Question: What characterizes an amphiphilic molecule?
About the article "Biochemistry", section "Lipids_1".

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A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase.
Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail.
Since the 1980s a large number of other materials have been employed to produce Langmuir monolayers, some of which are semi-amphiphilic, including macromolecules such as polymers.
Langmuir monolayers are extensively studied for the fabrication of Langmuir-Blodgett film (LB films), which are formed by transferred monolayers on a solid substrate.
A Gibbs monolayer or soluble monolayer is a monolayer formed by a compound that is soluble in one of the phases separated by the interface on which the monolayer is formed. 
Surface pressure can also refer to the change of surface tension as a function of the area of water surface available to each molecule in a solution.
The measurement of surface pressure is key in determining the monolayer properties of an amphiphilic material.
The surface pressure describes the depression of the surface tension below the value for an uncontaminated interface.
See also: Pressure: Surface pressure. 
The lipophilic group is typically a large hydrocarbon moiety, such as a long chain of the form CH3(CH2)n, with n > 4.
The hydrophilic group falls into one of the following categories:  Charged groups * Anionic.
Examples, with the lipophilic part of the molecule represented by an "R", are: ** carboxylates: RCO2−; ** sulfates: RSO4−; ** sulfonates: RSO3−.
** phosphates: The charged functionality in phospholipids.
* Cationic.
Examples: ** ammoniums: RNH3+.
Polar, uncharged groups.
Examples are alcohols with large R groups, such as diacyl glycerol (DAG), and oligoethyleneglycols with long alkyl chains.
Often, amphiphilic species have several lipophilic parts, several hydrophilic parts, or several of both.
Proteins and some block copolymers are such examples.
Amphiphilic compounds have lipophilic (typically hydrocarbon) structures and hydrophilic polar functional groups (either ionic or uncharged).
As a result of having both lipophilic and hydrophilic portions, some amphiphilic compounds may dissolve in water and to some extent in non-polar organic solvents.
When placed in an immiscible biphasic system consisting of aqueous and organic solvents, the amphiphilic compound will partition the two phases.
The extent of the hydrophobic and hydrophilic portions determines the extent of partitioning. 
The idea of a Langmuir–Blodgett (LB) film was first proven feasible in 1917 when Dr. Irving Langmuir (Langmuir, 1917) showed that single water-surface monolayers could be transferred to solid substrates.
18 years later, Dr. Katharine Blodgett made an important scientific advance when she discovered that several of these single monolayer films could be stacked on top of one another to make multilayer films (Blodgett 1935).
Since then, LB films (and subsequently the troughs to make them) have been used for a wide variety of scientific experimentation, ranging from 2D crystallization of proteins to Brewster angle microscopy.
The LB trough's general objective is to study the properties of monolayers of amphiphilic molecules.
An amphiphilic molecule is one that contains both a hydrophobic and hydrophilic domain (e.g.
soaps and detergents).
The LB trough allows investigators to prepare a monolayer of amphiphilic molecules on the surface of a liquid, and then compress or expand these molecules on the surface, thereby modifying the molecular density, or area per molecule.
This is accomplished by placing a subphase (usually water) in a trough, spreading a given amphiphile over the surface, and then compressing the surface with barriers (see illustration).
The monolayer’s effect on the surface pressure of the liquid is measured through use of a Wilhelmy plate, electronic wire probes, or other types of detectors.
An LB film can then be transferred to a solid substrate by dipping the substrate through the monolayer.