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Surface activity of modified dendrimers at high compression
Physica B 248 (1998) 184—190
Surface activity of modified dendrimers at high compression G.F. Kirton!, A.S. Brown!, C.J. Hawker", P.A. Reynolds!, J.W. White!,* ! Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia " IBM Almaden Research Centre, 650 Harry Road, San Jose, CA 95120-6099, USA
Abstract The effect of peripheral substitution on the surface activity of poly(aryl ether) dendrimers on water are being examined by measurements of the P—A isotherms and by neutron reflectometry. Contrast variation is used to define the molecular film structure at the air—water interface at high compressions. Monolayers are present for the methyl ester-substituted and cyanide-substituted dendrimers, whilst a partial second layer is formed for the unsubstituted [G-4]-OH dendrimer. The effect of temperature on these molecular films is also being investigated. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Neutron reflectometry; Dendrimers; Surfactants
1. Introduction In previous experiments we have studied polyether dendrimers spread at the air—water interface to form Langmuir films by neutron reflectometry [1], a useful technique for such films [2,3]. The isotherm data show a scaling behaviour with molecular weight and generation [1], and the lower generation polyether dendrimers have surface pressure—surface area (P—A) isotherms with an interesting pathological behaviour. One great virtue of using dendrimers, compared with other polymeric materials, to study the behaviour of insoluble surfactants at the air—water interface is the exceptionally monodisperse nature of the products generated by
* Corresponding author. Tel.: #61 6 249 3578; fax: #61 6 249 4903.
an iterative convergent synthesis [4,5]. We are thus able to attribute the differences seen in the isotherms to differences in the molecular geometry produced by changing the extent of the hydrophilic character of the molecules, rather than to any artefacts associated with variable polymer molecular weight. We report a sensitivity to peripheral substitution of the dendrimer by cyanide or methyl ester groups, so as to make the molecule more hydrophilic, of both the P—A isotherms and the neutron reflectivity of dendrimer films at the air—water interface. The generic fourth generation dendrimer structure is shown in Fig. 1. We use the unsubstituted dendrimer (R"H), the methyl ester-substituted dendrimer (R"CO Me), and the cyanide2 substituted dendrimer (R"CN). In our previous experiments on unsubstituted dendrimers the P—A isotherms indicated that for generations higher
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 2 2 9 - 4
G.F. Kirton et al. / Physica B 248 (1998) 184—190
Fig. 1. General fourth generation poly(aryl ether) dendrimer structure, R -[G-4]-OH, where R is the site of substitution. 16
than the third, [G-3]-OH, the molecules behaved in the same way as classical incompressible monolayers [1]. The limiting area per molecule corresponded well with the expected area from an elliptically elongated molecule whose major axis is approximately perpendicular to the air—water interface.
2. Experimental The P—A isotherms were measured using an automated custom-built PIC trough (area 200 cm2),
185
as well as a commercial automated NIMA 601A trough (area 560 cm2). The troughs were temperature controlled to $0.2°C. A suspended bob served as the pressure sensor in the PIC trough, whilst a paper Wilhelmy plate was used for the NIMA trough. The unsubstituted dendrimers were spread by microsyringe from a dilute (&0.5 mg/ml) toluene solution, whereas the substituted dendrimers required spreading from a dilute chloroform solution. On the NIMA trough, the slowest practical barrier speed of 20 cm2/min was used throughout. The cleanliness of the Millipore water surface after cleaning by suction and the solvent purity were checked by rapid compression to detect the presence of surface active impurities by significant rises in surface pressure. The neutron reflectometry was performed using the CRISP and SURF time-of-flight instruments at the ISIS Facility, Rutherford—Appleton Laboratories, U.K. The theory of specular neutron reflectometry is given elsewhere [6,7]. An incident angle of 1.5° and the neutron wavelength range of 0.5—6.6 A_ resulted in a range of momentum transfer perpendicular to the air—water interface, Q , of ; 0.05—0.65 A_ ~1. The reflectivity of D O provided a 2 scale factor for absolute reflectivity in subsequent measurements. Contrast was provided by the use of two subphases, D O, and air contrast-matched 2 water of zero mean scattering length density, ACMW, for films of the same material under the same conditions. The reflectivity profiles were fitted by a least squares routine to optical models involving multiple slabs, in which the thicknesses, scattering length densities and roughnesses could be varied. The program is a development of l—mulfit [8], and allows for simultaneous refinement of multiple data sets in a single model. This program has proven useful in minimizing ambiguities inherent in the modelling of reflectivity data. Starting values for the refinements were obtained by calculating the scattering length densities for the poly(aryl ether) dendrimers, (1.7]10~6 A_ ~2 for both the unsubstituted and the methyl ester, 2.0]10~6 A_ ~2 for the cyanide, based on a material density of 1.0 g cm~3), and previous reflectometry results [1]. Note that the unsubstituted dendrimer d -[G112 4]-OH is deuterated on the outer benzyl groups
186
G.F. Kirton et al. / Physica B 248 (1998) 184—190
to provide scattering length density contrast (3.7]10~6 A_ ~2) against the fully hydrogenated [G-4]-OH.
3. Results and discussion 3.1. P—A isotherms In Fig. 2 we show the P—A isotherms taken at 10°C for the unsubstituted [G-4]-OH dendrimer and the two substituted forms. In general, the isotherms initially exhibit a gentle rise in surface pressure, the gaseous phase region, followed by a steep rise in surface pressure corresponding to a condensed, solid-like phase. Note that a feature, which we refer to as a nucleation peak, is only present in the unsubstituted dendrimer P—A isotherms. It characterizes the pathology of the P—A isotherms for the lower generation poly(aryl ether) dendrimers [1] which is also present in the polystyrene and fatty acid isotherms [9,10]. This is interpreted to be the point at which a compressed monolayer of surfactant collapses into a multilayer structure. Movement of molecules into the upper layers relieves the stress on the lower layer, with a subsequent fall in surface pressure. A steep rise in the isotherm follows when the multilayer system is compressed more and further multilayer formation is not possible. The limiting specific surface areas associated with the unsubstituted : methyl ester substituted :
cyanide substituted dendrimer films are in the ratio of 1 : 2.0 : 2.2, respectively, from the most condensed phase. If one takes the limiting area before the nucleation peak in the unsubstituted dendrimer, the ratios are approximately 1 : 1.4 : 1.5. If we assume the latter ratios apply to the monolayer, the footprints of the substituted dendrimers are distinctly larger than that for the unsubstituted dendrimer. Scaling of the molecular weight cannot account for much of this difference, so that a difference in molecular conformation between the substituted and unsubstituted dendrimer is inferred. Another point of interest is that the ratio of limiting areas for the unsubstituted dendrimer is approximately 1.4, indicating that the multilayer system is a partial bilayer structure. For several surfactant systems, the ratio is close to integral (2 or 3) corresponding to a bilayer or trilayer structure, respectively. The non-integral value indicates that there are conformational differences in the molecules between the layers, or that the second layer is not complete. A complementary technique such as neutron reflectometry is required to further elucidate this point.
3.2. Neutron specular reflectivity The reflectivity profiles obtained for the dendrimer molecular films at the air—water interface have been modelled as either a single or a double slab structure. Experimentation with various constraints have proved the utility of maintaining a thickness constraint for the films across the two subphases and a constant roughness of 2.5 or 3.0 A_ . Results of the modelling are outlined in Tables 1—3. The most appropriate fits to the reflectivity, R, presented in terms of RQ4 versus the momentum z transfer, Q , are given in Figs. 3—5. The areas in the z legend for each of the plots are the average specific areas for the films in each constrained fit.
3.3. Unsubstituted dendrimer films at 10°C
Fig. 2. P—A isotherms for R -[G-4]-OH dendrimers at the 16 air—water interface at 10°C; barrier speed"33.0 mm/min.
The previous data at 20°C were fitted, each data set individually, and required multislab models [1]. Here we apply further constraints by simultaneous
G.F. Kirton et al. / Physica B 248 (1998) 184—190
187
Table 1 Two slab, thickness constrained, models of the neutron reflectivity for the compression of d -[G-4]-OH films at the air—water interface 112 at 10°C. For this and the following tables, the layers are numbered sequentially from air to the subphase; A is the area per molecule in .0A_ 2; d is the thickness of layer i in A_ ; Nb is the scattering length density of layer i in 10~6 A_ ~2; s2 is the overall goodness of fit for the i i constrained models. The errors in the least significant figure(s) are indicated by the values in parantheses. The error in the film area per molecule is $2 A_ 2. Subphase
A
D O 2 ACMW D O 2 ACMW D O 2 ACMW
194 194 153 153 99 99
.0-
d 1
Nb 1
d 2
Nb 2
v2
18.5(1.3) 18.5(1.3) 20.7(1.0) 20.7(1.0) 24.7(6) 24.7(6)
1.25(13) 0.16(10) 1.01(6) 0.11(8) 1.37(5) 0.06(6)
38.0(6) 38.0(6) 41.1(4) 41.1(4) 44.6(4) 44.6(4)
5.56(4) 3.35(3) 5.33(3) 3.28(3) 5.07(3) 2.25(3)
1.34
Table 2 One slab, thickness constrained, models of the neutron reflectivity for the compression of (MeO C) -[G-4]-OH films at the 2 16 air—water interface at 20°C and 8°C d 1
Nb 1
s2
292 293 155 154 100 100
22.4(3) 22.4(3) 29.0(2) 29.0(2) 30.2(2) 30.2(2)
1.92(5) 2.03(2) 2.58(6) 1.90(2) 2.60(6) 1.56(2)
1.06
293 293 154 154 100 100
20.2(3) 20.2(3) 26.2(2) 26.2(2) 27.9(2) 27.9(2)
2.16(11) 2.15(3) 2.43(5) 1.95(2) 2.46(7) 1.61(2)
1.75
Subphase
A
(i) ¹"20°C D O 2 ACMW D O 2 ACMW D O 2 ACMW (ii) ¹"8°C D O 2 ACMW D O 2 ACMW D O 2 ACMW
.0-
1.94 2.96
1.92 4.06
refinement of data sets, with fewer disposable parameters, in a two slab model. For the bottom slab (closest to the subphase), the thickness increases from 39.0 to 44.6 A_ over the given film area range. In comparing this thickness to the expected (spherical) molecular diameter of 22 A_ for the dendrimer molecule [1], this slab corresponds to either a bilayer of undistorted molecules, or a monolayer of highly distorted molecules. The top slab is thinner, and increases in thickness upon compression (18.5 to 24.7 A_ ). This
1.33 1.10
Table 3 One slab, thickness constrained, models of the neutron reflectivity for the compression of (NC) -[G-4]-OH films at the 16 air—water interface at 20°C and 8°C d 1
Nb 1
s2
211 211 187 186 148 148
29.2(3) 29.2(3) 30.1(3) 30.1(3) 32.0(3) 32.0(3)
2.38(6) 2.11(2) 2.55(7) 2.08(2) 2.79(13) 1.98(2)
3.14
211 210 184 186 148 148
22.7(2) 22.7(2) 24.9(2) 24.9(2) 27.2(2) 27.2(2)
2.38(11) 2.24(11) 2.26(10) 2.21(10) 2.52(10) 2.17(9)
1.84
Subphase
A
(i) ¹"20°C D O 2 ACMW D O 2 ACMW D O 2 ACMW (ii) T"8°C D O 2 ACMW D O 2 ACMW D O 2 ACMW
.0-
3.77 7.59
1.65 4.03
corresponds well to a layer of undistorted spherical molecules. The high scattering length densities for the bottom slab indicates that the layer is essentially filled with the dendrimer surfactant, with some degree of compaction. A difference in the scattering length densities for the films across the two subphases is consistent with the uptake of subphase into the surfactant layers, as observed in the previous experiments at 20°C [1]. It is interesting to note the decrease in scattering length densities for the films
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G.F. Kirton et al. / Physica B 248 (1998) 184—190
Fig. 3. RQ4 vs. Q plots and two slab model fits for d -[G-4]112 OH films at the air—water interface at 10°C.
on both subphases with compression. These trends are suggestive of a loss of surfactant into the subphase. The scattering length densities for the top slab are only a small fraction of those for the bottom slab, being far less so for the film on ACMW. The scattering length densities for the top slab decrease with compression for the film on ACMW, whilst they are relatively static for the film on D O. This confirms that the top slab is an 2 incomplete dendrimer layer. The s2 values affirm that the two slab models are a good representation of the unsubstituted dendrimer film structure. In comparing separate three slab models for the individual data sets derived from the unsubstituted films at 20°C [1], there is a resemblance in the bottom two slabs to the bottom slab of the two slab models. However, the reflectivity profiles for the films at 10°C do not support the partitioning of the scattering length densities for the bottom monolayer.
3.4. Methyl ester-substituted dendrimer films at 8°C and 20°C
Fig. 4. RQ4 vs. Q plots and one slab model fits for (MeO C) 2 16 [G-4]-OH films at the air—water interface at 8°C.
Fig. 5. RQ4 vs. Q plots and one slab model fits for (NC) -[G16 4]-OH films at the air—water interface at 8°C.
For the methyl ester-substituted dendrimer film at 20°C (Table 2), a single slab model shows an increase in film thickness with compression, the increase being more dramatic between the first two points of measurement. The thicknesses for these films (22.4—30.2 A_ ) are significantly less than those of the substituted dendrimer at similar film areas per molecule, showing that the edge substitution has resulted in a flatter molecular profile of the surfactant, which is also consistent with the P—A isotherms illustrated in Fig. 2. Comparable values of the thickness are obtained for the methyl ester-substituted dendrimers at 8°C, as given in Table 2. The thickness of the films is slightly less at the lower temperature. The scattering length densities of the methyl ester-substituted dendrimer films at both temperatures show a decrease with compression on ACMW but an increase with compression on D O. 2 These trends, in contrast to the unsubstituted dendrimer, are indicative of an uptake of subphase into the surfactant film during compression. The values are not significantly affected by the change in
G.F. Kirton et al. / Physica B 248 (1998) 184—190
temperature. When the trends in scattering length density and thickness are combined, there is a suggestion that during compression, loss of the surfactant into the subphase is occuring. The s2 values for the methyl ester-substituted dendrimers show that single slab, thickness constrained, models are reasonable representations of the surfactant structure at both temperatures. We find no marginal benefit in applying a two slab model. The results suggest that the single slab models are best improved by using larger interfacial roughness values.
3.5. Cyanide-substituted dendrimer films at 8°C and 20°C The single slab models for the cyanide-substituted dendrimer films at 20°C and 8°C are given in Table 3. The model results for the cyanide-substituted dendrimer films are very similar to the those found for the methyl ester-substituted dendrimers. At 20°C, the thickness increase with compression (29.2—32.0 A_ ) is close to that for the methyl estersubstituted dendrimer, other than the lack of a large initial increase. As with the methyl estersubstituted dendrimers, the cyanide-substituted dendrimer films are thinner (22.7—27.2 A_ ) at the lower temperature of 8°C. The two substituted dendrimers are closely similar in film thickness at equivalent compressions and distinct from the unsubstituted dendrimer. The trends in the scattering length densities are essentially the same as for the methyl ester-substituted dendrimers, with the values being higher for the films on D O at both temperatures. In 2 addition, the trends at both temperatures are consistent with subphase incorporation and loss of dendrimer as the film is compressed, as is observed for the methyl ester-substituted dendrimer films. The v2 values for the cyanide-substituted dendrimers films are reasonable, but at 20°C, the models are evidently not as good as for the methyl ester-substituted dendrimer films. As for the methyl-ester dendrimer films, applying the double slab model is not a significant improvement.
189
4. Conclusions The unsubstituted dendrimer when compressed past the nucleation peak, has a film structure which consists of a complete monolayer, where the molecules are distorted into a prolate conformation, and a partial second layer of undistorted spherical molecules. This is confirmed by the necessity for a two slab model for sufficiently accurate representation of the film structures. We speculate that the second layer is formed in compression past the nucleation peak, when molecules move out of the monolayer with the accompanying release of surface pressure. The introduction of methyl ester or cyanide groups onto the surface of the dendrimer molecule results in a very different film structure. Both of the substituted dendrimers form films which are well represented by compact monolayers which are significantly thinner than those of the unsubstituted dendrimer films. The thicknesses of these compact monolayers are consistent with the hypothesis of an increased interaction between the dendrimer and the subphase due to the more hydrophilic nature of the methyl ester or the cyanide groups. These hydrophilic groups are thus seen to act as additional anchoring points to the water surface that tend to spread the molecule across the interface, and to strongly inhibit the formation of the second layer which is present in the films of the unsubstituted dendrimer. A common feature of the dendrimer films at high compression is the loss of surfactant into the subphase. Evidently, there is a greater tendency for the dendrimer molecules to move into the subphase rather than to form multilayered structures, even though only partially expressed in the unsubstituted dendrimer. This is expected, since all these dendrimers have hydrophilic ether linkages in their open architecture which are accessible to the subphase. All the films incorporate the water subphase, again expected due to the ether linkages, but the substituted dendrimers differ from the unsubstituted dendrimers in their increase in water content on compression. This again is a result of their greater hydrophilicity than the unsubstituted dendrimer.
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G.F. Kirton et al. / Physica B 248 (1998) 184—190
It is clear that peripheral substitution has a significant effect on the high compression film structure for the poly(aryl ether) dendrimers. We are currently investigating whether this effect also applies to the dendrimer films at lower compressions. In particular, the low compression film structure for the unsubstituted dendrimer should prove fruitful in explaining the presence or absence of the nucleation peak phenomenon. References [1] P.M. Saville, P.A. Reynolds, J.W. White, C.J. Hawker, J.M.J. Fre´chet, K.L. Wooley, J. Penfold, J.R.P. Webster, J. Phys. Chem. 99 (1995) 8283.
[2] J. Majewski, T.L. Kuhl, M.C. Gerstenberg, J.N. Israelachvili, G.S. Smith, J. Phys. Chem. B 101 (1997) 3122. [3] L.T. Lee, E.K. Mann, O. Guiselin, D. Langevin, B. Farnoux, J. Penfold, Macromolecules 26 (1993) 7046. [4] C.J. Hawker, J.M.J. Fre´chet, J. Chem. Soc., Chem. Commun. 15 (1990) 1010. [5] C.J. Hawker, J.M.J. Fre´chet, J. Am. Chem. Soc. 112 (1990) 7638. [6] J. Penfold, in: Neutron, X-ray and Light Scattering, Elsevier, Amsterdam, 1991, p. 223. [7] J. Lekner, Theory of Reflection, Martnius Nijoff Publishers, Dordrecht, 1987. [8] J. Penfold, J.R.P. Webster, Rutherford-Appleton Laboratories, UK. [9] P.M. Saville, I.R. Gentle, J.W. White, J. Penfold, J.R.P. Webster, J. Phys. Chem. 98 (1994) 5935. [10] C. McFate, D. Ward, J. Olmsted III, Langmuir 9 (1993) 1036.
Surface activity of modified dendrimers at high compression G.F. Kirton!, A.S. Brown!, C.J. Hawker", P.A. Reynolds!, J.W. White!,* ! Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia " IBM Almaden Research Centre, 650 Harry Road, San Jose, CA 95120-6099, USA
Abstract The effect of peripheral substitution on the surface activity of poly(aryl ether) dendrimers on water are being examined by measurements of the P—A isotherms and by neutron reflectometry. Contrast variation is used to define the molecular film structure at the air—water interface at high compressions. Monolayers are present for the methyl ester-substituted and cyanide-substituted dendrimers, whilst a partial second layer is formed for the unsubstituted [G-4]-OH dendrimer. The effect of temperature on these molecular films is also being investigated. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Neutron reflectometry; Dendrimers; Surfactants
1. Introduction In previous experiments we have studied polyether dendrimers spread at the air—water interface to form Langmuir films by neutron reflectometry [1], a useful technique for such films [2,3]. The isotherm data show a scaling behaviour with molecular weight and generation [1], and the lower generation polyether dendrimers have surface pressure—surface area (P—A) isotherms with an interesting pathological behaviour. One great virtue of using dendrimers, compared with other polymeric materials, to study the behaviour of insoluble surfactants at the air—water interface is the exceptionally monodisperse nature of the products generated by
* Corresponding author. Tel.: #61 6 249 3578; fax: #61 6 249 4903.
an iterative convergent synthesis [4,5]. We are thus able to attribute the differences seen in the isotherms to differences in the molecular geometry produced by changing the extent of the hydrophilic character of the molecules, rather than to any artefacts associated with variable polymer molecular weight. We report a sensitivity to peripheral substitution of the dendrimer by cyanide or methyl ester groups, so as to make the molecule more hydrophilic, of both the P—A isotherms and the neutron reflectivity of dendrimer films at the air—water interface. The generic fourth generation dendrimer structure is shown in Fig. 1. We use the unsubstituted dendrimer (R"H), the methyl ester-substituted dendrimer (R"CO Me), and the cyanide2 substituted dendrimer (R"CN). In our previous experiments on unsubstituted dendrimers the P—A isotherms indicated that for generations higher
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 2 2 9 - 4
G.F. Kirton et al. / Physica B 248 (1998) 184—190
Fig. 1. General fourth generation poly(aryl ether) dendrimer structure, R -[G-4]-OH, where R is the site of substitution. 16
than the third, [G-3]-OH, the molecules behaved in the same way as classical incompressible monolayers [1]. The limiting area per molecule corresponded well with the expected area from an elliptically elongated molecule whose major axis is approximately perpendicular to the air—water interface.
2. Experimental The P—A isotherms were measured using an automated custom-built PIC trough (area 200 cm2),
185
as well as a commercial automated NIMA 601A trough (area 560 cm2). The troughs were temperature controlled to $0.2°C. A suspended bob served as the pressure sensor in the PIC trough, whilst a paper Wilhelmy plate was used for the NIMA trough. The unsubstituted dendrimers were spread by microsyringe from a dilute (&0.5 mg/ml) toluene solution, whereas the substituted dendrimers required spreading from a dilute chloroform solution. On the NIMA trough, the slowest practical barrier speed of 20 cm2/min was used throughout. The cleanliness of the Millipore water surface after cleaning by suction and the solvent purity were checked by rapid compression to detect the presence of surface active impurities by significant rises in surface pressure. The neutron reflectometry was performed using the CRISP and SURF time-of-flight instruments at the ISIS Facility, Rutherford—Appleton Laboratories, U.K. The theory of specular neutron reflectometry is given elsewhere [6,7]. An incident angle of 1.5° and the neutron wavelength range of 0.5—6.6 A_ resulted in a range of momentum transfer perpendicular to the air—water interface, Q , of ; 0.05—0.65 A_ ~1. The reflectivity of D O provided a 2 scale factor for absolute reflectivity in subsequent measurements. Contrast was provided by the use of two subphases, D O, and air contrast-matched 2 water of zero mean scattering length density, ACMW, for films of the same material under the same conditions. The reflectivity profiles were fitted by a least squares routine to optical models involving multiple slabs, in which the thicknesses, scattering length densities and roughnesses could be varied. The program is a development of l—mulfit [8], and allows for simultaneous refinement of multiple data sets in a single model. This program has proven useful in minimizing ambiguities inherent in the modelling of reflectivity data. Starting values for the refinements were obtained by calculating the scattering length densities for the poly(aryl ether) dendrimers, (1.7]10~6 A_ ~2 for both the unsubstituted and the methyl ester, 2.0]10~6 A_ ~2 for the cyanide, based on a material density of 1.0 g cm~3), and previous reflectometry results [1]. Note that the unsubstituted dendrimer d -[G112 4]-OH is deuterated on the outer benzyl groups
186
G.F. Kirton et al. / Physica B 248 (1998) 184—190
to provide scattering length density contrast (3.7]10~6 A_ ~2) against the fully hydrogenated [G-4]-OH.
3. Results and discussion 3.1. P—A isotherms In Fig. 2 we show the P—A isotherms taken at 10°C for the unsubstituted [G-4]-OH dendrimer and the two substituted forms. In general, the isotherms initially exhibit a gentle rise in surface pressure, the gaseous phase region, followed by a steep rise in surface pressure corresponding to a condensed, solid-like phase. Note that a feature, which we refer to as a nucleation peak, is only present in the unsubstituted dendrimer P—A isotherms. It characterizes the pathology of the P—A isotherms for the lower generation poly(aryl ether) dendrimers [1] which is also present in the polystyrene and fatty acid isotherms [9,10]. This is interpreted to be the point at which a compressed monolayer of surfactant collapses into a multilayer structure. Movement of molecules into the upper layers relieves the stress on the lower layer, with a subsequent fall in surface pressure. A steep rise in the isotherm follows when the multilayer system is compressed more and further multilayer formation is not possible. The limiting specific surface areas associated with the unsubstituted : methyl ester substituted :
cyanide substituted dendrimer films are in the ratio of 1 : 2.0 : 2.2, respectively, from the most condensed phase. If one takes the limiting area before the nucleation peak in the unsubstituted dendrimer, the ratios are approximately 1 : 1.4 : 1.5. If we assume the latter ratios apply to the monolayer, the footprints of the substituted dendrimers are distinctly larger than that for the unsubstituted dendrimer. Scaling of the molecular weight cannot account for much of this difference, so that a difference in molecular conformation between the substituted and unsubstituted dendrimer is inferred. Another point of interest is that the ratio of limiting areas for the unsubstituted dendrimer is approximately 1.4, indicating that the multilayer system is a partial bilayer structure. For several surfactant systems, the ratio is close to integral (2 or 3) corresponding to a bilayer or trilayer structure, respectively. The non-integral value indicates that there are conformational differences in the molecules between the layers, or that the second layer is not complete. A complementary technique such as neutron reflectometry is required to further elucidate this point.
3.2. Neutron specular reflectivity The reflectivity profiles obtained for the dendrimer molecular films at the air—water interface have been modelled as either a single or a double slab structure. Experimentation with various constraints have proved the utility of maintaining a thickness constraint for the films across the two subphases and a constant roughness of 2.5 or 3.0 A_ . Results of the modelling are outlined in Tables 1—3. The most appropriate fits to the reflectivity, R, presented in terms of RQ4 versus the momentum z transfer, Q , are given in Figs. 3—5. The areas in the z legend for each of the plots are the average specific areas for the films in each constrained fit.
3.3. Unsubstituted dendrimer films at 10°C
Fig. 2. P—A isotherms for R -[G-4]-OH dendrimers at the 16 air—water interface at 10°C; barrier speed"33.0 mm/min.
The previous data at 20°C were fitted, each data set individually, and required multislab models [1]. Here we apply further constraints by simultaneous
G.F. Kirton et al. / Physica B 248 (1998) 184—190
187
Table 1 Two slab, thickness constrained, models of the neutron reflectivity for the compression of d -[G-4]-OH films at the air—water interface 112 at 10°C. For this and the following tables, the layers are numbered sequentially from air to the subphase; A is the area per molecule in .0A_ 2; d is the thickness of layer i in A_ ; Nb is the scattering length density of layer i in 10~6 A_ ~2; s2 is the overall goodness of fit for the i i constrained models. The errors in the least significant figure(s) are indicated by the values in parantheses. The error in the film area per molecule is $2 A_ 2. Subphase
A
D O 2 ACMW D O 2 ACMW D O 2 ACMW
194 194 153 153 99 99
.0-
d 1
Nb 1
d 2
Nb 2
v2
18.5(1.3) 18.5(1.3) 20.7(1.0) 20.7(1.0) 24.7(6) 24.7(6)
1.25(13) 0.16(10) 1.01(6) 0.11(8) 1.37(5) 0.06(6)
38.0(6) 38.0(6) 41.1(4) 41.1(4) 44.6(4) 44.6(4)
5.56(4) 3.35(3) 5.33(3) 3.28(3) 5.07(3) 2.25(3)
1.34
Table 2 One slab, thickness constrained, models of the neutron reflectivity for the compression of (MeO C) -[G-4]-OH films at the 2 16 air—water interface at 20°C and 8°C d 1
Nb 1
s2
292 293 155 154 100 100
22.4(3) 22.4(3) 29.0(2) 29.0(2) 30.2(2) 30.2(2)
1.92(5) 2.03(2) 2.58(6) 1.90(2) 2.60(6) 1.56(2)
1.06
293 293 154 154 100 100
20.2(3) 20.2(3) 26.2(2) 26.2(2) 27.9(2) 27.9(2)
2.16(11) 2.15(3) 2.43(5) 1.95(2) 2.46(7) 1.61(2)
1.75
Subphase
A
(i) ¹"20°C D O 2 ACMW D O 2 ACMW D O 2 ACMW (ii) ¹"8°C D O 2 ACMW D O 2 ACMW D O 2 ACMW
.0-
1.94 2.96
1.92 4.06
refinement of data sets, with fewer disposable parameters, in a two slab model. For the bottom slab (closest to the subphase), the thickness increases from 39.0 to 44.6 A_ over the given film area range. In comparing this thickness to the expected (spherical) molecular diameter of 22 A_ for the dendrimer molecule [1], this slab corresponds to either a bilayer of undistorted molecules, or a monolayer of highly distorted molecules. The top slab is thinner, and increases in thickness upon compression (18.5 to 24.7 A_ ). This
1.33 1.10
Table 3 One slab, thickness constrained, models of the neutron reflectivity for the compression of (NC) -[G-4]-OH films at the 16 air—water interface at 20°C and 8°C d 1
Nb 1
s2
211 211 187 186 148 148
29.2(3) 29.2(3) 30.1(3) 30.1(3) 32.0(3) 32.0(3)
2.38(6) 2.11(2) 2.55(7) 2.08(2) 2.79(13) 1.98(2)
3.14
211 210 184 186 148 148
22.7(2) 22.7(2) 24.9(2) 24.9(2) 27.2(2) 27.2(2)
2.38(11) 2.24(11) 2.26(10) 2.21(10) 2.52(10) 2.17(9)
1.84
Subphase
A
(i) ¹"20°C D O 2 ACMW D O 2 ACMW D O 2 ACMW (ii) T"8°C D O 2 ACMW D O 2 ACMW D O 2 ACMW
.0-
3.77 7.59
1.65 4.03
corresponds well to a layer of undistorted spherical molecules. The high scattering length densities for the bottom slab indicates that the layer is essentially filled with the dendrimer surfactant, with some degree of compaction. A difference in the scattering length densities for the films across the two subphases is consistent with the uptake of subphase into the surfactant layers, as observed in the previous experiments at 20°C [1]. It is interesting to note the decrease in scattering length densities for the films
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Fig. 3. RQ4 vs. Q plots and two slab model fits for d -[G-4]112 OH films at the air—water interface at 10°C.
on both subphases with compression. These trends are suggestive of a loss of surfactant into the subphase. The scattering length densities for the top slab are only a small fraction of those for the bottom slab, being far less so for the film on ACMW. The scattering length densities for the top slab decrease with compression for the film on ACMW, whilst they are relatively static for the film on D O. This confirms that the top slab is an 2 incomplete dendrimer layer. The s2 values affirm that the two slab models are a good representation of the unsubstituted dendrimer film structure. In comparing separate three slab models for the individual data sets derived from the unsubstituted films at 20°C [1], there is a resemblance in the bottom two slabs to the bottom slab of the two slab models. However, the reflectivity profiles for the films at 10°C do not support the partitioning of the scattering length densities for the bottom monolayer.
3.4. Methyl ester-substituted dendrimer films at 8°C and 20°C
Fig. 4. RQ4 vs. Q plots and one slab model fits for (MeO C) 2 16 [G-4]-OH films at the air—water interface at 8°C.
Fig. 5. RQ4 vs. Q plots and one slab model fits for (NC) -[G16 4]-OH films at the air—water interface at 8°C.
For the methyl ester-substituted dendrimer film at 20°C (Table 2), a single slab model shows an increase in film thickness with compression, the increase being more dramatic between the first two points of measurement. The thicknesses for these films (22.4—30.2 A_ ) are significantly less than those of the substituted dendrimer at similar film areas per molecule, showing that the edge substitution has resulted in a flatter molecular profile of the surfactant, which is also consistent with the P—A isotherms illustrated in Fig. 2. Comparable values of the thickness are obtained for the methyl ester-substituted dendrimers at 8°C, as given in Table 2. The thickness of the films is slightly less at the lower temperature. The scattering length densities of the methyl ester-substituted dendrimer films at both temperatures show a decrease with compression on ACMW but an increase with compression on D O. 2 These trends, in contrast to the unsubstituted dendrimer, are indicative of an uptake of subphase into the surfactant film during compression. The values are not significantly affected by the change in
G.F. Kirton et al. / Physica B 248 (1998) 184—190
temperature. When the trends in scattering length density and thickness are combined, there is a suggestion that during compression, loss of the surfactant into the subphase is occuring. The s2 values for the methyl ester-substituted dendrimers show that single slab, thickness constrained, models are reasonable representations of the surfactant structure at both temperatures. We find no marginal benefit in applying a two slab model. The results suggest that the single slab models are best improved by using larger interfacial roughness values.
3.5. Cyanide-substituted dendrimer films at 8°C and 20°C The single slab models for the cyanide-substituted dendrimer films at 20°C and 8°C are given in Table 3. The model results for the cyanide-substituted dendrimer films are very similar to the those found for the methyl ester-substituted dendrimers. At 20°C, the thickness increase with compression (29.2—32.0 A_ ) is close to that for the methyl estersubstituted dendrimer, other than the lack of a large initial increase. As with the methyl estersubstituted dendrimers, the cyanide-substituted dendrimer films are thinner (22.7—27.2 A_ ) at the lower temperature of 8°C. The two substituted dendrimers are closely similar in film thickness at equivalent compressions and distinct from the unsubstituted dendrimer. The trends in the scattering length densities are essentially the same as for the methyl ester-substituted dendrimers, with the values being higher for the films on D O at both temperatures. In 2 addition, the trends at both temperatures are consistent with subphase incorporation and loss of dendrimer as the film is compressed, as is observed for the methyl ester-substituted dendrimer films. The v2 values for the cyanide-substituted dendrimers films are reasonable, but at 20°C, the models are evidently not as good as for the methyl ester-substituted dendrimer films. As for the methyl-ester dendrimer films, applying the double slab model is not a significant improvement.
189
4. Conclusions The unsubstituted dendrimer when compressed past the nucleation peak, has a film structure which consists of a complete monolayer, where the molecules are distorted into a prolate conformation, and a partial second layer of undistorted spherical molecules. This is confirmed by the necessity for a two slab model for sufficiently accurate representation of the film structures. We speculate that the second layer is formed in compression past the nucleation peak, when molecules move out of the monolayer with the accompanying release of surface pressure. The introduction of methyl ester or cyanide groups onto the surface of the dendrimer molecule results in a very different film structure. Both of the substituted dendrimers form films which are well represented by compact monolayers which are significantly thinner than those of the unsubstituted dendrimer films. The thicknesses of these compact monolayers are consistent with the hypothesis of an increased interaction between the dendrimer and the subphase due to the more hydrophilic nature of the methyl ester or the cyanide groups. These hydrophilic groups are thus seen to act as additional anchoring points to the water surface that tend to spread the molecule across the interface, and to strongly inhibit the formation of the second layer which is present in the films of the unsubstituted dendrimer. A common feature of the dendrimer films at high compression is the loss of surfactant into the subphase. Evidently, there is a greater tendency for the dendrimer molecules to move into the subphase rather than to form multilayered structures, even though only partially expressed in the unsubstituted dendrimer. This is expected, since all these dendrimers have hydrophilic ether linkages in their open architecture which are accessible to the subphase. All the films incorporate the water subphase, again expected due to the ether linkages, but the substituted dendrimers differ from the unsubstituted dendrimers in their increase in water content on compression. This again is a result of their greater hydrophilicity than the unsubstituted dendrimer.
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It is clear that peripheral substitution has a significant effect on the high compression film structure for the poly(aryl ether) dendrimers. We are currently investigating whether this effect also applies to the dendrimer films at lower compressions. In particular, the low compression film structure for the unsubstituted dendrimer should prove fruitful in explaining the presence or absence of the nucleation peak phenomenon. References [1] P.M. Saville, P.A. Reynolds, J.W. White, C.J. Hawker, J.M.J. Fre´chet, K.L. Wooley, J. Penfold, J.R.P. Webster, J. Phys. Chem. 99 (1995) 8283.
[2] J. Majewski, T.L. Kuhl, M.C. Gerstenberg, J.N. Israelachvili, G.S. Smith, J. Phys. Chem. B 101 (1997) 3122. [3] L.T. Lee, E.K. Mann, O. Guiselin, D. Langevin, B. Farnoux, J. Penfold, Macromolecules 26 (1993) 7046. [4] C.J. Hawker, J.M.J. Fre´chet, J. Chem. Soc., Chem. Commun. 15 (1990) 1010. [5] C.J. Hawker, J.M.J. Fre´chet, J. Am. Chem. Soc. 112 (1990) 7638. [6] J. Penfold, in: Neutron, X-ray and Light Scattering, Elsevier, Amsterdam, 1991, p. 223. [7] J. Lekner, Theory of Reflection, Martnius Nijoff Publishers, Dordrecht, 1987. [8] J. Penfold, J.R.P. Webster, Rutherford-Appleton Laboratories, UK. [9] P.M. Saville, I.R. Gentle, J.W. White, J. Penfold, J.R.P. Webster, J. Phys. Chem. 98 (1994) 5935. [10] C. McFate, D. Ward, J. Olmsted III, Langmuir 9 (1993) 1036.