Osmosis is the spontaneous net movement of a solvent across a Semipermeable membrane toward the side with higher solute concentration, driven by a gradient in solvent chemical potential (often described as water potential in biological contexts). Classical definitions emphasize movement of water in biological systems, but the process applies to any solvent and occurs wherever a membrane is permeable to solvent but restricts solute passage. The effect can be halted by applying a counterpressure equal to the solution’s osmotic pressure. Encyclopaedia Britannica; IUPAC Gold Book.

Historical development

Early observations of osmosis through natural membranes were reported by the French abbé Jean-Antoine Nollet in 1748 using pig bladder coverings on flasks, foreshadowing membrane selectivity in solvent flow. Wikipedia—Jean-Antoine Nollet cites Nollet’s experiment and contemporary sources. The general term “osmose” (from which “osmosis” derives) and the systematic study of “osmotic force” were introduced by Scottish chemist Thomas Graham in his 1854 Bakerian Lecture to the Royal Society, which established key terminology and distinctions between diffusion and osmotic phenomena. Royal Society—Graham, On Osmotic Force; Royal Society Archive record.

Quantitative measurements were advanced by Wilhelm Pfeffer in 1877 using artificial copper ferrocyanide membranes (the "Pfeffer cell") to measure the minimal counterpressure required to prevent solvent entry—defining osmotic pressure experimentally and showing its proportionality to solute concentration and temperature in dilute solutions. Wellcome Collection—Pfeffer, Osmotische Untersuchungen (1877); Embryo Project—Pfeffer Cell Apparatus; ChemistryViews—Pfeffer anniversary.

A thermodynamic framework was provided by Jacobus Henricus van 't Hoff, who in 1887–1888 derived the relation for ideal dilute solutions that osmotic pressure (π) is proportional to the molar concentration of solute and temperature (π = iMRT), establishing an analogy with the ideal gas law. His papers also connected osmotic pressure with other colligative properties. van ’t Hoff 1887 (Proceedings of the Physical Society of London); van ’t Hoff 1888 (Philosophical Magazine); RSC—education article on van ’t Hoff and electrolyte solutions.

Physical chemistry and thermodynamics

Osmotic pressure is defined as the excess pressure required to maintain osmotic equilibrium between a solution and the pure solvent across a solvent-permeable membrane. A rigorous expression uses solvent activity and partial molar volume, while the van ’t Hoff form applies to ideal dilute solutions. The IUPAC definition gives Π = −(RT/V_A) ln a_A; for ideal dilute solutions Π ≈ c_B RT (with c_B the solute amount concentration). IUPAC Gold Book. Osmotic pressure is a colligative property dependent on the number of solute particles, not their identity, and is closely related to freezing point depression and boiling point elevation. Encyclopaedia Britannica; Nature (1913) commentary on osmotic-pressure measurements.

The terms osmolarity (osmol/L) and osmolality (osmol/kg) quantify osmotic concentration; clinical practice favors osmolality because it is temperature and pressure independent, while IUPAC discourages the term “osmolarity” in favor of thermodynamically rigorous quantities. StatPearls—Serum Osmolality; IUPAC Gold Book.

Biological roles

In cells, osmosis governs water balance, volume, and turgor. Plant cells maintain turgor pressure as water enters due to more negative solute (osmotic) potential inside the cell; loss of water can produce plasmolysis and wilting. Water potential (Ψ) combines solute potential (Ψ_s) and pressure potential (Ψ_p), directing water from higher to lower Ψ. OpenStax Biology 2e—Passive Transport; OpenStax Biology—Transport of Water and Solutes in Plants. In animals, maintaining osmotic balance underlies osmoregulation and clinical management of fluids; effective blood tonicity depends on osmotically active solutes (e.g., sodium, glucose), and measured osmolality provides diagnostic insight. StatPearls—Plasma Osmolality and Oncotic Pressure; PubMed—Serum osmolality overview.

At the molecular level, specialized membrane proteins called Aquaporin water channels enable rapid, selective water permeation and are central to osmotic water movement in tissues such as kidney and secretory glands. The discovery and characterization of aquaporins, recognized by the 2003 Nobel Prize in Chemistry (Peter Agre), provided direct molecular evidence for facilitated osmosis in biological membranes. NobelPrize.org—Agre Facts; NobelPrize.org—Popular Information; Britannica—Peter Agre.

Measurement and units

Osmotic pressure can be determined with classical osmometers (e.g., Pfeffer cell) by measuring the counterpressure needed to stop solvent inflow; modern laboratory osmometers infer osmolality from colligative properties (commonly freezing point depression) or from vapor pressure. Embryo Project—Pfeffer Cell Apparatus; StatPearls—Osmometer. In clinical settings, serum and urine osmolality are measured directly and compared to calculated estimates to assess solute disturbances and the osmolal gap. PubMed—Serum osmolality.

Technological and environmental applications

Reverse osmosis (RO) applies pressure in excess of the osmotic pressure to force solvent (usually water) to permeate a dense, selective membrane from the concentrated to the dilute side, producing treated water (permeate) and a concentrated brine. RO is the leading desalination technology by installed capacity and is widely used in potable water treatment and ultrapure water production. USGS—Reverse osmosis desalination overview; US EPA—Point‑of‑Use Reverse Osmosis; FDA—Reverse Osmosis (definition and operation); IUPAC Gold Book—definition of reverse osmosis.

Dialysis therapies exploit Diffusion and osmotic and hydrostatic gradients across a semipermeable dialyzer membrane to remove solutes and water in kidney failure; ultrafiltration refers specifically to fluid removal driven by transmembrane pressure. National Kidney Foundation—Ultrafiltration; StatPearls—Hemodialysis; LWW—Hemodialysis principles.

In food preservation, high external solute concentrations (e.g., salt or sugar) generate osmotic gradients that draw water from microbial cells, inhibiting growth; in laboratories, dialysis and osmotic dehydration techniques similarly leverage osmotic fluxes. General principles and definitions apply from the sources above: Encyclopaedia Britannica; IUPAC Gold Book.

Distinctions and related concepts

• –Osmosis vs. Diffusion: diffusion is random thermal motion leading to net solute movement down concentration gradients without a membrane requirement; osmosis is solvent flow across a selective membrane driven by chemical potential differences. Encyclopaedia Britannica.
• –Osmosis vs. solvent flow under pressure: in RO, applied pressure exceeding osmotic pressure reverses the natural osmotic flux. IUPAC Gold Book—Reverse osmosis; FDA—Reverse Osmosis.
• –Osmotic measures: osmolality (osmol/kg) is generally preferred for physiological and clinical work; osmolarity (osmol/L) is common but temperature dependent and discouraged in formal IUPAC usage. StatPearls—Serum Osmolality; IUPAC Gold Book.

Key formulas

• –van ’t Hoff relation (ideal dilute solutions): π = iMRT, where i is the van ’t Hoff factor, M is solute molarity, R the gas constant, and T absolute temperature. van ’t Hoff 1887/1888; RSC education article.
• –Thermodynamic definition (solvent basis): Π = −(RT/V_A) ln a_A (IUPAC). IUPAC Gold Book.

Notable contributors

• –Thomas Graham (coined “osmose”; 1854). Royal Society—On Osmotic Force.
• –Wilhelm Pfeffer (first precise osmotic pressure measurements; 1877). Wellcome Collection—Pfeffer (1877).
• –Jacobus Henricus van 't Hoff (osmotic pressure law; 1887–1888). Philosophical Magazine—1888 paper.
• –Aquaporin discovery by Peter Agre (Nobel Prize in Chemistry, 2003). NobelPrize.org.