Suspensions

 Suspension

In this blog we are discussing about the suspensions, First of all define what is suspension and we cover its related important topics like what are ideal properties of suspension, what is interfacial properties of suspended particles and what are types of suspension. so friends read the blog till the end for full information.

What is suspension?

 Pharmaceutical suspension may be defined as a dispersion in which insoluble solids (drugs) are suspended in a liquid medium.

• The word 'pharmaceutical suspension' means that the solids are drugs and / or pharmaceutically useful agents.
• Suspension are also called as heterogenous system, or more precisely biphasic system.
• A good pharmaceutical suspension is one which the particle size distribution lies between 1 and 50 μm.
• In coarse suspension, the insoluble solids may have a size range from 10 to 1000 μm.
• The solid particles may be mono-size or poly-size distribution.
• The liquid medium is usually water or water base and are administered by several routes.

what are ideal properties of suspension?

• Suspension molecules should not settle rapidly.
• Prevent from cake formation at bottom.
• Easy to pour.
• Stable.
• Easily flow out from syringe needle.

what is interfacial properties of suspended particles?

In suspension, the solid particles is uniformly dispersed in liquid medium. the interface is formed in between the two phases, which influence the stability of suspension.

1. Surface free energy:- 

During formulation of suspension, the solid reduce to fine powder for better mixing in continuous medium. 

• Due to very fine particles, surface area of particles are increase and also their surface free energy increase.
• Surface free energy increase = stability of suspension decrease.
• Due to excess surface free energy, particles become highly energetics and tends to aggregates it self's and resulting in formulation of compact cake.
• Equation = 

△G=ΥSL △A

                        △G = Surface free energy.

                        ΥSL = Interfacial tension b/w solid & liquid.

                         △A = Change in surface area.

• To reduce surface free energy and make suspension stable, we use wetting agents and surfactants.
• Also system make floccules to reduce the surface free energy.
• e.g., poly sorbate-80 (wetting agents) etc._
  

2. Formation of electrical double layer:-

The ionic species present in soluble get absorbed at the surface. Due to ionization charges on the surface may arise. this produce the formulation of electric double layer.

• When surface change is negative then their will be layer of tightly bound solvent molecules which is stem layer.
• Where as the layer of positive charge, adjacent to the surface is called diffuse layer.
• Low electrolyte concentration- Deflocculated system.
• More (increasing) electrolyte concentration- flocculated system.

[OR]

• Interaction (attraction) force b/w particles ↑ = Flocculated.
• Repulsive force b/w particles ↑ = Deflocculated.

what is settling of suspension?

• Also known as rate of sedimentation, because settling of suspension is depend on the sedimentation rate. 
• The setting down of suspension particles of suspension or also sedimentation.
• Rate of sedimentation ↑ = Rate of settling.
• The more rate of sedimentation, faster the particles settle of sedimentation, faster the particles settle down and faster the formation of cake.
• The rate of settling of suspended particles is explained by stake's law.

                                                                  V = d218η(ρ1​−ρ2​)g

    

Where 

      V = Rate of sedimentation / Rate of settling.

      d2  = diameter of  particle.

      ρ1 = Density of  solvent [dispersion medium].
​

      ρ2 = Density of solvent [dispersion medium].

      g = Gravity

• Rate of settling ↑ = stability ↓
• To make suspension stable:- 
• Increase viscosity of dispersion medium by using some agents such as (methylcellulose, natural gums, acacia, tragacanth etc.) 

          [structural vehicle method].

• Stake's law is valid for those suspension which does not contained more than 2% solid.

What are type of suspension?

Suspension are two type (a) Flocculated (b) Deflocculated suspension.

(a). Flocculated:- Flocculated suspension are those suspension in which particles of dispersed phase aggregates together to form bench like structures called floccules.

(b). Deflocculated:- These are those in which particles of dispersion medium.

* Difference between flocculated and deflocculated suspension.

Flocculated	Deflocculated
Rate of sedimentation is high.	Rate of sedimentation is law.
More stable.	Less stable.
It have large voids, so on setting down it form soft cake.	It have less voids, it forms hard cake.
On shaken, it redisperse and come back to stable condition.	On shaken, it’s redispersion is very tough due to hard cake.
More stable	Less stable

 

                 

 

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