Surfactants and Interfacial Ordering

SFG [4.309, 4.310] uses visible and infrared lasers for generation of their sum frequency. Tuning the infrared laser in a certain spectral range enables monitoring of molecular vibrations of adsorbed molecules with surface selectivity. SFG includes the capabilities of SHG and can, in addition, be used to identify molecules and their structure on the surface by analyzing the vibration modes. It has been used to observe surfactants at liquid surfaces and interfaces and the ordering of interfacial... 

The low interfacial tensions between two liquids have been measured for different systems by using the pendant drop method. In the case of the quaternary system Ci2ll25S 3 tNa+H20+n-Butanol+Toluene, the interfacial data as measured by pendant drop method are compared with reported literature data, using other methods (with varying NaCl concentration). In order to understand the role of co-surfactant, ternary systems were also investigated. The pendant drop method was also used for measuring the interfacial tension between surfactant-H20/n-alcohol (with number of carbon atoms in alcohol varying from 4-10). The interfacial tension variation was dependent on both the surfactant and alcohol. 

Ford and coworker [104] have studied HIPEs of water-in-xylenes, stabilised by a variety of surfactants, and postulated three properties which an emulsifier should possess in order to form stable w/o HIPEs of high volume fraction a) a lowering of the interfacial tension between water and oil phases, b) the formation of a rigid interfacial film and c) rapid adsorption at the interface. 

In impact tests the order was the same except that Tritonal/D-2 95/5 and Picratol 52/48 were reversed Addnl work was conducted on the use of surfactants for reducing the interfacial tension between TNT and paraffin waxes. Refs 98 100 report results of tests using the n-octadecyl, n-hexa-decyl, n-dodecyl and n-hexyl esters of 2,4,6-trinitrobenzoic acid to permit stable homogeneous blends of paraffin waxes in molten TNT. It is reported that all these materials act as surfactants and that the interfacial activity increases as a linear function of the alkyl chain length of the ester. N-octadecyl 2,4,6-trinitrobenzoic acid also acts as a surfactant for the beeswax/TNT system. It was concluded, however, that the degree of reduction in the interfacial tension of... 

In order to understand how surfactant reduces surface and interfacial tension, one must first need to understand the concept of surface and interfacial tension. 

The nature of the surfactant and its concentration is expected to play a role. To achieve a mechanically strong interfacial film, which can ensure the stability of the emulsion, the interfacial film of adsorbed surfactant molecules should be condensed in order to have strong lateral intermolecular interactions. A blend of two surfactants with different areas of head groups rather than an individual surfactant can more easily generate a close-packed and mechanically strong interfacial film. 

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

The droplet deformation increases with increases in the Weber number which means that, in order to produce small droplets, high stresses (i.e., high shear rates) are require. In other words, the production of nanoemulsions costs more energy than does the production of macroemulsions [4]. The role of surfactants in emulsion formation has been described in detail in Chapter 10, and the same principles apply to the formation of nanoemulsions. Thus, it is important to consider the effects of surfactants on the interfacial tension, interfacial elasticity, and interfacial tension gradients. 

A general pattern of microemulsion phase behavior exists for systems containing comparable amounts of water and a pure hydrocarbon or hydrocarbon mixture together with a few percent surfactant. For somewhat hydrophilic conditions, the surfactant films tend to bend in such a way as to form a water-continuous phase, and an oil in water microemulsion coexists with excess oil. Drops in the microemulsion are spherical with diameters of order 10 nm. Both drop size and solubilization expressed as (VJVX the ratio of oil to surfactant volume in the microemulsion, increase as the system becomes less hydrophilic. At the same time interfacial tension between the microemulsion and oil phases decreases. Just the opposite occurs for somewhat lipophilic conditions. That is, a water in oil microemulsion coexists with excess water with drop size and solubilization of water (VJV,) increasing and interfacial tension decreasing as the system becomes less lipophilic. When the hydrophilic and lipophilic properties of the surfactant films are nearly balanced, a bicontinuous microemulsion phase coexists with both excess oil and excess water. For a balanced film (VJV,) and (VJV ) in the microemulsion are nearly equal, as are 7, 0 and... 

From the above, it is clear that a pre-requisite of low water/oil interfacial tensions is the complete saturation of the water-rich and oil-rich phases as well as the water/oil interface by surfactant molecules. Of course, this pre-requisite is fulfilled if one of the phases considered is a microemulsion. Furthermore, since the pioneering work of Lang and Widom [81] it is known that if a system is driven through phase inversion the interfacial tensions may become ultra-low. However, about 20 years ago, a number of experimental investigations were devoted to clarifying the origin of the ultra-low interfacial tensions [15, 17, 39, 71, 81-85]. In order to understand this correlation between phase behaviour and interfacial... 

The choice of suitable surfactants and additional chemicals for the decontamination of source zones largely depends on the type of pollutant and the structure of the soil (mainly on adsorption behaviour and hydraulic conductivity). Adsorbed and solid pollutants or very viscous liquid phases cannot be mobilised. Preformed microemulsions, co-solvents or co-surfactants can be favourably used for such contaminations in order to enhance the solubilisation capacity of surfactants. NAPL with low viscosity can easily be mobilised and also effectively solubilised by microemulsion-forming surfactant systems. Mobilisation is usually much more efficient. It is achieved by reducing the interfacial tension between NAPL and water. Droplets of organic liquids, which are trapped in the pore bodies, can more easily be transported through the pore necks at lower interfacial tension (see Fig. 10.2). The onset of mobilisation is determined by the trapping number, which is dependent on... 

---