Subphase attraction, chain

One end of each surface-active molecule in a monolayer is anchored firmly to the Uquid surface by the attraction of the polar head group for the aqueous subphase, while the hydrophobic portion is displaced easily from it. If the molecules are separated widely as in a gaseous monolayer, the simple two-dimensional gas law is approached, namely, irA = kT, where k is the Boltzmann constant. The hydro-phobic chains are free to assume almost any orientation above the surface and may sweep out circles with radii as long as their tails by rotating around their point of attachment at the head group. However, intermolecular translational movements are restricted to the two-dimensional interfacial plane because the hydrophilic head groups cannot leave the aqueous surface. 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

Fig. 5. Schematic of Langmuir-Blodgett transfers. The gray circles represent the hydrophilic head groups, and the thin black lines represent the alkyl chains. The molecules have been spread on an aqueous subphase, (a) A hydrophilic substrate is being withdrawn from the dipping well and is attracting the hydrophihc head groups that drive the film transfer, (b) A hydrophobic substrate is being pushed down through the monolayer. The alkyl chains are transferred, and the monolayer is pulled off the surface by hydrophobic interactions.

The formation of polymer films at the air-water interface includes slightly different requirements than those known for amphiphiles of small molar mass. In particular, films of homopolymers can be built [19-21], in which cases no amphiphilic balance can be discussed to the contrary, the balance of interactions between macro-molecular chains and between macromolecules and water controls the polymer spreadability. In other words, the bulk (subphase) cohesion has to be overcome by the attraction for the interface for the polymer to build stable monolayers [22]. It is not necessary that the polymer is water-insoluble, but individual monomers must be surface-active, i.e. must be able to adsorb to the interface from bulk solution. 

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