Effects of Organic Solvent Addition on the Aggregation and Micellar

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Langmuir 2007, 23, 11496-11505

Effects of Organic Solvent Addition on the Aggregation and Micellar Growth of Cationic Dimeric Surfactant 12-3-12,2BrAmalia Rodrı´guez,† Marıá del Mar Graciani,† Manuel Angulo,‡ and Marıá Luisa Moya´*,† Departamento de Quı´mica Fı´sica, UniVersidad de SeVilla, c/o Profesor Garcıá Gonza´ lez 2, 41012 SeVilla, Spain, and SerVicio de RMN, UniVersidad de SeVilla, Apartado 1203, E-41071 SeVilla, Spain ReceiVed July 28, 2007. In Final Form: August 30, 2007 The micellization and micellar growth of cationic dimeric surfactant propanediyl-R-ω-bis(dodecyldimethylammonium) bromide, 12-3-12,2Br-, have been studied in several water-organic solvent mixtures. The organic solvents were ethylene glycol, glycerol, 1,2-propylene glycol, 1,3-propylene glycol, acetonitrile, dioxane, formamide, and N,Ndimethylformamide. Results showed that the aggregation process was less favored in the binary mixtures than in pure water, which was explained by considering the influence of the solvophobic effect on micellization. The addition of organic solvents was accompanied by a diminution in the average aggregation number, Nagg, of the dimeric micelles. o , to the Gibbs energy This diminution was due to the decrease in the interfacial Gibbs energy contribution, ∆Ginterfacial of micellization caused by the decrease in the hydrocarbon/bulk-phase interfacial tension. As a result of the micelle size diminution, the concentration at which the sphere-to-rod transition occurred, C*, was higher in the mixtures than in pure water. Micelle size reduction is accompanied by a decrease in the ionic interactions and in the extra packing contribution to the deformation of the surfactants tails, making the formation of cylindrical micelles less favorable.

Introduction work,1

In recent the effects of ethylene glycol, EG, addition on the aggregation and micellar growth of three didodecyl dicationic dibromide dimeric surfactants, 12-s-12,2Br- (where s ) 3-5 methylene groups), were investigated. The scope of this work was to study the influence of the bulk phase (water-EG mixtures) characteristics on morphological transitions. The addition of polar organic solvents to aqueous micelle solutions alters the tendency of the amphiphile molecules to aVoid contact with the solvent; therefore, the surfactant concentration range in which sphere-to-rod transitions take place is expected to change.1 Results showed that an increase in the EG content present in the dimeric micellar solutions caused an increase in the surfactant concentration where the sphere-to-rod transitions occurred. The authors proposed that the main factor responsible for this observation was the decrease in the interfacial Gibbs energy contribution, ∆Gointerfacial, to the Gibbs energy of micellization, ∆GoM, produced by the increase in the amount of EG.2 To investigate this subject further, the sphere-to-rod transition of cationic dimeric surfactant propanediyl-R-ω-bis(dodecyldimethylammonium) bromide,3 12-3-12,2Br-, has been studied in several water-organic solvent mixtures. Only organic solvents that remain mainly in the bulk phase of the micellar solutions were chosen. Solvents that incorporate to some degree into the micelles cause changes in the characteristics of the aggregates not only because of variations in the bulk-phase properties (solvophobic effect) but also because of their incorporation into the micellar aggregates. The contributions of the two effects cannot be separated. With this in mind, aqueous binary mixtures * Corresponding author. E-mail: [email protected]. Homepage: www. centro.us.es/coloides. † Departamento de Quı´mica Fı´sica. ‡ Servicio de RMN. (1) Rodrı´guez, A.; Graciani, M. M.; Munõz, M.; Robina, I.; Moya´, M. L. Langmuir 2006, 22, 9519. (2) (a) Camesano, T. A.; Nagarajan, R. Colloids Surf., A 2000, 167, 165. (b) Nagarajan, R.; Wang, C.-C. Langmuir 2000, 16, 5242. (3) (a) Menger, F. M.; Keiper, J. N. Angew. Chem., Int. Ed. 2000, 39, 1906. (b) Zana, R. J. Colloid Interface Sci. 2002, 248, 203.

of ethylene glycol (EG), glycerol (GLY), N,N-dimethylformamide (DMF), formamide (FM), acetonitrile (ACN), 1,2-propanediol (1,2-PROP), 1,3-propanediol (1,3-PROP), and dioxane (DO) were used in the study of the sphere-to-rod transition of 12-3-12,2Brmicelles. Conductivity, surface tension, NMR, and steady-state fluorescence measurements were carried out in order to elucidate the results. Practical aspects such as the rheological behavior and solubilization capacity of micellar solutions depend on the aggregate morphology. Therefore, the investigation proposed in this work is of interest in both pure and applied sciences. Besides, to our knowledge, no previous studies on the influence of bulkphase characteristics on micellar growth have been carried out. Experimental Section Materials. 6-Methoxy-N-(3-sulfopropyl)quinolinium, SPQ, was purchased from Molecular Probes, Inc. and used as received. The organic solvents were from Fluka and were used without further purification. Pyrene-3-carboxaldehyde was from Fluka, and pyrene was from Aldrich; the later was purified before use by methods reported in the literature.4 Hexadecylpyridinium chloride was from Aldrich and was recrystallized from acetone before use. Ethylene glycol-d6 was from Across Organics (99%), and D2O was from Merck (99.8%). The synthesis of the dimeric surfactant was done as described in ref 3a. The surfactant was characterized by 1H NMR, 13C NMR, and elemental analysis (CITIUS, University of Seville), and the results were in agreement with those previously reported. Conductivity Measurements. Conductivity was measured with a Crison GLP31 conductimeter, connected to a water-flow cryostat maintained at 298 K. A Crison Burette 1S dispenser was programmed to add adequate quantities of a concentrated surfactant solution in order to change [surfactant] from concentrations well below the cmc up to at least 2 to 3 times the cmc concentration. This method allows one to obtain a large number of experimental conductivity data, causing the estimation of the cmc to be more accurate. The conductimeter was calibrated with KCl solutions of the appropriate concentration range. (4) Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1973, 95, 6885.

10.1021/la702293d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007

Cationic Dimeric Surfactant 12-3-12,2BrSteady-State Fluorescence Measurements. Fluorescence measurements were made by using a Hitachi F-2500 fluorescence spectrophotometer. The temperature was kept at 298 K by a waterflow thermostat connected to the cell compartment. cmc Determination by Using SPQ as a Probe. The water-organic solvent 1 × 10-6 M SPQ surfactant solutions were prepared in doubly distilled water. The fluorescence intensities were measured at 443 nm by the excitation at 346 nm, as indicated in ref 5. Excitation and emission slits were 5 and 10 nm, respectively, and a scan speed of 240 nm was used. AVerage Micellar Aggregation Numbers Determination. To obtain the average aggregation number, Nagg, of the dimeric micelles in the different water-organic solvent mixtures, the study of the fluorescence quenching of pyrene by N-hexadecylpyridinium chloride, CPyC, was carried out in those media. Solutions of pyrene (1 × 10-6 mol dm-3) in the different aqueous binary mixture micellar solutions were prepared as in ref 6. In all cases, a surfactant concentration equal to 0.02 M was used. Pyrene was excited at 335 nm, and its emission was recorded at 373 and 384 nm, which correspond to the first and third vibrational peaks, respectively, with the use of excitation and emission slits of 2.5 and 2.5 nm. A scan speed of 60 nm/min was used. The low probe concentration avoided excimer formation, and the quencher concentration was varied in such a way that [pyrene]/ [micelles] and [quencher]/[micelles] ratios were low enough to ensure a Poisson distribution.7 The aggregation number obtained for 123-12,2Br- micelles in water was in agreement with literature data obtained by the same method.5,8 Second Inflection Point, C*, Determination. The water-organic solvent 1 × 10-6 M SPQ surfactant solutions were prepared in doubly distilled water. The fluorescence intensities were measured at 443 nm by the excitation at 346 nm, as indicated in ref 5. Surfactant concentrations were well above the cmc. Study of the Polarity of the Micellar Interfacial Region. Solutions of pyrene-3-carboxaldehyde (10-5 mol dm-3) in the different waterorganic solvent 12-3-12,2Br- micellar solutions were prepared in the same way as were the pyrene solutions. Pyrene-3-carboxaldehyde was excited at 356 nm, and fluorescence spectra were recorded at between 400 and 600 nm. A scan speed of 240 nm/min was used, and the excitation and emission slits were each 10 nm. The fluorescence maxima shown in the spectra are strongly dependent on the polarity of the medium.9 To check the reliability of the results, the fluorescence emission spectra of DTAB (0.1 M) was recorded. The fluorescence maximum was 446 nm, in good agreement with literature data.9 Surface Tension Measurements. The air/water-organic solvent mixtures surface tensions were measured by the du Nouÿ ring method using a KSV 703 digital tensiometer (Finland) equipped with an automatic device to set the time between two consecutive measurements and to select the rising velocity of the platinum ring. A waterjacketed sample beaker connected to a cryostat was used to control the sample temperature. Prior to each measurement, the ring was rinsed with ethanol and then heated briefly by holding it above a Bunsen burner until it glowed. The vessel was cleaned by using chromic sulfuric acid, boiled in distilled water, and then heated with a Bunsen burner flame before use. The precision of the measurements was (1 mN m-1. NMR Measurements. NMR samples were prepared by dissolving the corresponding amount of the surfactant in 1 mL of D2O or a mixture D2O/ethylene glycol-d6 (35 wt % EG), followed by brief sonication. NMR experiments were recorded on a Bruker Avance 500 spectrometer (500.2 MHz for 1H) equipped with a 5 mm inverse probe and a Great 1/10 pulsed-gradient unit capable of producing (5) Kuwamoto, K.; Asakawa, T.; Ohta, A.; Miyagishi, S. Langmuir 2005, 21, 7691. (6) Domıńguez, A.; Fernańdez, A.; Gonza´lez, N.; Iglesias, E.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227. (7) (a) P. Infelta, M. Gratzel, J. Chem. Phys. 1979, 70, 179. (b) Infelta, P. P. Chem. Phys. Lett.1980, 61, 88. (c) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152. (8) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412. (9) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176.

Langmuir, Vol. 23, No. 23, 2007 11497 Table 1. Critical Micelle Concentrations (cmc), Micellar Degrees of Ionization (r), and Gibbs Energy of Micellization (∆ GoM) for Water-Organic Solvent 12-3-12,2Br- Micellar Solutions at T ) 298 Ka bulk phase

103(cmc)/M

R

-∆GoM/kJ mol-1

water 20 wt % EG 35 wt % EG 20 wt % GLY 35 wt % GLY 20 wt % 1,3-PROP 35 wt % 1,3-PROP 35 wt % 1,2-PROP 35 wt % DMF 15 wt % FM 35 wt % FM 10 wt % ACN 35% DO

0.97 1.26 1.98 1.12 1.41 1.80 3.30 2.55 3.90 2.14 5.90 1.88 3.87

0.21 0.22 0.23 0.20 0.22 0.22 0.23 0.24 0.30 0.26 0.33 0.33 0.29

70.0 67.0 62.9 68.9 65.7 64.6 58.9 60.5 55.0 61.9 51.9 59.3 55.6

a EG - ethylene glycol; GLY - glycerol; 1,3-PROP - 1,3-propylene glycol; 1,2-PROP - 1,2-propylene glycol; DMF - N,N-dimethylformamide; FM - formamide; ACN - acetonitrile; and DO - dioxane.

magnetic field gradients in the z direction of about 50 G cm-1. The chemical shifts in the 1H NMR spectra were referenced to the residual HDO10,11 or DOCHD-CD2OD signal. For the determination of the self-diffusion coefficients through the DOSY spectra, a BPPLED pulse sequence with a longitudinal eddy current delay of 5 ms was employed. The gradient length and the diffusion time were optimized for each sample (δ ) 1.5-3.5 ms and ∆ ) 180-400 ms). A linear ramp of 16 steps was used to increment the gradient strength between 2 and 95%. All experiments were performed at 298 K. The singlet corresponding to the NMe2 group was selected for the DOSY analysis.

Results and Discussion Effects of Organic Solvent Addition on Micellization. Table 1 shows the critical micelle concentrations, cmc’s, and the degrees of micellar ionization of various water-organic solvent 12-312,2Br- solutions. These values were obtained through conductivity measurements (Figure 1), and the cmc and R values were determined from inflections in plots of conductivity, κ, against the surfactant concentration (Williams method12), as described in ref 13 (solid lines in Figure 1). cmc and R values corresponding to the aqueous 12-3-12,2Br- solution are in agreement with literature data.14 However, because criticisms have arisen about this method,15 the authors also used a fluorescence method developed by Asakawa et al.16 to estimate the cmc and R values. 6-Methoxy-N-(3-sulfopropyl)quinolinium, SPQ, is a zwitterionic inner salt that is very soluble in water and has a low octanol-water partition coefficient.17 It was confirmed by gel filtration that SPQ remains in the aqueous bulk phase without being trapped in cationic micelles. The SPQ fluorescence gives a single broad emission peak centered at 443 nm by the excitation at 346 nm. The fluorescence is quenched by halide ions via a collisional mechanism. The variation of fluorescence intensity is related to the concentration of bromide ions by the (10) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 75127515. (11) Wu, D.; Chen, A.; Jonson, C. S., Jr. J. Magn. Reson., Ser. A 1995, 115, 260-264. (12) Williams, R.; Phillips, J. N.; Mysels, K. J. Trans. Faraday Soc. 1955, 51, 728. (13) Rodrı´guez, A.; Graciani, M. M.; Munõz, M.; Moya´, M. L. Langmuir 2003, 19, 7206. (14) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (15) Sarmiento, F.; del Rıó, J. M.; Prieto, G.; Atwood, D.; Jones, M. N.; Mosquera, V. J. Phys. Chem. 1995, 99, 17628. (16) Asakawa, T.; Kitano, H.; Ohta, A.; Miyagishi, S. J. Colloid Interface Sci. 2001, 242, 284. (17) Verkman, A. S.; Seller, M. C.; Chao, A. C.; Leung, T.; Ketchman, R. Anal. Biochem. 1989, 178, 355.

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Figure 1. Dependence of the specific conductivity, κ/µS cm-1, on surfactant concentration for water-GLY 12-3-12,2Br- solutions with 20 wt % GLY at 298 K. The inset shows the effect of the gemini surfactant concentration on SPQ fluorescence quenching in the same binary mixture.

Stern-Volmer relation,17 Io/I ) 1 + KSV[Br-], where Io and I are the fluorescence intensities in the absence and presence of the quencher, respectively, and KSV is the Stern-Volmer constant. The inset in Figure 1 shows the Stern-Volmer plot for the quenching of SPQ fluorescence by the 12-3-12,2Br- surfactant in water-glycerol, with 20 wt % GLY. The observed quenching can be ascribed to the free bromide ions dissociated from the surfactant. Data in the inset of Figure 1 show a distinct break, giving the cmc. The variation in the Stern-Volmer plot slope can be attributed to the counterion binding of micelles. The micellar degree of ionization can be evaluated as illustrated by the inset in Figure 1. The extrapolation of the linear line below the cmc was used as the calibration line to high concentrations above cmc. The values of R were calculated using eq 1:5

R)

Cfree - cmc CT - cmc

(1)

Here, Cfree and CT are the concentrations of the free bromide ions and the surfactant, respectively. Cfree at a total surfactant concentration of CT is estimated, and then the micellar degree of ionization is calculated by using eq 1 (inset in Figure 1). The cmc and R values obtained by the two methods are in agreement. Data in Table 1 show that the addition of organic solvents results in an increase in the cmc. The explanation for this has been pointed out previously:18 the transfer of the surfactant tail from the bulk phase into the micellar core and that of the alkyl chains in the head groups and in the spacer from the bulk phase into the micellar surface (or deeper if the chains are sufficiently long) are less favorable when the amount of organic solvent in the mixture increases. This is due to the water-organic solvent mixtures being better solvents for the surfactant molecules than pure water.19 The cmc also depends on other Gibbs energy contributions to the Gibbs energy of micellization, but they are (18) Rodrı´guez, A.; Munõz, M.; Graciani, M. M.; Fernańdez-Pachoń, M. S.; Moya´, M. L. Colloids Surf., A 2007, 298 and references therein. (19) Marcus, Y. Ion SolVation; Wiley: London, 1986.

always much weaker when compared to the dependence on the solvophobic Gibbs energy contribution.2 In relation to the micellar degree of ionization, the addition of organic solvents produces an increase in R, although for some mixtures the variation is small. This increase can be rationalized by taking the cmc increase caused by the organic solvent addition into account. The increment in the cmc results in an increase in the ionic strength (the monomer concentration increases). Hence, the electrostatic repulsions between the charged surfactant head groups in micelles would decrease (screening effects), and as a consequence, R would increase. It was found that the increase in the micellar degree of ionization of monomeric surfactant micelles, such as those of DTAB, is usually larger than that found for didodecyl dicationic dibromide surfactant micelles in the same water-organic solvent mixtures.1,20 This was explained by considering the nonuniformity in charge distribution at the micellar surface of the cationic dimeric micelles.1,2b,3 The Gibbs energy of micellization, ∆GoM, of ionic micelles depends on both the cmc and R. In the case of ionic dimeric surfactants, it can be calculated by21

∆GoM ) 2RT(1.5 - R)ln cmc

(2)

where cmc is on the molar fraction scale. The ∆GoM values estimated by using eq 2 are listed in Table 1. One can see that, in all cases, the presence of the organic solvent makes the micellization process less favorable, with the increase in the cmc being the main factor controlling the increment in ∆GoM. That is, the solvophobic effect principally determines the changes in ∆GoM when an organic solvent, which remains in the bulk phase, is added to the aqueous surfactant solution. In regard to the influence of the solution characteristics on the experimental ∆GoM changes, one of the properties more frequently used for rationalizing solvophobic effects is the polarity (20) Moya´, M. L.; Rodrı´guez, A.; Graciani, M. M.; Carmona, A. M. J. Colloid Interface Sci. in press; see references therein. (21) Zana, R. AdV. Colloid Surface Sci. 2002, 97, 205.

Cationic Dimeric Surfactant 12-3-12,2Br-

Figure 2. Plot of the Gibbs energy of micellization, ∆GoM, against the Gordon parameter of the bulk phase for the water-organic solvent 12-3-12,2Br- surfactant solutions studied in this work. T ) 298 K.

of the bulk phase. The static dielectric constant, , also called the permittivity, is often considered to describe the polarity of binary mixtures (in particular, the relation 1/19). Because solvents with low polarity usually solubilize organic molecules more easily than those that are highly polar, one would expect that the lower the dielectric constant of a binary solvent mixture, the higher ∆ GoM would be as compared to that in pure water. To investigate this idea, the Gibbs energy of micellization was plotted against 1/ (inset of Figure 2). values corresponding to the different binary mixtures were taken from the literature.22 One can see that different binary mixtures with similar 1/ values do not have similar ∆GoM values. That is, the polarity of the bulk phase, described through its dielectric constant, is not an adequate parameter to account for the observed changes in ∆GoM. This is clearly shown if one takes into account the ∆GoM changes in water-formamide mixtures. Formamide is an organic solvent that is more polar than water. (The dielectric constants for pure water and pure formamide at 298.2 K are 78.5 and 111.0, respectively.19) Therefore, an increase in the formamide weight percentage present in the mixture results in an increase in its polarity. However, this increase is accompanied by an increase in ∆GoM (triangles in the inset of Figure 2). This is so because, in spite of being more polar, formamide is a better solvent for organic molecules than is water. The ability of a given bulk phase to bring about the selfassociation of conventional amphiphiles can be characterized by its Gordon parameter, G ) γ/Vm1/3, where γ is the air/mixture surface tension and Vm the molar volume of the bulk phase.23 This parameter is considered to be a measure of the cohesive energy density of the solution. It is interesting that the cohesive energy of a liquid is related to the solubility behavior because the same intermolecular attractive forces have to be overcome during vaporization and during the solvation process. G values can be estimated for the binary mixtures used in this work from the experimental surface tension values of the mixtures, and assuming that the molar volume for the mixture can be determined by averaging the molar volumes of the individual solvents by volume, Vmixture ) XwaterVwater + Xorganic solventVorganic solvent. Figure 2 shows the plot of ∆GoM values against the Gordon (22) (a) Franks, F. In Water: A ComprehensiVe Treatise; Franks, F., Ed.; Plenum Press: New York, 1973; Vol. 1, p 1. (b) Asuero, A. G.; Herrador, M. A.; Gonza´lez, A. G. Talanta 1993, 4, 479. (23) Ramadan, M.; Evans, D. F.; Lumry, R.; Philson, S. J. Phys. Chem. 1985, 89, 3405.

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Figure 3. Influence of the quencher (N-hexadecylpyridinium chloride, CpyCl) concentration on the intensity of pyrene fluorescence in water-EG 12-3-12,2Br- micellar solutions with 35 wt % EG at 298 K.

parameter corresponding to the different bulk phases. An approximately linear dependence (r ≈ 0.94) was found for all of the bulk phases, with the exception of water-formamide mixtures. Figure 2 indicates that the cohesive energy density of the bulk phase, measured through the Gordon parameter, seems to play a significant role in determining the contribution of the solvophobic effect on the Gibbs energy of micellization in waterorganic solvent 12-3-12,2Br- solutions, at least for organic solvents that are less polar than water. The same result was found for monomeric conventional surfactants.20 The deviation of ∆GoM values in water-formamide mixtures was also observed for the aggregation process of monomeric conventional surfactants in water-binary mixtures.20 The ∆GoM values for these mixtures indicate that they are better solvents for surfactant molecules than was expected. Perhaps the explanation could be related to the higher polarity of these mixtures as compared to that of pure water. Water-formamide solutions would be better solvents than water for the polar head groups of surfactants, and at the same time, they would also be better solvents for the hydrophobic chains of surfactants. This could explain the deviation of the ∆ GoM values corresponding to water-FM mixtures in Figure 2. This subject is under investigation. Average micellar aggregation numbers of 12-3-12,2Brmicelles in the different bulk phases at [12-3-12] ) 0.02 M were obtained by using the fluorescence quenching of pyrene by hexadecylpyridinium chloride (Figure 3). A surfactant concentration of 0.02 M was chosen because, for all of the bulk phases studied, at that [12-3-12,2Br-] one would expect the dimeric micelles to be spherical. (See below.) Dimeric surfactant concentrations higher than 0.02 M could correspond to different micellar shapes depending on the nature of the bulk phase; consequently, average aggregation numbers obtained under those conditions could not be compared. The Nagg value in pure water is in good agreement with those obtained by other authors using the same method.5,8 However, it is worth noting that the average aggregation numbers obtained through steady-state fluorescence quenching, SSFQ, for didodecyl dicationic dibromide surfactants are smaller than those obtained by means of time-resolved fluorescence quenching, TRFQ.3b,8 Nevertheless, at least in water-EG mixtures, the variations in Nagg obtained by the two methods upon varying the weight percent of EG were similar.1 Besides, SSFQ methods were found to be adequate for estimating average micellar aggregation numbers in several water-organic

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Figure 4. Plot of the average aggregation number, Nagg, against the air/bulk-phase surface tension, γ/mN m-1, for water-organic solvent 12-3-12,2Br- micellar solutions. T ) 298 K.

solvent mixtures.24 On these bases, Nagg values obtained in this work have to be considered to be approximate, and attention will be paid to the Nagg changes by varying the bulk phase. In this work and in previous work,1,24 it was found that the addition of organic solvents results in a decrease in the average aggregation numbers of micelles (for conventional ionic, nonionic, and zwitterionic surfactants as well as for cationic dimeric surfactants). The organic solvent addition can influence the magnitude of the size-dependent Gibbs energy contributions to the micellization Gibbs energy. The interfacial Gibbs energy contribution takes into account that the formation of a micelle creates an interface allowing for contact between the hydrophobic core and the bulk phase. It is a large positive term that decreases as the micelle size increases, favoring micellar growth, and is proportional to the hydrocarbon micelle core/bulk-phase interfacial tension.2,25 It is reasonable to expect that the variations in the hydrocarbon/bulk-phase interfacial tension are proportional to the changes in the air/bulk-phase surface tension, γ, when an organic solvent is added. With this in mind, ∆Gointerfacial is expected to decrease as the amount of organic solvent increases because the air/bulk-phase surface tension decreases upon increasing the organic solvent content in the bulk phase. If this decrease in ∆Gointerfacial is primarily responsible for the decrease in the average aggregation number of the dimeric micelles upon organic solvent addition, then a dependence of Nagg on γ would be expected. Figure 4 shows that, with the exception of waterformamide mixtures, an approximately (r ≈ 0.93) linear dependence was observed. Nagg values in Figure 4 correspond to the number of hydrophobic chains making up one micelle hydrophobic core. A similar dependence was found for conventional monomeric surfactants,1,2,20 where Nagg values for water-FM mixtures also deviate. This result points out that changes in the air/bulk-phase surface tension (that is, in the (24) (a) Graciani, M. M.; Munõz, M.; Rodrı´guez, A.; Moya´, M. L. Langmuir 2005, 21, 3303. (b) D’Errico, G.; Ciccarelli, D.; Ortona O. J. Colloid Surface Sci. 2005, 286, 747. Aguiar, J.; Molina-Bolivar, J. A.; Peula, J. M.; Carnero-Ruiz, C. J. Colloid Interface Sci. 2005, 255, 382, (c) Carnero-Ruiz, C.; Dıáz-Lo´pez, L.; Aguiar, J. J. Colloid Interface Sci. 2007, 305, 293. (d) Rodrı´guez, A.; Munõz, M.; Graciani, M. M.; Moya´, M. L. J. Colloid Interface Sci. 2006, 298, 942. (e) Ray, A.; Ne´methy, G. J. Phys. Chem. 1971, 75, 809. (f) Carnero-Ruiz, C.; MolinaBolivar, J. A.; Aguiar, J.; MacIsaac, G.; Moroze, S.; Palepu, R. Langmuir 2000, 17, 6831. (g) Carnero-Ruiz, C.; Molina-Bolivar, J. A.; Aguiar, J.; MacIsaac, G.; Moroze, S.; Palepu, R. Colloid Polym. Sci. 2003, 281, 531. (h) Glenn, K. M.; Moroze, S.; Palepu, R. M.; Bhattacharya, S. C. J. Dispersion Sci. Technol. 2005, 26, 79. (25) Maibaum, L.; Dinner, A. R.; Chandler, D. J. Phys. Chem. B 2004, 108, 6778.

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hydrocarbon/bulk-phase interfacial tension) significantly influence the changes in micelle size. In water-FM mixtures, other energy terms also contribute to the variations in Nagg. A decrease in the aggregation number would bring about an increase in the polarity of the micellar interfacial region.18 A less-packed micelle makes the penetration of solvent molecules (water and organic solvent) in the palisade layer easier; as a consequence, in spite of the decrease in the polarity of the bulk phase when adding organic solvents (except formamide), an increase in the polarity of the interfacial region of the dimeric micelles was expected. The excitation of dilute solutions of pyrene-3-carboxaldehyde, P3C, leads to blue-violet fluorescence. The fluorescence is due to the monomeric excited singlet state and displays a strong solvent dependence.9 The fluorescence maximum, λmax, shows a red shift with increasing solvent polarity. For several pure solvents and solvent mixtures, a linear relation between the fluorescence maximum and the bulk dielectric constant has been found. NMR and UV spectral studies have shown that when pyrene-3-carboxaldehyde molecules are solubilized in micelles their hydrophobic (aromatic) moiety goes into the micellar core and the hydrophilic group protrudes into or is anchored at the micellar surface or double layer.26 Therefore, the values of λmax in micellar solutions could provide a direct measure of the polarity of the micelle-bulk interface region. With this in mind and in order to get information about the polarity of the interfacial region of the spherical dimeric micelles (previous to the morphological transition), fluorescence emission spectra of P3C in the different water-organic solvent 12-312,2Br- micellar solutions, with [surfactant] ) 0.02 M, were recorded. It was found that the emission spectra were not simple but were the result of the sum of those corresponding to the P3C molecules localized in the bulk phase and in the micellar interfacial region. To resolve the two contributions to the total fluorescence emission intensity, a deconvolution program was used. This program was kindly provided by Professor Manuel Domıńguez and Professor Domingo Gonza´lez from the Department of Physical Chemistry of the University of Seville. It is a curvefitting method based on a least-square analysis and is described in ref 27. Fluorescence emission spectra of P3C in the waterorganic mixtures used as bulk phases (in the absence of dimeric surfactant) were also recorded to obtain λmax(bulk phase). These λmax(bulk phase) values showed an approximately linear dependence on the dielectric constant of the mixture, as was expected. The introduction of the λmax(bulk phase) experimental values in the program made the resultant deconvolutions more reliable (Figure 5). The λmax(interfacial region) values obtained from the fittings are listed in Table 2. One can say that, generally speaking, the polarity of the interfacial region increases (λmax increases) when the micellar aggregation number decreases, in agreement with expectations. Effects of Organic Solvents Addition on Micellar Growth. Once the effects of organic solvent addition on micellization and on dimeric micelles characteristics, at low surfactant concentrations, have been examined, the influence of bulk composition on the sphere-to-rod transition will be considered. 12-3-12,2Brmicelles show a sphere-to-rod transition, sometimes called the second cmc, C*, upon increasing surfactant concentration in aqueous solution.3 In previous work,1 it was shown that this transition takes place at [12-3-12,2Br-] ) 0.025 M in pure water. This result is in agreement with those obtained by other authors (26) (a) Gratzel, M.; Kalyanasundaram, K.; Thomas, J. H. J. Am. Chem. Soc. 1974, 96, 7869. (b) Erikkson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (27) (a) Sevilla, J. M.; Domıńguez, M.; Garcıá-Blanco, F.; Bla´zquez, M. Comput. Chem. 1989, 13, 197.


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Figure 6. SPQ fluorescence quenching in water-dioxane 12-312,2Br- micellar solutions with 35 wt % DO at high surfactant concentrations. T ) 298 K. Inflection point C* can be assigned to the sphere-to-rod transition.

Figure 5. Deconvolution of the fluorescence emission spectra of pyrene-3-carboxaldehyde, P3C, in water-GLY (with 20 wt % GLY) and water-1,2-PROP (with 35 wt % 1,2-PROP) 12-3-12,2Brmicellar solutions. [12-3-12,2Br-] ) 0.02 M and T ) 298 K. Table 2. Wavelength of the Fluorescence Maximum of Pyrene-3-carboxaldehyde (λmax) at Low and High Surfactant Concentrations in Water-Organic Solvent 12-3-12,2BrMicellar Solutions and Values of the 12-3-12,2BrConcentrations at Which a Sphere-to-Rod Transition Occurs (C*) in Those Binary Mixtures When T ) 298 Ka bulk phase

λmax([12-3-12,2Br-] ) 0.02 M)/nm

C*

λmax([12-3-12,2Br-] > C*)/nmb

water 20 wt % EG 35 wt % EG 20 wt % GLY 35 wt % GLY 20 wt % 1,3-PROP 35 wt % 1,3-PROP 35 wt % 1,2-PROP 35 wt % DMF 15 wt % FM 35 wt % FM 10 wt % ACN 35% DO

440.1 441.6 443.9 441.4 443.0 445.1 446.0 447.5 450.0 446.0 449.3 443.0 451.9

0.025 0.047 0.077 0.036 0.047 0.060 0.081 0.090 0.100 0.075 0.076 0.088 0.094

437.1 (0.035) 436.8 (0.057) 440.6 (0.085) 436.9 (0.046) 438.5 (0.057) 439.7 (0.070) 436.2 (0.090) 439.5 (0.100) 442.8 (0.110) 440.7 (0.085) 437.7 (0.085) 436.7 (0.105) 443.1 (0.100)

a EG - ethylene glycol; GLY - glycerol; 1,3-PROP - 1,3-propylene glycol; 1,2-PROP - 1,2-propylene glycol; DMF - N,N-dimethylformamide; FM - formamide; ACN - acetonitrile; and DO - dioxane. b Surfactant concentrations at which the spectra were recorded are in parentheses.

through steady-state fluorescence,5 calorimetry,28 and cryogenic transmission microscopy29 measurements. To investigate how (28) Fisicaro, E.; Compari, C.; Duce, E.; Constabili, C.; Viscardi, G.; Quagliotto, P. J. Phys. Chem. B 2005, 109, 1744.

the addition of organic solvents (which remain mainly in the bulk phase) affects this sphere-to-rod-transition, the C* value for 12-3-12,2Br- in several water-organic solvents mixtures was determined. The fluorescence intensity of SPQ is unaffected by dissolved oxygen and solvent polarity5 and is insensitive to not only surfactant monomers but also micelles.29 The bound bromide ions electroneutralized with micelles were insensitive to fluorescence quenching. Thus, it is possible to evaluate the concentration of free bromide ions dissociated from surfactants by the SPQ fluorescence quenching behavior. Because micellar growth is accompanied by changes in the micellar degree of ionization, SPQ fluorescence quenching is expected to be a good method for detecting the sphere-to-rod transitions. Figure 6 shows the SPQ fluorescence quenching in water-dioxane 12-3-12,2Br- micellar solutions with dioxane 35 wt % at high surfactant concentrations, with [surfactant] well above the cmc (cmc ) 3.87 mM, see Table 1). The slope of the Stern-Volmer plot was found to remain approximately constant from the cmc up to 0.088 M, but it changed with increasing 12-3-12,2Br- concentration in the high-concentration region. This decrease in the slope indicates a decrease in the micellar degree of ionization, suggesting micellar growth at high concentrations.31 This second inflection point, C*, could be assigned to the transition for spherocylindrical micelles. C* values obtained in the different binary aqueous mixtures investigated are summarized in Table 2. One can see that C* depends strongly on the amount and nature of the organic solvent present in the bulk phase of the dimeric micellar solutions. To explain this result, the different contributions to the Gibbs energy of micellization, ∆GoM, will be considered next. For all surfactants, the formation of aggregates in a given bulk phase is made possible by the negative solvophobic Gibbs energy contribution to ∆GoM.2 The dependence of the different Gibbs energy contributions to ∆GoM on micellar size was investigated by Camesano and Nagarajan to determine which terms favor micellar growth and which ones limit their growth.2b The solvophobic Gibbs energy and the coverage Gibbs energy (29) (a) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (b) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (30) Asakawa, T.; Ishino, S.; Hansson, P.; Almgrem, M.; Ohta, A.; Miyagishi, S. Langmuir 2004, 20, 6998. (31) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 60, 5435.

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contributions are independent of the size of the aggregate. The Gibbs energy interfacial term, ∆Gointerfacial, decreases as micelle size increases, favoring micellar growth. Steric interactions consider the repulsions caused by the crowding of head groups at the micelle surface, and they limit the growth of micelles because they increase in magnitude as the micelle size increases. The tail deformation Gibbs energy accounts for the stretching and deforming of surfactant tails so as to pack within the micelle core, meeting liquidlike density constraints. It also limits the micellar growth because its magnitude increases upon increasing micellar size, and it is larger for cylindrical than for spherical micelles. In the case of dimeric surfactants, an extra Gibbs energy packing term has to be considered. It accounts for the packing constraints on the tails, which are connected by the spacer, and favors micelle growth because it decreases as micelle size increases. It also decreases as the aggregate curvature decreases (from spheres to cylinders to lamellae), favoring the transition from spherical to cylindrical micelles. The ionic term, which accounts for the repulsive electrostatic interactions between head groups, is large in magnitude and limits the growth of micelles. When the spacer is short, the two ionic groups of a dimeric surfactant are forced to be closer to one another than they are to the charged groups of adjacent dimeric surfactant molecules, resulting in a nonuniform distribution of charges at the micellar surface. This nonuniformity results in a reduction of the electrostatic repulsive energy. Taking this into account, for short spacers the diminished electrostatic repulsions permit larger micelles to form (such as the threadlike and wormlike micelles experimentally observed3). The organic solvent addition can influence the magnitude of the size-dependent Gibbs energy contributions to the micellization Gibbs energy. As was mentioned above, the interfacial Gibbs energy contribution is proportional to the hydrocarbon micelle core/bulk-phase interfacial tension.2,25 It is expected to decrease as the amount of organic solvent increases because the air/bulkphase surface tension diminishes upon augmenting the organic solvent content in the bulk phase. It was shown in this work that changes in ∆Gointerfacial are mainly responsible for the observed decrease in Nagg of 12-3-12,2Br- micelles at low surfactant concentrations when the organic content in the bulk phase increases. The tail-deformation Gibbs energy and the extra Gibbs energy packing contributions are not expected to depend directly on the nature and content of organic solvent present in the bulk phase. However, the decrease in micelle size originating with the organic solvent addition causes a decrease in the magnitude of both terms, making the sphere-to-rod transition less favorable. Similarly, the steric repulsions between the head groups and the electrostatic repulsions are expected to decrease because the area per head group, ao, is expected to increase when Nagg decreases. Besides, in spite of the decrease in the dielectric constant of the bulk phase when the organic solvent is added (except for waterformamide mixtures), the substantial increase observed in the cmc (Table 1) causes an increase in the ionic strength, which would result in a screening of the ionic charges at the micellar surface. That is, the smaller the 12-3-12,2Br- aggregates are in a given bulk phase, the less favored the sphere-to-rod transition would be. Because changes in micelle size at low surfactant concentration are mainly controlled by variations in the hydrocarbon/bulk-phase interfacial tension, that is, in the air/bulkphase surface tension (Figure 4), a dependence of C* on the air/bulk-phase surface tension, γ, could be expected. Figure 7 shows that C* depends linearly (r ≈ 0.98) on γ for the different water-organic solvent mixtures studied, with the exception of water-formamide mixtures. This deviation was expected given

Rodrı´guez et al.

Figure 7. Plot of the second inflection point, C*, against the air/ bulk-phase surface tension, γ/mN m-1, for water-organic solvent 12-3-12,2Br- micellar solutions. T ) 298 K. EG - ethylene glycol, GLY - glycerol, 1,2-PROP - 1,2-propylene glycol, 1,3-PROP - 1,3propylene glycol, DMF - N,N-dimethylformamide, FM - formamide, DO - dioxane, DMSO - dimethylsulfoxide, and CAN - acetonitrile.

that water-formamide mixtures diverged from the general dependence of Nagg on γ (Figure 4). The linear dependence of C* on γ shown in Figure 7 can be taken as indicative that the decrease in micelle size, caused by the decrease in the interfacial Gibbs energy term, is principally responsible for the observed increase in the surfactant concentration range where the sphereto-rod transition occurs when organic solvents are present in the 12-3-12,2Br- micellar solutions. Fluorescence emission spectra of P3C in the different waterorganic solvent 12-3-12,2Br- micellar solutions, with [surfactant] > C*, were recorded. As was found for [surfactant] ) 0.02 M, the emission spectra were the sum of those corresponding to the P3C molecules localized in the bulk phase and in the micellar interfacial region. In this case, in the presence of high surfactant concentrations, the fluorescence emission contribution of P3C localized in the bulk phase was smaller than in the presence of [2-3-12,2Br-] ) 0.02 M. After applying the deconvolution program, the λmax value corresponding to the P3C localized in the interfacial region of the 12-3-12,2Br- spherocylindrical micelles was obtained. They are listed in Table 2. One can see that in all cases λmax([surfactant] > C*) was lower than λmax([surfactant] ) 0.02 M); that is, the polarity of the interfacial region of micelles decreases when spherocylindrical aggregates form. This is an expected result because the sphere-to-rod transition is accompanied by an increase in head group counterion association (Figure 6) and dehydration at the micelle surface.32 λmax([surfactant] > C*) values listed in Table 2 correspond to different surfactant concentrations because C* values are different for the various bulk phases investigated. An additional way of explaining the results is through the consideration of the packing parameter, P ) V/aolc.33 This factor was used to rationalize the behavior of the micellar morphology, the sphere-to-rod transitions, for example, with the length (lc) and volume (V) of the tail chain of the surfactant and the optimum surface area occupied by a surfactant head group at the micelle core-bulk-phase interface (ao). A packing parameter of less than 1/3 is characteristic of spherical micelle-forming surfactants (32) (a) Geng, Y.; Romsted, L. S.; Menger, F. F. J. Am. Chem. Soc. 2006, 128, 492. (b) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Caran, K. L.; Romsted, L. S. Langmuir 2000, 16, 9095. (33) Israelachvili, J.; Mitchell, D.; Niham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601.


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Figure 8. Concentration dependence of 1H NMR spectra on surfactant concentration in aqueous 12-3-12,2Br- micellar solutions: (a) [12-3-12,2Br-] ) 5 × 10-4 M; (b) [12-3-12,2Br-] ) 0.01 M; and (c) [12-3-12,2Br-] ) 0.06 M. T ) 298 K.

and is within the range of 1/3 < P < 1/2 for cylindrical micelleforming surfactants. The length and volume of the tail chain of the surfactant do not depend on bulk composition. ao is expected to increase upon decreasing Nagg; therefore, the packing parameter will decrease, favoring the formation of spherical micelles. That is, on the basis of Tanford’s model,34 the tendency of the 123-12,2Br- surfactant to form spherical micelles increases when the air/bulk-phase surface tension decreases. NMR measurements are helpful for investigating micellar solutions at a molecular level35 and have been used to get information about morphological transitions.36 The authors tried to get information on the growth of 12-3-12,2Br- micelles in water and in water-EG solutions, with 35 wt % EG, by using 1H NMR and diffusion NMR (DOSY) spectroscopy. The measurements were also carried out in DTAB water solutions for the sake of comparison. Dodecyltrimethylammonium bromide, DTAB, can be considered to be the monomer of the dimeric surfactant 12-3-12,2Br- because DTAB has a single hydrophobic chain identical to those of the dimeric surfactant and the ionic head group and counterion are the same. Besides, it is known (34) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (35) So¨derman, O.; Stilbs, P.; Price, W. S. Concepts Magn. Reson. 2004, 231, 121. (36) (a) Onoda-Yamamuro, N.; Yamamuro, O.; Tanaka, N.; Nomura, H. J. Mol. Liq. 2005, 117, 139. (b) Bello, C.; Bombelli, C.; Borocci, S.; Di profio, P.; Mancini, G. Langmuir 2006, 22, 933. (c) Nakamura, K.; Shikata, T. Langmuir 2006, 22, 9853.

that DTAB micelles remain spherical up to high surfactant concentrations in aqueous solutions.37 Therefore, NMR spectra of DTAB solutions, at different surfactant concentrations (below and above the cmc), will give information about changes in the chemical shifts and in the diffusion coefficients of the species present in the solution as a result of the micellization process but not a subsequent morphological transition. Three different types of samples were prepared for the NMR studies: 12-3-12,2Br- in D2O and in D2O/EG-d6 (35 wt % EGd6) and DTAB in D2O. For the 12-3-12,2Br- solutions, surfactant concentrations below the cmc, between the cmc and C*, and above C* were used. In the case of DTAB, one concentration below the cmc and four above it were studied (where no morphological transition happens). Figure 8 shows the 1H NMR spectra obtained for 12-3-12,2Br- solutions investigated in D2O. From the analysis of the 1H NMR spectra at different concentrations, one can evaluate the changes experienced by the chemical shifts of selected signals accompanying an increase in the surfactant concentration. These changes have been previously used to determine the cmc values of other surfactants.35,37 In the case of DTAB and 12-3-12,2Br-, the chemical shift variations upon changing [surfactant] are more pronounced for the protons located closer to the head ionic groups of the surfactant. With this in mind, the signal corresponding to the N-methyl group was (37) Guerrero-Martıńez, A.; Gonza´lez-Gaitano, G.; Vinãs, M. H.; Tardajos, G. J. Phys. Chem. B 2006, 110, 13819.

11504 Langmuir, Vol. 23, No. 23, 2007

Figure 9. Influence of surfactant concentration on (a) variations in the 1H NMR chemical shift of the N-CH3 group of surfactants, δ; (b) variations in the line width at half-height of the 1H NMR signal corresponding to the N-CH3 group of surfactants, B; and (c) variations in the self-diffusion coefficient of the surfactant, D. Variations are referenced to the value of the magnitude obtained in the presence of [surfactant] < cmc (subscript 1). (b) 12-3-12,2Brin D2O, (2) 12-3-12,2Br- in D2O-EG-d6 (35 wt % EG-d6), and (9) DTAB in D2O. T ) 298 K.

selected with the scope of determining if changes in the chemical shifts upon increasing surfactant concentration can give information about micellar morphological transitions. Furthermore, this signal has the advantage of being the only singlet in the 1H NMR spectra of the surfactants investigated in this work. Figure 9a shows the dependence of 102x|(δ1 - δi)/δ1| on surfactant concentration, with δi being the 1H chemical shift of the N-CH3

Rodrı´guez et al.

group of the surfactants. Subscript 1 refers, in each case, to the solution where the surfactant concentration is below the cmc. One can see that, in all cases, a large variation in the chemical shift is found when [surfactant] goes from below the cmc to above the cmc (cmc(DTAB) ) 14.5 mM, cmc(12-3-12,2Br-) ) 0.97 mM, and cmc(12-3-12,2Br-in 35 wt % EG) ) 1.98 mM). For surfactant concentrations higher than the cmc, the observed changes upon increasing [surfactant] are much smaller. Therefore, the study of the dependence of δ on surfactant concentration does not seem to be useful for getting information about the changes in the size and shape of micelles. The line width at half-height, B, is intimately related to the dynamics of the molecule in solution (transverse relaxation), which, in turn, is affected, among other factors, by the size and shape of the molecule or the aggregate. Changes in this parameter upon varying [surfactant] could be correlated to changes in the micellar morphology.38 The dependence shown for the increment of the line width at half-height of the signal corresponding to the protons of the N-methyl group on [surfactant], with respect to that observed for the surfactant solutions at [surfactant] < cmc, B1, is shown in Figure 9b. This Figure shows that B remains constant in DTAB solutions when the surfactant concentration increases, going from [DTAB] < cmc to [DTAB] well above the cmc. For the dimeric surfactant investigated, a small change in B accompanies the increase in surfactant concentration when [surfactant] goes from below the cmc to above the cmc. However, a much larger variation in B is observed when [surfactant] increases to above the cmc, particularly in the absence of ethylene glycol-d6. The fact that no significant changes were observed for the DTAB solutions, where no morphological transition occurs, indicates that variations in line width B upon changes in [surfactant] can be a useful tool for investigating morphological transitions in micelles. It is worth noting that no significant changes in the line width of the residual HDO were observed in the range of surfactant concentrations studied in D2O or in D2O + EG-d6 (35 wt % EG-d6), thus indicating that no appreciable changes in the viscosity of the solutions take place. Therefore, no substantial effects of viscosity variations on the measured line widths are expected. C* values for the dimeric surfactant in pure water and waterEG mixtures, with 35 wt % EG, are 0.025 and 0.077 M, respectively (Table 2). In the deuterated solvents, the sphereto-rod transitions can occur at somewhat different surfactant concentrations, but for [12-3-12,2Br-] equal to 0.06 and 0.1 M in D2O and D2O-EG-d6 (35 wt % EG-d6), respectively, the morphological transitions are expected to have already happened. Figure 9b shows that B varies more rapidly by increasing the surfactant concentration in D2O than in D2O-EG-d6 (35 wt % EG-d6). This can be taken as indicative that the growth rate of the dimeric micelles is slower when deuterated ethylene glycol is added to deuterated water. Diffusion-ordered NMR (DOSY) spectroscopy provides a means to measure the self-diffusion coefficients, D, of the molecules in solution, and the analysis of the obtained values can reveal changes in the molecular size and shape. This methodology has been widely used in the study of surfactants, and the cmc of different surfactants have been reported from the analysis of DOSY spectra.37,39 For the solutions studied, a diminution of the self-diffusion coefficient of the surfactant, D, was observed when the surfactant concentration increased. The self-diffusion coefficients of the residual HDO were determined (38) Groth, C.; Nyden, M.; Holmberg, K.; Kanichy, J. R.; Shah, D. O. J. Surfact. Det. 2004, 7, 247. (39) Guerrero-Martıńez, A.; Palafox, M. A.; Tardajos, G. Chem. Phys. Lett. 2006, 432, 486.


for all of the samples, and they showed no substantial variations with the concentration of the surfactant, coherent with the small variation of the viscosity deduced from the analysis of the line widths. Figure 9c shows the plots of 102x|(D1 - Di)/D1| against surfactant concentration, with Di being the self-diffusion coefficients of the surfactants obtained from the DOSY spectra. As expected, a significant reduction of the self-diffusion coefficient is observed when the concentration of the surfactant is increased from below to above the cmc. Additionally, a noticeable reduction is also observed when a comparison between concentrations below and above C* is made, as in the case of the line width, in agreement with micellar growth accompanying an increase in surfactant concentration. Again, the most important change in the selfdiffusion coefficient upon increasing [surfactant] corresponds to 12-3-12,2Br- in D2O, whereas for the same surfactant in D2O/ EG-d6 (35 wt % EG-d6), the changes are smaller. This points out that micellar aggregates grow more rapidly when [surfactant] increases in D2O than in D2O-EG-d6 (35 wt % EG-d6), in agreement with the results obtained by means of the B-variation study. For DTAB solutions, a decrease in D by increasing [DTAB] is found when going from 0.02 to 0.06 M, although the observed changes in D for the dimeric surfactants were higher in percentage. It is interesting that the cmc of the dimeric surfactant is 1 order of magnitude smaller than that of DTAB. This means that points 2 and 3 in Figure 9 for the dimeric surfactant solutions are 10 or 60 times the cmc, whereas for DTAB solutions the highest [surfactant] is about 6 times the cmc. Even so, the change in D when [DTAB] increases is smaller than in the case of the dimeric surfactant, in agreement with the DTAB micelles not growing substantially upon increasing the surfactant concentration. From these results, one can conclude that the study of changes in the self-diffusion coefficient of the surfactants by increasing [surfactant] can provide information about micellar growth accompanying morphological transitions.

Concluding Remarks The effects of organic solvent addition on the aggregation and micellar growth of the 12-3-12,2Br- surfactant were investigated. From these results, we can conclude the following: 1. The addition of an organic solvent, which remains mainly in the bulk phase, makes the aggregation process less favorable. This was due to the water-organic solvent mixtures being better solvents for the cationic dimeric surfactant molecules than pure water (solvophobic effect).

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2. The cohesive energy of the mixtures, described through the Gordon parameter, plays a significant role in the determination of the Gibbs energy of micellization in the different mixtures. 3. The decrease in the average micellar aggregation number of 12-3-12,2Br- micelles, at low surfactant concentration, was principally due to the decrease in the Gibbs energy interfacial contribution to ∆GoM. This decrease originated in the diminution in the hydrocarbon/bulk-phase interfacial tension (air/bulk-phase surface tension) following the addition of the organic solvent. 4. An increase in the 12-3-12,2Br- concentration where the sphere-to-rod transition occurs, C*, is observed when an organic solvent is present in the bulk phase. The amount and nature of the organic solvent influences C* principally through the decrease in the hydrocarbon/bulk-phase interfacial tension (air/bulk-phase surface tension) caused by the presence of the organic solvent. 5. NMR measurements were shown to be helpful in the investigation of morphological transitions in the absence and presence of organic solvents. In particular, for the surfactant used in this work, the study of changes in the line width, B, of the 1H NMR signal of the N-CH3 groups upon increasing surfactant concentration provides information about the micellar growth rate accompanying this increase. The dependence of the self-diffusion coefficients of surfactants, D, on [surfactant] can also be useful, although these changes are smaller than those in B. Point 4 is particularly relevant because from the changes in the air/bulk-phase surface tension caused by the addition of an organic solvent to an aqueous solution, the surfactant concentration at which the sphere-to-rod transition will take place could be approximately predicted. This result is of interest because practical aspects such as rheological behavior and solubilization capacity of micellar solutions depend on aggregate morphology. Results obtained in water-formamide mixtures deviate from the general trends found in this work for the rest of the organic solvents used. This behavior could be related to formamide being a more polar solvent than water. However, this subject needs to be investigated further. Acknowledgment. This work was financed by the DGCYT (grant BQU2006-00597) and Consejerıá de Innovacioń, Ciencia y Empresa de la Junta de Andalucıá (FQM-274). LA702293D

Effects of Organic Solvent Addition on the Aggregation and Micellar

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