Effect of tetrabutylammonium bromide on the micelles of sodium dodecyl sulfate

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) Effect of tetrabutylammonium bromide on the micelles of sodium dodecyl sulfate J. Mata a,, D. Varade a, G. Ghosh b, P. Bahadur a a Department of Chemistry, South Gujarat University, Surat , India b IUC-DAEF, Mumbai Centre, B.A.R.C., Trombay, Mumbai , India Received 14 November 2003; accepted 6 July 2004 Available online 28 August 2004 Abstract Micellar behavior of sodium dodecyl sulfate (NaDS) was examined in the presence of tetrabutylammonium bromide (TBABr) by surface tension, viscosity, dynamic light scattering (DLS), dye solubilization, and cloud point measurements. NaDS showed enhanced solubilization properties and a remarkable decrease in surface tension and critical micelle concentration (CMC) in presence of TBABr. Both viscosity and DLS showed growth in NaDS micelles (50 mm) above 100 mm TBABr concentration; sphere-to-rod transition and micellar growth observed till 200 mm, above which solution undergoes phase separation. The results are explained on the basis of the binding ability of bulky tetrabutylammonium ion on NaDS Elsevier B.V. All rights reserved. Keywords: CMC; Micellar growth; Cloud point; DLS; Solubilization 1. Introduction Ionic surfactants get adsorbed from solution on to interfaces and aggregates in solution to form charged micelles through a cooperative association process. The micellar formation takes place above a certain surfactant concentration, the so-called critical micelle concentration (CMC), below which surfactant molecules are present as monomers [1 3]. Such self-assembled systems have been examined for several decades; the most extensively studied surfactant being sodium dodecyl sulfate [4,5]. Micelles provide important functional properties to surfactants, which, in turn, can change in the presence of additives like electrolyte or nonelectrolytes. It is thus not surprising that the effect of different kinds of electrolytes, both with inorganic and organic counter ions, on the critical micelle concentration (CMC), aggregation number (N agg ) and micellar transition for surfactants has been examined in detail [4,5]. Corresponding author. Tel.: ; fax: / address: jitendramata@hotmail.com (J. Mata). Ionic micelles can attract ions or exclude them depending on the electrical charge of the ions and the surfactant head group and thus influence micellar behavior. Inorganic salts decrease CMC, increase N agg and favour sphere-to-rod transitions due to the condensation of counter ions on the charged micelles. Organic counter ions (weakly surface active hydrotropic substances) influence micellar characteristics, due to hydrophobicity of the counter ion and thus a very strong change is sometimes anticipated [6,7]. Mixed surfactants are often used to improve performance, as the mixtures are superior to single surfactant due to the interaction between two differently charged surface-active species [8 10]. Aqueous mixtures of anionic and cationic surfactants have been found to exhibit fundamentally different properties than the corresponding solutions of pure surfactant or mixtures of an ionic and a nonionic surfactant [11,12]. Strong interaction dominated by electrostatic forces has been shown to be present in such oppositely charged systems, which often limit their use due to the formation of insoluble complex. Sodium dodecyl sulfate (NaDS) and tetrabutylammonium bromide (TBABr) show clouding, a phenomenon that is usually observed with nonionic surfactants [13 15] /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.colsurfa

2 70 J. Mata et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) Zana et al. [16] studied the micellar properties of tetraalkylammonium dodecyl sulfates (TAADS) and concluded that the large size of the tetraalkylammonium ions probably limits the value of aggregation number of TAADS micelles and restrict their growth upon increasing surfactant concentration. Our aim was to characterize the micellar properties of NaDS in presence of TBABr, a weakly surface-active material by determining the values of critical micelle concentration (CMC), sphere-to-rod transition, size and shape of the micelle using the combination of several techniques. 2. Materials and methods The anionic surfactant sodium dodecyl sulfate (NaDS) was Purum grade sample from Fluka, Switzerland and was used as received. Surface tension measurements gave a sharp break point for NaDS at CMC ( M) without showing any minimum. Tetrabutylammonium bromide (TBABr) from Fluka, Switzerland was used as received. Water used to prepare solutions was triple distilled from alkaline permanganate. For DLS measurement Milli-Q water having specific resistance 18.7 M was used Surface tension The surface tension of NaDS solutions in absence and presence of TBABr was measured by drop weight method using a modified stalagmometer [17]. The assembly consists of Pyrex glass bulb of spherical shape with a capillary tube attached at filling and dropping ends. The capillary tube at the dropping end was blown into a two-fold U shape and the tip of the end was grounded in the form of a fine cone. By this way, not only the formation of drops of uniform shape and size was ensured but also drops were allowed to break under their own weight. A thoroughly stoppered weighing bottle was attached to the dropping end through a rubber septum. The weighing bottle attached to a dropping capillary tube was placed suspended in another closed long glass tube. The stalagmometer assembly along with the pre-dried and pre-weighed weighing bottle was lowered into a thermostatic water bath maintained at 25 ± 0.1 C. A 30 min time of equilibrium was always allowed. Then a known number of drops (>20) of given solution and reference triple distilled water were allowed to fall into the weighing bottle in separate runs. The weight of solutions as well as triple distilled water drawn from separate runs was instantly recorded on single pan balance. The surface tension of the individual solution was then calculated from known values of surface tension of water, densities and weight of solution and water Dye solubilization For dye solubilization experiments, a water insoluble dye, orange-ot (1-o-tolyl azo-2-naphthol, mol. wt. = 262.3) synthesized by coupling of o-toluidine and 2-naphthol in our laboratory was used. It was recrystallized twice from ethanol. The dye was shaken with an aqueous solution of the surfactant for 48 h at room temperature and then the residue was removed by means of centrifugation and filtration. The absorbance of the resultant solution was then measured at λ = 470 nm using Shimadzu (UV-160A) spectrophotometer Cloud point (CP) Cloud point was determined at fixed concentration of the NaDS (50 mm) in the absence and presence of varying amount of added TBABr (0 200 mm) by gently heating the solution in thin 20 ml glass tubes immersed in a beaker containing water well-stirred with a magnetic bar while being heated. The heating rate of the sample was controlled to 1 C/min. The first appearance of turbidity was taken as the cloud point Viscosity The viscosity measurements were carried out using an Ubbelohde suspended level capillary viscometer. The viscometer was always suspended vertically in a thermostat at 25 ± 0.1 C. The viscometer was cleaned and dried every time before each measurement. The flow time for constant volume of solution through the capillary was measured with a calibrated stopwatch Dynamic light scattering (DLS) The DLS experiments were carried out in a home-built set-up. The incident beam is generated from a vertically polarized He Ne laser source (λ = nm) fixed at one arm of a goniometer. The scattered beam is passed through a vertical polarizer and counted by a photo-multiplier tube (PMT) at 90, mounted on the other arm of the goniometer. Surfactant solutions were filtered through 0.22 m filter papers (Millipore) and loaded into an optical quality 8 ml borosilicate cell, which was placed inside a borosilicate cuvette consisting of index matching liquid and aligned with the axis-of-rotation of the goniometer. Scattered photocurrent from PMT was suitably amplified and digitized before it was fed to a channel digital correlator (Malvern, UK, model auto-sizer 4700). The whole assembly was placed on a vibration free table. All correlation spectra were recorded at 25 C. All spectra were analyzed using the CONTIN [18] software provided by Malvern. 3. Results and discussion The surface tension (γ) of NaDS solutions was measured for a range of concentrations above and below the critical micelle concentration (CMC). As shown in the Fig. 1, a decrease in surface tension of NaDS was observed with increase in concentrations up to the CMC, beyond which no considerable

3 J. Mata et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) Table 1 Critical micelle concentration of NaDS in the absence and presence of TBABr [TBABr] (M) CMC (mm) ST DS Fig. 1. Surface-active behavior of TBABr ( ); surface-active behavior of NaDS in absence and presence of TBABr concentrations. ( ) M, ( ) M, ( ) M, ( ) M, ( ) M, ( ) M. change was noticed. This is a common behavior shown by surfactants in solution and is used to determine their purity and CMCs. Surface-active behavior of TBABr in water shows a marked resemblance to that obtained by Tamaki [19], who reported that quaternary halides do not form micelles but the surface activity increases progressively with the number of carbon atoms in the alkyl group. The surface tension could be decreased up to 50 mn m 1 at 1.0 M, the highest TBABr concentration studied. Such a weakly surface-active behavior of organic hydrophobic ions put them in the class of hydrotropes. Tetrabutylammonium ions show a tendency to be rejected from the aqueous phase as a result of the strong cohesive force between water-water bonding, which is responsible for the adsorption of this large, hydrophobic cation at the interface in contact with their aqueous solution. Surface tension data for NaDS performed in presence of varied TBABr concentrations are also shown in the same figure. The electrical atmosphere in the aqueous NaDS solution altered in presence of TBABr, which neutralizes the effective head group charge resulting in reduced electrostatic repulsion between the polar head groups. Thus micelles are formed at much lower concentration as compared to that in pure water. The CMC of NaDS decreased in the presence of TBABr, the decrease being dependent upon the concentration of TBABr. The CMC data obtained from the break point in the γ log concentration plots for NaDS in absence and presence of TBABr are recorded in Table 1. Solubilizing power is one of the most important properties of surfactants. A number of approaches have been taken to measure solubilizing behavior of surfactants, in which the solubilization of water insoluble dye Orange OT in the surfactant micelles was studied. The obtained results were plotted as absorbance versus concentration of the surfactants. Sol- ubilization plots (Fig. 2) reveal that the amount of the dye solubilized was little up to the CMC of NaDS and thereafter a sudden and steep rise was observed with the formation of micelles in the bulk. It is evident that the solubilizing power of NaDS markedly increases in the presence of TBABr because the micelle swelling becomes more favorable with the reduction of micellar surface charge density. The CMC value for NaDS obtained by surface tension and dye solubilization method agrees well with each other. Variations in CMC values depending on the method of determination have been reported in literature [20,21]. The knowledge of micellar behavior and clouding phenomenon is of both practical and theoretical interest, since the solubilizing power of surfactants depends on their state of aggregation and homogeneous solutions are always demanded in the most practical applications of surfactants. Fig. 3 shows the variation of cloud point (CP) with NaDS (50 mm) in the presence of different concentrations of TBABr. The CP decreases with increase of TBABr concentration. It is well known that nonionic surfactants always have CP properties [22]. While in case of ionic surfactants CP phenomenon is rarely observed [13,23,24]. Clouding behavior has been attributed to dehydration of the hydrophilic group of the surfactant with increasing temperature [25]. It is believed that large Fig. 2. Absorbance of NaDS in absence and presence of dissolved TBABr. ( ) M, ( ) M, ( ) M.

4 72 J. Mata et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) Fig. 3. Cloud point of NaDS (50 mm) as a function of TBABr. amount of added salt, which screen the electrostatic repulsion is thought to be responsible for ionic micellar systems exhibiting CPs. The difference between TBABr and an ordinary salt is that TBA + has a symmetrical structure with four butyl chains and thus possesses a large hydrophobic volume. The butyl chains of such a quaternary salt may penetrate into the micellar interior due to the hydrophobic interaction [26]. Van der Waals attraction and the penetration effect will help in attracting two micelles together, while the electrical repulsion will prevent the micellar contact. Dehydration of the ionic heads of the surfactant monomers takes place on raising the temperature that increases the interaction between anionic heads and TBA +. At the CP the collapse of the micelle occurs, which results in the formation of two phases. More TBABr concentration will replace more structured water and the phase separation is expected to appear at a lower temperature since the NaDS concentration is constant. This is clearly reflected from the figure. Fig. 4 shows that relative viscosity (η rel ) of NaDS (50 mm) increases with increasing TBABr. It can be seen that η rel increases with increasing TBABr concentration. The water structure around the butyl chains penetrating into the micellar core will break down, resulting into the entropy increase of the system, which may be a driving force for the micellar growth. Also the positive charge on the N-atom will decrease the effective charge of the anionic micelle and replacement of water from micellar surface by butyl chains will be responsible for micellar growth [27]. This explanation is well supported by Kabir-ud-Din and coworkers [14] on similar systems by SANS measurements. The variation in micellar size and shape upon addition of TBABr to NaDS (50 mm) solution were measured by DLS. Diffusion coefficients at different TBABr concentrations were obtained by fitting the field correlation curve g 1 (τ) =ʃ(γ )exp( Γτ), where Γ (=q 2 D) is the particle relaxation rate, D is the diffusion coefficient, τ is the decay time. The corresponding micellar sizes were obtained us- Fig. 4. Relative viscosities of NaDS (50 mm) as a function of TBABr. Fig. 5. Apparent diffusion coefficient of NaDS (50 mm) as a function of TBABr concentration. ing Stoke Einstein equation [28]. In Fig. 5 we have shown diffusion coefficient (D) at different TBABr, which clearly indicates the transition in NaDS micelle from one structure to other. Sharp decrease of diffusion coefficient is clearly noticed around 100 mm TBABr concentration, which indicates structural transition of NaDS micelles from sphereto-rod [29]. To obtain the length (L) of the rod, we have used the simple formula for the radius of gyration for a rod, R 2 g = (R2 /2) + 12L 2, where R g = 3/5R for sphere, R H is the measured (using Stoke Einstein equation [28]) hydrodynamic radius of micelle and R was taken as the chain length of NaDS surfactant ( 1.75 nm). 4. Conclusion This study aims at a better understanding of binding ability of bulky tetrabutylammonium ion on NaDS micelles. NaDS

5 J. Mata et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) shows a remarkable decrease in surface tension, CMC and enhanced solubilization power in presence of TBABr. A sharp decrease of diffusion coefficient noticed around 100 mm TBABr concentration clearly indicates structural transition of NaDS micelles from sphere-to-rod. Increased hydrophobic interaction in NaDS-TBABr system shows clouding phenomena, which generally occur with nonionic surfactants. Acknowledgement We are thankful to Dr. P.S. Goyal, Director, IUC-DAEF, BARC, Mumbai for DLS measurement and CSIR Project no.: 01(1827)/02/EMR-II for financial support. References [1] C. Tanford, The Hydrophobic Effect-Formation of Micelles and Biological Membranes, second ed., Wiley, New York, 1980 (Chapter 6). [2] M.J. Rosen, Surfactants and Interfacial Phenomena, second ed., Wiley, New York, 1989 (Chapter 3). [3] D. Myers, Surfactant Science and Technology, second ed., VCH, New York, 1992 (Chapter 3). [4] P. Mukerjee, K.J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Systems, NSRDC-NBS-36, Washington, DC, [5] N.M. van Os, J.R. Haak, L.A.M. Rupert, Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants, Elsevier, Amsterdam, [6] O.R. Pal, V.G. Gaikar, J.V. Joshi, P.S. Goyal, V.K. Aswal, Langmuir 18 (2002) [7] P.A. Hassan, S.J. Candau, F. Kern, C. Manohar, Langmuir 14 (1998) [8] J.F. Scamehorn, Phenomena in mixed surfactant systems, in: J.F. Scamehorn (Ed.), ACS Symposium Series 311, Am. Chem. Soc., Washington, DC, [9] K. Ogino, M. Abe, Mixed surfactant systems, in: P.M. Holland, D.N. Rubingh (Eds.), ACS Symposium Series, Am. Chem. Soc., Washington, DC, [10] P.M. Holland, D.N. Rubingh (Eds.), Mixed Surfactant Systems, Am. Chem. Soc., Washington, DC, [11] M. Bhat, V.G. Gaikar, Langmuir 15 (1999) [12] H. Gandhi, D. Varade, P. Bahadur, Tenside Surf. Det. 39 (2002) 16. [13] S. Kumar, D. Sharma, Kabir-ud-Din, Langmuir 16 (2000) [14] S. Kumar, V.K. Aswal, A.Z. Naqvi, P.S. Goyal, Kabir-ud-Din, Langmuir 17 (2001) [15] S. Kumar, D. Sharma, Z.A. Khan, Kabir-ud-Din, Langmuir 17 (2001) [16] M. Benrraou, B.L. Bales, R. Zana, J. Phys. Chem. 107 (2003) [17] D.V.S. Jain, S. Singh, Ind. J. Chem. 10 (1972) 629. [18] S.W. Provencher, Comput. Phys. Commun. 27 (1979) 227. [19] K. Tamaki, Bull. Chem. Soc. Jpn. 40 (1967) 38. [20] M.E. Haque, A.R. Das, S.P. Moulik, J. Phys. Chem. 99 (1995) [21] A. Ray, G. Nemethy, J. Phys. Chem. (1971) 75. [22] M.J. Schick, Nonionic Surfactants: Physical Chemistry, second ed., Marcel Dekker, New York, [23] G.G. Warr, T.N. Zemb, M. Drifford, J. Phys. Chem. 94 (1990) [24] Z.J. Yu, G. Xu, J. Phys. Chem. 93 (1989) [25] J.B. Hayter, M. Zulauf, Colloid Polym. Sci. 260 (1982) [26] M. Almgren, S. Swarup, J. Phys. Chem. 87 (1983) 876. [27] V.E. Haverd, G.G. Warr, Langmuir 16 (2000) 157. [28] B.J. Berne, R. Pecora, Dynamic Light Scattering: With Applications to Chemistry, Biology and Physics, Wiley, New York, [29] P.A. Hassan, S.R. Raghavan, E.W. Kaler, Langmuir 18 (2002) 2543.