Role of Incorporation of Multiple Headgroups in Cationic Surfactants in

Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India. ReceiVed: April 23, 2001; In Final Form: September 5, 2001. Three new single-chain...
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J. Phys. Chem. B 2001, 105, 12803-12808

12803

Role of Incorporation of Multiple Headgroups in Cationic Surfactants in Determining Micellar Properties. Small-Angle-Neutron-Scattering and Fluorescence Studies Jayanta Haldar,† Vinod K. Aswal,‡ Prem S. Goyal,§ and Santanu Bhattacharya*,† Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India, and Solid State Physics DiVision, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India, and IUC-DAEF, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India ReceiVed: April 23, 2001; In Final Form: September 5, 2001

Three new single-chain surfactants bearing one, two, and three headgroups have been synthesized. Each of these surfactants formed micelles upon solubilization in water. The critical micellar concentrations (cmc) of these surfactants were determined using a micelle-solubilized, extrinsic fluorescent probe, pyrene. The cmc values were found to increase with every increase in the number of headgroups of the surfactant. Detailed small-angle-neutron-scattering (SANS) studies were performed with the micellar solutions in D2O to study their aggregate properties. The data were analyzed using the Hayter and Penfold model for macroion solution to compute the interparticle structure factor, S(Q), taking into account the screened coulomb interactions between the micelles. SANS analysis has shown that the extent of aggregate growth of these cationic micelles depends on the number of the headgroups present in the surfactants. It has been observed that the micelles become progressively smaller in size with every increase in the number of headgroups of the surfactants. The aggregation number (N) is continually decreased and the fractional charge (R) is gradually increased with the increase in the number of headgroups. The semiminor axis and semimajor axis of the micelle decrease strongly with the increase in the number of headgroups. To mitigate charge repulsion, the hydrocarbon chains in the multiheaded surfactants appear to take up bent conformations. The effects of concentration and temperature on micellar growth have also been examined. While the N and R for the micelles of single-headed surfactant increased and decreased, respectively, with concentration, no significant alteration in these parameters was observed with micelles of surfactants with double and triple headgroups upon increase in concentration. Increase in temperature brought about transformation of micelles of surfactant with one headgroup more toward spherical morphology. However, such temperature-induced changes were less-pronounced with micelles of doubleand triple-headed surfactants.

Introduction Micelles are formed by the self-organization of the surfactant molecules in their aqueous solution above the critical micellar concentration (cmc).1 The aggregates formed are of various types, shapes, and sizes. These include spherical, ellipsoidal, cylindrical, or threadlike micelles, disklike micelles, membranes, and even vesicles.2 The aggregation behavior of surfactants in solution is modulated by parameters such as concentration, temperature, and the presence of additives, such as electrolytes, alcohols, and amines, etc.3 However, instances are also known in which the molecular architectures of surfactants profoundly influence the properties of these aggregates. Different types of cationic surfactants are known. These include surfactants consisting of a single long alkyl chain connected to one polar headgroup such as cetyltrimethylammonium bromide (CTAB). Another class of surfactant, called “gemini” surfactants, has been recently introduced. These consist of two hydrophobic chains and two headgroups covalently attached by a flexible or rigid spacer.4 These surfactants form * Corresponding author and Swarnajayanti Fellow (DST). Fax: +9180-360-0529. E-mail: [email protected]. Also located at the Chemical Biology Unit of JNCASR, Bangalore 560 012, India. † Indian Institute of Science. ‡ Solid State Physics Division, Mumbai Centre, Bhabha Atomic Research Centre. § IUC-DAEF, Mumbai Centre, Bhabha Atomic Research Centre.

micellar aggregates. On the other hand, the surfactants bearing one polar headgroup with two or more alkyl, acyl, or mixedchain hydrophobic chains produce vesicles on dispersion in water.5,6 Recently, surfactants bearing four headgroups and four hydrocarbon chains have also been introduced.7 These examples illustrate how tailoring of surfactant molecules influences their aggregation behavior. However, until recently, there has been no attempt to systematically examine the influence of enhancing the number of headgroup units while keeping the hydrophobic part unaltered. If several charged headgroups are incorporated onto one end of a single hydrocarbon chain of the surfactant molecule, it is not clear whether a surfactant will form micelles. This is because with every rise in the number of charges, the micellar surface demands a progressively smaller aggregation number, which makes it increasingly difficult to fill the micellar volume. Several questions then arise. Would the surfactant behave like a salt, for example, NaCl, in which the salt remains fully ionized as Na+ and Cl- ions? In that case, would there be any aggregation of cations? How many charged headgroups can be attached onto a single hydrocarbon chain before micelle formation would no longer be possible? Can one regulate the micellar growth and surfactant packing by the increase in the headgroup charges? How will the counterion distribution be governed in micelles from surfactants with multiple headgroups?

10.1021/jp011523b CCC: $20.00 © 2001 American Chemical Society Published on Web 12/04/2001

12804 J. Phys. Chem. B, Vol. 105, No. 51, 2001 To address such fundamental questions, we have synthesized a novel class of single-chain cationic surfactants with multiple headgroups.8

The micelles formed from these new surfactants were characterized in detail. Small-angle-neutron-scattering (SANS),9 a reliable technique for examining the sizes and shapes of micelles, was employed. This method has been extensively used for the examination of different membrane structures.10 Recently, we exploited SANS to examine the micellar solutions of gemini surfactants.11 Herein, we report the micelle formation from these new multiheaded surfactants and present the SANS characterization of the solutions of different surfactants at various concentrations and temperatures. Experimental Section Materials. CTAB, pyrene, and other chemicals used in this study were purchased from Aldrich. The D2O (99.4 atom % D) was obtained from Heavy Water Division, BARC, Trombay, India. The micellar solutions were prepared by dissolving known amounts of surfactants in D2O. All of the reagents and solvents used in this study were of the highest grade available commercially. Millipore-grade water was used for all physical measurements. All of the surfactants with h ) 1, 2, and 3 were synthesized and purified as described earlier.8 1H-NMR, FTIR, ESI-MS and elemental analysis confirmed the structures assigned to these surfactants. Thin-layer chromatography (TLC) on precoated silica gel plates confirmed that each of the presented surfactants was highly pure and no other spot was detectable on tlc plate. Methods. Critical Micellar Concentration (cmc). The steadystate fluorescence technique was used to determine the critical micellar concentrations.12 Pyrene, a fluorescence probe with a spectral signature that changes with the formation of aggregates such as micelles, was chosen as a probe. Fluorescence measurements were performed in a Hitachi F-4500 spectrofluorimeter equipped with a thermostated water-circulating Julabo model F10 bath. All measurements were carried out at 25 °C and used a 3 cm3 cell. Excitation wavelength was fixed at 310 nm, and emission spectra of the region 360-410 nm were studied keeping the bandwidths fixed at 2.5 nm for the emission and 10 nm for the excitation. Pyrene, a micelle-soluble fluorescence probe, exhibits fine structure in its emission spectra that is quite dependent on the surfactant concentration. Peaks 3 and 1 show the greatest solvent dependency. Therefore, plots of the surfactant concentration against the ratio (I3/I1) of the intensities of the first (I1) and the third (I3) vibronic peaks in the fluorescence emission spectra due to micelle-solubilized pyrene provide breaks indicating the

Haldar et al. onset of micellization. This break point is assigned as the systemic cmc value for a given micelle. Surfactant solutions of different concentrations in water were doped with same amount of pyrene for the cmc determination. Small-Angle-Neutron-Scattering (SANS) Measurements. Data Collection. The small-angle-neutron-scattering (SANS) experiment was carried out on micellar solutions of cationic surfactants containing a single hydrocarbon chain with single, double, and triple cationic headgroups. Because the details of the micellar properties of CTAB are well-documented in the literature, the present study allows us to compare these results directly with that of CTAB, which is a single-headed cationic surfactant. All final solutions used in the neutron-scattering experiments were prepared in D2O. This provides a good contrast between the micelle and the solvent in a SANS experiment. Neutron-scattering measurements were performed on the SANS instrument at the DHRUVA Reactor, Trombay.13 The mean wavelength of the neutron beam was 5.2 Å, and the data were collected in the wave vector transfer, Q (Q ) 4π sin 1/ φ/λ, where λ is the wavelength of the incident neutrons and 2 φ is the scattering angle), range of 0.02-0.20 Å-1. The solutions were held in a 0.2 cm path-length UV-grade quartz sample holder with tight fitting Teflon stoppers, sealed with Parafilm. The effect of different concentrations on the SANS distribution was studied for micellar solutions of all three surfactants for concentrations in the range of 25-100 mM at 40 °C. The effect of temperature in the range of 40-70 °C was also investigated at a fixed surfactant concentration of 50 mM. The measured distributions were corrected for the background, empty-cell scattering and sample transmission. Solvent intensity was subtracted from that of the sample. The resulting corrected intensities were then normalized to absolute cross section units. The experimental points were fitted using a nonlinear leastsquares routine as described below. Comparisons between the experimental and the calculated cross sections are shown in Figures 1-7. SANS Analysis. Calculation of the Scattering Intensity. In small-angle-neutron-scattering, one measures the coherent differential cross section (dΣ/dΩ) per unit volume. For a system of monodisperse interacting micelles, dΣ/dΩ is given by

dΣ ) n(Fm - Fs)2V 2[〈F(Q)2〉 + 〈F(Q)〉2(S(Q) - 1)] + B dΩ (1) where n denotes the number density of the micelles, Fm and Fs are, respectively, the scattering length densities of the micelle and the solvent, and V is the volume of the micelle.14 The aggregation number, N, of the micelle is related to the micellar volume, V, by the relation, V ) NV, where V is the volume of the surfactant monomer. The volumes of the surfactant monomers of the single-, double-, and triple-headed surfactants are 653, 875, and 1097 Å3, respectively. It is calculated by taking into account the volume of the hydrocarbon chain up to the 15th carbon to be 431 Å3, as obtained from Tanford’s formula.15 The volume of each headgroup N+(CH3)3 is 102 Å3, as taken from literature, and the volume of each linker (-CH2OCOCH2-) part is 120 Å3 (volume of the linker V ) M/dNA, where M ) 72 is the molecular weight of the linker, d ) 1 g cm-3 is density, and NA is Avogadro’s number).16 The scattering length densities of single-, double-, and triple-headed surfactants are -0.062 × 1010, 0.14 × 1010, and 0.25 × 1010 cm-2, respectively. The scattering length density of D2O is 6.38 × 1010 cm-2. F(Q) is the single particle form factor, and S(Q) is the interparticle structure factor. B is a constant term that

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represents the incoherent scattering background, which is mainly due to hydrogen in the sample. The single particle form factor has been calculated by treating the micelle as a prolate ellipsoidal. For such an ellipsoidal micelle,

〈F2(Q)〉 )

∫01[F(Q,µ)2 dµ]

(2)

∫01F(Q,µ) dµ]2

(3)

〈F(Q)〉2 ) [ F(Q,µ) )

3(sin x - x cos x) x3

x ) Q[a2µ2 + b2(1 - µ2)]1/2

(4) (5)

where a and b are, respectively, the semimajor and semiminor axis of the ellipsoidal micelle and µ is the cosine of the angle between the directions of a and the wave vector transfer Q. Structure Factor for Interacting Micelles. In general, micellar solutions of ionic surfactants show a correlation peak in the SANS distribution. The peak arises because of the corresponding peak in the interparticle structure factor S(Q) and indicates the presence of electrostatic interactions between the micelles. S(Q) specifies the correlation between the centers of different micelles, and it is the Fourier transform of the radial distribution function g(r) for the mass centers of the micelle. In the analysis for ellipsoidal micelles, S(Q) has been calculated using mean spherical approximation as developed by Hayter and Penfold.17 In the approximation, the micelle is assumed to be a rigid equivalent sphere of diameter σ ) 2(ab2)1/3 interacting through a screened coulomb potential, which is given by

u(r) ) u0σ

exp[-κ(r - σ)] , r>σ r

(6)

where κ is the Debye-Huckel inverse screening length and is calculated by

κ)

[ ] 8πNAe2I

1/2

103kBT

(7)

defined by the ionic strength, I, of the solution

1 I ) cmc + RC 2

(8)

In pure micellar solution, I is determined by the concentration of the surfactant monomers, which is equal to the cmc and dissociated counterions from the micelles. The fractional charge, R (R ) Z/N, where Z is the micellar charge), is the charge per surfactant molecule in the micelle and is a measure of the dissociation of the counterions of the surfactant in the micelle. C presents the concentration of the surfactant. The contact potential, u0, is given by

u0 )

Z2e2 π0σ(2 + κσ)2

(9)

where  is the dielectric constant of the solvent medium, 0 is the permittivity of free space, and e is the electronic charge. Although micelles may produce polydisperse systems, we have assumed them as monodisperse for the simplicity of the calculation and to limit the number of unknown parameters in the analysis. The dimensions of the micelle, the aggregation number, and the fractional charge have been determined from

Figure 1. SANS distributions from micellar solutions of single- (h ) 1), double- (h ) 2), and triple-headed (h ) 3) surfactants and CTAB at 50 mM at 40 °C. The lines shown are theoretical fits and the solid marks are experimentally determined data points.

the analysis. The semimajor axis (a), semiminor axis (b ) c), and the fractional charge (R) are the parameters in analyzing the SANS data. The aggregation number is calculated by the relation N ) 4πab2/(3V). It may be pointed out here that the volumes of the surfactant molecules presented herein are calculated data. The volumes of the headgroup, spacer, and alkyl chain have all been taken from the literature.15,16 The reason for keeping the volume per molecule and minor-axis “b” as constant is to limit the number of unknown parameters in the analysis. In fact, it was verified earlier that these parameters are found to be similar even if they are kept as fitting parameters. Results and Discussion Micelle Formation. At concentrations up to ∼5 mM, all of the surfactants were found to be readily soluble in water at 25 °C. However, solubilization of the single-headed surfactant (h ) 1) in pure D2O beyond 10 mM required heating, and the solution for the single-headed surfactant (h ) 1) in water in the 25-100 mM concentration regime appeared translucent. Because SANS studies required high concentrations of surfactants (25-100 mM) to avoid optical scatter, all of the experiments were performed at 40 °C or higher temperatures. Surfactants with h ) 2 and h ) 3 were, however, highly soluble in D2O at ambient temperature. Solutions of these surfactants in the concentration ranges from 25 to 100 mM were thus found to be quite transparent at 25 °C. Higher solubility of the surfactants with h ) 2 or h ) 3 in water may be due to the increasing hydrophilic character of these surfactants with higher number of headgroups. The critical micelle concentrations (cmc’s) for each surfactant, corresponding to h ) 1, 2, and 3, were determined at 25 °C by plotting the values of the pyrene intensity ratio, I3/I1, against the respective surfactant concentrations. For each surfactant, reproducible breaks were observed indicating the onset of micellization. The cmc values obtained using this procedure for the surfactants were ∼1.0 (h ) 1), 3.5 (h ) 2), and 3.7 mM (h ) 3), and the value obtained using the same method for CTAB (0.8 mM) was in good agreement with the cmc value reported.12 Neutron Scattering. Next, we measured neutron cross sections from micellar solutions of various surfactants including CTAB in D2O at a fixed surfactant concentration (50 mM) at 40 °C. All of the distributions showed well-defined peaks characteristic of suspensions of charged particles (Figure 1. These results confirm that micelles are indeed formed from all of the surfactants irrespective of the number of headgroups (h) per alkyl chain. Usually this peak occurs at Qm ≈ 2π/d, where d is the average distance between micelles. Because the Qm was found to vary with the number of headgroups (h), it may be

12806 J. Phys. Chem. B, Vol. 105, No. 51, 2001

Haldar et al.

TABLE 1: Effect of Increasing Number of Headgroups (h) in the Surfactants on Q Valuea surfactant

N

R

b ) c (Å)

a (Å)

a/b

h)1 h)2 h)3 CTAB

244 48 20 135

0.08 0.23 0.27 0.14

24.5 17.3 14.5 22

66.9 36.2 27.3 37.3

2.73 2.1 1.88 1.7

a All of the SANS spectra were taken at 40 °C using a 50 mM micellar solution of each surfactant.

inferred that intermicelle distance and hence the number density (n) of micelles are not the same for different surfactants even at identical surfactant concentrations. This suggests that the aggregation number (N) of the micelle varies with the number of headgroups (h). Further, it is clear that the aggregation number (N) decreases with an increase in h value (Table 1). This is understandable because larger headgroup sizes and enhanced electrostatic repulsion, as in the cases with h ) 2 and h ) 3, will require more space per individual surfactant. This also means that it would be possible to accommodate fewer surfactant molecules (with h ) 2 and h ) 3) to pack into a single micellar aggregate. It may be mentioned that at 50 mM surfactant concentration, micelles adopt a prolate ellipsoidal shape (a * b ) c) for all of the samples including CTAB. The lengths of the respective semimajor axis (a) and the semiminor axes (b ) c) are given in Table 1. It is seen that both a and b and the axial ratio a/b decrease when the value of h is increased. It is interesting to note that compared to the surfactant with h ) 1, CTAB micelles have a considerably lower aggregation number (N ) 135) at identical concentration and temperature. This is despite the fact that CTAB also possesses only one Me3N+ headgroup and a single hydrocarbon chain. In a micellar aggregate, the surfactant with h ) 1 is expected to be rather “tightly” associated because of the presence of an ester unit [-O-C(O)-] that links the Me3N+ headgroup with the hydrocarbon chain. The presence of an ester linkage has been shown to facilitate intermolecular association among surfactants through dipolar interactions in organized assemblies.18 In the present instance, because these ester linkages are located near the Stern-layer region19 of the micelles, hydrogen-bonding interactions among the surfactant molecules may also operate via interfacially adhering water molecules.5b,20 Both the semimajor (a) and the semiminor axis (b ) c), as well as the axial ratio (a/b), decrease with an increase in the number of headgroups (h). This is due to the fact that with every rise in the number of Me3N+ units in the headgroup, the charge repulsion between each surfactant at the headgroup level becomes increasingly severe. To alleviate such unfavorable electrostatic consequences, the hydrocarbon chains in the micelles of the multiheaded surfactants most likely adopt folded, s-gauche conformations. This in turn would lead to the formation of increasingly “disorderly” collections of the multiheaded surfactant molecules in the resulting micellar aggregate. In such disorganized aggregates, the hydrophobic core of the micelle should experience relatively more aqueous environment. By comparison in micelles of surfactants with h ) 1, the hydrocarbon chains would be expected to be in their more extended conformation, leaving the terminal CH3 group significantly buried deep inside the core. Fractional charge (R) provides a measure of the dissociation of the counterion from the surfactant cations present in the micelle. Analysis of the SANS data (Table 1) gives an R value of 0.08 for the surfactant with h ) 1. This means that only 8% of the Br- counterions are dissociated from the micelle of the surfactant with h ) 1, leaving the remaining 92% of counterions

Figure 2. SANS distributions from micellar solutions of single-headed surfactant (h ) 1) at different concentrations: 25, 50, 75, and 100 mM at 40 °C.

virtually bound. Inspection of Table 1 further reveals that the fractional charge on the micelle increases with the number of headgroups on the surfactant. This is not surprising if the equilibrium dissociation constant depends linearly on the number of cationic headgroups per surfactant molecule. Then, one could expect that Rd ) 2Rs and Rt ) 3Rs where Rs, Rd, and Rt represent the fractional charges on the single-, double-, and triple-headed surfactants, respectively. However, from Table 1, we see that Rd > 2Rs, suggesting a far greater dissociation of the counterions in the case of micelles of the surfactant with h ) 2 than would be expected merely from doubling the number of headgroups of the surfactant with h ) 1. Thus with the double-headed surfactant, considerably enhanced dissociation of counterions occurs from the corresponding micellar surface. This could be due to the fact that double-headed surfactants require more room at the headgroup level than that needed for the packing of two single-headed surfactants in a micelle. This is expected because the positive charges located on each of the two Me3N+ heads of h ) 2 would like to stay as far apart as possible because of electrostatic repulsion. With an increase in charge, the headgroup hydration also increases, facilitating greater ionization. Although each Me3N+ unit on the triple-headed surfactant (h ) 3) would tend to stay even farther apart, the intermolecular repulsion between individual surfactants in the micelle increases steeply. As a consequence, the aggregation number of these micelles is considerably lowered. To mitigate the headgrouplevel repulsion and to accommodate the volume necessary for a micelle to be formed, the hydrocarbon chains in these micelles should also adopt severely bent conformations. While increased charge facilitates hydration, the hydrocarbon chain coiling regulates the extent of increase in fractional charge (Rt) in h ) 3. The surface charge densities for the micelles in Table 1 for h ) 1, 2, and 3 and CTAB are 1.33 × 10-3, 1.81 × 10-3, 1.35 × 10-3, and 2.2 × 10-3 e/Å2, respectively. These numbers have been obtained by dividing the total charge on the micelle by the surface area of the micelle. It may be noted that the decrease in the surface charge density for h ) 3 may originate from the necessity to accommodate the bigger effective physical headgroup size at the micellar surface. Role of Surfactant Concentration. The effects of variation of concentration for the three new surfactants on SANS distributions at 40 °C are shown in Figures 2-4. With increase in concentration, the interparticle distance decreases and the peak shifts to higher Q values. Figure 2 shows the effect of concentration variation of micellar solutions of the single-headed surfactant (h ) 1) at 40 °C. The concentration variation range examined was from 25 mM to 100 mM. The volume of a surfactant molecule in the micelle was taken to be independent of the concentration of surfactant. The monomer volume (V) for the surfactant h )1 was found to be about 653 Å3. It is

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TABLE 2: Effect of Concentration on Q Value for Single(h ) 1), Double- (h ) 2), and Triple-Headed (h ) 3) Surfactant Systems at 40 °Ca surfactant

C (mM)

N

R

b ) c (Å)

a (Å)

a/b

h)1

25 50 75 100 25 50 75 100 25 50 75 100

215 244 269 390 45 48 47 48 18 20 20 21

0.09 0.08 0.08 0.06 0.23 0.23 0.22 0.22 0.25 0.27 0.33 0.38

24.5 24.5 24.5 24.5 17.3 17.3 17.3 17.3 14.5 14.5 14.5 14.5

58.8 66.9 73.7 107.0 33.8 36.2 35.4 36.2 24.6 27.3 27.3 28.6

2.40 2.73 3.01 4.37 1.95 2.10 2.05 2.10 1.70 1.88 1.88 1.97

h)2

h)3

a

Volume of the surfactant and semiminor axis b for individual surfactants are kept fixed in the fitting procedures.

Figure 5. SANS distributions from a 50 mM micellar solution of single-headed surfactant (h ) 1) at various temperatures: 40, 50, 60, and 70 °C.

TABLE 3: Effect of Temperature on Q Value for Single- (h ) 1), Double- (h ) 2), and Triple-Headed (h ) 3) Surfactant Systems at 50 mM Concentrationa surfactant

T (°C)

N

R

b ) c (Å)

a (Å)

a/b

h)1

40 50 60 70 40 50 60 70 40 55 70

244 188 153 124 48 45 40 35 20 18 15

0.08 0.11 0.15 0.20 0.23 0.27 0.35 0.42 0.27 0.29 0.32

24.5 24.5 24.5 24.5 17.3 17.3 17.3 17.3 14.5 14.5 14.5

66.9 51.5 41.8 33.7 36.2 33.8 30.0 26.1 27.3 24.6 20.5

2.73 2.10 1.71 1.38 2.10 1.95 1.73 1.51 1.88 1.70 1.41

h)2

h)3

Figure 3. SANS distributions from micellar solutions of double-headed surfactant (h ) 2) at different concentrations: 25, 50, 75, and 100 mM at 40 °C.

Figure 4. SANS distributions from micellar solutions of triple-headed surfactant (h ) 3) at different concentrations: 25, 50, 75, and 100 mM at 40 °C.

seen that the calculated distributions give the peak positions in dΣ/dΩ with a good correspondence with the experimentally determined points. As the concentration of the surfactant with h ) 1 is decreased, it is found that the peak in the measured distribution shifts to lower Q values as the distance between the micelles increases. The micellar shape changes from a/b ) 2.40 to a/b ) 4.37 upon increase in concentration of h ) 1 from 25 mM to 100 mM. (Table 2). The aggregation number (N) is increased from 215 to 390. The fractional charge (R) also decreased from 0.09 to 0.06 upon increase in concentration. Interestingly however, with the surfactants having double headgroups (h ) 2) and triple headgroups (h ) 3), no significant changes in the micellar parameters were observed with the increase in concentration (Figures 3 and 4). In particular, it is seen that micelles in these solutions have almost the same aggregation number (N), fractional charge (R), and a/b ratio (Table 2). This suggests that micellar shape does not change significantly with increases in concentration. This is understandable because larger headgroup sizes and enhanced electrostatic repulsion, as in the cases with h ) 2 and h ) 3, will require

a

Volume of the surfactant and semiminor axis b for individual surfactants are kept fixed in the fitting procedures.

more room and would only be able to accommodate a smaller number of surfactant molecules to pack into a single micelle. To form the larger micelles, the aggregation number has to go up with every increase in concentration. This in turn would need a greater degree of folding of the long-chain hydrocarbon, which might not be thermodynamically favored. Effect of Temperature Variation. Figure 5 shows the variation of neutron cross sections for micellar solutions of the surfactant with h ) 1 at different temperature from 40 to 70 °C and at 50 mM concentration. The neutron cross section decreases, and the peak shifts to the higher Q values as the temperature is increased. Table 3 records the information about the micelles from the surfactant with h ) 1 as a function of temperature. The degree of ionization, which plays an important role for modification of the magnitude of electrostatic repulsion at the level of headgroups, increases with the increase in temperature. This leads to the increase in the effective headgroup area of the surfactant in the micelle and results in a decrease in the aggregation number (N) from 244 to 124. The effective fractional charge per monomer, however, appears to increase from 0.08 to 0.20 with an increase in temperature. Because a smaller fractional charge indicates a more ellipsoidal morphology, increasing temperature appears to induce an ellipsoid-tosphere-like transition by changing the a/b value from 2.73 to 1.38. In the case of double-headed (h ) 2) and triple-headed (h ) 3) surfactants (Figures 6 and 7), the micellar morphology also changes with temperature (Table 3). But the effect is not as pronounced as that of h ) 1.With the increase in temperature, the aggregation number (N) decreases from 48 to 35 for h ) 2, and for h ) 3, it changes from 20 to 15. The fractional charge also increases from 0.23 to 0.42 and from 0.27 to 0.32 for the surfactants h ) 2 and h ) 3, respectively. This indicates that

12808 J. Phys. Chem. B, Vol. 105, No. 51, 2001

Haldar et al. Acknowledgment. This work was supported by a grant from the Inter University Consortium. References and Notes

Figure 6. SANS distributions from a 50 mM micellar solution of double-headed surfactant (h ) 2) at various temperatures: 40, 50, 60, and 70 °C.

Figure 7. SANS distributions from a 50 mM micellar solution of tripleheaded surfactant (h ) 3) at various temperatures: 40, 55, and 70 °C.

micelles become more spherical with the increase in temperature. This is also supported by the decrease in a/b values upon increase in temperature. Conclusions Micelle formation from three new surfactants that vary in terms of the number of headgroups was investigated The critical micellar concentrations increase with the increase in the number of headgroups. However, the rise in cmc was modest when one compares the cmc of the surfactant with h ) 2 with that of h ) 3. From the measurements of small-angle-neutron-scattering cross sections, it is shown that the micellar morphology changes with the increase in the number of surfactant headgroups. SANS analysis further indicates that the micelles become progressively smaller in size as the number of headgroups in the surfactant increases. To fill the micellar volume, the hydrocarbon chains fold increasingly with the increase in the number of headgroups. It is also noticed that aggregation number (N) is dramatically decreased and the fractional charge (R) is increased with the rise in the number of headgroups. For the single-headed surfactant, the effects of concentration and temperature variation are more pronounced than those of its double- and triple-headed counterparts. Thus, the properties of the surfactant micelles are significantly influenced by the number of charges at the headgroup.

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