Rheological Properties of Disperse Systems & Semisolids

Rheological Properties of Disperse Systems & Semisolids

M. PRIYANKA

M. Pharmacy 1st year

School Of Pharmaceutical Sciences & Technologies, JNTUK

CONTENTS

• Introduction to disperse systems

• Classification of disperse systems

• Size of disperse systems

• Rheological properties of colloidal systems

• Factors affecting rheology of colloidal dispersions

• Rheological properties of coarsely dispersed systems

• Rheological properties of semisolids

08-07-2016 [email protected] 2

INTRODUCTION TO DISPERSE SYSTEMS

Disperse systems are the formulations consisting of two or more phases with a highly developed interface between them.

In disperse systems, the disperse phase is distributed in the form of small particles in the other continuous phase, the dispersion medium.

According to the particle size or dispersity of the disperse phase, the disperse systems are divided into two types


CLASSIFICATION OF DISPERSE SYSTEMS

DISPERSE SYSTEMS

Finely dispersed or colloidal systems

Coarsely dispersed

Suspensions

Emulsions


Size of disperse systems

In colloidal systems, the particle size varies from 10-4, 10-5 to 10-7 cm.

In coarsely dispersed systems, the particles have sizes of 10-4 cm.


RHEOLOGICAL PROPERTIES OF COLLOIDAL SYSTEMS

DEFINITION OF COLLOIDAL DISPERSION:-

Colloidal dispersion or colloidal system is a heterogeneous system which is made up of dispersed phase & dispersion medium. In colloidal dispersion one substance is dispersed as very fine particles in another substance called dispersion medium.

Generally, colloidal dispersions are non-newtonian in nature


FACTORS AFFECTING RHEOLOGY OF COLLOIDAL DISPERSIONS

Spherical particles Low viscosity

Linear shape particles High viscosity

1. Shape of the dispersed particles:-

2. Affinity of particles to the medium:-

If the linear particles are placed in a medium having low affinity for particles, these

tend to assume spherical shape & the viscosity falls.


3. Molecular weight of the particles:-

Higher the molecular weight of the particles of the disperse phase, greater will be

the viscosity & vice versa


VISCOSITY:-

Viscosity is an expression of the resistance to flow of a system under an applied stress.

the flow of a dilute colloidal system is expressed by an equation developed by Einstein

𝜼 = 𝜼𝝄 𝟏 + 𝟐. 𝟓𝝓

Where,

𝜼 = viscosity of the dispersion

𝜼𝝄= viscosity of the dispersion medium

𝝓 = volume fraction of particles


Typical flow curve for a colloidal dispersion

Newtonian Plateau

Shear Thinning

Shear Thickening & / or

2nd Newtonian Plateau

𝜼

𝒍𝒐𝒈𝒓


Determination of molecular weight of polymers by measuring viscosity of colloidal dispersion

By measuring the viscosity of the colloidal dispersion, we can determine the

molecular weight of polymers.

Higher the molecular weight of the dispersed particles greater will be the viscosity.

Therefore, by measuring the viscosity, molecular weight of the polymers can be

determined.

The approximate molecular weight of the dispersed phase can be estimated using

intrinsic viscosity.

Where,

K & a = constants

𝜼 = intrinsic viscosity

M = mol.wt

[𝜂] = 𝐾𝑀𝑎


Constants k & a are characteristics of a particular polymer-solvent system. These constants are obtained by calibrating with a colloidal dispersion of known molecular weight(M) & characteristics.

Once k & a are known, molecular weight ‘M’ can be calculated by experimentally determining the intrinsic viscosity. Capillary viscometer is used for this purpose.

Intrinsic Viscosity

Viscosity – Concentration plot used for the determination of molecular weight of polymers08-07-2016 [email protected] 12

RHEOLOGICAL PROPERTIES OF COARSELY DISPERSED SYSTEMS

Suspensions & emulsions falls under the category of coarse dispersion

systems.


In suspensions & emulsions, the flow properties have major influence in

the manufacture, during storage & administration of drugs.

The flow properties such as pseudo plastic & thixotropy are important

for physical stability of suspensions. For emulsions flow type may vary

according to their concentration


RHEOLOGICAL PROPERTIES OF SUSPENSIONS

During storage, the suspensions exhibit gel like structure & lowers therate of settling. On moderate shaking, the product assumes sol-likebehaviour, which permit pouring of the product from the bottle. The sol likebehaviour also helps in uniform spreading of dermatological preparations.Therefore, in order to maintain these properties the flow properties shouldbe studied.

Dispersion medium is largely responsible for determining the rheologyof the suspensions. The preformulation studies include evaluation ofvehicles used in the suspension. An optimum viscosity of this mediumshould be selected through experimentation.


The experimental methods involve the study of the system from lower to higher rates of shear. The resultant thixotropic curves are compared with those of the standard products.

Shear stress

rpm

Flow curves for 5% suspending agents in water showing thixotropy


The flow behaviour in the above curves indicates that a combination of 5% CMC &5% micro bentonite is a more suitable vehicle when compared to individualsuspending agents.

Rheological evaluation is used as a quality control parameter for comparingproducts. The consistency of suspensions are evaluated using cup & bob or cone &plate viscometers. These are not applicable for flocculated suspensions because thestructure of the flocs are destroyed during the analysis.


BROOKFIELD VISCOMETER

This instrument may be ideal to study the behaviour of settling. It is mounted onhelipath stand with T-spindle. The T bar rotates & changes its position continuously.At the same time, it also descends slowly into the suspension with the help ofsynchronous motor. The path traced by the spindle is a helix. As the T bar moves,the sediment offers resistance. The dial reading indicates the magnitude ofresistance. Thus using T spindle & helipath, the dial reading can be plotted againstthe number of turns of the spindle.


Since the T bar continuously encounters the undisturbed layer, the results indicatehow the particles are settling with respect to time

As T bar continuously descends newer layers of suspension, this technique providesinformation about the level at which the flocs network is greater due to aggregation.

The effect of aging & storage conditions can be evaluated using this method. Thedata indicates whether any undesirable changes are taking place.

In the screening study, good suspensions show a lesser rate of increase of dialreading as the spindle turns i.e., the curve is horizontal for a longer period.


RHEOLOGICAL PROPERTIES OF EMULSIONSThe following flow related attributes are desirable for the overall performance

of an emulsion

Removal of an emulsion from a bottle or tube

Flow of an emulsion through a hypodermic needle

Spreadability of an emulsion on the skin

Stress induced flow changes during manufacture


The rheology of emulsions has many similar features to that of suspensions.However, they differ in three main aspects

The liquid/liquid interface that contains surfactant or polymer layers introduces aresponse to deformation & one has to consider the interfacial rheology

The dispersed phase viscosity relative to that of the medium has an effect on therheology of the emulsion

The deformable nature of the dispersed phase droplets, particularly for largedroplets, has an effect on the emulsion rheology


In general, dilute emulsions exhibit “Newtonian flow”. As the viscosity of theemulsion increases, flocculation of globules will be reduced because the mobility ofglobules is restricted, leads to creaming. Due to this antagonistic effect, an optimumviscosity is desirable for good stability.

Concentrated emulsions exhibit “non-Newtonian flow”. Multipointviscometers are used for viscosity analysis.


RHEOLOGICAL PROPERTIES OF SEMISOLIDS

Viscosity of the semisolid dosage forms may directly influence thediffusion rate of the drug. Therefore, the product behaviour must be monitoredat the time of application. This also helps to monitor batch-to-batchconsistency.


Most topical semisolids, when applied on the surface of the skin, shownon-Newtonian behaviour. The structures formed within semisolid drugproducts during manufacturing can show a wide range of behaviours including,thixotropy & irreversible or reversible structure damage.

A number of rheological properties play an important role indetermining how a material behaves as it moves from storage to handlingenvironment. For example, the force or stress required to initiate flow ofsemisolid products plays a significant role in the shelf-life, storage, transfer,packaging & end-use performance of those materials


Semisolids do not flow at low shear stresses but undergo reversibledeformation like elastic solids. When a characteristic shear stress is exceeded,they flow like liquids.

Semisolids with high yield valve are described as hard. When theirplastic viscosity is high, they are described as stiff. Best instrument fordetermining the rheological properties of pharmaceutical semisolids isrotational viscometer. E.g. cone-plate viscometer, cup & bob viscometer,Brookfield viscometer


REFERENCES1. Pharmaceutical engineering principles & practices by C.V.S.

Subrahmanyam, J.Thimma setty, Sarasija suresh, V.Kusum devi

2. The theory & practice of industrial pharmacy by Leon. Lachman &Herbert A. Lieberman

3. www.google.com


Thank you

Priyanka Modugu


Rheological Properties of Disperse Systems & Semisolids

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Transcript of Rheological Properties of Disperse Systems & Semisolids