Grafting of Diblock Copolymers onto Adsorbed Surfactant Films

Collective stochastic resonance in shear-induced melting of sliding bilayers. Moumita Das , G. Ananthakrishna , Sriram Ramaswamy. Physical Review E 20...
0 downloads 0 Views 398KB Size
Download PDF
1634

Langmuir 2002, 18, 1634-1640

Grafting of Diblock Copolymers onto Adsorbed Surfactant Films C. Robelin,†,‡ F. P. Duval,§ P. Richetti,*,†,‡ and G. G. Warr§ CNRS/Rhodia Complex Fluids Laboratory, Rhodia Inc., CN 7500, Prospect Plains Road, Cranbury, New Jersey 08512-7500, Centre de Recherche Paul Pascal, Avenue Schweitzer, 33600 Pessac, France, and School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia Received July 18, 2001. In Final Form: November 27, 2001 We studied the grafting of diblock copolymers onto mica surfaces in aqueous solutions. Because the copolymers are nonadsorbing on mica, grafting is achieved in the presence of adsorbing cationic surfactants. The surfactant film makes a sublayer onto which the hydrophobic moiety of macromolecules can end anchor. By using both a surface force apparatus (SFA) and an atomic force microscope (AFM), we studied how the interactions between the surfactant-coated surfaces are modified by the presence of copolymer and show how these modifications relate to changes in the morphology of the adsorbed surfactant film.

Introduction Mixtures of polymers and surfactants are present in a wide range of solutions for applications as diverse as detergency, hair and body-care, and DNA transfection. Such applications have motivated many applied and fundamental studies. The bulk properties of polymer/ surfactant solutions have been discussed in many review articles or monographs dedicated to the field.1,2 However, the surface properties of these mixtures, a determining factor in many applications, are beginning to receive more attention.3-8 Fleming3 et al. detected a competition between cationic polymer and anionic surfactant adsorbing onto graphite from aqueous solution by atomic force microscopy (AFM). Giasson et al.4 previously demonstrated this competition effect between a cationic surfactant and a polymer in mineral oil. Shubin et al.6 and Anthony et al.7 studied how the interactions between cationic polymers adsorbed on mica are influenced by an anionic surfactant. Argilier et al.8 similarly studied cationic and nonionic polymers adsorbed onto mica in the presence of anionic surfactants. The effect of depletion forces has also been studied when the degree of association between the surfactant and the macromolecule is varied.9,10 In this paper, we discuss how a nonadsorbing polymer can be * Author to whom correspondence should be addressed. E-mail: [email protected]. † Rhodia Inc. ‡ Centre de Recherche Paul Pascal. § University of Sydney. (1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (3) Fleming, B. D.; Wanless, E.; Biggs, S. Langmuir 1999, 15, 8719. (4) Giasson, S.; Weits, D. A.; Israelachvili, J. N. Colloid Polym. Sci. 1999, 277, 403. (5) Claesson, P.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (6) Shubin, V. Langmuir 1994, 10, 1093. (7) Anthony, O.; Marques, C.; Richetti, P. Langmuir 1998, 14, 6086. (8) Argillier, J. F.; Ramachandran, R.; Harris, W. C.; Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242. (9) Ruth, M.; Yoshisawa, H.; Fetters, L. J.; Israelachvili, J. N. Macromolecules 1996, 29, 7193. (10) Kuhl, T.; Guo, Y.; Alderfer, J. L.; Berman, A. D.; Leckband, D.; Israelachvili, J.; Wen Hui, S. Langmuir 1996, 12, 3003.

induced to adsorb by mixing with a surfactant chosen to associate with the polymer and to adsorb onto the substrate. Mica is a model hydrophilic surface and has been extensively used to study both the adsorption of surfactants11-14 and the interactions15-17 between such coated surfaces. Adsorbed structures on mica are now welldescribed for a wide variety of amphiphilic molecules.11-14 It is of fundamental interest to determine how the structure of such films can be modified when polymer is gradually incorporated among adsorbed amphiphilic molecules, as well as the consequences for interactions between such coated surfaces. To assess the influence of polymer incorporation, both a surface forces apparatus (SFA) and an atomic force microscope (AFM) have been employed. The polymers under investigation are neutral hydrophilic-hydrophobic poly(oxyethylene) diblock copolymers, and the surfactant is the cationic surfactant dodecyltrimethylammonium bromide (DTAB). AFM images of DTAB adsorbed films on mica in solutions above the critical micelle concentration (cmc) have shown aggregates interpreted to be flexible cylinders.12 Interactions between cationic-surfactant-coated mica surfaces are dominated by a double-layer force at large separations, typically greater than 5 nm.15 Upon addition of neutral hydrophilic-hydrophobic diblock copolymers, we expect that the macromolecules will end-graft onto the adsorbed surfactant film by means of their hydrophobic blocks. Polymers grafted onto surfaces by various means, including functionalization of the polymers and covalent bonding, have been intensively studied, but to our knowledge, none have involved anchoring in an adsorbed surfactant layer. Previous studies have shown that the influence of various parameters such as grafting density, solvent quality, and (11) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. V.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (12) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (13) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (14) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5, 88. (15) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (16) Ke´kicheff, P.; Christenson, H. K.; Ninham, B. W.Colloids Surf. 1989, 40, 31. (17) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. Langmuir 1991, 7, 2694.

10.1021/la0111194 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002

Grafting of Diblock Copolymers onto Adsorbed Surfactant Films

Langmuir, Vol. 18, No. 5, 2002 1635

polydispersity can lead to pancake, mushroom, or brushlike self-assembly of the polymer layers.18 The paper is organized as follows. In section 2, we describe the materials and the experimental techniques used in this work. Section 3 is devoted to the experimental results obtained with the two techniques. The modification of the force profiles as a function of the copolymer concentration, Cp, is then presented. For comparison, AFM pictures of the adsorbed layers are also presented for the same concentration range. Section 4 consists of the discussion. Experimental Section Dodecyltrimethylammonium bromide, DTAB (99.9 atom %, Aldrich); poly(ethylene oxide) stearates (Myrj 53, Mw ) 2484 g/mol; Myrj 59, Mw ) 4684 g/mol) obtained from Sigma and referred to here as D50 and D100, respectively; and diblock polyoxyethylene-polystyrene (Mw ) 58 700 g/mol) from Polymer Expert, referred to as D1300, were used as received. These diblock copolymers are water-soluble polymers consisting of a hydrophobic and hydrophilic sequences. We denote these polymers Dn, where n indicates the mean number of oxyethylene monomers in the hydrophilic moiety. The hydrophobic moieties are always shorter, consisting of a C18 alkyl chain for D50 and D100 and a styrene chain of 14 monomers for D1300. These diblock copolymers are expected to self-assemble into micelles in water at very low concentrations,19 0.184 wt %. DTAB/D1300 Solutions. When D1300 copolymer is added to a 1 cmc DTAB solution, the force profile is no longer exponential as was the case for the two other copolymers. Interactions remain repulsive and reversible over loading-unloading cycles. However, as illustrated in Figure 5, they are much longer-range than those in the reference profile at Cp ) 0 wt %. The observed interactions

Grafting of Diblock Copolymers onto Adsorbed Surfactant Films

Figure 5. Comparison of force profiles between two mica surfaces immersed in aqueous solutions of 1 cmc DTAB surfactant alone (+) and in the presence of D1300 diblock copolymer (b, compression; O, decompression). The solid line corresponds to the best fit obtained with the Alexander-de Gennes model (eq 2).

can be reasonably fitted by the Alexander-de Gennes24,25 model for the interaction between two grafted polymer brushes

[(

2L0 F 16πkBTL0 ) 7 3 R D - 2τ 35s

)

5/4

+5

(

)

D - 2τ 2L0

7/4

]

- 12 (2)

where L0 is the thickness of a brush, s is the mean separation between two anchoring sites, and D is the separation between the two surfaces. τ is the thickness of the adsorbed surfactant sublayer into which the polymer is anchored, and it is set here to (approximately) 3 nm. A fit to eq 2 is shown in Figure 5 for the profile of a 0.636 wt % D1300 copolymer solution. From the best fit, we obtain a brush thickness of L0 ≈ 18 nm. This value lies between twice the radius of gyration of the PEO sequence if it is assumed to be Gaussian (2Rg ≈ 10 nm) and if it as in a good solvent (2Rg ≈ 24 nm); see below. The fit also yields the distance between two grafted chains, s ≈ 9.5 nm, which is close to the value of Rg under good-solvent conditions. These values suggest that the grafting density is in the crossover domain between the mushroom and brush regimes.26 AFM Imaging. Adsorbed Film Structure. Figure 6 shows 200 × 200 nm deflection images of adsorbed DTAB layers on a mica surface immersed in a 1 cmc solution in the absence and presence of D50 diblock copolymer. In the absence of D50 (Figure 6a), the adsorbed film appears as parallel stripes meandering across the surface. The average center-to-center separation of the stripes is about 5 nm, and they exhibit some preferential orientation over a large area. Figure 6a is similar to results reported elsewhere,12 which have been interpreted as full cylinders lying on the surface.12 At low D50 concentration (0.01 wt %) (Figure 6b), the cylinder structure is qualitatively preserved. However, at larger Cp, the cylinders are gradually transformed into globular aggregates. The morphology transition appears to be continuous, because, for intermediate copolymer concentrations, the adsorbed film consists of a mixture of small cylinders and spherical aggregates as exemplified in Figure 6c (0.053 wt %). At larger Cp values, the transition is complete, and the aggregates are all globular in shape, as shown in Figure 6d (0.5 wt %). Fourier transforms of these images show that the mean separation between the aggregates in(24) Alexander, S. J. Phys. (Paris) 1977, 38, 983. (25) de Gennes, P. G. C. R. Hebd. Seances Sci. 1985, 300, 839. (26) Omarjee, P.,; Hoerner, P. ; Riess, G.; Cabuil, V.; Mondain-Monval, O. Eur. Phys. J. E 2001, 4, 45.

Langmuir, Vol. 18, No. 5, 2002 1637

creases from approximately 5 to 7 nm as the aggregate shape changes. Table 1 lists the observed aggregate structures as a function of the bulk copolymer concentration for D50/DTAB mixtures as well as for the others two polymers studied. The same general behavior is observed with both the D100/DTAB and D1300/DTAB systems; Figure 7 shows an image in the final stage for both systems, with Cp equal to 0.18 wt % for D100/DTAB and 1 wt % for D1300/DTAB. The AFM images result from a constant force interaction between the tip and the surface. This means that the Fourier-transformed radius of the cylindrical or globular micelle images corresponds to the centerto-center separation, ∆, between adsorbed aggregates. It is believed that the aggregate diameters are on the order of magnitude of twice the chain length of the surfactant hydrophobic tail, i.e., 3.3 nm. The 5-7-nm spacing corresponds to that observed for similar surfactants on mica or for the same surfactant on other charged surfaces. This distance is believed to be determined by the electrostatic interaction between the globular micelles. The maximum gap between the micelles should therefore be around 3.7 nm. An alternative interpretation of these images would be that the bright and dark spots correspond to the underlying DTAB covered and uncovered, respectively, by the grafted diblock copolymer. Indeed, one might expect that the presence of the polymer would locally modulate the interaction between the tip and the coated surface. However, as shown above, similar results for different PEO molecular weights of very different characteristic sizes led us to reject this explanation. Discussion Force curves and AFM images together show that the diblock copolymers are adsorbed onto the surfactant film and that they change both the structure of the adsorbed surfactant layer and the interactions between the coated mica surfaces. Morphology Transition and Shift in Dc. As Table 1 shows, an increase in polymer concentration leads to a morphology transition from cylinders to globules on the surface for all three diblock copolymers, regardless of the PEO and hydrophobic tail sizes. This demonstrates clearly that these three nonionic diblock polymers are all adsorbed onto the surfactant-coated mica. The shape transition arises from the anchoring of the polymer hydrophobic tail into the DTAB film, which can be regarded as creating defects in the cylindrical micelles. Because of the experimental noise, it is difficult to precisely define a limiting concentration above which the surface micelles are completely globular; however, the data strongly suggest that it is a function of the PEO size. For larger PEO chains, the shape transition to globules is completed at lower (molar) concentrations (see Table 1). As it has been shown that PEO chains do not aggregate in solution27,28 or adsorb onto cationic surfactant micelles at room temperature,29 we describe an individual diblock copolymer molecule as a hydrophobic chain attached to a PEO sphere of radius Rg. As Gaussian chains, Rg would be given by l(n/6)1/2, where l is the length of a monomer, i.e., 0.35 nm for an EO monomer,10,30 and n is the number of monomers in the chain. This leads to Rg values of approximately 1.0, 1.4, and 5 nm for n ) 50, 100, and (27) Devanand, K.; Selser, J. C. Macromolecules 1991, 24, 5943. (28) Kinugasa, S.; Nakahara, H.; Fudagawa, N.; Koga, Y. Macromolecules 1994, 27, 6889. (29) Anthony, O.; Zana, R. Langmuir 1994, 10, 4048. (30) Borbely, S. Langmuir 2000, 16, 5540.

1638

Langmuir, Vol. 18, No. 5, 2002

Robelin et al.

Figure 6. AFM images of the adsorbed-layer structure of DTAB on mica at 1 cmc with progressive addition of D50 diblock copolymer. (a) DTAB only (Cp ) 0 wt %) and (b) Cp ) 0.01 wt % show long cylinders, whereas (c) Cp ) 0.053 wt % shows a mixture of short rods and spheres or globules, and (d) Cp ) 0.5 wt % shows complete conversion of the surfactant layer into globular aggregates. Table 1. Morphologies of Aggregates in the Adsorbed Layer Observed by AFM as a Function of Diblock Copolymer Concentrationa D50 D100 D1300

cylinders

globules

Cp e 0.02% (0.08 mM) Cp e 0.053% (0.11 mM) Cp e 0.08% (0.014 mM)

0.14% (0.56 mM) e Cp 0.18% (0.38 mM) e Cp 1% (0.17 mM) e Cp

a A gradual transformation from cylinders into spheres is observed at intermediate concentrations.

1300, respectively. However scattering studies27,31,32 have shown that water is a rather good solvent for PEO, leading to an exponent nearer to 0.633 and yielding Rg values of 1.7 ( 0.2, 2.5 ( 0.3, and 12 ( 1 nm for D50, D100, and D1300, respectively. The Rg value calculated for D1300 is greater than onehalf the distance between two adsorbed DTAB micelles, ∆/2. This means that only one diblock copolymer per (31) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (32) Kawagushi, S.; Imai, G.; Suzuki, J.; Miyahhara, A.; Kitano, T.; Ito, K. Polymer 1997, 38, 2885. (33) Flory, P. J. Statistical Mechanics of Chain Molecules; WileyInterscience: New York, 1969

globular micelle is needed for complete coverage of the surface. However, Rg(D50) < Rg(D100) < ∆/2, so each globular DTAB micelle can accommodate more than one of the smaller anchored copolymers; hence, surface saturation might be expected to occur at higher copolymer concentrations. It is of interest to note that the shift in Dc observed in the force profiles (Figure 3) correlates well with the change in morphology of the adsorbed films. This is further evidence of the polymers anchoring themselves to the adsorbed surfactant. However, in contrast with the gradual change in adsorbed-layer morphology (Figure 6), Dc changes abruptly above a certain concentration (Figure 3). This can be explained by considering that the anchored polymer molecules can migrate away from the load region in the SFA, so that the surface morphology in the load region (and Dc) corresponds to one with a lower Cp. If we assume a uniform mixed film, then this migration would be easier at lower graft density. At higher grafting densities, the lateral interactions between grafted chains will limit any reorganization, and lateral migration will become more difficult. Above the Cp for complete transformation into globules, no migration can occur, so that,

Grafting of Diblock Copolymers onto Adsorbed Surfactant Films

Langmuir, Vol. 18, No. 5, 2002 1639

Figure 7. AFM images of the adsorbed-layer structure of DTAB on mica at 1 cmc in the presence of sufficient diblock copolymer to complete the morphology transformation to globular aggregates: (a) D100, Cp ) 0.18 wt %; (b) D1300, Cp ) 1 wt %.

at such concentrations, Dc necessarily reflects the globular adsorbed-layer structure. Comparison of Steric and Double-Layer Forces. In each of the three different polymers adsorbed on the coated mica, the interactions measured by SFA are somewhat different. To describe these systems we introduce three regimes by comparing the Debye length, ξ-1 with the radius of gyration of the PEO chains, Rg. Recall that ξ-1 is fixed at 2.4 nm by the surfactant concentration, which was kept at 1 cmc throughout this study. Rg . ξ-1. The radius of gyration of the D1300 copolymer is estimated to be about 12 nm, i.e., much larger than ξ-1. For this system (0.64 wt %), we find that the force profile is dominated by long-range steric repulsions and is welldescribed by the Alexander-de Gennes model. This model was developed for two interacting brushes. The fitted grafting density (s ≈ 9.5 nm) corresponds to the intermediate domain between the mushroom and brush regimes and is also consistent with our estimate of Rg for D1300 (s ≈ Rg). It has been shown that the Alexander-de Gennes model also holds in the crossover domain between the mushroom and brush regimes.26 The grafted copolymers are certainly only lightly stretched. This is also consistent with the surface structure observations by AFM. Because of the large radius of gyration of the PEO chain, we expect only one D1300 chain to attach to each globular DTAB micelle. This means that, for Cp ) 1 wt %, where the shape transformation from cylinders to spheres is complete and the surface is saturated with D1300, s should equal the intermicellar spacing of approximately 7 nm. At Cp ) 0.64 wt %, the surface is not yet saturated with D1300, so we would expect the AFM images to show some short cylindrical micelles (as observed at 5 wt %) and the average spacing between anchored polymers to be somewhat greater than 7 nm, consistent with the fitted s ≈ 9.5 nm from the force curves. Rg < ξ-1. The mean radius of gyration of the PEO chains of D50 is estimated to be 1.7 nm, i.e., for this system, Rg < ξ-1. When D50 is added to the surfactant solution, the corresponding force profiles do not present any characteristic signature of steric repulsion as they do for the larger copolymer D1300. The force profiles are exponential at every value of Cp, with a decay length given by the expected Debye length. The fact that the force profile remains dominated by double-layer repulsion can only be

Figure 8. Fitting of a modulated force profile obtained for DTAB in the presence of D100 diblock copolymer (Cp ) 0.18 wt %). The solid line corresponds to the best fit obtained by combining the Alexander-de Gennes and Debye-Hu¨ckel interactions. The dotted lines show contributions from each component of the force.

due to the charge on the adsorbed surfactant layer, which is largely unaffected by the subsequent addition of polymer. Rg ≈ ξ-1. For D100, the mean radius of gyration of the PEO chains is estimated to be about 2.5 nm, i.e., very close to the Debye length for the DTAB solution, and hence, both steric and double-layer repulsions contribute to the overall interaction. The thicknesses of the DTAB (3 nm) and the grafted D100 (2Rg ≈ 5 nm) layers gives an overall layer thickness of 8 nm, which means that we can expect steric interactions for separations of less than 16 nm if the PEO chains are not stretched. This is precisely the range in which we observe the modulation of the DLVO force. However, these two forces are not merely additive. Figure 8 shows the best fit obtained to a simple model combining both Alexander-de Gennes (eq 2) and DebyeHu¨ckel (eq 1) terms for a DTAB/D100 solution with Cp ) 0.18 wt %. To get good agreement, the fitted Debye length, ξ-1, must be much lower than that given by the ionic strength, viz., 1.7 instead of 2.3 nm. Such a discrepancy points out that the profiles are not merely the sum of eqs 1 and 2. Several reasons for this can be outlined. According to the picture discussed above, the copolymer chains are endgrafted on the surfactant films. Adsorbed DTAB films are believed to be liquidlike, so the anchored chains can diffuse

1640

Langmuir, Vol. 18, No. 5, 2002

laterally within the film. As noted above, under compression, the anchored polymers can migrate laterally out of the load region to release the applied stress, and migration is expected to be easier at lower graft density (i.e., larger s). Interestingly, we note that the modulation in the force profiles is more marked at lower Cp (see Figure 2b). This stress-induced mechanism implies that both s and also the local charge density (i.e., K in eq 1) of the mixed film are D-dependent, contrary to what is assumed in eqs 1 and 2. However, no indication of such modulation is clearly observable in the force profile obtained with the D1300 copolymers (Figure 4), although this system is mainly dominated by steric repulsion due to the compression of chains and is at relatively high coverage of copolymer. Depletion effects9,10 are ruled out because these experiments were performed at low molecular weight and low polymer concentration. Moreover, as already discussed, the modulation is strongest for low concentrations, which is opposite to a depletion interaction. An alternative origin for the modulation would be that the mixed layers undergo an elastic deformation. In recent theoretical papers,34 it was predicted that a membrane is locally elastically stretched around the anchoring point of a grafted polymer chain, thus allowing the chain to have more degrees of freedom. In this framework, the adsorbed surfactant films decorated by diblock copolymers must be thought of as swollen membranes. Under decompression, the surfactant film gradually returns to its undeformed thickness.34 In this description, the substrate is no longer incompressible, as assumed in both eqs 1 and 2, and thus both the charge and anchoring plane move with the surface separation, D, when the two surfaces interact. Note however that, within experimental resolution, no modulation is observed in the force profiles obtained with the D50 copolymers (Figure 2a). It is not clear why the decompressed film mechanism would be relevant for D100 and not for D50 copolymers. This elastic deformation also suggests a mechanism for the morphological transformation. An anchored polymer locally exerts a radially symmetric pressure field on the adsorbed layer that might be sufficient to “pinch off” the adsorbed cylinder of the surfactant sublayer into a globule, also producing two short cylinders. Increasing concentra(34) Bickel, T.; Marques, C.; Jeppeson, C. Phys. Rev. E 2000, 62, 1124.

Robelin et al.

tions of anchored polymer would then produce a gradual transformation of cylinders into spheres, as observed. An additional consideration is the change in mean curvature of the adsorbed surfactant aggregate, which will be affected by the hydrophobic group of the diblock copolymer. The hydrophobic tails of the diblock copolymers are longer than the alkyl chains of the cationic surfactant in all cases studied, and this situation is known to cause structural changes in bulk micelles.36 From this point of view, the polystyrene hydrophobe of D1300 would be expected to cause a greater perturbation in the adsorbed micelles than the stearyl groups of D50 and D100. We are currently investigating these factors in more detail. Conclusion In this study, we have shown that nonadsorbing diblock copolymers can be rendered adsorbing by mixing them with adsorbing (cationic) surfactants. The copolymers likely anchor their hydrophobic moieties into the adsorbed surfactant film. Above a threshold grafting concentration, the polymer induces a transition in the morphology of the adsorbed film from cylinders to spheres or globules. The presence of anchored copolymer also modifies the force profile between two surfactant-coated surfaces to include a steric repulsion when the radius of gyration, Rg, is comparable to or larger than the Debye length, ξ-1. When Rg is comparable to or smaller than ξ-1, polymer adsorption leads to an offset in the electrostatic repulsion of about 3 nm that accompanies the change in adsorbed film morphology. Some unusual features of the interactions are observed that are ascribed to lateral mobility of the diblock copolymer chains anchored in the surfactant sublayer. Acknowledgment. We thank Dr. C. Marques for much helpful advice and many suggestions. Parts of this work were funded by the Australian Research Council. G.G.W. benefited from a visiting appointment at the University of Bordeaux during the preparation of this manuscript. LA0111194 (35) Bickel, T.; Marques, C.; Jeppeson, C. C. R. Acad. Sci. Paris, Se´ rie IV 2000, 661. (36) Warr, G. G.; Grieser, F. Chem. Phys. Lett. 1985, 116, 505.