Insoluble monolayers, surface tension

The automated pendant drop technique has been used as a film balance to study the surface tension of insoluble monolayers [75] (see Chapter IV). A motor-driven syringe allows changes in drop volume to study surface tension as a function of surface areas as in conventional film balance measurements. This approach is useful for materials available in limited quantities and it can be extended to study monolayers at liquid-liquid interfaces [76],... 

It is not uncommon for this situation to apply, that is, for a Gibbs mono-layer to be in only slow equilibrium with bulk liquid—see, for example. Figs. 11-15 and 11-21. This situation also holds, of course, for spread monolayers of insoluble substances, discussed in Chapter IV. The experimental procedure is illustrated in Fig. Ill-19, which shows that a portion of the surface is bounded by bars or floats, an opposing pair of which can be moved in and out in an oscillatory manner. The concomitant change in surface tension is followed by means of a Wilhelmy slide. Thus for dilute aqueous solutions of a methylcellu-... 

In actual practice the soluble component usually is injected into the substrate solution after the insoluble monolayer has been spread. The reason is that if one starts with the solution, the surface tension may be low enough that the monolayer will not spread easily. McGregor and Barnes have described a useful injection technique [265]. 

The significance of the above discussion is to point out that the orientation of the hydrocarbon chains with respect to the surface and to each other can be monitored, and controlled, as a function of the surface pressure (which in turn is directly proportional to the surface tension if the monolayer is insoluble in the subphase). 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

Next let us consider some of the physical properties of the spread monolayer we have described. Equation (1) states that the surface tension of the covered surface will be less than that of pure water. It is quite clear, however, that the magnitude of 7 must depend on both the amount of material adsorbed and the area over which it is distributed. The spreading technique already described enables us to control the quantity of solute added, but so far we have been vague about the area over which it spreads. Fortunately, once the material is deposited on the surface, it stays there —it has been specified as insoluble and nonvolatile for precisely this reason. This means that some sort of barrier resting on the surface of the water may be used to corral the adsorbed molecules. Furthermore, moving such a barrier permits the area accessible to the surface film to be varied systematically. In the laboratory this adjustment of area is quite easy to do in principle. As we see below, the actual experiments must be performed with great care to prevent contamination. 

The principal requirements for an ideal gaseous film are that the constituent molecules must be of negligible size with no lateral adhesion between them. Such a film would obey an ideal two-dimensional gas equation, ttA kT, i.e. the it-A curve would be a rectangular hyperbola. This ideal state of affairs is, of course, unrealisable but is approximated to by a number of insoluble films, especially at high areas and low surface pressures. Monolayers of soluble material are normally gaseous. If a surfactant solution is sufficiently dilute to allow solute-solute interactions at the surface to be neglected, the lowering of surface tension will be approximately linear with concentration - i.e. 

The derivative d In c ldT is calculated for each adsorption isotherm, and then the integration in Equation 5.5 is carried out analytically. The obtained expressions for J are listed in Table 5.2. Each surface tension isotherm, oCEi), has the meaning of a two-dimensional equation of state of the adsorption monolayer, which can be applied to both soluble and insoluble surfactants. ... 

Tn his classic book (I) N. K. Adam discussed the behavior of very dilute - monolayers at the air/water (A/W) interface and using measurements published ear her by Jessop and himself (2, 3, 4), he showed that surface pressure (n)-area (A) isotherms for insoluble uncharged species, when plotted on a nA vs. n basis, suggested a limit of IkT at zero II. The same limit was also suggested by Schofield and Rideal s plot (5) of Frumkin s surface tension data (6) using the Gibbs adsorption isotherm to calculate A. Adam (I) stressed that n should be measured to the second decimal place to establish this limit unequivocally Adam and Jessop (4) provide one of the few sound extrapolations to this limit with their data on the esters of some dicarboxylic acids. 

Surface self-diffusion is the two-dimensional analogue of the Brownian motion of molecules in a liquid bulk. Measurements of self-diffusion have to be performed in complete absence of any Marangoni flow caused by surface tension differences. Such experimental conditions are best established in an insoluble monolayer where one part consists of unlabelled and the other of radio-tracer labelled molecules. The movement of molecules within the surface monolayer can be now observed by using a Geiger-Miiller counter. There are possible effects of liquid convective flow in the sublayer which was discussed for example by Vollhardt et al. (1980a). With e special designed apparatus Vollhardt et al. (1980b) studied the self-difihision of different palmitic and stearic acid and stearyl alcohol and obtained self-diffusion coefficients between l-i-4 lO cm /s. 

All surface-active molecules, such as soaps and lipids, can be prepared as monomolecular layers at the air-water interface. Driven by the reduction of the surface free energy of water, these molecules spread when applied to the surface, for example, from a volatile solvent (Fig. 10). The physical properties of these monolayers were first investigated in the 1940s by Langmuir after his first work on molecules in insoluble monolayers [41]. A so-called Langmuir trough filled with water defines an exactly known area for the spread molecules (Fig. 10). At low lateral density, these molecules behave like a quasi-two-dimensional gas. If the area for these molecules is reduced by a movable barrier, this lateral compression will eventually lead to a measurable lateral pressure n (force F per unit length of barrier). Which can be measured by a so-called Wilhelmy balance (see Kuhn et al. [42]), It is the difference between the surface tension of the free, y, and the layer-covered, y, water surface ... 

Modification of the solid surface. The surface can be completely coated by monolayers of low surface energy compounds on which oil drops having higher surface tension will not spread, or covered with narrow ring coatings of insoluble polymers of low surface energy, such as fluorocarbon derivatives, which surround the oil drop. 

It is known that adsorption of a surface-active substance (surfactant) on the interface results in a formation at this surface of an oriented monolayer that lowers the surface tension. Typical water solutions of surfactants contain organic molecules with long hydrocarbon tails and polar heads [13]. Hydrocarbons are practically insoluble in water, and water is a highly polar liquid. Figure 17.3 shows how molecules of ideal surfactant are adsorbed on the water surface. The polar heads of molecules penetrate into water, while hydrocarbon tails remain in the gaseous medium. Formation of the monolayer requires a relatively small number of molecules. 

Lest the reader think that 2-D foams are just figments of the imagination, it must be pointed out that they can be generated - or at least closely approximated - by squeezing a 3-D foam between two narrowly spaced, wetted, transparent plates (2, 31-35). Structurally even closer realizations may be obtained in phase-coexistence regions of insoluble monolayers of surface-active molecules at the air-water interface (36), where the role of surface tension is taken over by the line tension at the phase boundaries.]... 

Mukerjee and Kushnick (167) showed that at low frequency the demulsifier behaves as a soluble mono-layer, and at high frequency as an insoluble monolayer. Variation in interfaeial tension from a local change in area is virtually instantaneous. This gradient is short circuited when the demulsifier moleeule moves to and from the surface to bulk or is sufficiently soluble in the bulk phase. 

Consider now the situation on the right in which an essentially insoluble, but still surface-active, material such as stearic acid is placed on one side of the barrier. Assuming that the barrier does not leak, the system may be left for any practical period of time and when the surface tension of each side is measured, it will be found that a for the side to which stearic acid has been added has been lowered, while the other side remains that of the pure water, (To. Stearic acid may be added until a monolayer is formed with no change in (for the clean side. The added stearic add molecules, being insoluble in water, cannot be dissolved and transported through the water to be adsorbed on the other side of the barrier. The monolayer ultimately formed is an insoluble monolayer. 

The lowering of surface tension of lipids, which form insoluble monolayers, is identical to the spreading pressure of the monomolecular surface (see Section 8.4), and some surface film data of n-fatty acids are given in Table 8.14. 

Flow or transport may also produce spatial variations in interfadal tempraa-ture or concraitration and hence in intrafadal tension. Indeed, we have already seen in Chapter 5 that flow assodated with surface wave motion causes surfactant concentration gradients to develop at an interface with an insoluble monolayer. The resulting inteifacial tension gradiraits were found to significantly enhance the damping of wave motion at hquid-gas interfaces, with resulting adverse effects on transfer of heat or mass across the interface. 

Gibbs and Insoluble Monolayers The adsorption of surfactant molecules at the surface of a liquid can be so strong that a monomolecular film (Gibbs monolayer) of unidirectionally ordered surfactants is formed (Fig. 5). Since the decrease in surface tension is directly related to the surface excess adsorption of the surfactant by the Gibbs adsorption equation (Eq. 6), the formation of the Gibbs mono-layer can be monitored by decrease of the surface tension. The maximum number of molecules filling a given area depends upon the area occupied by each molecule. 

Experimentally, one can spread a known amount of material (too small to saturate the starting area) on a subphase in a trough at constant temperature and move a barrier to compress an insoluble monolayer formed on the surface by slowly decreasing the available area. When the surface excess F increases by compression of the film, the lowering of the surface tension is expected from the Gibbs equation (Eq. 6). A pressure-area (n — A) isotherm measured by the above procedures gives us very important information about physical nature of the film and molecular characteristics of the adsorbed materials. 

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