Adsorbed monomolecular films

Jo = surface fension befween air and the pure aqueous phase y = surface fension in fhe presence of a monolayer 

Surface pressure has unifs of millinewfons per meter that is, it is the two-dimensional analogue of fhree-dimensional pressure that has units of millinewtons per square meter. When different compounds are studied by the film balance, fhey display a range of properfies. In film balance sfudies, if is usual to spread a known amount of a relafively insoluble surface-active compound from solution onto a clean surface. The surface area can fhen be 

---

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

Condensation can, therefore, take place in narrow capillaries at pressures which are lower than the normal saturation vapour pressure. Zsigmondy (1911) suggested that this phenomenon might also apply to porous solids. Capillary rise in the pores of a solid will usually be so large that the pores will tend to be either completely full of capillary condensed liquid or completely empty. Ideally, at a certain pressure below the normal condensation pressure all the pores of a certain size and below will be filled with liquid and the rest will be empty. It is probably more realistic to assume that an adsorbed monomolecular film exists on the pore walls before capillary condensation takes place. By a corresponding modification of the pore diameter, an estimate of pore size distribution (which will only be of statistical significance because of the complex shape of the pores) can be obtained from the adsorption isotherm. 

Emphasis was first placed on the adsorptive behavior of Compound D on the surface of chromium because that metal has the following desirable properties (a) it is an excellent adsorbent for carboxylic acid groups (5) (b) a large body of data is available on the properties of adsorbed, monomolecular films of aliphatic (16), partially fluorinated (13), fully fluorinated (2), and chloro-fluoro carboxylic acids (2) (c) the metal surface can be readily and reproducibly cleaned by standard metal-lographic polishing techniques and (d) there is a hard, coherent, thin-film oxide on the surface (18). 

However, in view of the detailed knowledge about the constitution of adsorbed monomolecular films (their structure and the successful application of van der Waals dispersion forces to account for their properties), it seems reasonable to require that the pressure-dependent shear strength concept be treated with equivalent detail and precision. Otherwise the concept is essentially data-fitting and leaves the door open for another empirical explanation which is strongly supported by observation namely, that in actual experimentation there are enough defects in the adsorbed film to permit significant contributions to the overall observed friction from microscopic regions not protected by the film. 

EFFECT OF ADSORBED MONOMOLECULAR FILMS ON THE EVAPORATION OF VOLATILE ORGANIC LIQUIDS. 

It is of interest to try to relate the adsorption characteristics of a surfactant to the stability of an emulsion stabilized solely by an adsorbed monomolecular film. The toM number of molecules that can be adsorbed in a given interfacial area wiUbe controlledmainly by the effective area per molecule of the adsorbing species. That is, how many of the molecules can fit into the limited space of the interface For most normal surfactant species, the area per molecule is determined primarily by the hydrophilic group and its hydration layer. The relative solubility of the surfactant in the two phases will also affect the result, but that factor is difficult to determine and is most often ignored. A few representative molecular areas at the oil-water interface are given in Table 11.2. 

Because an adsorbed monomolecular film will have a thickness on the order of 2.5 nm, while the surface asperities present on all but the finest surfaces will seldom be less than 5-10 nm, it is important to have a clear picture of the mechanism of boundary lubrication at the molecular level. A typical situation is shown schematically in Figure 18.12, where it can be seen that there are two types of contact between the two surfaces in the total contact area A contact between the adsorbed lubricant films (area cuA the figure) and that between the actual surfaces where the adsorbed film has broken down (area I3A). The total frictional force between the two will be the sum of each contribution... 

In the case of boundary layer lubrication, in which the adsorption of mono-molecular films is required, the best protection is provided by materials such as fatty acids and soaps that can adsorb strongly at the surface to form a solid condensed film. Less durable but effective protection can be obtained with polar groups such as alcohols, thiols, or amines. The least effective protection is obtained with simple hydrocarbons that adsorb more or less randomly and through dispersion forces alone. For adsorbed monomolecular films, best results are obtained when the hydrocarbon tail has at least 14 carbons. In some cases fluorinated carboxylic acids and silicones may provide a lower initial coefficient of friction, but their weaker lateral interaction sometimes results in a less durable surface film that melts at a lower temperature, ultimately resulting in less overall protection. If a polar lubricant can form a direct chemical bond to the surface, as in the formation of metal soaps, even better results can be expected. 

Surfactants that are adsorbed at oil/water interfaces to form monomolecular films and reduce interfacial tension. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

A recent study by DePalma and Tillman [ 10] also demonstrates the potential of surface modification by self-assembled monolayers of low surface energy fiuoroalkyl-containing silanes. Fatty acids, amines and alcohols have long been known to adsorb as monomolecular films on metals. Silane coupling agents have featured strongly in new studies to develop more robust films, covalently bound together and to the metal substrate. 

Theoretical consideration of the IR spectroscopy of monolayers adsorbed on a metal surface showed that the reflection-absorption spectrum is measured most efficiently at high angles of incidence, and that only parallel component of incident light gives measurable absorption species (23). Figure 4 presents a schematic description of a monomolecular film on a mirror, with the incident light and direction of the polarization. Figure 5 presents, in detail, an alkyl thiol molecule on a metal surface. Note the direction of the different transition dipoles. Thus, while both the... 

A very versatile approach to the formation of multilayer films has been developed by Decher, based on polyelectrolytes. If a solid substrate with ionic groups at the surface is dipped into a solution of a complementary polyelectrolyte, an ultrathin, essentially monomolecular film of the polyion is adsorbed [340]. The adsorption is based on pairing of surface bound ionic sites with oppositely charged ions, bound to the macromolecule. The polymers adsorb in an irregular flattened coil structure and only part of the polymer ions can be paired with the surface ions (Figure 29a). Ionic sites which remain with small counterions provide anchor points for a next layer formed by a complementary polyelectrolyte [342,343]. This way multilayer polyelectrolyte films can be prepared layer-by-layer just by dipping a suitable substrate alternately in an aqueous solution of polyanions and polycations. The technique can be employed with nearly all soluble charged polymers and results in films with a... 

The Gibbs adsorption isotherm shows the dependence of the extent of adsorption of an adsorbent on its bulk concentration or pressure. However, we also need to know the state of the adsorbate at the surface. These are interrelated because the extent of material adsorb-tion on a surface depends on the state of the surface. The behavior of the molecules in the surface film is expressed by a surface equation of state which relates the spreading pressure, n, which is the difference between the solvent and solution surface tensions, %= % - y to the surface concentration of the adsorbent. This equation is concerned with the lateral motions and interactions of the molecules present in an adsorbed film. In general, the surface equation of state is a two-dimensional analogue of the three-dimensional equation of state of fluids, and since this is related to monomolecular films, it will be described in Sections 5.5 and 5.6. It should be remembered that on liquid surfaces, usually monolayers form, but with adsorption on solid surfaces, usually multilayers form (see Section 8.3). 

The crucial condition that characterizes failure of a monomolecular film is inability to persist in a state of structured adsorption on the bounding surfaces consequently atoms in these surfaces can approach one another and interact directly. Molecules of non-polar liquids are not adsorbed as persistently at the bounding surfaces as are molecules of polar "boundary" lubricants and are more easily disoriented and desorbed under the influence of increased temperature and shear stress. 

During the period 1937-1940 Higuchi proposed a modified capillary condensation theory to explain the isotherms of 18 sorbates on titania gel of the same lot. The new theory proposes that in sorption phenomena vapors may be adsorbed in two ways (a) adsorption due to the surface force of solid sorbents which is usually accomplished by forming a monomolecular film in the relatively low pressure range and (b) capillary condensation of sorbates into pores whose radii are larger than ca. 10 A and covered by an adsorption film. The capillary condensation is undoubtedly due to the vapor pressure depression of the sorbate liquid described by the Thompson equation. 

Monolayer Adsorption Adsorption in which a first or only layer of molecules becomes adsorbed at an interface. In monolayer adsorption, all of the adsorbed molecules are in contact with the surface of the adsorbent. The adsorbed layer is termed a monolayer or monomolecular film. 

---