Influence of Headgroup on the Aggregation and Interactional Behavior

Aug 26, 2013 - Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India. ‡. Solid State Physics Division, Bhabha Atomic Research C...
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Influence of Headgroup on the Aggregation and Interactional Behavior of Twin-Tailed Cationic Surfactants with Pluronics Rajwinder Kaur,† Sugam Kumar,‡ Vinod K. Aswal,‡ and Rakesh Kumar Mahajan*,† †

Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

‡

S Supporting Information *

ABSTRACT: The surface tension measurements have been employed to characterize the micellar and interfacial behavior of pure and mixed systems of twin-tailed cationic surfactants: dimethylene bis(decyldimethylammonium bromide) (10−2− 10), didecydimethylammonium bromide (DDAB), and 1,3didecyl-2-methylimidazolium chloride (DDIC) with pluronics P84 and F108 in the aqueous solution. The interactions of each surfactant with both pluronics are found to be nonideal and synergistic except for the mixed system of 10−2−10 + F108, for which interactions are antagonistic and every interaction has been studied on the basis of headgroup disparity. Dynamic light scattering (DLS), zeta (ζ) potential, and small angle neutron scattering (SANS) measurements have been used to determine the influence of the mixing ratio on the morphology of the various mixed aggregates that are formed. Pure DDAB is found to form unilamellar vesicles whereas pure 10− 2−10 and DDIC form prolate ellipsoidal micelles. The unilamellar vesicles of DDAB are destructed to yield spherical mixed micelles on addition of pluronics via expansion or contraction of vesicles. However, the pure pluronics and their mixed systems with 10−2−10 and DDIC form charged spherical micelles, and charge is confirmed by thenfractional charge and ζ values. The ζ values of pure surfactants are found to decrease on addition of pluronics, indicating a decrease in surface charge on inclusion of pluronics.

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INTRODUCTION Pluronics, the triblock copolymers, made up of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) blocks with the general formula (PEO)n-(PPO)m-(PEO)n have gained much interest recently in the biomedical field for their use as drug or gene delivery vehicles or sensitizers for drug-resistant cells to improve drug transport or to repair biological membranes damaged by thermal burns or intense ionizing radiation exposure.1−4 They are highly surface-active compounds and form micelles above the critical micelle concentration (cmc) or critical micelle temperature (cmt), resembling the micelle formation of short-chain surfactants whose hydrophobic core is formed by weakly solvated PPO blocks and surrounded by an outer shell of fully hydrated PEO end chains.5−8 In the area of surfactant science and technology, keeping in view the rising demands, mixed micellar systems including cationic surfactants and pluronics are receiving much attention. In practical applications, mixtures of surfactants are often used because some effects that are not expected in single surfactant systems can take place in aqueous solution containing mixed surfactants.9,10 The performance superiority exhibited by the mixtures of surfactants is attributed to the synergistic or the antagonistic interaction, depending on the properties of the surfactants.11−13 A knowledge of the © 2013 American Chemical Society

mechanism of interactions between surfactant and pluronic molecules could provide important insight into the potential for the use of certain surfactants as vectors in drug delivery. The interactions between pluronics and conventional surfactants have been extensively studied using a number of techniques.14−17 In contrast, there have been only a few studies of interactions between twin-tailed surfactants and pluronics.18−20 The twin-tailed surfactants are known to organize into vesicles or bilayers. Vesicles have been considered to be models for biological membranes and have been used in applications such as microreactors, drug encapsulation, and drug delivery. There is increasing interest in investigating surfactant aggregates that mimic biological membranes, such as phospholipid liposomes or synthetic amphiphile vesicles, because the architecture of these artificial membranes is considerably simpler than that of cell membranes.21 A number of techniques have been used to investigate the structure, morphology, and phase behavior of vesicle−micelles transitions in mixed surfactant systems.22−25 As part of a comprehensive study of the mixed cationic surfactant + pluronic systems, we have carried out surface Received: May 16, 2013 Revised: August 17, 2013 Published: August 26, 2013 11821

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series of measurements was repeated at least three times. The reproducibility of surface tension measurements in determining the cmc values was estimated to be within ±0.1 × 10−5 M. Dynamic Light Scattering (DLS) Measurements. The DLS measurements were performed using a Malvern 4800 autosizer employing a 7132 digital correlator at 30 ± 0.1 °C. The light source was an argon ion laser that operated at 514.5 nm with a maximum output power of 2 W. The scattering intensity obtained is a function of scattering wave vector q, which is given by the relation q = [4πn sin(θ/2)]/λ, with n being the refractive index of the solvent, λ being the wavelength of laser light, and θ being the scattering angle. The apparent diffusion coefficient (D) was obtained from the relation Γ = Dq2, where Γ is the average decay rate. The average hydrodynamic diameters (Dh) were calculated using the Stokes−Einstein equation, which is given by the relation D = kBT/3πηDh, where kB is the Boltzmann constant and η is the solvent viscosity. At least 5 series of 10 measurements were performed for each independent sample. Zeta (ζ) Potential Measurements. The ζ potentials were measured at 30 ± 0.1 °C with a Zetasizer Nano Z from Malvern Instruments Ltd. using a laser Doppler microelectrophoresis technique that operates with a 5 mW He−Ne laser at 633 nm. Each ζ potential value is taken as an average over 10 independent measurements. The ζ potential is obtained from the electrophoretic mobility (μE) using the Smoluchowsky equation ζ = μEη/ε, where η and ε are the solution viscosity and the dielectric constant of the medium, respectively. SANS Measurements and Theoretical Details. Small-angle neutron scattering (SANS) measurements were carried out using an indigenously built SANS instrument operating at Dhruva Reactor, Bhabha Atomic Research Centre (BARC), Trombay, India.30 The mean incident neutron beam wavelength (λ) was 5.2 Å with a wavelength resolution (Δλ/λ) of approximately 15%. The scattered neutrons were detected in an angular range of 0.5−15° using a linearposition-sensitive detector (PSD). The scattered neutrons were measured for scattering vector Q (Q = 4π sin(θ/2)/λ, where θ is the scattering angle) in the range of 0.015−0.3 Å−1. The measured SANS data were corrected for the background, the empty cell contribution, and the transmission and were presented on an absolute scale using the standard protocols. For SANS measurements, samples were prepared in D2O in order to minimize the incoherent scattering and to increase the contrast.31 All of the data were recorded at 30 ± 0.1 °C. In a SANS experiment, one usually measures the differential scattering cross section (dΣ/dΩ) per unit volume as a function of wave vector transfer (Q). In the case of monodisperse particles in a medium, it can be written as

tension, dynamic light scattering (DLS), zeta (ζ) potential, and small-angle neutron scattering (SANS) measurements to examine the interactions of cationic twin-tailed surfactants such as dimethylene bis(decyldimethylammonium bromide) (10−2−10), didecydimethylammonium bromide (DDAB), and 1,3-didecyl-2-methylimidazolium chloride (DDIC) with pluronics P84 and F108. All three surfactants have two similar hydrophobic tails but different headgroups as shown in Scheme 1. Thus, the influence of the nature of the headgroup of cationic Scheme 1. Molecular Structure of the Surfactants and Pluronics Employed

twin-tailed surfactants on the mixing behavior with pluronics has been studied. There are only few studies reported on the micellization behavior of DDAB that features studies of DDAB with nonionics such as poly(oxyethylene glycols) with cationic surfactants,26,27 but there is no report on the interactional behavior of DDAB with pluronics. Also, DDIC is a novel surfactant with two hydrophobic tails attached to an imidazolium headgroup, and no report is available in the literature to the best of our knowledge regarding its micellization behavior and interactions with pluronics. The aggregation behavior of pure twin-tailed cationic surfactant, pure pluronics, and their mixtures at varying concentrations of pluronics (P84 and F108) has been investigated with SANS and DLS measurements. In addition, zeta potential measurements have been employed to determine the surface charge on the mixed micelles with the change in composition of the components.

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EXPERIMENTAL SECTION

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METHODS

dΣ = NF(Q ) S(Q ) + Bincoherent dΩ

(1)

where N is the particle number density. F(Q) is the form factor that is characteristic of the specific size and shape of the scatterer. S(Q) is the structure factor that depends upon the spatial arrangement of the particles and thereby gives information about the interparticle interaction. Bincoherent is a constant term that represents the incoherent background coming mainly from the hydrogen atoms present in the sample. Form factors for different shapes (spherical, ellipsoidal, and long bilayer) of particles have been used in the SANS data analysis. For spherical particles of radius R, it is given by

Materials. Didecyldimethylammonium bromide (DDAB) and 1,3didecyl-2-methylimidazolium chloride (DDIC) with a purity of >99% were obtained from Sigma-Aldrich and were used without further purification. Gemini surfactant 10−2−10 was synthesized as reported in the literature.28,29 Pluronics P84 and F108 were obtained from Sigma-Aldrich and were used as such. The molecular structure of all of the surfactants (10−2−10, DDAB, DDIC) and two pluronics with their hydrophilic lipophilic balance (HLB) are shown below in scheme 1.

F(Q ) =

Surface Tension Measurements. The surface tension measurements of pure systems and their associated mixtures were carried out with a Kruss Easy Dyne tensiometer from Kruss Gmbh (Hamburg, Germany) using the Wilhelmy plate method. The samples were thermostatted at 30 ± 0.1 °C. The surface tension of doubly distilled water, 71.6 ± 0.4 mN m−1, was used for calibration purposes. The

⎡ sin(QR ) − (QR )cos(QR ) ⎤2 16π 2 (ρp − ρs )2 R6⎢3 ⎥ 9 ⎣ ⎦ (QR )3

(2)

where ρp and ρs are the scattering-length densities of particle and solvent, respectively. For vesicles having inner radius R and thickness dR 11822

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Figure 1. Surface tension as a function of log[surfactant] for (a) pure surfactants 10−2−10, DDAB, and DDIC (inset of part a showing the surface tension as a function of log[pluronic] for P84 and F108) and (b) mixed system DDAB + F108 at different mole fractions of pluronic F108 (αF108).

F(Q ) =

⎡ ⎛ Z + 1⎞ ⎤ ⎛ Z + 1 ⎞Z + 1 Z 1 f (R ) = ⎜ ⎟ R exp⎢− ⎜ ⎟R ⎥ ⎝ R cm ⎠ ⎣⎢ ⎝ R cm ⎠ ⎥⎦ Γ(Z + 1)

16π 2 (ρp − ρs )2 9 ⎡ ⎢(R + dR )3 ⎣ sin Q (R + dR ) − Q (R + dR )cos Q (R + dR ) Q 3(R + dR )3 2 sin QR − QR cos QR ⎤ − R3 ⎥ ⎦ Q 3R3

where Rcm is the mean and Z is the width of the distribution. Γ is the mathematical gamma function. The polydispersity of this distribution is given by σ = 1/(Z + 1)1/2. Throughout the data analysis, corrections were made for instrumental smearing. The calculated scattering profiles were smeared by the appropriate resolution function to compare with the measured data. The parameters in the analysis were optimized by means of a nonlinear least-squares fitting program.

(3)

■

For prolate ellipsoidal particles, 16π 2 F(Q ) = (ρp − ρs )2 (ab2)2 9

∫0

1

[F(Q , μ)]2 dμ

RESULTS Surface Tension Measurements. Micellar Parameters: cmc and cmc*. Representative plots of γ versus log[surfactant] for pure surfactants (10−2−10, DDAB, DDIC) and the mixed system of DDAB + F108 at different mole fractions of pluronic (i.e., αF108) are shown in Figure 1a,b, respectively. Similar plots for the other mixed systems 10−2−10 + P84/F108, DDAB + P84, and DDIC + P84/F108 are shown in Figure S1a−e in the SI. The cmc values are summarized in Table 1, and the cmc value for pure surfactants 10−2−10,35 pluronic P84,36 and pluronic F10836 are in good agreement with literature value. The cmc values of pure pluronics P84 and F108 are 3.17 and 4.05 mM. The lower cmc value of P84 is attributed to its hydrophobicity, which is greater than that of F108. The cmc values of pure surfactants vary in the order 10−2− 10 > DDIC > DDAB; this observed trend can be explained on the basis of the structure of their respective hydrophilic headgroups and counterions. In case of DDIC and DDAB, there is only one headgroup per molecule whereas in 10−2−10 there are two headgroups per molecule, resulting in increased intermicellar electrostatic repulsions. However, because of the bulkiness of the imidazolium headgroup in DDIC, it is more effective in screening the electrostatic repulsions among the polar headgroups; moreover, the charge on the imidazolium ring is delocalized. However, the greater degree of hydration of the chloride ion as compared to that of bromide ions makes the reduction of the electrostatic repulsions among polar headgroups less effective as a result of poor binding at the micellar surface. Thus DDIC, because of its bulky headgroup and the presence of chloride ions as counterions, has a greater cmc

(4)

where the functions are given by F(Q , μ) =

3(sin x − x cos x) x3

and

x = Q [a 2μ2 + b2(1 − μ2 )]1/2 where a and b are, respectively, semimajor and semiminor axes of a prolate ellipsoid (a > b = c). The variable μ is the cosine of the angle between the directions of a and Q. For an oblate ellipsoid (b = c > a), a and b can be interchanged in the above equations. S(Q) for charged particles is calculated by using the Hayter and Penfold analysis under the rescaled mean spherical approximation (RMSA),32,33 which assumes a screened Coulomb interaction between charged particles. However, S(Q) in the case of nonionic micelles was determined from the analytical solution of the Ornstein−Zernike equation in the Percus−Yevick approximation employing a hardsphere potential.34 Additional information such as molecular volumes, scattering lengths, and scattering-length densities of pure components and their mixtures is given in Supporting Information (SI) under section S1. For a solution of polydisperse interacting particles, eq 2 can be expressed as dΣ (Q ) = dΩ

∫ ddΩΣ (Q , R)f (R) dR + Bincoherent

(6)

(5)

where f(R) is the size distribution and usually accounted for by the Schultz distribution as given by 11823

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Table 1. Micellar,a Interfacial,b and Thermodynamicc Parameters β

γcmc, mN m−1

0.09/0.18 0.31/0.47 0.63/0.67 0.77/0.83 0.74/0.95

1.0 1.4 −1.03 −0.80 −4.52

34.7 27.5 31.3 34.7 35.4 35.5 26.0

0.14/0.04 0.25/0.13 0.48/0.45 0.62/0.76

−1.96 −1.51 0 −1.81 −3.07

27.1 28.3 29.5 30.0 33.5 25.1

0.17/0.04 0.28/0.14 0.33/0.28 0.50/0.48 0.61/0.78

−2.34 −1.87 −0.66 −3.19 −3.83

31.8 28.3 34.0 34.5 34.4 32.7

4.05 4.57 5.01 5.13 5.37 4.31

0.28/0.15 0.43/0.41 0.61/0.61 0.76/0.79 0.93/0.93

−1.75 −0.45 0.24 3.47 11.5

36.9 28.4 32.2 35.1 36.2 37.1

1.01 1.02 1.64 1.48 0.84

0.15/0.03 0.27/0.11 0.26/0.22 0.45/0.39 0.56/0.72

−2.48 −2.39 −0.46 −1.87 −5.67

27.5 28.5 28.5 29.3 36.0

1.09 0.83 0.76 0.60 1.09

0.16/0.03 0.32/0.12 0.41/0.23 0.48/0.41 0.58/0.73

−2.53 −3.57 −4.24 −5.67 −4.95

33.5 35.3 35.2 35.8 36.2

cmc (mM) αP84 1.0 0.1 0.3 0.5 0.7 0.9 0.0 αP84 0.1 0.3 0.5 0.7 0.9 0.0 αP84 0.1 0.3 0.5 0.7 0.9 0.0 αF108 1.0 0.1 0.3 0.5 0.7 0.9 αF108 0.1 0.3 0.5 0.7 0.9 αF108 0.1 0.3 0.5 0.7 0.9

3.17 6.58 6.41 3.47 3.34 1.92 6.46 1.04 1.13 2.21 1.33 1.40 1.13 1.07 1.12 1.55 1.02 1.20 1.23

X1/X1,ideal

Γmax × 106, mol m−2 10−2−10 + P84 0.61 1.33 0.98 0.46 0.30 0.29 1.79 DDAB + P84 1.58 1.40 1.27 1.13 0.49 2.11 DDIC + P84 1.52 1.40 0.90 0.67 0.44 1.98 10−2−10 + F108 0.55 1.68 0.96 0.52 0.27 0.40 DDAB + F108 1.45 0.86 1.28 0.95 0.31 DDIC + F108 0.81 0.55 0.55 0.47 0.39

Amin, Å2

−ΔG0m, kJ mol−1

−ΔG0ads, kJ mol−1

274.3 124.2 169.9 358.7 546.9 575.4 94.34

24.63 22.78 22.85 24.39 24.49 25.89 22.83

85.12 55.94 63.97 104.62 145.16 150.37 48.31

105.4 118.5 131.0 147.5 339.1 79.9

27.43 27.23 25.53 26.81 26.69 27.22

55.59 58.15 58.68 63.63 104.44 49.26

108.8 118.4 183.7 248.4 375.5 83.5

27.36 27.24 26.43 27.48 27.07 27.01

53.55 58.18 68.20 82.85 111.62 46.66

301.6 98.7 173.7 318.8 612.8 413.3

24.01 23.70 23.47 23.41 23.29 23.85

87.09 49.42 64.51 93.60 154.41 110.10

114.3 193.1 129.6 175.2 531.7

27.51 27.48 26.29 26.54 27.97

57.92 77.59 59.96 71.07 142.81

203.8 302.6 301.9 356.5 430.4

27.32 28.00 28.22 28.82 27.32

74.35 94.00 94.41 104.99 118.08

a Critical micelle concentration (cmc), micellar compositions (X1, X1,ideal), interactional parameter (β), . bSurface tension at cmc (γcmc), surface excess (Γmax) and minimum area per molecule (Amin). cFree energy of micellization (ΔG0m) and free energy of adsorption (ΔG0ads) of the pure surfactants and their mixed systems at 30 ± 0.1 °C.

value that does DDAB, resulting in the observed trend in cmc values. The variations in the cmc* values calculated by using Clint’s equation (eq s3 in section S2 of the SI) from the experimental cmc values of the mixed systems at different mole fractions of pluronics are shown in Figure 2. It is evident from Figure 2a that the experimental cmc values of 10−2−10 and pluronic P84 show positive departures from the cmc* values at αP84 < 0.5 and the values above this show a negative departure. Whereas opposite results have been observed for the 10−2−10 and F108 mixed system as observed from Figure 2b, the experimental cmc values of 10−2−10 and pluronic F108 show a negative departure from the cmc* values at αF108 ≤ 0.5, and the values above this show a positive departure. For the other mixed systems (DDAB/DDIC + P84/F108), the cmc values show a

negative departure from the ideal behavior over the entire mole fraction range of both pluronics (i.e., αP84/F108). A negative departure of cmc values from cmc* indicates that mixed micelle formation occurs because of more favorable mixing in the mixed states whereas opposite results are obtained from unfavorable (antagonistic) mixing in the mixed state. Interaction Parameter. The magnitude of the nonideality between the two components in the mixed systems is expressed in terms of the interaction parameter (β) calculated with the Rubingh equation (eqs s4 and s5 in the SI). It is evident from Table 1 that for mixed system of DDAB/DDIC + P84/F108 the X1 values are higher than the X1,ideal values over all other mole fractions of P84 and F108 (i.e., αP84/F108 except at αP84/F108 = 0.9). A large value of X1 shows that the mixed micelles are richer in their respective pluronic than its expected value 11824

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Figure 2. Plots of cmc (experimental) and cmc* (predicted from the Clint equation) vs αpluronic for (a) 10−2−10/DDAB/DDIC + P84 and (b) 10− 2−10/DDAB/DDIC + F108.

whereas small values indicate the predominance of surfactant in their respective mixed micelles. However, it has been observed from Table 1 that the X1 values are less than the X1,ideal values over most of the mole fraction range of pluronic P84 (i.e., αP84) for mixed system 10−2−10 + P84. The X1 values do not vary much from the X1,ideal values over most of the mole fraction range of pluronic F108 for mixed system of 10−2−10 + F108. The value of β is positive for the 10−2−10 + P84 mixed system, and its magnitude decreases with increased concentration of P84, and the value of β changes its sign from positive to negative, indicating an increase in synergism. However, for the 10−2−10 + F108 mixed system the value of β changes its sign from negative to positive, indicating an increase in antagonism with increasing concentration of F108. Within the framework of RST, the negative β values can be related to the synergistic interactions, whereas the positive values can be related to the antagonistic (unfavorable) behavior. The β values are found to be negative over the entire mole fraction range of pluronics (P84 and F108) (i.e., αF108/P84 for the mixed systems of DDAB + P84/F108 and DDIC + P84/F108 indicating synergistic interactions). Interfacial Parameters at the Air/Solution Interface. The values of surface tension at the cmc (γcmc) of the pure surfactant, pure pluronics, and their associated mixed systems are given in Table 1. The γcmc values of pure surfactants vary in the order DDIC > 10−2−10 > DDAB, indicating that the surface activity increases in reverse order (i.e., DDAB is the most surface-active surfactant). The γcmc values of the pure pluronics are greater than those of all three surfactants, indicating that they are less effective in lowering the surface tension of water. The γcmc values of all of the mixed systems increase progressively with increasing concentration of pluronic, which shows that mixed systems are more surfaceactive at higher concentrations of surfactant. The maximum surface excess concentration (Γmax) and Amin can be calculated with the following Gibbs adsorption equations37 Γmax =

− 1 ⎛ dγ ⎞ ⎜ ⎟ nRT ⎝ d ln C ⎠

A min =

1020 NA Γmax

(8)

where dγ/d ln c is the maximum slope obtained in the plots of γ versus log[surfactant]. R, T, c, and NA are the gas constant, temperature (K), concentration of surfactant, and Avogadro’s number, respectively. The value of n is the number of solute species whose concentration at the interface changes with the change in surfactant concentration. The value of n is taken as 3 for 10−2−10 and its associated mixed systems, and the value of n is taken as 2 for DDAB/DDIC and their associated mixtures with both pluronics. The values of Γmax for 10−2−10, DDAB, and DDIC are 1.79, 2.11, 1.98 μmol/m2, respectively. The Γmax value of DDAB is greater than that of the two other surfactants, indicating that it is more surface-active than the other two surfactants. It is well established that the size and polarity of the hydrophilic headgroup are the dominant factors in determining the Γmax values. 10−2−10 has two headgroups per molecule, so the headgroup−headgroup repulsions are maximized, resulting in the lowest surface density and hence large Amin values. Although DDAB and DDIC have one headgroup per surfactant molecule, DDIC has less surface density because of the bulkiness of the imidazolium headgroup of DDIC. The pure cationic surfactants have higher Γmax values than the pure pluronics (F108 and P84), indicating that they are more hydrophobic in nature as a result of the presence of two hydrophobic tails in them. The structure of pure pluronic at the air/water interface was found to contain an uppermost layer, which extends outside the water phase and contains only PPO chains, and a layer in the aqueous phase comprising all of the PEO and a few PPO chains.38 The Γmax values of mixed systems are found to be higher than that of pure pluronic but less than those of pure surfactants. The Γmax values of all of the mixed systems are found to decrease with increase concentration of pluronics in the mixture. At lower surfactant concentrations (i.e., αP84/F108> 0.5), the surfactant molecules interact with only PEO units, and the thickness of the mixed monolayer is reduced. At higher concentrations of surfactants in mixed composition, the surfactant molecules penetrate the PPO region of the

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Figure 3. SANS data pattern (with error bars in experimental data) for (a) pure surfactant DDAB (fits with varying dR) and (b) pure surfactants 10−2−10 and DDIC.

variations in Dh and ζ values of pure 10−2−10 with varying concentration of pluronics (P84 and F108) in the mixed system of total concentration of 5 wt % are shown in Figure S2 of theSI. SANS Measurements. Typical scattering curves for 2 wt % of pure 10−2−10, DDAB, and DDIC are shown in Figure 3. A close quantitative analysis of the scattering curves of pure DDAB reflects the formation of unilamellar vesicles (ULVs). The large ULVs are seen as a bilayer in the limited Q window of SANS measurements and therefore would show a linear scattering pattern having a slope of −2 on a log−log scale, which is also seen in the present case. Moreover, because no correlation peak coming from the repetition of lamella was observed in the SANS data, the multilamellar structure of the vesicles is ruled out. Therefore, the SANS data of DDAB has been fitted for unilamellar vesicles using eq 3. The cutoff in the higher-Q region in the SANS spectra decides the thickness of the vesicles. It may be mentioned that the calculated thickness of the vesicles (19.5 Å) is much smaller than twice the extended length of the hydrophobic tail, which is about 14.5 Å. Our analysis shows that a significantly smaller thickness may possibly arise from some coiling or interdigitation of hydrophobic tails. SANS mostly sees the hydrophobic regions of the lamellar structure because of the low contrast for their headgroups. The scattering from the headgroup region is very small and can be neglected. It is generally accepted that for the aggregates formed from surfactants some water molecules (in the case of SANS, D2O) penetrate the headgroup region to some extent and this penetration of D2O will decrease the contrast, thus reducing the apparent bilayer thickness measured by SANS.41 We have in fact varied the lamellar thickness and checked its dependence on the fitting of the data as shown in Figure 3a. Similar types of thickness values have also been reported elsewhere for the shell thickness of other twin-tailed surfactants (e.g., didodecyldimmethylammonium bromide (DDAB) and dihexadecyldimmethylammonium bromide (DHDAB)42). The extended lengths of the hydrophobic tails of DDAB and DHDAB are about 16.7 and 21.7 Å and the reported bilayer thicknesses are 24 and 27 Å, respectively. It is possible that the smaller value of the thickness may arise by diffuse scattering from monomers contributing to the scattering

monolayer, resulting in an increase in the thickness of the mixed monolayer at the air/solution interface. Consequently, an opposite trend has been observed for the Amin values of pure and mixed systems. Penfold38 et al. have reported similar results for the surface adsorption behavior of the mixture of dodedcyltrimethyl ammonium chloride (DTAC) and pluronics. Thermodynamic Framework of Micellization. The standard free energy of micellization per mole (ΔG0mic) and the standard Gibbs energy of adsorption (ΔG0ads) have been evaluated by using the following equations39 ΔG 0 m = RT ln cmc ΔG 0 ads =

ΔG 0 m − πcmc Γmax

(9)

(10)

where the surface pressure values at the cmc (πcmc) were obtained from the relation πcmc = γo − γcmc. Table 1 show that the ΔG0ads and ΔG0m values are negative for all of the pure and mixed systems, indicating that both processes are spontaneous. ΔG0ads values are more negative than their corresponding ΔG0m values, indicating that surface adsorption is more spontaneous than micellization. DLS and Zeta Potential Measurements. DLS and zeta potential (ζ) measurements are carried out to obtain further information on the physicochemical properties of the mixed aggregates of surfactants and pluronics. The ζ values are a measure of the electric field potential at the micelle’s plane of zero shear and are affected by the size, shape, and surface charge of the structure.40 The ζ value of the pure pluronics (F108, P84) was close to 0 mV as expected from the electroneutral nature of the pluronics. The ζ values of the mixed systems (surfactant + pluronic) in water were positive and dependent upon the concentration of surfactant. The Dh of pure DDAB at a concentration of 1 wt % is 234.0 ± 11.7 nm, having a ζ value of 64.63 ± 0.21 mV. Changes in the Dh and ζ values on addition of pluronics to DDAB have been observed. The Dh of pure 10−2−10 is 4.6 ± 0.4 nm at a concentration of 5 wt %, having a ζ value of 25.6 ± 1.4 mV. The Dh values of pure P84 and pure F108 at a concentration of 5 wt % are 15.8 ± 0.5 and 18.2 ± 0.9 nm, respectively. The 11826

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Table 2. Various Key Model Parameters at 30 ± 0.1 °Ca surfactant (wt %)

a (Å)

b (Å)

αc

polyd

10−2−10 (2%) DDIC (2%)

58.5 ± 1.6 32.4 ± 1.2

15.2 ± 0.4 13.3 ± 0.2

0.20 ± 0.02 0.28 ± 0.02

0.20 ± 0.02 0.28 ± 0.03

d (Å)

Nagg 95 ± 8 40 ± 3

161.2 95.2

a Semimajor axis (a), semiminor axis (b), fractional charge (αc), polydispersity (polyd), mean distance between micelles (d), and aggregation numbers (Nagg).

Figure 4. SANS data pattern (with error bars in experimental data) for (a) DDAB + P84 and (b) 10−2−10 + F108. The inset in plot b shows a linear scale for the scattering cross-section.

Table 3. Various Key Model Parameters (a, b, Rhs, polyd, ϕ, αc, d, and Nagg) for DDAB + P84/F108 at 30 ± 0.1 °C system (wt %) 4% DDAB + 1% P84 3% DDAB + 2% P84 3%DDAB + 2% F108 2% DDAB + 3% F108

a (Å) 68.1 46.2 75.8 61.7

± ± ± ±

2.0 1.4 2.4 1.9

b (Å) 13.0 15.4 11.0 11.5

± ± ± ±

0.3 0.4 0.2 0.2

Rhs 119.0 99.0 129.0 126.0

± ± ± ±

ϕ

polyd 5.0 2.5 5.2 5.1

0.30 0.20 0.20 0.22

intensity at high Q values. However, all of our experiments are performed at concentrations much above the cmc, where the contribution from monomers may be neglected. Unlike the case of DDAB, SANS data of other two surfactants (10−2−10/DDIC) show correlation peaks, which is an indication of the formation of interacting charged micelles. Usually, this peak occurs at Qmax ≈ 2π/d, where d is the mean distance between the micelles. The ionic micelles of these surfactants are fitted with a form factor of prolate ellipsoidal shape and a structure factor as calculated by Hayter and Penfold analysis for charged macroions under a rescaled mean spherical approximation. The key model parameters are summarized in Table 2. DDIC surfactant (Nagg ≈ 40) is found to form smaller micelles compared to 10−2−10 (Nagg ≈ 95), which is also evident from Figure 3b where the scattering for DDIC decreases and the peak position is shifted to higher Q value. The smaller size of DDIC micelles can be understood from the bulkiness of the imidazolium headgroup and the increase in surface charge on the micelles. As already discussed in the Micellar Parameters: cmc and cmc* section, the binding of the chloride ions to the micellar surface will be poor because of the greater degree of hydration. Thus, poor binding of counterions will lead to more surface charge on the DDIC micelles as shown by the αc value. Similar to ionic micelles, SANS data of pure pluronics (P84and F108) also show correlation peaks. In the case of nonionic micelles, the correlation peak arises because of hard

± ± ± ±

0.03 0.02 0.02 0.02

0.19 0.18 0.18 0.16

± ± ± ±

αc 0.01 0.01 0.02 0.01

0.01 0.03 0.01 0.02

± ± ± ±

d (Å) 0.005 0.01 0.007 0.01

261.9 219.0 285.7 273.3

Nagg 385 176 392 241

± ± ± ±

29 14 27 16

sphere repulsion between the particles and is usually seen at a much higher concentration than for ionic micelles. These micelles can be considered to consist of a spherical core−shell particle with differing scattering-length densities of the hydrophobic (PPO) core and hydrophilic shell (PEO) interacting via a hard sphere potential. The scattering-length density from the PEO shell is not significantly different from that of solvent as a result of the high degree of solvation of hydrophilic PEO units; therefore, the scattering intensity is mainly governed by the core only. The scattering data of pure 5 wt % pluronics F108 and P84 have been analyzed by considering F(Q) for spherical micelles and S(Q) for the hard sphere potential. The hard sphere radii of both pluronics are found to be much larger than their respective core radii because the interaction radius also involves the shell of the PEO block. The volume fractions thus obtained are greater than that corresponding to 5 wt % because of the D2O penetration in the hydrophilic core. The representative plots of SANS data with fitted curves of mixed systems such as DDAB + P84 and 10− 2−10 + F108 are shown in Figure 4a,b, respectively. The other plots for mixed systems DDAB + F108, 10−2−10 + P84, and DDIC + P84/F108 are shown in Figure S3 of the SI. The variations in the form of the scattering data of DDAB on addition of P84 as shown in Figure 4a illustrate that the shape of the aggregates is strongly dependent upon the surfactant composition. As the concentration of P84 increases (i.e., at 2 and 3 wt %), we have not observed the linear scattering pattern 11827

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Table 4. Various Key Model Parameters (Core Radius (Rc), Rhs, polyd, αc, ϕ, d, and Nagg) for Mixed Micelles of Surfactant and Pluronics P84/F108 at 30 ± 0.1 °C system (wt %) 5% 5% 4% 1% 4% 3% 2%

P84 F108 10−2−10 + 1% P84 10−2−10 + 4% P84 DDIC + 1% P84 DDIC + 2% P84 DDIC + 3% P84

Rc (Å) 45.1 37.9 21.0 33.5 17.2 19.2 21.9

± ± ± ± ± ± ±

1.3 1.2 0.6 1.0 0.6 0.7 0.7

Rhs (Å) 72.1 94.7 41.8 60.3 39.4 39.7 45.1

± ± ± ± ± ± ±

2.6 2.9 1.4 2.0 1.2 1.0 1.4

± ± ± ± ± ± ±

0.04 0.06 0.02 0.03 0.03 0.02 0.03

having a slope of −2 on the log−log scale as observed for pure DDAB. Various models corresponding to particle geometries have been used to fit the experimental SANS data. Only the oblate ellipsoid model captured the shape of the mixed aggregates in the measured Q range. The scattering data at these concentrations have been analyzed using the oblate ellipsoid model, taking into account the intermicellar interactions as described by Hayter and Penfold in RMSA calculations. This model incorporates the elliptical growth with a minor/major axis ratio (ν) DDIC + pluronics as shown in Table 4 (Table S5 in the SI for surfactant + F108). The observed trend in the size of mixed micelles can be explained by the headgroups and fractional charge. Although DDIC has one headgroup per molecule, as a result of the bulkiness of the imidazolium ring, the intercalation of pluronic molecules into micelles of DDIC occurs to a lesser extent. Also, the surface of the mixed micelles of DDIC + pluronics is more charged as observed from the values of fractional charge, and the intermicellar repulsions are more pronounced. Moreover DDIC + pluronics mixed systems have the lowest Nagg because of a less dense packing of the micelles resulting from steric hindrance and pronounced electrostatic repulsions among the charged headgroups.

■

Corresponding Author

*Fax: +91 183 2258820. E-mail: [email protected].

■

Notes

The authors declare no competing financial interest.

CONCLUSIONS This work is aimed at obtaining further knowledge of the ionic surfactant + pluronic mixed systems, keeping in mind their wider practical applications. Surface tension measurements have been used to characterize the micellar and surface properties of the pure and mixed systems in an aqueous solution of twintailed cationic surfactants 10−2−10, DDAB, and DDIC with pluronics P84 and F108. The cmc values of pure surfactants vary in the order of 10−2−10 > DDIC > DDAB. The interactions of each surfactant with both pluronics are found to be nonideal and synergistic except for the mixed system of 10− 2−10 + F108, for which interactions are antagonistic. This difference in the interactions of surfactants with pluronics stems from the dissimilarities in the size and hydration of the headgroups and the nature of the counterions. It has been observed that the structure of the headgroups of surfactants and the hydrophobic character of pluronics affect the structure of the final aggregate. Pure DDAB is found to form ULVs, and the addition of P84 or F108 results in strong structural changes. From the combined analysis of DLS, ζ potential, and SANS measurements for the DDAB + P84/F108 mixed system, it has been concluded that the vesicle to micelle (V−M) transition comes about in three stages: (i) at much lower concentration of pluronics, the PPO units of the pluronic are intercalated into a bilayer of vesicles, resulting in an expansion or contraction of vesicle size; (ii) after saturation, the vesicles are ruptured to yield oblate ellipsoidal micelles; and (iii) the oblate ellipsoidal micelles are transformed to smaller mixed micelles on further addition of pluronics. Pure 10−2−10 and DDIC are found to form charged prolate ellipsoids. The 10−2−10 + P84/F108 and DDIC + P84/F108 mixed systems are found to form spherical charged mixed micelles. The inclusion of 10−2−10/DDIC molecules in pluronic micelles will impart positive charge to the mixed aggregates in comparison to the uncharged large spherical micelles of pluronics, which is confirmed by the ζ values of the mixed micelles. Overall, the study reported here can help in gaining further insight into the aggregation and interactional behaviors of ionic surfactant + pluronic mixed systems.

■

AUTHOR INFORMATION

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ACKNOWLEDGMENTS Financial assistance from UGC-DAE (Bhabha Atomic Research Center, B.A.R.C), Trombay, India, and a research project (ref. no. CRS-M-153) is thankfully acknowledged. We gratefully acknowledge Dr. P. A. Hassan, B.A.R.C., for providing the DLS facilities.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Tables containing molecular volumes, scattering lengths, and scattering-length densities of pure pluronics and surfactants and their mixed systems. Surface tension as a function of 11831

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