Langmuir 1988, 4 , 1266-1269
1266
Polymer-Nonionic Micelle Complexation. Formation of Poly(propylene oxide)Xomplexed n -0ctyl Thioglucoside Micelles Josephine C. Brackman,t Nico M. van Os,$ and Jan B. F. N. Engberts*it Department of Organic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands, and KoninklijkelShe11-Laboratorium,Amsterdam, P.O. Box 3003, 1003 AA Amsterdam, The Netherlands Received April 29, 1988. I n Final Form: June 14, 1988 The possible formation of polymer-micelle complexes of the water-soluble nonionic polymers poly(ethy1ene oxide) (PEO),poly(propy1ene oxide) (PPO), poly(N-vinylpyrrolidone)(PVP),poly(viny1alcohol)-poly(viny1 acetate) copolymer (PVA-PVAc, 17% acetate content), and (hydroxypropy1)cellulose (HPC) with n-octyl thioglucoside (OTG), n-octyltriethylene glycol ether (C8E3),and several zwitterionic surfactants has been investigated. The finding that the critical micelle concentration of these surfactants is not influenced by the presence of polymer does not exclude polymer-micelle complexation since PPO-OTG interactions have been definitely established by using microcalorimetry. PPO-OTG complexation is further supported by the change in turbidity of the PPO solution at the cmc of OTG, the perturbed clouding behavior of PPO, and the reduced Krafft temperature of OTG.
Introduction In the past 2 decades, there has been considerable interest in the association of surfactant micelles with water-soluble nonionic polymers.'p2 These polymer-micelle complexes are of great industrial importance, for instance, in cosmetic products, paints, and coatings and in tertiary oil recovery applications. Furthermore, they are of biological interest since they may function as models for aspects of protein-membrane interactions. The majority of studies in this area concentrates on complexes of anionic surfactants (almost exclusively sodium dodecyl sulfate3 and homologues) with polymers such as poly(ethylene oxide) (PEO),4-9poly(propy1ene oxide) (PPO)? and poly(N-vinylpyrrolidone)(PVP).8JoJ1 Cationic surfactants (largely alkylammonium ~ a l t s ) ~ * form ~ @only J very weak complexes or do not interact a t all.4p7 Nonionic surfactants4p7are usually considered to be totally indifferent toward polymers, although n-nonylphenol-polyethylene glycol ether interacts with PE012 and (hydroxyethy1)cellulose (HEC).13 The former complexation has been attributed to an affinity of the phenol moiety for PE0.14 By contrast, viscosimetric measurements did not provide evidence for interaction between n-octyl phenolethoxylate and PE0.7 Viscosity and clouding point measurements have provided evidence for interactions between surfactants of the polyethylene glycol ether type and some mildly hydrophobic (co)polymers and poly(carboxy1ic acid)s.15 In order to explain and quantify the influence of surfactant head-group structure on interaction with polymers, both Nagarajan7 and Ruckenstein4 developed detailed models. On the basis of different points of view, both authors stress the importance of the relative contributions of stabilization of the water-hydrophobic core interface by the polymer on one hand and the unfavorable interaction between the surfactant head groups and polymer segments on the other hand. According to Nagarajan? the latter interaction stems from steric repulsion; according to Ruckenstein: the interfacial tension between the head groups and water is unfavorably influenced by polymer association. Since nonionic surfactants invariably possess bulky head groups, the area of hydrophobic core-water contact is limited, and, as a result, association with poly+ University
of Groningen.
* Koninklijke/Shell-Laboratorium. 0743-7463/88/2404-1266$01.50/0
mers is predicted not to O C C U ~ . ~ JHowever, in Nagarajan's model the free energy of transfer of the polymer from the aqueous phase to the micellar pseudophase is not taken into account. In Ruckenstein's treatment this quantity is implicitly accounted for in the experimental method for estimating the change in interfacial tension induced by the polymer. But this experimental method cannot be used for water-soluble polymers such as PP0l6 and HPC,3b which are soluble in nonpolar solvents. Exactly these polymers are known to show the strongest interaction with sodium dodecyl sulfate (SDS)3i6and cetyltrimethylammonium bromide (CTAB)3s6and are likely candidates for favorable interactions with nonionic surfactants. In the present study we provide strong evidence for the association of PPO with micelles formed from the nonionic surfactant n-octyl thioglucoside (OTG)." It is suggested that the predicted destabilizing effect of PPO on the micelle is more than compensated by a favorable free energy of transfer of the polymer from water to the micelle.
Experimental Section Materials. The surfactants C8E, (Kwant) and OTG (n-octyl thioglucopyranoside, Sigma) were used as received. The zwitterionic surfactants were synthesized and kindly provided by K. (1)Breuer, M. M.; Robb, I. D. Chem. Ind. (London) 1972,13, 530. (2)Goddard. E. D. Colloids Surf. 1986. 19,255. (3)(a) Turro, N. J.; Bonetz, B. H.;Kwd, P.'L. Macromolecules 1984, 17, 1321. (b) Winnik, F. M.; Winnik, M. A.; Tazuke, S. J.Phys. Chem. 1987,91, 594. (4)Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987,3,382. (5)Shirahama, K.;Ide, N. J. Colloid Interface Sci. 1976,54, 450. (6)(a) Witte, F. M.; Engberts, J. B. F. N. J. Org. Chem. 1987,52,4767. (b) Witte, F. M.; Engberta, J. B. F. N. J. Org. Chem. 1988,53,3085. (7)(a) Nagarajan, R. Colloids Surf. 1985,13, 1. (b) Nagarajan, R.; Kalpacki, B. In Microdomaina in Polymer Solution; Dubin, P., Ed.; Plenum: New York, 1985;p 357. (c) Nagarajan, R.; Kalpacki, B. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1982,23(1),41. (d) Nagarajan, R. Adu. Coll. Interface Sci. 1986,26, 205. (8)Witte, F.M.; Buwalda, P. L.; Engberta, J. B. F. N. Colloid Polym. Sci. ... 1987. , 265. - - - ,42.
(9)Cabane, B. J. Phys. Chem. 1977,81,1639. (10)Lange, H. Colloid Polym. Sci. 1971,243,101. (11)Fadnavis, N.W.; Van den Berg, H. J.; Engberts, J. B. F. N. J.Org. Chem. 1985,50,48. (12)Szmerkovg, V.; Krfilik, P.; Berek, D. J . Chromatogr. 1984,285, 188. (13)Boscher, Y.;Lafuma, F.; Ouivoron, C. Polym. Bull. 1983,9,533. (14)Pletnev, M.Y.; Trapeznikov, A. A. Kolloidn. Zh. 1978,40,948. (15)For a review, see: Saito, S. In Nonionic Surjuctants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987;Chapter 15. (16)Molyneux, P.Water-SolubleSynthetic Polymers: Properties and Behauior; CRC: Boca Raton, FL, 1984;Vol. I. (17)Tsuchiya, T.; Saito, S. J. Biochem. 1984,96,1593.
0 1988 American Chemical Society
Polymer-Nonionic Micelle Complexation
Langmuir, Vol. 4, No. 6, 1988 1267
Table I. cmc Values (mM) for Micelles in Water and in the Presence of Polymers' surfactant T, O C HZO PEO PPO HPC PVA-PVAC n - C 1 ~ H 2 i ~ N * - l C H 2 ~ ~ W ~
44
2.5'
2.5'
2.6'
~ ~ - C I I H ~ ~ ~ * - C H ~ - C O ;
45
0.32'
0.33'
0.33'
n-Cl2HZS~N*-(CH2)2S09-
55
0.36'
0.38'
0.35'
25 20
C8E3 OTG OTG
6.14c
6.17c
10.ld 8.05O 8.7d 9.2'
25
PVP
6.17c
10.ld
s.10e
8.10e
8.7d 8.8'f
'
'Polymer concentration 0.5 g.dL-' for the zwitterionic surfactants and OTG; 1.0 gdL-' for CBE3 Bromophenol blue absorption method. Surface tension method. Pyrene fluorescence method. e Microcalorimetric method. f In DzO: 8.2 mM. - A 7 1 0 ) x lo3 0 0 0 A A
A
A
A
A
A
0
20
1 160
0
& A
,eo
o!o
140icc 120 000
c
I
240
480
1LI40
0
mM
[OTG]
-
c
A A A A A A A
960
o
o
, 720 [OTG]
n 960 mM
~
o
~
' 1200
1
I
1440
1680
Figure 1. Bromophenol blue absorption and turbidity in a 0.5 g 4 - l PPO solution as a function of the 0% concentration. A710 is a measure of the turbidity; Aem-A710 denotes the 620-nm absorption of bromophenol blue corrected for the turbidity. Hovius and A. Kuiterman.ls The purifications of PEO (Fluka, weight-averagedMW 1 O O O O ) ~PVP (Kolloidon-90,BASF),18and PVA-PVAc (acetate content 17%,Mowiol3-83, Hoechst)" have been described previously. PPO (Janssen, weight-averagedMW 1000), HPC (Aldrich, average MW lOOOOO), and the probes bromophenol blue (Merck) and pyrene (Aldrich) were used as received. cmc Measurements. Spectrophotometric measurements of the cmc were performed by determining the absorption of bromophenol blue at a suitable wavelength between 600 and 620 nm at a probe concentration of 6 X lo4 M with a Perkin-Elmer A5 spectrophotometer. In the case of OTG in the presence of PPO (measurements at 610 nm), a small correction had to be made to account for the change in turbidity. This was done by subtraction of the absorption at 710 nm, outside the bromophenol blue absorption band (Figure 1). The pyrene fluorescence method for determining cmc values is a well-documented pr~cedure.~ Measurements were performed on a SLM-Aminco SPF-500 C spectrofluorometer. Surface tension measurements were carried out by using the Wilhelmy-platemethod. Plots of surface tension vs C8E3concentration showed no minimum. In all cases thermostated sample solutions were used (see Table I). Microcalorimetry. Microcalorimetric measurements were performed by using a LKB 2277 heat-flow microcalorimeter described e1se~here.l~ (18) Fadnavis, N.; Engberta, J. B. F. N. J. Am. Chem. SOC.1984,106, 2636. (19) (a) Wadsoe, I. Thermochim. Acta 1985,85, 245. (b) Nordmark, M. G.; Laynez, J.; Schoen, A.; Suurkuusk, J.; Wadsoe, I. J. Biochem. Biophys. Methods 1984, 10, 187. (c) van Os, N. M.; Haandrikman, G. Langmuir 1987, 3, 1051.
Table 11. Clouding Temperatures of PPO" medium cloud temp, O C H,O 26-37 26-37 DiO 30 HzO + OTG (20mM) DzO + OTG (15 mM) 25 "0.5 gdL-'. Clouding Points and Krafft Temperatures. Clouding points and Krafft temperatures were determined by recording the transmission at 500 nm of vigorously stirred dispersions as a function of temperature by using a Perkin-Elmer A5 spectrophotometer. The clouding point of PPO was taken as the temperature representing the midpoint of the change in transmission in the case of a narrow transition region (1-2 "C) or as the temperature range in the case of a broad transition region (=lo "C). The Krafft temperature of OTG is taken as the onset of the sudden increase in transmission in a 20 mM OTG dispersion in H20 or a 15 mM OTG dispersion in DzO.
Results and Discussion One of the most convincing indications for polymermicelle interactions has always been a reduced value of the cmc in the presence of polymer.12J0 We will show that this criterion appears not to be generally valid. Table I lists cmc values of several zwitterionic20and nonionic surfactants in water and in the presence of various nonionic polymers. For all surfactants, the cmc values are virtually unchanged by the presence of polymer. Even HPC? which is able to lower the cmc of SDS by a factor of 15 and that of CTAB by a factor of 4, has no effect on the cmc of OTG. Although the conclusion that these nonionic and zwitterionic surfactants do not interact with the polymers would be attractively in accord with theories for polymer-micelle interaction (vide supra)?' it is definitely not true for the combination PPO/OTG. It is known that the rather hydrophobic PPO is folded spirally in tightly coiled disks in aqueous solution.20 We find that even at 25 O C these disks tend to aggregate slightly, producing a nearly visible turbidity. This turbidity is, however, suddenly reduced upon addition of OTG in a concentration equal to or beyond the cmc, presumably because of interactions between PPO and OTG micelles. The change in turbidity is the result of the perturbed clouding behavior of PPO in the presence of OTG micelles (Table 11). Clouding of PPO in H20 and D20 is a gradual process taking place in a temperature range of over 10 OC. However, in the presence of OTG clouding occurs abruptly within 2 OC, indicating a cooperative process. In D20, OTG shifts the (20) Hovius,K.; Kuiterman, A.; Engberta, J. B. F. N., to be published. (21) (a) Sandell, L. S.; Goring, D. A. I. Macromolecules 1970,3,50. (b) Sandell, L. S.; Goring, D. A. I. Makromol. Chem. 1970, 138,77.
1268 Langmuir, Vol. 4 , No. 6,1988
Brackman et al.
15 min
top -
9
I
1.4o/uw
10
11 12
bottom
17 19
18
20
21
Figure 2. Top: Microcalorimeter response curve upon titration of a concentrated OTG solution into a PPO solution with the final OTG concentration remaining below the cmc. The encircled numbers refer to the titration steps; i.e., 9 corresponds to the ninth titration step, see also Figure 3. Each response consists of an endothermic and an exothermic peak. Bottom: Similar data, but now the final OTG concentration is beyond the cmc. Note the increase of the endothermic signal relative to that shown in the top part. The exothermic effect has disappeared completely beyond titration step no. 18. Signal noise is caused by the stirrer. Temperature is 25 "C. Table 111. Krafft Temperatures of OTG medium Krafft temp, O C HZO
