Colloidal Nanoscience Notes

Electrostatic vs Steric Stabilization

Electrostatic stabilization involves the repulsion of like charges between approaching particles, driving them apart as their double layers overlap.

Steric stabilization uses anchored polymers (ligands) on the particle surface to prevent neighboring particles from getting too close. The polymer layers repel each other upon approach, preventing agglomeration due to Van der Waals attraction.

Functionalization of Nanoparticle Surfaces

Modular design of surface ligands for biocompatible nanomaterials:

• (a) Anchor links: Capping molecules bind directly to the nanocrystal surface through hydrophobic interactions.

• (b) Stabilizer region: Incorporates polyethylene glycol (PEG) or zwitterionic chains to bind water molecules, imparting hydrophilicity and preventing non-specific adsorption.

• (c) Reactive groups: Capping molecules feature reactive groups for covalent conjugation to biofunctional molecules after ligand exchange.

• (d) Biofunctional units: Offer targeting, therapeutic, or sensing capabilities.

Surface Plasmon Resonance (SPR)

SPR occurs when collective oscillation of electrons on the plasmonic nanoparticle surface results in strong extinction of light (absorption and scattering) at a specific wavelength (frequency).

The wavelength of light at which SPR occurs is dependent on:

• Nanoparticle size

• Shape

• Surface

• Agglomeration

Examples of plasmonic nanoparticles: Au, Ag, Cu.

SPR Mechanism

The electric field of light induces movement of conduction electrons, creating an electric dipole at the nanoparticle (NP) surface. This charge accumulation generates an electric field opposite to that of the light.

SPR: Gold Example

Gold nanoparticles with different shapes exhibit varying absorbance spectra:

• Nanospheres

• Nanorods

• Nanotriangles/prisms

• Porous nanospheres

• Nanocorals

• Nanostars/urchins

• Nanoshells (core-shells)

• Nanocubes

• Nanododecahedra

• Nanorices (core-shells)

• Nanocages/rattles

SPR Sensing

SPR-based sensing applications utilize:

• Colorimetric change by NP aggregation

• SPR shift due to changes in the physical-chemical environment

• Au NP structural changes

Plasmonic nanoparticles (Au, Ag) can be analyzed using UV-visible spectroscopy. Nanoparticle enlargement results in a color shift and increased optical extinction (scattering and absorption) in the visible spectrum.

Factors Affecting SPR

• Adsorption

• Surface interactions

• Aggregation (colorimetric change)

• Particle growth (change in size)

Colloids

A colloid is a system where one substance is finely divided and evenly distributed throughout another. It's not a solution but often called a sol.

• Dispersed phase: solid, liquid, or gas

• Dispersion medium (continuous phase): solid, liquid, or gas

Colloidal properties are typically observed when the dispersed phase size ranges from 1 to 1000 nm.

Colloidal Dispersions

Colloidal matter exists as colloidal-sized phases of solids, liquids, or gases uniformly dispersed in a separate medium (dispersion phase).

• Disperse phase: The substance being dispersed.

• Continuous phase: The medium in which the disperse phase is suspended (e.g., egg white foam).

Colloidal Systems in Food Science

Sol vs Gel

A sol is a colloid with solid particles dispersed in a liquid continuous phase.

A gel forms when the solid particles in a sol create a network (continuous phase) that traps the liquid (disperse phase). Examples include proteins, starches, pectin, and agar.

Emulsions

When water and oil are shaken, they form an unstable emulsion where oil separates over time due to immiscibility.

A stable emulsion requires an emulsifier, a third substance that stabilizes the mixture.

• Oil-in-water (o/w): Oil droplets dispersed in water (e.g., milk).

• Water-in-oil (w/o): Water droplets dispersed in oil (e.g., butter).

Emulsifiers

An emulsifier has a hydrophilic head and a hydrophobic tail. The tail sits within the oil, and the head sits within the water, preventing coalescence (joining) of oil or water bubbles.

Example: Mayonnaise uses egg yolk (lecithin) as an emulsifier to stabilize oil and vinegar.

Foams

Foams consist of gas bubbles (usually air) dispersed in a liquid (e.g., egg white foam).

Mechanical action unfolds proteins, creating a network that traps air. Heating the foam coagulates proteins and drives off moisture, forming a solid foam (e.g., meringue).

Examples: Ice cream, bread, and cakes are solid foams.

Particle Size Ranges in Colloidal Domain

The colloidal domain lies between atoms/ions/molecules and larger particles like silt and sand, ranging from approximately 1 nm to 1 μm (10^-9 to 10^-6 meters).

Colloidal Behavior

Surface atoms or molecules have distinct properties compared to those in the bulk phase.

As bulk phase is subdivided, the surface-to-bulk ratio increases, making surface properties significant. Key factors influencing colloidal behavior include:

• Particle size and shape

• Surface properties (chemical and physical)

• Continuous phase chemical and physical properties

• Particle-particle interactions

• Particle-continuous phase interactions

Coagulation and Flocculation

Coagulation is the process where colloidal particles aggregate into a tight, dense structure due to the inability to maintain a dispersed state. This is usually irreversible without significant energy input.

Flocculation involves the formation of flocs, which are loose, open aggregates of colloidal particles. Flocs can sometimes be reversible with minimal energy input.

Coagulation vs Flocculation

• Coagulation: A chemical process often involving the addition of electrolytes (salts) to induce aggregation. No mixing is required. Coagulation happens above a minimum electrolyte concentration. Electrolytes have different coagulation values (smaller value = larger coagulating power).

• Flocculation: A physical process using polymers to induce settling of particles into larger flakes. Mixing is required.

Degrees of Coagulation

Classification based on average zeta-potential:

• Maximum: 0 to +3

• Excellent: -4 to -1

• Fair: -10 to -5

• Poor: -20 to -11

Process of Coagulation and Flocculation

1. Repulsive force of negatively charged particles (measured by zeta-potential).

2. Neutralize negative charge by adding positively charged ions (takes 1-2 seconds) to overcome zeta-potential and create micro-flocs.

3. Different degrees of coagulation achieved based on zeta potential.

4. Van der Waals forces (attractive forces) come into effect when charges are neutralized, forming flocs.

5. Polymers help keep flocs together.

6. Flocs enlarge and settle out during sedimentation.

Optical Properties of Colloidal Particles

Colloidal dispersions scatter light, unlike true solutions. The scattering depends on the ratio of particle size to the wavelength of light.

John Tyndall discovered that a light beam passing through a colloidal dispersion spreads out (Tyndall scattering), even in transparent dispersions.

White appearance: is due to intense light scattering dispersing light in all directions (e.g., milk or clouds).

Brownian Motion of Colloidal Particles

Colloidal particles collide due to Brownian motion, convection, gravity, and other forces. Such collisions can lead to coagulation and destabilization.

Scottish botanist Robert Brown discovered Brownian motion in 1827, observing pollen particles in water moving irregularly.

Brownian motion results from collisions of liquid molecules with the solid particle. Smaller particles are more susceptible to random fluctuations in collisions, causing the irregular movement.

Colloidal Stability

Colloidal stability refers to the long-term integrity of a dispersion and its ability to resist sedimentation or particle aggregation. It's defined by how long particles remain suspended.

Metastable colloids may still exhibit instabilities over time, leading to coagulation and larger particles. Emulsions, suspensions, and foams should be assessed for colloidal stability to determine aging properties and resistance to shear stress.

Coalescence

Coalescence is the tendency of suspended media to reassemble and form larger masses, usually irreversibly altering the macroscopic condition of the colloid. Homogenous mixtures are no longer suitable for the intended purpose.

Particle Aggregation

Particle aggregation is the aggregating potential of dispersed phase media in a colloidal system.

Intermolecular interactions (Van der Waals, electrostatic, depletion) cause solid particles to cluster, affecting colloidal stability and reducing product efficacy.

Aggregated particles remain distinct and do not merge. Recovering the initial dispersion state requires strong mechanical stress, which may not always be possible.

• Controlled/reversible particle aggregation

• Uncontrolled/irreversible particle aggregation

Self Assembly

Self-assembly is the spontaneous self-organization of objects to minimize free energy. Surface properties, shape, and size homogeneity influence self-assembly of nanoscale building blocks. The shape determines the directionality of interactions.

Examples include nanoparticle superlattices and assembly of magnetic nanoparticles.

Classes of Self Assembly

• Static and dynamic self-assembly

• Co-assembly, hierarchical self-assembly, and directed self-assembly

• Intersections: systems obtained by hierarchical co-assembly or hierarchical directed self-assembly

Coassembly

Coassembly involves two or more different components self-assembling contemporaneously and interdependently to form a complex architecture where the components are typically segregated and not entirely mixed.

Hierarchical Self-Assembly

Hierarchical self-assembly comprises fundamental building blocks self-assembling into primary structures held together by short-range forces. These primary structures then form secondary structures via different, longer-range forces. This process can repeat until the highest level in the hierarchy is attained.