Surfactant Activity in Chemistry

Surfactant Activity & Application

• Surfactants are amphipathic, possessing both hydrophobic (water-repelling) and hydrophilic (water-attracting) groups.
• Hydrophobic groups are typically carbon chains, while hydrophilic groups may involve ionic or polar functionalities.
• Water's high degree of structure due to hydrogen bonding is disrupted by ionic or strongly polar solutes. Surfactants compensate by interacting with water molecules, allowing ionic/polar materials to dissolve freely.
• Non-polar substances resist solubilization in water. Water molecules cluster around non-polar molecules, creating a negative entropy change.
• Surfactant solubility depends on the polar group's interaction with water to overcome hydrophobic/hydrophobic interactions.
• Surfactants orientate at surfaces or interfaces, with hydrocarbon chains away from the aqueous phase.
• Increased surfactant chain length increases the energetic favorability for adsorption at interfaces, thus raising concentration at the interface.
• This adsorption lowers water's surface tension.
• Water-water attractions are more substantial than water-hydrocarbon or hydrocarbon-hydrocarbon interactions.
• Surfactant molecules disrupt water-water bonding and decrease surface tension.

HLB (Hydrophilic-Lipophilic Balance)

• HLB values predict surfactant use based on partitioning between phases.

• Low HLB values indicate oil solubility, while high HLB values suggest water solubility.

• Calculated as [hydrophilic component percentage] / 5.

• Different HLB values correlate with different applications:

  • ≥13: solubilization (easily soluble, clear solutions)
  • 8-10: oil-in-water (o/w) emulsification
  • 6-8: solubility with agitation (wetting)
  • 3-6: Poor water solubility (water-in-oil (w/o) emulsification)
  • 1-3: Insoluble, antifoaming

• Fats and oils have required HLB values. Choosing an emulsifier requires matching the HLB of the emulsifier to the HLB of the oil phase.

Calculation of Required HLB of an Oil Phase

• Determine percentages of oils and emulsifiers within the oil phase.
• Find the required HLB of each oil or emulsifier within the oil phase.
• Multiply the percentage of each component by its respective HLB.
• Sum the resultant values to compute the total required HLB for the oil phase.

Gibbs Equation

• Gibbs derived an equation that determines the extent of surfactant adsorption.
• Dynamic equilibrium occurs between surface and bulk molecules.
• Surface excess concentration is the measure of excess amount in surface phase versus bulk phase.
• Calculated using the Gibbs Equation: Г = - (Cb/RT).(dy/dCb). (Γ = surface excess concentration, Cp = bulk concentration (mol dm-3), R=gas constant, and dy/dCp = gradient of surface tension versus bulk concentration)

Solubilization

• Surfactant molecules are primarily in the bulk, not the surface.
• Adding surfactants leads to pronounced surface tension decreases, followed by a plateau or leveling off near the critical micelle concentration(cmc)
• CMC is the surfactant concentration at which micelle formation starts.
• Below CMC, monomers (single surfactant molecules) exist; above CMC, micelles (aggregates of surfactant molecules) form.
• Micelle formation achieves minimum free energy.

Micelle Formation

• State of minimum free energy: hydrophobic interactions drive surfactant aggregation into micelles.
• Negative entropy of water due to restructuring around hydrophobic groups is overcome by aggregation.
• Micelle formation is more energetically favorable at higher concentrations.
• Surfactants self-aggregate into micelles because they are not restricted by the water structure, and therefore have freedom to move.
• Dynamic equilibrium with monomers constantly breaks and reforms.

Micelle Factors

• Non-ionic micelles generally larger than ionic micelles (asymmetrical).
• Oxyethylene chains tend to hydrate the micelles.
• Factors affecting the critical micelle concentration (CMC) and micellar size include:
  • Hydrophobic group structure (chain length). Longer chains decrease CMC and increase size.
  • Hydrophilic group structure. Increased hydrophilic component increases CMC.

Ionic Surfactant Properties

• Non-ionic surfactants have lower CMC values and higher aggregation numbers versus ionic surfactants.
• Polyoxyethylene chains increase hydrophilicity in non-ionic surfactants.
• Counter ions' effect on ionic micelles include:
  • Weakly hydrated ions have higher absorption into the micelle. This reduces the repulsive force between head groups, thus decreasing charge, and consequently, size and CMC.
  • Adding electrolytes reduces micelle size and CMC.

Temperature Effects

• Heating aqueous non-ionic surfactant solutions may cause cloudiness at the cloud point temperature.
• This phenomenon is generally reversible.
• Solubility of ethoxylate surfactants tends to decrease at increased temperatures.
• Krafft point: Temperature above which the solubility of ionic surfactants increases significantly.

Molecular Aggregation Sites during Solubilization

• Sites for solubilization depend on the surfactant's properties.
• Water-soluble surfactants dissolve in water.
• Medium/long-chain polar surfactants aggregate.
• Short-chain polar surfactants also aggregate.
• Non-polar molecules interact with other non-polar molecules.

Conductivity Methods of Surfactants

• Plot conductivity versus surfactant concentration shows two linear regions. This conductivity method is used to determine the critical micelle concentration (CMC).
• During micellization, viscous drag from the solvent decreases and fewer ions are free for movement; resulting in less conductivity.
• Ionic atmosphere around the micelles reduces the number of mobile ions, therefore decreasing conductivity.
• Bound counter ions within micelles reduce the overall charge, leading to reduced conductance due to opposing forces and the overall retardation force.