed knowledge of the internal structure and dynamics of the aggregate. In this respect, knowledge of alkyl chain conformations is important. In this study 13C NMR chemical shifts of tetradecyltrimethylammonium bromide (TTAB) adsorbed onto silica particles and in micellar solution are measured in the temperature range 8−78 °C in order to monitor possible variations in alkyl chain conformations and aggregate structure. Chemical shifts are determined by the degree of magnetic shielding of the nucleus by surrounding electrons, and variations in local molecular density can shift the resonance of the nucleus upfield (smaller chemical shift values) or downfield (higher chemical shift values) in the NMR spectrum. It has been shown that changes in 13C chemical shifts reflect changes in surfactant chain conformations, where downfield shifts indicate an increase in alkyl chain trans conformers and upfield shifts indicate an increase in gauche conformers.7,24−27 Our current understanding of adsorbed surfactant properties comes from a range of analytical techniques. Imaging techniques, such as atomic force microscopy (AFM)20,28,29 and Brewster angle microscopy (BAM),30 have previously been Received: February 24, 2012 Revised: March 27, 2012 Published: March 30, 2012 6569

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temperature and then centrifuged at 18 000 rpm, after which the supernatant was removed and analyzed. The surfactant adsorption density, Γ, is calculated using the equation

used to observe the morphological phase behavior of adsorbed surfactants. Liu and Ducker28 investigated the morphology of adsorbed alkyltrimethylammonium bromide (CnTAB) surfactants using AFM, and found that the equilibrium morphology at the solid−liquid interface was similar above and below TK. However, the mechanical properties of the adsorbed aggregate changed below TK, where slower molecular motion was indicated. AFM studies showed a spherical, micelle-like aggregation of TTAB on silica.29 These studies are conducted on flat surfaces where surface corrugation is less than the surfactant dimensions; hence, the morphologies observed by these techniques cannot be assumed for surfactants adsorbed on colloidal particles.31 Thermotropic phase transitions of oleate species adsorbed on flat surfaces have been studied with FT-IR spectroscopy.32−34 Here, the number of gauche conformers was found to increase with elevating temperatures, with a frequency shift occurring between 25 and 40 °C.33 In a similar FT-IR study, Clark and Ducker20 focused on the rates of adsorption, desorption, and exchange of TTAB adsorbed on silica from D2O solutions, and found that these rates were approximately the same 11 °C above and 8 °C below TK (∼14 °C for TTAB in D2O). Furthermore, it is suggested that TTAB forms surface micelles at the silica surface at concentrations above ∼3 mM. Soederlind and Stilbs24 investigated the 13C NMR chemical shifts of several adsorbed cationic surfactants at 298 K, and observed downfield shifts for several of the carbons in the alkyl chain, indicating an increase in the number of trans conformers upon aggregation. In the study presented here, a similar methodology is applied to investigate variations in 13C chemical shifts of both adsorbed and micellar surfactants over the temperature range 8−78 °C. Additionally, the surfactant adsorption density, Γ, is monitored to observe possible variations with temperature.

Γ=

Δc·V m·as

(1)

where Δc is the concentration difference (mol/L) in the surfactant solution prior to and after adsorption, V is the volume of the liquid phase, m (g) is the mass of the solid material, and as (m2/g) is the surface area of the solid. The TTAB concentration in the supernatant solution was estimated by comparing 1H NMR resonance intensities to those of a solution with known concentration.39 2.2. NMR. All NMR experiments were carried out on a Bruker Avance 500 Ultrashield spectrometer operating at 500 MHz for protons. For the silica-containing samples, a double resonance 4 mm MAS probe head was used. Single pulse (SP MAS) 13C spectra with high power proton decoupling were recorded with MAS spinning speeds of 4 kHz. The 90° pulse length was estimated to be approximately 6 μs. About 10 000 scans are sufficient to determine the 13 C chemical shifts (see Figure 1).

2. MATERIALS AND METHODS The cationic surfactant tetradecyltrimethylammonium bromide (TTAB, >99%) was used as received from the producer (Sigma Aldrich). The CMC of TTAB is ∼3.8 mM at 298 K.2,4,35,36 The Krafft temperature of TTAB was found to be below 4 °C, lower than the temperature range studied here (8−78 °C). The thermal stability of TTAB is good, and possible decomposition in the temperature range studied here is insignificant.35 Aerosil OX50 fumed silica particles with an average diameter of 40 nm and surface area of 50 ± 15 m2/g were used as adsorbent. These are nonporous silica particles with a high chemical purity, making them well suited for homogeneous samples.37 2.1. Sample Preparation. Samples for the 13C NMR studies were prepared by mixing ∼0.5 g silica particles with ∼5.0 g of a 5.3 mM TTAB solution. This is sufficient in order to obtain plateau adsorption on silica.23,38 The samples were tumbled for 24 h at room temperature and then centrifuged at 18 000 rpm, after which the supernatant was removed. The high speed centrifugation ensures that the particles are closely packed, thus reducing the contribution of surfactant bulk monomers on the NMR results. Immediately after centrifugation, the samples were packed into 4 mm ZrO2 MAS (magic angle spinning) rotors and sealed with rotor caps. To ensure similar conditions, new samples were made by this procedure for every 13C spectra recorded. This avoids possible effects caused by storage or fast sample spinning for longer periods. There was no significant difference in the silica/liquid ratios between parallels of similar samples. The 5.3 mM TTAB/SiO2 sample weight consisted of 60% liquid and 40% SiO2. Samples for adsorption density experiments were prepared in a similar way, by mixing ∼0.5 g silica particles with ∼5.0 g of a 5.3 mM TTAB solution preheated to the desired temperature. The samples were then tumbled for 24 h in an oil bath regulated to the desired

Figure 1. Bottom: 13C SP spectrum of a 1.0 mM TTAB solution. Top: 13 C SP MAS spectrum (4 kHz spinning) adsorbed on silica particles, recorded using 10 000 scans. Both spectra are recorded at 300 K. The TTAB molecular structure is displayed, where the alkyl chain carbons are numbered in accordance with the labeling of the 13C resonances in the bottom spectrum. CN denotes the nitrogen bound methyl groups. The C4−C11 peaks in the bottom spectrum are enhanced for better clarity. The ppm axis is calibrated with an external TMS sample.

3. RESULTS 3.1. 13C Chemical Shifts. The chemical shifts of surfactants are generally affected by two factors: (1) direct effects of the environment (medium effects), and (2) the average conformation of the molecules (conformation effects). Whereas 1H chemical shifts are affected by both factors, it has been shown that 13C chemical shifts of the surfactant alkyl chain are almost exclusively dependent on conformation effects. For hydrocarbons in general, this is called the γ effect and is thoroughly discussed elsewhere.40 Thus, changes in 13C chemical shifts can 6570

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1. Here, all but the C4−C11 carbon resonances are assigned; however, these resonances are nonresolved in the adsorbed TTAB spectra, and no assignments of these peaks were therefore attempted in this study. It is apparent from Figures 1 and 2 that the 13C resonances experience downfield shifts upon both adsorption and

to a good approximation be ascribed to conformational changes.25,27 The main advantage of using colloidal and close-packed nonporous silica particles is that it reduces the bulk phase (see later). In suspensions of solid particles in surfactant solution, surfactants exists both as “free” monomers in the bulk and in surface bound aggregates. Assuming the chemical exchange rate between free and aggregated surfactants is fast on an NMR time scale, the observed chemical shift, δobs, will be a weighted average between the chemical shift of aggregated surfactants, δagg, and free surfactants, δfree: δobs = pagg δagg + (1 − pagg )δfree

(2)

Here, pagg is the fraction of aggregated surfactants. Depending on the system and sample preparation, previous NMR studies have indicated both fast and slow chemical exchange.24,41 For centrifuged (close-packed) particles, previous 2H NMR studies have shown that chemical exchange is slow on the NMR time scale, and that the spectra are entirely dominated by resonances of adsorbed molecules.24,41 Additionally, in the system presented here it is reasonable to assume that pagg ≫ pfree, given the surface area to solution volume ratio; thus, no corrections of the observed shifts are done. However, eq 2 also applies for micellized TTAB, and δobs must therefore be adjusted accordingly in order to observe correct values. This was done by measuring the concentration dependence of δobs (see Supporting Information).42,43 In these calculations it is assumed that δmic (the chemical shift of micellized TTAB) is concentration independent, i.e. that there is no change in micellar shape in the concentration range studied that would change the gauche to trans ratio. This assumption is well supported by a previous study, which shows that sphere-to-rod transitions only occur for alkyltrimethylammonium bromide surfactants with longer hydrocarbon chains than TTAB, and then only at considerably higher concentrations than that used here.2 Furthermore, a factor which should be considered when evaluating the plots presented in Figure 4 is whether or not the ratio between free and micellized TTAB changes with temperature. This would affect the calculated chemical shift compensation, Δδ, added to δobs according to eq 2. The CMC of TTAB has been shown to increase slightly with increasing temperature.2,4,36 On the basis of the results of refs 2, 4, and 36, the 5.3 mM TTAB solution used in Figure 4 is likely above CMC within the temperature region used. To investigate whether the ratio between free and micellized TTAB changes with temperature, similarly to the CMC, Δδ was calculated at 280, 300, 320, and 350 K and found to decrease with temperature (see Supporting Information). This means that more surfactants are aggregated in micelles at higher temperatures. Considering the increase in CMC this is not surprising. In the plots presented in Figure 4 this has been accounted for. Approximate CMC values used in the calculations of Δδ at different temperatures were estimated on the basis of average results from previous studies (see Supporting Information).2,4,36 13 C NMR spectra of a 1.0 mM TTAB solution (below CMC) and TTAB adsorbed on silica particles are displayed in Figure 1. As shown in Figure 1, the middle C4−C11 carbon resonances, as well as the C3 and C13 resonances, could not be resolved for adsorbed TTAB, despite MAS spinning speeds of 4 kHz. 13C signal assignments have been reported previously for CnTAB,24,44 and are indicated in the lower spectrum of Figure

Figure 2. 13C chemical shift differences at 300 K for the alkyl chain carbons between adsorbed and free TTAB (△), and micellar and free TTAB (■). Downfield shifts (increased number of trans conformers) are taken as positive. The free TTAB corresponds to a 1.0 mM TTAB solution. Chemical shift values for micellar TTAB are corrected according to eq 2 (see Supporting Information). The C4−C11 resonances could not be separated for the adsorbed TTAB sample; thus, the chemical shift values for these carbon atoms are average values, also for micellar (and free) TTAB.

micellization, indicating an increased number of trans conformers. An exception is the C1 carbon which experiences an upfield shift upon micellization (Figure 2). The 13C chemical shifts of micellized TTAB (δmic) are smaller than those of adsorbed TTAB (δads), indicating more trans conformers in the adsorbed aggregate compared to in micelles. However, the features shown in Figure 2 are fairly similar for both adsorbed and micellized TTAB, indicating some similarity for the conformations of the alkyl chain carbons in the two states. Soederlind and Stilbs24 obtained results for micellized and adsorbed DTAB (dodecyltrimethylammonium bromide) and CTAB (hexadecyltrimethylammonium bromide) that show similar features to those found in Figure 2. Furthermore, a 2 H NMR study by the same authors indicates bilayer structure for both the adsorbed DTAB and CTAB.41 MD simulations show that a high number of trans conformers supports formation of surfactant bilayers,45 thus also supporting the bilayer arrangement of the adsorbed TTAB in this study. Micelles have in general been shown to have a more liquid-like interior, and presence of both gauche and trans conformers.46 The average areas occupied per adsorbed TTAB molecule on silica listed in Table 1 are similar to values found for CTAB Table 1. Adsorption Densities, Γ, in mol/m2, and the Corresponding Average Area, A, Occupied per Adsorbed Molecule in nm2 for a 5.3 mM TTAB Solution Adsorbed on OX50 Silica Particles at Varying Equilibration Temperatures

6571

equilibration temperature

Γ (mol/m2) × 10‑25

A (nm2)

25 °C 45 °C 75 °C

6.41 5.91 6.18

0.39 0.36 0.37

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Figure 3. Left: 13C chemical shift variations with temperature for TTAB adsorbed on silica particles. The stapled line indicates the chemical shift of free TTAB monomers (1.0 mM solution at 300 K). Lines are drawn as guides for the eye. Right: Corresponding 13C MAS spectra recorded at 280, 300, 320, and 350 K with 4 kHz sample spinning.

(0.36 nm2) and DTAB (0.35 nm2) at room temperature.24 In comparison, estimates of average areas occupied by TTAB in micelles range between 0.47 and 0.51 nm2.2,47 For the values of A (nm2) in Table 1 the structure of the adsorbed layer has been disregarded. Thus, a reduced area occupied per molecule for adsorbed TTAB is expected assuming a bilayer arrangement due to neighboring molecules having opposite spatial orientations and, thus, reduced electrostatic repulsion between the positively charged head groups in comparison to micelles. From Figure 2, it is apparent that the largest downfield shift occurs for the C4−C11 carbons in the adsorbed TTAB molecules which are shifted 2.1 ppm on average. This result is comparable to results from previous studies on similar systems.24,27 Furthermore, the largest difference between adsorbed and micellized TTAB is for the C1 carbon. This carbon atom is positioned so that it is not affected by the γeffect, and 13C chemical shift variations for the C1 resonance are thus not related to gauche−trans conformations. However, the C1 carbon is positioned so that it will be more affected by water molecules residing between the TTAB head groups. It is therefore possible that the difference between δmic and δads for this specific carbon is related to the average amount of water molecules in its vicinity; in a micelle all head groups are facing water, whereas in an adsorbed bilayer a large amount of head groups are facing the silica surface. Furthermore, increased local atomic charge density at C1 upon micellization can explain the upfield shift compared to free TTAB,48 and can also contribute to the observed difference between adsorbed and micellar TTAB. Despite MAS spinning speeds of 4 kHz, the 13C resonances of the adsorbed surfactants show considerable line broadening, indicating severe motional restrictions for the adsorbed TTAB compared to free TTAB. Furthermore, there are variations in the degree of line broadening for different carbon atoms in the

alkyl chain. Due to overlapping of some of the resonances and narrowing of the line widths due to MAS, a detailed discussion of the line widths is not included here. However, from Figure 1 it is obvious that the resonance line broadening is particularly severe for carbons C1 and C2. This was also found in a previous study.24 Furthermore, the C1 resonance is not visible at temperatures below 300 K (with 10 000 accumulated FIDs), while the C2 resonance is only barely visible. Figure 3 shows how the 13C chemical shifts of adsorbed TTAB vary with temperature. It is apparent that the 13C chemical shifts have a nonlinear dependency on temperature. The dotted lines in the plots indicate the chemical shift for the specific carbon(s) in a 1.0 mM TTAB solution of nonaggregated molecules at 300 K. At temperatures of about 70− 80 °C (with some variation depending on the carbon in question) the 13C chemical shifts are similar to those of nonaggregated molecules. The adsorption density measurements (Table 1), however, indicate no significant desorption in the temperature range studied (the variations in A and Γ are too small to indicate any trend). This also indicates that no major structural phase transition occurs in the temperature range studied. Consequently, the surfactants remain aggregated at the surface despite a large number of alkyl chain gauche conformers. Assuming a bilayer arrangement, this indicates that the interior of the bilayer becomes increasingly liquid-like with increasing temperatures, similar to a gel to liquid crystalline phase transition. The resonance representing the C4−C11 carbons (Figure 1) shows a single peak in the entire temperature region studied, and it was thus not possible to quantify the number of surfactants with gauche conformers in the alkyl chain, such as some authors have done previously.27 Furthermore, due to the gradual upfield movement of the resonances with increasing temperature, the observed reso6572

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Figure 4. Left: 13C chemical shift variations with temperature for a 0.0053 M TTAB solution. The stapled line indicates the chemical shift of the specific carbon in free TTAB monomers (0.001 M solution at 300 K). The plots for carbons C4−C11 are numbered from 1 to 6 according to the position of the corresponding 13C resonances in the NMR spectrum, from high to low chemical shift values. Right: Corresponding 13C spectra of the 0.0053 M TTAB solution at 280, 300, 320, and 350 K.

Figure 5. Illustration of a probable sample arrangement, assuming an idealized sample with spherical and uniform particles. The average minimum distance between particles is displayed, as well as the radius of an octahedral hole between particles. The TTAB all-trans alkyl chain length is 2 nm.49

with increasing temperature. This indicates that the micelle interior becomes more liquid-like at higher temperatures. By comparing Figures 3 and 4 it is clear that the 13C chemical shift differences between high and low temperature regions for a specific carbon is significantly smaller for mizelliced TTAB than for adsorbed TTAB. This indicates that less energy is needed to induce conformational disorder in an adsorbed bilayer than in micelles.

nance shifts are likely average values due to dynamical averaging of trans and gauche states (see later). The various alkyl chain 13C chemical shifts of micellized TTAB (Figure 4) vary differently with temperature than for adsorbed TTAB (Figure 3). In fact, for the C1 carbon of micellized TTAB the 13C chemical shift values increase exponentially with increasing temperature. For the C2−C13 carbons, however, there is a linear decrease in 13C chemical shift values, and a corresponding increase in gauche conformers, 6573

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4. DISCUSSION 4.1. Adsorbed Aggregate Structure at 300 K. In contrast to AFM and IR studies of TTAB adsorbed on a flat surface,20,28,33 which indicate adsorbed micelles, the results presented here for silica particles (40 nm diameter) indicate a bilayer structure. This is due to differences between δads and δmic (Figure 2), the average area occupied by each molecule (Table 1), and correlation with results from previous 2H and 13 C NMR studies on similar systems. 24,41 A possible explanation for a bilayer arrangement of TTAB adsorbed on sufficiently small silica particles is that it is more favorable for the surfactants to form aggregates that follow the curvature of the particle. However, no definite equilibrium structure can be deduced from the results in this study alone. The silica particles in this study have an average diameter of 40 nm. A close packing of the particles is implied by the silica/ liquid volume ratio of approximately 1:1.5. A hypothetical sample of spherical, uniform, and hexagonal close-packed particles will by simple mathematical calculations consist of 74 vol % particles and 26 vol % space. However, the silica liquid ratio for the 5.3 mM TTAB/SiO2 samples in this study shows that 60% of the sample volume is occupied by liquid. Assuming hexagonal packing and spherical uniform particles, the particles must on average be separated by 9.1 nm to accommodate this amount of liquid, giving a maximum radii of 10.2 nm for a sphere placed in an octahedral hole between particles. This situation is depicted in Figure 5. The maximum length of a TTAB alkyl chain is approximately 2 nm;49 thus, it is not unlikely that the increased amount of liquid in the 5.3 mM TTAB/SiO2 samples are due to the particles being separated by adsorbed surfactants. In our real samples, however, deviance from particle uniformity will lead to some deviation from the theoretical silica/liquid ratio and thus also the size of the “pores” in between the particles. Figure 5 shows an idealized situation for the samples in this study, assuming uniform particles, and illustrates that there is little space available for a bulk phase in the samples. Thus, the majority of surfactant molecules in the samples will be associated with the silica surface. 4.2. Effects of Temperature, 8−78 °C. 4.2.1. Adsorbed TTAB. Infrared spectroscopy has previously shown that there are few “double-gauche” sequences (two adjacent gauche conformers) in adsorbed bilayers, and that the dominant gauche types are “end-gauche” (the terminal methyl group is in the gauche conformation relative to the methylene groups three carbon atoms away) and the “gauche−trans−gauche” sequence, referred to as a “kink”.27 In comparison, micelles (SDS) show a larger amount of double-gauche sequences.50 Furthermore, infrared spectroscopy indicates that there is no more than one kink per chain in an adsorbed bilayer. The observed upfield shifts with increasing temperature for all the central alkyl chain carbons are therefore likely due to a single kink that is mobile along the length of the chain, and hence affects all carbons. A recent theoretical study supports this assumption.51 Thus, the upfield movement observed for the TTAB 13C resonances as the temperature increases likely reflects changes in the ratio between all-trans chains and chains containing a single kink sequence moving along the alkyl chain at a rate fast on an NMR scale. For the C2−C13 carbons the alkyl chain conformation varies similarly with temperature for all carbons. The nonlinear increase in chains containing gauche conformers with increasing

temperature can be indicative of a melting point for the bilayer, similar to that found for lipid bilayers. Figure 3 shows that a steeper decline in 13C chemical shifts occurs at roughly 320 K (∼50 °C) for the C2−C13 carbons, and this may thus indicate a transition temperature for the adsorbed TTAB bilayer. For vesicles and lipid bilayers, the melting point is also associated with an increase in lateral diffusion and “flip−flop” motions within the bilayer. Molecules adsorbed on a surface, however, will have a larger barrier for lateral diffusion due to interaction with the solid surface, and the melting points of such systems may thus have a different nature than vesicular bilayers. As mentioned, a previous study shows that the adsorption, desorption, and exchange rates of adsorbed TTAB remain unchanged with increasing temperature.20 However, the conformational disorder occurring at higher temperatures appears similar for adsorbed and vesicular bilayers. For instance, a vesicular DPPC (dipalmitoylphosphatidylcholine) bilayer exists in a gel phase below 35 °C, and obtains a liquid crystalline state at 42 °C. At 35 °C the 13C chemical shifts of the central alkyl chain carbons of DPPC move approximately 2 ppm upfield,8 similar to the total upfield shift observed for the central carbons in adsorbed TTAB between 280 and 350 K. At the transition to the liquid crystalline phase at 42 °C a smaller additional upfield shift of about 0.7 ppm is observed for DPPC. The results from the study presented here suggest that the transition to a state with higher bilayer fluidity occurs more gradually for the adsorbed TTAB bilayer than for lipid bilayers. It is possible that the chemical shifts will stabilize above a certain temperature when all TTAB alkyl chains experience the gauche effect. Another scenario is that the bilayer TTAB structure will dissociate when the gauche population reaches a certain level. However, it was not possible to do measurements at higher temperatures in this study due to equipment constraints. Micelles. In contrast to what is found for a bilayer, the linear increase in gauche conformers in micelles demonstrates that there is no defined phase transition temperature for micelles. Furthermore, the small difference in chemical shifts, and consequently small increase in conformational disorder, between low and high temperature regions for micellar TTAB may be due to the density of alkyl chains in micelles. If the molecules occupy a similar area in the adsorbed bilayer as in a micelle, one would expect the average area occupied by each TTAB molecule (A) in the bilayer to be approximately half of that in a micelle, i.e., about ∼0.25 nm2. However, a value for A of ∼0.37 nm2 is obtained for adsorbed TTAB, which is only about 25% lower than the value found for micellized TTAB. Assuming the adsorbed molecules are in fact arranged in a bilayer, this could explain the observed differences in temperature dependency between δmic and δads; due to increased chain separations, the bilayer structure allows for more conformational disorder as the temperature is increased, causing δads to move further upfield with increasing temperature than δmic. It is well-known that the shape of a surfactant aggregate in solution to a good approximation depends on the “critical packing parameter”, equal to V/(a0lc), where V and lc are the volume and the critical length of the hydrocarbon chain, respectively, and a0 is the headgroup area. Smaller values of the critical packing parameter give increasing positive curvature of the aggregates. Thus, from this simple model it is apparent that a larger value of V is required to transition from a micelle to a bilayer structure. 6574

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(4) Aguiar, J.; Molina-Bolivar, J. A.; Peula-Garcia, J. M.; Carnero Ruiz, C. Thermodynamics and micellar properties of tetradecyltrimethylammonium bromide in formamide−water mixtures. J. Colloid Interface Sci. 2002, 255, 382−390. (5) Cerichelli, G.; Mancinit, G. NMR techniques applied to micellar systems. Curr. Opin. Colloid Interface Sci. 1997, 2, 641−648. (6) Davis, J. H. Deuterium magnetic resonance study of the gel and liquid crystalline phases of dipalmitoyl phosphatidylcholine. Biophys. J. 1979, 27, 339−358. (7) Batchelor, J. G.; Prestegard, J. H.; Cushley, R. J.; Lipsky, S. R. Conformational analysis of lecithin in vesicles by 13C NMR. Biochem. Biophys. Res. Commun. 1972, 48, 70−75. (8) Mavromoustakos, T.; Theodoropoulou, E.; Yang, D.-P. The use of high-resolution solid-state NMR spectroscopy and differential scanning calorimetry to study interactions of anaesthetic steroids with membrane. Biochim. Biophys. Acta, Biomembr. 1997, 1328, 65−73. (9) Cameron, D. G.; Casal, H. L.; Gudgin, E. F.; Mantsch, H. H. The gel phase of dipalmitoyl phosphatidylcholine. An infrared characterization of the acyl chain packing. Biochim. Biophys. Acta, Biomembr. 1980, 596, 463−467. (10) Leekumjorn, S.; Sum, A. K. Molecular studies of the gel to liquid-crystalline phase transition for fully hydrated DPPC and DPPE bilayers. Biochim. Biophys. Acta, Biomembr. 2007, 1768, 354−365. (11) Marrink, S. J.; de Vries, A. H.; Tieleman, D. P. Lipids on the move: Simulations of membrane pores, domains, stalks and curves. Biochim. Biophys. Acta, Biomembr. 2009, 1788, 149−168. (12) Paria, S.; Manohar, C.; Khilar, K. C. Experimental studies on adsorption of surfactants onto cellulosic surface and its relevance to detergency. J. Instit. Eng., Singapore 2003, 43, 34−44. (13) Paria, S.; Khilar, K. C. A review on experimental studies of surfactant adsorption at the hydrophilic solid-water interface. Adv. Colloid Interface Sci. 2004, 110, 75−95. (14) Scamehorn, J. F. H. Surfactants in Chemical/Process Engineering; Marcel Dekker: New York, 1988. (15) Standnes, D. C.; Austad, T. Wettability alteration in carbonates: Interaction between cationic surfactant and carboxylates as a key factor in wettability alteration from oil-wet to water-wet conditions. Colloids Surf., A 2003, 216, 243−259. (16) Aoudia, M.; Al-Maamari, R. S.; Nabipour, M.; Al-Bemani, A. S.; Ayatollahi, S. Laboratory study of alkyl ether sulfonates for improved oil recovery in high-salinity carbonate reservoirs: A case study. Energy Fuels 2010, 24, 3655−3660. (17) Yongfu Wu, P. J. S.; Blanco, M.; Tang, Y; Goddard, W. A. An experimental study of wetting behavior and surfactant EOR in carbonates with model compounds. Soc. Pet. Eng. J. 2008, 13, 26−34. (18) Ziegler, V. M. Effect of temperature on surfactant adsorption in porous media. Soc. Pet. Eng. J. 1981, 21, 218−228. (19) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Mechanism of cationic surfactant adsorption at the solid-aqueous interface. Adv. Colloid Interface Sci. 2003, 103, 219−304. (20) Clark, S. C.; Ducker, W. A. Exchange rates of surfactant at the solid−liquid interface obtained by ATR-FTIR. J. Phys. Chem. B 2003, 107, 9011−9021. (21) Cases, J. M.; Villieras, F. Thermodynamic model of ionic and nonionic surfactants adsorption-abstraction on heterogeneous surfaces. Langmuir 1992, 8, 1251−1264. (22) Denoyel, R.; Rouquerol, J. Thermodynamic (including microcalorimetry) study of the adsorption of nonionic and anionic surfactants onto silica, kaolin, and alumina. J. Colloid Interface Sci. 1991, 143, 555−572. (23) Stodghill, S. P.; Smith, A. E.; O’Haver, J. H. Thermodynamics of micellization and adsorption of three alkyltrimethylammonium bromides using isothermal titration calorimetry. Langmuir 2004, 20, 11387−11392. (24) Soederlind, E.; Stilbs, P. Chain conformation of ionic surfactants adsorbed on solid surfaces from carbon-13 NMR chemical shifts. Langmuir 1993, 9, 1678−1683. (25) Persson, B. O.; Drakenberg, T.; Lindman, B. Amphiphile aggregation number and conformation from carbon-13 nuclear

Another factor is that, due to the anchoring of TTAB to the surface, additional energy supplied to an adsorbed bilayer may to a greater extent result in increased conformational disorder, whereas in a micelle other processes, such as increased lateral motion and exchange with bulk monomers, are more likely to occur. A previous study showed that the aggregation number of TTAB decreases with increasing temperature, along with a decrease in charge density.2 As mentioned, the adsorption density of adsorbed TTAB does not change with temperature (Table 1). This demonstrates that different processes take place in the micellar and adsorbed state when additional energy (heat) is introduced to the system.

5. CONCLUSIONS In this study, the differences in alkyl chain conformational disorder for TTAB in the adsorbed and micellar state have been demonstrated between 8 and 78 °C. The adsorbed state best fit a bilayer structure, and seems to possess a transition temperature at about 50 °C, above which the conformational disorder increases more rapidly. Such a transition temperature is not observed for micelles. Furthermore, no significant desorption or changes in aggregate structure were observed for adsorbed TTAB. The total increase in gauche conformers for the adsorbed TTAB bilayer within the temperature range studied is similar to that found for gel to liquid crystalline phase transitions in vesicular bilayers. For micellar TTAB, the ratio between free and micellized TTAB changed with increasing temperature toward a larger fraction of micellized TTAB. Apart from the C1 carbon, for which the number of trans conformers increased with increasing temeprature, the number of gauche conformers increases linearly with temperature for all alkyl chain carbons in TTAB micelles; however, the total increase in gauche conformers is significantly smaller for micellar than for adsorbed TTAB.

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ASSOCIATED CONTENT

S Supporting Information *

Information concerning corrections done to δobs for the TTAB micellar solution. Plots of the 13C chemical shift difference between δfree and δmic versus concentration for all carbon resonances at 280, 300, 320, and 350 K. Table with values of Δδ. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 47-55-583353. Fax: 47-55-589400. E-mail: Willy. [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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