Surface properties, dilational

The techniques used to measure dilational surface properties have been described in detail (4, 6). Surfaces of surfactant solutions are subjected to small amplitude, sinusoidal area variations, and the dilational modulus is given by the ratio between surface tension change measured and fractional area change applied. The modulus generally depends on the frequency at which the experiment is carried out, and this frequency... 

Callaghan and Neustadter [31] have made a study of the foam stabilities of air-crude oil and natural gas-crude oil systems using a variety of light crude oils of viscosities 14 mPa s. This study, at ambient temperature using a sparging method, concerned so-called dead oils from which natural gas had been separated. It also involved a comparison of the foam behavior with critical film rupture thicknesses, bulk phase, and surface shear viscosities together with dilatational surface properties. 

A Prlns, Dynamic Surface Tension and Dilational Interfacial Properties, in New Physico-chemical Techniques for the Characterization of Complex Food Systems, E. Dickinson, Ed., Blackie Academic (1995), chapter 10. (Review of dynamic techniques.)... 

As for pure LMWE films, the surface dilatational modulus is higher for protein-saturated-LMWE than for protein-unsaturated-LMWE mixed films at every surface pressure. The surface dilatational properties of mixed protein-emulsifier films also depend on the presence of some food components (ethanol and sucrose) in the aqueous phase. In general, a decrease in the dilatational rheological properties on the addition of ethanol was found for protein-water-insoluble LMWE. That is, the static and... 

A. Prins. Dynamic Surface Tension and Dilational Interfacial Properties. In E. 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

The situation changes for non-equilibrium systems. The dynamic surface properties of micellar solutions depend strongly on the concentration in a broad range of surface life time and/or of the frequency of surface compression and dilation. First of all this is related to the fact that the adsorption rate of surfactants increases with concentration for both sub-micellar and micellar solutions. As an example, dynamic surface tensions of SDS in 0.1 M NaCl measured by Fainerman and Lylyk [77] are shown in Fig. 7. As one can see entirely different values of the dynamic surface tension and of the adsorption can correspond to the same surface age at c > CMC. 

This relation describes not only periodic deformations of a liquid surface. Using methods of integral transformations it is possible to show that the dynamic surface elasticity is a fundamental surface property and its value determines the system response to a small arbitrary surface dilation [161]. With this method it is also possible to determine the dynamic elasticity of liquid-liquid interfaces where the surfactant is soluble in both adjacent phases [133]. Moreover, similar transformations lead to an expression for the dynamic surface elasticity for the case when the mechanism of the slow step of micellisation is determined by scheme (5.185) or for frequencies corresponding to the fast step of micellisation [133,134]. However, as stated above, it is the slow process which mainly influences the adsorption kinetics from micellar solutions. 

The Boussinesq-Scriven constitutive law postulates that = rj id. The surface viscosity, however, being a property of the adsorption layer itself, should be related to the relative displacement of the adsorbed molecules. As a result the dilatational surface viscous stress should be proportional to the rate of local deformation, i.e.. For insoluble... 

Drop and bubble methods have been developed significantly during the last years. These techniques provide access to dynamic properties of liquid interfaces. The drop and bubble shape technique as well as the fast oscillating drops and bubbles are described essentially as tools for dilational surface rheology. 

Another surface property that will be used in this chapter is the surface dilational modulus... 

Another important property is the surface dilational viscosity, k... 

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

Ed is the dilatational elasticity, and rid is the dilatational viscosity. It is characteristic for a stable foam to exhibit a high surface dilatational elasticity and a high dilatational viscosity. Therefore effective defoamers should reduce these properties of the foam. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

The surface rheological properties of the /3-lg/Tween 20 system at the macroscopic a/w interface were examined by a third method, namely surface dilation [40]. Sample data obtained are presented in Figure 24. The surface dilational modulus, (E) of a liquid is the ratio between the small change in surface tension (Ay) and the small change in surface area (AlnA). The surface dilational modulus is a complex quantity. The real part of the modulus is the storage modulus, e (often referred to as the surface dilational elasticity, Ed). The imaginary part is the loss modulus, e , which is related to the product of the surface dilational viscosity and the radial frequency ( jdu). 

Experiments with the /3-lg/Tween 20 system were performed at a macroscopic a/w interface at a /3-lg concentration of 0.2 mg/ml [40]. The data obtained relate to the properties of the interface 20 minutes after formation. Up to R = 1, the storage modulus (dilational elasticity) was large and relatively constant, whereas the loss modulus (dilational viscosity) increased with increasing R. As R was increased to higher values there was a marked decrease in the storage modulus (dilational elasticity) and a gradual increase in the loss modulus (dilational viscosity). In summary, the data show the presence of a transition in surface dilational behavior in this system at a solution composition of approximately R = 1. At this point, there is a transformation in the adsorbed layer properties from elastic to viscous. 

Furthermore, surface reconstruction may be viewed, for the variation of certain properties, simply as a response to depression , or to a strain of dilatation, in the surface layer. This approach has been applied with success to account for the surface phonon, and for the global destabilization (blue shift) of the surface exciton. In contrast, the modification of the surface excitonic structure is not amenable to such a simple image of the reconstruction. 

---