Control Of Intermolecular Forces At Interfaces

The building units for interfacial self-assembly can be categorized in three different groups: amphiphilic building units, building units with functional group(s), and nonamphiphilic ones. Each group shows a distinctive packing pattern. This section describes its details and the fine-tuning possibility of the intermolecular forces among the building units in each group, which ultimately could provide us a better opportunity to control the structures and properties of the self-assembled aggregates.

6.2.1. Packing Geometry: Balance with Attractive and Repulsive Forces

Figure 6.4 shows two different types of packing geometry that can be anticipated from the interfacial self-assembly of amphiphilic building units. Each mode is determined based on the types of interfaces, the external forces that are applied during self-assembly, and the degree of the surface coverage (surface density). It is also affected by the molecular or experimental factors such as molecular structure and properties, deposition rate and time, pH, types and concentration of salts, and so on. The upright mode (a) is typically expected when there is an applied external pressure on the lateral direction (such as the Langmuir monolayer) or when the surface coverage is very high. Depending on the specific interaction of the head groups or the tails with the interfaces, two submodes are possible: head-down and head-up. For both submodes, the amphiphiles are usually self- assembled with some degree of tilted geometry. The flat mode is expected when the surface coverage of the building units is relatively low.

For both upright and flat packings, the concept and application of the packing geometry (g) stand exactly the same, as shown in Figure 3.8. It should be possible

(a) upright packing g g g g gas or liquid phase liquid or solid phase

(b) flat packing

(b) flat packing

Figure 6.4. Two different types of amphiphile packing at interfaces.

liquid or solid phase

Figure 6.4. Two different types of amphiphile packing at interfaces.

to predict the structure of the self-assembled aggregates from the change of this value. It should also work for the colloidal building units, once the force balance is well designed and played as described in Figure 5.13.

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