Surface Tension

Detailed Description

Surface tension is a property of liquid surfaces that causes them to behave as if they were covered with a stretched elastic membrane. It arises from the cohesive forces between liquid molecules at the surface, which experience an imbalance of molecular attractions compared to molecules in the bulk of the liquid. In the context of a Safety Data Sheet (SDS), surface tension provides important information for understanding how a liquid will behave in various applications and environmental scenarios.

Surface tension (γ) is typically expressed as force per unit length and is defined as the work required to increase the surface area by one unit:

γ = dW/dA

Where:

γ (gamma) is the surface tension
dW is the differential work
dA is the differential increase in surface area

Key concepts related to surface tension include:

Cohesion: The attraction between molecules of the same substance, which is the primary cause of surface tension.
Adhesion: The attraction between molecules of different substances, which affects how a liquid interacts with surfaces.
Capillary action: The ability of a liquid to flow against gravity in narrow spaces, driven by surface tension and adhesive forces.
Wetting: The ability of a liquid to maintain contact with a solid surface, influenced by the balance of cohesive and adhesive forces.
Surfactants: Substances that lower the surface tension of a liquid, enhancing its spreading and wetting properties.
Temperature dependence: Surface tension generally decreases with increasing temperature.
Interfacial tension: The tension at the interface between two immiscible liquids, analogous to surface tension at a liquid-air interface.

Importance in Safety Data Sheets

Surface tension information in an SDS is important for several reasons:

Spreading behavior: Indicates how a liquid will spread on surfaces, affecting spill containment strategies.
Wetting properties: Determines how a liquid will interact with various surfaces, including protective clothing and equipment.
Absorption potential: Affects how a liquid may be absorbed into porous materials, including soil and protective fabrics.
Emulsion stability: Influences the stability of emulsions and the effectiveness of emulsion-breaking treatments.
Foaming tendency: Relates to a liquid's propensity to form stable foams, which can be relevant for fire-fighting and process safety.
Droplet formation: Affects spray and mist formation characteristics, which is relevant for inhalation exposure assessment.
Environmental mobility: Influences how a substance will move through soil and water systems.
Application properties: Critical for products designed to coat, clean, or wet surfaces.

Measurement Methods

Several techniques are used to measure surface tension:

Method	Description	Advantages	Limitations
Du Noüy Ring Method	Measures the force required to pull a platinum ring from the liquid surface	Widely used, good accuracy, suitable for many liquids	Requires correction factors, sensitive to vibration
Wilhelmy Plate Method	Measures the force exerted on a thin plate partially immersed in the liquid	Good accuracy, suitable for dynamic measurements	Requires precise alignment, sensitive to contamination
Pendant Drop Method	Analyzes the shape of a drop hanging from a needle, which is affected by surface tension	Small sample volume, suitable for high temperatures and pressures	Requires image analysis, more complex setup
Sessile Drop Method	Analyzes the shape of a drop resting on a surface	Provides contact angle information, small sample volume	Requires image analysis, affected by surface properties
Maximum Bubble Pressure Method	Measures the maximum pressure needed to form a bubble at the end of a capillary	Suitable for dynamic measurements, works with opaque liquids	More complex interpretation, sensitive to flow rate
Capillary Rise Method	Measures the height to which a liquid rises in a capillary tube	Simple equipment, fundamental method	Less accurate, requires precise measurement of capillary dimensions
Spinning Drop Method	Analyzes the shape of a drop in a rotating tube	Excellent for very low interfacial tensions	Specialized equipment, complex setup
ASTM Methods	Standardized procedures (e.g., ASTM D1331, D1590)	Standardized, reproducible, widely accepted	May not be optimized for all liquid types

Surface Tension Units and Typical Values

Surface tension is typically expressed in the following units:

Unit	Symbol	Equivalent in SI Units	Common Usage
Newton per meter	N/m	1 N/m	SI unit, scientific applications
Millinewton per meter	mN/m	0.001 N/m	Common in scientific literature, most practical measurements
Dyne per centimeter	dyn/cm	0.001 N/m	Older literature, equivalent to mN/m
Erg per square centimeter	erg/cm²	0.001 N/m	Older literature, equivalent to mN/m

Surface tension values for common liquids at 20°C:

Liquid	Surface Tension (mN/m)	Category	Notes
Water	72.8	Very High	Reference liquid, high due to hydrogen bonding
Mercury	485.5	Very High	Highest among common liquids, forms spherical drops
Glycerol	63.4	High	Viscous liquid with strong hydrogen bonding
Ethylene Glycol	47.7	High	Common antifreeze component
Olive Oil	32.0	Moderate	Typical vegetable oil
Ethanol	22.1	Low	Common alcohol
Acetone	23.7	Low	Common solvent, good wetting properties
n-Hexane	18.4	Very Low	Non-polar hydrocarbon
Diethyl Ether	17.0	Very Low	Very volatile solvent
Surfactant Solutions	25-40	Low	Depends on surfactant type and concentration
Liquid Nitrogen	8.9	Very Low	Measured at -196°C

Factors Affecting Surface Tension

Temperature

Surface tension generally decreases with increasing temperature, approaching zero at the critical temperature. This relationship can often be approximated by:

γ(T) = γ₀(1 - T/T_c)ⁿ

Where:

γ(T) is the surface tension at temperature T
γ₀ is a constant
T_c is the critical temperature
n is an empirical exponent (often approximately 1.2)

For water, surface tension decreases by about 0.15 mN/m for each 1°C increase in temperature.

Impurities and Additives

Surface tension is highly sensitive to surface-active impurities:

Surfactants: Even at very low concentrations, surfactants can dramatically reduce surface tension.
Alcohols and organic compounds: Many organic compounds lower the surface tension of water.
Dissolved salts: Inorganic salts typically increase the surface tension of water slightly.
Particulate matter: Fine particles can adsorb at interfaces and affect apparent surface tension.

Molecular Structure

The molecular structure significantly affects surface tension:

Intermolecular forces: Stronger intermolecular forces (hydrogen bonding, dipole-dipole interactions) generally result in higher surface tension.
Molecular weight: Within a homologous series, surface tension often increases with molecular weight.
Molecular shape: Branched molecules typically have lower surface tension than linear molecules of the same molecular weight.
Functional groups: Polar functional groups often increase surface tension, while non-polar groups decrease it.

Environmental Factors

Several environmental factors can influence surface tension measurements:

Pressure: Surface tension is relatively insensitive to pressure except at very high pressures.
Humidity: Ambient humidity can affect measurements, particularly for hygroscopic liquids.
Surface age: Newly formed surfaces may have different properties than equilibrated surfaces.
Electric fields: Strong electric fields can influence surface tension (electrocapillary effect).

Surface Tension and Safety Considerations

Surface tension has several important safety implications:

Spreading behavior: Liquids with low surface tension spread more readily on surfaces, potentially increasing the area affected by spills.
Penetration into materials: Low surface tension liquids can more easily penetrate protective clothing, porous materials, and soil.
Capillary action: Surface tension drives capillary action, which can cause liquids to wick through small openings or porous barriers.
Misting and atomization: Liquids with lower surface tension form smaller droplets when sprayed or agitated, potentially increasing inhalation exposure.
Foaming: Surface tension affects foam stability, which can be relevant for fire-fighting foam effectiveness.
Emulsion formation: Surface tension influences the formation and stability of emulsions, affecting cleanup and separation processes.
Dermal absorption: Liquids with lower surface tension may enhance skin penetration of dissolved substances.

Regulatory Considerations

While surface tension is not specifically mandated by GHS for all substances, it is often included in Section 9 of Safety Data Sheets as supplementary information that helps users assess potential hazards and appropriate handling procedures.

Surface tension information is particularly relevant for:

Surfactants and detergents: Surface tension reduction is a primary function of these products.
Pesticides and agricultural chemicals: Surface tension affects spreading on plant surfaces and soil penetration.
Cleaning products: Wetting properties are critical to performance and safety assessment.
Petroleum products: Surface tension affects spreading behavior in spills and environmental fate.
Firefighting agents: Surface tension is critical to performance and environmental impact assessment.

For substances that significantly alter the surface tension of water (e.g., surfactants), this property may be relevant for environmental hazard classification under various regulatory frameworks.

Best Practices

When reporting surface tension in an SDS:

Express the value in standard units (preferably mN/m or dyn/cm)
Clearly indicate the temperature at which the measurement was made
Specify the measurement method used if available (e.g., Du Noüy ring, Wilhelmy plate)
For concentration-dependent properties (e.g., surfactant solutions), specify the concentration
For temperature-sensitive materials, consider providing values at multiple temperatures
For substances where surface tension is not applicable (e.g., solids, gases), clearly state this
For surfactants and surface-active agents, consider providing information on their effect on the surface tension of water
For mixtures with complex behavior, provide appropriate ranges or additional explanatory information
Ensure consistency between surface tension information and other related properties in the SDS (e.g., wetting properties, emulsion characteristics)