Tuning the Structure of SDS Micelles by Substituted Anilinium Ions

Jan 5, 2005 - DLS studies indicate a progressive growth of SDS micelles with addition of .... m-toluidine were obtained from Sisco Research Labs, Mumb...
0 downloads 0 Views 123KB Size
Download PDF
1340

J. Phys. Chem. B 2005, 109, 1340-1346

ARTICLES Tuning the Structure of SDS Micelles by Substituted Anilinium Ions Gunjan Garg,† P. A. Hassan,*,† V. K. Aswal,‡ and S. K. Kulshreshtha† NoVel Materials and Structural Chemistry and Solid State Physics DiVisions, Bhabha Atomic Research Centre, Mumbai - 400 085, India ReceiVed: June 23, 2004; In Final Form: NoVember 2, 2004

The growth behavior of aggregates formed in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the cationic hydrophobic salts o-toluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) has been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS studies indicate a progressive growth of SDS micelles with addition of less than equimolar concentrations of hydrophobic salts. A prolate ellipsoidal model is used to analyze the DLS data, which is further supported by SANS measurements. We explain the propensity for the strong growth of micelles in the presence of OTHC and MTHC by the high charge neutralization provided by these salts as the aromatic counterions are adsorbed on the surface of the micelles. When the substitution is at the meta position, i.e., for MTHC, micellar growth is favored at lower salt concentrations than for OTHC. The variation in growth behavior is explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. Micellar parameters obtained from SANS data at elevated temperature also support enhanced growth of micelles in the presence of MTHC as compared to OTHC.

Introduction The micellization of amphiphilic molecules has been a topic of considerable interest for many years. The aggregates formed through the micellization of amphiphiles can be of various shapes and sizes such as spherical or ellipsoidal, cylindrical or threadlike, and disklike micelles; bilayers; vesicles; and so on.1-9 The geometry of the aggregates formed by the amphiphilic molecules is a result of a delicate balance of two opposing forces. The attractive tail-tail hydrophobic interaction provides the driving force for the aggregation of surfactant molecules, while the electrostatic repulsion between headgroups limits the size that a micelle can attain. As a result, the characteristics of these aggregates can be easily controlled by changes in the solution conditions such as temperature, concentration, and ionic strength. Charged spherical micelles can grow into elongated rodlike structures by the addition of electrolytes that screen the electrostatic repulsion. Addition of electrolytes can change the ionic strength of the medium, thereby changing the range of repulsive interactions.10-13 This decreases the effective repulsion between headgroups, thus decreasing the average headgroup area per molecule at the surface of the micelles. A decreased headgroup area favors the formation of rodlike micelles consistent with the geometrical packing model of Israelachvili.14 Organic salts that contain a hydrophobic part and have a charge opposite to that of the surfactant molecule are highly efficient in promoting a structural transition in ionic micelles. Sodium salts of organic acids such as salicylates,15,16 p-toluene * Corresponding author. Tel.: + 91-22 25592327. Fax: + 91-22 25505151. E-mail: [email protected]. † Novel Materials and Structural Chemistry Division. ‡ Solid State Physics Division.

sulfonates,17 chlorobenzoates,18 hydroxy naphthalene carboxylates,19-21 and alkyl sulfates22 can change the structure of cationic micelles at molar concentrations less than that of the surfactant. It is now recognized that, with the addition of aromatic counterions, these ionic globular micelles undergo uniaxial growth to form threadlike micelles and exhibit nonNewtonian flow characteristics analogous to those of semidilute polymer solution.23 Direct evidence for the presence of long thread like micelles in some systems has been obtained from cryo-TEM observations.24 Such formation of polymer-like micelles is not limited to aqueous surfactant solutions. Reverse micelles, formed in nonpolar solvents by surfactants such as lecithin, are also polymer-like in nature.25 Recently, we demonstrated a sphere-to-rodlike transition in anionic sodium dodecyl sulfate (SDS) micelles induced by the addition of the aromatic salt p-toluidine hydrochloride (PTHC).26,27 Dynamic light scattering (DLS) studies on SDS micelles in the presence of aniline hydrochloride (AHC) also revealed a similar transition.28 However, the manifestation of uniaxial growth of the micelles with the addition of AHC to 50 mM SDS was observed at a higher concentration of salt than with the parasubstituted analogue, PTHC. A similar difference in growth behavior in cationic micelles with the changing nature and position of substitution in aromatic counterions has also been reported. For example, 4-chlorobenzoate and 3,4 or 3,5 dihlorobenzoate anions induce viscoeleasticity in cetyltrimethylammonium (CTA+) micelles, whereas 2-chlorobenzoate and 2,6dichlorobenzoate do not show similar effect.29 This difference was attributed to the difference in the intercalation and penetration of the aromatic counterions at the interface of ionic micelles. The size, shape, and flexibility of elongated micelles are significantly influenced by the pattern of substitution on aromatic

10.1021/jp0472663 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/05/2005

Tuning the Structure of SDS Micelles

J. Phys. Chem. B, Vol. 109, No. 4, 2005 1341 dΩ) as a function of scattering vector q. For a system of charged, monodisperse interacting particles dΣ/dΩ can be written as32

dΣ ) n(Fm - Fs)2V2{〈F(q)2〉 + 〈F(q)〉2[S(q) - 1]} + B (1) dΩ Figure 1. Chemical structures of different aromatic salts.

counterions. Knowledge of the effect of positional substitution on the aromatic counterions in inducing sphere-to-rod transitions in anionic micelles is scarce and requires further experimental efforts. We present here DLS and small-angle neutron scattering (SANS) data on SDS-water system in the presence of otoluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) and compare the results with those obtained for AHC and PTHC. The change in the microstructure of the micelles upon addition of different cationic hydrophobic salts in terms of its shape and size was investigated.

where n denotes the number density of micelles; Fm and Fs are the scattering length densities of the micelle and the solvent, respectively; and V is the volume of the micelle. F(q) is the single-particle form factor, and S(q) is the interparticle structure factor. B is a constant term that represents the incoherent scattering background. The single-particle form factor was calculated by treating the micelles as prolate ellipsoids32

〈F(q)2〉 )

(A) Chemicals. SDS (electrophoresis grade), o-toluidine, and m-toluidine were obtained from Sisco Research Labs, Mumbai, India. AHC was obtained from Fluka. To prepare the salts OTHC and MTHC, an ether solution of the corresponding amines was extracted with an equimolar amount of dilute HCl and crystallized by slow evaporation of the solvent. The chemical structures of the salts used are given in Figure 1. All chemicals were used as received. Deionized water from a Millipore MilliQ system (resistivity ≈ 18 MΩ cm) was used in all cases to prepare aqueous solutions. For SANS measurements, samples were prepared in D2O (99.4 atom % D purity). (B) DLS. Dynamic light scattering (DLS) measurements were performed using a Malvern 4800 Autosizer employing a 7132 digital correlator. The light source was an Ar ion laser operated at 514.5 nm with a maximum power output of 2 W. The correlation functions were analyzed by the method of cumulants.30 Measurements were made at five different angles ranging from 50° to 130°. The samples of micellar solutions were filtered through 0.2-µm filters (Acrodisc) to avoid interference from dust particles. DLS measurements were carried out on SDS solutions in the presence of different concentration of OTHC and MTHC. The concentration of surfactant was kept constant (50 mM), and the salt-to-surfactant concentration ratio (xsalt ) [salt]/[surfactant]) was varied from 0.3 to 0.8. (C) SANS. Small-angle neutron scattering experiments were carried out using the SANS diffractometer at the Dhruva Reactor, Bhabha Atomic Research Centre, Trombay, India.31 The diffractometer makes use of a beryllium oxide filtered beam of mean wavelength (λ) 5.2 Å. The angular distribution of the scattered neutrons was recorded using a one-dimensional position-sensitive detector (PSD). The accessible wave vector transfer [q ) (4π sin θ)/λ, where 2θ is the scattering angle] range of the diffractometer is 0.02-0.3 Å-1. The PSD allows simultaneous recording of data over the full q range. The samples were held in a quartz sample holder of 0.5-cm thickness. In all measurements, the temperature was kept fixed at 30 °C. The measurements were also made for 30 mM salt samples at 50 °C. The measured SANS data were corrected and normalized to a cross-sectional unit, using standard procedures. (D) SANS Analysis. SANS studies were carried out on a micellar solution of SDS (50 mM) in the presence of 10, 20, and 30 mM AHC, OTHC, and MTHC. In SANS one measures the differential scattering cross section per unit volume (dΣ/

(2)

∫01F(q,µ) dµ]2

(3)

〈F(q)〉2 ) [ F(q,µ) )

Experimental Section

∫01[F(q,µ)2 dµ]

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 axes of the ellipsoidal micelles and µ is the cosine of the angle between the directions of a and the wave vector transfer q. The interparticle structure factor, 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 centers of mass of the micelles. Unlike the calculation of F(q), it is quiet complicated to calculate S(q) for any shapes other than spherical. This is because S(q) depends on the shape as well as the orientation of the particles. To simplify this calculation, prolate ellipsoidal micelles are assumed to be equivalent spheres. We calculated S(q) as derived by Hayter and Penfold33 using the Ornstein-Zernike equation and the rescaled mean spherical approximation.34 The micelles were assumed to be rigid equivalent spheres of diameter σ ) 2(ab2)1/3 interacting through a screened Coulomb potential. It is to be noted that calculations using an “included sphere” considering the semimajor axis as the radius of the sphere in comparison will lead to a higher value of fractional charge for the micelles than that obtained with the equivalent sphere radius. This is evident from the fact that the surface potential decreases with increasing diameter of the effective sphere. Although micelles can produce polydisperse systems, to simplify calculations and limit the number of unknown parameters in the fit, we assumed them to be monodisperse ellipsoids.35,36 The dimensions of the micelle, aggregation number and fractional charge were determined from the analysis. The semimajor axis (a) and the fractional charge (R) are the parameters used for analyzing the SANS data. The aggregation number was calculated by the relation N ) 4πab2/3V, where V is the volume of the surfactant monomer. The surfactant monomer volume was estimated using the Tanford formula.37 With the addition of a hydrophobic salt, a change in the volume fraction of the micelle is expected as the salt is solubilized into the micelle surface. The volume occupied by the aromatic salt (215 Å3 for AHC and 238 Å3 for OTHC and MTHC) is included in the volume fraction of the micelles, assuming that all of the salt molecules are solubilized in the micelles. The volume of the surfactant molecules was also calculated from density measurements made with a pycnometer. The volume of the surfactant molecule excluding counterions and hydrophilic

1342 J. Phys. Chem. B, Vol. 109, No. 4, 2005

Garg et al.

Figure 2. Representative plot of the intensity correlation function for 50 mM SDS with xOTHC ) 0.5 at a scattering angle of 90°. The solid line is a fit to the data using the method of cumulants.

oxygen was found to be 352 Å3, which is similar to the value obtained from Tanford formula (350 Å3). Because of the penetration of water molecules into the headgroup region of the micelles, the scattering length density of the headgroup region is not significantly different from that of solvent. Thus, the semiminor axis (b) of the ellipsoid is taken as the length of the hydrocarbon chain (16.7 Å). The scattering profiles as calculated by eq 1 were smeared by the appropriate resolution function for comparison with the measured data.31 The parameters were optimized by means of a nonlinear least-squares fitting program, and the errors were calculated by standard methods.38 The scaling factors give the scaling of the experimental data with the calculate values (Tables 1-3).

Figure 3. Angular variation of the average decay rates (Γ) of the electric field correlation function as a function of q2 for 50 mM SDS at different xOTHC and xMTHC values.

Results and Discussion DLS Results. A representative plot of the intensity correlation function for 50 mM SDS with xOTHC ) 0.5 and at a scattering angle of 90° is shown in Figure 2. Measurements of the autocorrelation function were not made for xsalt < 0.3 because of the very low scattering intensity of ionic SDS micelles. The solid line in Figure 2 represents the theoretical fit to the experimental data by the method of cumulants with a relaxation rate (Γ) of 0.03 µs-1 and a polydispersity index [variance/ (mean)2] of 0.34, and it reveals a reasonably good fit with this model. Analysis using the constraint regularization method CONTIN39,40 also revealed a unimodal distribution of relaxation rate, supporting the validity of cumulant results. Thus, the average Γ value obtained from cumulant analysis was used to calculate the diffusion coefficient of the micelles. The angular variation of the average decay rates (Γ) of the electric field correlation functions obtained from the fits at different concentrations of OTHC and MTHC is shown in Figure 3 as a function of q2. For all of the samples, the data fall on a linear plot passing through the origin indicating translational diffusion of the scatterers and negligible contribution from rotational diffusion, if any anisotropic structure exist. This is expected for conventional surfactant micelles because of their very small dimension as compared to the wavelength of the light. A progressive decrease in the slope of the Γ vs q2 plots with increasing concentration of salt was observed, which suggests a decrease in the apparent diffusion coefficient of the micelles. The variation of apparent diffusion coefficient, Da, of SDS micelles (50 mM) at different values of xOTHC and xMTHC is

Figure 4. Variation of the apparent diffusion coefficient, Da (obtained from the slope of the plots of Γ vs q2), at different values of xOTHC and xMTHC for 50 mM SDS.

depicted in Figure 4. For xOTHC ) 0.3, Da and the corresponding hydrodynamic diameter, calculated using the Stokes-Einstein relation, are in the range expected for small globular micelles. As the salt concentration increases, Da decreases steadily, reflecting an increase in the average dimension of the micelles. The decrease in Da of the micelles is more pronounced when MTHC is used as the salt instead of OTHC. On comparing these data with those from the earlier-reported DLS studies on SDS micelles in the presence of AHC28 and PTHC,26 it is observed that, for a given salt concentration, the average size of SDS micelles in the presence of OTHC is similar to that in the presence of AHC. On the other hand, in the presence of MTHC, the micellar size was found to be in the same range as in the presence of PTHC. The difference in the growth behaviors of SDS micelles in the presence of these salts suggests that the presence of methyl groups at different positions in the benzene ring of aromatic salts plays an important role in inducing the growth of the micelles. The observed growth behavior of SDS micelles with differences in the position of substitution on the aromatic counterion can be explained in terms of the orientation of the counterions

Tuning the Structure of SDS Micelles on the micellar surface. Aromatic counterions are known to adsorb on the surface of ionic micelles through electrostatic and hydrophobic forces. Several groups have used proton NMR spectroscopy to appraise the orientational details for substituted benzoate counterions bound to CTA+ micellar surfaces; whether the substituents are hydroxy,41 chloro,42,43 or methyl,44 substantial penetration of the micellar interface was inferred for all of the positional isomers. Recently, NMR studies revealed that PTHC resides on the surface of the SDS micelles in such a way that the NH2 group and the ortho protons experience a hydrophilic environment whereas the meta and para protons experience hydrophobic environment.26 Because the ortho group is directed toward the hydrophilic part, the presence of a methyl group at ortho position of aniline hydrochloride does not alter the growth behavior of SDS micelles as compared to AHC. However, a substitution at the meta position of aniline hydrochloride influences the growth behavior significantly due to the fact that the substituent is directed toward the hydrophobic core of the micelles, thereby increasing its hydrophobic contribution toward adsorption on the micelle surface. Such adsorption of the counterions decreases the surface charge of the micelles because of the oppositely charged nature of the counterions. A decrease in the surface charge density of the micelles decreases the effective repulsion of the headgroups of the surfactants, which, in turn, reduces the effective headgroup area per surfactant. At high enough salt concentration, xsalt ) 0.8, the Da value of the SDS micelles decreases by 1 order of magnitude. Such a large difference in Da cannot be explained for spherical micelles, as any increase in radius beyond the length of the surfactant chain leads to the energetically unfavorable formation of a vacuum or water pool at the core of the micelle. According to Tanford’s formula, the maximum extended length of a dodecyl chain is 16.7 Å,37 so the observed equivalent sphere diameter of the micelles suggests anisotropic growth of the micelles as prolate or oblate ellipsoids upon addition of salt. On the basis of geometrical packing considerations, Israelachvili et al.14 suggested that a structural transition in micelles could be envisioned with an increase in the dimensionless packing parameter, V/aol (where V is the volume of the surfactant monomer, ao is the area of the headgroup, and l is the length of the alkyl chain). A lower headgroup area increases the packing parameter of the micelles and this would induce a transition from a spherical to a rodlike structure. Further, the extra endcap energy of rodlike micelles due to the difference in curvature between the cylindrical body of the micelle and the hemispherical end caps leads to a rapid growth of the micelles.45 On the basis of viscosity and SANS measurements, it has been reported that the addition of PTHC to SDS micelles induces uniaxial growth of the micelles as prolate ellipsoids.26,27 In the case of cationic surfactants, there are also reports showing an evolution of micelles from sphere to prolate ellipsoids in the presence of aromatic counterions.45,46 Thus, the DLS data at high salt concentrations were analyzed in terms of a prolate ellipsoidal structure. The lengths of the micelles were estimated using Perrin’s formula,47 which relates the average diffusion coefficient to the minor and major axes of the ellipsoid. A correction for the measured Da value for possible hydrodynamic and thermodynamic contributions was applied during the data analysis as reported earlier.26 The semiminor axis of the ellipsoid was taken as constant at 16.7 Å, and only the semimajor axis was varied. The average length L of the micelle as obtained from the major axis of the ellipsoid is depicted in Figure 5. The results are also compared with those obtained from SANS

J. Phys. Chem. B, Vol. 109, No. 4, 2005 1343

Figure 5. Variation of the length of SDS micelles calculated by DLS and SANS as a function of varying concentrations of OTHC and MTHC.

Figure 6. SANS data from 50 mM SDS micellar solution with varying concentrations of the salts AHC, OTHC, and MTHC at 30 °C.

data (open triangles), which will be discussed further. A marked increase is observed in the length of the micelles upon the addition of salt. The length of the micelles in the presence of MTHC was found to be higher than that in the presence of OTHC. The above results are further supported by SANS measurements.

1344 J. Phys. Chem. B, Vol. 109, No. 4, 2005

Garg et al.

TABLE 1: Micellar Parametersa from 50 mM SDS with Varying Concentrations of the Salt AHC xAHC

temp (°C)

Nagg

Rb

a (Å)

b (Å)

ao (Å2)

V/aol

scaling factor

0 0.2 0.4 0.6 0.6

30 30 30 30 50

75 ( 18 97 ( 7 147 ( 8 230 ( 19 152 ( 12

0.21 (0.24) 0.14 (0.21) 0.08 (0.17) -

22.5 ( 5.5 32.5 ( 2.5 55 ( 3 94 ( 8 62 ( 5

16.7 16.7 16.7 16.7 16.7

57.8 53.9 51.2 50.0 50.8

0.36 0.39 0.41 0.42 0.41

1.13 1.00 1.02 1.00 0.95

a Nagg ) aggregation number, R ) fractional charge, a ) semi-major axis, b ) semiminor axis, ao ) effective headgroup area/surfactant, V/aol ) packing parameter. b Values in parentheses indicates R calculated using the dressed micelle model as described in ref 49.

TABLE 2: Micellar Parametersa from 50 mM SDS with Varying Concentrations of the Salt OTHC xOTHC

temp (°C)

Nagg

Rb

a (Å)

b (Å)

ao (Å2)

V/aol

scaling factor

0 0.2 0.4 0.6 0.6

30 30 30 30 50

75 ( 18 95 ( 6 144 ( 10 235 ( 33 163 ( 12

0.21 (0.24) 0.14 (0.21) 0.09 (0.17) -

22.5 ( 5.5 32.5 ( 2.0 55 ( 4 99 ( 14 69 ( 5

16.7 16.7 16.7 16.7 16.7

57.8 53.9 51.2 50.0 50.6

0.36 0.39 0.41 0.42 0.41

1.13 0.97 0.98 0.98 0.93

a Nagg ) aggregation number, R ) fractional charge, a ) semi-major axis, b ) semiminor axis, ao ) effective headgroup area/surfactant, V/aol ) packing parameter. b Values in parentheses indicates R calculated using the dressed micelle model as described in ref 49.

TABLE 3: Micellar Parametersa from 50 mM SDS with Varying Concentrations of the Salt MTHC xMTHC

temp (°C)

Nagg

Rb

a (Å)

b (Å)

ao (Å2)

V/aol

scaling factor

0 0.2 0.4 0.6 0.6

30 30 30 30 50

75 ( 5.5 103 ( 9 197 ( 16 256 ( 28 234 ( 31

0.21 (0.24) 0.13 (0.20) -

22.5 ( 5.5 35 ( 3 75 ( 6 108 ( 12 98 ( 13

16.7 16.7 16.7 16.7 16.7

57.8 53.4 49.7 49.9 50.0

0.36 0.39 0.42 0.42 0.42

1.13 1.03 1.04 1.02 0.93

a Nagg ) aggregation number, R ) fractional charge, a ) semi-major axis, b ) semiminor axis, ao ) effective headgroup area/surfactant, V/aol ) packing parameter. b Values in parentheses indicates R calculated using the dressed micelle model as described in ref 49.

SANS Results. The evolution of the SANS spectra for 50 mM SDS solutions at room temperature in the presence of different concentrations of the salts AHC, OTHC, and MTHC is shown in Figure 6. The solid lines in Figure 6 represent the calculated scattering patterns obtained by assuming prolate ellipsoidal micelles. In the absence of any salt, the SANS spectrum shows the characteristic correlation peak indicating the presence of repulsive intermicellar interactions between the negatively charged SDS micelles. The correlation peak usually occurs at qmax ≈ 2π/d, where d is the average distance between the micelles and qmax is the value of q at the peak position.48 Upon addition of salt, this correlation peak broadens and shifts to lower q values. The broadening of the correlation peak at constant volume fraction of the micelles is an indication of a decrease in the range of electrostatic interactions. This can be attributed to the decrease in the Debye screening length due to an increase in the ionic strength of the medium and a decrease in the surface charge of the micelles due to the adsorption of the hydrophobic counterions on micellar surface. The shift in qmax toward lower q values upon addition of salt suggests an increase in the value of d. This observation together with the merging of the SANS spectra in the high-q region for all salt concentrations suggests that the micelles grow uniaxially upon the addition of the salt. The results of quantitative analysis of the data in the presence of different salts, i.e., AHC, OTHC, and MTHC, are summarized in Tables 1-3, respectively. The parameters observed for SDS micelles in the absence of any added salt are in good agreement with those reported earlier.2,11,12 The micelle aggregation number and the semimajor axis were found to increase upon addition of the salt, whereas the fractional charge on the micelles decreased. The calculated SANS spectra are sensitive to the surface charge of the micelles only when a correlation peak is present. Thus, at higher salt concentrations, when no correlation peak was present, no attempts were made to estimate the fractional charge on the micelles. In such cases, the surface charge was fixed as close

to zero, and the semimajor axis was used as the fitting parameter. The effective headgroup area per surfactant molecule (ao) was found to decrease with increasing concentration of different hydrophobic salts. This is due to the screening of the repulsive charge at the surface of the micelles, which, in turn, reduces the effective surface area per surfactant molecule of the micelles. Because of the decrease in the headgroup area of the surfactant molecule, the packing parameter increases with increasing concentration of salt and induces a transition from spherical micelles to a rodlike structure. We also calculated the fractional charge using the dressed model of micelles developed by Hayter.49 This is an analytical form for the degree of micellar ionization from a self-consistent solution of the original dressed micelle theory of Evans, Mitchell, and Ninham.50 As per this model, the degree of ionization (R) can be computed with the expression

R)1-

[

]

4 [(1 + ω2)1/2 - (1 + xo2)1/2] xos

(6)

where xo ) κR; κ-1 is the Debye screening length; R is the equivalent sphere radius of the micelle; and s is the surface charge density, given by

s)

e2 oκaokBT

(7)

where o is the permittivity of free space,  is the dielectric constant of the medium, ao is the area per surfactant headgroup, kB is the Boltzmann constant, e is the electronic charge, and T is the absolute temperature. The parameter ω in eq 6 is given by ω ) xo(z + 1)/2, z being obtained by an iterative solution of the equation

(4z2 - u)2(z + 1) - t2(z - 1) ) 0 where u ) s2 +4 and t ) 8s/xo.

(8)

Tuning the Structure of SDS Micelles

Figure 7. SANS data from a 50 mM SDS micellar solution in the presence of 30 mM AHC, OTHC, and MTHC at 50 °C.

The fractional charge of ionic micelles calculated by considering the excess adsorption of ions about the micelles was found to agree reasonably well with experimental values measured by SANS. The fractional charge (R) obtained from the SANS analysis using a screened coulomb potential was compared with the theoretical R computed from the dressed micelle model, and the results are reported in parentheses in Tables 1-3. For pure SDS micelles, the values of R obtained from SANS and calculated by theory are in good agreement. However, in the presence of hydrophobic salts, the fractional charges obtained with the dressed model is much larger than those obtained by SANS. This marked difference could be due to the hydrophobic nature of the counterions, which is not taken into account in the dressed micelle model. Recent reports show that nonelectrostatic forces such as dispersion forces also play a role in explaining the specific binding of counterions in the micelles51 and counterion condensation on polyelectrolytes.52 It should be noted that dispersion forces are not the only ones that are important, but others such as hydration, structural, oscillatory, hydrophobic, and depletion forces are also prevalent in the literature. Ninham has discussed the importance of various non-DLVO forces in the present theories of colloidal interaction.53 We do not include this non-DLVO forces here to limit the number of free parameters and complexity in SANS analysis. The comparative study of the micellar parameters (Tables 1-3) shows that, in the presence of small concentrations of AHC, OTHC, and MTHC (xsalt ) 0.2), the dimensions of the SDS micelles are comparable. With increasing salt concentration (xsalt ) 0.4), the micellar growth is faster in the presence of MTHC as compared to AHC and OTHC. This is consistent with the results obtained from DLS measurements as discussed earlier. However, upon further addition of the salt (xsalt ) 0.6), no significant difference in the micellar length could be observed, which might be due to the limit of the instrument at high length scales. To verify this aspect, experiments were carried out at higher temperature (50 °C) where the micelle length decreases and becomes amenable to SANS measurements. SANS spectra on micellar solutions of SDS in the presence of AHC, OTHC, and MTHC (xsalt ) 0.6) at elevated temperature (i.e., 50 °C) are depicted in Figure 7. With increasing temperature, the dissociation of counterions increases, which results in a stronger repulsion between the charged headgroups. As a result, the formation of smaller micelles takes place, which is reflected by a decrease in the absolute cross section. A quantitative estimate of the micellar parameters (Tables 1-3) suggests that the aggregation number and semi-

J. Phys. Chem. B, Vol. 109, No. 4, 2005 1345 major axis decrease with increasing temperature. The effective headgroup area per surfactant molecule increases with increasing temperature because of the greater micelle ionization and hence stronger repulsion between the charged headgroups, as a result of which the packing parameter decreases. The parameters obtained from the fit reveal that the growth of the micelles is more significant in the presence of MTHC as compared to OTHC. The lengths of the prolate ellipsoidal micelles in the presence of OTHC and MTHC obtained from DLS and SANS measurements are compared in Figure 5. The results obtained from SANS measurements are in accordance with the data obtained from DLS studies. The smaller length of the micelles obtained from SANS measurements as compared to DLS at xsalt ) 0.6, probably arises from the poor sensitivity to the data at length scales closer to the limit of SANS instrument. At low salt concentrations, the micellar dimensions are in the range accessible from SANS measurements. As the micellar length increases, its characteristic feature shifts to lower q values and finally moves out of the experimentally accessible q range. This restricts our SANS measurements to salt concentrations below xsalt ) 0.6. On the other hand, estimations of the micellar length from DLS data could not be carried out at lower concentrations because of the small aspect ratio of SDS micelles. Conclusions A comparative study of the growth behavior of SDS micelles in the presence of AHC, OTHC, and MTHC has been carried out using dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS and SANS results indicate that the micelles exhibit linear growth resulting in the formation of prolate ellipsoidal micelles. The strong growth of SDS micelles in the presence of AHC, OTHC and MTHC salts occurs because of the adsorption of aromatic counterions on the surface of the micelles. The growth of the micelles is much more pronounced with the addition of MTHC as compared to AHC and OTHC. This difference in the growth behavior can be explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. The present study offers a strategy to tune the size and shape of the SDS micelles by changing the nature and concentration of organic counterions. Acknowledgment. The authors are thankful to Dr. C. Manohar of Indian Institute of Technology, Mumbai, India, for many fruitful discussions. P.A.H. is thankful to Prof. E. W. Kaler of University of Delware for initiating a research program on hydrotrope induced growth of anionic micelles. G.G. gratefully acknowledges Board of Research in Nuclear Science, DAE, India for the award of a Dr. K. S. Krishnan Research Associateship. References and Notes (1) Degiorgio, V.; Corti, M. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; North-Holland: Amsterdam, 1985. (2) Chen, S. H. Annu. ReV. Phy. Chem. 1986, 37, 351. (3) Zana, R. Surfactant Solutions: New Methods of InVestigation; Marcel Dekker: New York, 1987. (4) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279. (5) Zemb, T.; Dubois, M.; Deme, B.; Gulikkrzywicki, T. Science 1999, 283, 816. (6) Dubois, M.; Deme, B.; Gulikkrzywicki, T.; Dedieu, J. C.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672. (7) Magid, L. J.; Han, Z.; Li, Z.; Butler, P. D. Langmuir 2000, 16, 149. (8) Magid, L. J. J. Phys. Chem. B 1998, 102, 4064.

1346 J. Phys. Chem. B, Vol. 109, No. 4, 2005 (9) Lu, K.; Jacob, J.; Thiyagarajan, P.; Conticello, V. P.; Lynn, D. G. J. Am. Chem. Soc. 2003, 125, 6391. (10) Aswal V. K.; Goyal P. S.; Thiyagarajan, P. J. Phys. Chem. B 1998, 102, 2469. (11) Aswal, V. K.; Goyal, P. S. Phys. ReV. E 2000, 61, 2947. (12) Magid, L. J.; Li, Z.; Butler, P. D. Langmuir 2000, 16, 10028 and references therein. (13) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5435. (14) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525. (15) Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92, 4712. (16) Ait Ali, A.; Makhloufi, R. Phys. ReV. E 1997, 56, 4474. (17) Soltero, J. F. A.; Puig, J. E. Langmuir 1996, 12, 2654. (18) Magid, L. J. Colloids Surf. 1986, 19, 129. (19) Brown, W.; Johansson, K.; Almgren, M. J. Phys. Chem. 1989, 93, 5888. (20) Hassan, P. A.; Valaulikar, B. S.; Manohar, C.; Kern, F.; Bourdieu, L.; Candau, S. Langmuir 1996, 12, 4350. (21) Hassan, P. A.; Candau, S. J.; Kern, F.; Manohar, C. Langmuir 1998, 14, 6025. (22) Oda, R.; Narayanan, J.; Hassan, P. A.; Manohar, C.; Salkar, R. A.; Kern, F.; Candau, S. J. Langmuir 1998, 14, 4364. (23) Clausen, T. M.; Vinson, P. K.; Minter, J, R.; Davis, H. T.; Talhom, Y.; Miller, W. C. J. Phys. Chem. 1992, 96, 474. (24) Magid, L. J.; Gee, J. C.; Talmon, Y. Langmuir 1990, 6, 1609. (25) Schurtenberger, P.; Scartazzini, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3695. Schurtenberger, P.; Cavaco, C. Langmuir 1994, 10, 100. (26) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (27) Hassan, P. A.; Gerhard, F.; Kaler, E. W. J. Colloid Interface Sci. 2003, 257, 154. (28) Hassan, P. A.; Sawant, S. N.; Bagkar, N. C.; Yakhmi, J. V. Langmuir 2004, 20, 4874. (29) Carver, M.; Smith, T. L.; Gee, J. C.; Delichere, A.; Caponetti, E.; Magid, L. J. Langmuir 1996, 12, 691. (30) Brown, J. C.; Pusey, P. N.; Dietz, R. J. Chem. Phys. 1975, 62, 1136.

Garg et al. (31) Aswal, V. K.; Goyal, P. S. Curr. Sci. 2001, 79, 947. (32) (a) Hayter, J. B.; Penfold, P. Colloid Polym. Sci. 1983, 261, 1022. (b) Sheu, E. Y.; Wu, C. F.; Chen, S. H. J. Phys. Chem. 1986, 90, 4179. (33) Hayter, J. B.; Penfold, P. Mol. Phys. 1981, 42, 109. (34) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (35) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. Phys. ReV. E 1998, 57, 776. (36) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. Phys. ReV. E 1999, 59, 3116. (37) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980. (38) Bevington, P. R. Data Reduction and Error Analysis for Physical Sciences; McGraw-Hill: New York, 1969. (39) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (40) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229. (41) (a) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 5, 379. (b) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286. (c) Valaulikar, B. S.; Mishra, B. K.; Bhagwat, S. S.; Manohar, C. J. Colloid Interface Sci. 1991, 144, 304. (42) Smith, B. C.; Chou, L. C.; Zakin, J. L. J. Rheol. 1994, 38, 73. (43) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699. (44) Bachofer, S. J.; Turbitt, R. M. J. Colloid Interface Sci. 1990, 135, 325. (45) Cates, M. E.; Candau, S. J. J. Phys. Condens. Matter 1990, 2, 6869. (46) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (47) Berne, B. J.; Pecora, R. Dynamic Light Scattering: With Application to Chemistry Biology and Physics; J. Wiley and Sons: New York, 1976. (48) Chen, S. H.; Sheu, E. Y.; Kalus, J.; Hoffmann, H. J. Appl. Crystallogr. 1988, 21, 751. (49) Hayter, J. B. Langmuir 1992, 8, 2873. (50) Evans, D. F., Mitchell, D. J., Ninham, B. W. J. Phys. Chem. 1984, 88, 6344. (51) Bostrom, M., Williams, D. R. M., Ninham, B. W. Langmuir 2002, 18, 6010. (52) Bostrom, M., Williams, D. R. M., Ninham, B. W. J. Phys. Chem. B 2002, 106, 7908. (53) Ninham, B. W. AdV. Colloid Interface Sci. 1999, 83, 1.