Surfactants surface forces

The surface forces apparatus (Section VI-3C) has revealed many features of surfactant adsorption and its effect on the forces between adsorbent surfaces [180,181]. A recent review of this work has been assembled by Parker [182]. 

Yamada S and Israelachvili J N 1998 Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus J. Rhys. Chem. B 102 234-44... 

Rutland M W and Parker J L 1994 Surface forces between silica surfaces in cationic surfactant solutions adsorption and bilayer formation at normal and high pH Langmuir 0 1110-21... 

Pashley R M and Israelachvili J N 1981 A comparison of surface forces and interfacial properties of mica in purified surfactant solutions Colloids Surf. 2 169-87... 

Pashley R M, McGuiggan P M, Ninham B W, Brady J and Evans D F 1986 Direct measurements of surface forces between bilayers of double-chained quaternary ammonium acetate and bromide surfactants J. Phys. Chem. 90 1637-42... 

Molecularly motivated empiricisms, such as the solubility parameter concept, have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inadequate for mixtures that involve macromolecules, associating entities like surfactants, and rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these systems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in solution, the rheological and mechanical properties of these solutions, and, more generally, the fluid mechaiucs of non-Newtonian liquids. 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

J.L. Parker Surface Force Measurements in Surfactant Systems. Prog. Surf. Sci. 47, 205 (1994). 

It is probable that numerous interfacial parameters are involved (surface tension, spontaneous curvature, Gibbs elasticity, surface forces) and differ from one system to the other, according the nature of the surfactants and of the dispersed phase. Only systematic measurements of > will allow going beyond empirics. Besides the numerous fundamental questions, it is also necessary to measure practical reason, which is predicting the emulsion lifetime. This remains a serious challenge for anyone working in the field of emulsions because of the polydisperse and complex evolution of the droplet size distribution. Finally, it is clear that the mean-field approaches adopted to measure > are acceptable as long as the droplet polydispersity remains quite low (P < 50%) and that more elaborate models are required for very polydisperse systems to account for the spatial fiuctuations in the droplet distribution. 

In this situation, the equilibrium thickness at any given height h is determined by the balance between the hydrostatic pressure in the liquid (hpg) and the repulsive pressure in the film, that is n = hpg. Cyril Isenberg gives many beautiful pictures of soap films of different geometries in his book The Science of Soap Films and Soap Bubbles (1992). Sir Isaac Newton published his observations of the colours of soap bubbles in Opticks (1730). This experimental set-up has been used to measure the interaction force between surfactant surfaces, as a function of separation distance or film thickness. These forces are important in stabilizing surfactant lamellar phases and in cell-cell interactions, as well as in colloidal interactions generally. 

Between hydrophobic surfaces a completely different interaction is observed. Hydrophobic surfaces attract each other [184], This attraction is called hydrophobic interaction. The first direct evidence that the interaction between solid hydrophobic surfaces is stronger than the van der Waals attraction was provided by Pashley and Israelachvili [185,186], With the surface force apparatus they observed an exponentially decaying attractive force between two mica surfaces with an adsorbed monolayer of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Since then the hydrophobic force has been investigated by different groups and its existence is now generally accepted [189]. The origin of the hydrophobic force is, however, still under debate. 

Special probes were introduced to measure surface forces in colloidal systems as a function of the ionic strength and the concentration of surfactant molecules [214]. A so-called colloid probe can be prepared by gluing a silica sphere onto a conventional Si3N4 tip [179]. 

The surface forces technique measures the force between molecules (eg. surfactants, polymers) adsorbed on mica sheets. In the case of large molecules such as polymers, the measurement is most sensitive to the regions closest to the solution and provides little direct information about the region adjacent to the surface. As it is a measurement between macroscopic surfaces, it is unable to provide information on microscopic chemical differences at the interface. Infrared spectroscopy could provide additional information about the quantity of adsorbed material on the mica surface, the identity and orientation of the adsorbed species, and possibly the nature of the surface linkage. 

The role of surfactants in stabilization/destabilization of foam (air/liquid dispersions) is similar to that for emulsions. This is due to the fact that foam stability/instability is determined by the surface forces operative in liquid films between air bubbles. In many industrial applications, it is essential to stabilize foams against collapse, e.g., with many food products, foam in beer, fire-fighting foam, and polyurethane foams that are used for furniture and insulation. In other applications, it is essential to have an effective way of breaking the foam, e.g., in distillation... 

Recent molecular dynamics simulations of water between two surfactant (sodium dodecyl sulfate) layers, reported by Faraudo and Bresme,14 revealed oscillatory behaviors for both the polarization and the electric fields near a surface and that the two fields are not proportional to each other. While the nonmonotonic behavior again invalidated the Gruen—Marcelja model for the polarization, the nonproportionality suggested that a more complex dielectric response of water might, be at the origin of the hydration force. The latter conclusion was also supported by recent molecular dynamics simulations of Far audo and Bresme, who reported interactions between surfactant surfaces with a nonmonotonic dependence on distance.15... 

P. M. Claesson, Surface Forces Apparatus Studies of Polymers, Polyelectrolytes and Polyelectrolyte-Surfactant Mixtures at Interfaces (P. Dubin and R. Farinato, eds.), Colloid-Polymer Interactions From Fundamentals to Practice, Wiley, New York, 1999, p. 287. 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

The h(Cei) dependences (at pG = const and t = const) are studied also for other surfactants, phospholipids and polymers, and they have proved to be very informative with respect to not only the CBF/NBF transition and Cei,cr values, but also surface forces acting in black foam films (see Sections 3.3. and 3.4). 

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