Overview of Common Solubility Factors

Solubility is the degree in which a substance (termed “solute”) dissolves into a solvent, thereby forming a solution. Solubility is often measured in grams of solute per 100 mL of solvent, at a particular temperature. Solutes and solvents can both exist in various forms, like solids, liquids, gas, or as supercritical fluids. Solubility will vary greatly depending on the composition of the solutes and solvents used. For example, the solubility of table salt (NaCl) in water is ~36g/100mL, while the solubility of chlorine in water is <1g/100 mL (both at room temperature). In the body, by far the most common solvent is water that aids in dissolving molecular components like proteins, carbohydrates, lipids, and nucleic acids into key areas of the microenvironment. Solubility is affected by a number of factors, and with every solute-solvent combination there is a limit to which no more solute can be added at a given temperature and pressure, termed the saturation limit.

Temperature

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In general, the solubility of a reaction will increase with temperature. As the temperature of the reaction increases, the kinetic energy of the solute molecules increases as well. This increases the rate of dissolution in multiple ways. First, the solvent molecules collide with solute molecules with greater frequency and with more force, causing the solute to break down into subcomponents quicker. Second, the increased kinetic energy weakens intermolecular attractions and forces within the solute, which causes these molecules to dissolve more readily.

For many solutes that are miscible in water, there exist solubility curves that provide graphical data on the relationship between solubility and temperature. It is important to remember that different solute-solvent combinations, however, react differently. Generally:

• The solubility of solid solutes increases with temperature, while the solubility of gaseous solutes decreases with temperature.
• If the process is endothermic, increasing temperature will increase dissolution. If the process is exothermic, increasing temperature will decrease it.

Pressures

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Although external pressures have very little effect on the solubility of liquid and solid solute-solvent combinations, the solubility of gases greatly increases with pressure. In the body, this is particularly important for ensuring the equilibrium balance between key gases, like nitrogen (N2), carbon dioxide (CO2), and oxygen (O2), within the blood. According to Henry's law:

C=kP
Where:
C = concentration of a dissolved gas at equilibrium
P = partial pressure of the gas
K = Henry's law constant (mol/L x atm) x 10^-4. Empirically determined for each gas, solvent, and reaction temperature.

Biomolecular Chemistry

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The solubility of a molecule is also dependent on its specific molecular chemistry. Molecules that contain sequences that are enriched with charged amino acids like arginine (R), lysine (K), and aspartic acid (D) exhibit a higher level of solubility. This also means that changing the pH of a reaction, which can change the charge states of residues, can influence the solubility of a solute-solution combination.

Solutes and solutions whose molecular makeup is very similar will have similar intermolecular forces and will usually be highly soluble in each other. The opposite goes for substances whose molecular makeup is significantly different, and these combinations will not mix or solubilize efficiently. When molecular chemistry is not highly specialized, on a molecular level these solutes and solvents do not bind well, which may impact dissociation down the line. Generally, for any laboratory endeavor it is recommended that the solute-solution combination be highly compatible, so that the solution does not separate unintentionally mid-experiment.

Tools:

Protein Solubility Calculator

Polarity

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In many cases, hydrophilic (polar) solutes will dissolve in hydrophilic solvents while a hydrophobic (nonpolar) solute will dissolve a hydrophobic solvent. For example, nonpolar solutes, like fats, oils, and greases, will not dissolve in polar solvents, like water, and vice versa. Hydrophilic compounds, like amino acids, proteins, vitamins, and carbohydrates, generally consist of O-H or N-H groups that easily form hydrogen bonds with water molecules. Hydrophobic compounds, however, usually contain C-H bonds that will not interact readily with water. Some examples of common hydrophobic compounds in the body include lipids and nucleic acids.

FAQs:

What is a solubility test?

Molecular Composition

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Another factor that affects solubility is the molecular composition; the larger and more complex the solute, the slower it will dissolve. This concept is true for compounds at any size. For example, a sugar cube will take longer than sugar granules to fully dissolve, as more surface area is exposed.

Generally, small, simple molecules have a large surface area to volume ratio, which allows them to interact more easily with a solvent. Conversely, large molecules with complex structure tend to have a lower surface area to volume ratio and may exhibit stronger intermolecular forces that may be more difficult to be broken up by a solvent. For example, table salt dissolves into water faster than sugar because of the intermolecular hydrogen bonds that sugar molecules possess.

Motion

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As mentioned, the dissolving rate of a solute in a solvent depends, in part, on the collisions that occur between the solvent and solute molecules. By stirring the solution, the frequency of those collisions increases, which increases the rate of dissolution. Say, instead, a reaction is not stirred. In this case, only the layer of solvent that comes in direct contact with the solute will become saturated, preventing full dissolution of the solution. Importantly, stirring a solution during a reaction will increase how quickly a solute dissolves, but it will not impact how much solute can dissolve into a solvent, like other solubility factors.

Resources:

Solubility Reference Table

Products

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References

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Solubility
Biochemistry, Dissolution and Solubility
Nanoencapsulation of hydrophobic and low-soluble food bioactive compounds within different nanocarriers