Adsorption/aggregation of surfactants and their mixtures at solid-liquid interfaces

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Materials Science

Background:

• Adsorption of surfactants and polymers at solid-liquid interfaces is crucial for modifying interfacial properties in diverse industrial applications.
• Interfacial behavior is governed by various forces including electrostatic, covalent, hydrogen, and hydrophobic bonding, alongside solvation effects.
• Understanding these forces from a thermodynamic perspective is essential for process optimization.

Purpose of the Study:

• To review the influence of various forces on adsorption behavior at solid-liquid interfaces from a thermodynamic viewpoint.
• To present experimental results from microcalorimetric and spectroscopic studies of adsorbed layers.
• To elucidate the thermodynamic driving forces behind surfactant and polymer adsorption.

Main Methods:

• Microcalorimetry was used to obtain thermodynamic data for surfactant adsorption on alumina.
• Spectroscopic techniques, including fluorescence with a pyrene probe and electron spin resonance (ESR), were employed.
• Analysis of calorimetric data for anionic and non-ionic surfactants, and their mixtures, on alumina.

Main Results:

• Anionic surfactant adsorption on alumina was initially highly exothermic, attributed to electrostatic interactions.
• Further adsorption and hemimicelle formation were identified as primarily entropy-driven processes.
• Entropy effects were more significant for anionic-non-ionic surfactant mixtures compared to anionic surfactants alone.

Conclusions:

• Thermodynamic analysis provides critical insights into adsorption mechanisms at solid-liquid interfaces.
• Electrostatic interactions dominate initial adsorption, while entropy drives subsequent aggregation.
• Spectroscopic studies reveal the impact of surface aggregation and molecular conformation on dispersion and wettability.