Spread films of a poly(ethylene oxide)-polystyrene diblock copolymer were studied at the h a t e r and heptanewater interfaces by using the Wilhelmy plate technique and surface quasi-elastic light scattering (SLS) from capillary waves. At the air-water interface, the dynamic f i l m viscoelastic parameters, Le., surface pressure, surface dilational elasticity and viscosity, and transverse viscosity, were deduced from the SLS measurements. The static surface tension and dilational elasticity were compared to their dynamic counterparts from SLS. At the heptanewater interface, the dynamic surface pressure and transverse viscosity were evaluated, and the latter was found to be zero for the whole surface concentration range. Some speculations about possible chain conformations of the copolymer at the two interfaces are offered.

The static and dynamic properties of pentadecanoic acid at the air-water interface have been examined by using the surface light scattering technique combined with the Wilhelmy plate method. Surface pressure ( ) vs. area per molecule (A) plots for films studied by stepwise compression or by one-shot spreading, over the range of 500-20 Á2, exhibited a distinct plateau between the liquid-expanded (LE) to liquidcondensed (LC) phase transition at 31.2 Á2. Surface scattering measurements of capillary waves at well-defined wavenumbers for the same monolayer allowed estimation of fs, the frequency shift at maximum intensity, and A/gc, the instrument-width-corrected spectral full width at half-height. In contrast to previously reported surface scattering data with less pure pentadecanoic acid, a sharp first-order discontinuity was observed at the LE/LC phase transition. From 500 Á2 to the phase transition, and the corresponding dynamic counterpart increased monotonically, showing little change from pure water at A > 50 Á2. However, A/a c, a measure of the temporal damping of the capillary waves, exhibited a significant increase from 100 to 50 Á2, followed by a continued decrease to the LE/LC transition. Static and dynamic elastic moduli were compared and their correspondence and disparity were found at different regions of surface concentration.

The technique of quasi-elastic light scattering from interfacial capillary waves was implemented to determine the dynamic surface pressure of dipalmitoylphosphatidylcholine (DPPC) spread at the heptane/water interface in a wave vector range of 330-520 cm-1. The interfacial concentration of DPPC was spanned for about a factor of 10 such that the range of area per molecule, A, covered was 40-350 Á2 3. By

Langmuir 1987, 3, 31-35 31 tendency form aqueous-two-phase systems with PEG. Lithium and multivalent cations, because of their interaction with the ether oxygens of PEG, reduce the tendency of the salt to form a two-phase system. 2.Among the various Na salts tested, the higher the valency of the anion, the lower the concentration required for phase formation.3. The effectiveness of various salts to form two-phase systems with PEG is similar to their tendency to depress the cloud point of polyethoxylates. 4.Factors leading to incompatibility between PEG and inorganic salts in aqueous solution are suggested to be solvation, repulsive interaction between the ether oxygens and the anions, and image forces arising from the dielectric discontinuity at the polymer molecule-water interface.5. The aqueous two-phase formation boundary can be characterized by the Setschenow equation for electrolyte-nonelectrolyte interaction. Also, along the phase boundary, the activity of Na2S04 remains almost constant.It is therefore possible to predict the entire phase boundary based on the information for a single point on the phase boundary.6. The higher the temperature, the lower the amount of salt required for phase formation. Acknowledgment.We thank E. Fu and R. Lopresti for their help with the experiments.Note Added in Proof. Further work has indicated that clouding and phase separation in the above systems in fact represent the same phenomenon. These results will be reported elsewhere.A surface light scattering technique for probing capillary waves in combination with the Wilhelmy plate method of surface tension measurement has been used to study the dynamic properties of vinyl stearate (VS) and poly(viny1 stearate) (PVS) monolayers spread at the air/water interface. Two molecular weight grades of PVS, 5300 and 3700, gave identical surface pressure-area isotherms and light scattering results. Estimation off,, the frequency shift of the power spectrum at maximum intensity, and the instrumenticorrected full width at half-height, allowed estimation of surface viscoelastic parameters of the monomer and its corresponding polymer. Whereas VS exhibited behavior in complete accord with that observed earlier for small amphiphiles such as pentadecanoic acid and dipalmitoylphosphatidylcholine, i.e., small surface viscosity relative to surface elasticity, above a surface concentration equivalent to 55 A2/monomer PVS exhibited significantly higher surface viscosities, i.e., N surface poise, and a predominant contribution over surface elasticity to the overall dynamic modulus (e*). Thus both a quantitative and qualitative difference in surface viscoelasticity for "unconnected" and "connected" monomers in a monolayer are reported for the first time. IntroductionThe study of polymer monolayers spread at the air/ water interface provides a basis for understanding the interfacial properties of polymers adsorbed to any interface. To date, significant attention has been given to the static or thermodynamic properties of protein and synthetic polymer ...