J. Phys. Chem. 1995,99, 10865-10878
10865
Polymer-Induced Microstructural Transitions in Surfactant Solutions Xiangbing Li, Zuchen LinJ Jim Cai? L. E. Scriven, and H. T. Davis* Department of Chemical Engineering and Materials Science, and Center for Inte~acialEngineering, University of Minnesota, Minneapolis, Minnesota 55455 Received: November 30, 1994; In Final Form: April IO, 1 9 9 9
The interactions of nonionic polymers poly(viny1 methyl ether) (PVME), poly(propy1ene oxide) (PPO), poly(acrylic acid) (PAA), and ionic poly(sodium 4-styrenesulfonate) (PSS) with the wormlike micelles in aqueous solutions of nonionic hexaethylene glycol monohexadecyl ether (C1&), pentaethylene glycol monododecyl ether (C12E5), and cationic surfactant cetyltrimethylammonium bromide (CTAB)/sodium salicylate (Nasal) have been investigated by cryo-transmission electron microscopy and shear rheology. All the surfactant solutions were viscous, wormlike micellar solutions in the absence of polymers. The hydrophobic nonionic PPO induced a wormlike micelle to ribbon-shaped discoid micelle transition in c1&6solution, and there is no appreciable change in C12E5 upon the addition of PPO and PVME. The results indicate the surfactantpolymer interaction is enhanced with an increase in surfactant alkyl chain length. Both PVME and PPO induced a wormlike-to-spherical micelle transition in the CTAB/NaSal solution. The contrast in the transitions between the c16E6 and CTAB with PPO addition is due to the difference in the head groups which results in a difference in where the polymer resides in the surfactant aggregates. PSS has little effect on the CiE, systems, and PAA does not destroy the wormlike micelles of the CiEj before precipitation. The effects of nonionic polymer are interpreted in terms of the theory developed by Nagarajan.
Introduction A lot of work has appeared on aqueous solutions of polymers and surfactants. The work stems in past from their practical importance, for example, in pharmaceutical, cosmetical, petroleum, paint and coating, and mineral processing industries,'s2 and in past from their potential as a model for understanding of some fundamental biological problems3 such as proteinmembrane interactions. Despite the work that has been done on surfactant-polymer systems, the precise morphologies and mechanisms of the polymer-micelle associations are still not well understood. Theoretical papers based on thermodynamic considerations have been p ~ b l i s h e d ~to- ~explain the nature of complexation of nonionic polymer molecules with surfactant aggregationssuch as globular micelles, rodlike micelles, bilayers, and microemulsions. The polymer-surfactant association is generally visualized as polymer molecules wrapped around the surfactant aggregate at its hydrophobic core-water interface. Three competing factors are emphasized in the determination of the occurrence of the complexes: favorable enhanced shielding of the hydrophobic core from water, unfavorable increased steric repulsion at the aggregate surface, and favorable hydrophobic interaction between polymer and aggregate, the magnitude of which depends on the hydrophobicity of the polymer. The nonionic surfactant is predicted to be indifferentto neutral polymer because the nonionic surfactant usually possesses a bulky head group which already limits the interfacial area between hydrophobic core and water. However, this prediction has not been thoroughly tested and some recent work has shown that although the critical micelle concentration (cmc) is not influenced by the presence of nonionic polymer, there are 435
+Present address: The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 398707, Cincinnati, OH 45239-8707. Present address: Xerox Corporation, 800 Phillips Road 103/05B, Webster, NY 14580. * Address correspondence to this author. Abstract published in Advance ACS Abstracrs, June 15, 1995.
*
@
0022-3654/95/2099-10865$09.00/0
interactions in some cases, for example, between poly(propy1ene oxide) and n-octylthioglucoside7and also between (hydroxypropy1)cellulose and some nonionic surfactants.8 Furthermore, also unanswered is the question of whether a polyelectrolyte with high charge density and possible strong electrostatic effect or a slightly less hydrophobic polymeric acid will have any effect on the aggregation behavior of nonionic surfactant at relatively high concentrations compared to their cmc. On the other hand, the association between polymer and ionic surfactant is predicted to be strong and to lead to a stabilization of micelles as reflected by the reduced value of the c ~ c . ~ - I ' The stabilization is thought to originate from a stabilization of the interface between hydrocarbon core and ~ a t e r , 4 *at~which 3 ~ the polymer resides.I2 Nonionic hexaethylene glycol monohexadecyl ether (C16E6) in aqueous solution has been shown to form entangled wormlike micelles of about 60 %, in diameter and from several hundred angstroms to more than a micrometer in length, with or without electrolyte at a temperature range from 30 to 40 OC.I39I4 Another nonionic pentaethylene glycol monododecyl ether (CI 2E5) also forms in aqueous solution wormlike micellesI5that are shorter in length and less entangled. Cetyltrimethylammoniumbromide (CTAB) is by far the most studied cationic surfactant with a fascinating ability to form viscoelastic wormlike micellar solutions at low concentrations in the presence of salicylate anions.l6-Is CI& and C12E5 have different alkyl chain lengths, but both have approximately equally bulky head groups, whereas C&6 and CTAB have the same alkyl chain length but different head group characteristics. So by investigating their different responses to the addition of slightly hydrophobic poly(viny1 methyl ether) (PVME), poly(propy1ene oxide) (PPO),less hydrophobic poly(acry1ic acid) (PAA), and sodium 4-styrenesulfonate polyelectrolyte (PSS) will be helpful in understanding the factors affecting polymer-surfactant interactions. In the past decades, various techniques, such as neutron s ~ a t t e r i n g , 'nuclear ~ - ~ ~ magnetic resonance (NMR),21,22 fluorescence s p e c t r o ~ c o p y ,conductivity ~~-~~ measurements,I0 micro0 1995 American Chemical Society
10866 J. Phys. Chem., Vol. 99, No. 27, 1995
Li et al. 1 oo
*i
t
c
U J
10'
E
Experimental Section Materials. Hexaethylene glycol monohexadecyl ether ( C I A ) and pentaethylene glycol monododecyl ether (C12E5) were obtained from Nikko Chemical Co. Ltd. (Japan). Cetyltrimethylammonium bromide (CTAB, 99%), poly(viny1 methyl ether) (PVME) in the form of 50 wt % aqueous solution, poly(propylene oxide) (PPO), and poly(sodium 4-styrenesulfonate) (PSS) were from Aldrich Chemicals, Milwaukee, WI. Sodium salicylate (Nasal) was purchased from Mallinckrodt, MO, and poly(acry1ic acid) was obtained in powder from Scientific Polymer Product. All chemicals were used without further purification. The molecular weights of PPO are lo00 and 4000; PSS has a molecular weight of 70 000. The molecular weight of PVME obtained from the same source has been determined to be about 27 OO0.7 The viscosity averaged molecular weight of PAA was determined to be 600 000. Solutions of Cl6E6 and Cl6Edpolymer were made by adding doubly-distilled water to weighted quantities of surfactant and polymer and then put in a water bath at 32 "C for several hours. Solutions of CTABNaSal, CTAB/NaSal/polymer,C12E5, and C 12E5/polymer were prepared by adding appropriate amounts of doubly-distilledwater to weighted constituents; the samples were then stirred for overnight at room temperature with a magnetic stirrer rotating at about 2 Hz. Methods. Cryo-TEM samples were prepared in the controlled . environment vitrification system (CEVS), which is described in detail elsewhere.28 In the CEVS, temperature was controlled to within 0.1 "C. Before the sample was introduced into the CEVS, the environmental chamber was brought to steady state at the desired temperature and near saturation of water (95-99% relative humidity). The humidification of the chamber was accomplished with sponges extending upward
'
' " ' # " I
' """I
'
'
"
'
1
0
Figure 1. Cryo-TEM image of 2% C16E6 in water at 32 "C. Long entangled wormlike micelles are formed. (F denotes a frost; B and E are the possible start and end of long wormlike micelles.)
~alorimetry,~ and r h e ~ m e t r y , ' ~have ; ~ ~been used to probe the microstructure of association colloids. Some of these techniques measure the change in cmc and aggregation number with added compounds. In such cases, the concentrations of surfactant solutions are limited to be about or below the cmc. Cryo-TEM is suitable for direct visualization of microstructuresof the size scale of micelles26without the artifacts associated with staining or drying.27 In this paper, cryo-TEM and rheometry are used to study polymer-induced microstructure transitions in the aqueous surfactant solutions.
'
0
10-l
1 oo
10'
Frequency (radls)
1 o2
Figure 2. (a) Steady shear measurement for 2% C16& in water at 32 "C. (b) Dynamic shear measurement for 2% c&6 in water at 32 OC.
from liquid reservoirs. The air inside the chamber was circulated across the sponges to reduce temperature and composition gradients in the vapor. The high relative humidity within the chamber reduced evaporation of water from the sample and prevented the artifacts that results from drying. The steady state of the CEVS was set to 32 "C for the C&6 and Cl,&/polymer samples, and those, samples were put in a water bath at 32 "C before being introduced into the CEVS. All other samples were made at 25 "C. Thin films of sample were formed by placing a 3 pL drop of solution on a holey polymer support film which had been coated with carbon and mounted on the surface of a standard TEM grid.29 The drop was blotted with filter paper so that thin (10500 nm) films of the sample remained, and these spanned the 2-8 pm holes in the support film. After some delay (20-30 s for the CTAB system, 10-20 s for the C&6 system, and about 10 s for the C12E5 system) following the blotting, the entire assembly was then plunged through a synchronous shutter at the bottom of the environmental chamber and into liquid ethane at its freezing point. The 10-30 s delay was used in view of the finding by previous investigators1*and our study that shear alignment of wormlike micelles by blotting relaxed within this period. The vitrified specimens were examined at 120 kV in the conventional TEM mode of an analytical electron microscope (JEOL 1210, JEOL U.S.A., Boston, MA) installed with a minimum dose system (MDS) and a cryo-transfer holder (Model 626, Gatan, Inc., Warendale, PA). The cryo-transfer holder temperature was maintained below - 170 "C during imaging. The condenser lens aperture was set at 100pm, and the objective aperture was set at 50 pm in diameter. The specimen was imaged at a nominal underfocus of 2-4 pm of microscope objective lens in order to achieve adequate phase contrast, which is mainly responsible for gradients of optical density in the images. Images were recorded on Kodak SO-163 film at 20 000-30 OOOx (f5%) and 1-2 s under the MDS mode. The recorded films were developed with full-strength D- 19 developer
J. Phys. Chem., Vol. 99, No. 27, I995 10867
Microstructural Transitions in Surfactant Solutions
lo2
lo2 Eta*(Pa.r)
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
e e e e
e e e e e e e
6
o n
0
1
frequency ,sweep from 0.1 to 100 rads. A couette geometry, i.e. concentric cylinder, was used, and the cup radius was 17 mm and the bob radius and length were 16 and 33 mm, respectively. The c1&6 and Cl6Edpolymer solutions were measured at 32 "C, and all CTAB and C12E5 solutions, with and without added polymer, were measured at 25 "C.
100
Results Long entangled wormlike micelles have a diameter about 60 (Figure l), agreeing with the results obtained before.l3.l4 It is difficult to trace one individual micelle from end to end because the electron micrograph is a two-dimensional projection of microstructures contained in a three-dimensionalspecimen. But some micelles can be estimated to be about 0.1 ,um long. The viscosity from steady shear measurement and the storage modulus G', loss modulus G", and complex viscosity Iq*l from oscillatory shear measurements are shown in Figure 2, a and b, respectively, reflecting the viscoelastic behavior expected from its entangled micellar microstructure. A dynamic strain sweep measurement was also done on the specimen to determine the linear response region of the solution. The results shown in Figure 3 indicate a linear region up to 80% strain. For consistency among results, experiments reported here were collected at a 20% strain ratio. Figure 4 shows cryo-TEM images of 1,2, and 5 g/L PVME added to 2% C1&. No apparent change in the wormlike
A were observed in the cryo-TEM image of a 2%C1&
1 oo
10'
Strain (%)
1 o2
Figure 3. Dynamic strain sweep measurement for 2% C16& in water at 32 "C.
(Eastman Kodak Co., Rochester, NY) for 12 min and photographically enlarged. Steady shear and small-amplitudeoscillatory dynamic shear measurements were performed on a Rheometrics fluid spectrometer RFS II supported by FWIOS software which can convert the measured torque into either G' (the storage modulus) and G" (the loss modulus) in oscillatory shear or q (viscosity) in steady shear experiments. The 10 g.cm transducer was used to detect torque and normal force simultaneously. For steady shear measurements, data were averaged for clockwise and counterclockwisedirections with shear rate from 0.02 to lo00 Us. The dynamic shear measurements were take over the
Figure 4. Cryo-TEM image of 2% C1& in water at 32 "C with (a) 1 g/L PVME, (b) 2 gL PVME, and (c) 5 gL PVME. All show ~ ~ r m l i k e micelles. (d) An unrelaxed sample with 1 g/L PVME shows shear alignment.
Li et al.
10868 J. Phys. Chem., Vol. 99, No. 27, 1995 lo-' n
p?
Q
e w b
c
I
->z (ID
(ID
L c v)
1 o-2
io-'
loo
10' IO* Shear Rate (1h)
lo3
Figure 5. Steady shear viscosity measurement for 2% C1& with the addition of PVME at 32 "C. Some reduction in viscosity with 5 g/L PVME.
micelles microstructure is observed as compared with that of the polymer free c16E6 surfactant solution. Figure 4d is an unrelaxed sample where threadlike micelles were ordered from the shear alignment. Steady shear experiments on ClaEdpVME solutions show in Figure 5 some decrease in viscosity at a concentration of 5 g/L PVME. This corresponds to the loose micellar microstructure shown in Figure 4c. Figure 6 is an image of 5 g/L PVME-2% c1&6 at 38 "C. Under these conditions the solution looked turbid, indicating that phase separation caused the formation of an interesting network structure in which 10-20 wormlike micelles bunch together forming the network. Such a network microstructure has not been reported previously. An unexpected but interesting structural change in the aggregates formed from nonionic c&6 with the addition of nonionic PPO is shown in Figure 7. At 1 g/L PPO, there are elongated micelles, but they look more like straight rods than wormlike structures. At 2 and 5 g/L,elongated discoid micelles or ribbonlike micelle structures are formed. Note the diameter
of the threadlike micelles and the thickness of the ribbon-shaped micelles are larger than the diameter of the wormlike micelles in polymer-free 2% c1&6 solution. Corresponding viscosity measurements also reflected this transition. As shown in Figure 8, there is little change in viscosity with the addition of 1 g/L PPO , but with addition of 2 and 5 g/L PPO there is a large reduction in the viscosity and the solutions show a viscoelastic to Newtonian transition. Figures 9 and 10 are the cryo-TEM images and steady shear viscosity results of the 2% C1& with added 1, 2, and 5 g/L PSS solutions. Both the electron micrographs and viscosity measurements indicate that wormlike micellar microstructure and viscoelastic rheological behavior persist. The cryo-TEM images of the 2% c16& with 1 and 2 g/L PAA are shown in Figure 11, a and b. The steady viscosity of the solutions (Figure 12) is increased by a factor of about 5 with the added PAA. The 2% c1&6solutions with 1 g/L PAA or 2 g/L PAA have the same the rheology behavior. Heavy precipitation occurred when 5 g/L PAA was added to the 2% c1&6solution, which made cryo-TEM and rheology measurements unsuitable for microstructure study. To check the possible viscosity changes brought by the viscosity of the polymer solutions themselves, the steady shear viscosity of the polymer solutions added to the surfactant solution was also measured at 1 and 5 g/L in concentration. Figure 13 shows that all those dilute polymer solutions exhibit Newtonian behavior. Their viscosities are about 1-2 orders of magnitude lower than the viscosity of the surfactantlpolymer solutions at low frequencies. Therefore, the viscosity variation in the surfactantlpolymer solutions can be safely attributed to the change in surfactant microstructurescaused by the interactions between surfactant and the added polymers. The cryo-"EM image of another nonionic surfactant, C12E5 (2%), that forms wormlike micelles in water is shown in Figure 14a. These micelles are not as long as those in c&6 systems.
Figure 6. Cryo-TEM image of the network structure of 2% C1a-5 g/L PVME at 38 OC. About 10-20 wormlike micelles are bunched together between the arrows.
Microstructural Transitions in Surfactant Solutions
J. Phys. Chem., Vol. 99, No. 27, 1995 10869
Figure 7. Cryo-TEM images showing the change of microstructure 2% C1& with the addition of PPO at 32 “C.(a) 1 g/L PPO,stiffened rodlike micelles (R). (b) 2 g/L PPO,disc-shaped bilayer micelles (D) coexist with possible rodlike micelles. (c) 5 g/L PPO,mostly ribbon structure, some are in horizontal positions (H) and some in vertical position with short edge up (Vl) or long edge up (V2).
Figure 14b-g and Figure 15 are the cryo-TEM images of 2% C12E5 solutions with the addition of the same polymer (PVME, PPO, PSS, PAA) at the same concentrations (1, 2, 5 g/L) discussed above. Wormlike micelles are the dominant features in all those images. The same results were obtained whether the molecular weight of PPO added to 2% C12E5 is 4000 or 1000, indicating minimum effect of the polymer molecular weight here. At 5 g/L PAA, there was precipitation from the 2% C12E5 solution, and the sediment was less compact than that from the 2% C I A . The cryo-TEM specimen of the 5 g/L PAA-2% C12E5 solution was extremely radiation sensitive, and we were not able to get its electron microscope image even with great care to minimize the electron dose to the limit of suitable focusing and imaging. This enhanced radiation damage might attribute to the dissolved polymer in the solution.30 The steady shear viscosity (Figures 16 and 17) of the C12E5 and
ClzES/polymer solution is about 1 order of magnitude lower than the C I A systems, and its behavior is essentially Newtonian. G’ and G” are too small to be detected reliably. The addition of the PVME and PSS at all the concentrations and the addition of PAA at 1 and 2 g/L have no effect on the viscosity. The addition of PPO at 5 g/L and PAA at 5 g/L lowers the viscosity. [CTAB]/[NaSal], a cationic surfactant, also forms very viscous long wormlike micellar solutions. A cryo-TEM image of the [CTAB] = 25 mM/[NaSal] = 15 mM solution shows long entangled wormlike micelles of about 60 8, in diameter (Figure 18), and some micelles can be estimated to be about 1 pm long. These long wormlike micelles and their entanglement are consistent with the dynamic viscoelastic behavior of the solution shown in Figure 19. With the addition of polymer PVME and PPO to the CTAB/NaSal solution, its viscoelastic
Li et al.
10870 J. Phys. Chem., VoZ. 99,No. 27, I995
-* ->g c
u) (I)
5
+PPO
c
(2glL)
v)
1o
-~ IO-'
loo
10' Shear Rate (l/s)
lo2
lo3
Figure 8. Steady shear viscosity measurement for 2% C1& with the addition of PPO at 32 "C.Little change with 1 g/L PPO. At 2 and 5 g/LPPO, almost Newtonian behavior with large reduction in viscosity.
behavior disappears. This change is reflected in the steady shear viscosity measurement shown in Figure 20, where it is shown that the addition of even 1 g/L PVME or PPO to the CTABI NaSal solution suffices to transform the solution into a Newtonian fluid with a viscosity close to that of pure water. And the cryo-TEM images in Figure 21 show the corresponding wormlike-to-spherical micelle transition with the addition of the PVME and PPO solutions at different concentrations. At 1 g/L PVME (Figure 21a), there are some wormlike micelles and numerous spherical micelles. The length of wormlike micelles has been reduced to several hundred angstroms. At 2 g/L PVME (Figure 21b), there are mostly spherical micelles and a few short wormlike segments, whereas at 5 g/L PVME (Figure 21c) there are only spherical micelles. A similar sequence of this transition is observed with the addition of 1000 MW PPO to the CTAB/NaSal solution (Figure 21d-f). The same transition is obtained with the addition of 4000 M W PPO to the CTAB/NaSal solution. This polymer-induced breakdown of rodlike micelles and striking transition from non-Newtonian to Newtonian fluids in CTAB/NaSal solutions with the addition of PVME and PPO were also found in Brackman and Engberts's work.31v32The additions of PAA and PSS (1, 2, and 5 g/L)to the CTAB/NaSal solution all cause heavy precipitation, and no rheology and cryo-TEM work were done on those solutions.
Discussion A change in cmc is often used as an apparent and convincing indication of polymer-surfactant interactions, but for the
surfactant solutions with concentrations far above their cmc, this method is not applicable. Cryo-transmission electron microscopy has been used to identify the structure changes in those solutions caused by the introduction of polymers. Coupled with the rheological results, our micrographs provide insight into traditional theories and suggest new hypotheses concerning polymer-surfactant interactions. Effect of Surfactant Alkyl Chain Length. From the geometrical properties of rodlike micelles and Gibbs free energy of polymer-free and polymer-bounded rodlike micelles, a parameter K which is a measure of the preference for the formation of rodlike micelles compared to that of spherical micelles can be expressed as5 for a polymer-free situation 1nK---
k+a, - acy)- In 0
gSP
(
lnl--
(1) acy
asp
acy
for a polymer-bound situation
where the g,, is the number of surfactant molecules in the largest spherical micelle, the 0 is the macroscopic interfacial tension between the hydrocarbon and water, aspis the area per molecule of the largest spherical micelle, and ucyis the area per molecule of the infinite long rodlike micelle. aspand acyequal ~ V & I and 2v&, respectively, where vo is the molecular volume of the hydrophobic tail of the surfactant and ZO is the extended length of the hydrophobic tail, so v& is the cross-sectional area of an alkane hydrophobic tail. For a single alkyl chain it equals approximately 21 A2. up is the cross-sectional area of the polar head group of the surfactant, aPl is the contact area between the polymer and the hydrophobic surfactant surface of the aggregate per molecule of the surfactant, and $I is the electrostaticinteraction parameter describing repulsion between surfactant polar head groups at the aggregate surface. 4 equals zero for nonionic surfactant. For singly-charged ionic am-
Figure 9. Cryo-TEM images of 2% C&6 at 32 "C with addition of (a) 1 g/L PSS, (b) 2 g/L PSS, and (c) 5 g/L PSS. Mostly long rigid micelles.
Microstructural Transitions in Surfactant Solutions
J. Phys. Chem., Vol. 99, No. 27, 1995 10871
enough to destabilize the cylindrical geometry. Whereas in 2% Cl&6 solution, the wormlike to ribbon shaped micelle microstructural transition is clearly shown in the cryo-TEM images f (Figure 7) and further supported by the shear viscosity measure& ment (Figure 8). At 1 g/LPPO, there are mostly rigid rodlike micelles which are less curved as the wormlike micelles in the *RRRRRRRRR polymer free 2% c1&6solution. At 2 g/L PPO, there are some small discs or ribbon-shaped micelles which consist of a planar L m 8 part whose boundary is made up of a hemicylindrical rim. At c u) 5 g/L PPO, long ribbon-shaped discoid micelles dominate. It should be pointed out here that this wormlike to ribbon-shaped A 1. 0-- 2 micelles transition is different from the wormlike to globular IO' loo IO' lo2 lo3 Show Rate (lls) micelles transition; the latter has been relatively well studied and e s t a b l i ~ h e d ~ - ~ ~ 'The ~ ~ ~wormlike ~ ~ ~ ' . ~ ~to ribbon-shaped Figure 10. Steady shear viscosity measurement for 2% C16EdpSS at 32 "C. All show shear thinning similar to polymer-free 2% C,& micelles transition is a new finding here and requires an solution. explanation. PPO is a rather hydrophobic molecule and is found to be folded spirally in tightly coiled discs in aqueous solution' to minimize its contact with water, and so it strongly tends to reside in the more hydrophobic region, as suggested from the fact that PPO is more soluble in apolar hydrocarbon solvent than in the polar water solvent . As a result, PPO prefers to get very deep where vo is the molecular volume of the hydrophobic tail of the into hydrophobic micellar core than staying at the hydrocarbon amphiphile (27 A3for each methylene group and 54 A3 for the core-water interface. This PPO residence in the interior of terminal methyl group), ,!? is a constant which modifies the micelles increases the volume of hydrophobic core and also the Debye-Huckel estimate of the interionic interaction energy packing parameter, favoring transition to a less curved structure. (=0.46), E is the dielectric constant of water (=go), K is the When the PPO concentration is low, less PPO gets in the reciprocal Debye length (=(Cl C,dd)1'2/3.08 A-1 at 25 "C) hydrophobic and wormlike micelles become rodlike micelles. and is the radius of the counterion of the ionic head group of When more PPO is absorbed into the hydrocarbon core at higher the amphiphile, lo is the length of the hydrophobic tail of the PPO concentration, cylindrical micelles are transferred into surfactant (=1.256 A for each methylene group and 2.765 A ribbon-shaped discoidal micelles. The diameter of the rodlike for each terminal methyl group), and 1 is the distance of micelles (Figure 7a,b) and thickness of the ribbon-shaped separation between the hydrophobic core of the segregate and discoidal micelles (Figure 7b,c) in the C16EdPPO are obviously the surface at which the ionic charges or dipoles are located. larger than the diameter of the wormlike micelles in the polymer From eq 1 we can see that the magnitude of K depends on free CI& solution, and this c o n f m s the interior of the c1& the length of surfactant tail through its dependence on g,. Since micelles does grow with the addition of PPO. And this larger the largest sphericalmicelle size g,, increaseswith the surfactant micelle core supports the explanation that some PPO gets into tail length, the longer the length of surfactant alkyl chain is, the micelle interior. If the polymer were absorbed only at the the larger the rodlike micelle to spherical micelle propensity hydrophobic core-water interface, the polymer would behave K. as if grafted to the head group of the surfactant, which would The cross-sectional areas upof the polar head grou of c16& result in high steric repulsion and transfer the wormlike micelles and C12E5 are almost the same (approximately 35 12), so the into spherical micelles. The transition into spherical micelles value on the right-hand side of eq 1 is also the same for c1&6 caused by the increased lateral steric repulsion from the grafted and C12E5 . Since C16E6 has a longer alkyl chain than C I Z E,~ polymers at the head group regions has been well recogthe rodlike micelle to spherical micelle propensity K is larger Very recently, Kwak et al.35also concluded in his for CI&, which is illustrated by the fact that 2% CI& produces NMR study on the system o-phenyl decanoatePE0 that PEO much longer and more entangled wormlike micelles than 2% resides in the interior of the micelle. The mechanism postulated C,,E5 does (compare Figure 1 and Figure 14a), the longer and here concerning the wormlike to ribbon-shaped micelle transition more entangled wormlike micelles in 2% CI& are also reflected is shown schematically in Figure 22. It should also be pointed in the viscosity measurements (Figure 2a and Figure 16) where out that some PPO can also stay outside of the micelles, and the steady shear viscosity of 2% c1&6is more than an order of for these PPO inside the micelle, some of their segments can magnitude larger than that of 2% C12E5. extend to the micelle core-water interface from the interior. Figure 7 and Figure 14e-g show that cl& and CIZES More research using NMR and S A N S is under consideration respond very differently to the addition of poly(propy1eneoxide). to find more compelling evidence that some PPO are in the c ] &has , a wormlike to ribbon-shaped micelle transition C,,& micelle core. whereas the C12E5 micelle remains unchanged with the PPO The same role the alkyl chain length plays in strengthening addition. This different behavior suggests that the nonionic the polymer-micelle interactions has been reported previpolymer-nonionic surfactant interaction is greatly enhanced by ously?-11,36~37 For example, Brackman et al. found that n-octyl increasing the alkyl chain length. C I ~ has E ~little affinity for phosphate micelles do not interact with PEO whereas n-decyl the nonionic PPO: Figure 14e-g indicates that wormlike phosphate micelles do; they also found PEO has a more micelles remain the most prevalent structures upon addition of pronounced interaction with sodium n-dodecyl sulfate (SDS) PPO (MW = lO00) in the 2% C12E5. It follows from these results that either the polymer-micelle interaction in the C I ~ E ~ / micelles than the interaction with sodium n-decyl sulfate (SDeS) micelles. Because the hydrophobic effect is the key factor in PPO solution is not very strong or, if the polymer is adsorbed, determining the nonionic polymer and nonionic surfactant it is incorporated into the micelle in such a way as not to upset complexation, it is not surprising that longer alkyl chain leads the geometric balance of the head and tail group of the C12E5
...........
i
+
Li et al.
10872 J. Phys. Chem., Vol. 99, No. 27, 1995
Figure 11. Cryo-TEM images of 2% C I A at 32 "C.(a) Addition of 1 g/L PAA; some wormlike micelles are bunched together in threads (T) and some of them cross (C) each other perpendicularly, forming a kind of domain structure. (b) Addition 2 of g/L PAA. 1o2 n
OOOOOOOOOOOOOOOOOOOO
-* c
6
(R
-
Ig/L PVYE
0
I g l L PPO
00
z
A
c 0)
5glL PSS
v)
10.4
IO-'
loo
10' lo2 Shear Rate (l/s)
lo3
Figure 12. Steady shear viscosity measurement for 2% CI& / PAA at 32 "C.
to stronger polymer-micelle interaction because the longer alkyl chain, the stronger the hydrophobic interaction. Effect of Ionic Head Group of Surfactant. Ionic surfactants, because of their repulsive electrostatic interionic interac-
10.5
104
10.3
10.2 10-1 100
Shear Rate (l/s)
io1
io*
103
Figure 13. Steady shear viscosity of the dilute polymer solutions.
tion, usually have relatively larger equilibrium areas per head group than nonionic surfactant of the same alkyl chain length and the same head size. For example, CTAB and c1&6 have the same alkyl chain length of about 21 A,whereas the former has an equilibrium area per head group of 65 A2, much larger
Microstructural Transitions in Surfactant Solutions
J. Phys. Chem., Vol. 99, No. 27, 1995 10873
b
Figure 14. (a) Cryo-TEM image of 2% C12Es in water, wormlike micelles about 5 nm in diameter. Cryo-TEM images of 2% CnEs with addition of PVME: (b) 1 g/L PVME, wormlike micelles appear unaffected by the addition of polymer; (c) 2 g/L PVME; (d) 5 g/L PVME, still wormlike micelles. Cryo-TEM images of 2% C12E5 with addition of PPO: (e) 1 g/L PPO ( M W = 1O00), wormlike micelles appear unaffected by the addition of polymer. (f) 2 g/L PPO. (g) 5 g/L PPO, still wormlike micelles.
than that of the c&6, which is about 42 A2. Because of this large equilibrium area per head group, alkyltrimethylammonium bromide cationic single chain amphiphiles form spherical micelles. Only when the repulsive electrostatic interionic interaction is suppressed by the addition of suitable electrolytes, as CTABRVaSal shows, will the spherical micelles be transformed into wormlike micelles. Quite differently from the behavior of nonionic c16E6 and C12E5, the ionic wormlike micelles in the CTABRVaSal solution undergo a wormlike to spherical micelle transition with the addition of PVME or PPO, as can be seen from Figure 21. This transition were also studied by Brackman and E n g b e r t ~ ~byl - ~ ~
viscosity measurement. Other wormlike or rodlike micelle to spherical micelle transitions in ionic surfactant solutions were also observed by Hoffmann and U l b i ~ h twhen ~ ~ they added aliphatic and aromatic hydrocarbons to wormlike micelle systems. They explained this transition on a simple model which is based on geometrical packing consideration^.^^ Since it is the Nasal molecules that induce the spherical to wormlike transition in the first place, one could be tempted to draw the conclusion that the effect of the added polymer is to extract the Nasal from the surfactant layer and reverse the process. By N M R studies, however, Wong et a1.22have shown that the molecular environment of the Nasal does not change
10874 J. Phys. Chem., Vol. 99, No. 27, 1995
appreciably upon the addition of the polymer. We must therefore look for a direct interaction between the added polymer and micelle to understand the mechanism of the transition. In terms of packing volume heuristics, we hypothesize that, in the ionic surfactant case, polymer is adsorbed near the micelle water-ionic head group region. This heuristic argument has been made more quantitative by Nagarajan4Jwith construction of a Gibbs free energy of polymer-micelle association. The topology of the polymer-micelle aggregate is visualized in his model as some of the polymer segments penetrating the interfacial region of micelles and shielding a part of the micellar core from water. The effectiveness of this shielding is described
Li et al.
by a parameter aPl, which is the area per surfactant molecule of the micellar core shielded by the polymer. The quantity uP1 gives rise to three competing contributions to the Gibbs free energy of micelles. First, a decrease in hydrophobic surface area of the micelle exposed to water OCCUTS, which decreases the Gibbs free energy and favors the formation of polymer-bound micelles. Second, steric repulsion arises between the polymer segments and the surfactant head groups at the micellar surface, which increases the Gibbs free energy of the head groups and opposes the formation of the polymerbound micelles. Finally, unfavorable polymer-water contact is reduced when the polymer molecule is removed from water
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and transferred to an essentially hydrophobic surface of micellar core, which decreases the Gibbs free energy and favors the formation of the polymer-bound micelles. From his calculations Nagarajan has concluded that in the case of ionic surfactants the increase in the equilibrium area per molecule of the aggregate due to the steric repulsion arising from the presence of polymer at head group region is much smaller than polymer-surfactant contact area upl. So polymerbound ionic micelles have lower Gibbs free energy although
higher effective head group area. So the packing parameter is smaller and favors the formation of the smaller spheriodal micelles. The mechanism discussed for the wormlike to spherical transition is shown schematically in Figure 23. This morphological transition driven by the tendency of hydrophobic polymers to wrap around the surface of surfactant aggregates was also discussed in Brackman and Engberts's work.31*32 We should note that, upon addition of PPO, the morphological transitions in CTAB and c1&6 are strikingly different. The former has wormlike to spherical micelle transition whereas the latter has wormlike to ribbon-shaped micelle transition, although both form long wormlike micelles in polymer-free solution and they differ only in head group. Two different transitions are explained by the two mechanisms discussed in the previous sections, and they differ in the location of micelle where PPO resides. The differences are clearly brought by the head group difference: charged or uncharged. Effect of Hydrophobicity of Polymer and Surfactant. Strong hydrophobic interaction between polymer and surfactant is necessary for the polymer-induced microstructuralchange to be appreciable. That means the hydrophobicity of either polymer or hydrophobic portion of surfactant or both has to be large enough. Larger hydrophobicityin the C1& tail compared to C12E5 leads to a wormlike to ribbon micelle transition, which is not seen in C12E5. More hydrophobic PPO induces a transition in c1&6which less hydrophobic PVME does not. In the case of alkyltrimethylammoniumsalts, the bulky head group already shields most of the core from contact with water.4o Therefore, only sufficiently hydrophobic polymers will adsorb on the micelles of the alkyltrimethylammonium salts, because the additional reduction in Gibbs free energy due to the transfer of polymer segments from aqueous phase to the hydrophobic micellar core is needed to help to drive the adsorption. Thus, if the polymer is not hydrophobic enough, it will not induce the wormlike to spherical transition. In confirmation of this expectation, poly(acry1ic acid) does not destroy wormlike micelles formed by tetradecyltrimethylammonium bromide (?TAB) and Nasal solution^.^^ Interaction with Polyelectrolyte. Polyelectrolytes have strong interaction with oppositely charged surfactant^^^^^^ due
Figure 18. Cryo-TEMimage of CTAB/NaSal solution without any polymer added in. Long wormlike micelles were formed.
Li et al.
10876 J. Phys. Chem., Vol. 99, No. 27, 1995 t
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complexes with proton-donating poly(carboxylic acid)s in water by cooperation of many hydrogen bonds and the hydrophobic effect, and below a pH around 3 the complexes were insoluble with a stoichiometric composition with respect to proton donor and a~ceptor.~~J" As a result of binding, the configuration of polymers in solution was shrunk. Similarly, in the case of PEOtype nonionic surfactants (like CiEj), the interactions of nonionic surfactant with PAA were reinforced by the presence of a big hydrophobic m0iety.4~9~~ At a high ratio of PMsurfactant the complex contracts whereas at the low ratio it expands. Poly(acry1ic acid) (PAA) (1 and 2 g/L) raises the steady shear viscosity of 2% c1&6 by a factor of 3 as can be seen from Figure 12. Cryo-TEM images of those solutions reveal more entangled microstructures (Figure 1l), where the wormlike micelles are somewhat cross-linked. All those results indicate interaction between PAA and C1&. The high molecular weight PA4 polymer, because of its very long polymer segments, could associate with several wormlike micelles at the same time through both hydrophobic interaction and hydrogen bonds between carboxyl and ethers. The result is to bind the wormlike micelles together and create physical entanglements which contribute to the higher viscosity and multilinked microstructures. Higher PAA concentrations (5 g/L) decrease the pH, increase the binding between nonionic c1&6and PAA, and lead to shrinking of the complex and finally to precipitation. Similar results were reported by Saito et al. 47 PAA solutions (1 and 2 g/L) also raise the steady shear viscosity of 2% CnE5 by a factor of 3 as can be seen from Figure 17, but cryo-TEM images (Figure 15e-f) show little change in the microstructure of the 2% Ci2E5 solutions as PAA is added. These results indicate that either there is only a weak interaction between ClzE5 and PAA because of the weak hydrophobic interaction due to shorter alkyl chain or that there is some PAA-CI~E~ association, but due to shorter wormlike micelles, it is not easy to create an appreciable physical entanglement. The precipitation from 2% Ci2E5 with 5 g/L PAA supports the latter possibility. The sediment from 2% C12E5 is less compact than that from 2% C16E6, indicating less intense binding between PAA with C,,E5 than with C16E6. Our previous study on tetradecyltrimethylaonium bromide (TTAB)-PAA association with cryo-TEM found the wormlike structure of TTAEVNaSal solution was not destroyed with the addition of PAA. In this study, the heavy precipitation upon the addition of PAA to CTAB/NaSal indicates a stronger interaction between PAA-CTA33, which can be attributed to stronger hydrophobic interactions.
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to the electrostatic attractions between the polyion and the ionic head group of the surfactant. Almgren et al.41recently showed that, in the alkyltrimethylammonium surfactants C12TA+ and C16TA+ in dilute solutions of sodium poly(styrenesulfonate), several aggregates are formed at each polyelectrolyte chain and linked by polyelectrolyte segments at a critical surfactant aggregation concentration far below the cmc. The aggregation numbers do not change with increasing surfactant concentration while more aggregates are formed at each polyelectrolyte chain. In our experiments, the concentration of the CTAB (25 mM) is far above the cmc (0.85 mM), and all the additions of PSS (1-5 g/L) cause precipitates, which is due to charge neutralization and the reduced solubility of aromatic compounds. On the other hand, the nonionic surfactants C16E6 and Ci2E5 are much more indifferent to polyelectrolyte. Viscosity results (Figures 10 and 17) show little change upon the addition of PSS to both 2% C16E6 and 2% C12E5 solutions. Cryo-TEM images (Figure 16) also indicate no appreciable changes in 2% C12E5 whereas the wormlike micelles in 2% C16E6 are only stiffened into more rigid rods (Figure 9) which could be attributed to slightly stronger hydrophobic interactions. So there is no strong attraction between nonionic surfactants and polyelectrolytes to break the wormlike micelles, and strong interaction occurs only when the hydrophobic interaction and hydrogen bonding between polyelectrolyte and surfactant are strong enough. Recently, Iliopoulos and Olsson4*used hydrophobically-modified poly(sodium acrylate) (HMPA) (with octadecyl chains randomly anchored with a density less than 5 mol %) to investigate the association with nonionic surfactants ( C I ~ and E~ ClzE8). Strong interactions were found and were attributed to the hydrophobic alkyl chain of the HMPA that dissolved in the surfactant aggregations. Interaction with Polymeric Acid. Many water-soluble nonionic polymers, like PEO and PPO, are known to form
Conclusions The PPO-induced microstructural transitions C16E6 in solutions provide further evidence that the interactions between nonionic polymer and nonionic surfactant can be significant. A new type of wormlike micelle to ribbon-shaped discoid micelle transition has been identified in the PPo-c&6 solutions. The difference between c1& and C12E5 in their response to the addition of PPO indicates the enhancement of polymersurfactant complexation by an increase in the surfactant alkyl chain length. Both PPO and PVME caused a wormlike micelle to spherical micelle transition in the CTAB/NaSal solution. The difference in the final shape of the transformed microstructures between CI& and CTAB suggests that the polymers are in the hydrophobic core of the nonionic Cl& aggregation while in the ionic CTAB situation the polymer wraps micelle around the ionic head group region. Polyelectrolyte (PSS) had little effect on either the microstructure or the viscosity of the CiE, systems. Polymeric acid interacts with surfactants through the
J. Phys. Chem., Vol. 99, No. 27, I995 10877
Microstructural Transitions in Surfactant Solutions
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hydrophobic effect and hydrogen binding, and PAA, although causing some increase in the physical entanglement and
Figure 23. Schematic illustration of CTAB/NaSal ith the ai lition of nonionic polymer. (a) Wormlike micelles of CTAB/NaSal. (b) Polymers wrap at head group regions and increase effective head group area. (c) Finally the wormlike micelles are transformed into spherical micelles.
viscosity, did not destroy the wormlike micelles in the CiEj systems before precipitation due to the shrinking of the complex. It is also shown that, in correlation with rheology, cryo-TEM
10878 J. Phys. Chem., Vol. 99, No. 27, 1995
is a uniquely useful technique in probing the polymer-surfactant associations.
Acknowledgment. We thank Professor Y.Talmon at the Technion-Israel Institute of Technology for helpful discussions. This work was supported by National Science Foundation Center for Interfacial Engineering at University of Minnesota. References and Notes (1) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 13, 531. (2) Robb, I. D. In Anionic Surfactants-Physical Chemistry of Surfacrant Action; Lucassen-Reynders, E. H., Ed.; Dekker: New York, 1981; p 109. (3) Steinhardt, J.; Reynolds, J. A. Multiple Equilibrium in Proteins; Academic Press: New York, 1969. (4) Nagarajan, R. Colloids Surf:1985, 13, 1. (5) Nagarajan, R. J. Chem. Phys. 1989, 3, 1980. (6) Ruckenstein, E.; Huber, G.; Hoffman, H. Langmuir 1987, 3, 382. (7) Brackman. J. C.: van Os. N. M.: Eneberts. J. B. F. N. LanPmuir 1988,4, 1266. (8) Winnik. F. M. Lanamuir 1990. 6. 522. (9) Goddad, E. D. Colibids Surf:1986, 19, 255. (10) Brackman, J. C.; Enaberts, J. B. F. N. Lannmuir 1991, 7, 2097. (11) Brackman, J. C.; EGberts, J. B. F. N. J. Colloid Interface Sci. 1989, 132, 250. (12) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (13) Cummins, P. G.; Staples, E.; Penfold, J.; Heenan, R. K. Lungmuir 1989, 5, 1195. (14) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (15) Lin, Z. Ph.D. Dissertation, University of Minnesota, 1993. (16) Gravsholt, S. Rheology; Plenum Press: New York, 1981. (17) Shikata, T.; Sakaiguchi, Y.;Uragami, H.; Tamura, A.; Hirata, H. J. Colloid Interface Sei. 1987, 119, 29 1. (18) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (19) Cabane, B.; Duplessix, R. Colloids Surf:1985, 13, 19. (20) Leung, P. S.; Goddard, H. C.; Glinka, C. J. Colloids Surf:1985, 13, 47. (21) Smith, M. L.; Muller, N. J. Colloid Interface Sei. 1975, 52, 507. (22) Wong, T. C.; Liu C.; Chiu-Duen, P.; Davis, K. Langmuir 1992, 8, 460. (23) Ananthapadmanabhan, K. P.; Leung, P. S.; Goodard, E. D. Colloids Surf:1985, 13, 63.
Li et al. (24) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam,J. V. Langmuir 1992, 8, 2405. (25) Saito S.In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; pp 881-926. (26) Vinson, P. K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J. Colloid Interface Sei. 1991, 142, 74. (27) Kilpatrick, P K.; Miller, W. G.; Talmon, Y. J. Colloid Intelface Sci. 1985, 107. 146. (28) Bellare, J. R.; Davis, H. T.; Scriven. L. E.; Talmon, Y. J. Electron Microsc. Technol. 1988, 10, 87. (29) Vinson, P. K. Proc. 45th Annu. Meeting MSA 1987, 644-645. (30) Talmon Ishi, Private communication, Feb. 1994. (31) Brackman, J. C.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1990, 112, 872. (32) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (33) Needham, D.; McIntosh, T.; Lasic, D. Biochim Biophys. Acta 1992, 1108, 40. (34) Kenworthy, A.; McIntosh, T.; Needham, D.; Hristova, K. Biophys. J. 1993, 64, A348. (35) Gao, Z.; Washylishen, R. E.; Kwak, C. T. J. Colloid Inte$ace Sei. 1990, 137, 137. (36) Perron, G.; Francoeur, J.; Desnoyers, J. E.; Kwark, J. C. T. Can. J. Chem. 1987, 65, 990. (37) Shirahama, K.; Ide, N. J. Colloid Interface Sei. 1976, 54, 450. (38) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sei. 1989, 129, 388. (39) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (40) Berr, S.S.;Caponetti, E.; Johnson, J. S., Jr.; Jones, R. R. M.; Magid, L. J. J. Phys. Chem. 1986, 90, 5766. (41) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J. V. Langmuir 1992, 8, 2405. (42) Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500. (43) Ikawa, T.; Abe, K.; Honda, K.; Tsuchida, E. J. Polym. Sci., Polym. Chem. Educ. 1975, 13, 1505. (44)Inoue, M.; Otsu, T. J. Polym. Sci., Polym. Chem. Educ. 1976, 14, 1933. (45) Saito, S.; Taniguchi, T. J. Colloid Interface Sei. 1973, 44, 114. (46) Musabekov, K. B.; Abdiev, K. Zh.; Aidarova, S. B. Kolloid Zh. 1984, 46, 374. (47) Saito, S.; Taniguchi, T.; Matsuyama, H. Colloid Polym. Sci. 1976, 254, 882. JW43 173H
