Micellization of sodium dodecyl sulfate and polyoxyethylene dodecyl ethers in solution

Transcription

1 Colloid Polym Sci (2009) 287: DOI /s z ORIGINAL CONTRIBUTION Micellization of sodium dodecyl sulfate and polyoxyethylene dodecyl ethers in solution Tejas Patel & Goutam Ghosh & Vinod Aswal & Pratap Bahadur Received: 4 April 2009 /Accepted: 1 July 2009 /Published online: 22 July 2009 # Springer-Verlag 2009 Abstract The effect of polyoxyethylene type nonionic surfactants (C 12 E n n=3, 4, 5, 6, 7 and 8) on the aqueous solution of sodium dodecyl sulfate (SDS) in absence and presence of NaCl was examined using small-angle neutron scattering (SANS), dynamic light scattering (DLS), and viscosity measurements. Upon addition of C 12 E n, micellar size of SDS was found to increase significantly, and such micellar elongation was further enhanced in the presence of NaCl. Micellar growth is most significant in presence of shorter moieties of C 12 E n (e.g., n=3, 4) as compared to higher ethereal oxygen content. The results of structural investigations with SANS and DLS to confirm this assumption are reported. The cloud point of C 12 E n has increased upon addition of SDS and decrease with NaCl, and a typical behavior is observed when both SDS and NaCl were present. Keywords Sodium dodecyl sulfate. Polyoxyethylene dodecyl ethers. SANS. Micellar growth T. Patel (*) : P. Bahadur Department of Chemistry, Veer Narmad South Gujarat University, Surat , India tejasrp@gmail.com G. Ghosh UGC-DAE Consortium for Scientific Research, Bhabha Atomic Research Centre, Trombay, Mumbai , India V. Aswal Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai , India Introduction The study of mixed micelles and monolayers is of considerable interest for both theoretical and practical reasons as mixed surfactant systems often provide improvement in the performance [1, 2]. Usually, marked interaction in ionic + nonionic surfactant mixtures has been observed which results in increase in cloud point of nonionics, decrease in Kraft point of ionics, increase in surface activity, and decrease in critical micelle concentration, each one contributing favorably to practical applications of surfactants [3]. Polyoxyethylene dodecyl ethers provide a systematic approach by variations in the hydrophobic/ hydrophilic character that influences surface properties and performance behavior to a great extent [4]. The availability of nonionics as research grade surfactants with varying moles of oxyethylene with low polydispersity provide a possibility for its application in both biochemical research and numerous technological applications. It provides opportunity to understand interaction of nonionic surfactants with ionic ones in mixtures [4]. There have been few recent reports on the mixed surfactant systems comprising of sodium dodecyl sulfate (SDS) and nonionic surfactants [5 13]. For example, Halide et al. [5], using viscosity measurements of mixed micelles composed of sodium dodecyl sulfate and polyoxyethylene dodecyl ethers (C 12 E n with n=4, 10, and 23) in water and 0.1 M sodium chloride solution, showed that the relative viscosity varies at the mixed molar fraction of SDS, between 0.2 and 0.3, and then the relative viscosity of mixed systems decreases with the increasing mole fraction of SDS. Nettesheim et al. [6] have reported the influence of SDS on structure and rheological properties of aqueous solutions of C 12 E 4. Joshi et al. [7] have reported the interaction between SDS and C 12 E 12 /C 12 E 15 in water at

2 1176 Colloid Polym Sci (2009) 287: different mole fractions (0 1) using surface tension, viscometry, and dynamic light scattering (DLS) measurements. Feitosa and Brown [8] have studied the solution properties of mixed micelles formed between the anionic surfactant sodium dodecyl sulfate and the nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C 12 E 5 ) over a wide range of surfactant concentration and temperature by dynamic light scattering, surface tension, and clouding temperature, while Paulo et al. [9] studied the mixed micelles of sodium dodecyl sulfate and tetraoxyethylene dodecyl ether (C 12 E 4 or Brij 30) by surface tension, zeta potential, and fluorescence spectroscopy measurements. However, the effect of nonionic head group on the formation and properties still remains to be completely clarified. In this context, we investigate the effect of the size of the surfactant head group, i.e., the chain length of oxyethylene (EO) of surfactants, and composition of nonionic polyoxyethylene dodecyl ether (C 12 E n ) on the micellization of aqueous anionic SDS. Our study is entirely different in which we have used viscometry, cloud point measurements, DLS, and small-angle neutron scattering (SANS) techniques to gain better understanding of the mechanisms of micellar growth. Materials and methods Sodium dodecyl sulfate and nonionic surfactants polyoxyethylene dodecyl ethers (C 12 E n, n=3, 4, 5, 6, 7 and 8) were from Fluka and used as received. The Kraft point and critical micelle concentration (CMC; at 30 C) of SDS were 8 C and 8.0 mm, respectively. The CMCs of C 12 E n (at 25 C) and cloud points are in good agreement with the literature values [14]. Surfactant C 12 E 3 C 12 E 4 C 12 E 5 C 12 E 6 C 12 E 7 C 12 E 8 Cloud point < ( C) CMC (mm) Triple-distilled water from an all-pyrex glass apparatus was used for the preparation of solutions. D 2 O (>99.4% atom% D) was obtained from heavy water division of Bhabha Atomic Research Centre, Trombay. The micellar solutions of 50 mm SDS were prepared in H 2 O (or D 2 O for SANS measurements) in presence of 0, 0.1, 0.25, 0.5 and 1 M NaCl and varying concentration of nonionic C 12 E n Viscosity The viscosities were measured 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 and after each measurement. The flow time for constant volume of solution through the capillary was measured with a calibrated stopwatch. Dynamic light scattering The DLS experiments were carried out in a homebuilt set up. The incident beam is generated from a vertically polarized 100 mw He Ne laser source (λ=532 nm) fixed at one arm of a goniometer. The scattered beam is passed through a vertical polarizer and counted by a photomultiplier tube (PMT) at 90, mounted on the other arm of the goniometer. Mixed polymer surfactant solutions were equilibrated for 8 h before measurement. Surfactant solutions were filtered through 0.22-μm filter papers (Millipore) and loaded into an optical quality 8 mm diameter cylindrical quartz cell, which was placed inside a borosilicate cuvette consisting of index matching liquid (trans-decalin) 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 (7132 correlator with 4700 autosizer software, Malvern, UK). Details about the setup could be seen elsewhere [15]. All measurements were carried out at 30 C. The average decay rate (Γ) of the electric field autocorrelation function, g 1 (τ), was estimated using the method of cumulants [16]. The apparent diffusion coefficients (D) of the micelles were obtained from the relation, Γ ¼ Dq 2 ð1þ Where q is the magnitude of the scattering vector, given by q ¼ ½4pn sinðq=2þ=lš ð2þ n being the refractive index of the solvent, λ the wavelength of laser light, and θ the scattering angle. The corresponding hydrodynamic diameters (d) were calculated using the Stokes Einstein relation. For all the solutions, Γ varies linearly with q 2 indicating translational diffusion of the scatters. Small-angle neutron scattering The small-angle neutron scattering experiments were performed using a diffractometer at the Dhruva reactor, BARC, Trombay, India [17]. The data were normalized to a cross-sectional unit using standard procedures. The scattering cross section per unit volume measured as a

3 Colloid Polym Sci (2009) 287: function of scattering wave vector can be expressed as [18], hd dσ dω ¼ n ð r m r s Þ 2 V 2 FðQÞ 2 E þ hfðqþ i i 2 ðsðqþ 1Þ þ B ð3þ where n denotes the number density of micelles, and ρ m and ρ s represent scattering length densities of the micelle and the solvent, respectively. V is the volume of the micelle. F(Q) ands(q) are the form factor and structure factor, respectively, and B represents incoherent scattering background due to hydrogen atoms in the solvent. The single particle form factor has been calculated by treating the micelle as prolate ellipsoidal. The corresponding form factor is given by [19]. Z F 2 1 ðqþ ¼ 0 h i FQ; ð mþ 2 dm ð4þ Fig. 2 Viscosity behavior of 50 mm SDS in 0.1 M NaCl in presence of C 12 E n at 30 C. Squares C 12 E 3, circles C 12 E 4, triangles C 12 E 5, inverted triangles C 12 E 7, diamonds C 12 E 8 hfðqþi 2 ¼ FQ; ð mþ ¼ 2 4 Z 1 0 FQ; ð m 3 2 Þdm5 3 ð sin x x cos x Þ x 3 ð5þ ð6þ x ¼ Qa 2 m 2 þ b 2 1 m 2 1=2 ð7þ where a and b are the semi major and semi minor axes, respectively, and μ is the cosine of angle between the directions of a and wave vector Q. The inter-particle structure factor S(Q) for ellipsoidal micelles is calculated using mean spherical approximation as developed by Hayter and Penfold [20]. In SANS data analysis, semi-minor axis (a), semi-major axis (b), and effective fractional charge per head (α) are taken as fitting parameters. The aggregation number is calculated by the relation N=4πab 2 /3v, where v is the volume of the surfactant tail. Fig. 1 Clouding behavior of 50 mm C 12 E 7 in different concentrations of NaCl in presence of SDS. Squares H 2 O, circles0.1 M NaCl, triangles 0.25 M NaCl, inverted triangles 0.5 M NaCl, diamonds 1.0 M NaCl Fig. 3 Hydrodynamic diameters values for 50 mm SDS in 0.1 M NaCl in presence of C 12 E n at 30 C. Squares C 12 E 3, circlesc 12 E 4, trianglesc 12 E 5, inverted trianglesc 12 E 7, diamondsc 12 E 8

4 1178 Colloid Polym Sci (2009) 287: Table 1 Micellar parameters of 50 mm SDS in 0.1 M NaCl in presence of 40 mm C 12 E n for n=3, 4, 5, 7, and 8 as obtained from the DLS data analysis at 30 C. The radius of each aggregate was taken as r=16.7å Number Aggregation number N Experimental value of R h (Å) Calculated length of rod (Å) SDS Results and discussion Viscosity and DLS measurements Clouding behavior The cloud points (CPs) of nonionic surfactants solutions (50 mm) in water and salt solutions in the presence of SDS were measured. A representative figure for C 12 E 7 is shown in Fig. 1. It can be seen that CP shows a sharp increase on addition of little amount of SDS. The CP increases to 100 C at SDS concentration as low as 1 mm. The addition of ionic surfactant to micelles of C 12 E 7 micelles results in formation of mixed micelles with some surface charge. This will result in charge repulsion between the micelles, hindering their aggregation and coacervation to subsequently increase the CP. Addition of NaCl provides a pronounced minimum in the CP value. Earlier reports [21, 22] have investigated mixed micellization of Triton X SDS. This is due to the fact that with presence of NaCl, the original charge distribution of the mixed micelles is screened; therefore, more amount of nonionic surfactant is required to attain CP in presence of added salt. The viscosity and the DLS measurements were performed under identical conditions with aqueous solution of SDS fixed at 50 mm and varying concentrations of nonionic C 12 E n surfactants (0 60 mm). Figure 2 shows the relative viscosity, η rel, of 50 mm SDS solutions, in presence of 0.1 M NaCl, as a function of C 12 E n for n=3,4,5,7,and8. The viscosity remained unchanged up to 30 mm, and only a slight increase was observed at higher concentration for C 12 E 7 and C 12 E 8 with long polyoxyethylene moieties. On the other hand, significant increase in η rel was observed for surfactant with shorter EO chain length, e.g., for n=3, 4, and 5, where C 12 E 3 was the most effective, and the solution became highly viscous above 25 mm. In Fig. 3, from DLS experiments, the change in hydrodynamic diameter, D h, in above solutions with addition of C 12 E n are shown as measured. Here also, the micellar hydrodynamic diameter (D h ) increases where the relative viscosity increases as discussed above. Therefore, increase in viscosity signifies the increase in frictional Fig. 4 Viscosity behavior of 50 mm SDS at different concentrations of NaCl in presence of C 12 E 4 at 30 C. Squares 0.1 M NaCl, circles 0.25 M NaCl, triangles 0.5 M NaCl, inverted triangles 1.0 M NaCl Fig. 5 Change in hydrodynamic diameters for 50 mm SDS in different concentrations of NaCl in presence of C 12 E 4 at 30 C. Squares 0.1 M NaCl, circles 0.25 M NaCl, triangles 0.5 M NaCl, inverted triangles 1.0 M NaCl

5 Colloid Polym Sci (2009) 287: Fig. 6 SANS plot for 50 mm SDS in 0.1 M NaCl in presence of 50 mm C 12 E n at 30 C. Triangles C 12 E 3, squares C 12 E 5, circles C 12 E 8 force at the interface between micelle and water which essentially is due to the unidirectional growth of the micelle [23]. Using an own developed MATLAB program for free search of shape based on the input experimental values of hydrodynamic diameter (D h ), we analyzed the micellar shape for all concentrations and molecular moieties of C 12 E n applying different shape relationships, like Perrin's relations for ellipsoids and Broesmer's relation for rod [24]. In doing so, we have considered the inter-micellar interaction to be negligible. In Table 1, we have shown the structural parameters for pure 50 mm SDSandcomplexesof50mMSDSand40mMC 12 E n with n=3,4,5,7,and8inpresenceof0.1mnacl,as obtained from above analyses. These results clearly indicate that the micelle of SDS transforms from a spherical to a rodlike shape upon addition of C 12 E n.the transformation is more significant if the shorter polyethylene chain length of C 12 E n (e.g., for n 5), i.e., for higher lipophilic/lipophobic ratios (R). This result is significant as R has a contribution to the Gibbs' free energy of micellization. Hence, both viscosity and DLS results are in consistent with each other. The D h in DLS results show a plateau at higher concentrations of C 12 E n, which is not observed from viscosity results. This could be interpreted in terms of its partitioning in aqueous phase, which affects the water structure and causes some sort of destabilization of the micelle. Because of partitioning, the concentration of C 12 E 3 at micelle water interface would increase the size of the micelles, while aqueous partitioning would decrease the size. These two opposite tendencies may impart a near constancy to the D h values at higher [C 12 E 3 ]. It is to be noted here that the effect of C 12 E n on cationic micelles is much stronger [25] as compared to SDS micelles. Also, the appearance of the plateau region at higher concentration of C 12 E n in SDS is not as clear as in case of cationic micelles. These facts may indicate that the intramicellar binding in SDS micelles are relatively stronger than those of cationic surfactants. The viscosity and DLS measurements for above mixed systems were also performed at varying NaCl concentrations. The effect of NaCl on the size of SDS micelles in 50 mm solution in presence of C 12 E 4 is shown in Figs. 4 and 5. As depicted from Fig. 4, the viscosity increases sharply with increase in NaCl concentration, attributed to the sphere-to-rod transition of micelles [25, 26]. The viscosity of 50 mm SDS solution in presence of 0.1 M NaCl remains almost unchanged up to 50 mm C 12 E 4, beyond which the value increased slightly. As the NaCl concentration (say, 0.25 M) was increased in the solution of mixed SDS and C 12 E 4, the viscosity was also seen to increase, which could be due to the micellar growth, and the value suddenly increased at higher NaCl concentrations (i.e. above 0.5 M), indicating a sphere-to-rod transition in the micellar structure. Viscosity result is also supported by DLS measurements, as shown in Fig. 5, where a sharp increase in size (D h ) at higher NaCl concentration is observed. The presence of NaCl induces intake of surfactant molecules in the micellar core. The charge neutralization on head group decreases the repulsion at the shell region. This creates hydrophobic environment to facilitate accumulation of surfactant molecules and increase the size. Fig. 7 SANS plot for 50 mm SDS in 10 mm C 12 E 4 at 30 C. Squares 0.1 M NaCl, circles 0.25 M NaCl, triangles 0.5 M NaCl, diamonds 1.0 M NaCl

6 1180 Colloid Polym Sci (2009) 287: Table 2 Micellar parameters of 50 mm SDS in 0.1 M NaCl in presence of 50 mm C 12 E n as obtained from the SANS data analysis at 30 C [C 12 E n ] (mm) Aggregation number N Semimajor axis a (Å) Semiminor axis b=c (Å) Fractional charge C 12 E C 12 E C 12 E Small-angle neutron scattering Small-angle neutron scattering measurement was carried out on the aqueous solution of SDS/C 12 E n mixture in order to provide a supportive structural evidence for the DLS and viscosity data. SANS results for 50 mm SDS in 0.1 M NaCl in presence of 50 mm C 12 E n at 30 C are shown in Fig. 6. The SANS profiles showing the effect of different concentration for NaCl in 50 mm SDS in 10 mm C 12 E 4 are shown in Fig. 7. It is clearly observed that, on addition of C 12 E n, the scattering intensity increases drastically, which indicates micellar growth. SANS results show the large scattering cross section for SDS in the low-q region indicating the strong effect of C 12 E 3 in inducing the growth of micelles. The axial ratio attains almost constant value at higher concentration of additive, which is in good agreement with the DLS results discussed earlier. As shown in Table 2, the aggregation number for SDS micelles shows a dramatic increase in presence of C 12 E 3. While in case of higher C n E m, the tendency to induce micellar growth was found to decrease with increase in size of polyoxyethylene moiety. This phenomenon can be explained in terms of their relative solubility in water. Increase in polyoxyethylene segment adds to solubility of a certain C n E m in aqueous phase. We would like to point out here that the aggregation numbers obtained from DLS data analyses (Table 1) significantly match with those obtained from SANS data analyses, which indicates the consistency of the technique. Among the nonionics examined, C 12 E 8 with large oxyethylene units has highest solubility in water, while C 12 E 3 has poor solubility due to lesser content of oxyethylene. C 12 E 3 tries to get solubilized in the inner core region and induces micellar growth. The higher oxyethylene chain forces the nonionic surfactant to remain in aqueous solution, away from hydrophobic micellar core region. The extent of influencing the micellar size was found in order of C 12 E 3 >C 12 E 5 >C 12 E 8. In Table 3 are recorded micellar parameters for 50 mm SDS in presence of 10 mm C 12 E 4 at different salt concentrations. It can be seen that aggregation number increases gradually with increase in salt concentration. This can easily be understood in terms of charge neutralization at the shell that subsequently reduces the repulsion between the head groups, and resultant induction of hydrophobicity allows aggregation of more surfactant molecules. The fractional charge was calculated for SDS aggregates in presence of 0.1 M NaCl which showed lesser value (0.29) in comparison to salt-free solution due to charge neutralization. In presence of higher content of salt, the large sizes of micelles are found to be due to neutralization of charge at the head group. It is difficult to calculate this decrease in the fractional charge of the micelles at higher salt concentrations since no correlation peak is observed as a result of the strong screening of the charge between the micelles. In these systems, data are fitting considering S(Q)~1. Conclusion We have studied the effect of polyoxyethylene type nonionic surfactants (C 12 E n n=3,4,5,6,7,and8)on the aqueous solution of SDS in absence and presence of NaCl using SANS, DLS, and viscosity measurements. Viscosity and DLS results are qualitatively supported by SANS measurements. Addition of C 12 E n induces micellar elongation of SDS which is further enhanced in the presence of NaCl. Micellar properties (elongation) can be easily tuned by either increasing the C 12 E n concentration or by a decrease in n (smaller headgroup size) at a fixed concentration of C 12 E n indicating that the lipophilic Table 3 Micellar parameters of 50 mm SDS in 10 mm C 12 E 4 in presence of different concentrations of NaCl as obtained from the SANS data analysis at 30 C [NaCl] (M) Aggregation number N Semimajor axis a (Å) Semiminor axis b=c (Å)

7 Colloid Polym Sci (2009) 287: contribution inside the aggregate plays a major role in the growth of micelle. In addition to this, it is also possible to achieve this by increasing the temperature at a fixed composition. It is interesting to point out that such sharp mixed systems may have a huge capability as a smart material. Acknowledgment Financial assistance from CSIR project no. 01 (2068)/06/EMR-II is gratefully acknowledged. References 1. Ogino K, Abe M (1992) Mixed surfactant systems. Marcel Dekker, New York 2. Shiloach A, Blankschtein D (1998) Measurement and prediction of ionic/nonionic mixed micelle formation and growth. Langmuir 14: Abe M, Scamehorn JF (2005) Mixed surfactant systems, 2nd edn. Marcel Dekker, New York 4. Rubingh D, Mittal K (1979) Solution chemistry of surfactants. Plenum, New York 5. Halide A, Taliha S, Mehmet I (2003) Effect of polyoxyethylene chain length and electrolyte on the viscosity of mixed micelles. Turk J Chem 27: Nettesheim F, Zipfel J, Lindner P, Richtering W (2001) Influence of SDS on structure and rheology of aqueous solutions of nonionic surfactant tetraethylene glycol mono - dodecylether (C 12 E 4 ). Colloids Surf A: Physicochem Eng Aspects : Joshi T, Mata J, Bahadur P (2005) Micellization and interaction of anionic and nonionic mixed surfactant systems in water. Colloids Surf A: Physicochem Eng Aspects 260: Feitosa E, Brown W (1998) Mixed micelles of the anionic surfactant sodium dodecyl sulfate and the nonionic pentaethylene glycol mono-n-dodecyl ether in solution. Langmuir 14: Paulo H, Oliveira M, Gehlen MH (2002) Characterization of mixed micelles of SDS & tetraoxyethylene dodecyl ether in aqueous solution. Langmuir 18: Fan Y, Cao M, Yuan G, Wang Y, Yan H, Han C (2006) Aggregation behavior in mixed system of doubled chain anionic surfactant with single chained nonionic surfactant in aqueous solution. J Colloid Inter Sci 299: Glenn K, Bommel A, Bhattacharya S, Palepu R (2005) Self aggregation of binary mixtures of sodium dodecyl sulfate and polyoxyethylene alkyl ethers in aqueous solution. Colloid Polym Sci 283: Penfold J, Tucker I, Thomas R, Staples E, Schuermann R (2005) Structure of mixed anionic/nonionic surfactant micelles: experimental observations relating to the role of headgroup electrostatic and steric effects and the effects of added electrolyte. J Phys Chem B 109: Islam MN, Kato T (2005) Effect of temperature on the surface phase behavior and micelle formation of a mixed system of nonionic/anionic surfactants. J Colloid Inter Sci 282: Van Os NM, Haak JR, Rupert LAM (1993) Physico-chemical properties of selected anionic, cationic and nonionic surfactants. Elsevier, Amsterdam 15. Mata J, Varade D, Ghosh G, Bahadur P (2004) Effect of tetrabutylammonium bromide on the micelles of sodium dodecyl sulfate. Colloids Surf A: Physicochem Eng Aspects 245: Bern B, Pecora R (1976) Dynamic light scattering. Wiley, New York 17. Aswal VK, Goyal PS (2000) Small-angle neutron scattering diffractometer at dhruva reactor. Curr Sci 79: Chen SH, Lin TL (1987) Colloidal solutions. In: Skold K, Price DL (eds) vol. B 23. Academic Press, New York 19. Pedersen JS (1997) Analysis of small-angle scattering data from colloids and polymer solutions: modeling and least-squares fitting. Adv Colloid Interface Sci 70: Hayter JB, Penfold J (1981) An analytic structure factor for macroion solutions. Mol Phys 42: Mata J (2006) Hydrodynamic and clouding behavior of Triton X SDS mixed micellar systems in the presence of sodium chloride. J Disp Sci Technol 27: Marszall L (1988) Cloud point of mixed ionic-nonionic surfactant solutions in the presence of electrolytes. Langmuir 4: Kabir-ud-Din, Siddiqui US, Ghosh G (2009) Growth of gemini surfactant micelles under the influence of additives: DLS studies. J Disp Sci Technol 30: Young CY, Missel PJ, Mazer NA, Benedek GB, Carey MC (1978) Deduction of micellar shape from angular dissymmetry measurements of light scattered from aqueous sodium dodecyl sulfate solutions at high sodium chloride concentrations. J Phys Chem 82: Kadam Y, Patel T, Ghosh G, Bahadur P (2009) Mixed micellization of n-alkyltrimethylammonium bromides and dodecyl polyoxyethylene ethers. J Disp Sci Technol 30: Patel T, Ghosh G, Aswal V, Bahadur P (2009) Structural characteristics of the aqueous mixed nonionic-cationic surfactants: effect of chain length, head group and temperature. Colloids Surf A: Physicochem Eng Aspects 33: