Effect of salt on the micelles of cetyl pyridinium chloride

Colloids and Surfaces A: Physicochem. Eng. Aspects 259 (2005) 95–101

Effect of salt on the micelles of cetyl pyridinium chloride D. Varade a,∗ , T. Joshi a , V.K. Aswal b , P.S. Goyal c , P.A. Hassan d , P. Bahadur a a

Department of Chemistry, South Gujarat University, Surat 395007, India Solid State Physics Division, BARC, Trombay, Mumbai 400085, India c IUC-DAEF, Mumbai Centre, BARC, Trombay, Mumbai 400085, India Novel Materials and Structural Chemistry Division, BARC, Trombay, Mumbai 400085, India b

d

Received 12 June 2004; accepted 14 February 2005 Available online 16 March 2005

Abstract Surface tension, conductance, viscosity, dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements were carried out in order to investigate the effect of added salt NaCl and NaBr on micellization and structure of cetylpyridinium chloride (CPyCl) micelles. Critical micelle concentration (CMC) of CPyCl decreases with increase in salts concentration, while CMC and counter ion dissociation (β) increases with increase in temperature. At a given CPyCl concentration, the micelle size increases with increasing salt concentration above a threshold value. Micellar growth in presence of salts is clearly reflected by an increase in viscosity, hydrodynamic diameter (d) from DLS and aggregation number from SANS measurements. The effect of NaBr on CPyCl micelles was more pronounced in comparison to NaCl. The difference in solution properties of Br− and Cl− is that Cl− counter ions being more hydrated are less effective to neutralize the charge on the micellar surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Micellization; SANS; DLS; Micellar growth

1. Introduction Micelles of ionic surfactants formed in aqueous media are aggregates of surfactant molecules with different sizes and shapes depending on given solution conditions. As for the structure a micelle of ionic surfactants is composed of a compressive core surrounded by a less compressive surface structure [1,2]. In addition, counter ions are bound to micelle surface. Looking at counter ion behavior, the binding states are different depending on charge, size and polarity in counter ions species themselves. The solution properties of surfactant are all reflected from surfactant ions comprising various combinations of hydrophobic tail with hydrophilic head and from counter ion species. ∗ Corresponding author. Tel.: +91 261 2258384; fax: +91 261 2256012/2227312. E-mail address: [email protected] (D. Varade).

0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.02.018

Cationic surfactants offer some additional advantages over other class of surfactants [3–6]. These substances besides their surface activity do show antibacterial properties and are used as cationic softeners, lubricants, retarding agents, antistatic agents and in some cases consumer use etc. Ionic micelles of various cationic surfactants having chloride or bromide as counter ion undergo the salt induced sphereto-rod transition, when NaCl or NaBr is added across a certain threshold salt concentration. Experimentally, many techniques have been used in order to investigate the surfactant concentration, added salt concentration and temperature dependences of micellar solution properties [7–10]. Cetyltrimethylammonium bromide (CTABr) forms rodlike micelles at concentration above 0.27 M in water at 28 ◦ C, on the other hand, cetyltrimethylammonium chloride (CTACl) do not show such a behavior, the aggregates remaining globular up to 0.7 M [11]. This point out the specific role played by the counter ion and existence of spherical and nonspher-

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ical aggregates can be understood in terms of more or less efficient repulsions between the polar groups at the micellar surface. It is well known that salts such as KBr and sodium salicylate (NaSal) induce pronounced growth of CTABr micelles due to charge neutralization at the micellar surface by these salts [12,13]. The counter ion distribution or condensation and size of the ionic micelles play an important role to decide the effect of salt on the micellar systems [14]. Weican et al. [15] by means of laser light scattering, 1 H NMR measurements and fluorescence probe reported that CTABr forms rod-like micelles at 0.1 mol L−1 KBr and the worm like micelles are formed at above 0.2 mol L−1 KBr. In this paper we have reported systematic studies on influence of NaCl and NaBr salt on the micellization and structure of the micelles in cetylpyridinium chloride (CPyCl) solution as studied by surface tension, conductance, viscosity, dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements. The results are interpreted in terms of counter ion condensation on the ionic micelles, which depends on the hydrated size of the counter ions.

measurement. The flow time for constant volume of solution through the capillary was measured with a calibrated stopwatch. 2.4. Dynamic light scattering DLS measurements were performed using a Malvern 4800 Autosizer employing 7132 digital correlator. The light source was argon ion laser operated at 514.5 nm with a maximum output power of 2 W. The average decay rate (Γ ) of the electric field autocorrelation function, g1 (τ), was estimated using the method of cumulants. The apparent diffusion coefficients (D) of the micelles were obtained from the relation Γ = Dq2 (q is the magnitude of the scattering vector given by q = [4πn sin(θ/2)]/λ, n being the refractive index of the solvent, λ, the wavelength of laser light and θ is the scattering angle) and the corresponding hydrodynamic diameters (d) were calculated using the Stokes–Einstein relationship. For all the solutions, Γ varies linearly with ‘q2 ’ indicating translational diffusion of the scatterers. 2.5. Small angle neutron scattering

2. Materials and methods The cationic surfactant cetylpyridinium chloride (CPyCl), salt NaCl and NaBr were highly pure samples purchased from Fluka, Switzerland and were used as received. Triply distilled water from an all Pyrex glass apparatus was always used for the preparation of solutions for surface tension, conductance and viscosity measurements. For DLS measurement Milli-Q water having specific resistance 18.2 M cm was used. The samples for SANS experiments were prepared in D2 O. 2.1. Conductance Conductometric measurements were made with a digital conductivity meter (Phillips, India) using a dip-type cell. All measurements were done in a jacketed vessel, which was maintained at the appropriate temperature (±0.1 ◦ C). The errors in the conductance measurements were within ±0.5%. The conductance was measured after thorough mixing and temperature equilibrium at each dilution.

For SANS measurements, the sample solutions were prepared in D2 O. The use of D2 O as solvent instead of H2 O provides better contrast in neutron scattering experiments. In all the measurements, CPyCl concentration (0.25 M) and temperature (30 ± 0.1) ◦ C were kept constant. The solutions were held in a quartz cell of 0.5 cm thickness with tight-fitting Teflon stoppers. The SANS experiments were performed using a SANS diffractometer at the Dhruva reactor, BARC, Trombay [17]. The diffractometer uses a polycrystalline BeO filter as a monochromator. The mean wavelength of the in˚ with a wavelength resolution cident neutron beam is 5.2 A ( λ/λ) of approximately 15%. The angular distribution of the scattered neutron was recovered by a linear 1 m long He3 position sensitive detector (PSD). The data were recorded in ˚ −1 . All the measured SANS distrithe Q range of 0.02–0.24 A butions were corrected for the background and solvent contributions and the data were normalized to the cross-sectional unit using standard procedures [17].

2.2. Surface tension

3. Results and discussion

The surface tension of CPyCl solutions in absence and presence of salts was measured by drop weight method using a modified stalagmometer [16].

Conductivity measurements were performed in water for CPyCl at various temperatures (30–60 ◦ C at an interval of 10 ◦ C) in order to evaluate the CMC and the degree of counter ion dissociation, β. The plots of conductance data are presented as specific conductance versus concentration in Fig. 1. The values of CMC and β at various temperatures for CPyCl solution are recorded in Table 1. The values of CMC and β increase with temperature in the range investigated. The effect of temperature on the CMC of surfactants in aqueous solution is usually analyzed in terms of two opposing factors. First, as the temperature increases the degree of hydration of

2.3. Viscosity The viscosity measurements were carried out using an Ubbelohde suspended level capillary viscometer. The viscometer was always suspended vertically in a thermostat with a temperature stability of ±0.1 ◦ C in the investigated region. The viscometer was cleaned and dried every time before each

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Table 1 The CMCs, degree of counter ion dissociation (β), standard molar Gibbs energies ( G◦ ), enthalpies ( H◦ ) and entropies ( S◦ ) of micellization for CPyCl in water at different temperatures Temperature (K)

CMC (mM)

β

G◦ (kJ mol−1 )

H◦ (kJ mol−1 )

S◦ (kJ mol−1 )

303.2 313.2 323.2 333.2

0.98 1.11 1.30 1.45

0.41 0.43 0.45 0.46

−27.59 −28.17 −28.65 −29.23

−10.17 −10.85 −11.55 −12.28

0.057 0.055 0.053 0.050

the hydrophilic group decreases, which favors micellization; however, an increase in temperature also causes the disruption of the water structure surrounding the hydrophobic group and this is unfavorable to micellization. It seems from the data in Table 1 that this second effect is predominant in the temperature range studied. On the other hand, β, also increases regularly with temperature increase. This observed increase in β is probably due to a decrease in the charge density at the micellar surface caused by the decrease in the aggregation number of the micelle. The CMC values, as determined at various temperatures, were further used for calculation of the thermodynamic parameters of micellization. In the charged pseudo-phase model of micelle formation, the standard free energy of micelle formation per mole of surfactant is given by G◦m = (2 − β)RT ln XCMC

(1)

where R is the gas constant, T is the temperature in Kelvin scale and XCMC stands for the CMC in the mole fraction unit. The standard enthalpy of micelle formation ( Hm◦ ) can be derived by the van’t Hoff equation
  ∂ ln XCMC Hm◦ = −(2 − β)RT 2 (2) ∂T The enthalpy of micellization can be obtained if the dependence of the CMC on temperature is known. One can see that the standard enthalpy of micelle formation is more negative or exothermic on higher temperature side. A linear plot was observed of ln XCMC against T for CPyCl as shown in

Fig. 1. Plots of specific conductivity vs. concentration of CPyCl in water at different temperatures.

Fig. 2. The slope in the plot was taken as [(∂ln XCMC )/∂T]. ◦ ) was The standard molar entropy of micelle formation ( Sm calculated from ◦ Sm




Hm◦ − G◦m = T

 (3)

The entropy change is positive in all cases. However, it decreases with increasing temperature. The thermodynamic parameters of micellization obtained by following the above procedure are summarized in Table 1. The surface tension (γ) of CPyCl solutions in absence and presence of NaCl and NaBr was measured for a range of concentrations above and below the critical micelle concentration (CMC). As shown in the Fig. 3 (for NaCl), a linear decrease in surface tension was observed with increase in surfactant concentrations up to the CMC, beyond which no considerable change was noticed. The electrical atmosphere in the aqueous surfactant solution altered in presence of NaCl which neutralizes the effective head group charge resulting in reducing the electrostatic repulsion between the polar head groups and the micelles are formed at much lower concentration as compared to that in pure water. The CMC of the surfactant decreased in the presence of NaCl and NaBr, the decrease being dependent upon the concentration of added salt. The effect was more pronounced in case of NaBr. The area occupied by the ˚ 2 , at the air–water interface at satusurfactant molecules in A ration monolayer was estimated using the surface tension (γ)

Fig. 2. Plots of ln XCMC vs. temperature for CPyCl in water.

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Fig. 3. Plots of surface tension vs. log CPyCl concentration in absence and presence of varying amount of NaCl at 30 ◦ C.

Fig. 4. Relative viscosity vs. concentration of CPyCl in absence and presence of NaCl concentration at 30 ◦ C.

data at temperature T and the Gibbs adsorption isotherm:   1 dγ Γs − (4) nRT d ln C where Γ s is surface excess, R is the universal gas constant, C is the concentration of the surface active compound and n is taken as 2 because of the ionic nature of the surfactant. Interfacial properties of CPyCl in water at 30 ◦ C are recorded in Table 2. The viscosity measurement for micellar solution of CPyCl were made in absence and presence of NaCl and NaBr at 30 ◦ C in order to study the transition of micelle shape induced by the change in ionic strength. The results obtained in NaCl were quite interesting (Fig. 4). CPyCl shows a gradual increase in viscosity with increase in concentration in absence of NaCl. But the electrical atmosphere in the aqueous surfactant solution is profoundly altered by the added NaCl and this is turn influences the viscous flow of CPyCl. Initially, the solution of CPyCl becomes less viscous with increase in the concentration in presence of lower NaCl concentration, but a dramatic increase in viscosity was observed in presence of 1.0 M NaCl. This is evidently an indication of decrease in the electrostatic interaction between micelles which will be reflected as a decrease in the “effective volume fraction” in the presence of low NaCl concentration [19]. From the above results we can conclude that only above a certain salt concentration there is a transition from globular to rodlike micelles. Almgren and coworkers [20] has also studied the effect of varied NaCl concentration on the relative viscosity of hexadecyltrimethylammonium bromide (CTABr, 25 mM) solution, in which they have reported the similar initial de-

Fig. 5. Variation of hydrodynamic diameter of 0.25 M CPyCl in varying salt concentration at 30 ◦ C.

crease in viscosity at low salt concentration followed by a dramatic increase at high salt concentration. CPyCl shows an increase in viscosity (figure not shown) at much lower concentration of NaBr (0.2 M) as compared to NaCl (1.0 M). The changes in the aggregation behavior of CPyCl micelles in presence of NaCl and NaBr can be inferred from DLS measurements. The apparent hydrodynamic diameter (d) of the micelles in presence of different amounts of NaCl and NaBr are depicted in Fig. 5. Considering the ionic nature of the surfactant, in the absence of any salt, the measured diffusion coefficients will be modified due to the presence of repulsive intermicellar interactions. Such repulsive interactions will lead to an increase in the diffusion coefficient and hence a decrease in the apparent diameter of the micelles.

Table 2 Interfacial properties of CPyCl in water at 30 ◦ C Surfactants

CPyCl

CMC (mM) Experimental

Literature [reference]

0.95

1.00 [18]

C␲ = 20 (mM)

γ CMC (mN m−1 )

˚ 2) Area/molecule (A

0.5

42.0

42.0

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1/2

Thus, the observed diameter of the micelles in the absence of salts will be slightly underestimated as no account of interparticle interaction has been taken into consideration. This is evident from the smaller diameter of micelles (2 nm) as compared to that expected from the length of hydrocarbon chain of the surfactant (3.2 nm). With addition of salts, an increase in the apparent diameter of the micelles from ∼2 nm to ∼8 nm is observed suggesting an increase in size of the micelles. This observed change in apparent size arises from both decrease of intermicellar interaction and growth of the micelles. Considering large changes in the relative viscosity of the solution with addition of salt, a prolate ellipsoidal or rod-like growth of the micelles can be envisioned. The observed micellar growth is significantly different for NaCl and NaBr. In presence of NaCl, the apparent diameter reaches a value of 8 nm at a salt concentration of 1.2 M while the same effect could be observed for NaBr even at 0.2 M. This suggests that the nature of the condensation of counter ions on the micelle surface is different for Cl− and Br− . In SANS experiments one measures the differential scattering cross-section per unit volume (dΣ/dΩ) as a function of scattering vector Q. For a system of monodisperse interacting micelles (dΣ/dΩ) is given by [21]

where a and b are, respectively, the semimajor and semiminor axes of the ellipsoidal micelle and µ is the cosine of the angle between the directions of a and the wave vector transfer Q. In general, micellar solutions of ionic surfactants show a correlation peak in the SANS distribution [24]. 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 Fourier transform of the radial distribution function g(r) for the mass centers of the micelle. Unlike the calculation of P(Q), it is quite complicated to calculate S(Q) for any other shape than spherical. This is because of S(Q) depends on the shape and as well as on the orientation of the particles. To simplify this, prolate ellipsoidal micelles are assumed to be equivalent spherical. We have calculated S(Q) as derived by Hayter and Penfold [25] from the Ornstein–Zernike equation and using the mean spherical 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

dΣ (Q) = n(ρm − ρs )2 V 2 [F (Q)2  dΩ

u(r) = u0 σ

+F (Q)2 (S(Q) − 1)] + B

(5)

where n denotes the number density of the micelles, ρm and ρs are, respectively, the scattering length densities of the micelle and the solvent 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, which is mainly due to hydrogen in the sample. The micelles formed at the CMC are spherical. If the solution conditions (e.g., concentration, ionic strength, etc.) of the micellar solutions are changed that favors the growth of the micelles, they grow along one of the axial directions of the micelles. The growths of the micelles along other two axial directions are restricted by the length of the surfactant molecule to avoid the energetically unfavorable any empty space or water penetration inside the micelle [22,23]. The prolate ellipsoidal shape (a = b = c) of the micelles is widely used in the analysis of SANS data because it also represents the other different possible shapes of the micelles such as spherical (a = b) and rod-like (a  b). For such an ellipsoidal micelle  1 F 2 (Q) = [F (Q, µ)2 dµ] (6) 0



1

F (Q)2 =

2 F (Q, µ) dµ

(7)

3(sin x − x cos x) x3

(8)

0

F (Q, µ) =

x = Q[a2 µ2 + b2 (1 − µ2 )]

exp[−κ(r − σ)] , r

(9)

r > σ,

(10)

where κ is the Debye–Huckel inverse screening length and is calculated by 1\2
8πNA e2 I (11) κ= 103 εkB T defined by the ionic strength I of the solution I = CMC + 21 αC + Cs,

(12)

where I is determined by the CMC, dissociated counter ions from the micelles and the salt concentration. The fractional charge α (=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 counter ions of the surfactant in the micelle. C and Cs present the concentrations of the surfactant and salt in the solution, respectively. The contact potential u0 is given by u0 =

Z 2 e2 πεε0 σ(2 + κσ)2

(13)

where ε is the dielectric constant of the solvent medium, ε0 is the permittivity of free space and e is the electronic charge. The dimensions of the micelle, aggregation number and the fractional charge have been determined from the analysis. The semimajor axis (a), semiminor axis (b = c) and the fractional charge (α) are the parameters in analyzing the SANS data. The aggregation number is calculated by the relation N = 4πab2 /3v, where v is the volume of the surfactant monomer. Throughout the data analysis corrections were made for instrumental smearing [26]. The parameters

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Table 3 Micellar parameters from 0.25 M CPyCl in presence of varying NaCl and NaBr concentration at 30 ◦ C Salt

[Salt] (M)

Aggregation number, N

Fractional charge, α

˚ Semiminor axis, b = c (A)

NaCl

0.0 0.1 0.5 1.0

104 113 131 229

0.2 0.2 0.2 0.2

18.1 18.7 19.5 18.4

44.3 45.3 48.4 94.4

2.4 2.4 2.5 5.1

NaBr

0.1 0.2

139 368

0.2 0.07

19.2 19.5

52.7 136.0

2.7 7.0

in the analysis were optimized by means of nonlinear leastsquare fitting program and the errors of the parameters were calculated by the standard methods used [27]. Figs. 6 and 7 show the SANS data from 0.25 M CPyCl micellar solution with and without the addition of salt NaCl and NaBr at temperature 30 ◦ C, respectively. The SANS distribution from a pure 0.25 M CPyCl shows a well defined ˚ −1 . This corpeak at the wave vector transfer Q ∼ 0.076 A

Fig. 6. SANS data from 0.25 M CPyCl micellar solution with varying concentration of NaCl.

˚ Semimajor axis, a (A)

Axial ratio a/b

relation peak is an indication of strong repulsive interaction between the positively charged CPyCl micelles. The peak usually occurs at Qm ∼ 2π/d, where d is the average distance between the micelles and Qm is the value of Q at the peak position. As shown in the figure cross-section increases and the peak position shift to lower Q values with the increase in the salt concentration. This reflects the increase in the size of the micelles in the presence of salt [7,14]. The broadening of the correlation peak is due to screening of charge by the salt between the micelles. The micellar parameters obtained for these systems are given in Table 3. It is seen that in case of NaCl the fractional charge on the micelle remains nearly same but the aggregation number increases when the NaCl concentration in the micellar solution is increased. This indicates that the counter ion has equal affinity to stay in water as well as condense on the micelles. For the same concentration of the salt NaCl and NaBr, the charge neutralization on the ionic CPyCl micelles is different for these salts, and this leads to the formation of different micellar structures when NaCl and NaBr are added. For example the aggregation number of CPyCl micellar solution increases from 104 to 229 upon addition of 1 M NaCl; while aggregation number increases upto 368 by the addition of only 0.2 M NaBr. These results are well supported by viscosity and DLS measurements discussed earlier. The charge neutralization at the surface of the micelle by the increase in the counter ion condensation decreases the effective head group area for the surfactant monomer to occupy in the micelle and hence the increase in the aggregation number of the micelle. The difference in solution properties of Br− and Cl− is that Cl− counter ions are more hydrated. This suggests that the hydrated size of the counter ion similar to the size of the micelle plays an important role to control the properties of micellar solution. The comparison of the micellar parameters in Table 3 suggests that counter ion condensation is more effective to neutralize the charge on the micellar surface when the hydrated size is small. The solid lines in Figs. 6 and 7 are fitted curves to the experimental data using Eq. (5).

4. Conclusions Fig. 7. SANS data from 0.25 M CPyCl micellar solution with varying concentration of NaBr.

We have studied the effect of added salt NaCl and NaBr on micellization and structure of cetylpyridinium chloride

D. Varade et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 259 (2005) 95–101

(CPyCl) micelles. It was found that the micelle size increases with increasing salt concentration above a threshold value. The observed micellar growth is significantly different for NaCl and NaBr. The effect of NaBr on CPyCl micelles was more pronounced in comparison to NaCl. The results are interpreted based on the dependence of the counter ion condensation. Acknowledgements Financial assistance from IUC-DAEF Project No. IUC/CRS-M-103/2001 and help from J. Mata for DLS measurements is gratefully acknowledged. References [1] Y. Moroi, Micelles Theoretical and Applied Aspects, Plenum Press, New York, 1992. [2] D.M. Bloor, J. Gormally, E.W. Jones, J. Chem. Soc. Faraday Trans. 80 (1984) 1915. [3] E. Jungerman, Cationic Surfactants, Marcel Dekker, New York, 1969. [4] J. Cross, E.J. Singer, Cationic Surfactants: Analytical & Biological Evaluation, Marcel Dekker, New York, 1994. [5] , P.M. Holland, D.N. Rubingh (Eds.), Cationic Surfactants: Physical Chemistry, Marcel Dekker, New York, 1991. [6] J.M. Richmond, Cationic Surfactants: Organic Chemistry, Marcel Dekker, New York, 1990. [7] V.K. Aswal, P.S. Goyal, Chem. Phys. Lett. 357 (2002) 491.

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