Langmuir 1991, 7, 2097-2102
2097
Influence of Polymers on the Micellization of Cetyltrimethylammonium Salts Josephine C. Brackman and Jan B. F. N. Engberts' Department of Organic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received February 11, 1991. I n Final Form: April 26, 1991 The critical micelle concentrationvalues and aggregationnumbers of cetyltrimethylammoniumbromide (CTAB) micelles have been measured in the presence and absence of the polymers poly(viny1 methyl ether) (PVME), poly(propy1ene oxide) (PPO),poly(ethy1ene oxide) (PEO), and poly(vinylpyrro1idone) (PVP), Association of the micelles with the polymers PVME and PPO is apparent from a reduction in both the cmc and the aggregation number. PEO and PVP do not influence these properties. The influence of PVME on the transition from sphericalto rodlike micelles of cetyltrimethylammoniumtosylate (CTATs) has been studied by measuring viscosities. The shift to higher concentrations of the sudden viscosity increase in the presence of PVME is interpreted as resulting from the formation of spherical, polymerbound micelles in preference to rodlike micelles. A similar interpretation applies to the transformation of a viscous and non-Newtonian solution of CTABfsodium salicylate (Nasal) into a waterlike, Newtonian fluid upon addition of PPO or PVME. PVP, ethanol, and 2-methyl-2-propanol do not influence the rheology of a CTAB/NaSal solution, but PEO exerts a modest effect.
Introduction Cetyltrimethylammonium salts (CTAX), particularly the bromide and chloride, are by far the most widely studied cationic surfactants. Although this one-sided interest probably stems from their easy availability, they are indeed interesting surfactants. The formation of viscoelastic solutions at extremely low concentrations (ca. 10-4M)in the presence of salicylate anions' is an especially fascinating phenomenon. Notwithstanding these interesting properties, CTAX salts, as well as the relatively few other cationic surfactants that have been investigated, have a poor reputation in the field of polymer-micelle interactiom2 This stems from the fact that they give only significantinteraction with rather hydrophobic polymers,H though recently a modest propensity for binding to more hydrophilic polymers has been The association between ionic micelles and polymers usually leads to a stabilization of the micelles, which is apparent from the reduced value of the critical micelle concentration (cmc).2 The stabilization is thought to originate from a stabilization of the interface between the hydrocarbon core and water,2tg at which the polymer resides.lO However, in the case of alkyltrimethylammo(1) (a) Graveholt,S. Naturwissenschaften 1979,66,263. (b) Gravsholt,
S.InRheology;htarita,G.,Marucci, G., Nicolais, L., Eds.;Plenum Preee:
New York, 1981; Vol. 3, p 629. (2) Goddard, E D. Colloids Surf. 1986, 19, 255. (3) (a) Witte, F.M.; Engberta, J. B. F.N. Colloids Surf. 1989,36,417. (b) Witte, F. W. Ph.D. Thesis, University of Groningen, 1988. (4) Winnik, F.M.;Winnik, M. A.; Tazuke, S.J. Phys. Chem. 1987,91, 594. (5) (a) KarlsMm, G.; Carbon, A.; Lindman, B. J. Phys. Chem. 1990, 94,6005. (b) Carleeon, A.;Karlstrtim, G.; Lindman, B. J . Phys. Chem. 1989,93,3673. (c) Carbon, A.;Lindman, B.; Watanabe, T.; Shirahama, K. Langmuir 1989,5,1250. (6) (a) Shirahama, K.; Himuro, A.; Takiaawa, N. Collid Polym. Sci. 1987, 266, 96. (b) Shirahama, K.; Oh-ishi, M.; Takisawa, N. Colloids Surf. 1989, 40,261. (7) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (8)Perron, G.;Frantpeur, J.; Desnoyers, J. E.; Kwak, J. C. T. Can.J. Chem. 1987,65, 990. (9) (a) Nagarajan, R.;Kalpakci, B. In Microdomains in Polymer Solutions; Dubin, P., Ed.,Plenum Press: New York, 1985; p 369. (b) Nagarajan, R. Colloids Surf. 1986,13,1. (c) Nagarajan, R.Adu. Colloid InterfaceSei. 1986,26,205. (d)Nagarajan, R.;Kalpacki, B. Polym. P r e p . (Am. Chem. SOC.,Diu. Polym. Chem.) 1982,23 (l),41. (e) Nagarajan, R. J. Chem. Phys. 1989,90,1980. (10) Cabane, B.J. Phys. Chem. 1977,81, 1639.
0743-7463/91/2407-2097$02.50/0
nium surfactants, the bulky head group already shields most of the core from contact with water," and furthermore, steric repulsions between head groups and polymer segments may produce an unfavorable contribution to the free energy of formation of polymer-bound micelle^.^ Therefore, only sufficiently hydrophobic polymers will interact with CTAX micelles, because of the additional reduction in free energy due to the transfer of polymer segments from the aqueous phase to the micellar surroundings. Recently, we have found that the loss in electrostatic repulsion among the head groups upon formation of the smaller polymer-bound micelles constitutes another important contribution to the reduction of the free energy of micellization and, thus, the reduction in cmc.lZ So far, only the interaction of spherical cationic micelles with polymers has been discussed. Certain cetyltrimethylammonium salts, however, are well-known for the formation of rodlike (or wormlike; no stiffness is implied) micelles.1J*16 Surprisingly, Nagaragan%appears to be the only author who considered rodlike micelles in studies of polymer-micelle interactions. He predicted theoretically that rodlike micelles of SDS, formed in the presence of NaC1, will be transformed into polymer-bound ellipsoidal micelles in the presence of PEO. Rodlike micelles of would be unaffected by PEO. However, Nagaragan's predictions were not experimentally tested. In this paper, we report on the interaction of micelles of CTAB (X = bromide), CTATs (X = tosylate), and CTASal (X = salicylate) with polymers. The relatively hydrophobic polymers PVME and PPO both induce a decrease in aggregation number of CTAB. In the case of CTATs, interaction with these polymers leads to an increase in the concentration at which the spherical micelles are transformed into rodlike micelles. These findings (11) (a)Tabony,J. MoLPhys. 1984,51,975. (b)Berr,S. S.;Caponetti, E.; Johnson,J. S., Jr.; Jones, R. R. M.; Magid, L. J. J.Phys. Chem. 1986, 90,5766. (12) Brackman, J. C.; Engberta, J. B. F.N. Submitted for publication. (13) Hoffmann, H.; Ebert, G.Angew. Chem., Int. Ed. Engl. 1988,27, 902. (14) Reo, U. R.K.; Manohar, C.; Valaulikar, B. S.;Iyer, R.M. J. Phys. Chem. 1987, 91, 3286. (15) Johnson, I,;Olofsaon, G.J . Colloid Interface Sci. 1986,106,222. (16) Anet, F. A. L. J.Am. Chem. SOC.1986,108,7102.
0 1991 American Chemical Society
2098 Langmuir, Vol. 7,No. 10,1991
support t h e view t h a t the disappearance of t h e gellike and viscoelasticpropertiesof a CTASal solution in the presence of PVME or PPO originate from a preferential binding of t h e polymers to spherical micelles rather t h a n t o rodlike micelles.
Experimental Section Materials. CTAB (Merck)was purified as described by Duynstee and Grunwald." CTATs (Sigma),Nasal (Merck), and PPO (weight-averaged molecular weight (MW) 1O00, Aldrich) were used as received. The purification of PEO (Sigma, weightaveraged molecular weight 20 O00)'8 and PVP (Kolloidon-90, BASF)l8has been described before. PVME (50% (w/w) solution in water, inherent viscosity 0.57, Aldrich) was freeze-dried. The yellowish residue was dissolved in ethanol and heated with activated carbon. After filtration, the solvent was evaporated and the residue was dissolved in water, dialyzed, and freezedried. The polymer was stored as a 20% (w/w) solution in water. The molecular weight of PVME (27 000) was determined by viscosity measurements in butanone. The intrinsic viscosity equals K X (MW)a, in which K = 137 X lO-9 mL gl and a = 0.56 at 30 "C for this combination of polymer and solvent.20 The quencher 9-methylanthracene (Janssen) was used as received. The fluorophore bis(2,2'-bipyridyl)(4,4'-didecyl-2,2'bipyridyl)ruthenium(II) perchlorate was a gift from Dr. L.A. M. Rupert of the Koninklijke/Shell Laboratories, Amsterdam. The water used in all experiments was demineralized and distilled twice in an all-quartz distillation unit. Conductivity Measurement. Conductivities were measured by using a Wayne-Kerr autobalance universal bridge, B642, fitted with a Philips conductance cell, PW 9512101, with a cell constant of 0.71 cm-l. The solutions were thermostated in the cell at 25 f 0.1 "C for at least 15 min before measurements were initiated. The conductivity cell was equipped with a magnetic stirring device. The surfactant concentrations were varied by the addition (microsyringe) of appropriate portions (10-50 pL) of a concentrated solution of the surfactant to the conductivity medium. Concentrations were corrected for volume changes. Cmc values were taken from the intersection of the tangents drawn before and after the first break in the conductivity vs concentration plot. In the case of PPO and PVME solutions, no clear break could be observed since the conductivity varies nonlinearly with the concentration above the cmc. In these cases, the cmc values were taken from the discontinuity in the plot of the first derivative of the conductivity vs the concentration. These values obtained by this technique deviate from those determined by Witte.3 Fluorescence Measurements. Stock solutions of fluorophore and quencher were prepared in 96 ?6 Uvasol-gradeethanol (Merck). In a typical experiment, 2 pL of the fluorophore stock solution was injected into 2 mL of the surfactant solution, yielding probe concentrations between 1W6 and lo4 M. Subsequently 2-pL aliquots of the appropriate quencher solution were injected. The concentration of the quencher solution was chosen to yield a quencher-to-micelle ratio of ca. 0.8 after injection of between 8 and 20 pL. The solution in the cuvette was stirred with a magnetic device and thermostated at 25 f 0.1 "C. Fluorescence intensities were measured with a SLM-Aminco (SPF-500C)spectrofluorometer. Excitation and emission wavelengthswere 453.5 and 626 nm, respectively. The aggregation numbers were determined from plots of lnZ(0) -In I([Q]) versus [Q]/([CTAB] - cmc) according to the method of Turro and Yekta.21 UV Measurements. UV measurements were performed with a Perkin-Elmer A5 spectrophotometer, using cuvettes with 1-mm path lengths. The results were reproducible to within 0.001 absorbance unit. (17) Duynstee, E.F. J.; Grunwald, E. J. Am. Chem. SOC.1959,81,4540,
_-
ASA2
(18)Witte, F. M.; Buwalda, P. L.; Engberta, J. B. F. N. Colloid Polym. Sci. 1987,265, 42. (19) Fadnavis, N.; Engberta, J. B. F. N. J . Am. Chem. SOC.1984,106, 2636. (20) Brandrup,J.;Immergut,E. H.PolymerHandbook, Pnded., Wiley: New York, 1975. (21) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978,100, 5951.
Brackman and Engberts Table I. Critical Micelle Concentration of CTAB in the Absence and Presence of Polymers. at 25 "C medium
HzO
cmc. mM
0.95b
PVME 0.46 PPO 0.37 PEO 0.95* Polymer concentration: 0.5 g dL-1. Taken from ref 3b. Rheological Measurements. Solutions were prepared at least 1h in advance. CTATs solutions were prepared by dilution of a clear stock solution of 40 mM CTATs with either water or an aqueous polymer solution. CTASal solutions were prepared from appropriate fresh stock solutions of CTAB, Nasal, and polymer. Rheologicalmeasurements were performed with a Brabender Rheotron rheometer with either cone-and-plate geometry (P7) or cylindrical geometry (Al). The rheometer was equipped with a Normal F-sensor which allowed the measurement of first normal stress differences when cone-and-plate geometry is used. For the measurement of shear stress, from which the (apparent) viscosity is calculated as shear stress divided by shear rate, the cylindrical geometry produces the more accurate data. The sample solution was thermostated at 25 f 0.1 "C during the measurements.
Results Rodlike Micelles of CTAB. Table I lists the cmc values of CTAB in t h e absence and presence of polymers. These cmc values were determined by conductivity measurements. Usually, a clear break is observed in the conductivity vs concentration plot, indicative of the cmc. I n the presence of PVME or PPO, however, the conductivity changes gradually. The most likely cause for this behavior is the fact that the degree of counterion binding is less for the polymer-bound micelles than for the normal micelles. The reduction in counterion binding will be most pronounced just above t h e cmc when the polymer to micelle ratio is relatively large and the aggregation number relatively small compared to the same data near saturation (vide infra). At increasing surfactant concentrations the aggregates grow and the counterion binding will increase in order to reduce electrostatic repulsion between the head groups, which come nearer together. Furthermore, the cooperativity of the monomer-to-aggregate transition will be less for smaller aggregates and this will also widen the concentration range for the micellization process. T h e aggregation numbers of CTAB micelles in the absence and presence of polymers have been measured using by t h e T u r r o and Yekta2Imethod, extended by Warr and Grieser22for application to cationic and nonionic micelles. T h e method is based on static quenching of the fluorescent probes tris(2,2'-bipyridyl)ruthenium(II) perchlorate (Ru(bpy)32+) or bis(2,2'-bipyridyl)(4,4'-didecyl2,2'-bipyridyl)ruthenium(II)perchlorate (Ru(bpy)z(bpyL ~ o ) ~by + )the hydrophobic quencher 9-methylanthracene. This method is very practical. However its applicability has been t h e subject of severe debate,22-27 and t h e method should be employed with care. Lianos and ZanaF3 for instance, found a too low value for the aggregation number of SDS/NaCl using R~(bpy)3~+/9-methylahthracene. This was attributed to a failure of t h e assumption of static (22) (a) Warr, G. G.; Grieser, F. J . Chem. SOC.,Faraday Trans. 1 1986, 82,1829. (b) Warr, G. G.; Grieser, F. Chem. Phys. Lett. 1986,116,506. ( c ) Warr, G. G.; Grieser, F. J. Chem. SOC.,Faraday Trans. 1 1986,82, 1813. (23) Lianos, P.; h a , R. J . Phys. Chem. 1980,84,3339. (24) Almgrem, M.; LBfroth, J.-E.J. Chem. Phys. 1982, 76, 2734. (25) Infelta, P. P. Chem. Phys. Lett. 1979,61,88. (26) Malliaris, A. Prog. Colloid Polym. Sci. 1987, 73, 161. (27) Moroi, Y.; Humphry-Baker, R.; Gritzel, M. J. Colloid Interface Sci. 1987, 119, 588.
Langmuir, Vol. 7, No. 10, 1991 2099
Influence of Polymers on CTAB Micelles Table 11. Aggregation Number of CTAB Micelles in the Absence and Presence of Polymers. at Various Surfactant Concentrations' ICTABLmM HsO PVME PPO PEO PVP 10 70 25 30 65 70 20 36 37 30 69 42 40 67 68 47 42 40 50 69 51 43 65 69 a Polymer concentration: 0.5 g dL-1. Temperature: 25 O C .
quenching a t high aggregation numbers.24 This failure and the errors induced by polydispersity do not immediately result in nonlinear Stern-Volmer plots, which are used to obtain the aggregation number.22 Furthermore, high fluorophore-to-micelle ratios should be avoided,25 although later this prerequisite also has been disputed.24 Another method that has often been applied is based on pyrene excimer formation.2g30 Moroi et al.27used both static quenching of Ru(bpy)s2+ and pyrene excimer formation to obtain aggregation numbers for micelles of SDS and alkylsulfonic acids and found a satisfactory agreement. Lissi and Abuin31used the Ru(bpy)g2+/9-methylanthracene system for the determination of the aggregation numbers for PEO/SDS and PVP/SDS complexes. If the experimental conditions are chosen with care, that is, low aggregation numbers and a low fluorophore concentration, good results can be obtained.26 Using quenching of the probe Ru(bpy)2(bpyLd2+,a value of 70 (Table 11)is found for the aggregation number of CTAB micelles at 25 "C. Other values reported in the literature include 5432(steady-statefluorescence,pyrene); 88,30 90,m82,23and 96 f 1033 (time-resolved fluorescence, pyrene); 10454(timeresolved fluorescence, l-methylpyrene/ tetradecylpyridinium chloride); 80 f lg5(time-resolved fluorescence, 1,5-dimethylnaphthalene/cyclicazoalkane); 9230 and (light scattering); and 14511b (small angle neutron scattering). It is clear that a value must be grossly in error to fall outside the literature range (54-145)! Several authors have questioned the use of pyrene in the case of tetraalkylammonium surfactants since both specific intera~tions~as well as the induction of micellar groWth24*38 have been observed. Scattering techniques are only applicable to polymer-bound micelles if several conditions are met,39which is probably not the case with hydrophobic polymers like PPO and PVME. Altogether, we contend that quenching of Ru(bpy)2(bpyLl0)~+ by 9-methylanthracene is a satisfactory choice and most likely suitable for studies of the influence of polymers on the aggregation number of CTAB micelles (Table 11). Sphere-to-Rod Transition of CTATs. One of the counterions that is able to induce the formation of rodlike micelles from cetyltrimethylammonium surfactants is to(28)Infelta, P.p.; Griitzel, M. J. Chem. Phys. 1979,70,179. (29)Atik, S.5.;Nam, M.; Singer, L. A. Chem. Phys. Lett. 1979,67,75. (30)Lianos, P.;Zana, R. J. Colloid Polym. Sci. 1981,84, 100. (31)Lissi, E. A.; Abuin, E. J.Colloid Interface Sci. 1985,105, 1. (32)Selinger, B. K.; Watkins, A. R. Chem. Phys. Lett. 1978,56, 99. (33)Turro, N.J.; Tanimoto, Y. Photochem. Photobiol. 1981,34,157. (34)Roelanta, E.; De Schrijver, F. C. Langmuir 1987,3,209. (35)Aikawa, M.;Yekta, A.; Turro, N. J. Chem. Phys. Lett. 1979,68, 285. (36)Ekwall, P.;Mandell, L.; Solyom, P. J. J. Colloid Interface Sci. 1971,35,519. (37)Anacker, E. W.:Rush. R. M.: Johnson, J. S. J. Phrs. Chem. 1964, 68,81. (38)Offen, H.W.; Daweon, D. R.; Nicoli, D. F. J . Colloid Interface Sci. 1981,80,118. (39)Cabane, B.; Duplessix, R. Colloids Surf. 1985,13,19.
Table 111. Apparent Viscosities of Micellar CTATs Solutions in Aqueous Solutions in the Absence and Presence of PVME at 25 OC
vapp, Pa 8 [CTATs], mM
HzO
10 15 20 25 30 35 40 45 50
0.0017 0.0046 0.016 0.047 0.53 5.09
0.25 g dL-' PVME 0.5 g dL-1 PVME 0.0016 0.0017 0.0047 0.013 0.081 0.24 1.28 3.55 10.01
0.0040 0.021 0.067 0.32 1.25 2.54
sylate (Ts). Septilveda and co-workersN studied the rheology of solutions of CTATs and of other CTAX surfactants. They also reported cmc values, degrees of dissociation, and the free energy of transfer for the counterion from water to the micelle.a The tosylate ion is less rod-inducing than the salicylate (Sal) ion. As a result globular micelles of CTATs are initially formed above the cmc (2.6 X lo4 M).Nb However, these micelles start to g r o d Oabove a critical rod concentration (crc) of around 15mM. Thus, CTATs provides the possibility of studying the sphere-to-rod transition and the influence of polymer on the critical concentration at which this transition takes place. In the case of CTASal rodlike micelles are formed directly above the cmc. Sepulveda et al.4Ob~cestablished the transition concentration for CTATs, by examining the increase in relative viscosity (crc ca. 20 mM), the decrease in partial molar volume (crc = 8-12 mM), and the break in the plot of UV absorbance versus concentration (crc = 15mM). We could not reproduce the break in UV absorbance around 15 mM, reported by Sepulveda.ab Therefore, we used viscosity measurements to obtain the concentration at which the sphere-to-rod transition of CTATs takes place, in the absence and presence of PVME. Usually the apparent viscosity of a solution of rodlike micelles drops rapidly when the shear rate is increased. Only a t low shear rates (or at very high shear rates) is the viscosity Newtonian, that is, independent of shear rate. In Table I11these low shear (Newtonian)viscositiesare listed for solutions containing various concentrations of CTATs in H2O and in the presence of 0.25 and 0.5 g dL-' of PVME (measured with cylindrical geometry). For the highly viscous solutions, shear rates as low as 6 X s-l were used. It is hard to identify the sphere-to-rod transition with a well-defined concentration, since the viscosity increases nonlinearly with the CTATs concentration (Figure 1). The viscosity of a 15 mM CTATs solution in H2O is already 4 times as high as that of water (1cP). At 18 mM of CTATs, a first normal stress difference, indicating viscoelastic behavior and thus the presence of rods, can be observed above a shear rate of 476 s-l (using cone-and-plate geometry). Such viscoelastic behavior can also be observed visuallyas the recoil of trapped air bubbles when a swirlingmotion of the solution is abruptly stopped. From 20 mM CTATs onward, thixotropic behavior is definitely displayed using a cone-and-plate measuring device (between 119 and 476 s-l) and from 25 mM CTATs onward using a cylindrical measuring device (between 60 and 1199-1). Thixotropicbehavior indicates the occurrence of a decrease of the apparent viscosity with increasing time and is revealed in this case after an initial stepwise (40)(a) Bartet, D.;Gamboa, C.; Sepdlveda, L. J.Phys. Chem. 1980, 84,272. (b) Gamboa, C.; Sepdlveda, L. J. Phys. Chem. 1989,945640. (c) Gamboa, C.; Sepdlveda, L. J. Colloid Interface Sei. 1986,113,566.
Brackman and Engberts
2100 Langmuir, Vol. 7,No. 10,1991 log
VISCOSlty. CP
500
1
li ic
-2
0
0
20
10
30
50
LO
-3
[CTATs], mM
Figure 1. Viscosity at low shear rates (Newtonian behavior) of micellar CTATs solutions in H10 (O),0.25 g dL-1 of PVME (m), and 0.5 g dL-l of PVME (0) at 25 "C,measured with cylindrical geometry. Extrapolation of the lines is based on the data from Table 111. Table IV. Effect of Sodium Salicylate and Several Monomeric and Polymeric Additives on the Viscosity of a Micellar CTAB Solution [CTAB], mM [Nasal], mM additivea viscosity, CP 25 25 25 25 25 25 25 25 25 25
( a p p a r e n t viscosity)
I
PVME PEO 15 15 15 15 15 15 15
1.08 f 0.02 1.510 f 0.0006 1.26 h 0.02 2711) 8.8' 1.630 f 0.006 1.080 f 0.006 274) 16.9 2817) 15.4' 3055,b8.1' 2213) 8.9'
PVME PPO PEO PVP EtOH t-BuOH 0 [additive] = 0.5 g dL-1. Shear rate = 0.2985 rate = 471.6 s-1,
*
5-1.
Shear
increase in shear rate. The thixotropy and the viscoelasticity and non-Newtonian behavior are indicative for changes in the internal structure of the solution. Those changes originate from alignment and disruption of the rodlike micelles by the shear f o r ~ e s . ~ ~ ? ~ ~ Rodlike Micelles of CTASal. Cetyltrimethylammonium salicylate is the archetype of a cationic surfactant that forms rodlike micelles even in dilute (ca. M) solutions.' At higher concentrations, CTASal solutions become viscoelastic and behave in a strongly non-Newtonian manner. The maximum in viscosity lies at a [Sal]/ [CTA+]ratio below We used 25 mM of CTAB and 15 mM of Nasal. It is easily seen that the presence of 0.5 g dL-l of PVME or PPO completely eliminates the gellike properties and reduces the viscosity to about that of water. Addition of the more hydrophilic polymers PEO or PVP does not induce such a transition. We have used rheological measurements to quantify the effect. The (apparent) viscosities of micellar CTAB solutions in the absence and presence of sodium salicylate,polymers, and low molecular weight additives are listed in Table IV. These values have been obtained by using a measuring device with cylindrical geometry. The CTAB/NaSal (41) Strivene, T. A. Colloid Polym. Sci. 1989, 267, 269. Bird, R. B.; Curtias, C. F.; Armetrong, R. C.; Haeeeger, 0. Dynamics of Polymeric Liquids; Wiley: New York, 1980, Vole. 1-2. (42) (a)Rehage, H.; Wunderlich, I.; Hoffmann, H. h o g . Colloid Polym. Sci. 1986, 72, 51. (b) Kalue, J.; Hoffmann, H.; Ibel, K. Colloid Polym. Sci. 1989, 267, 818. (43) Rehage, H.; Hoffmann, H. J . Phys. Chem. 1988,92, 4712.
f
-1
1
I
I
I
0
1
2
3 log
(shear r a t e )
Figure 2. Double logarithmic plot of apparent viscosity vs shear
rate for the following aqueous solutions of CTAB (25 mM): no additives, (m); + PEO 20k (0.5g dL-l), (A);+ PVME (0.5g dL-'), ( 0 ) ; Nasal (15mM), (0); + Nasal (15 mM) and PEO 20k (0.5 g dL-11, (A);+ NaSal(l5 mM) and PVME (0.5 g dL-9, (0).The data were measured with cylindrical geometry.
+
solutions, whether or not in the presence of PVP, ethanol, or 2-methyl-2-propano1, and, to a slightly lesser extent, CTAB/NaSal/PEO (20k),clearly exhibit non-Newtonian behavior. That is, the apparent viscosities4 vary dramatically with changes in shear rate (Table IV, and Figure 2). By contrast, the apparent viscosities of CTAB/NaSal in the presence of PVME or PPO, and of CTAB solutions without Nasal, are orders of magnitude lower and are independent of shear rate, indicative of Newtonian behavior. The shear rate dependence of the viscosity of the CTAB/ NaCl solutions is not affected by PVP, ethanol, or 2-methyl-2-propanol, and only slightly by PEO (Figure 2). Generally speaking, three regions may be discerned in a plot of apparent viscosity versus shear rates4 At very low shear rates, Newtonian behavior will be displayed. Our data do not include low enough shear rates to observe this region. In the second region, the internal structure of the solution is altered by the shear forces. In the case of rodlike micelles, this causes a drop in apparent viscosity, due to aligning and disruption of the rods. This region is very obvious in Figure 2. In the third region, a t higher shear rates, the structural changes are completed and Newtonian behavior can be observed again. For the present system, this transition occurs around a shear rate of 100 s-l. Wolff et a1.45reported the same shear rate of 100 s-l, above which Newtonian flow was observed for 20-25 mM of CTAB containing 9-11.3 mM of g-anthracenecarboxylic acid. A closer inspection of our data in the second region, in which structural changes occur, leads to surprising results. In this region (below 84 s-l), the shear stress (0.8 Pa, cylindrical geometry) does not change with changing shear rate (Figure 3), which implies a power law exponent of zero. This is very unliquidlike behavior. However, visual observation convinced us that the CTAB/NaCl solution is not a solid body. The shear stress obtained with coneand-plate geometry reveals the same plateau region below (44) Bird,R.B.;Curtisa,C.F.;Armstrong,R.C.;Haseager,O.Dyr"ics of Polymeric Liquids; Wiley: New York, 1980; Vols. 1-2.
(45) Wolff, T.; Emming, C. S.; Suck, T. A.; von Bijnau, G. J. Phys. Chem. 1989,93,4894.
Langmuir, Vol. 7, No. 10, 1991 2101
Influence of Polymers on CTAB Micelles Log (shear ntress)
1.0
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O
0
Figure 3. Double logarithmic plot of shear stress, measured with cylindrical geometry, va shear rate for aqueous solutions of CTAB (25 mM)/NaSal (15 mM) without additives ( 0 )and in the presence of 0.5 g dL-l of PEO 20k ( 0 ) .
476 s-l a t a shear stress of 1Pa. In the presence of PEO only a slight shoulder can be detected (Figure 3). This anomalous shear stress behavior has, to the best of our knowledge, only been noted before by Strivens,4I also for the CTASal system. A phenomenon known as “walllip''^^^^ may lie at the origin of these observations. It has been postulated that (apparent) slip effects are due to the formation of a thin low-viscosityfluid layer near the wall of the flow channel. Wunderlich et tried to relate the dependence of the flow curve of a CTASal solution on the measuring device to such slip effects. The attempt was in vain, however, since the curves could not be explained in terms of the slip velocity concept. A rate-independent shear stress is only expected for a solid which clearly does not apply for a CTAB/ NaCl solution. A definite feature of the liquidlike properties of this solution is the observation of thixotropy and rheopexy. Rheopexy (the opposite of thixotropy) denotes an increase in viscosity with increasing time a t a constant shear rate. For CTAB/NaCl solutions, this is observed upon stepwise decreasing the shear rate. It results from the reversibility of the alignment and breakdown of the rodlike micelles. For the CTAB/NaCl solution both thixotropy and rheopexy are observed below a shear rate of 476 s-l and for the CTAB/NaSal/PEO system below 76 s-l using cone-and-plate geometry. The disappearance of these phenomena coincideswith the end of the shear stress plateau. The 3-5 min, which are needed in the case of stepwise-increased shear rates, are normal times for CTASal solutions41and are not greatly affected by the presence of PEO. The visually observed viscoelasticity has also been quantified through the measurement of first normal stress differences using cone-and-plate geometry (Figure 4). Especially at shear rates below 2000 s-l, the first normal stress differences of the CTAB/NaSal solution, in the presence of PEO, are significantly larger than those of the aqueous CTAB/NaSal solution. However, below 200 s-l in the presence of PEO,the first normal stress difference drops sharply to values that are too low to measure. The higher first normal stress difference of the CTAB/NaSal/ (46) (a) Cohen, Y.;Metzner, A. B. J.Rheol. 1985,29,67. (b) Kraynik, A. M.; Schowalter, W. R. J. Rheol. 1981,25,95. (e) Carreau, P.J.; Bui, Q.H.; Leroux, P. Rheol. Acta 1979, 18,600. (47) Kiljanski, T.Rheol. Acta 1989,28,61. (48) Wunderlich, A. M.; Brunn, P. 0. Colloid Polym. Sci. 1989,267, 627.
PEO solution compared to that of the CTAB/NaSal solution is probably related to the fact that the viscosity of the former solution is also higher in this range of shear rates (Figure 2).
Discussion Spherical Micelles of CTAB. The hydrophobic polymers PVME and PPO induce a reduction in the cmc of CTAB, which points to polymer-micelle interaction (Table I). The more hydrophilic polymer PEO does not exert any influence on the cmc. Though that observation does not exclude interaction, literature data reveal that there is no interaction comparable to that in systems like PEO/SDS or CTAB with hydrophobic polymers. However, some very weak interaction of PEO presumably with surfactant monomers may occur.8-8 PVP definitely does not associate with CTAB. Addition of PVME or PPO results in a appreciable reduction of the aggregation number ( n ) (Table 11). Such a decrease in n has also been found for SDS in the presence of PE0,3*31*49tmPVP,31pm and PPO.3 As anticipated, we find that the aggregation number of CTAB micelles is not altered by the presence of PEO and PVP. Reduction in n upon binding of the hydrophobic polymers is understandable since the interacting polymers, which most likely bind at the hydrocarbon core-water interface,21°J0require space in order to keep head group-polymer repulsions to a minimum. Furthermore, electrostatic inter-head-group repulsions will be diminished.I2 Altogether comparatively large surface-to-volume ratios of the micelles will be favored in the presence of an interacting polymer. This implies the formation of a smaller aggregate. The dependence of n for the polymer-bound micelles on the surfactant concentration (observed for both CTAB/ PVME and CTAB/PPO) has precedent in the literature on anionic micelles of SDS in the presence of polymer^.^+^^ The polymer-bound aggregatesprobably grow asthe degree of saturation of the polymer is increased due to a decrease (49) FranGoia, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1986,21, 165. (50)(a) Zana, R.; Lang, J.; Lianoa, P. In Microdomuins in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985; p 357. (b) Zana, R.;Lang, J.; Lianoe, P. Polym. R e p . (Am. Chem. SOC.,Div. Polym. Chem.) 1982, 39 (l),39.
2102 Langmuir, Vol. 7, No. 10,1991 of the local concentration of polymer segments at the micellar interface39 and an increase in intermicellar repulsions. Sphere-to-Rod Transition of CTATs. Although the concentration for the sphere-to-rod transition of CTATs cannot be clearly defined, it seems obvious from Figure 1 that the presence of PVME shifts the sphere-to-rod transition to higher concentrations. However, there may be a pitfall in this alluring conclusion. In 1985 Hoffmann et al.51a stated "that all theories which try to explain the viscoelasticproperties of micellar solutions on models that are based on the existence of well-defined rods, without taking into account the transient nature of the micelles, sooner or later must fail". They illustrated this statement with the behavior of n-tetradecylpyridinium salicylateand n-tetradecylammoniumsalicylate. These compoundshave similar cmc values, critical rod concentrations, and light scattering behavior, which suggests that the micellar structures and the interactions between them should also be the same. In spite of these similarities, the viscosities of aqueous solutions of these two compounds differ by almost 2 orders of magnitude. The differences between the structural relaxation times of the micelles were shown to lie at the origin of this difference. For these surfactants the relaxation time stems from the kinetics of formation and dissociation of the micelle, whether stepwise per monomer or via coalescence or fragmentation of the entire micelle, and not from the rotation of the rods. Since this relaxation time may be influenced by the presence of additivesl3*51such as l-butanol or l-pentanol, it is conceivable that the shift in concentration where the viscosity increase of the CTATs solution takes place, induced by PVME, is also due to similar effects and not to a shift in concentration of the sphere-to-rod transition. However, we submit that this is not the case (vide infra) and that indeed a shift in transition concentration upon addition of PVME takes place. We propose that PVME preferentially binds to spherical micelles of CTATs, for which the surface to volume ratio is more favorable for interaction with the polymer, for the same reasons as in the case of CTAB (vide supra). When the CTATs concentration exceeds the saturation concentration of PVME, free micelles will be formed, which grow into rods upon increasing the concentration. This view is based on circumstantial evidence: (i) a reduction in the size of the aggregate is also found for CTAB micelles in the presence of PVME; (ii) the transition regions in the viscosity plots of the CTATs solution in HzO, 0.25 g dL-' aqueous PVME, and 0.5 g dL-l aqueous PVME are virtually superimposable; (iii) the shift in transition concentration is almost proportional with the polymer concentration, which points to saturation of the polymer playing a role; (iv) 0.5 g dL-l of ethanol, 2-methyl-2-propano1,or noninteracting polymers hardly or do not perturb the viscosity of a CTASal (51) (a) Hoffmann, H.; LBbl, H.; Rehage, H.; Wunderlich, I. Tenside Deterg. 1985,22,6. (b) Wunderlich, I.; Hoffmann, H.; Rehage, H. Rheol. Acta 1987,26,532. (c) Hoffmann, H.; Ulbricht, W. J . Colloid Interface Sci. 1989, 129, 388.
Brackman and Engberts solution, contrary to interacting polymers such as PVME (vide infra). Rodlike Micelles of CTASaLS2 The CTAB/NaSaI solution is transformed from a viscous, non-Newtonian to a waterlike, Newtonian fluid by the presence of the polymers PPO and PVME. This polymer-induced transition is, like in the case of CTATs, attributed to preferential binding of spherical rather than rodlike micelles onto the hydrophobic polymers. This is consistent with the reduction in aggregate size of CTAB micelles in the presence of PVME and PPO and the shift to higher surfactant concentrations for the sphere-to-rod transition of CTATs by the presence of PVME. The hydrophilic polymers PEO and PVP do not bind to CTAX micelles and, therefore, do not exert dramatic effects on the rheology of a solution of these aggregates. The alcohols ethanol and 2-methyl-2-propanoldo not noticeably affect the rodlike micelles either. However, PEO has a (slight) influence on the rheology of the CTAB/NaSd solution. This may be due to (i) interference of the polymer chains with the intermicellar ordering and flow of the rods or (ii) to a modest interaction of PEO with CTAX monomers or aggregateseither influencingthe structural relaxation time or the structures themselves. There appears to be no reason, however, why interference should occur for PEO but not for PVP, whereas there are indications that PEO has a very small but detectable effect on CTAB aggregation, which PVP has not.68 Thus the explanation in terms of a modest effect on the aggregate morphology is more likely. Conclusion The above results indicate that an appropriate polymer may have a dramatic effect on the aggregation behavior of even alkyltrimethylammonium surfactants, which are rather discriminative toward polymers. Only slightly hydrophobic (though still water-soluble) polymers PPO and PVME associate with these surfactants, in contrast with the more hydrophilic polymers PEO and PVP. From the reduction in aggregation number of CTAB micelles upon association with the former polymers, it is already apparent that the polymer-bound micelles favor a higher surface to volume ratio of the micelle than do unperturbed micelles. In the case of CTASal and CTATs the free energy change upon association with PPO and PVME is sufficient to overcome the initial preference for a cylindrical micelle, in which the head groups are quite close together, and accommodate the polymer at the interface of a spherical micelle instead. The free energy gain originates from a favorable transfer of polymer segments from the aqueous to the micellar phase. Furthermore, electrostatic inter-head-group interactions are diminished, whereas the extra core-water contact is stabilized by the rather hydrophobic polymers. Registry No. CTAB,57-09-0;PVME,9003-09-2; PPO,25322PVP,9003-39-8;CTATs, 138-32-9;Na69-4;PEO,25322-68-3; Sal, 54-21-7. (52) Preliminary account: Brackman, J. C.; Engberta, J. B. F. N. J. Am. Chem. SOC. 1990,112,872.
