- Email: [email protected]
Effect of counterion size on the viscosity behaviour of sodium dodecyl sulphate micellar solutions
ournaJof
S \-~.:. , , ' . j J
0 LEC U LA R
LIQUIDS ELSEVIER
Journal of Molecular Liquids 75 (1998) 25-32
Effect of counterion size on the viscosity behaviour of sodium dodecyl sulphate micellar solutions* Kabir-ud-Din*, Sara Liz David and Sanjeev Kumar Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India Received 12 August 1996; accepted 28 April 1997
The present studies were performed to see the influence of counterion size on the viscosity behaviour of 0.3M sodium dodecyl sulphate micellar solutions at 30 ° C. Some symmetrical quaternary ammonium salts (R4NBr where R=H, CH3, C=Hs, n-C3H7 or n-C4HQ)and two organic salts (thiamine hydrochloride and guanidine hydrochloride, THCI and GuHCI)) ware used for deriving the counterions. The viscosity decreased, increased and again decreased as the concentration of R4NBr was progressively increased (no initial fall was detected with (n-C4Ho)4NBr). The h,I was always higher with the combination SDS-(C=Hs)4NBr than with SDS-(n-C3HT)4NBr. The results are interpreted in the light of adsorption/intercalation of the counterions or their electrostrictive/hydrophobic nature. Similar reasoning is invoked for THCI addition. The viscosity results of GuHCI addition are discussed with reference to its water structure breaking nature. © 1998 Elsevier Science B.V.
1. INTRODUCTION It is well known that surfactant molecules can organise themselves into spherical aggregates when dissolved into water near cmc. However, in some cases it is possible to obtain other aggregates such as cylindrical micelles, vesicles, etc [1]. On a molecular scale, this change in the aggregate morphology is facilitated by an increase in counterion binding and dehydration of the surfactant head groups and bound counterions [2-4]. Apart from magnitude of the charge, the influence of a counterion has been related to its size [5], its degree of hydration [6,7], its closeness of approach to the micellar surface, its effect on the water structure [8], etc. The less hydrated an ion, better it binds to an oppositely charged surface. Counterions are 'bound' primarily by strong electric field but also by specific interactions that depend upon the nature of the head group and the type of the counterion. Specific counterion effects on a variety of micellar properties
•The authors acknowledge financial support from Inter-University Consortium for Department of Atomic Energy Facilities, India (Grant No. IUC: (PB-41): 94-9512590). • Author to whom correspondence should be addressed.
0167-7322/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PI! SO167-7322(97) 00025-1
26 generally follow a Hofmeister series [9]. However, specificity may also depend upon hydrogen bonding interactions between hydrated counterions and head groups or the partial disruption of the hydration layers of the head groups and counterions, and the possibility that a fraction of the counterions are site bound to the surfactant head groups, e.g., contact ion-pair formation, cannot be excluded. A large number of studies have been performed using surfactants with monovalent head groups that are anionic, e.g., -OSO~-, with alkali metal, alkaline earth, or quaternary ammonium counterions, or cationic, e.g., -N'Me 3, with halide, or hydrophilic mono- and divalent organic and inorganic counterions [10-15]. The quaternary ammonium ions (R4N÷) interact both electrostatically and hydrophobically with the micellar surface. In contrast to metal cations, R4N* cations are essentially non-hydrated. These have been found to displace loosely bound Na÷in the micelles composed of decyl phosphate monoanions by increasing the ionic-strength and lowering the cmc but cannot displace Na÷ ions tightly associated with the head groups. R4N÷ ions are "wrapped in a plastic bag" and cannot interact specifically, but only coulombicallywith anionic head groups and hydrophobicallywith exposed hydrocarbons at the micellar surface. For such type of counterions, an effect on the electrostatic energy is also possible, both directly, because the bulkiness of the ions keeps the positive and negative charges apart, and indirectly, via a decrease of the electric permittivity at the micelle surface when the bulky groups become inserted between the head groups. A detailed account of SDS micelles in presence of simple inorganic counterions can be found in the literature [16-21]. Recently, counterion effect with R4N* type cations is gaining recognition in micellar systems [2,22-25]. Structural interactions in aqueous solutions of these ions have been interpreted as showing that solvent-induced attractive forces exist between two hydrophobic solutes and repulsive forces between a hydrophobic solute and a hydrophilic one [26,27]. These interpretations would suggest that structural interactions are involved in salting-out of non-electrolytes capable of hydrophobic interactions by hydrophilic salts, e.g., NH4Brand (CH3)4NBr; and salting-in by hydrophobic salts, e.g., (n-C3H~)4NBrand (n-C4Hs)4NBr. The effectiveness of cations to salt-out is in the order K* > Na* > NH4*
~
(CH3)4N* > (C2Hs)4N* > (n-C3H;,)4N*
:> ( n - C 4 H g ) 4 N +.
The sequence shows that the salting-out efficiency decreases with increase in both the ion size (Table 1) and the ability of the ion to alter the degree of structure in water (hydrophobic bonding). R4N salts exhibit an ambivalent nature in aqueous solutions. In these ions the single positive charge is buried in a paraffin shell. The salting-in effects of these salts are in contrast to the salting-out effects of the small inorganic salts. In many studies itwas shown that, above a single concentration of electrolyte, micellar shape transforms from spherical to cylindrical (s ---~c) [2-4,16,17]. Typical values of 1:1 electrolyte concentrations to initiate s --~ c transitions are relatively high. Most of the time these studies were preformed using inorganic salts. Since R4N÷cations modify the structure of water around them in a similarway as some simple hydrocarbons do, it could be of considerable interest to see how this interaction affects the viscosity behaviour of surfactant micellar solutions. The reason for selecting R4N salts in this study orginates
2? from the fact that the degree of H-bonding in water alters dramatically along this series of salts [28], e.g., NH4* to (n-C4Hs)4N+. Visualizing the significant properties of R4N+ cations, it was thought worthwhile to pursue a study in order to understand the role of counterion size in concentrated micellar solutions. In this studyviscosity measurements have been performed in 0.3M solutions of sodium dodecyl sulphate with different symmetrical quaternary ammonium counterions. Effect of adding two unique organic cations viz. thiamineH ÷(TH*) and guanidinium (Gull +) were also studied (the organic cations were derived successively from thiamine hydrochloride, I, and guanidine hydrochloride, II): NH2' HCt N ~"
C[-
+ "'r-- CH2 - - N"-~---~ cH3
/
NH2
H2N,.,C H3C
.
I
CH2CH20H
CI.-
~ NH2
II
2. EXPERIMENTAL Sodium dodecyl sulphate (CPC-USA, purity > 99%) was used as supplied. Tetramethyl-and tetraethylammonium bromides were reagent grade chemicals from BDH, England, with stated purities of 98.5 and 98%, respectively, while tetra-n-propyl(>99%) and tetra-n-butylammonium bromides (>99%) were from Merck-Schuchardt, Germany. Thiamine hydrochloride (99%) and guanidine hydrochloride (99%) were from Sigma, USA, and Riedel de-Haen, Hannover, respectively. All the salts were dried for at least 72 h before use in a vacuum drying oven. The temperature during drying was maintained according to the thermal stability and fusion point of the salt. The dried salts were stored over P2Os. Distilled water used for making the solutions was further purified by distillation (twice) over basic permanganate in an all-glass still. The surfactant solutions were prepared by weighing out the requisite amount each time and dissolving it into a freshly prepared salt solution. At higher [salt], viscosities were dependent on the rate of flow. To obtain viscosities under Newtonian flow conditions, a wide U-shaped tube containing water was connected to one of the limbs of Ubbelohde viscometer. This arrangement allowed us to vary the pressure (P) under which the solution flows and thus to determine viscosity vlaues at various rates of flow from the slope of the straight line obtained by variation of P vs lit (where t is the time of flow of the solution). All the experiments were performed at a controlled temperature of 30.0 _+0.1 °C.
28 3. RESULTS The relative viscosities, l~r (---- 1'1/'11o, where -q and 11oare the viscosities of the sample solution and solvent water, respectively), of aqueous micellar solutions of SDS were obtained in presence of various quaternary ammonium salts, THCI and GuHCI. Figures 1 and 2 show the variation of ~, with the salt concentrations.
4. DISCUSSION Figure 1, which depicts the variation of In 11, of 0.3M SDS micellar solutions with [salt] for different quaternary ammonium salts, shows that in few systems (e.g., NH4*) the viscosity, first of all, decreases slightly, then increases and finally decreases again. Here the sufractant concentration is constant (0.3M) and the [salt] is slowly increased which will cause growth of already persent ellipsoidal micelles [29] with simultaneous increase in interparticle distance. The former factor will tend to increase the I'q,I while the latter will decrease it; this effect possibly dominates in the beginning and causes a decrease in TIr. The appearance of minimum was observed with the other quaternary ammonium salts too but was less pronounced. Another point worth noting is that higher the value of m (the carbon chain-length of R4N*), lower is the salt concentration required for appearance of minimum (no minimum was observed with m=4). On further increase in salt concentration, micellar growth (size of the micelle) dominates the overall effect and is responsible for the viscosity rise. In earlier studies, strong counterion binding of R4N÷ ions to anionic micelles have been reported [2, 23]. Compared to inorganic counterions, the R4N÷ are essentially non-hydrated (Table 1) and, therefore, will have only limited objections against binding to the micelle. On the other hand, the ions have a low charge density and a hydrophobic nature. All these factors will contribute towards the interactions with micelles and, therefore, to the bulk viscosity of the solutions.
Table 1 Formal and hydrated radii (A) of some alkali metal and R4N÷ cations Ion Na÷ K÷ NH4* (CHs)4N* (C2Hs)4N÷ (n-C3HT)4N÷ (n-C4Hg)4N* a
Radius Hydrated
Formal
Solvation layer thickness
Reference
3.6 3.3 3.31 3.67 4.00 4.52 4.94
0.95 1.33 1.48 3.47 4.00 4.52 4.94
2.65 1.97 1.83 0.20 0 0 0
4 4 a a a a a
E.R. Nightingale, Jr., J. Phys. Chem., 63 (1959) 1381.
29
3.0
I
2"0 i.-
r-
1"0
I
0
0,2
t,,
0"4
i
i,
0,6
I~
'
J
0.CJ
1.3
1,7
[ SALT]~M Figure 1. Variation of relative viscosities, -q,, of 0.3 M SDS micellar solutions with R4N+ cations at 30°C :R=NH 4, -X-; CH~, a • C~H5, O n-C3H7, A ; n-C4Hg, 4. Viscosity shows steep rise with (n-C4H~)4N÷, although electrostatic interaction will not be as effective as other salts of the series; the reason being its bigger size and hence the charge density on the ion will be less[25]. NH4÷ and all the other members of the series will try to get adsorbed on the negatively charged micelle, the (n-C4Ho)4N*, in addition, will also try to intercalate between head groups of monomers in the micelle (due to hydrophobic interaction of the bigger alkyl part). The intercalation of this hydrophobic cation will provide an opportunity to interact closely (although its size will restrict it). The closer approach due to intercalation will decrease the electrostatic interactions in addition to increased hydrophobic interactions. These two interrelated factors will increase Rp (the so-called MitchelI-Ninham parameter[30]) which, in turn, will be responsible for steep rise in the viscosity of 0.3M SDS with (n-C~H~)~N*cation (our smallangle neutron scattering (SANS) results [31] obtained on few of these systems also confirm the view point as drastic changes in micellar size (semi-major axis, b) and charge per monomer (c0 of the micelle were observed in case of 0.3M SDS + (n-C4Hg)4N*). Another noteworthy observation to be gleaned from the results in Figure 1 is the higher viscosities obtained with (C=Hs)4N+than with (n-C3HT)4N+. This could be interpreted from the point of view that for (C2Hs)4N* there is a balance between the electrostrictive
30 nitrogen charge center and the hydrophobic alkyl groups, but the same balance is not existing for (n-C3HT)4N*. Thus, the resultant of the two forces seems to be the deciding factor for the aggregate size as well as the bulk viscosity of the solution. The fall in viscosity (after the maximum), which occurred at relatively higher [salt], can be understood by considering the following facts. Salt addition will definitely neutralize the charge of the anionic micelle and the micelle would then be like a non-ionic one (at higher [salt]). The electrostatic effect will then work in an opposite way as further addition of the salt will cause adsorption of the counterion. This adsorption will impart a positive charge to the micelles (charge reversal) which diminish destructively to smaller sizes because of the electrostatic repulsion among the micelles (now positively charged) and form the basis of viscosity decrease. Charge reversal of cationic micelles due to excess adsorption of salicylate or benzoate anions with resultant viscosity decrease have been demonstrated successfully by making rheological and electrophoretic light scattering measurements [32-34]. Figure 2 shows the effect of addition of two organic cations (TH÷and Gull*) on the viscosity behaviour of 0.3M SDS micellar solutions. The electrostatic effect of TH ÷will
4.0
tO" ¢=
2.0
---..Z~.C~ 0
L
I
,
0"5
I 1.0
I
I 1.5
[SALT],M Figure 2. Variation of relative viscosities, cations TH* (O) and Gull ÷ (4) at 30 ° C.
"l~r,
of 0.3M SDS micellar solutions with organic
31 be similar but the hydrophobic portion may facilitate the intercalation of this ion between head groups of similar charge (as in the case of (n-C4Hg)4N+). These interactions should cause micellar growth and seems to do so as regular increase in viscositywas observed with SDS - TH ÷ combination. Guanidinium cation (Gull ÷) is known as a strong water structure breaker which is also strongly hydrated [35]. Further, this ion takes coplanar, resonance structures and possesses high polarizability [36]. These effects, in a way, would affect the interactions of Gull ÷with SDS micelle and a compensation seems to take place due to the structure breaking nature of Gull ÷ opposite to that of electrostatic effect. Consequently, almost no variation in ~lroccurred with SDS-GuH ÷ combination. In conclusion, we can say that the viscosities of micellar solutions (and the so-called micellar growth), apart from electrostatic nature of the added ion, are dependent upon the hydrophobic volume as well as the water structure breaking ability of the ion.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. A. Sein and J. B.F.N. Engberts, Langmuir, 11 (1995) 455. J.R. Lu, A. Morrocco, T. Su, R.K. Thomas and J.J. Penfold, J. Colloid Interface Sci., 158 (1993) 303. S.G. Oh and D.O. Shah, J. Phys. Chem., 97 (1993) 284. S. Berr, R.R.M. Jones and S. Johnson, Jr., J. Phys. Chem., 96 (1992) 5611. P. Mukerjee, K.J. Mysels and P. Kapaun, J. Phys. Chem., 71 (1967) 4166. H. Gustausson and B. Lindman, J. Am. Chem. Soc., 100 (1978) 4647. MJ. Schick, J. Phys. Chem., 68 (1964) 3585. K.J. Mysels, Introduction to Colloid Chemistry, Interscience, New York, 1959. C.A. Bunton, F. Nome, F.H. Quina and L.S. Romsted, Acc. Chem. Res., 24 (1991) 357. B. Lindman and H. Wennerstrom, Top, Curr. Chem., 87 (1980) 32. K. L. Mittal and B. Lindman (eds.), Surfactants in Solution, Plenum Press, NewYork, 1982. K.L. Mittal and E.J. Fendler (eds.), Solution Behaviour of Surfactants, Theoretical and Applied Aspects, Plenum Press, New York, 1982. C.A. Bunton and G. Savelli, Adv. Phys. Org. Chem., 22 (1986) 213. M. Gratzel and K. Kalyanasundaram (eds.), Kinteics and Catalysis in Microheterogeneous Systems, Marcel Dekker, New York, 1991. S. Hayashi and S. Ikeda, J. Phys. Chem., 84 (1980) 744. S. Ikeda, S. Hayashi and T. Imae, J. Phys. Chem., 85 (1981) 106. EY. Sheu, C.F. Wu and S.H. Chen, J. Phys. Chem., 90 (1986) 4179. P.J. Missel, N.A. Mazer, M.C. Carey and G.B Benedek, J. Phys. Chem., 93 (1989) 8354. D. Nguyen and G.L. Bertrand, J. Phys. Chem., 96 (1992) 1994. L. S. Romsted and C.O. Yoon, J. Am. Chem. Soc., 115 (1993) 989.
32 22. B. Kachar, D.F. Evans and B.W. Ninham, J. Colloid Interface Sci., 100 (1964) 287. 23. M.Jansson, L. Eriksson and P. Skagerlind, Colloids Surf., 53 (1991) 157. 24. A. Wagenaar, L Streefland, D. Hoekstra and J.B.F.N. Engberts, J. Phys. Org. Chem., 5 (1992) 451. 25. J. Eastoe, B.H. Robinson and R.K. Heenan, Langmuir, 9 (1993) 2820. 26. J.E. Desnoyers, M. Arel, G. Perron and C. Jolicoeur, J. Phys. Chem., 73 (1969) 3346. 27. J.E. Desnoyers and F.M. Ichhaporia, Can. J. Chem., 47 (1969) 4639. 28. W.F. Furter (ed.), Thermodynamic Behaviour of Electrolytes in Mixed Solvents, Adv. Chem. Series 155, Am. Chem. Soc., Washington, 1976. 29. Kabir-ud-Din, S. Kumar, V.K. Aswal and P.S. Goyal, J.Chem. Soc., Faraday Trans., 92 (1996) 2413. 30. J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc., FaradayTrans. 2, 72 (1976) 1525. 31. b changed from 33.4 to 45.8 A and 33.4 to 101.2 Afor (CH3)4N÷ and (n-C4H~)4N÷, respectively, as [R4N÷] was changed from 0 - 0.3 M. However, c~remained more or less constant (0.14 _+0.1) with R=CH s but dropped drastically (~ 0.07) with R = n-C4H9. 32. T. Imae, J. Phys. Chem., 94 (1990) 5953. 33. T. Imae and T. Kasaka, J. Phys. Chem., 96 (1992) 10030. 34. H. Rehage and H. Hoffmann, Mol. Phys., 74 (1991) 933. 35. D.B. Wetlaufer, S.K. Malik, L. Stoller and R.L.Coffin, J. Am. Chem. Soc., 86 (1964) 508. 36. J.T. Edsall and J. Wyman, Biophysical Chemistry, Academic Press, New York, 1958.
S \-~.:. , , ' . j J
0 LEC U LA R
LIQUIDS ELSEVIER
Journal of Molecular Liquids 75 (1998) 25-32
Effect of counterion size on the viscosity behaviour of sodium dodecyl sulphate micellar solutions* Kabir-ud-Din*, Sara Liz David and Sanjeev Kumar Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India Received 12 August 1996; accepted 28 April 1997
The present studies were performed to see the influence of counterion size on the viscosity behaviour of 0.3M sodium dodecyl sulphate micellar solutions at 30 ° C. Some symmetrical quaternary ammonium salts (R4NBr where R=H, CH3, C=Hs, n-C3H7 or n-C4HQ)and two organic salts (thiamine hydrochloride and guanidine hydrochloride, THCI and GuHCI)) ware used for deriving the counterions. The viscosity decreased, increased and again decreased as the concentration of R4NBr was progressively increased (no initial fall was detected with (n-C4Ho)4NBr). The h,I was always higher with the combination SDS-(C=Hs)4NBr than with SDS-(n-C3HT)4NBr. The results are interpreted in the light of adsorption/intercalation of the counterions or their electrostrictive/hydrophobic nature. Similar reasoning is invoked for THCI addition. The viscosity results of GuHCI addition are discussed with reference to its water structure breaking nature. © 1998 Elsevier Science B.V.
1. INTRODUCTION It is well known that surfactant molecules can organise themselves into spherical aggregates when dissolved into water near cmc. However, in some cases it is possible to obtain other aggregates such as cylindrical micelles, vesicles, etc [1]. On a molecular scale, this change in the aggregate morphology is facilitated by an increase in counterion binding and dehydration of the surfactant head groups and bound counterions [2-4]. Apart from magnitude of the charge, the influence of a counterion has been related to its size [5], its degree of hydration [6,7], its closeness of approach to the micellar surface, its effect on the water structure [8], etc. The less hydrated an ion, better it binds to an oppositely charged surface. Counterions are 'bound' primarily by strong electric field but also by specific interactions that depend upon the nature of the head group and the type of the counterion. Specific counterion effects on a variety of micellar properties
•The authors acknowledge financial support from Inter-University Consortium for Department of Atomic Energy Facilities, India (Grant No. IUC: (PB-41): 94-9512590). • Author to whom correspondence should be addressed.
0167-7322/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PI! SO167-7322(97) 00025-1
26 generally follow a Hofmeister series [9]. However, specificity may also depend upon hydrogen bonding interactions between hydrated counterions and head groups or the partial disruption of the hydration layers of the head groups and counterions, and the possibility that a fraction of the counterions are site bound to the surfactant head groups, e.g., contact ion-pair formation, cannot be excluded. A large number of studies have been performed using surfactants with monovalent head groups that are anionic, e.g., -OSO~-, with alkali metal, alkaline earth, or quaternary ammonium counterions, or cationic, e.g., -N'Me 3, with halide, or hydrophilic mono- and divalent organic and inorganic counterions [10-15]. The quaternary ammonium ions (R4N÷) interact both electrostatically and hydrophobically with the micellar surface. In contrast to metal cations, R4N* cations are essentially non-hydrated. These have been found to displace loosely bound Na÷in the micelles composed of decyl phosphate monoanions by increasing the ionic-strength and lowering the cmc but cannot displace Na÷ ions tightly associated with the head groups. R4N÷ ions are "wrapped in a plastic bag" and cannot interact specifically, but only coulombicallywith anionic head groups and hydrophobicallywith exposed hydrocarbons at the micellar surface. For such type of counterions, an effect on the electrostatic energy is also possible, both directly, because the bulkiness of the ions keeps the positive and negative charges apart, and indirectly, via a decrease of the electric permittivity at the micelle surface when the bulky groups become inserted between the head groups. A detailed account of SDS micelles in presence of simple inorganic counterions can be found in the literature [16-21]. Recently, counterion effect with R4N* type cations is gaining recognition in micellar systems [2,22-25]. Structural interactions in aqueous solutions of these ions have been interpreted as showing that solvent-induced attractive forces exist between two hydrophobic solutes and repulsive forces between a hydrophobic solute and a hydrophilic one [26,27]. These interpretations would suggest that structural interactions are involved in salting-out of non-electrolytes capable of hydrophobic interactions by hydrophilic salts, e.g., NH4Brand (CH3)4NBr; and salting-in by hydrophobic salts, e.g., (n-C3H~)4NBrand (n-C4Hs)4NBr. The effectiveness of cations to salt-out is in the order K* > Na* > NH4*
~
(CH3)4N* > (C2Hs)4N* > (n-C3H;,)4N*
:> ( n - C 4 H g ) 4 N +.
The sequence shows that the salting-out efficiency decreases with increase in both the ion size (Table 1) and the ability of the ion to alter the degree of structure in water (hydrophobic bonding). R4N salts exhibit an ambivalent nature in aqueous solutions. In these ions the single positive charge is buried in a paraffin shell. The salting-in effects of these salts are in contrast to the salting-out effects of the small inorganic salts. In many studies itwas shown that, above a single concentration of electrolyte, micellar shape transforms from spherical to cylindrical (s ---~c) [2-4,16,17]. Typical values of 1:1 electrolyte concentrations to initiate s --~ c transitions are relatively high. Most of the time these studies were preformed using inorganic salts. Since R4N÷cations modify the structure of water around them in a similarway as some simple hydrocarbons do, it could be of considerable interest to see how this interaction affects the viscosity behaviour of surfactant micellar solutions. The reason for selecting R4N salts in this study orginates
2? from the fact that the degree of H-bonding in water alters dramatically along this series of salts [28], e.g., NH4* to (n-C4Hs)4N+. Visualizing the significant properties of R4N+ cations, it was thought worthwhile to pursue a study in order to understand the role of counterion size in concentrated micellar solutions. In this studyviscosity measurements have been performed in 0.3M solutions of sodium dodecyl sulphate with different symmetrical quaternary ammonium counterions. Effect of adding two unique organic cations viz. thiamineH ÷(TH*) and guanidinium (Gull +) were also studied (the organic cations were derived successively from thiamine hydrochloride, I, and guanidine hydrochloride, II): NH2' HCt N ~"
C[-
+ "'r-- CH2 - - N"-~---~ cH3
/
NH2
H2N,.,C H3C
.
I
CH2CH20H
CI.-
~ NH2
II
2. EXPERIMENTAL Sodium dodecyl sulphate (CPC-USA, purity > 99%) was used as supplied. Tetramethyl-and tetraethylammonium bromides were reagent grade chemicals from BDH, England, with stated purities of 98.5 and 98%, respectively, while tetra-n-propyl(>99%) and tetra-n-butylammonium bromides (>99%) were from Merck-Schuchardt, Germany. Thiamine hydrochloride (99%) and guanidine hydrochloride (99%) were from Sigma, USA, and Riedel de-Haen, Hannover, respectively. All the salts were dried for at least 72 h before use in a vacuum drying oven. The temperature during drying was maintained according to the thermal stability and fusion point of the salt. The dried salts were stored over P2Os. Distilled water used for making the solutions was further purified by distillation (twice) over basic permanganate in an all-glass still. The surfactant solutions were prepared by weighing out the requisite amount each time and dissolving it into a freshly prepared salt solution. At higher [salt], viscosities were dependent on the rate of flow. To obtain viscosities under Newtonian flow conditions, a wide U-shaped tube containing water was connected to one of the limbs of Ubbelohde viscometer. This arrangement allowed us to vary the pressure (P) under which the solution flows and thus to determine viscosity vlaues at various rates of flow from the slope of the straight line obtained by variation of P vs lit (where t is the time of flow of the solution). All the experiments were performed at a controlled temperature of 30.0 _+0.1 °C.
28 3. RESULTS The relative viscosities, l~r (---- 1'1/'11o, where -q and 11oare the viscosities of the sample solution and solvent water, respectively), of aqueous micellar solutions of SDS were obtained in presence of various quaternary ammonium salts, THCI and GuHCI. Figures 1 and 2 show the variation of ~, with the salt concentrations.
4. DISCUSSION Figure 1, which depicts the variation of In 11, of 0.3M SDS micellar solutions with [salt] for different quaternary ammonium salts, shows that in few systems (e.g., NH4*) the viscosity, first of all, decreases slightly, then increases and finally decreases again. Here the sufractant concentration is constant (0.3M) and the [salt] is slowly increased which will cause growth of already persent ellipsoidal micelles [29] with simultaneous increase in interparticle distance. The former factor will tend to increase the I'q,I while the latter will decrease it; this effect possibly dominates in the beginning and causes a decrease in TIr. The appearance of minimum was observed with the other quaternary ammonium salts too but was less pronounced. Another point worth noting is that higher the value of m (the carbon chain-length of R4N*), lower is the salt concentration required for appearance of minimum (no minimum was observed with m=4). On further increase in salt concentration, micellar growth (size of the micelle) dominates the overall effect and is responsible for the viscosity rise. In earlier studies, strong counterion binding of R4N÷ ions to anionic micelles have been reported [2, 23]. Compared to inorganic counterions, the R4N÷ are essentially non-hydrated (Table 1) and, therefore, will have only limited objections against binding to the micelle. On the other hand, the ions have a low charge density and a hydrophobic nature. All these factors will contribute towards the interactions with micelles and, therefore, to the bulk viscosity of the solutions.
Table 1 Formal and hydrated radii (A) of some alkali metal and R4N÷ cations Ion Na÷ K÷ NH4* (CHs)4N* (C2Hs)4N÷ (n-C3HT)4N÷ (n-C4Hg)4N* a
Radius Hydrated
Formal
Solvation layer thickness
Reference
3.6 3.3 3.31 3.67 4.00 4.52 4.94
0.95 1.33 1.48 3.47 4.00 4.52 4.94
2.65 1.97 1.83 0.20 0 0 0
4 4 a a a a a
E.R. Nightingale, Jr., J. Phys. Chem., 63 (1959) 1381.
29
3.0
I
2"0 i.-
r-
1"0
I
0
0,2
t,,
0"4
i
i,
0,6
I~
'
J
0.CJ
1.3
1,7
[ SALT]~M Figure 1. Variation of relative viscosities, -q,, of 0.3 M SDS micellar solutions with R4N+ cations at 30°C :R=NH 4, -X-; CH~, a • C~H5, O n-C3H7, A ; n-C4Hg, 4. Viscosity shows steep rise with (n-C4H~)4N÷, although electrostatic interaction will not be as effective as other salts of the series; the reason being its bigger size and hence the charge density on the ion will be less[25]. NH4÷ and all the other members of the series will try to get adsorbed on the negatively charged micelle, the (n-C4Ho)4N*, in addition, will also try to intercalate between head groups of monomers in the micelle (due to hydrophobic interaction of the bigger alkyl part). The intercalation of this hydrophobic cation will provide an opportunity to interact closely (although its size will restrict it). The closer approach due to intercalation will decrease the electrostatic interactions in addition to increased hydrophobic interactions. These two interrelated factors will increase Rp (the so-called MitchelI-Ninham parameter[30]) which, in turn, will be responsible for steep rise in the viscosity of 0.3M SDS with (n-C~H~)~N*cation (our smallangle neutron scattering (SANS) results [31] obtained on few of these systems also confirm the view point as drastic changes in micellar size (semi-major axis, b) and charge per monomer (c0 of the micelle were observed in case of 0.3M SDS + (n-C4Hg)4N*). Another noteworthy observation to be gleaned from the results in Figure 1 is the higher viscosities obtained with (C=Hs)4N+than with (n-C3HT)4N+. This could be interpreted from the point of view that for (C2Hs)4N* there is a balance between the electrostrictive
30 nitrogen charge center and the hydrophobic alkyl groups, but the same balance is not existing for (n-C3HT)4N*. Thus, the resultant of the two forces seems to be the deciding factor for the aggregate size as well as the bulk viscosity of the solution. The fall in viscosity (after the maximum), which occurred at relatively higher [salt], can be understood by considering the following facts. Salt addition will definitely neutralize the charge of the anionic micelle and the micelle would then be like a non-ionic one (at higher [salt]). The electrostatic effect will then work in an opposite way as further addition of the salt will cause adsorption of the counterion. This adsorption will impart a positive charge to the micelles (charge reversal) which diminish destructively to smaller sizes because of the electrostatic repulsion among the micelles (now positively charged) and form the basis of viscosity decrease. Charge reversal of cationic micelles due to excess adsorption of salicylate or benzoate anions with resultant viscosity decrease have been demonstrated successfully by making rheological and electrophoretic light scattering measurements [32-34]. Figure 2 shows the effect of addition of two organic cations (TH÷and Gull*) on the viscosity behaviour of 0.3M SDS micellar solutions. The electrostatic effect of TH ÷will
4.0
tO" ¢=
2.0
---..Z~.C~ 0
L
I
,
0"5
I 1.0
I
I 1.5
[SALT],M Figure 2. Variation of relative viscosities, cations TH* (O) and Gull ÷ (4) at 30 ° C.
"l~r,
of 0.3M SDS micellar solutions with organic
31 be similar but the hydrophobic portion may facilitate the intercalation of this ion between head groups of similar charge (as in the case of (n-C4Hg)4N+). These interactions should cause micellar growth and seems to do so as regular increase in viscositywas observed with SDS - TH ÷ combination. Guanidinium cation (Gull ÷) is known as a strong water structure breaker which is also strongly hydrated [35]. Further, this ion takes coplanar, resonance structures and possesses high polarizability [36]. These effects, in a way, would affect the interactions of Gull ÷with SDS micelle and a compensation seems to take place due to the structure breaking nature of Gull ÷ opposite to that of electrostatic effect. Consequently, almost no variation in ~lroccurred with SDS-GuH ÷ combination. In conclusion, we can say that the viscosities of micellar solutions (and the so-called micellar growth), apart from electrostatic nature of the added ion, are dependent upon the hydrophobic volume as well as the water structure breaking ability of the ion.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. A. Sein and J. B.F.N. Engberts, Langmuir, 11 (1995) 455. J.R. Lu, A. Morrocco, T. Su, R.K. Thomas and J.J. Penfold, J. Colloid Interface Sci., 158 (1993) 303. S.G. Oh and D.O. Shah, J. Phys. Chem., 97 (1993) 284. S. Berr, R.R.M. Jones and S. Johnson, Jr., J. Phys. Chem., 96 (1992) 5611. P. Mukerjee, K.J. Mysels and P. Kapaun, J. Phys. Chem., 71 (1967) 4166. H. Gustausson and B. Lindman, J. Am. Chem. Soc., 100 (1978) 4647. MJ. Schick, J. Phys. Chem., 68 (1964) 3585. K.J. Mysels, Introduction to Colloid Chemistry, Interscience, New York, 1959. C.A. Bunton, F. Nome, F.H. Quina and L.S. Romsted, Acc. Chem. Res., 24 (1991) 357. B. Lindman and H. Wennerstrom, Top, Curr. Chem., 87 (1980) 32. K. L. Mittal and B. Lindman (eds.), Surfactants in Solution, Plenum Press, NewYork, 1982. K.L. Mittal and E.J. Fendler (eds.), Solution Behaviour of Surfactants, Theoretical and Applied Aspects, Plenum Press, New York, 1982. C.A. Bunton and G. Savelli, Adv. Phys. Org. Chem., 22 (1986) 213. M. Gratzel and K. Kalyanasundaram (eds.), Kinteics and Catalysis in Microheterogeneous Systems, Marcel Dekker, New York, 1991. S. Hayashi and S. Ikeda, J. Phys. Chem., 84 (1980) 744. S. Ikeda, S. Hayashi and T. Imae, J. Phys. Chem., 85 (1981) 106. EY. Sheu, C.F. Wu and S.H. Chen, J. Phys. Chem., 90 (1986) 4179. P.J. Missel, N.A. Mazer, M.C. Carey and G.B Benedek, J. Phys. Chem., 93 (1989) 8354. D. Nguyen and G.L. Bertrand, J. Phys. Chem., 96 (1992) 1994. L. S. Romsted and C.O. Yoon, J. Am. Chem. Soc., 115 (1993) 989.
32 22. B. Kachar, D.F. Evans and B.W. Ninham, J. Colloid Interface Sci., 100 (1964) 287. 23. M.Jansson, L. Eriksson and P. Skagerlind, Colloids Surf., 53 (1991) 157. 24. A. Wagenaar, L Streefland, D. Hoekstra and J.B.F.N. Engberts, J. Phys. Org. Chem., 5 (1992) 451. 25. J. Eastoe, B.H. Robinson and R.K. Heenan, Langmuir, 9 (1993) 2820. 26. J.E. Desnoyers, M. Arel, G. Perron and C. Jolicoeur, J. Phys. Chem., 73 (1969) 3346. 27. J.E. Desnoyers and F.M. Ichhaporia, Can. J. Chem., 47 (1969) 4639. 28. W.F. Furter (ed.), Thermodynamic Behaviour of Electrolytes in Mixed Solvents, Adv. Chem. Series 155, Am. Chem. Soc., Washington, 1976. 29. Kabir-ud-Din, S. Kumar, V.K. Aswal and P.S. Goyal, J.Chem. Soc., Faraday Trans., 92 (1996) 2413. 30. J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc., FaradayTrans. 2, 72 (1976) 1525. 31. b changed from 33.4 to 45.8 A and 33.4 to 101.2 Afor (CH3)4N÷ and (n-C4H~)4N÷, respectively, as [R4N÷] was changed from 0 - 0.3 M. However, c~remained more or less constant (0.14 _+0.1) with R=CH s but dropped drastically (~ 0.07) with R = n-C4H9. 32. T. Imae, J. Phys. Chem., 94 (1990) 5953. 33. T. Imae and T. Kasaka, J. Phys. Chem., 96 (1992) 10030. 34. H. Rehage and H. Hoffmann, Mol. Phys., 74 (1991) 933. 35. D.B. Wetlaufer, S.K. Malik, L. Stoller and R.L.Coffin, J. Am. Chem. Soc., 86 (1964) 508. 36. J.T. Edsall and J. Wyman, Biophysical Chemistry, Academic Press, New York, 1958.