ISO 4311:1979

Overview

ISO 4311:1979 defines a standard laboratory method for determining the critical micellization concentration (c.m.c.) of anionic and non‑ionic surfactants by measuring surface tension. The procedure uses a tensiometer with a plate, stirrup or ring to record surface tension across a series of aqueous solutions whose concentrations bracket the expected c.m.c. The c.m.c. is identified as a singular point on the plot of surface tension versus the logarithm of concentration (a sharp change of slope or a minimum).

Key topics and requirements

• Scope: Applies to purified or unpurified anionic and non‑ionic surfactants soluble in water. Temperature constraints:
  • Non‑ionic: cloud point at least 5 °C above test temperature.
  • Anionic: Krafft temperature at least 5 °C below test temperature.
• Apparatus: Tensiometer per ISO 304 using plate, stirrup or ring; low‑form beakers, watch‑glasses, temperature‑controlled water bath (±1 °C from 20 to 80 °C).
• Reagents: Use distilled water complying with ISO 2456.
• Procedure summary:
  • Prepare ~10 solutions (50 g each) covering the expected c.m.c. region.
  • Measure surface tension following ISO 304; then prepare six closely spaced concentrations around the approximate c.m.c. for fine determination.
  • Allow equilibration (typically up to 3 h or until values stabilize; for concentrations above c.m.c., equilibrium may be ~15 min).
  • Take three successive measurements at each concentration without drying the probe; rinse probe between concentration changes with ethanol then distilled water.
  • Plot surface tension (mN/m) vs log concentration (g/L) and determine the c.m.c. from the curve.
• Units & expression: c.m.c. is given in g·L‑1 (or mol·L‑1 for chemically pure products); surface tension in mN·m‑1.
• Quality & reproducibility: Interlaboratory difference on the same sample should not exceed ±10% of the mean.

Applications and users

ISO 4311 is used for quality control, formulation and research where the c.m.c. of surfactants matters:

• Detergent and cleaning product manufacturers
• Cosmetic and personal‑care formulators
• Chemical manufacturers of surfactants and emulsifiers
• Academic and industrial laboratories studying micellization, interfacial behavior and formulation stability
• Regulatory and standards laboratories verifying product specifications

Knowing the c.m.c. helps optimize dosage, predict foaming and emulsification behavior, and tailor formulations for performance and cost.

Related standards

• ISO 304 - Surface active agents: determination of surface tension
• ISO 2456 - Water used as a solvent for tests (specification for distilled water)
• ISO 607 / draft revisions - Sample division and preparation
• ISO/R 862 - Surface active agents: glossary (definitions such as micelle, Krafft temperature)

Keywords: ISO 4311, critical micellization concentration, c.m.c., surface tension, surfactant testing, anionic surfactants, non‑ionic surfactants, tensiometer, ring/plate/stirrup method.