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Edit Comment Close Premium member Presentation Transcript Slide 1: NOYES WHITNEY DISSOLUTION RATE Prepared by Chintan Akabari Guided by B.G. Prajapati Slide 2: Dissolution Dissolution is a process in which a solid substance solubilize in a given solvent i.e mass transfer from the solid surface to the liquid phase. Slide 3: THEORIES TO EXPLAIN DRUG DISSOLUTION 1.Dissolution layer model or film theory 2.Danckwert’s Model or Surface Renewal Theory 3.Interfacial Barrier Model or Limited Solvation Theory Slide 4: Simplest and most common theory for dissolution The process of dissolution of solid particle in a liquid, in the absence of reactive or chemical force Consists of two consecutive stages: STAGE 1:First is an interfacial reaction that results in the liberation of solute molecules from the solid phase. This involves a phase change so that molecules of solid become molecules of solute in the solvent in which the crystal is dissolving.The solution in contact with the solute will be saturated(because it is in direct contact with undissolved solid. Its concentration will be Cs , a saturated solution. In short solution of the solute form a thin film or layer at the solid/liquid interface called as stagnant layer or diffusion layer or boundary layer This step is usually rapid. DIFFUSION LAYER MODEL Slide 5: STAGE 2: After this , the solute molecules must migrate through the boundary layers surrounding the crystal to the bulk of the solution,at which time its concentration will be Cb. This step involves the transfer of these molecules away from the solid liquid interface into the bulk of the liquid phase under the influence of diffusion or convection. This step is slow step. Slide 6: BOUNDARY LAYERS Boundary layers are static or slow moving layers of liquid that surround all wetted solid surfaces. The rate of flow of fluid over an even surface will be dependant upon the distance from the surface. The velocity, which will be almost zero at the surface , increases with the increasing distance from the surface until the bulk of the fluid is reached and the velocity becomes constant. In short, the region over which differences in velocity are observed is referred to as boundary layers. Its depth is dependant upon the viscosity of the fluid and the rate of flow in the bulk fluid. If high viscosity and low flow rate then thick boundary. If low viscosity and high flow rate then thin boundary. Boundary layers arises because of the intermolecular forces between the liquid molecules and solid surface. They are important barrier for heat and mass transfer. Slide 7: Now,Mass transfer takes place more slowly trough these static or slow moving layers ,which inhibit the movement of solute molecules from the surface of the solid to the bulk of the solution. The concentration of the solution in the boundary layers changes therefore from being saturated (Cs) at the crystal surface to being equal to that of the bulk of the solution (Cb) at its outmost limit. Slide 8: RATE LIMITING STEP Like any reaction that involves consecutive stages, the overall rate of dissolution will depend on whichever of this steps is the slowest. Here the interfacial step-1 is rapid and step-2 is slower and so the rate of dissolution will be determined by the rate of slower step-2, of diffusion of dissolved solute across the static boundary layers of liquid that exists at a solid liquid interface. Slide 9: FICK’S SECOND LOW OF DIFFUSION Noyes Whitney equation based on Fick’s second low of diffusion. The rate of diffusion will obey fick’s law of diffusion, i.e the rate of change in concentration of dissolved material with time it directly proportional to the concentration difference between the two sides of diffusion layer. i.e, dc/dt C----------(1) dc/dt = kC--------------(2) where the constant k is the rate constant(sec-1) and C is the difference in concentration of solution at solid surface(Cs) and the bulk of the solution(Cb). So, dc/dt = k(Cs-Cb)--------------(3) Slide 10: MODIFIED NOYES WHITNEY EQATION It was developed to defined the dissolution from a single spherical particle. It is based on the Fick’s fist law of diffusion. The rate of mass transfer of solute molecules or ions through a static diffusion layer (dm/dt) is directly proportional to the surface area available for molecule or ionic migration (A) , the concentration difference (Cs-Cb) across the boundary layer,is inversely proportional to the thickness of the boundary layer (h). Where, dm/dt =Rate of mass transfer D=diffusion coefficient(m2/s) A=surface area Kw/o=water/oil partition coefficient of the drug (Cs-Cb)=concentration gradient V=volume of dissolution media h=Thickness of boundary layer Slide 11: Dissolution of drug particles according to diffusion layer model Slide 12: Above Equation represents first order dissolution process ,the driving force for which is the concentration gradient (Cs-Cb). Under such a situation , dissolution is said to be under non sink conditions. This is true in case of in vitro dissolution in a limited dissolution medium. Dissolution in such a situation slow down after sometime due to build up in the concentration of drug in the bulk of solution . Sink condition The in vivo dissolution is always rapid than in vitro dissolution because the moment the drug dissolves, it is absorbed into the systemic circulation. As a result Cb=0 and dissolution is at its maximum. Thus, under in vivo conditions, there is no concentration build up in the bulk of the drug i.e Cs>>Cb and sink condition are maintained. Non sink condition Slide 13: Under sink condition ,If the volume and surface area of solid are kept constant so, above equation reduce to dc/dt = k -----------(4) Equation (4) represents that the dissolution rate is constant under sink conditions and follows zero order kinetics i.e yield linear plot Conc. of dissolved drug Time Zero order dissolution under sink condition First order dissolution under non sink condition Dissolution rate under non sink and sink condition Slide 14: To obtained good in vitro in vivo dissolution rate correlation, the in vitro dissolution must be carried under sink condition,this can be achieved by: Bathing the dissolving solid in fresh solvent from time to time. Increasing the volume of dissolution. Removing the drug by partitioning it from the aqueous phase of the dissolution fluid into an organic phase placed either above or below the dissolution fluid –for example ,hexane or chloroform. Adding a water miscible solvent such as alcohol to the dissolution fluid, or By adding selected adsorbents to remove the dissolved drug. The in vitro sink conditions are so maintained that Cb is always less than 10% of CS Slide 15: Problem 1: Calculate the dissolution rate of a hydrophobic drug having the following physicochemical characteristics: surface area = 2.5 x 103 cm2 saturated solubility = 0.35 mg/mL (at room temperature) diffusion coefficient = 1.75 x 10-7 cm2/s thickness of diffusion layer = 1.25 µm [Note: need to convert to cm, so 1 µm = 1 x 10-4 cm and 1.25 x 10-4 cm] conc of drug in bulk = 2.1 x 10-4 mg/mL dM = DS(CS-Cb) dt h dM = (1.75 x 10-7)(2.5 x 103)( 0.35 mg/mL - 2.1 x 10-4 mg/mL) dt 1.25 x 10-4 dM = (1.75 x 10-7)(2.5 x 103)( 0.349 mg/mL) dt 1.25 x 10-4 dM = (1.75 x 10-7)(2.5 x 103)( 0.35 mg/mL - 2.1 x 10-4 mg/mL) dt 1.25 x 10-4 dM = 1.53 x 10-4 = 1.22 mg/sec dt 1.25 x 10-4 Factor affecting in-vitro dissolution rate of solid in liquid : Factor affecting in-vitro dissolution rate of solid in liquid 1] Diffusion Co-efficient - greater the value faster the dissolution - Affected by viscosity of dissolution medium and size of diffusing molecules. - Diffusion decreases as the viscosity of the dissolution medium increases. 2] SURFACE AREA - Greater the surface area faster the dissolution @ AFFECTED BY : 1) Size of solid particles - A1/Particle size. Particle size will change during dissolution process, because large particle will become smaller and small particles will disappear. 2) Dispersibility of powdered solid in dissolution medium. - If particles tend to form coherent mass in the dissolution medium then the surface area available for dissolution is reduced.This effect may be overcome by addition of wetting agent. 3) Porosity of solid particle. - Pores must be large enough to allow access of dissolution medium and outward diffusion of dissolved solute molecules. Slide 17: 3] Water oil partition co-efficient : Higher the value , more the hydrophiliciity and faster the dissolution in aqueous fluid. 4] Concentration gradient : Greater the concentration gradient , faster the diffusion and drug dissolution. Can be increased by increasing drug solubility and volume of dissolution media. This is affected by : 4.1) Volume of dissolution media -If volume is small Cb will approach Cs,if volume is large Cb may be negligible WRT Cs i.e apparent sink condition. 4.2) Temperature -Dissolution may be an exothermic or endothermic process. -According to thermodynamic equation, G= H- T S -When H is positive,the dissolution process is endothermic i.e heat is absorbed when dissolution occurs. -In endothermic process, a rise in temperature will lead to an increase in the solubility of the solid with positive heat of solution. -In exothermic dissolution,an increase in temp will lead to decrease in solubility. Solubility curve are often used to describe the effect of temperature.For e.g. sodium Sulphate exist as the decahydrate form up to 32.5°C, and its dissolution in water is endothermic process. Its solubility therefore increases with rise in temp up to 32.5°C. Above this temp it is converted in to anhydrous form and the dissolution of this is an Exothermic process. The solubility therefore exhibits a change from +ve to –ve slope as the temp exceed the transition value. Slide 18: 4.3) Nature of dissolution media. -Co solvents -Some mixtures are used to increase the solubility of solid.This is achieved by using co solvents such as ethanol or propylene glycol, which are miscible with water and which act as better solvent for solute. -For e.g..the aqueous solubility of metronidazole is about 100mg in 10 ml.The solubility of this drug can be increased by the incorporation of water miscible co solvent to the 500mg in 10 ml. Slide 19: - pH -If the pH of the solution of either weakly acidic drug or a salt of a drug is reduced then the proportion of unionized molecules in the solution increases. -Precipitation may occur because the solubility of the unionized species is less than that of ionized form. In case of solution of weakly basic drugs or their salts,precipitation is favoured by an increase in pH. 4.4) Molecular structure of solid. -A small change in the molecular structure of the compound can have a marked effect on its solubility in a given liquid For e.g.. The introduction of hydrophilic hydroxyl group can produce a large improvement in water solubility as evidenced by more than 100 fold difference in the solubility of phenol and benzene. -The conversion of weak acid to its sodium salt led to a much greater degree of ionic dissociation of the compound.A specific e.g. of this effect is provided by comparison of aqueous solubility of salicylic acid and its sodium salt,which are 1:550 and 1:1 respectively 5] Thickness of stagnant layer : more the thickness , lesser the diffusion and drug dissolution. Can be decreased by increasing agitation. Affected by degree of agitation , which depends on speed of stirring and shaking , shape , size , and position of stirrer , volume of dissolution medium , shape and size of container, viscosity of dissolution medium. References : References Pharmaceutics_the science of dosage form design , second edition by M.E.Aulton Remington_The science and practice of pharmacy , 21st edition , volume 1 Biopharmaceutics and pharmacokinetics by D.M.Bramankar and Sunil B. Jaiswal Martin’physical pharmacy and pharmaceutical science ,5th edition www.wikipidea.com Slide 21: THANK YOU You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.
NOYES WHITNEY DISSOLUTION RATE chintan akabarichintan Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 2614 Category: Entertainment License: All Rights Reserved Like it (1) Dislike it (0) Added: August 07, 2010 This Presentation is Public Favorites: 1 Presentation Description No description available. Comments Posting comment... By: nikhilbhowalu (15 month(s) ago) Hi sir ur ppt is excelent.. plz sent me copy of yr ppt on my email bhowalu.nikhil@rocket mail.com Thank yuu sir Saving..... Post Reply Close Saving..... Edit Comment Close By: mouni.568 (15 month(s) ago) hai,....sir ur ppt is excelent .....plz allow me to download ur ppt or send it on my mail mouni.568@gmail.com Saving..... Post Reply Close Saving..... Edit Comment Close By: zareenkhan (17 month(s) ago) hai,....sir ur ppt is excelent .....plz allow me to download ur ppt or send it on my mail mahi.krazy7@gmail.com Saving..... Post Reply Close Saving..... Edit Comment Close By: jay_143143 (17 month(s) ago) Hi chintan, Jalaram Thakkar here, I want your PPT of NOYES WHITNEY DISSOLUTION RAT. Please allow me to download or send it on my mail jay_143143@yahoo.com Saving..... Post Reply Close Saving..... Edit Comment Close By: rhtsarda (19 month(s) ago) hi, your presentation is really nice. i want your presentation for explaining the phenomenon in class. please send it to me. email: rhtsarda@gmail.com Saving..... Post Reply Close Saving..... Edit Comment Close Premium member Presentation Transcript Slide 1: NOYES WHITNEY DISSOLUTION RATE Prepared by Chintan Akabari Guided by B.G. Prajapati Slide 2: Dissolution Dissolution is a process in which a solid substance solubilize in a given solvent i.e mass transfer from the solid surface to the liquid phase. Slide 3: THEORIES TO EXPLAIN DRUG DISSOLUTION 1.Dissolution layer model or film theory 2.Danckwert’s Model or Surface Renewal Theory 3.Interfacial Barrier Model or Limited Solvation Theory Slide 4: Simplest and most common theory for dissolution The process of dissolution of solid particle in a liquid, in the absence of reactive or chemical force Consists of two consecutive stages: STAGE 1:First is an interfacial reaction that results in the liberation of solute molecules from the solid phase. This involves a phase change so that molecules of solid become molecules of solute in the solvent in which the crystal is dissolving.The solution in contact with the solute will be saturated(because it is in direct contact with undissolved solid. Its concentration will be Cs , a saturated solution. In short solution of the solute form a thin film or layer at the solid/liquid interface called as stagnant layer or diffusion layer or boundary layer This step is usually rapid. DIFFUSION LAYER MODEL Slide 5: STAGE 2: After this , the solute molecules must migrate through the boundary layers surrounding the crystal to the bulk of the solution,at which time its concentration will be Cb. This step involves the transfer of these molecules away from the solid liquid interface into the bulk of the liquid phase under the influence of diffusion or convection. This step is slow step. Slide 6: BOUNDARY LAYERS Boundary layers are static or slow moving layers of liquid that surround all wetted solid surfaces. The rate of flow of fluid over an even surface will be dependant upon the distance from the surface. The velocity, which will be almost zero at the surface , increases with the increasing distance from the surface until the bulk of the fluid is reached and the velocity becomes constant. In short, the region over which differences in velocity are observed is referred to as boundary layers. Its depth is dependant upon the viscosity of the fluid and the rate of flow in the bulk fluid. If high viscosity and low flow rate then thick boundary. If low viscosity and high flow rate then thin boundary. Boundary layers arises because of the intermolecular forces between the liquid molecules and solid surface. They are important barrier for heat and mass transfer. Slide 7: Now,Mass transfer takes place more slowly trough these static or slow moving layers ,which inhibit the movement of solute molecules from the surface of the solid to the bulk of the solution. The concentration of the solution in the boundary layers changes therefore from being saturated (Cs) at the crystal surface to being equal to that of the bulk of the solution (Cb) at its outmost limit. Slide 8: RATE LIMITING STEP Like any reaction that involves consecutive stages, the overall rate of dissolution will depend on whichever of this steps is the slowest. Here the interfacial step-1 is rapid and step-2 is slower and so the rate of dissolution will be determined by the rate of slower step-2, of diffusion of dissolved solute across the static boundary layers of liquid that exists at a solid liquid interface. Slide 9: FICK’S SECOND LOW OF DIFFUSION Noyes Whitney equation based on Fick’s second low of diffusion. The rate of diffusion will obey fick’s law of diffusion, i.e the rate of change in concentration of dissolved material with time it directly proportional to the concentration difference between the two sides of diffusion layer. i.e, dc/dt C----------(1) dc/dt = kC--------------(2) where the constant k is the rate constant(sec-1) and C is the difference in concentration of solution at solid surface(Cs) and the bulk of the solution(Cb). So, dc/dt = k(Cs-Cb)--------------(3) Slide 10: MODIFIED NOYES WHITNEY EQATION It was developed to defined the dissolution from a single spherical particle. It is based on the Fick’s fist law of diffusion. The rate of mass transfer of solute molecules or ions through a static diffusion layer (dm/dt) is directly proportional to the surface area available for molecule or ionic migration (A) , the concentration difference (Cs-Cb) across the boundary layer,is inversely proportional to the thickness of the boundary layer (h). Where, dm/dt =Rate of mass transfer D=diffusion coefficient(m2/s) A=surface area Kw/o=water/oil partition coefficient of the drug (Cs-Cb)=concentration gradient V=volume of dissolution media h=Thickness of boundary layer Slide 11: Dissolution of drug particles according to diffusion layer model Slide 12: Above Equation represents first order dissolution process ,the driving force for which is the concentration gradient (Cs-Cb). Under such a situation , dissolution is said to be under non sink conditions. This is true in case of in vitro dissolution in a limited dissolution medium. Dissolution in such a situation slow down after sometime due to build up in the concentration of drug in the bulk of solution . Sink condition The in vivo dissolution is always rapid than in vitro dissolution because the moment the drug dissolves, it is absorbed into the systemic circulation. As a result Cb=0 and dissolution is at its maximum. Thus, under in vivo conditions, there is no concentration build up in the bulk of the drug i.e Cs>>Cb and sink condition are maintained. Non sink condition Slide 13: Under sink condition ,If the volume and surface area of solid are kept constant so, above equation reduce to dc/dt = k -----------(4) Equation (4) represents that the dissolution rate is constant under sink conditions and follows zero order kinetics i.e yield linear plot Conc. of dissolved drug Time Zero order dissolution under sink condition First order dissolution under non sink condition Dissolution rate under non sink and sink condition Slide 14: To obtained good in vitro in vivo dissolution rate correlation, the in vitro dissolution must be carried under sink condition,this can be achieved by: Bathing the dissolving solid in fresh solvent from time to time. Increasing the volume of dissolution. Removing the drug by partitioning it from the aqueous phase of the dissolution fluid into an organic phase placed either above or below the dissolution fluid –for example ,hexane or chloroform. Adding a water miscible solvent such as alcohol to the dissolution fluid, or By adding selected adsorbents to remove the dissolved drug. The in vitro sink conditions are so maintained that Cb is always less than 10% of CS Slide 15: Problem 1: Calculate the dissolution rate of a hydrophobic drug having the following physicochemical characteristics: surface area = 2.5 x 103 cm2 saturated solubility = 0.35 mg/mL (at room temperature) diffusion coefficient = 1.75 x 10-7 cm2/s thickness of diffusion layer = 1.25 µm [Note: need to convert to cm, so 1 µm = 1 x 10-4 cm and 1.25 x 10-4 cm] conc of drug in bulk = 2.1 x 10-4 mg/mL dM = DS(CS-Cb) dt h dM = (1.75 x 10-7)(2.5 x 103)( 0.35 mg/mL - 2.1 x 10-4 mg/mL) dt 1.25 x 10-4 dM = (1.75 x 10-7)(2.5 x 103)( 0.349 mg/mL) dt 1.25 x 10-4 dM = (1.75 x 10-7)(2.5 x 103)( 0.35 mg/mL - 2.1 x 10-4 mg/mL) dt 1.25 x 10-4 dM = 1.53 x 10-4 = 1.22 mg/sec dt 1.25 x 10-4 Factor affecting in-vitro dissolution rate of solid in liquid : Factor affecting in-vitro dissolution rate of solid in liquid 1] Diffusion Co-efficient - greater the value faster the dissolution - Affected by viscosity of dissolution medium and size of diffusing molecules. - Diffusion decreases as the viscosity of the dissolution medium increases. 2] SURFACE AREA - Greater the surface area faster the dissolution @ AFFECTED BY : 1) Size of solid particles - A1/Particle size. Particle size will change during dissolution process, because large particle will become smaller and small particles will disappear. 2) Dispersibility of powdered solid in dissolution medium. - If particles tend to form coherent mass in the dissolution medium then the surface area available for dissolution is reduced.This effect may be overcome by addition of wetting agent. 3) Porosity of solid particle. - Pores must be large enough to allow access of dissolution medium and outward diffusion of dissolved solute molecules. Slide 17: 3] Water oil partition co-efficient : Higher the value , more the hydrophiliciity and faster the dissolution in aqueous fluid. 4] Concentration gradient : Greater the concentration gradient , faster the diffusion and drug dissolution. Can be increased by increasing drug solubility and volume of dissolution media. This is affected by : 4.1) Volume of dissolution media -If volume is small Cb will approach Cs,if volume is large Cb may be negligible WRT Cs i.e apparent sink condition. 4.2) Temperature -Dissolution may be an exothermic or endothermic process. -According to thermodynamic equation, G= H- T S -When H is positive,the dissolution process is endothermic i.e heat is absorbed when dissolution occurs. -In endothermic process, a rise in temperature will lead to an increase in the solubility of the solid with positive heat of solution. -In exothermic dissolution,an increase in temp will lead to decrease in solubility. Solubility curve are often used to describe the effect of temperature.For e.g. sodium Sulphate exist as the decahydrate form up to 32.5°C, and its dissolution in water is endothermic process. Its solubility therefore increases with rise in temp up to 32.5°C. Above this temp it is converted in to anhydrous form and the dissolution of this is an Exothermic process. The solubility therefore exhibits a change from +ve to –ve slope as the temp exceed the transition value. Slide 18: 4.3) Nature of dissolution media. -Co solvents -Some mixtures are used to increase the solubility of solid.This is achieved by using co solvents such as ethanol or propylene glycol, which are miscible with water and which act as better solvent for solute. -For e.g..the aqueous solubility of metronidazole is about 100mg in 10 ml.The solubility of this drug can be increased by the incorporation of water miscible co solvent to the 500mg in 10 ml. Slide 19: - pH -If the pH of the solution of either weakly acidic drug or a salt of a drug is reduced then the proportion of unionized molecules in the solution increases. -Precipitation may occur because the solubility of the unionized species is less than that of ionized form. In case of solution of weakly basic drugs or their salts,precipitation is favoured by an increase in pH. 4.4) Molecular structure of solid. -A small change in the molecular structure of the compound can have a marked effect on its solubility in a given liquid For e.g.. The introduction of hydrophilic hydroxyl group can produce a large improvement in water solubility as evidenced by more than 100 fold difference in the solubility of phenol and benzene. -The conversion of weak acid to its sodium salt led to a much greater degree of ionic dissociation of the compound.A specific e.g. of this effect is provided by comparison of aqueous solubility of salicylic acid and its sodium salt,which are 1:550 and 1:1 respectively 5] Thickness of stagnant layer : more the thickness , lesser the diffusion and drug dissolution. Can be decreased by increasing agitation. Affected by degree of agitation , which depends on speed of stirring and shaking , shape , size , and position of stirrer , volume of dissolution medium , shape and size of container, viscosity of dissolution medium. References : References Pharmaceutics_the science of dosage form design , second edition by M.E.Aulton Remington_The science and practice of pharmacy , 21st edition , volume 1 Biopharmaceutics and pharmacokinetics by D.M.Bramankar and Sunil B. Jaiswal Martin’physical pharmacy and pharmaceutical science ,5th edition www.wikipidea.com Slide 21: THANK YOU