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Glycerol mono-oleate monolayers at the air-water interface have been investigated by quasielastic light scattering from thermally excited capillary waves over a wide range of wave numbers. Using a relatively novel data analysis procedure four surface viscoelastic properties were deduced ab initio from the light scattering data : surface elastic moduli and viscosities governing shear normal to the monolayer (≡ tension) and dilation in the film plane. The tension and dilational modulus were compared with classical, equilibrium values in the first rigorous comparison of its kind. Various effects suggested that the two moduli were affected by rather different relaxation processes : discrepancies between the light scattering and equilibrium values of the two elastic moduli occurred in different states of the monolayer, and the two surface viscosities (both zero for the clean subphase) behaved very differently on monolayer compression. These effects were observed to be frequency dependent. In the fully compressed monolayer state the transverse shear modulus was characterised by an exponential relaxation, of time scale ∼ 9 μs. This relaxation time fell exponentially on monolayer expansion, reaching 100 ns for molecular areas ∼ 60 Å 2. Slower processes than these were rigorously excluded. The dilational modulus was generally less well determined than that affecting transverse shear. However in the expanded monolayer state, the data sufficed to demonstrate much slower relaxation, τ ∼ 290 μs. Possible molecular mechanisms are briefly discussed

The phase transitions of monolayers of carefully purified n-pentadecanoic acid at the air/water interface have been investigated using both classical and laser scattering methods. The equilibrium pi -A isotherms showed flat coexistence regions between the liquid-expanded and liquid-condensed states. Such clear first-order transitions appear only to be observable for pure pentadecanoic acid. In transitions at temperatures above the triple point ( approximately 17 degrees C) the light scattering clearly showed up phase separation within the monolayer, again demonstrating the first-order nature of the transitions. The liquid domains in the liquid/vapour transition were about 1 cm across. In the liquid-expanded/liquid-condensed transition the denser phase regions were of the order of 0.4 mm in size. These estimates are compatible with surface potential fluctuations which have been observed for this system. In the condensed/vapour transition below the triple point the monolayer behaved quite differently, no fluctuations being observed. At a surface concentration such that half of the pentadecanoic acid was in each surface phase, the viscoelastic properties of the film changed abruptly from close to those of the clean subphase to those of a viscoelastic medium. It is hypothesized that the molecular aggregates formed in this transition interact, when sufficiently close together, to form some kind of surface superstructure, which acts as a homogeneous surface phase.

The viscoelastic properties of monolayers of n-pentadecanoic acid at the air-water interface have been studied using surface light scattering. The monolayers displayed first-order liquid-condensed to liquid-expanded transitions: the surface properties reported are those of well defined, reproducible monolayer states. Two viscoelastic moduli were determined: for shear transverse to the surface and for uniaxial dilation in the surface. For experiments both above and below the triple point the surface viscoelasticity was found to depend upon the monolayer phase in a complex fashion. Both moduli displayed viscoelastic relaxation, the strength and timescales being different for the two moduli, as well as depending upon the monolayer state. At one point in the phase diagram it was shown that the high-frequency dilational viscosity measured by light scattering could be identified with the conventional surface shear viscosity. The results confirm the complexity of the viscoelastic behaviour of molecular films which is not adequately represented by a single surface viscosity.

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