8860
J. Phys. Chem. B 2007, 111, 8860-8867
1H
NMR and Viscometric Studies on Cationic Gemini Surfactants in Presence of Aromatic Acids and Salts Kabir-ud-Din,*,† Waseefa Fatma,† Ziya Ahmad Khan,† and Aijaz Ahmad Dar‡ Department of Chemistry, Aligarh Muslim UniVersity, Aligarh - 202 002, India, and Department of Chemistry, UniVersity of Kashmir, Srinagar - 190 006, Jammu and Kashmir, India ReceiVed: January 30, 2007; In Final Form: May 11, 2007
In this paper, we are reporting the influence of addition of aromatic acids (anthranilic and benzoic acid) and their sodium salts on the micellar morphological changes in three cationic gemini surfactant solutions, viz. 5 mM tetramethylene-1,4-bis(N-hexadecyl-N,N-dimethylammonium bromide), 10 mM pentamethylene-1,5-bis(N-hexadecyl-N,N-dimethylammonium bromide), and 10 mM hexamethylene-1,6-bis(N,-hexadecyl-N,Ndimethylammonium bromide). The solubilization site of the counterions (obtained from the additives) near the micellar surface are inferred by 1H NMR. The behavior is explained in the light of binding of counterions to the micelle as well as the nature of the functional group attached to the additive.
Introduction Gemini or dimeric surfactants are amphiphillic molecules consisting of two hydrophobic tails and two hydrophilic headgroups covalently attached through a spacer or linker rather than one hydrophilic and one/two hydrophobic group(s) of conventional surfactants. The nature of spacers, hydrophobic tails, and headgroups show a great deal of variation in gemini surfactants.1-4 Considerable number of studies have shown their superior performance as catalysts in organic reactions,5 high surface activity,2,6,7 unusual viscosity changes with an increase in surfactant concentration,8 unusual micelle structure,9,10 aberrant aggregation behavior,11 strong interactions with different surfactants,12,13 and behavior in nonpolar nonaqueous solvents.14 The greater efficiency and effectiveness of geminis over comparable conventional surfactants make them highly costeffective as well as environmentally desirable. Micelles can undergo structural transition from spherical-torod like under appropriate conditions of salinity, temperature or addition of some organic additives.15-19 The formation of such supramolecular structures induces strong viscoelasticity in the solution. This feature has been observed in a number of systems containing conventional cationic surfactants especially with some specific organic counterions such as salicylate, 3-hydroxy-2-naphalene carboxylate, alkyl sulfonate, anthranilate, etc.18-22 The occurrence of viscoelasticity depends on factors including packing constraint, ionic strength, surfactant headgroup, alkyl chain length,23,24 counterion binding to micelle surface, and solubilization site vis-a-vis orientation of the hydrophobic additives in the micelle.19,20,25 More studies have been devoted to understanding the solubilization site of such additives in conventional surfactants followed by their viscoelastic behavior using varied techniques such as electron microscopy,26,27 NMR,19,25,28,29 small-angle neutron scattering (SANS),30-33 static and dynamic light scattering,34,35 viscosity,36 etc. The exclusive attention given to such surfactant/additive systems is the result of the fact that surfactants are always used * Corresponding author. E-mail:
[email protected]. † Aligarh Muslim University. ‡ University of Kashmir.
in combination with one or more additives simultaneously for their scientific, experimental, industrial and theoretical applications.37-39 The organic additives (salts or polar compounds) may influence the morphology of micelles in a manner that depends upon the extent of their penetration/intercalation into the micelles.40 A counterion-mediated micellar growth study suggests that aromatic counterions are more effective in penetrating headgroup region leading to reduction in headgroup repulsions and hence facilitating micellar growth. The extent of penetration of aromatic counterions depends upon hydrophobicity, nature, and placement of substituents in aromatic ring and solvation effects.23,25 However, solubilization of additives in the micellar interior produces only swollen spherical micelles41 and does not contribute much toward micellar growth. In view of this, viscosity studies, being very sensitive to the shape/size of macroscopic objects in a colloidal solution, and 1H NMR studies, being used to probe the location and orientation of molecules in and around a micelle by means of chemical shift changes for surfactant and additive proton resonances, are found to be extremely helpful for interpreting the relationship between additive structure and micellar growth of a surfactant. Since the gemini surfactants have shown promise in skin care, antibacterial regimen, construction of high-porosity materials, analytical separations and solubilization processes,4 it is likely that they can be used in mixtures with various additives in future. In this endeavor, we have explored for the first time the influence of sodium anthranilate on the overall micellar structural changes of 5 mM tetramethylene-1,4-bis(N-hexadecylN,N-dimethylammonium bromide), a gemini surfactant, using 1H NMR studies.42 In continuation, we now report the influence of aromatic acids/salts (anthranilic acid, HAn; benzoic acid, HBen; sodium anthranilate, NaAn; and sodium benzoate, NaBen) on the overall micellar structural changes and possible viscoelasticity phenomena in three cationic gemini surfactant solutions, viz. 5 mM tetramethylene-1,4-bis(N-hexadecyl-N,Ndimethylammonium bromide), 10 mM pentamethylene-1,5-bis(N-hexadecyl-N,N-dimethylammonium bromide) and 10 mM hexamethylene-1,6-bis(N-hexadecyl-N,N-dimethylammonium bromide), symbolyzed as 16-4-16, 16-5-16, and 16-6-16, respec-
10.1021/jp070782j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8861 1H
tively, using NMR spectroscopic and viscometric techniques. Such type of studies is very scarce in literature though extensively reported in case of conventional cationic surfactants.19,20,25 The studies would be helpful in (a) gaining an insight into the effect of hydrophobic counterions, An- and Ben-, on micellar growth and structure of above-mentioned gemini surfactants, (b) evaluating the influence of the aromatic acids, viz. HAn and HBen, being weakly dissociating counterparts of their sodium salts, on the specified surfactants, (c) evaluating the effect of spacer chain length of the gemini surfactants on undergoing micellar structural changes and viscoelasticity by the aromatic acids/salts, and (d) understanding the solubilization site of these additives in such surfactant systems. Experimental Section N,N-Dimethylhexadecylamine (g95%, Fluka), 1,4-dibromobutane (g98%, Fluka), 1,5-dibromopentane (g98%, Fluka), and 1,6-dibromohexane (g97%, Fluka) were used without further purifications. D2O (99.9%) was obtained from Aldrich. HAn (98%, Fluka), HBen (99.5%, Qualigens), NaAn (99%, CPC), and NaBen (99%, G.S. Chemicals, India) were used as received. A 1:2.1 equivalent mixture of corresponding R,ω-dibromoalkane (m ) 4,5,6) with N,N-dimethylhexadecylamine in dry ethanol was refluxed at ∼80 °C for 48 h. The solvent was removed under vacuum from the reaction mixture and the solid thus obtained was recrystallized several times from hexane/ethyl acetate mixtures to obtain the compounds in pure form. The purity of the surfactants were checked on the basis of C, H, N analysis. The cmc of the surfactants were obtained by conductivity and surface tension measurements which were in close agreement with the literature values.43-45 The presence of no minimum in the surface tension vs [surfactant] plots was taken as additional evidence regarding the purity of the surfactants. 1H NMR Measurements. Stock solutions of the geminis were prepared in D2O. Sample solutions containing acids/salts were prepared by taking requisite amounts in standard volumetric flasks and making up the volumes by the freshly prepared surfactant solutions. The lower concentrations of the acids/salts were obtained by the dilutions with stock surfactant solutions. The sample solutions were syringed in the NMR tubes and the spectra were recorded on a Bruker Cryomagnet Spectrometer working at 300 MHz. Chemical shifts are given on the δ (ppm) scale. The reproducibility of chemical shifts was within 0.01 ppm. The line widths were measured from the spectra and are accurate to ( 0.1 Hz. Viscosity Measurements. Viscosity measurements were carried out by an Ubbelohde viscometer thermostated at 25 ( 0.1 °C, as described earlier.46 The viscometer was cleaned and dried every time before use. In order to check the reproducibility, the time of fall for every viscosity measurement was noted at least two times (sometimes three). By doing so it was found that the viscosity values were reproducible within (1%. Results and Discussion 1H NMR spectra of pure 5 mM 16-4-16, 10 mM 16-5-16, and 10 mM 16-6-16 in D2O are shown in Figure 1. The spectra of these surfactants and the resonance assignments agree well with those reported by us and others in literature.42,44 However, since the concentration of gemini surfactants is much higher (>175 times of the cmc),14 the observed chemical shifts, δ, can be considered those of the micellized surfactants. Tables 1-3 show the chemical shift changes of all proton resonances of 16-4-16, 16-5-16, and 16-6-16 gemini surfactants,
Figure 1. 300-MHz 1H NMR spectra of 16-4-16 (5 mM), 16-5-16 (10 mM), and 16-6-16 (10 mM) gemini surfactants in D2O.
respectively, in the absence and presence of increasing concentrations of NaAn, NaBen, HAn and HBen. The surfactant resonance assignment for the various protons is same as shown in Figure 1. It is well-known that the intercalation of aromatic counterions between headgroups of the micelles changes the surfactant resonances in a characteristic fashion on changing concentration of such additives. Generally, 1H resonances of headgroup region protons shift upfield and those further down the alkyl chain shift downfield.25,40-42 In addition, the merging
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TABLE 1: 1H NMR Chemical Shifts (δ, ppm) of Gemini 16-4-16 (5 mM) with Various Concentrations of Additives at 25 °C additive HBen
HAn
NaBen
NaAn
a
chemical shift (δ, ppm) 4 5
[additive] (mM)
1
2
3
0.0 1.0 2.0 3.0 5.0 1.0 2.0 3.0 5.0 1.0 2.0 3.0 5.0 1.0 2.0 3.0 5.0
0.78 0.91 0.92 0.92 0.92 0.91 0.92 0.92 0.93 0.91 0.92 0.93 0.96 0.92 0.92 0.93 0.93
1.20 1.33 1.32 1.32 1.33 1.33 1.33 1.33 1.34 1.33 1.33 1.34 1.37 1.33 1.34 1.34 1.32
1.28 a a a a a a a a a a a a a a a a
1.67 1.75 1.71 1.72 1.67 1.72 1.73 disappear disappear 1.72 1.61 disappear disappear 1.73 1.65 disappear disappear
1.80 1.89 1.86 1.88 1.85 1.88 disappear disappear disappear 1.88 1.86 disappear disappear 1.87 1.80 disappear disappear
6
7
3.33 3.43 3.39 3.41 3.36 3.42 disappear disappear disappear 3.38 3.36 disappear disappear 3.41 3.35 disappear disappear
3.07 3.18 3.17 3.17 3.16 3.18 3.16 3.15 3.14 3.19 3.16 3.14 3.11 3.17 3.15 3.12 3.06
Peak merges with 2.
TABLE 2: 1H NMR Chemical Shifts (δ, ppm) of Gemini 16-5-16 (10 mM) with Various Concentrations of Additives at 25 °C additive HBen
HAn
NaBen
NaAn
a
chemical shift (δ, ppm) 4 5
[additive] (mM)
1
2
3
0.0 1.0 3.0 5.0 10.0 1.0 3.0 5.0 10.0 1.0 3.0 5.0 10.0 1.0 3.0 5.0 10.0
0.93 0.91 0.91 0.93 0.94 0.89 0.89 0.90 0.90 0.91 0.93 0.93 0.96 0.86 0.92 0.93 0.95
1.35 1.33 1.32 1.33 1.33 1.33 1.32 1.31 1.30 1.33 1.34 1.34 1.37 1.28 1.34 1.34 1.36
1.44 1.40 1.48 a a 1.40 a a a a a a a a a a a
1.82 1.77 1.74 1.71 1.65 1.77 1.74 1.70 1.61 1.76 1.72 1.65 disappear 1.73 1.71 1.68 disappear
1.91 1.87 1.85 1.82 1.80 1.87 1.83 1.80 1.72 1.86 1.82 1.75 disappear 1.81 1.79 b disappear
6
7
3.46 3.42 3.39 3.36 3.32 3.41 3.38 3.35 3.27 3.41 3.36 3.30 disappear 3.37 3.34 3.31 disappear
3.22 3.19 3.17 3.17 3.14 3.19 3.17 3.15 3.10 3.18 3.17 3.13 3.10 3.14 3.15 3.14 3.09
Peak merges with 2. b Peak merges with 4.
TABLE 3: 1H NMR Chemical Shifts (δ, ppm) of Gemini 16-6-16 (10 mM) with Various Concentrations of Additives at 25 °C additive HBen HAn
NaBen
NaAn
a
[additive] (mM)
1
2
3
0.0 1.0 5.0 10.0 1.0 3.0 5.0 10.0 1.0 3.0 5.0 10.0 1.0 3.0 5.0 10.0
0.93 0.92 0.94 0.93 0.91 0.92 0.92 0.94 0.91 0.92 0.93 0.94 0.91 0.92 0.94 0.95
1.35 1.33 1.34 1.32 1.33 1.33 1.33 1.34 1.33 1.33 1.34 1.35 1.33 1.34 1.35 1.35
1.43 a a a 1.40 a a a a a a a a a a a
chemical shift (δ, ppm) 4 5 1.52 1.48 a a 1.49 1.44 1.41 a 1.48 1.43 a a 1.47 a a a
1.81 1.78 1.74 1.69 1.79 1.74 1.71 1.64 1.77 1.73 1.69 1.58 1.76 1.72 1.69 disappear
6
7
3.20 3.18 3.16 3.12 3.18 3.15 3.14 3.14 3.17 3.15 3.13 3.08 3.16 3.14 3.13 3.05
3.42 3.38 3.34 3.27 3.39 3.34 3.31 3.25 3.38 3.33 3.29 3.21 3.37 3.32 3.28 disappear
Peak merges with 2.
and broadening of some of the peaks of protons in the alkyl chain indicate the micellar growth.29,42 In view of this, the results presented in Tables 1-3 can be easily explained. The hydrophobic moieties of the surfactants near the core of the micelles are highly shielded and, therefore, show resonances
at lower δ values (resonances for 1-, 2-, and 3-H protons). However, as we move toward the headgroup the presence of N atoms make the adjacent protons more deshielded and therefore, show higher δ values (resonances for 4-, 5-, 6-, and 7-H protons). With increase in the concentration of aromatic
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8863 salts/acids in all the three surfactants studied, there occurs, except after first additions, general increase in δ value for protons of core carbon atoms and decrease for protons of carbon atoms near the ammonium headgroup. Moreover, merging of proton signals of intermediate carbon atoms with the proton signal of inner core carbon atoms is observed (peaks of 3-H protons with 2-H protons). This suggests that the aromatic ions An- and Ben- from aromatic salts/acids (though weakly dissociating) are solubilized in the palisade layer of the micelles, which include the region between hydrophilic groups and first few carbon atoms of the hydrophobic groups that comprise the outer core of the micellar interior. The interaction between carboxylate anion and ammonium groups of gemini surfactants would lead to compensation of positive charge on N-atom, thereby reducing its tendency to withdraw electrons from the carbon atoms attached to it. Therefore, such protons would get more shielded and hence show upfield chemical shift with the concentration of additive. This is very well observed in all resonances of 6- and 7-H protons in all gemini surfactants with all the additives. This effect may be transmitted to other protons via C-C bond and thus affect 1H resonances of 4- and 5-H protons (upfield shift). This upfield shift is more pronounced in NaAn and NaBen compared to their acid counterparts. This is quite expected due to strong interaction in case of former than the latter with the surfactant headgroups. In addition, among acids and salts, HAn and NaAn show greater upfield shift of headgroup protons than HBen and NaBen, respectively, due to presence of -NH2 group in anthranilate anion which helps them to remain in a more polar environment. Apart from it, penetration of aromatic ring into the palisade layer of micelle has been shown to produce an aromatic induced upfield chemical shift of methylene protons of the surfactant, the magnitude of which is indication of degree of penetration into the micelle.47,48 The consistent upfield shift of 4-H proton resonances with the increase in additive concentration supports the fact that the penetration of aromatic ions into the palisade layer of micelles takes place. The incremental upfield shift is seen to be almost similar for aromatic salts and their corresponding acids indicating their similar extent of penetration in each micelle. However, at similar concentrations the upfield chemical shift is more for aromatic salts than for the corresponding acids for a given surfactant indicating that the salts interact more strongly with the micelles. Therefore, upfield shift of 4-H protons may be attributed to the combination of compensation of positive charge on N-atom and aromatic induced ring-current effects. Bachofer et al.47 have shown similar effect of substituted naphthoate counterion on tetradecyltrimethylammonium bromide micelles. Moreover, they found a slight downfield shift of inner core protons in contrast to upfield shift of palisade layer protons. Similar to their observation, 1- and 2-H protons in all the gemini surfactants show a slight downfield shift with the concentrations of HBen, NaBen, HAn and NaAn. It is interesting to note that 3-H proton peaks merge with the 2-H proton resonances at most of the concentrations in all the surfactant-additive systems coupled with general broadening of proton peaks of surfactant when surfactant-additive mole fraction is close to 1:1 in case of 16-5-16 surfactant. However, in 16-4-16, the broadening seems to be highest and starts at additive concentration much earlier than with 16-5-16. This peak broadening points toward the micellar growth by these additives.29,42 It is interesting to note here that the first addition of additives in 16-4-16 surfactant shows different trend compared to that of 16-5-16 and 16-6-16 surfactants for both surfactant core and headgroup protons. In case of core protons (1- and 2-H), an
increase in δ values is observed for 16-4-16 in contrast to usual decrease for 16-5-16 and 16-6-16. Moreover, there occurs first increase in δ values of headgroup protons (6- and 7-H) in case of 16-4-16 compared to increase for 16-5-16 and 16-6-16. After first addition, in all the additives, the trend in case of headgroup proton chemical shifts and that of core protons is more or less similar for all the surfactants and is already explained. This different behavior after first addition for 16-4-16 surfactant can be explained as follows: The cmc of 16-4-16 surfactant is smaller and hence its micelles possess tight packing compared to 16-5-16 and 16-6-16 (due to smaller spacer chain length).14,44 This results in crowding of N-methyl protons of 16-4-16 surfactant on the micelle surface relative to 16-5-16 or 16-616. Therefore, as a result of greater space interaction, N-methyl protons of the former are more shielded than the latter which is evident from the lesser δ values in case of the former. The tight packing of core protons also results due to smaller headgroup area in 16-4-16 and hence shows small δ values. When the aromatic salts/acids are added two effects come into play: (a) due to their intercalation within the palisade layer the distance between N-methyl protons on surface is increased on first addition, resulting in decrease in crowding of such protons which, in turn, reduces the electron density around protons with increase in δ values; and (b) the intercalation also reduces the positive charge on nitrogen atom due to compensation which favors the increase in electron density on N-methyl protons and hence lowers the δ values. These two factors act in opposition to decide the overall effect on δ values. Now, since the 16-416 is already tightly packed, first addition of additive results in increase in δ values of 6- and 7-H protons due to the more contribution of factor a, relative to factor b. Thereafter, the effect of factor b might be pronounced resulting in decrease in δ values. However, comparatively loose packing of surfactant monomers in the micelles of 16-5-16/16-6-16 may result in greater contribution of factor b than factor a, hence continuous decrease in δ values for headgroup protons takes place. Similarly, the core protons of 16-4-16 would experience a slight loosening due to intercalation of additives in the palisade layer of micelles. This would result, as observed, in an increase in δ values for 1- and 2-H protons. However, for 16-5-16/16-6-16, the larger magnitude of effect b on headgroup protons results in slightly tighter packing, hence their shielding. After continued addition of the additives, there may be increase in distance between the core protons resulting in continuous increase in δ values thereafter. The line width at half-height of the signals from the protons of N-methyl groups of the surfactants as a function of HBen, NaBen, HAn and NaAn concentration are shown in Figure 2. The data show increase in line width values of these protons of surfactants with increasing concentration of all the aromatic acids/salts in case of 16-4-16 gemini surfactant but no such increase is seen in 16-5-16 and 16-6-16 surfactants except in 16-5-16/NaAn and NaBen systems where a slight rise in line width values at higher additive concentrations is observed. Moreover, at 1:1 mole fraction, merging of these proton peaks occurs with 6-H protons. In addition, the increase in line width in case of 16-4-16 is much more in presence of NaAn and NaBen compared to in presence of their corresponding acids at similar concentrations supporting the fact that the former possesses strong interaction than the latter with the surfactant headgroups. Also, comparison between effect of NaAn and NaBen on line width values at similar concentrations shows the stronger interaction of An- with headgroup than Benanions. This may be due to presence of -NH2 group at ortho
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Figure 2. Plot of line width of the signals from protons of the N-methyl group of 16-6-16, 16-5-16 and 16-4-16 gemini surfactants at 25 °C against different concentrations of additives viz. (a) HBen, (b) NaBen, (c) HAn, and (d) NaAn.
position to COO- group in the former which facilitates its binding with the positively charged headgroup through electrostatic interaction between positive N-atom of surfactant and negative COO- group/lone pair on -NH2 group of additive. In this context, the inter-headgroup repulsions would be decreased to an appreciable extent thereby facilitating micellar growth. The (a) increase in N-methyl proton line widths of 16-4-16 with concentration of all additives, (b) slight increase in line width values for 16-5-16 with only aromatic salts at higher additive concentration, and (c) no change in line width values for 166-16 with all aromatic acids/salts indicate appreciable effect of the spacer of surfactants in undergoing micellar growth in presence of additives. Thus, nature of gemini surfactant as well as that of additive are both important for obtaining micellar growth. To support the explanation that the line width increase of N-methyl protons indicates the micellar growth and to further ascertain the effect of additives in inducing micellar growth, viscosity measurements were performed in all the surfactant systems with various additive concentrations. Variation of relative viscosity with different concentrations of HAn, HBen, NaAn and NaBen for 5 mM 16-4-16, 10 mM 16-5-16, and 10 mM 16-6-16 is shown in Figure 3. Again, the effect seems to be dependent on the nature of the surfactant’s spacer as well as additive and is in conformity with the observation made in case of increase in line width of N-methyl protons. The 16-4-16
gemini shows increase in relative viscosity with all the additives at each concentration (except in presence of HBen the effect is seen only at higher concentration). Again, aromatic salts are much effective than their corresponding acids. However, among acids, HAn is more effective than HBen, while among salts, NaAn is more effective than NaBen. This is in accord with the explanation furnished for line width increase and hence micellar growth. It must be mentioned that the micellar growth is accompanied with the large increase in the relative viscosity. In addition, no viscosity rise in case of 16-6-16 with any of the additives while increase in viscosity at higher concentrations of NaBen and NaAn (but not with HBen and HAn) in case of 16-5-16 conforms to the previous observations. The micellization and adsorption characteristics of cationic gemini surfactants are strongly dependent on the chain length and nature of spacer.4,13,44 For instance, cmc’s of gemini surfactants have been found to be smaller than their comparable conventional surfactant counterparts and found to increase with the increase in spacer chain length for a given hydrophobic chain length.14 The latter effect has been attributed to the increase in surfactant headgroup area.4,14 In contrast to conventional surfactant micelles, the distance between the headgroups in gemini surfactant micelles presents a bimodal distribution.4 The two headgroups of a given gemini surfactant molecule in a micelle are held at a distance dictated by the length of spacer and cannot be changed. However, distance between two headgroups of two
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8865
Figure 3. Plots of relative viscosities with additive (a) HBen, (b) NaBen, (c) HAn, and (d) NaAn concentrations for 16-6-16 (10 mM), 16-5-16 (10 mM), and 16-4-16 (5 mM) gemini surfactants at 25 °C.
surfactant molecules in a micelle is mainly dictated by interheadgroup repulsions that can be manipulated. Therefore, it is expected that the effective headgroup size would increase from 16-4-16 to 16-6-16. Electrostatic interaction between headgroups is one of the factors that governs the size and shape of micelles. The suppression of the electrostatic interaction between the headgroups can be achieved by adding electrolytes but more so by salts having hydrophobic nature.49 It is generally observed and also evident from our data, that aromatic salts/acids get intercalated between headgroups of micelles and hence reduce the surface charge of the micelles leading to promotion of micellar growth. If one adopts the formalism proposed by Israelachvilli et al.,50 the shape of a micelle is primarily determined by surfactant geometry and packing. Accordingly, the progression from spherical to cylindrical micelles necessitates an increase of the surfactant packing parameter, P ) V/al, where V is the volume of hydrophobic portion of the surfactant monomer, a is the surface area occupied per surfactant headgroup and l is the length of the hydrocarbon chain. Addition of salt or any additive that leads to reduction in a would induce sphere-to-rod transition in micelles and hence lead to viscosity increase. As per this view point, the extent of reduction in a in our gemini surfactants would be related to their spacer chain length. Therefore, for a given additive having certain ability to interact with the headgroups, the effective reduction of a in the selected gemini surfactants would be in the order 16-4-16 >
16-5-16>16-6-16. This is because of the fact that even though reduction in the distance between two headgroups of two surfactants in a micelle may be same in all the surfactants for a given additive, the distance between two headgroups of the same surfactant would remain the same dictated by its spacer chain length. This would ultimately lead to more net reduction of a in the surfactant with smaller spacer chain length. This explanation is consistent with our findings. It is evident that no additive at all concentrations, used in this study, is effective in bringing the viscosity increase in the 16-6-16 surfactant. However, the effect of reducing a by all the additives and hence micellar growth in case of 16-4-16 surfactant is greater. Only salts (NaAn and NaBen) are effective in case of 16-5-16 surfactant and hence form an intermediate case. As already mentioned, the effect of salts is greater than their corresponding acids since their strong interaction with surfactant headgroups leads to more effective decrease in a. Moreover, among the acids and salts, HAn and NaAn, respectively, impose greater effect due to their enhancement of polar character because of -NH2 group and hence interact strongly with the headgroup than HBen and NaBen. It may be mentioned that more concentration (>10 mM) of NaAn or NaBen may be needed to induce structural changes in 16-6-16 micelles. The intercalation of aromatic acids/salts between the headgroups of micelles is supported by the 1H NMR data of the additives obtained at various concentrations in micelles of
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Figure 4. 300 MHz 1H NMR spectra of NaAn: (b) 10 mM pure NaAn; at different concentrations of NaAn in presence of (d) 10 mM 16-5-16, and (e) 10 mM 16-6-16.
Figure 5. 300 MHz 1H NMR spectra of NaBen: (a) 5 mM pure NaBen; (b) 10 mM pure NaBen; at different concentrations of NaBen in presence of (c) 5 mM 16-4-16, (d) 10 mM 16-5-16, and (e) 10 mM 16-6-16.
gemini surfactants. Figures 4 and 5 show the 1H NMR spectra of ring protons of NaAn and NaBen, respectively, in their pure state (a, b) as well as at their different concentrations in presence of 5 mM 16-4-16 (c), 10 mM 16-5-16 (d), and 10 mM 16-6-16 (e) (the corresponding spectra of NaAn s 16-4-16 system are given in ref 42). The spectra are of first-order and consist of three multiplets in case of both pure NaAn and NaBen in D2O. The signals of amino protons for NaAn (see Figure 5 of
reference 42 and Figure 4b) do not appear separately as they are labile and thus merge with the solvent peak. The ring protons 3-, 4-, and 5-H of NaAn in all surfactants shift upfield while 6-H proton shifts downfield. This is due to presence of former protons in nonpolar environment while the latter in polar environment indicating intercalation of An- near headgroups of dimeric surfactants. However, the general peak broadening of ring protons of An- in case of 16-4-16 and 16-
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8867 5-16 (at higher concentrations, Figure 4d; see ref 42 also), but not in 16-6-16 (Figure 4e), indicates the presence of grown micelles in the first two systems. This fact is supported by the viscosity increase as discussed earlier. In addition, there occurs distinction of 3- and 5-H proton peaks in presence of micelles establishing slightly different environment of 3- and 5-H protons. This is obvious since 3-H protons would be affected by -NH2 group of An- ion. Likewise, 3-, 4- and 5-H ring protons of NaBen are shifted upfield while 2- and 6-H protons downfield in presence of all the surfactants (Figure 5). This supports the fact that Benembeds its aromatic ring in the palisade region of the micelle and its COO- group remains in close proximity with the positively charged headgroup of the surfactant. In addition, there occurs a progressive broadening and eventual collapse of the multiplets in presence of 16-4-16 surfactant at and above 1 mM concentration but beyond 3 mM concentration in presence of 16-5-16 surfactant. This observation is not very clear in 16-616 surfactant. Concomitant with this is the viscosity rise in 164-16 and 16-5-16 surfactants but no change in presence of 166-16 surfactant. This indicates strong interaction of Ben- ion with the surfactant headgroups and consequent reduction in a to the extent that micellar morphology changes in 16-4-16 and 16-5-16 dimeric surfactants. Perhaps similar effect could be observed with respect to 16-6-16 surfactant in presence of NaBen beyond 10 mM concentration. In presence of aromatic acids (data not shown), similar effects were observed in case of all the surfactants except a few distinctions. First, broadening of aromatic proton peaks and eventual collapse of multiplets in case of HAn are not observed in presence of 16-5-16 surfactant (as in the case of NaAn) concomitant with which is no rise in viscosity in the 16-5-16/ HAn system. This shows HAn embedding in the similar environment in the micelles (as NaAn) but not producing decrease in a to the extent that micellar growth may occur. As already pointed out, being a weakly dissociating counterpart of NaAn, it does not undergo strong interaction with the surfactant headgroups. In addition, broadening effect is quite large in the 16-4-16/NaAn system compared to 16-4-16/HAn system indicating the weaker effect of HAn in inducing viscosity rise as is evident from the magnitudes of the relative viscosity (Figure 3). Second, the broadening effect on aromatic proton peaks of HBen is smaller than that of NaBen in presence of 16-4-16 pointing toward less efficiency of the former to induce micellar growth. In contrast to collapse of multiplets in NaBen in 165-16 above 3 mM concentration there is no such observation in case of HBen in 16-5-16. This suggests no effect of HBen in inducing micellar growth. Finally, there is a tendency toward broadening and eventual collapse of mutiplet structure of NaAn/ NaBen aromatic protons as concentration of additive increases in case of 16-6-16 surfactant but no such observation in case of their weakly dissociating counterparts HAn/HBen indicating that the salts may induce structural transition of 16-6-16 surfactant micelles beyond the studied concentrations. Conclusion With the above discussion, we can conclude that the micellar growth of the gemini surfactants by addition of the organic acids/ salts depends not only on the presence of additional group on the benzene ring but also on the spacer chain length of the gemini. The effects of NaAn and NaBen are greater than their corresponding acids due to the strong interactions of their counterions with the headgroups of the micelles, which leads to decrease in the surface area occupied per surfactant headgroup (a) and results in the micellar growth. With smaller spacer chain
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