J. Phys. Chem. 1993,97, 293-294
293
Dendrimer and Polystyrene Surfactant Structure at the Air-Water Interface P. M. S a d e and J. W. White' Research School of Chemistry, Australian National University, P.O. Box 4, Canberra A.C.T., Australia 2601
C . J. Hawker Department of Chemistry, The University of Queenrland, Qld, Australia 4072 I(. L.
Wooley and J. M. J. FreChet
Department of Chemistry, Cornell University, Zthaca, New York 14853- 1301 Received: October 30, 1992 The II-A isotherms at the air-water interface and 25 OC for monomolecular films of the recently synthesized dendrimer surfactants are reported and compared with those of polystyrene terminated with a single hydroxyl group. There is a strong dependence ofthe isotherms on molecular weight (MW) for both systems, the lower MWs typified by an increase in the surface pressure through a liquid expanded phase, followed by a peaked collapse transition, indicating a nucleation and growth process, to a liquid condensed, phase. The higher molecular weight dendrimer and polystyrene isotherms are also similar, with a steep increase in the surface pressure being due to a direct transition to a solid phase.
LatrOd~tiOll
Recent developmentsin X-rayIp2and neutron s~attering~-~ have greatly broadened the understanding of the two-dimensional structures and phase transitions of classical surfactant molecules at the air-water interface. Structures perpendicular to the interface as revealed by neutron reflectivit~~-~ to quite high precision aredescribed well by two-layer models: the hydrophobic aliphatic chain and the head groupwater regions with parameters such as the in-plane packing and the interface penetration now quite well defined in these simple cases. The intralayer interactions and packing density determine many of the monolayer properties, the phases wbich are formed, and the rates at which transitions between phases occur. It is the purpose of this paper to explore the effects of radically changing those interactions and densities using polymeric surfactants of very different t p . The classical isotherm, for an aliphatic hydrocarbonchain with polar head group, is characterized by several phases which are &marked by changes in the surface pressure, 11,as the area per molecule is decreased. The phases include transitions from a gaseous to homogeneous liquid expanded state, LE, marked by an increase in 11,followed by a transition to the liquid condensed phase, LC, and finally a transition to a solid phase. These characteristics change as the molecular weight of the insoluble chain portion of the surfactant is increased. Steric interactions between neighboring chains begins to dominate the isotherm, and this is particularly notable for films of surfactants consisting of hydrophobic polymersterminated at one end with a hydrophilic group. Here the molecules may be synthesized long enough that the polymer-polymer interactions are of the same order as the energy of interaction of the hydrophilic groups with the aqueous substrate. Indead, if the size of the hydrophobic group becomes toolarge, the bulkpropertiesofthepdymcricpartofthesurfactant arc manifested and the hydrophilicgrouphaslittle or no interaction with the substrate as it becomes isolated. The ability on the one hand to synthesize highly monodisperse polymers such as poIyisoprene and polystyrene, terminated by a single hydrophilic g r ~ u p Pprovides *~ interestingcasm for the study of the transition between the classical and polymeric, or bulk, properties of surfactants through the phase changes described by the isotherms. A different class of polymers for studying the affectsof isolatingthe hydrophilicgroup is provided by the recently synthesized dendritic polymers made by divergent8-10 and To whom corrcspondmce should be a d d r d . 0022-3654/58/2097-0293$04.00/0
conv~rgent'~-l~ methods. The convergentmethod ischaractrrized by growth from a single functional group at the focal point of the molecule. This method tends to simplify purification and control over the molecular design. An example of such a molecule is shown in Figure 1, where the base unit is 3,5dihydroxybenzyl alcohol. In this case the fourth generation dendrimer, [G-41, has a focal point hydroxyl group with phenyl groups as the chain ends. These features give the dendrimer not only the outward appearance of a globular polystyrene but also a surfactant nature through the hydrophilic head group. Experimental Seetian The dendrimersused in this work were synthesizedas previously de3~ribed.Il-~~ The monomers added in the final generation of the synthesis, forming the "outer shell" of the dendrimer, were perdeuterated for neutron scattering studies. The moleculesw m characterized by GPC and NMR and shown to be monodisperse single molecules. The polystyrene chains were synthesized in a manner similar to that reported el~ewhere.~.~ The living polymer chains were terminated by the addition of ethylene oxide to produce the corresponding alcohol after workup with methanol. The molecular weight was confirmed by GPC, and the dispersity ratio, Mw/M., was less than 1.10. All surfactants were spread from a dilute toluene solution(about 5 X lo-* g/mL) at the air-water interface with isotherms being recorded at 20.0 OC. The toluene had been highly purified and multiply distilled. Aliquots 50 tima the spreading volume gave no spreading pressure after evaporation and comprwion on the Langmuir trough used whose minimum sensitivity WBB 0.06 mN m-1. R d b md DbcpMion Representative isotherms for polystyrene, Mw 1069 and 4500, anddendrimers, [G-41and [G-51,Mw3405 and6909, rsspectively, are shown in Figure 2. The lower molecular weight dendrimer and polystyrene have similar isotherms typified by an increase in the surface pressure, liquid expanded, LE, phase, followed by a peaked collapse transition, indicating a nucleation and growth process, to a liquid condensed, LC, phase. Further compression results in the formation of a solid film which exerts a mechanical pressure on the pressure measuring device through steric interactions of the hydrophobic groups. The higher molecular weight dendrimer and polystyreneisotherms are also similar, with a steep increase in the surface pressure being due to a direct transition to a solid phase. Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
294
Letters
100
200
300 Area /
500
400,
600
A' molecule'
Figure 3. Isotherms of [G4] and [G5] dendrimers showing differendue to compression rate and pause time before compression.
TABLE I: Limiting Areas of Dendrimer rad Polystyrene Isotherms
Figure 1. Fourth generation dendrimer, [G4], based on 3,Sdihydroxybenzyl alcohol. 20
--.PSOH Mw 1069 - -. Den[G-4]-OH Mw 3405 ...... Mw 4560 Den[G-B]-OH Mw 6909 - PSOH
I
YE
15
. B
5
1
I
I
I I
I
L
I
I
I
I
I
I
I
I
I
10
%
0 5
5
0 100
200
300
4d0
500
6
Area / A' molecule Figure 2. Isotherms of dendrimers [G4] and [G5], M, 3405 and 6909, and PSEtOH, M, 1069 and 4500.
The low molecular weight polystyrene surfactant has a fairly typical isotherm for a classical surfactant. The peak in the surface pressure between the LC and LE phases is representative of a nucleation and growth process's which indicates a slow formation of the LE phase. The second increase in the surface pressure (the transition to a solid phase) gives a limiting area, Ao, which is related to the cross sectional area of the polymer chain. In this case A0 is 58 A2/molecule, which is reasonable. The higher molecular weight polystyrene, however, forms a coil structure at the air-water interface in an analogous manner to that for hydrophobic polystyrene. Thus, the limiting area of this isotherm, here A0 = 323 A2/molecule, corresponds to the physical dimensions of the polymer coil that has been deposited on the surface as the spreading solvent has evaporated. The scaling of the coil size with molecular weight, at the interface, is analogous to the scaling of the radius of gyration of polystyrenes in a theta solvent.19 In the case of the dendrimers, the isotherm of [G-4] undergoes a very prominent nucleation and growth process between the LE and LC phases, indicating a slow nucleation's followed by a very quick growth. The addition of another generation, however, eliminates this feature through the combined effects of steric
surfactant
Ao, A2 molecule-1
compression rate, cm2 s-I
dendrimer[G4] dendrimer[G4] dendrimer[G5] dendrimer[G5] dendrimer[G5] FSOH (MW 1069) PSOH (MW 4500)
128 144 323 357 395 58 323
0.04 0.20 0.20 (1-h pause)
0.04 0.20
0.20 0.20
interactions and isolation of the hydrophilic group from the substrate. The limiting areas measured for the isotherms of the dendrimers shown in Figure 2 are A0 = 144 A2/molecule for [G-4] and A0 = 395 A2/molecule for [G-5]. These isotherms, run at a rather high compression rate of 0.2 cm2 s-1, show a marked difference in the limiting areas when compared to slower compression rates or having a delay between spreading and compression (Figure 3). This may be due to changes in the size of the molecule through solvent swelling. The resulting limiting areas of all the isotherms are shown in Table I. Future work will examine and compare the scaling of limiting area with molecular weight for the novel dendritic macromolecular architecture with normal linear polystyrenes. Acknowledgment. Partial support of this research by the Australian Research Council (Q.E. I1 Fellowship to C.J.H.)and the NSF (Grant DMR 8913278) is acknowledged with thanks. References and Notes (1) Kenn, R. M.; Whm, C.; MBhwald, H.; Kjaer, K.; Ala-Nielsen, J. In Surfuce X-Ray und Neutron Scattering, Zabel, H.. Robinson, I. K., Eds.; Springer-Verlag: Berlin, 1992; p 139. (2) Bloch. J. M.; Eisenberger, P. Nucl. Instrum. Methods 1988, 831, 468. (3) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J . Phys. Chem. 1992. 96. 1373. (4) Lu,J. R.;Simister, E.A.; Lee, E. M.; Thomas, R. K.;Rennie, A. R.; Penfold, J. Lungmuir 1992, 8, 1837. ( 5 ) Penfold, J.; Thomas, R. K. J . Phys.: Condens. Mutter 1990.2.1369. (6) Morton, M. Anionic Polymerization: Principles und Practice; Academic Press: New York, 1983. (7) Young, R. N.; Quirk, R. P.; Fetters, L. J. Anionic Polymerizations of Non-Polar Monomers Involving Lithium. In Advunces In Polymer Science; Springer-Verlag: Berlin, 1984; Vol. 56. p 1. (8) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.;Rocck, J.; Ryder, J.; Smith, P. Polym. J . 1985, 17, 117. (9) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K. J . Org. Chem. 1985. 50. 2004. --.--.--(10) Padias, A. B.; Hall, H. K., Jr.; Tomalia, D. A.; McConnell, J. R. J , Org. Chem. 1987, 52, 5305. ( 1 1) Hawker, C. J.; Frtchet. J. M.J.J. Am. Chem.Soc. 1990,112,7638. (12) Wooley, K.L.; Hawker, C. J.; Frtchet, J. M. J.J. Chem. Soc.,Perkin Trans. 1991, 1059. (13) Hawker, C. J.; Frtchet, J. M. J. Polymer 1992, 33, 1507. (14) Mourey, T. H.; Turner, S. R.;Rubinstein, M.; Frtchet, J. M. J.; Hawker, C. J.; Wooley, K. L. Macromolecules 1992, 25, 2401. (15 ) Brinkhuis, R. H. G.; Schouten, A. J. Mucromolecules 1991,241487, (16) Kumaki, J. Mucromolecules 1986. 19, 2258. (1 7) Kumaki, J. Mucromolecules 1988, 21, 749. (18) Kumaki, J. J. Polym. Sci. E Polym. Phys. 1990, 28, 105. (19) Huber, K.;Bantle, S.; Lutz, P.; Burchard, W .Macromolecules 1985, 18, 1461. ~
