Surface light scattering technique

In this paragraph we will briefly recall the main features of the surface light scattering technique. Further details can be found in reference (4). 

Water is extensively used to produce emulsion polymers with a sodium stearate emulsifrer. The emulsion concentration should allow micelles of large surface areas to form. The micelles absorb the monomer molecules activated by an initiator (such as a sulfate ion radical 80 4 ). X-ray and light scattering techniques show that the micelles start to increase in size by absorbing the macromolecules. For example, in the free radical polymerization of styrene, the micelles increased to 250 times their original size. 

Particles of a size of less than 2 turn are of particular interest in Process Engineering because of their large specific surface and colloidal properties, as discussed in Section 5.2. The diffusive velocities of such particles are significant in comparison with their settling velocities. Provided that the particles scatter light, dynamic light scattering techniques, such as photon correlation spectroscopy (PCS), may be used to provide information about particle diffusion. 

However, for the temperatures above 15°C the agreement is not quantitative. It is also important to remember that the kilohertz region has been explored by the surface Light Scattering (SLS) technique. The results obtained are compatible... 

It is very well known that the nature of the monolayer partially depends on the strength of interfacial interactions with substrate molecules and that of polymer in-tersegmental interactions. And it is normal to expect that the viscoelastic properties of polymer monolayer are also dependent on these factors. The static and dynamic properties of several different polymer monolayers at the air - water interface have been examined with the surface quasi-elastic Light Scattering technique combined with the static Wilhelmy plate method [101]. 

Ray and Schork (1980) have developed an on-Hne device for measuring surface tension. A bubble rise technique is used. If this method or the light scattering technique of Hamielec were used in a oMitrol system one could presumably alter the feed rate of a kq/ ingredient, probsbiy emulsifier, to prevent undesired transients. More work is needed to demonstrate the potential of these methods. 

We shall not pursue this approach here because it does not help us much in finding a molecular interpretation, certainly not for U°. Even in the absence of waves (solidified liquids), U° is substantial. Rather, this interpretation deals with a contribution to y than with y itself. However, we recall that the capillary wave connection had already occurred in the technique for measuring surface tensions from surface light scattering, see Mandelstam s equation [1.10.1], from which an explicit formula for y may be derived. 

Finally, we recall that surface light scattering is another modem technique. Essentially. thermal capillary waves can be studied and this enables us to derive interfacial tensions and binding constants. We discussed this matter in sec. 1.10. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

We would like to thank the S.E.R.C. and Unilever Research for support for one of us (DNS) Unilever Research also provided access to Si NMR and laser Raman spectrometers. We thank EKA AB, Surte, Sweden, for financial support for another of us (KRA) and for help with the ultrafiltration experiments. We are also very grateful to Mr. Kenneth Rosenquist of the Swedish Institute for Surface Chemistry, Stockholm, for his assistarce in applying the dynamic light scattering technique. Finally we are grateful to the PQ Corporation for making it possible for this paper to be presented at New York. 

The application of laser light scattering techniques to molecular characterization of dielectric films offers the ability to directly probe chemical bonding within the film and at the film-substrate interface. Real-time measurements can be carried out under ambient conditions or in hostile environments allowing transient film stability studies to be conducted. Such laser-based techniques require only an optically clear line of sight between sample and analyzer and offer several advantages over the high vacuum surface analytical techniques commonly applied to film characterization. These include nondestructive measurement capability, rapid data acquisition time, and ability to use the optical properties of the sample to enhance the sensitivity of the measurement. 

Elastic and inelastic light scattering are nowadays widely used techniques for the characterization of fluids. In particular, these techniques have been extensively and successfully used with microemulsion systems to obtain information about droplet sizes. Surface light scattering is a less common technique but has been used with microemulsion interfaces, in particular to measure the ultralow interfacial tensions found in these systems. In this chapter we discuss these aspects, first recalling the theoretical background and illustrating the potential of the techniques with experimental results. 

The above discussion has centered on wave motion imposed on a surface by, for instance, an oscillating bar. But thermal fluctuations cause wave motion of small amplitude even on interfaces that are not disturbed by external means. With laser light scattering techniques it is possible to measure interfadal tension from analysis of surface fluctuations. This method has been applied to the measurement of ultralow interfacial traisions between liquid phases (Bouchiat and Meunier, 1972 Cazabat et al., 1983 Zollweg et al., 1972). Presumably it could also be used to determine surface compressibility or other rheological properties. 

Light scattering techniques are useful tools to investigate both interfacial properties such as surface tension and viscoelasticity (4) and bulk properties such as droplet size and interaction forces between these droplets (5,6). It must be pointed out that in each case, light is probing thermal fluctuations in the medium but the fluctuations are of a very different nature surface roughness in the first case, and droplet concentration fluctuations in the case of bulk scattering. 

Light scattering techniques appear as a very useful tool to investigate the interfacial properties of oil-water mixtures, e.g., interfacial tension, surface viscosity, as well as micellar size and micellar interactions. 

---