Langmuir 1994,10, 4069-4072
4069
Growth of Sodium Dodecyl Sulfate Micelles in the Presence of n-Octylamine Sanjeev Kumar,? V. K. Aswal,* H. N. Singh,? P. S. Goyal,* and Kabir-ud-Din*9t Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India, a n d Solid State Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India Received May 2, 1994. I n Final Form: August 18, 1994@ The effect of addition of n-octylamine (OA)on structural transition in aqueous (D2O)micellar solutions of SDS (sodium dodecyl sulfate) was studied by viscosity and small-angle neutron scattering (SANS) at various temperatures. The distinct rise in relative viscosity of SDS solution on the addition of a certain OA concentration (-0.03 M)has been attributed to change in micellar shape from a sphere to a rod. At higher OA concentration, these rods overlap giving rise to large increase in viscosity. SANS distribution showed a well-defined peak for 0.3 M SDS. With the OA addition (0.02-0.065M)shift in the peak position toward low &-valueswas observed. This is due to increase in intermicellar distance. The SANS distribution of 0.3 M SDS 0.065 M OA, studied as a function of temperature, showed that micellar size decreases on heating. The SANS spectra were analyzed using various existing models. The micelle dimension (bla) and mean aggregation number ( 5 )were computed over a range of OA concentrations and temperatures. The observed results are interpreted in terms ofmixed micelle formation with concomitantmicellar growth.
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Introduction The simplest possible aggregate of surfactant molecules is the micelle.’ At the molecular level, a balance of interfacial forces controls the curvature of the surfactant film which, in turn, determines the shape ofthe surfactant aggregates. I t is now accepted that micelles are spherical near critical micellar concentration (cmch2 Transition of spherical to larger micelles for ionic surfactants occurs upon a reduction of interhead-group r e p u l ~ i o n . It ~ ?may ~ be caused by salt5 or surfactant addition^^,^ or solute s o l u b i l i ~ a t i o n .I~t~is~ not clear if, at fixed surfactant concentration, micellar growth by increasing salt concentration is because of enhanced screening of the electrostatic interactions or due to change in “electrical charge per head group”. The solution viscosity responds to changes in both the structure of aggregates and their mutual interactions. In dilute solutions (where these interactions are minimized) the technique is sensitive to the shapes of particles in the solution.1° Transition from small to large micelles is accompanied by a significant increase in viscosity11and appearance of anisotropic susceptibilities.12However, care must be taken in analyzingviscosity data in terms of shape transition because the equiviscous solutions of a surfactant system (surfactant additives) could have micelles which
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* To whom the correspondence should be addressed. of Chemistry, AMU. Solid State Physics Division, BARC. Abstract published in Advance ACSAbstracts, October 1,1994. (1)Chevalier, Y.;Zemb, T. Rep. Prog. Phys. 1990,53,279. (2)Mukerjee, P. Adv. Colloid Interface Sei. 1967,I, 241. (3)Mazer, N. A.; Benedek, G. B.; Carey, M. C. J.Phys. Chem. 1976, 80,1075. (4) Missel, P. J.;Mazer,N. A.; Carey, M. C.; Benedek, G. B. InSolution Behaviour of Surfactants; Mittal, K. L., Fender, E. J., Eds.; Plenum: New York, 1982;p 373. (5)Khatory,A.; Kern, F.; Lequeux, F.;Appel, J.;Porte, G.; Morie, M.; Ott, A.; Urbach, W. Langmuir 1993,9,933. (6)Sheu, E.Y.;Chen, S. H.; Huang, J. H. J.Phys. Chem. 1987,91, 1535. (7)Bezzobotonov, V. Y.;Borbely, S.; Cser, L.; Farago, B.; Gladkih, I. A.; Ostanevich, Y.M.; Vass, S. Z. J. Phys. Chem. 1988,92,5730. (8)Lindemuth, P. M.; Bertrand, G. L. J.Phys. Chem. 1993,97,7769. (9)Prasad, Ch. D.;Singh, H. N.; Goyal, P. S.; b o , K. S.J.Colloid Interface Sci. 1993,155, 415. (10)Wang, J. Colloid Surf. 1993,70,15. (11)Prasad, Ch. D.;Singh, H. N.Colloid Surf. 1991,59,27. (12)Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993,9,1456. t Department
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differ in sizes.13 Recent studies of microscopicaggregates by SANS,S coupled with the study of the macroscopic properties of the system (e.g., viscosity), highlight important links between the microscopic structures and bulk physical properties. SANS technique is more direct to determine aggregation numbers than many other techniques described in literature and is free of any choice of thermodynamic model. Porte e t al.14have shown that the addition of alcohol (1-hexanol) decreases the spontaneous curvature of the surfactant-alcohol film with a concomitant transition of spherical micelles to rods and finally to wide bilayers. Very recently, the sphere-to-rod transition was studied in SDSMaCl solution in presence ofvarious organic additives by solution calorimetry.8 To date, most of the studies on the topic are devoted to aqueous concentrated surfactant systems in conjunction with a salt having a common counterion. Effect of organic additives alone in micellar growth has not received much attention. Such type of study may be useful in industries and as reaction media where presence of a salt is undesirable. The above mentioned facts prompted us to study the role of n-octylamine in micellar growth of anionic SDS which is generally not popular for forming large micelles. It is believed that the formation of globular micelles is more common with SDS.15-17 This is also evident from the Mitchell-Ninham packing parameter for SDS.18 Results of our study on the effect of addition of noctylamine on shape and size of 0.3 M SDS micellar solution in D20, as studied by SANS and viscometrically a t different temperatures, are reported.
Experimental Section “Specially pure” SDS (99%)was obtained from BDH,Poole, U.K. It was further purified by crystallizingtwice with ethanolwater mixture. The purity of the SDS was ascertained by the absence of minimum in the plot of surface tension versus log (13)Rajgopalan, V.; Goyal, P. S.;Valaulikar, B. S.;Dasannacharya, B. A. Physica B 1992,180&181,525. (14)Porte, G.; Marignan, J.;Bassereau, P.; May, R. J.Phys. (Paris) 1988,49,511. (15)Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980,84,744. (16)Mysels, K. J.;Princen, L. H. J. Phys. Chem. 1969,63,1896. (17)Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967,71,1898. (18)Mitchell, D. J.;Ninham, B. W. J. Chem. SOC.,Faraday Trans. 2 1981,77,601.
0743-746319412410-4069$04.50/0 1994 American Chemical Society
Kumar et al.
4070 Langmuir, Vol. 10,No. 11, 1994
>
c-In
24t
22
c
18 16 7 14 s 12 s > 10 -
4
20
0
u
-
Y w
w
1 c
6
-I
w a
0
0
0.04
Y
0.08
w
8 -
0
C n - OCTYLAMINE
J(Molar)
6 -
Figure 1. Relative viscosity (vr) of 0.3 M SDS solution in DzO against concentration of added n-octylamine at 303.15 K.
42 -
01
0
I
1
0-oa
0.04
0
0 (A
I
I
0.12
0.16
-1 )
Figure 3. SANS spectra from 0.3 M SDS for different concentrations of added n-octylamine at 303.15 K the lines
are theoretical fits and the solid circles are data points. The correspondingmolarity of the added n-octylamineis identified by the numbers given above the runs. The runs are shifted vertically 0, 2, 4, 6, and 8 units, respectively.
2.45
3.05
3.15
I/T
(
Table 1. Relative Viscosities (qr) and Activation Free Energies (AG*)for the Viscous Flow of 0.3 M SDS Solutions in De0 in the Presence of n-Octylamine at Various Temperatures
3.25
lo%-'
vr against 1/T plots for 0.3 M SDS at different concentrations of n-octylamine. The molar concentration of added n-octylamine is given above the plots. Figure 2. In
[surfactant]. n-Octylamine (OA) was a Fluka (purum) product while DzO (99.8 atom % D)was obtained from the Heavy Water Division, BARC. These were used as supplied. The viscosity and SANS measurements were performed on 0.3 M SDS in DzO with various OA concentrations (0.0-0.065 M) at different temperatures (30-60 "C). The solvent flow time in the Ubbelohde viscometer was always longer than 200 s and no kinematiccorrectionswere introduced. SANS measurements were made usingthe spectrometerat the CIRUSreactor,BARC.l9 The solutionswere held in a quartz cell of 1.0 cm thickness. The instrument has accessiblewave vector transfer Q ( = 4 d I sin 8/21 range between 0.02 and 0.8 A-1 (Iand 6 represent wavelength of incident neutrons and scattering angle, respectively). The data were collected for Q up t o 0.16 A-l. The instrument has a resolution (AQ/Q) of about 15% at Q = 0.05 A-l. The measured distributionswere correctedfrom the background and the empty cell contributions and the data were normalized to the crosssection units.
Results Viscometry. Figure 1 shows the variation of relative viscosity vr(=r]/ro, 7 and 70being viscosities ofthe solution and solvent D20,respectively) versus OA concentration at 30 "C.The densities ofthe solutions were close to DzO; hence by neglecting kinematic corrections, values of the activation free energy for the viscous flow, AG*, were calculated from the slopes of straight lines obtained from plots of In vr versus 1/T (Figure 2) using eq lZo
In vr = In A
+ AG*IRT
(1)
The values of qr at different temperatures and AG* are given in Table 1.
n-octylamine concn(M) 0.000 0.010 0.020 0.035 0.040 0.050 0.060 0.065
vr in DzO at 30°C 1.51 1.60 1.62 2.05 2.62 8.60 11.40 20.20
AG* (kJ/mol)
40°C 1.47
50°C 1.46
60°C 1.43
1.58 1.89
1.54 1.67
1.50 1.64
1.89 6.64
3.23
2.32
1.84
28.14
5.99
4.22
2.52
55.80
1.44
SANS. The SANS distributions for the 0.3 M SDS/ OA/DzO system, as a function of added OA and temperature, are shown in Figures 3 and 4. It could be seen that the Q value at which intensity maximum occurs (Q,?.,)shifts to smaller Q as the concentration of the OA is increased while temperature increase shows a n opposite effect. The data have been analyzed using the following two approaches. (A) Since the solubility of OA is negligible in water,21 it is assumed to be fully solubilized in the SDS micelles. In this approach, mean aggregation numbers ii (which consist of the numbers of SDS monomers n, and of OA monomers nam in a micelle) based only on the peak are calculated. The different models of positions (Qm) packing used are face-centered cubic (fcc), simple cubic (scZ2),and an empirical relation.23 The relevant expres(19) Desa, J. A. E.; Mazumdar, s.;Sequeira, A.; Dasannacharya, B. A. Solid State Phys. (India) 1985, 28C, 318. (20) Sepulveda, L.; Gamboa, C. J. Colloid Interface Sci. 1987,118, R7
(21)CRC Handbook of Chemistry and Physics, 58th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1978. (22) Chen. S. H.; Shiu, E. Y.: Kalus, J.; H o h a n n , H. J. A .. ppl. Crystallogr. 1988,21, 751. (23) Wu,C. F.; Shen, E. Y.; Bendedouch, D.; Chen, S. H. In Studies o f Double Laver Interaction in Micelle and Protein Solution br SANS: Iknam: Mehco, 1987; p 37.
Growth of SDS Micelles
Langmuir, Vol. 10,No. 11, 1994 4071 are the size parameters (an ellipsoid's semimajor ( b )and semiminor (a)axes) and the charge on the micelle. These parameters can thus be obtained from SANS data by leastsquares fitting of the calculated to the measured distributions. Model for F(Q). The measured dZ(Q)/dQ a t large Q are independent of amine concentration. The large Q data analysis using the form factor of a sphere suggests that one dimension (a)of the micelle remains almost the same (i.e., a = 18.4 A; this is in agreement with the value obtained by Cabane et aLZ6).In view ofthis, it is plausible to assume that OA additionto SDS may form mixed micelle and promote structural growth in one dimension (b).This also explains the high viscosity value of SDS on OA addition (Figure 1). The data were analyzed assuming the above model (i.e., ellipsoidal micelles, a = c t b). The value of a is fxed to be equal to the semiminor axis of ellipsoidalmicelle (18.4A).The parameter forF(Q)is the number of SDS monomers per micelle (n.1. Once the n. is known, the n, can be calculated and the number of OA molecules per micelle (n,) can be found by distributing the OA molecules in all micelles equally. The volume of the micelle is given by
20 22
18
-
14 I5 12 e 10 16
c
Y
w 0
8 -
:I 2
01 0
I
I
0.04
I
I
0.08 0
0 (A
0.12
0.16
-1 )
Figure 4. SANS spectra from 0.3 M SDS
+
0.065
M
n-octylamine at different temperatures. The corresponding temperature of each run is identified by the numbers given above the runs. The runs are shifted vertically 2,4,6,and 8 units, respectively.
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Table 2. Mean Aggregation Number (ii) for 0.3 M SDS n-Octylamine Micellar Systems As Obtained from the Peak Position (Q-) Using Different Models of Packing ii
value using the model
n-octylamine temp concn(M) 0.000 0.020 0.035 0.050 0.065 0.065 0.065 0.065
empirical
("C)
30 30 30 30 30 40 50 60
0.0711 0.0632 0.0553 0.0501 0.0474 0.0553 0.0579 0.0658
fcc 162 246 384 540 665 418 364 249
sc 125 190 296 416 513 323 281 192
relation 175 282 475 712 913 518 439 279
sions relating Qm and ii are given e l s e ~ h e r e . The ~ calculated ii values using the above three models of packing are given in Table 2. (B)The SANS distributions have also been analyzed by Hayter and Penfold type analysis.24 The original model, which was valid for spherical micelles, has subsequently been modified for ellipsoidal shape. The modified form, under the assumption that there are no orientational correlations between micelles, is given asz5
Vm= n,Vs
+ namVm
(3)
where V. and V , are the volumes of SDS and OA monomers, respectively. b of the ellipsoidal micelle is calculated by the relation V , = 4za2bf3and em from em
= (nsbs + nambam)"
(4)
where b, and b , are the scattering lengths of the SDS and OA molecules, respectively. Model for S(Q). The S(Q) is related to the Fourier transform of the particle pair correlation function,g(r).In the approximation that the center of mass and angular correlation of the micelles can be decoupled, the SCQ) for an isotropic system has the e x p r e s s i ~ n ~ ~ ~ ~ ~
where CCQ) is given by
P(Q) is the square of the F(Q) and p is the cosine of the angle between the scattering vector Q and the direction of the major axis of the ellipse. The factor C(Q)tends to be unity as Q approaches zero. The analysis of the scattering cross section thus reduces to the calculation of g(r). In the present work it is obtained by considering equivalent spherical micelles interacting via the Coulomb potential uc(r),which is given by
uc(r)= u,a/r exp[-k(r - a)] where dX(Q)/dQis differentialscattering cross section from a surfactant solution containing ellipsoidal micelle of volume V , present a t number density n, and of coherent scattering length density emdispersed in a medium (DzO) of scattering lengthdensity es. The form factor F(Q) is defined as lNJV,eQi dr and the angular bracket denotes the average overall possible orientations of the ellipsoid with respect to Q. S(Q)is the structure factor which arises from interference effects between particles. The parameters required to calculate dZ(Q)/dQ in the above method (24)Hayter, J. B.;Penfold, J. Mol. Phys. 1981,42, 109. (25)Berr, S.S.J. Phys. Chem. 1987,91,4760.
(7)
where u is the equivalent particle diameter, k-l is the Debye-Huckel screening length (which depends on surfactant concentrationand fractional charge on the micelle, a),and uo is the contact potential given by ug =
4e2(ans)2
€42
+ kI2
(8)
(26)Cabane, B.;Duplessix, R.; Zemb, T. J. Phys. (Paris) 1985,46, 2161. (27)Chen, 5. H. Annu. Rev. Phys. Chem. 1986,37,351.
4072 Langmuir, Vol. 20, No. 11, 1994
Kumar et a2.
Table 3. Micellar Parameters for 0.3 M SDS-n-Octylamine System Obtained from Hayter-Penfold-type Analysis
to the difference in the curvature elasticity ofthe spherical end-caps and the cylindrical part of the micelles. The micelle reorganization in solution must require energy n-octylamine temp since the size decrease implies an increase in surface area, concn(M) ("C) a u(& b ( A ) blu n, nam ii different distribution of alkyl chains, and higher ionization degree.33 The viscosity is then controlled by both lifetime 30 0.067 18.4 34.70 1.88 139 0 139 0.000 30 0.056 18.4 45.00 2.44 171 12 183 0.020 of the micelles and reptation phenomenon34which explains 30 0.051 18.4 57.00 3.10 210 25 235 0.035 the high values of AG* (Table 1). 0.050 30 0.068 18.4 81.90 4.45 290 50 340 SANS-distribution from pure SDS (Figure 3) shows a 30 0.073 18.4 91.75 4.99 314 70 384 0.065 well-defined peak at Q 0.0711A-l. The detailed analysis 0.065 40 0.075 18.4 70.56 3.83 242 54 296 0.065 50 0.068 18.4 58.40 3.17 200 45 245 suggests that SDS micelles are ellipsoidal and have an 60 0.087 18.4 45.60 2.48 156 35 191 0.065 aggregation number 139 (which is in agreement with the literature value'). The values of given in Table 3 are ( E is dielectric constant of the solvent). The only paramsomewhat closer to those obtained from the sc model eters in calculating SCQ) are ii and a. (Table 2). However, the values ofii based on the empirical In the analysis, ii and a were taken as parameters of model are much larger than the results from the Hayterthe fit. The method of computing n, for the given Penfold type analysis. It is felt that the empirical formula aggregation number has been given above. The values of is not applicable. This was also noted in our earlier study.g the best fit parameters, namely n,, nam,ii (=ns + nam),and It may be seen from Table 3 that A increases with a are listed in Table 3. increase in OA concentration. However, the fractional charge on the micelle roughly remains constant with added Discussion OA. As mentioned earlier, n-alkylamines have been found Figure 1 shows that a t low concentrations of OA the to be solubilized in SDS by electrostatic effect and the viscosity remains almost constant but a distinct rise in protonated amine group is left on the surface of the viscosity a t higher OA concentrations is seen. The micelles m i ~ e l l e .This ~ ~ ~charge-induced ~~ solubilization of OA transform from sphere to rod and the length of rod inside the micelle decreases the surface area occupied per increases with increase in OA concentration. At high OA SDS monomer. Side-by-side, the presence of protonated concentration, the rods overlap and this gives rise to OA decreases the inter-head-group repulsion between increase inviscosity.28 OA has been found to be solubilized monomers of the micelle. These two inter-related factors in SDS micelles by electrostatic and hydrophobic effects, are responsible for a large value of ii and growth of the and the amine groups are left on the surface of the micelle (increased bla ratio) and shift in the peak position micelle.29 The solubilization of OA in SDS micelle (Figure 3). ameliorates the electrostatic repulsion between SDS At high temperature the peak shift (Figure 4) toward headgroups, thereby increasing aggregation number and thus promoting micellar growth. Amines, which may exist higher Q values indicates that intermicellar distances are in protonated form, have been noted to be more effective decreasing with the increase in temperature. It may be for electrostatic interactions with anionic surfactant^.^,^^ understood by considering the following facts. When the The availability of OA causes decrease in the surface area temperature is increased by a small value dT the total occupied per surfactant monomer with a concomitant energy added to the system is C, dT (C, being the heat decrease in surface charge density and increase in capacity a t constant pressure). This energy will partially Mitchell-Ninham parameter.18 If V, is the volume of be used in "evaporating" some of the SDS monomers hydrophobic portion of the SDS molecule, 1, is its length, initially attached to the micelles to the bulk solvent. At and A0 the area ofthe polar head, then the increase in this high temperature these SDS monomers are unable to parameter could be understood by considering the SDSremain in solution and form new micelles consisting of a OA couple as a single surfactant. This SDSIOA association smaller number ofmonomers in aggregates. Thus, as the would reduce Ao. Also, because of intercalation of pronumber of micelles increases, the distance between them tonated OA, the alkyl moiety would be embedded into the decreases with a concomitant shift of peak toward higher hydrophobic portion of the SDS monolayer. This pen&-region. These results are consistent with the viscosity etration will result in increasing the volume ofthe micellar data. core, which is equivalent to increasing Vc.31 This seems From the above discussion we can conclude that OA to result in increase in VJl&ovalue. Therefore, SDS-OA addition to SDS micellar solutions causes the formation should have a tendency to form long cylindrical micelles, of mixed micelles and promotes micellar growth. This and it seems to do so as reflected by increase in viscosity (Figure 1)and bla ratio (Table 3). Hartel and H ~ f f m a n n ~ ~ preposition was confirmed by increased value of bla. To put forth such arguments to design lyotropic nematics. our knowledge, ii and a for high SDS concentration and The data shown in Table 1indicate that viscosity of the with a protonated organic molecule (OA) which screens solution decreases distinctly on increase in temperature. out intramicellar Coulombicrepulsion have been obtained SANS data (Table 3) also suggest that micelles decrease for the first time. in size on heating. Table 3 also shows that f'i is exponentially decreasing with temperature and therefore Acknowledgment. We thank Dr. B. A. Dasannathe free energy for the viscous flow, AG*, can be linked charya and Dr. K. R. Rao for useful discussions. Financial support for this work by the Inter-University Consortium, (28) Ekwall, P.;Mandell, L.; Fontell, K. Mol. Cryst.Liq.Cryst. 1969, India, is gratefully acknowledged (IUC:(PB-19):91-92/ 9,157. (29)Yamashita,T.;Yano, H.; Harada, S.;Yasunaga, T.J.Phys. Chem. 1182). 1983,87,5482. (30) VenebIe, R. L.; Elders, K. L.; Fang, J. J. Colloid Interface Sci. 1986,109,330. (31)Lin, Z.;Cai, J. J.; Scriven, L. E.; Davis, H. T.J . Phys. Chem. 1994,98,5984. (32) Hartel, G.; Hoffmann, H. Liq.Cryst. 1989,5,1883.
(33) Lindman, B.; Wennerstrom, H. In Micelles: Topics in Current Chemistry; Springer-Verlag: BerlinlHeidelbergNew York, 1980; Vol. 87, p 5. (34) Cates, M. E.;Candau, S. J. Phys. Condens.Matter 1990,2,6869.
