Hydrophilic and Hydrophobic

FIELDS OF STUDY: Biochemistry; Chemical Engineering; Physical Chemistry

ABSTRACT

The characteristic properties of hydrophilic and hydrophobic materials are discussed. Hydrophilic materials readily interact with liquid water, while hydrophobic materials do not. These properties are important in biochemical systems and chemical engineering processes.

The Nature of Hydrophilic and Hydrophobic Materials

The terms "hydrophilic" and "hydrophobic" refer specifically to the interaction of a particular material with liquid water. The two words literally mean "water loving" and "water fearing," respectively, and they aptly describe the behavior of the corresponding materials: hydrophilic materials readily interact with water, while hydrophobic materials do not.

To understand this, one must understand the structure and electronic properties of the water molecule. A water molecule consists of two hydrogen atoms covalently bonded to a single oxygen atom. Each hydrogen atom contributes its single electron to the formation of the covalent bond. The electronegativity of the oxygen atom tends to keep the electron density of the oxygen-hydrogen bond locked between those two atoms, leaving the other side of each hydrogen atom bare of electron density and effectively exposing the positive charge of the hydrogen nuclei. At the same time, the oxygen has two lone pairs of electrons essentially bulging out from the other side of the molecule, creating a high electron density and negative charge. There is therefore a high degree of charge separation between the exposed nuclei of the hydrogen atoms and the lone electron pairs of the oxygen atom, giving the molecule as a whole a pronounced electrical polarity and creating what is known as a "dipole moment."

This dipole moment is the source of water’s rather unique physical properties. The polarity of the molecules effectively enables them to stick together, positive end to negative end, like little magnets. The resulting cohesion is due to the formation of hydrogen bonds between water molecules. This gives water very high melting and boiling points in comparison to other molecules of similar mass, as well as a great ability to dissolve other materials.

Hydrophilic materials are able to interact with the structural and electronic properties of water to different degrees. They may undergo solvation (that is, dissolve), ionic dissociation, or surface wetting (that is, adhere to the water molecules, thus becoming wet). Hydrophilic materials also tend to have polar molecular structures or similar electronic properties. Hydrophobic materials, on the other hand, are generally nonpolar covalent materials that do not dissolve in water and do not exhibit surface wetting.

Solvent Partitioning and Phase-Transfer Catalysis

Compounds have different solubilities in different solvents. Ionic compounds dissolve well in water but not in organic solvents such as diethyl ether or dichloromethane, while covalently bonded compounds, such as organic molecules, generally do not dissolve well in water. For any particular compound, solubility is a matter of degree, and manipulating the form of the molecule greatly affects its solubility. For example, a neutral amine will dissolve well in acidic water, but if the amine solution is neutralized or made basic by the addition of a stronger base, the amine will precipitate out of the solution as an undissolved solid. Organic acids behave in a similar manner with respect to stronger acids. By adjusting the acidity or basicity of a solution containing different components, it is a simple exercise to isolate the different compounds from each other with a second solvent that is immiscible (that is, does not mix) with the first. If one of the solvents is water or some other hydrophilic substance, the other must be a hydrophobic solvent that will not become dissolved in the existing solution. When the second solvent is added and mixed together with the solution, a material will migrate from one solution into the other, according to its solubility in each one. The process is called solvent partitioning.

A related process, phase-transfer catalysis, uses two immiscible solvents such as water and the organic compound benzene (C₆H₆). Each solution in the process is considered a "phase"—that is, matter with distinct properties that touches but does not mix with other types of matter—and the surface between them is known as the "phase boundary." This technique is used when two reactant materials will not dissolve in the same solvent. Such reactants are typically an ionic compound and a nonionic compound and may even be in different states of matter. In the procedure, each reactant is dissolved in the appropriate solvent, and the two solutions are stirred together vigorously. A third compound, the phase-transfer catalyst, is added and binds reversibly to one of the reactants to form an addition product, or adduct, that will dissolve well in the other solvent. The adduct transfers from one liquid phase into the other, where the two reactant species can contact each other and undergo the desired reaction. As in solvent partitioning, one solvent must be hydrophobic and the other must be hydrophilic.

Biological Systems

Hydrophilic and hydrophobic properties are essential in biological systems and are the functional basis of such fundamentally important aspects as cell structure. Every animal cell, and the various organelles that it contains, is enclosed within a cell membrane composed of phospholipid molecules. One common type of phospholipid consists of a glycerol molecule backbone to which are attached two long-chain fatty acids and a phosphate ion (PO₄³⁻) bonded to an alcohol group. The phosphate ion is bonded to one of the hydroxyl (−OH) substituents of the glycerol molecule, while the carboxyl groups (−COOH) of the fatty acids are bonded to the other two hydroxyl substituents of the glycerol. Overall, a phospholipid molecule has the following general structure:

A phospholipid molecule thus has two distinct regions. The phosphate end of the molecule is strongly hydrophilic, while the long-chain fatty acids, which are nonpolar and cannot form hydrogen bonds, are decidedly hydrophobic and are often referred to as "greasy." The fatty-acid section of the phospholipid molecule is actually much larger than the phosphate portion (the diagram is not to scale). There is a simple rule of solubility that "like dissolves like," meaning that polar solvents such as water generally dissolve polar materials, and nonpolar solvents such as hydrocarbons generally dissolve nonpolar materials. The two different regions of the phospholipid molecule therefore do not interact well with each other but are instead attracted to the corresponding sections of other molecules.

In large quantity in an aqueous environment, the long-chain fatty-acid portions of the phospholipid molecules are forced to aggregate and effectively blend into each other by the surrounding water molecules. The fatty acids cannot dissolve into each other completely, however, due to the repulsion between the phosphate groups to which they are attached. As a result, the molecules automatically form a sandwich-like structure known as a "bilayer." The two outer surfaces of the bilayer consist of the hydrophilic phosphate portions of the phospholipid molecules, often called the "heads." Between them is a thick, greasy hydrophobic layer of intertwined long-chain hydrocarbon groups, often called the "tails." This bilayer fully encloses the interior contents of animal cells, forming an integral membrane that can permit the passage of materials from one side to the other by various mechanisms. Being hydrophilic, the inner and outer surfaces can interact freely with the water-based fluids on either side of the membrane.

PRINCIPAL TERMS

• cohesion: the tendency for like molecules of a substance to stick together due to their shape and electronic structure.
• hydrogen bond: a weak type of chemical bond formed by the attraction of a hydrogen atom to an electronegative atom—an atom with a strong tendency to attract electrons—in the same or another molecule.
• polarity: a characteristic of a molecule or functional group in which there is a difference in the distribution of electronic charge, causing one part of the molecule or group to be relatively electrically positive and another part to be relatively electrically negative.
• repulsion: an oppositional force that pushes two entities apart, such as the electrostatic repulsion between particles of like electrical charge.
• solubility: the ability of a particular substance, or solute, to dissolve in a particular solvent at a given temperature and pressure.

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