Fundamentals of Pharmaceutical Processes Mixing

Main ideas / lessons about pharmaceutical mixing

Why mixing matters in pharmaceutics

Mixing is essential for many dosage forms across different system types, including:

• Solid–solid
• Solid–liquid
• Liquid–liquid
• Gas–liquid

The final formulation properties depend on how uniformly ingredients are mixed.

Definition of mixing

In pharmaceutics, mixing combines two or more components so that:

• each particle is as closely surrounded by particles of other components as possible
• the result is a uniform (homogeneous) mixture

Homogeneous mixture example concept:

• Particles of ingredient A are surrounded by particles of B
• Particles of B are surrounded by particles of A

Objectives of mixing (what mixing is used for)

Mixing is used to achieve different outcomes depending on the formulation goal:

• Physical mixture / uniform distribution

  • Produces a simple physical blend (e.g., powders, API + excipients during tablet preparation)
  • Intended no chemical interaction—just uniform physical distribution

• Dissolution / solution formation

  • Create solutions by dissolving one component in another (e.g., sugar water, salt water, dissolving an API in water, or an ingredient in oil)
  • Treated primarily as a physical change

• Facilitating chemical reactions

  • Mixing brings reactants into closer contact, increasing interaction and improving reaction efficiency

• Dispersion formation when components are immiscible

  • Even if components don’t dissolve in each other, mixing can create dispersions:
    • Emulsion: liquid dispersed in an immiscible liquid (e.g., oil-in-water emulsion like liquid paraffin emulsion)
    • Suspension: solid dispersed in a liquid (e.g., sand in water, and examples like Gelusil)

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Types of mixtures produced (and their behavior)

Positive mixtures (irreversibly mixed)

Once mixed, components:

• do not separate
• do not require extra energy to remain mixed

Examples:

• Miscible liquids (e.g., milk and water)
• Gases that mix readily and don’t separate easily

Negative mixtures (require continued mixing energy to stay mixed)

Examples:

• Oil and water

Typically require:

• agitation and/or heating

When agitation stops, they tend to de-mix. Examples listed:

• Suspensions
• Emulsions

Neutral mixtures

Examples:

• Pastes, ointments, powders

These require some energy input to mix, but unlike negative mixtures, they don’t spontaneously separate easily once mixed.

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Factors that affect mixing performance (with implications)

Particle size

• If particle sizes vary greatly:
  • smaller particles can fill void spaces
  • this may cause poor distribution or layering/settling
• If particle sizes are uniform:
  • mixing is easier
  • resulting products are more uniform

Particle shape

• Smoother, more uniform particles (e.g., spherical smooth) mix more easily
• Irregular/rough particles can interlock, forming clumps, making segregation less effective and mixing harder

Particle attraction (surface charge / adsorbed liquid films)

Attraction causes aggregation into larger particles, making mixing more difficult.

Material density

• Mixing high-density with low-density materials is harder
• Extremely low density can also be difficult due to handling/movement challenges
• Goal stated: bring density and size into an optimized workable range

Proportion and order of addition

Mixing is easier when materials have similar:

• texture, stickiness, finesse, density
• and similar proportions

Non-proportionate quantities and differing properties make mixing difficult. Mitigation mentioned:

• use lubricants or wetting agents to improve material properties and support uniform mixing

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Mixing mechanisms in liquid mixing (conceptual model)

Two main aspects of liquid mixing

Liquid mixing includes:

1. Localized mixing

   • added material interacts with its microenvironment
   • shear occurs between liquid and solid (or between phases)
   • described as micro-scale interaction mixing

2. Generalized movement

   • material must be transported through the bulk of the vessel
   • this supports homogeneity across the whole batch

Miscibility outcome

• Miscible components → homogeneous solution
• Immiscible components
  • liquid–liquid → heterogeneous mixture (e.g., emulsions)
  • solid–liquid → suspension

Forces and velocity components (impeller/shaft)

Three velocity components created by the impeller shaft:

• Longitudinal velocity: parallel to the shaft
• Radial velocity: perpendicular to the shaft (away/toward center)
• Tangential velocity: tangential to the circular flow

Combined effects generate:

• shear
• flow and interphase interaction
• conditions that help achieve uniform mixing

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Mixer design factors (engineering considerations)

Impeller design

Key considerations include:

• Blade form
  • smooth vs specific configurations (e.g., sigma ribbon)
• Position/orientation
  • vertical / tangential / horizontal
  • where inserted (top or side)
• Purpose: achieve proper mixing performance

Mixing unit/container design

• Container shape: cylindrical or square depending on the mixture
• Speed of rotation
• Diameter ratio (container diameter / mixing element diameter)
  • For low viscosity fluids: ratio ≈ 20
    • run at higher rotation speed
  • For high viscosity materials: ratio ≈ 1
    • run at lower rotation speed

Semi-solids note

Semi-solids require:

• sturdier mixers
• lower speed
• controllable rotation

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Powder mixing concepts (ordered vs random mixing)

Powders create neutral mixtures

Once arranged, powders do not segregate or demix easily unless an external force is applied.

Ordered vs random mixing

• Ordered mixing (goal)
  • each region of one “color” (e.g., white) is surrounded by the other (e.g., black)
  • indicates uniform distribution
• Random mixing (undesired)
  • patches are unevenly distributed
  • not uniform

Packing and porosity (after mixing)

Packing describes particle arrangement:

• Cubic packing
  • porosity ≈ 50%
• Rhombohedral packing
  • porosity ≈ 25%

Packing affects bed behavior through the resulting porosity.

Dilated bed requirement

A dilated bed is a porous powder bed that can be:

• fluidized
• handled
• mixed easily

If powder becomes compact and not fluidizable:

• segregation is less of an issue
• but handling, mixing, transport, and hopper flow become difficult

Target: porous, dilated powder for practical handling/mixing.

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Semi-solid mixing requirements (ointment/paste/cream/jelly)

Why semi-solid mixing differs

Semi-solids combine solids + liquids, so mixing differs from purely liquid–liquid or solid–solid systems.

Key operations

Requires:

• low shear smearing
• whipping
• folding
• stretching
• compressing

Reason given: high viscosity prevents reliance on very high speeds; instead, you need controlled deformation and movement.

Practical implication

Semi-solid mixing needs robust/sturdier mixers.

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Mixer types mentioned (by material class) + what they do

Solid mixers

• Cone blender
• Double cone blender
• Ribbon blender
• Sigma blade mixer

How examples work (from subtitles)

• V-cone blender

  • V-shaped container rotates; material is lifted and impacts during rotation
  • impact contributes to both size reduction and mixing

• Double cone blender

  • Two inverted cones; material is loaded and rotated to blend

• Ribbon blender

  • Ribbon-shaped shafts lift and cut through solids to mix them

• Sigma blade mixer

  • Single or double sigma blades
  • for solids, commonly uses a single sigma blade for blending

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Liquid mixers

• Propellers
• Turbines
• Air jet mixers

Key constraint mentioned

• Propeller/turbine operation should not produce vortexing
  • vortexing can trap air
  • trapped air can lead to oxidation

Liquids generally require more force than solids due to:

• propulsion through another liquid
• possible viscosity variability

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Semi-solid mixers

• Sigma blade mixers
• Planetary mixers
• Triple roller mills
• Colloidal mills

How examples work (from subtitles)

• Sigma blade mixers (semi-solids)

  • sigma-shaped blade in the center
  • material (solid + liquid in required proportions) added
  • the blade lifts/whips/drags material to move it from localized zones to bulk

• Planetary mixers

  • sturdy impeller shaft moves through bulk to mix solid + liquid

• Triple roller mills

  • three rotating rollers (same or different directions)
  • passing material through reduces size and disperses it in bulk

• Colloidal mill

  • applies pressure to reduce particles to the colloidal size range
  • used to produce colloid-based semi-solids

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Speakers / sources featured

• No specific speaker name, credit, or external source is identified in the provided subtitles.
• Only general “lecture” narration is present.