`ANSEL'S PHARMACEUTICrA,|s\,
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`DOSAGE FORMS AND
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`DRUG DELIVERY SYSTEM“
`TEN‘l?|-I EDITIQN
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`A
`‘V./'
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`Loyd V. Allen. Jr. PhD .
`Professor and Chair Emeritus
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`Department of Medicinal Chemistry and Pharmaceutics
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`College of Pharmacy
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`University of Oklahoma\
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`University of Oklahoma Health Sciences Center
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`Oklahoma City, Oklahoma
`Editor-in—C1'u'ef
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`International Ioumal ofPharmaceutical Compounding
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`-_
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`Howard c.AnseI. PhD
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`Professor and Dean Emeritus
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`College of Pharmacy
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`The University of Georgia
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`Athens, Georgia
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`0.Wolters Kluwer
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`_Insys Exhibit 2001
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`Page 1 of 64
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`Insys Exhibit 2001
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`Tenth Edition
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`Library of Congress Cataloging-in-Publication Data
`Allen, Loyd V., Ir., author.
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`Ansel’s pharmaceutical dosage forms and drug delivery systems / Loyd V. Allen, Jr, Howard C. Ansel. —
`Tenth edition.
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`p. ; cm.
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`Pharmaceutical dosage forms and drug delivery systems
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`Includes bibliographical references and index.
`ISBN 978-1-4511-8876-9
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`III. Title: Pharmaceutical dosage forms and drug delivery systems.
`ll. Title.
`I. Ansel, Howard C., 1933- author.
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`[DNLM: 1. Dosage Forms. 2. Drug Delivery Systems. QV 786]
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`RS200
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`615'.1—dc23
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`2013035677
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`The authors, editors, and publisher have exerted every effort to ensure that drug selection and dosage set
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`forth in this text are in accordance with the current recommendations and practice at the time of publication.
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`Page 2 of 64
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`Page 2 of 64
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`

`

`OBJECTIVES
`
`V i
`After reading this chapter, the student will be able to)’
`1. Differentiate between a suspension, an emulsion, algel, and onmagma
`2. Compare and contrast the different disperse systems, and list advcifitage
`V
`I
`and disadvantages of each system
`
`)
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`3. Compare and contrast the following emulsification theories: surface tension,
`oriented-wedge, and interfacial film
`
`. Define and differentiate the following terms from one another: lyophobic.
`lyophilic, hydrophobic, hydrophilic, amphiphilic, imbibition, swelling, syneresis,
`thixotropy, and xerogel
`
`. Evaluate and select a proper disperse system and delivery method fora
`given purpose, patient population, and/or patient circumstance
`
`This chapter includes the main types of liq-
`uid preparations containing undissolved or
`immiscible drug distributed throughout a
`vehicle. In these preparations, the substance
`distributed is referred to as the dispersed phase,
`and the vehicle is termed the dispersing phase
`or dispersion medium. Together, they produce
`a dispersed or disperse system.
`The particles of the dispersed phase are
`usually solid materials that are insoluble in
`the dispersion medium. In the case of emul-
`sions, the dispersed phase is a liquid that
`is neither soluble nor miscible with the liq-
`uid of the dispersing phase. Emulsification
`results in the dispersion of liquid drug as fine
`droplets throughout the dispersing phase. In
`the case of an aerosol, the dispersed phase
`may be small air bubbles throughout a solu-
`tion or an emulsion. Dispersions also consist
`of droplets of a liquid (solution or suspen-
`sion) in air.
`The particles of the dispersed phase vary
`widely in size, from large particles visible
`to the naked eye down to particles of col-
`loidal dimension, falling between 1.0 nm
`
`and 0.5 pm. A discussion on the difference
`between particles and molecules is pro-
`vided in Physical Pharmacy Capsule 14.1.
`Dispersions
`containing coarse particles,
`usually 10 to 50 mm, are referred to as coarse
`dispersions; they include the suspensions and
`emulsions. Dispersions containing particles
`of smaller size are termed fine dispersions (0.5
`to 10 um) and, if the particles are in the col-
`loidal range, colloidal dispersions. Magmas and
`gels are fine dispersions.
`Largely because of their greater size, par-
`ticles in a coarse dispersion have a greater
`tendency to separate from the dispersion
`medium than do the particles of a fine disper-
`sion. Most solids in dispersion tend to settle
`to the bottom of the container because of their
`
`greater density than the dispersion medium,
`whereas most emulsified liquids for oral use
`are oils, which generally have less density
`than the aqueous medium in which they are
`dispersed, so they tend to rise toward the
`top of the preparation. Complete and uni-
`form redistribution of the dispersed phase
`is essential
`to the accurate administration
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`445
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`Page 3 of 64
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`

`

`446
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`SECTION VI - LIQUID DOSAGE FORMS
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`PHYSICAL PHARMACY CAPSULE 'l4.l
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`Particles Versus Molecules
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`Particles of drug substances can actually range from an aggregation of two or more molecules
`to millions of molecules.The term ‘partic|e“ should not be confused with “molecule.” The mol-
`ecule is the smallest unit of any chemical compound that possesses all the native properties of
`that compound. Particles consist of numerous molecules, generally in a solid state (but can be
`liquid or gaseous). Dissolution is the solid to liquid transformation that converts solid drug par-
`ticles to individual, dissolved liquid molecules. Even the smallest invisible drug particle contains
`billions of molecules. Most nonprotein or small molecule organic drugs have formula weights
`ranging from 150 to 500.
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`EXAMPLE
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`Let's look at how many molecules may be present in a 1-ng particle of ibuprofen with a formula
`weight of 206:
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`(l ng)(l g) (6.02 x l 023 molecules)
`(particle) (1x10°) (206 g) (Mole)
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`= 2.923 X10" molecules
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`This illustrates that a 1—ng invisible particle will contain 2.923,000,000.000 molecules.
`
`of uniform doses. For a properly prepared
`dispersion, this should be accomplished by
`moderate agitation of the container.
`The focus of this chapter is on dispersions
`of drugs administered orally or topically. The
`same basic pharmaceutical characteristics
`apply to dispersion systems administered by
`other routes. Included among these are oph-
`thalmic and otic suspensions and sterile sus-
`pensions for injection, covered in Chapters
`17 and 15, respectively.
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`SUSPENSIONS
`
`Suspensions may be defined as prepara-
`tions containing finely divided drug par-
`ticles (the suspensoid) distributed somewhat
`uniformly throughout a vehicle in which the
`drug exhibits a minimum degree of solubil-
`ity. Some suspensions are available in ready-
`to-use form,
`that
`is, already distributed
`through a liquid vehicle with or without sta-
`bilizers and other additives (Fig. 14.1). Other
`preparations are available as dry powders
`intended for suspension in liquid vehicles.
`Generally, this type of product is a powder
`mixture containing the drug and suitable
`
`suspending and dispersing agents to be
`diluted and agitated with a specified quan-
`tity of vehicle, most often purified water.
`Figure 14.2 demonstrates preparation of this
`type of product. Drugs that are unstable if
`maintained for extended periods in the pres-
`ence of an aqueous vehicle (e.g., many anti-
`biotic drugs) are most frequently supplied as
`dry powder mixtures for reconstitution at the
`time of dispensing. This type of preparation
`is designated in the USP by a title of the form
`”for Oral Suspension.” Prepared suspen-
`sions not requiring reconstitution at the time
`of dispensing are simply designated as ”Oral
`Suspension.”
`
`Reasons for Suspensions
`
`reasons for preparing
`There are several
`suspensions. For example, certain drugs
`are chemically unstable in solution but
`stable when suspended. In this instance,
`the suspension ensures chemical stability
`while permitting liquid therapy. For many
`patients, the liquid form is preferred to the
`solid form of the same drug because of the
`ease of swallowing liquids and the flexibility
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`Page 4 of 64
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`CHAI-’lEl9 lzi
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`- DISPEPSE S‘/STEIVIS
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`447
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`FIGURE 14.2 Commercial antibiotic preparation for
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`oral suspension following reconstitution with purified
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`water. Left, dry powder mixture. Right, suspension after
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`reconstitution with the specified amount of purified
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`water.
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`the
`liquid dosage form of erythromycin,
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`result being Erythrornycin Estolate Oral
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`Suspension, USP. Use of insoluble forms of
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`drugs in suspensions greatly reduces the
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`difficult taste-masking problems of devel-
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`opmental pharmacists, and selection of the
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`flavorants to be used in a given suspension
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`may be based on taste preference rather
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`than on a particular f1avorant’s ability to
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`mask an unpleasant taste. For the most part,
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`oral suspensions are aqueous preparations
`with the vehicle flavored and sweetened to
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`suit the anticipated taste preferences of the
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`intended patient.
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`Features Desired in a
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`Pharmaceutical Suspension
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`There are many considerations in the devel-
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`opment and preparation of a pharmaceu-
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`tically elegant suspension.
`In addition to
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`therapeutic efficacy, chemical stability of the
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`components of the formulation, permanency
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`of the preparation, and aesthetic appeal of
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`the preparation—desirable qualities in all
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`pharmaceutical preparations~—a few other
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`features apply more specifically to the phar-
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`maceutical suspension:
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`1. A properly prepared pharrnaceutical sus-
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`pension should settle slowly and should
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`FIGURE 1 4.1 (.'ornmercial oral suspension.
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`in administration of a range of doses. This
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`is particularly advantageous for
`infants,
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`children, and the elderly. The disadvantage
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`of a disagreeable taste of certain drugs in
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`solution form is overcome when the drug
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`is administered as undissolved particles of
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`an oral suspension. In fact, chemical forms
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`of certain poor—tasting drugs have been
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`specifically developed for their insolubil-
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`ity in a desired vehicle for the sole pur-
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`pose of preparing a palatable liquid dosage
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`form. For example, erythromycin estolate
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`is a less water-soluble ester form of eryth-
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`romycin and is used to prepare a palatable
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`Page 5 of 64
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`Page 5 of 64
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`

`

`448
`
`SECTION VI - LIQUID DOSAGE FORMS
`
`be readily redispersed upon gentle shal<-
`ing of the container.
`. The particle size of the suspensoid should
`remain fairly constant throughout long
`periods of undisturbed standing.
`. The suspension should pour readily and
`.
`.
`evenly from its container.
`
`suspension,
`These main features of a
`which depend on the nature of the dis-
`persed phase, the dispersion medium, and
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`pharmaceutical adjuncts, will be discussed
`briefly.
`
`Sedimentation Rate Of the
`Particles Of (J Suspension
`
`The various factors involved in the rate of
`
`settling of the particles of a suspension are
`embodied in the equation of Stokes law,
`which is presented inthe Physical Pharmacy
`Capsule 14.2.
`
`PHYSICAL PHARMACY CAPSULE 14.2
`
`Sedimentation Rate and Stokes Equation
`
`fi=d’(n-99):;
`dt
`1811
`
`Stokes equation:
`
`where
`
`dx/dt is the rate of settling.
`d is the diameter of the particles.
`p, is the density of the particle.
`pa is the density of the medium.
`g is the gravitational constant, and
`n is the viscosity of the medium.
`
`A number of factors can be adjusted to enhance the physical stability of a suspension.
`including the diameter of the particles and the density and viscosity of the medium.The effect
`of changing these is illustrated in the following example.
`
`EXAMPLE
`
`A powder has a density of 1.3 g/mL and an average particle diameter of 2.5 pg (assuming
`the particles to be spheres). According to the Stokes equation, this powder will settle In water
`(viscosity of 1 CP assumed) at this rate:
`
`(2.5><‘|0“')2 (1 .3—1.o) (980)
`I8 x 0.01
`
`=l.02 x l0“cm/s
`
`If the particle size of the powder is reduced to 0.25 pm and water is still used as the disper-
`sion medium, the powder will now settle at this rate:
`
`-6
`(2.5 x 1 04)’ (1 .3 — 1.o)(9ao) _
`wxom
`—l.02xl0 cm/s
`
`As is evident, a decrease in particle size by a factor of 10 results in a reduction in the rate of
`settling by a factor of l00.This enhanced effect is a result of the d factor in the Stokes equation
`being squared.
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`Page 6 of 64
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`

`

`CHAPTER lél - DlSPE QSE SYSTEMS
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`449
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`PHYSICAL PHARMACY CAPSULE 14.2 CONT.
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`If a different dispersion medium, such as glycerin, is used in place ofwater. a further decrease
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`in settling will result. Glycerin has a density of 1.25 g)'rnL and a viscosity of 400 CF’. The larger
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`particle size powder (2.5 pm) will settle at this rate:
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`(2.5 x 1 04)? (1 .3 —i .25)(9s0)
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`'l8><4
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`= 4.25><lU“°cm/s
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`The smaller particte size (025 um) powder will now settle at this rate:
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`(2.5 ><l otif [1 .3— 1 .25}{9so)
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`l8><-fl
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`= 4.25x1o-‘° cmis
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`A summary of these results is shown in the following table:
`CONDITION
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`2.5 pm powder in water
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`0.25 pm powder in water
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`2.5 um powder in glycerin
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`0.25 pm powder in glycerin
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`RATE OF SETILING (OM13)
`
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`1.02 x TO“
`
`1.02 x 'lO'°
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`
`4.25 x 70'”
`
`
`4.25 x lU"°
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`As is evident from this table. a change in dispersion medium results in the greatest change
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`in the rate of settling of particles. Particle size reduction also can contribute significantly to sus-
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`pension stability. These factors are important in the formulation of physically stable suspensions.
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`The Stokes equation was derived for an
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`ideal situation in which uniform, perfectly
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`spherical particles in a very dilute suspen-
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`sion settle without producing turbulence,
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`without colliding with other particles of the
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`suspensoid, and without chemical or physi-
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`cal attraction or affinity for the dispersion
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`medium. Obviously,
`the Stokes equation
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`does not apply precisely to the usual pharma-
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`ceutical suspension in which the suspensoid
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`is irregularly shaped and of various particle
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`diameters, in which the fall of the particles
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`does result in both turbulence and collision,
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`and also in which the particles may have
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`some affinity for the suspension medium.
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`However, the basic concepts of the equation
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`do give a valid indication of the factors that
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`are important to suspension of the particles
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`and a clue to the possible adjustments that
`can be made to a formulation to decrease the
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`rate of sedimentation.
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`it
`is apparent that
`From the equation,
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`the velocity of fall of a suspended particle
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`is greater for larger particles than it is for
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`smaller particles, all other factors remaining
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`constant. Reducing the particle size of the
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`dispersed phase produces a slower rate of
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`descent of the particles. Also, the greater the
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`density of the particles, the greater the rate of
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`descent, provided the density of the vehicle
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`is not altered. Because aqueous vehicles are
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`used in pharmaceutical oral suspensions, the
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`density of the particles is generally greater
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`than that of the vehicle, a desirable feature.
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`If the particles were less dense than the
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`vehicle, they would tend to float, and float-
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`ing particles would be quite difficult to dis-
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`tribute uniformly in the vehicle. The rate of
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`sedimentation may be appreciably reduced
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`by increasing the viscosity of the dispersion
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`medium, and within limits of practicality,
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`this may be done. However, a product hav-
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`ing too high a viscosity is not generally desir-
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`able because it pours with difficulty and it is
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`equally difficult to redisperse the suspensoid.
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`Therefore,
`if the viscosity of a suspension
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`Page 7 of 64
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`Page 7 of 64
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`

`

`450
`
`SECTION VI - LIQUID DOSAGE FORMS
`
`is increased, it is done so only to a modest
`extent to avoid these difficulties.
`
`likely to become greatly larger or to form a
`solid cake upon standing.
`
`The viscosity characteristics of a suspen-
`sion may be altered not only by the vehicle
`used but also by the solid content. As the
`proportion of solid particles in a suspension
`increases, so does the viscosity. The viscos-
`ity of a pharmaceutical preparation may be
`determined through the use of a viscometer,
`such as a Brookfield viscometer, which mea-
`
`sures viscosity by the force required to rotate
`a spindle in the fluid being tested (Fig. 14.3).
`For the most part, the physical stability of
`a pharmaceutical suspension appears to be
`most appropriately adjusted by an alteration
`in the dispersed phase rather than through
`great changes in the dispersion medium. In
`most instances, the dispersion medium sup-
`ports the adjusted dispersed phase. These
`adjustments are concerned mainly with par-
`ticle size, uniformity of particle size, and sep-
`aration of the particles so that they are not
`
`Physical Features of the Dispersed
`Phase of a Suspension
`
`Probably the most important single consid-
`eration in a discussion of suspensions is the
`size of the particles. In most good pharma-
`ceutical suspensions, the particle diameter is
`1 to 50 pm.
`Generally, particle size reduction is accom-
`plished by dry milling prior to incorporation
`of the dispersed phase into the dispersion
`medium. One of the most rapid, conve-
`nient, and inexpensive methods of produc-
`ing fine drug powders of about 10 to 50 um
`size is micropulverization. Micropulverizers
`are high-speed attrition or impact mills that
`are efficient in reducing powders to the size
`acceptable for most oral and topical suspen-
`sions. For still finer particles, under 10 um,
`
`Synchronous motor
`
`Speed selector knob
`
`On—off toggle switch
`
`Clutch lever
`
`Knurled nut
`
`Handle
`
`Polnter
`
`Jewel bearing support /‘
`
`Spindle coupling nut
`
`Immersion mark
`
`Spindle body
`
`Gear train
`
`Circular
`bubble level
`
`Dial
`
`Calibrated
`spiral spring
`
`Upper shalt
`
`spindle guard
`
`Sample
`container
`
`FIGURE l4.3 The Brookfield viscometer. (Courtesy of Brooklield Engineering
`Laboratories.)
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`Page 8 of 64
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`

`

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`fluid energy grinding, sometimes referred to
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`as jet milling or micronizing, is quite effective.
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`By this process, the shearing action of high-
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`velocity compressed airstreams on the parti-
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`cles in a confined space produces the desired
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`ultrafine or micronized particles. The par-
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`ticles to be micronized are swept into vio-
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`lent turbulence by the sonic and supersonic
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`velocities of
`the airstreams. The particles
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`are accelerated to high velocities and collide
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`with one another, resulting in fragmentation.
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`This method may be employed when the
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`particles are intended for parenteral or oph—
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`thalmic suspensions. Particles of extremely
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`small dimensions may also be produced by
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`spray drying. A spray dryer is a cone—shaped
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`apparatus into which a solution of a drug is
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`sprayed and rapidly dried by a current of
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`warm, dry air circulating in the cone. The
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`resulting dry powder is collected. It is not
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`possible for a pharmacist to achieve the same
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`degree of particle size reduction with such
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`comminuting equipment as the mortar and
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`pestle. However, many micronized drugs are
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`commercially available to the pharmacist in
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`bulk, such as progesterone.
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`the
`As shown by the Stokes equation,
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`reduction in the particle size of a suspensoid
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`is beneficial to the stability of the suspen-
`sion because the rate of sedimentation of the
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`solid particles is reduced as the particles are
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`decreased in size. The reduction in particle size
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`produces slow, more uniform rates of settling.
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`However, one should avoid reducing the par-
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`ticle size too much because fine particles have
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`a tendency to form a compact cake upon set-
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`tling to the bottom of the container. The result
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`may be that the cake resists breakup with
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`shaking and forms rigid aggregates of parti-
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`cles that are larger and less suspendable than
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`the original suspensoid. The particle shape
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`of the suspensoid can also affect caking and
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`product stability. It has been shown that sym-
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`metrical barrel~shaped particles of calcium
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`carbonate produced more stable suspensions
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`than did asymmetrical needle—shaped par-
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`ticles of the same agent. The needle—shaped
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`particles formed a tenacious sediment cake
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`on standing that could not be redistributed,
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`whereas the barrel—shaped particles did not
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`cake upon standing (1).
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`
`
`CHAPTER M - DISPERSE SYSTEMS
`
`
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`
`451
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`To avoid formation of a cake, it is neces-
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`sary to prevent agglomeration of the particles
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`into larger crystals or into masses. One com-
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`mon method of preventing rigid cohesion of
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`small particles of a suspension is intentional
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`formation of a less rigid or loose aggregation
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`of the particles held together by compara~
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`tively weak particle-to—particle bonds. Such
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`an aggregation of particles is termed a floc or
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`a floccule, with flocculated particles forming
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`a type of lattice that resists complete settling
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`(although flocs settle more rapidly than fine,
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`individual particles) and thus are less prone
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`to compaction than unflocculated particles.
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`The flocs settle to form a higher sediment vol-
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`ume than unflocculated particles, the loose
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`structure of which permits the aggregates to
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`break up easily and distribute readily with a
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`small amount of agitation.
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`There are several methods of preparing
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`flocculated suspensions, the choice depend-
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`ing on the type of drug and the type of prod-
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`uct desired. For instance, in the preparation
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`of an oral suspension of a drug, clays such
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`as diluted bentonite magma are commonly
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`employed as the flocculating agent. The
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`structure of the bentonite magma and of
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`other clays used for this purpose also assists
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`the suspension by helping to support the
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`floc once formed. When clays are unsuit-
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`able as agents, as in a parenteral suspension,
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`frequently a floc of the dispersed phase can
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`be produced by an alteration in the pH of
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`the preparation (generally to the region of
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`minimum drug solubility). Electrolytes can
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`also act as flocculating agents, apparently by
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`reducing the electrical barrier between the
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`particles of the suspensoid and forming a
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`bridge so as to link them together. The care-
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`fully determined concentration of nonionic
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`and ionic surface—active agents (surfactants)
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`can also induce flocculation of particles in
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`suspension and increase the sedimentation
`volume.
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`Dispersion Medium
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`Oftentimes, as with highly flocculated sus-
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`pensions, the particles of a suspension settle
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`too rapidly to be consistent with what might
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`be termed a pharmaceutically elegant prepa~
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`ration. The rapid settling hinders accurate
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`Page 9 of 64
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`Page 9 of 64
`
`

`

`452
`
`SECTION Vi - LIQUID DOSAGE FORMS
`
`measurement of dosage and, from an aes-
`thetic point of view, produces too unsightly
`a supernatant layer. In many commercial
`suspensions, suspending agents are added to
`the dispersion medium to lend it structure.
`Carboxymethylcellulose (CMC), methylce1-
`lulose, microcrystalline cellulose, polyvinyl-
`pyrrolidone, xanthan gum, and bentonite are
`a few of the agents employed to thicken the
`dispersion medium and help suspend the
`suspensoid. When polymeric substances and
`hydrophilic colloids are used as suspending
`
`agents, appropriate tests must be performed
`to show that the agent does not interfere with
`availability of the drug. These materials can
`bind certain medicinal agents,
`rendering
`them unavailable or only slowly available for
`therapeutic function. Also, the amount of the
`suspending agent must not be such to render
`the suspension too viscous to agitate (to dis-
`tribute the suspensoid) or to pour. The study
`of flow characteristics is rheology. A sum-
`mary of the concepts of rheology is found in
`Physical Pharmacy Capsule 14.3.
`
`PHYSICAL PHARMACY CAPSULE 14.3
`
`Rheology
`
`Rheology, the study of flow, addresses the viscosity characteristics of powders, fluids. and
`semisolids. Materials are divided into two general categories, Newtonian and non-Newtonian,
`depending on their flow characteristics. Newtonian flow is characterized by constant viscosity,
`regardless of the shear rates applied. Non-Newtonian flow is characterized by a change in
`viscosity characteristics with increasing shear rates. Non-Nevvtonian flow includes plastic, pseu-
`doplastic, and dilatant flow.
`The Newton law of flow relates parallel layers of liquid: with the bottom layer fixed, when a
`force is placed on the top layer. the top plane moves at constant velocity, and each lower layer
`moves with a velocity directly proportional to its distance from the stationary bottom layer. The
`velocity gradient, or rate of shear (dv/dr), is the difference of velocity dv between two planes of
`liquid separated by the distance clr. The force (F’/A) applied to the top layer that is required to
`result in flow (rate of shear, G) is called the shearing stress (F). The relationship can be expressed:
`
`dv
`F’
`A ndr
`
`where 11 is the viscosity coefficient or viscosity.This relationship is often written:
`
`"=6
`
`where
`
`F = F’/A and
`G = dV/d|'.
`
`The higher the viscosity of a liquid, the greater the shearing stress required to produce a cer-
`tain rate of shear.A plot of F versus G yields a rheogram.A Newtonian fluid will plot as a straight
`line with the slope of the line being 11. The unit of viscosity is the poise. the shearing force
`required to produce a velocity of l cm/s between two parallel planes of liquid, each 1 cm? in
`area and separated by a distance of l cm.The most convenient unit to use is the centipoise,
`or cP (equivalent to 0.01 poise).
`These basic concepts can be illustrated in the following two graphs.
`
`Page 10 of 64
`
`

`

`CHAPTER 14 - DISPERSE SYSTEMS
`
`453
`
`PHYSICAL PHARMACY CAPSULE 14.3 CONT.
`
`RateofShear
`
`Shearing Stress
`
`Shear Rate
`
`EXAMPLE I
`What is the shear rate when an oil is rubbed into the skin with a relative rate of motion between
`
`the fingers and the skin of about 10 cm/ s and the film thickness is about 0.02 cm?
`-I
`10cm/s _
`0.02
`' 500 S
`
`G:
`
`The viscosity of Newtonian materials can be easily determined using a capillary viscometer,
`such as the Ostwald pipette, and the following relationship:
`
`where
`
`n’ = ktd
`
`n’ is viscosity;
`k is a coefficient, including such factors as the radius and length of the capillary, volume of
`the liquid flowing. pressure head, and so on;
`t is time: and
`
`d is density of the material.
`
`The official compendia, the USP and NF. use kinematic viscosity, the absolute viscosity divided
`by the density of the liquid, as follows:
`
`Kinematic viscosity = n’ / p
`
`The relative viscosity of a liquid can be obtained by using a capillary viscometer a