Nano Particles

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Nano Particles

• High fraction of atoms at or near the surface.
• Surface Tension liquids surfaces behave as
  though they are an elastic film.
• Kelvin Effect higher vapor pressure over
  smaller droplets
• Ostwald Ripening large particles grow at the
  expense of smaller particles
• Adsorption impurities tend to stick to surfaces
• Surface charge adsorption of ions can leave the
  nanoparticle electrically charged

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Classification of NanoParticle Suspensions
DISPERSED PHASE (nanoparticle) DISPERSED PHASE (nanoparticle) DISPERSED PHASE (nanoparticle) DISPERSED PHASE (nanoparticle)
CONTINUOUS PHASE (medium completely surrounding the nanoparticle) solid liquid gas
CONTINUOUS PHASE (medium completely surrounding the nanoparticle) solid solid suspension (solid sol) certain ceramics (Corel) alloys, ruby glass gel jello, jelly, cheese, certain rubbers, Tygon tubing solid foam foam rubber, marshmallow, Styrofoam
CONTINUOUS PHASE (medium completely surrounding the nanoparticle) liquid suspension (sol, in H2O hydrosol) muddy water, paint, ink emulsion mayonnaise, milk foam shaving cream, whipped cream
CONTINUOUS PHASE (medium completely surrounding the nanoparticle) gas smoke (aerosol) smoke, dust fog (liquid aerosol) fog, clouds does not occur (all gases are miscible) No Examples
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Homework Problem What Fraction of Atoms are on
the Surface?

• A sphere of radius R is composed of atoms of
  radius a. Make the assumption that the surface
  atoms occupy a spherical shell 2a thick. Use the
  packing fraction to correct for the interstitial
  volume. You do not need to consider the granular
  nature of the particle any further (ignore
  packing, stacking, surface corrugations, etc.).

Find the number of gold atoms (a 1.44 Å) in a
gold nanoparticle and the fraction of gold atoms
on the surface. The gold forms an FCC crystal.
Packing fractions FCC HCP 0.740
BCC 0.680 SC 0.524
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Surface Tension

• Fluids behave as though they have a surface
  composed of an elastic skin which is always in
  tension. There are many manifestations of
  surface tension you can observe everyday. Here
  are some fundamental properties of surface
  tension.

surface tension? ? units? force/length ?
typically given in dyne/cm
The force by a planar soap film supported on a
rectangular frame with one movable bar of length
l. The factor of two is introduced because the
soap film has two surfaces.
The work required to create new surface area.
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Pressure Difference Across a Curved Surface

• Forces on a liquid sphere of radius r

balance the forces
Pout
surface tension force
Pin
?2pr
In this example there is only one surface. For a
soap bubble, the force will be twice as great.
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Surface Tension Wetting Contact Angle

• Contact Angle for a Sessile Drop

V vapor L liquid S solid
Youngs Equation Horizontal Tensions balance
contact angle

• The critical surface tension ?c is an intrinsic
  characteristic of the surface.
• Liquids with ? lt ?c completely wet the surface (?
  0 º).
• Liquids with ? gt 90º are said to not wet the
  surface (?LS gt ?SV) .

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The Kelvin Equation

• The surface tension causes an increased chemical
  potential for a molecule inside a droplet. This
  is manifested as an increase in the vapor
  pressure P of the liquid droplet compared to that
  of the bulk liquid P0. The is described by the
  Kelvin equation. Two radii of curvature appear
  in the result, r1 and r2. For a sphere both
  terms are equal, but for a cylindrical surface
  one term vanishes because one radius is infinite
  (flat).

sphere
cylinder
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The Kelvin Effect
Atoms of liquid on the surface of a small droplet
are held less tightly compared to atoms on a flat
(bulk) liquid surface. High curvatures
effectively reduce the coordination number of the
surface atoms making them easier to evaporate.
Thus the liquid has a higher vapor pressure over
small liquid droplets compared to bulk liquid.
The effect of curvature on the vapor pressure of
liquids is the Kelvin effect. Positive
curvature liquid in drops has a higher vapor
pressure that bulk. Negative curvature liquid
in pores has a lower vapor pressure than
bulk. The vapor pressure P relative to the bulk
P0 can be found using the Kelvin equation, show
here for spherical surfaces of radius r.
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Example the Kelvin Effect on Water Drops
Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C Equilibrium Vapor Pressure Increase Over Pure Water Dropletas a Function of Droplet Radius at T 25 C
rp (µm) 1 0.3 0.1 0.03 0.01 0.003
P/P0 1.0011 1.0035 1.0107 1.0360 1.1118 1.4238
?P () 0.11 0.35 1.1 3.6 11 42
Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C Equilibrium Vapor Pressure Decrease of Pure Water inside a Poreas a Function of Pore Radius at T 25 C
rp (µm) 1 0.3 0.1 0.03 0.01 0.003
P/P0 0.9989 0.9964 0.9895 0.9653 0.8994 0.7023
?P () -0.11 -0.35 -1.1 -3.5 -10 -30
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Applications for Nanoparticles

• catalysis (high surface area, controlled crystal
  surfaces)
• optical properties (sun screen, hyperthermic
  cancer treatment, fluorescent tags)
• light scattering (smoke./fog screens)
• drug delivery (inhalation asthma, timed drug
  release.
• pesticide delivery (fogging and fumigation)
• magnetic recording (orient magnetic domain axis,
  important for hard drives, video audio tapes)
• pigments, inks, paints (coloring and opacity)