Surface Tension and interfacial phenomen


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Slide 1: 

1 Surface and Interfacial Phenomena Prepared by: Mr. jitendra patel

Slide 2: 

2 Interface is the boundary between two or more phases exist together The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interfacial phase. Several types of interface can exist depending on whether the two adjacent phases are in solid, liquid or gaseous state. Important of Interfacial phenomena in pharmacy: Adsorption of drugs onto solid adjuncts in dosage forms Penetration of molecules through biological membranes Emulsion formation and stability The dispersion of insoluble particles in liquid media to form suspensions. Interface

Slide 3: 

3 LIQUID INTERFACES Surface and Interfacial Tensions In the liquid state, the cohesive forces between adjacent molecules are well developed. For the molecules in the bulk of a liquid They are surrounded in all directions by other molecules for which they have an equal attraction. For the molecules at the surface (at the liquid/air interface) Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them. They can develop adhesive forces of attraction with the molecules of the other phase in the interface The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F .

Slide 4: 

4 To keep the equilibrium, an equal force must be applied to oppose the inward tension in the surface. Thus SURFACE TENSION [γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of dyne/cm INTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of dyne/cm. Invariably, interfacial tensions are less than surface tensions because an adhesive forces, between the two liquid phases forming the interface are greater than when a liquid and a gas phase exist together. If two liquids are completely miscible, no interfacial tension exists between them. Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST

Surface Tension : 

Surface Tension Surface tension as a force/ unit length can also be explained by means of 3 sided wire frame across which a movable bar is formed. A soap film is formed over the area of frame and can be stretched by applying a force f to the movable bar, length L. It acts against surface tension of soap film. When the force is removed the film will contract due its surface tension. So the surface tension is the function of the force applied to break the film over the length of movable bar. 5

Surface Tension : 

Surface Tension The soap film has two liq-gas interfaces, so the total length = 2 * length of bar Thus γ = fb/2L. 6

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Slide 8: 

8 Surface free energy is defined as the work required to increase the area of a liquid by 1 sq cm.

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SURFACE FREE ENERGY To move molecules from the inner layers to the surface, work needs to be done against the surface tension. Each molecules near to the surface posses more potential energy as compared to the molecules in the bulk of the liq. So if surface of liq increases, the energy of the liq also increases as energy is directly proportional to the size of the free surface. It is called surface free energy. 11


SURFACE FREE ENERGY Each molecules of the liq has a tendency to move inside the liq from the surface and with minimum surface free energy. For equilibrium, the surface free energy of a system must be at a minimum. Thus Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume. To increase the surface of the liq work must be done against the surface tension. γ = fb/2L. So fb = γ*2L 12

Slide 13: 

13 γ*2L*dx

Slide 14: 

14 The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2) erg = dyne . cm Its equivalent to the surface tension γ Thus the greater the area A of interfacial contact between the phases, the greater the free energy. W = γ ∆ A For equilibrium, the surface free energy of a system must be at a minimum. Thus Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume.

Slide 15: 

Surface tension is work per unit area to produce new surface. From thermodynamics surface tension at constant T and P, is increase in Gibbs free energy per unit area, 15 Γ = (dG/dA) T, P

Pressure Difference Across Curved Surface : 

Pressure Difference Across Curved Surface Surface tension can be also expressed in terms of pressure difference. Consider a soap bubble having a radius r. The total surface free energy W = 4πr2 γ. Suppose that bubble is caused to shrink so that its radius decreases by dr. Final surface free energy W = 4π(r-dr)2 γ. W = 4π r2γ- 8πγdr.r + 4πγ(dr)2 dr is very small compared to r. So the change in surface free energy is - 8πγdr.r 16

Pressure Difference Across Curved Surface : 

Pressure Difference Across Curved Surface The –ve sign indicate shrunken of the area. The opposite energy term depends on the pressure diff across the wall of the bubble. Pressure is force per unit area. So the work change brought about by decrease in radius dr is W = ΔP 4πr2 (-dr) At equilibrium this must equal change in surface free energy term and so - 8πγdr.r = ΔP 4πr2 (-dr) , ΔP =2γ/r As radius decreases P inside bubble increases 17

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Factors influencing Surface and Interfacial tension, : 

Factors influencing Surface and Interfacial tension, 19

Slide 20: 

20 Measurement of Surface and Inter­facial Tensions Methods for measuring surface and interfacial tension 1- Capillary rise method 2- Ring (Du Nouy) tensiometer 3- Drop weight method (Stalagmometer) 4- Bubble pressure The choice of the method for measuring surface and interfacial tension depend on: Whether surface or interfacial tension is to be determined. The accuracy desired The size of sample.

Slide 21: 

21 Capillary Rise Method When a capillary tube is placed in a liquid, it rises up the tube a certain distance. By measuring this rise, it is possible to determine the surface tension of the liquid. It is not possible, to obtain interfacial tensions using the capillary rise method. Cohesive force is the force existing between like mole­cules in the surface of a liquid Adhesive force is the force existing between unlike molecules, such as that between a liquid and the wall of a glass capillary tube When the force of Adhesion is greater than the cohesion, the liquid is said to wet the capillary wall, spreading over it, and rising in the tube. The Principle

Slide 22: 

22 If a capillary tube of inside radius =r immersed in a liquid that wet its surface, the liquid continues to rise in the tube due to the surface tension, until the upward movement is just balanced by the downward force of gravity due to the weight of the liquid a = γ cos Ө a = 2 π r γ cos Ө The upward component of the force resulting from the surface tension of the liquid at any point on the circumference is given by: Thus the total upward force around the inside circumference of the tube is Where Ө = the contact angle between the surface of the liquid and the capillary wall 2 π r = the inside circumference of the capillary. For water the angle Ө is insignificant, i.e. the liquid wets the capillary wall so that cos Ө = unity Cont. angle water and glass Cont. angle Mercury and glass

Slide 23: 

23 The downward force of gravity (mass x acceleration) is given by Where: π r 2 = the cross-sectional area h = the height of the liquid column to the lowest point of the meniscus (p – p o) = the difference in the density of the liquid p and its vapor po g = the acceleration of gravity w = the weight of the upper part of the meniscus. At Maximum height, the opposing forces are in equilibrium p o, Ө and w can usually be disregarded Hence the surface tension can be calculated. π r 2 h (p – p o) g + w 2 π r γ cos Ө = π r 2 h (p – p o) g + w 2 π r γ = π r 2 h p g γ = 1/2 r h p g

Slide 24: 

24 Ring (Du Nouy) Tensiometer the principle of the instrument depends on the fact that: the force necessary to detach a platinum-iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The force of detachment is recorded in dynes on a calibrated dial The surface tension is given by: Where: F = the detachment force R1 and R 2= the inner and outer radii of the ring. γ = F / 2 π (R1 + R2) For measuring surface and interfacial tensions. The principle

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25 If the volume or weight of a drop as it is detached from a tip of known radius is determined, the surface and interfacial tension can be calculated from Where m = the mass of the drop V = the volume of the drop p = the density of the liquid r = the radius of the tip g = the acceleration due to gravity Φ = a correction factor The correction factor is required as not all the drop leaves the tip on detachment The tip must be wetted by the liquid so as the drop doesn’t climb the outside of the tube. γ = Φ mg = Φ V pg 2 π r 2 π r Drop Weight and drop volume method

Slide 26: 

26 When a liquid such as oleic acid is placed on the surface of other liquid like water, it will spread as a film if the adhesion force is greater than the cohesive forces. The term film applies to a duplex film as opposed to monomolecular film. Duplex films are sufficiently thick so that surface and interface are independent of one another. Spreading coefficient

Slide 27: 

27 As surface or interfacial work is equal to surface tension multiplied by the area increment. The work of cohesion, which is the energy required to separate the molecules of the spreading liquid so as it can flow over the sub-layer= Where 2 surfaces each with a surface tension = γ L The work of adhesion, which is the energy required to break the attraction between the unlike molecules= Where: γ L =the surface tension of the spreading liquid γ S =the surface tension of the sub­layer liquid γ LS =the interfacial tension between the two liquids. Spreading occurs if the work of adhesion is greater than the work of cohesion, i.e. Wa > Wc or Wa - Wc > 0 Wc = 2 γ L Wa = γ L + γ S - γ LS

Slide 28: 

28 Spreading Coefficient is The difference between the work of adhesion and the work of cohesion S = Wa - Wc = (γ L + γ S - γ LS ) - 2 γ L S = γ S - γ L - γ LS S = γ S – (γ L + γ LS ) Spreading occurs (S is positive) when the surface tension of the sub-layer liquid is greater than the sum of the surface tension of the spreading liquid and the interfacial tension between the sub-layer and the spreading liquid. If (γ L + γ LS ) is larger than YS , (S is negative) the substance forms globules or a floating lens and fails to spread over the surface.

Spreading after equilibrium : 

Spreading after equilibrium After equilibrium water surface become saturated with spreading liq and spreading liq surface becomes saturated with water. If we use prime (‘) to denote values following equilibrium then new surface tension γs’,γL’. When mutual saturation has taken place, the spreading coefficient may be reduced or may become –Ve. It means although initial spreading of liq may occur, it can be followed by coalescence of excess material into a lens. The reversal of spreading take place when γs’, becomes less than (γLs’ +γL’). 29

Slide 30: 

30 Factor affecting Spreading Coefficient Molecular Structural: The greater the polarity of the molecule the more positive [S] as ethyl alcohol and propionic acid Non polar substances as Liquid petrolatum have negative [S] fail to spread on water For organic acids, as Oleic acid, the longer the carbon chain decrease in polar character decrease [S] Some oils can spread over water because they contain polar groups as COOH and OH Cohesive forces: Benzene spreads on water not because it is polar but because the cohesive forces between its molecules are much weaker than the adhesion for water.

Slide 31: 

31 Application of Spreading coefficient in pharmacy The requirement of film coats to be spreaded over the tablet surfaces. The requirement of lotions with mineral oils to spread on the skin by the addition of surfactants.

Slide 32: 

32 Adsorption at liquid Interface

Slide 33: 

ADSORPTION: It is surface effect. E.g. concentration of alkaloid molecule on the surface of clay. ABSORPTION: Gas or liq penetrates in to the capillary spaces of absorbing medium. The taking up of water by a sponge is absorption. 33

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Slide 35: 

35 A surfactant molecule is depicted schematically as a cylinder representing the hydrocarbon (hydrophobic) portion with a sphere representing the polar (hydrophilic) group attached at one end. The hydrocarbon chains are straight because rotation around carbon-carbon bonds bends, coils and twists them. Sodium Lauryl Sulfate molecule Surface Active Agents

Slide 36: 

36 Surface Active Agents Molecules and ions that are adsorbed at interfaces are termed surface active agents, surfactants or amphiphile The molecule or ion has a certain affinity for both polar and nonpolar solvents. Depending on the number and nature of the polar and nonpolar groups present, the amphiphile may be hydrophilic, lipophilic or be reasonably well-balanced between these two extremes. It is the amphiphilic nature of surface active agents which causes them to be adsorbed at interfaces, whether these be liquid/gas or liquid/liquid.

Slide 37: 

Surface active agent 12.8

Slide 38: 

38 A scale showing classification of surfactant function on the basis of HLB values of surfactants. The higher the HLB of a surfactant the more hydrophilic it is. Example: Spans with low HLB are lipophilic. Tweens with high HLB are hydrophilic. Hydrophilic Lipophilic Balance

Slide 39: 

39 Classification of Surface Active Agents Functional Classification According to their pharmaceutical use, surfactants can be divided into the following groups: Wetting agents Solubilizing agents Emulsifying agents Dispersing, Suspending and Defloculating agents Foaming and antifoaming agents Detergents

Slide 40: 

40 Hydrophilic Lipophilic Balance Determination of HLB 1.Polyhydric Alcohol Fatty Acid Esters (Ex. Glyceryl monostearate) HLB = 20 ( 1 – (S / A) ) 2.Surfactants with no Saponification no (Ex. Bees wax and lanolin) S = Saponification number of the ester A = Acid number of the fatty acid HLB =E + P / 5 E = The percent by weight of ethylene oxide P=The percent by weight of polyhydric alcohol group in the molecules Surfactants with hydrophilic portion have only polyoxyethylene (non-ionic surfactant) HLB =E / 5

Slide 41: 

3.The structure of the surfactant is divided in diff. component groups. Each group is assigned a number. The addition of these numbers for their respective groups permits the calculation of its HLB value. HLB = E(hydrophilic group number)-E(lipophilic group number)e.g. CH3(CH2)11SO4Na. SO4Na: 38.7, CH3 : 0.475, CH2: 0.475. HLB= 38.7-(0.475*12)+7= 38.7-5.7+7=40. 41

RHLB: : 

RHLB: A blend of surface active agents is used in preparation. Required HLB (Critical HLB) is the hydrophilic- lipophilic value that is desired in order to prepare a stable emulsion of o/w or w/o type. The required HLB value is calculated based on the oil phase and the type of emulsion. It is assumed that HLB of mixture of two surfactants containing fraction f, of A and (1-f) of B is algebraic mean of HLB values. HLB mix = f.HLBA + (1-f)HLBB 42

Soluble monomolecular films : 

Soluble monomolecular films When a small drop of polar short chain alcohol (amyl alcohol) is added to water, it spreads on water surface. As its conc increases, the molecules progressively get accumulated at the surface. At sp conc, the surface (interface) is completely cover with a monomolecular film of the added substance. Similarly, acids such as octanoic acid also form soluble monomolecular films. 43

Parameters evaluated : 

Parameters evaluated 1.Surface tension 2.Surface excess 3.Conc of amphiphiles in the bulk: can be estimated by drawing a sample from the bulk. The surface conc can be obtained by cutting and lifting the liq surface using a microtome blade. The collected surfactant molecules from the surface is estimated by suitable method. The diff between bulk conc and surface gives the surface excess. 44

Slide 45: 

The no of molecules per unit area of the surface can be estimated using Gibbs equation: Where Г : moles of solute adsorbed /unit area R: ideal gas constant T: ab. Temp. dγ : change in surface tension da2: change in solute activity at activity a2. Surface excess is the amt of the amphiphile per unit area of surface in excess of that in bulk liq. 45

Slide 46: 

For dilute soln of non electrolytes , activity term can be replaced by solute conc, c. Since, the term integral dc/c = d(ln c), the Gibbs eqn can be written as : This eqn is applicable to the adsorption of non-dissociating solutes such as surfactants. 46

Slide 47: 

If surface tension is plotted against ln C2. At lower conc, the surface is partially covered and surface tension is decreased slightly. As the conc increases, the surface tension also falls gradually. The linear portion of the plot, the slopes gives surface excess, Г. It can be calculated by multiplying slope with 2.303RT. 47 30 40 50 60 70 γ, dynes/cm ln C2

Slide 48: 

At particular conc of the surfactant, the surface tension gets leveled off indicating that the surface is completely covered with monomolecular layer. Beyond this conc, the surface active agents aggregate to form micelle and stay in the bulk of the liq. This pattern indicates the inflection point, which corresponds to the CMC. 48 30 40 50 60 70 γ, dynes/cm ln C2 The plot of y Vs. conc of amphiphile gives a linear relationship.

Slide 49: 


Adsorption at Solid interface : 

Adsorption at Solid interface A gas or liq is adsorbed on solid surface. The material used to adsorb gas or liq (solid) is known as adsorbent. The substance that is attached to the surface of the solid is called adsorbent. The degree of adsorption of gas by a solid depends on Nature of adsorbent and its surface area. Nature of adsorbate and the partial pressure of gas. Temp. 50

Type of adsorption. : 

Type of adsorption. 51 Combination of both type of adsorption is known as sorption. Desorption : adsorbed molecules or ions are removed from the solid surface.

Adsorption at solid/gas interface : 

Adsorption at solid/gas interface Adsorption of gas is important in following area Removal of objectionable odors from the room. Prevention of obnoxious gases entering in to the body (gas masks) Estimation of surface area and particle size of powders. The amount of gas adsorbed per unit area or unit mass of solid is measured at diff pressure of the gas at constant temp. The graph of amt of gas ad./unit A or m of solid Vs pressure is known as adsorption isotherm. 52

Freundlich isotherm : 

Freundlich isotherm The relationship between pressure of the gas and amt adsorbed at constant temp has been expressed by freundlich isotherm eqn: Where x = wt of gas adsorbed per unit wt of adsorbent,m P= equilibrium pressure, k and n = constant. This eqn gives curvilinear graph when (x/m) is plotted against pressure p. The constant k and n are evaluated from the exp and they depends on temp and nature of the adsorbent and adsorbate. 53

Freundlich isotherm : 

Freundlich isotherm Convert the eqn in logarithmic form Thus, plotting log (x/m)versus log p gives a straight line with the slope (1/n) and the intercept on the y axis is k. Liquefiable gases such as CO2, NH3, SO2, HCl get more readily adsorbed than other permeant gases such as O2, H2 and N2. Higher the critical temp of the gas, more is the adsorption of gases. 54

Freundlich isotherm : 

Freundlich isotherm 55 x/m p Log (x/m) Log p Slope = 1/n Intercept = k

Langmuir Adsorption : 

Langmuir Adsorption The following assumptions are made for this: The surface of solid posses fixed number of active sites for the adsorption of gases. At max adsorption, the gas layer that is found around the solid is of only one molecule thick. The rate of adsorption (condensation) is proportional to number of sites unoccupied. The rate of evaporation (desorption) is proportional to the number of occupied sites. 56

Langmuir Adsorption : 

Langmuir Adsorption The adsorption of gas on the solid surface depends on the pressure of the gas in the experimental conditions. At a particular pressure, p, Fraction of site occupied : Ө Fraction of sites unoccupied: (1-Ө) Rate of adsorption: r1 = k1(1-Ө)P, Rate of desorption: r2 = k2Ө, Where k1 and k2 are proportionality constants for the process of adsorption and desorption. 57

Langmuir Adsorption : 

Langmuir Adsorption At equilibrium : r1 = r2 If we consider, y= mass of gas adsorbed/g of adsorbrnt ym= mass of gas that 1 g of adsorbent can take up when a monolayer is complete. 58

Slide 59: 

The graph of P/y against P gives straight line. The value of ym and b can be obtained from slope and intercept. ym value is used to estimate sp surface of the solids. Langmuir isotherm is applicable to monomolecular layer adsorption. 59

Multimolecular adsorption : 

Multimolecular adsorption Sometimes, gases adsorb as multi-molecular layers on solids. Braunauer, Emmette and Teller have extended equation as: Where P = pressure of adsorbate, mmHg, y = mass of vapour per gram, Po = vapor pressure at saturation (monolayer) ym = amount of vapor adsorbed/unit mass of adsorbent, when the surface is covered with monomolecular layer. b = constant, proportional to heat of adsorption and latent heat of condensaton of subsequent layers. 60

Adsorption Isotherms : 

Adsorption Isotherms Adsorption isotherms are defined as the plots drawn between the amount of gas adsorbed on a solid against the equilibrium pressure or conc at constant temp. Types: Type I Type II Type III Type IV Type V 61

Adsorption Isotherm Type I : 

Adsorption Isotherm Type I It represents increase in adsorption with increase in pressure followed by a leveling off. This leveling off is due to the saturation of available specific chemical gps or entire surface is covered by a monomolecular layer. Same as Freundlich and Langmuir adsorption isotherm. E.g. adsorption of N2 gas on charcoal 62 Type I P Po Adsorbate (amt)

Adsorption Isotherm Type II : 

Adsorption Isotherm Type II Sigmoidal in shape and occurs when gases undergo physical adsorption on to nonporous solid. 1st inflection point represents formation of monolayer. As pressure increases multilayer formation occur. Represented by a BET eqn. where constant b is greater than 2. E.g. adsorption of N2 gas on iron or platinum catalyst at 78 oC. 63 Type II P Po Adsorbate (amt)

Adsorption Isotherm Type III : 

Adsorption Isotherm Type III It is seen rarely. The heat of adsorption of gas in the first layer is less than the latent heat of condensation of subsequent layers. Represented by BET eqn where constant b is smaller than 2. E.g. adsorption of bromine on silica or alumina. 64 Type III P Po Adsorbate (amt)

Adsorption Isotherm Type IV : 

Adsorption Isotherm Type IV Represents the adsorption of gases on porous solids. First point of inflection extrapolated to zero represents monomolecular layer adsorption. Condensation within the pores of the solid and the multimolecular layer is represented by further adsorption. E.g. adsorption of benzene on silica gel. 65 Type IV P Po Adsorbate (amt)

Adsorption Isotherm Type v : 

Adsorption Isotherm Type v It indicates capillary condensation. Here the adsorption reaches a limiting value before Po is attained. E.g. adsorption of water vapor on charcoal at 100 oC. 66 Type V P Po Adsorbate (amt)

Slide 67: 

67 Wetting agents Wetting agent is a surfactant that when dissolved in water, lower the contact angle and aids in displacing the air phase at the surface and replacing it with a liquid phase. Solids will not be wetted if their critical surface tension is exceeded than the surface tension of the liquid. Thus water with a value of 72 dynes/cm will not wet polyethylene with a critical surface tension of 3 1 dynes/cm. Based on this concept we should expect a good wetting agent to be one which reduces the surface tension of a liquid to a value below the solid critical surface tension.

Slide 68: 

68 According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it or no. which is the wettability between liquids and solids. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface and vice versa. Place a drop of a liquid on a smooth surface of a solid. According to the wettability, the drop will make a certain angle of contact with the solid. A contact angle is lower than 90°, the solid is called wettable A contact angle is wider than 90°, the solid is named non-wettable. A contact angle equal to zero indicates complete wettability.

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69 .

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Slide 71: 

71 Detergents

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72 Deflocculation of dirt in to suspension Convert Wash

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Zeta potential : 

Zeta potential Zeta potential is the electrical potential that exists at the shear plane of a particle, which is some small distance from the surface. Colloidal particles dispersed in a solution are electrically charged due to their ionic characteristics and dipolar attributes. The development of a net charge at the particle surface affects the distribution of ions in the neighboring interfacial region, resulting in an increased concentration of counter ions (ion of charge opposite to that of the particles) close to the surface. 74

Zeta potential : 

Zeta potential Each particle dispersed in a solution is surrounded by oppositely charged ions called fixed layer. Outside the fixed layer, there are varying compositions of ions of opposite polarities, forming a cloud-like area. Thus an electrical double layer is formed in the region of the particle-liquid interface. 75

Slide 76: 

This double layer may be considered to consist of two parts: a inner region which includes ions bound relatively strongly to the surface and an outer, or diffuse region in which the ion distribution is determined by a balance of electrostatic forces and random thermal motion. The potential in this region, therefore, decays with the distance from the surface, until at a certance distance it becomes zero (see the graph on the left). 76

Slide 77: 

When a voltage is applied to the solution in which particles are dispersed, particles are attracted to the electrode of the opposite polarity, accompanied by the fixed layer and part of the diffuse double layer. The potential at the boundary between this unit, that is to say at the above-mentioned shear plane between the particle with its ion atmosphere and the surrounding medium, is known as the Zeta Potential. 77

Slide 78: 

Zeta potential is a function of the surface charge of a particle, any adsorbed layer at the interface and the nature and composition of the surrounding medium in which the particle is suspended. 78

Slide 79: 

The principle of determining zeta potential is very simple.A controlled electric field is applied via electrodes immersed in a sample suspension and this causes the charged particles to move towards the electrode of opposite polarity. Viscous forces acting upon the moving particle tend to oppose this motion and an equilibrium is rapidly established between the effects of the electrostatic attraction and the viscosity drag. The particle therefore reach a constant terminal velocity. 79

Slide 80: 

80 Micellar Solubilization Surfactant molecules accumulate in the interfaces between water and water insoluble compound. Their hydrocarbon chains penetrate the outermost layer of insoluble compound which combine with the water­insoluble molecules. Micelles form around the molecules of the water­insoluble compound inside the micelles’ cores and bring them into solution in an aqueous medium. This phenomenon is called micellar solubilization. The inverted micelles formed by oil­soluble surfactant which dissolves in a hydrocarbon solvent can solubilize water-soluble compound which is located in the center of the micelle, out of contact with the solvent.

Slide 81: 

81 Micelles of nonionic surfactants consist of an outer shell containing their polyethylene glycol moieties mixed with water and an inner core formed by their hydrocarbon moieties. Some compounds like phenols and benzoic acid form complexes with polyethylene glycols by hydrogen bonding and/or are more soluble in liquids of intermediate polarity like ethanol or ethyl ether than in liquids of low polarity like aliphatic hydrocarbons. These compounds locate in the aqueous polyethylene glycol outer shell of nonionic micelles on solubilization. Drugs which are soluble in oils and lipids can be solubilized by micellar solubilization.

Slide 82: 

82 As Micellar solubilization depends on the existence of micelles; it does not take place below the CMC. So dissolution begins at the CMC. Above the CMC, the amount solubilized is directly proportional to the surfactant concentration because all surfactant added to the solution in excess of the CMC exists in micellar form, and as the number of micelles increases the extent of solubilization increases . Compounds that are extensively solubilized increase the size of micelles in two ways: The micelles swell because their core volume is augmented by the volume of the solubilizate. The number of surfactant molecules per micelle increases.

Slide 83: 

83 Foams are dispersion of a gas in a liquid (liquid foams as that formed by soaps and detergents ) or in a solid (solid foams as sponges ). Foaming and Anti Foaming agents Foaming agents Many Surfactants solutions promote the formation of foams and stabilize them, in pharmacy they are useful in toothpastes compositions. Anti Foaming agents They break foams and reduce frothing that may cause problems as in foaming of solubilized liquid preparations. in pharmacy they are useful in aerobic fermentations, steam boilers.

Slide 84: 

84 Structural Classification A single surfactant molecule contains one or more hydrophobic portions and one or more hydrophilic groups. According to the presence of ions in the surfactant molecule they may be classified into: Ionic surfactants Anionic surfactants: the surface active part is anion (negative ion ) e.g. soaps, sodium lauryl sulfate Cationic surfactants: the surface active part is cation (positive ion) e.g. quaternary ammonium salts Ampholytic surfactants: contain both positive and negative ions e.g. dodecyl-B-alanine.

Slide 85: 

85 Ionic surfactants They are the metal salts of long ­ chain fatty acids as lauric acid. Sodium dodecyl sulfate or Sodium Lauryl Sulfate is used in toothpaste and ointments Triethanolamine dodecyl sulfate is used in shampoos and other cosmetic preparations. Sodium dodecyl benzene sulfonate is a detergent and has germicidal properties. Sodium dialkvlsulfosuccinates are good wetting agents. Anionic surfactants

Slide 86: 

86 Cationic surfactants These are chiefly quaternary ammonium compounds. They have bacteriostatic activity probably because they combine with the carboxyl groups in the cell walls and of microorganisms by cation exchange, causing lysis. Among the most popular antiseptics in this category are benzalkonium chloride, cetylpyridinium chloride and cetyltrimethylammonium bromide, Ampholytic Surfactants These are the least common, e.g. dodecyl-β­alanine

Slide 87: 

87 Non-ionic surfactants Widely used in pharmaceutical formulations e.g. Tweens, Spans, Brij and Myrj. They are polyethylene oxide products. Surfactants based on sorbitan are of pharmaceutical importance. Esterification of the primary hydroxyl group with lauric, palmitic, stearic or oleic acid forms sorbitan monolaurate, monopalmitate, monostearate or monooleate These are water-insoluble surfactants called Span 20, 40, 60 or 80, respectively. Addition of about 20 ethylene oxide molecules produces the water-soluble surfactants called polysorbate or Tween 20, 40. 60 or 80.

Slide 88: 

88 At water-air interface Surface­active molecules will be adsorbed at water-air interfaces and oriented so that the hydrocarbon chains of are pushed out of the water and rest on the surface, while the polar groups are inside the water. Perhaps the polar groups pull the hydrocarbon chains partly into the water. At oil-water interface Surface­active molecules will be oriented so that the hydrophobic portion is inside the oil phase and the hydrophilic portion inside the water phase. As a Surface active substance contains a hydrophilic and a hydrophobic portions, it is adsorbed as a monolayer at the interfaces. Oriented Adsorption of surfactant at Interfaces

Slide 89: 

89 At low surfactant concentrations: The hydrocarbon chains of surfactant molecules adsorbed in the interface lie nearly flat oh the water surface. At higher concentrations: They stand upright because this permits more surfactant molecules to pack into the interfacial monolayer. As the number of surfactant molecules adsorbed at the water­air interface increased, they tend to cover the water with a layer of hydrocarbon chains. Thus, the water-air interface is gradually transformed into an non polar-air interface. This results in a decrease in the surface tension of water.

Slide 90: 

90 Micelle Formation When the surfactant molecules adsorbed as a monolayer in the water-air interface have become so closely packed that additional molecules cannot be accommodated with ease, the polar groups pull the hydrocarbon chains partly into the water. At certain concentration the interface and the bulk phase become saturated with monomers. Excess surfactants add will begin to agglomerate in the bulk of the solution forming aggregates called and the free energy of the system is reduced The lowest concentration at which micelles first appear is called the critical concentration for micelle formation [CMC ] Micelles.

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91 At a given concentration, temperature, and salt content, all micelles of a given surfactant usually contain the same number of molecules, i.e. they are usually monodisperse. For different surfactants in dilute aqueous solutions, this number ranges approximately from 25 to 100 molecules. The diameters of micelles are approximately between 30 and 80 Ao. Because of their ability to form aggregates of colloidal size, surfactants are also called association colloids. Micelles are not permanent aggregates. They form and disperse continually.

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93 a- Cone-shaped surfactant resulting in b-normal micelles c- Hampagne cork shaped surfactant resulting in d-reverse micelles with control of their size by the water content e- Interconnected cylinders. f- Planar lamellar phase. g- Onion-like lamellar phase. Surfactant shapes in colloidal solution

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94 Normal spherical micelles In dilute aqueous solutions micelles are approximately spherical. The polar groups of the surfactants are in the periphery and the hydrocarbon chains are oriented toward the center, forming the core of the micelles Inverted spherical micelles In solvents of low polarity or oils micelles are inverted. The polar groups face inward to form the core of the micelle while the hydrocarbon chains are oriented outward Cylindrical and lamellar micelles In more concentrated solutions of surfactants, micelles change from spherical either to cylindrical or lamellar phase.

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95 Changes occurred at the CMC I- A continuous decrease in Surface and interfacial tensions with surfactants concentration until CMC the Surface and interfacial tensions level become constant owing to crowding of surfactant molecules adsorbed at surfaces and interfaces. Properties of surfactant Solutions as functions of concentration: A, surface tension B, interfacial tension C, osmotic pressure; D, equivalent conductivity; E, solubility of compounds with low or zero solubility in water

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96 Changes occurred at the CMC II- The osmotic pressure (and all other colligative properties, lowering of the vapor pressure and of the freezing point), rises much more slowly with increasing surfactant concentration above than it does below the CMC because it depends on the number of dissolved particles. A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water

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97 Changes occurred at the CMC III- For ionic surfactants, the equivalent conductivity drops sharply above the CMC. Only the counterions of non-associated surfactant molecules can carry current. A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water

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98 Changes occurred at the CMC IV-Solubility of many drugs are increased after CMC. Other solution properties changing at the CMC: intrinsic viscosity and turbidity increase, while diffusion coefficient decreases A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water All these properties can be used to determine the CMC.

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99 Factors affecting CMC For nonionic surfactants Temperature CMC For ionic surfactants The CMC are higher for ionic than nonionic surfactants The charges in the micelle of ionic surfactant are in close, to overcome the resulting repulsion an electric work is required but nonionic surfactants have no electric repulsion to overcome in order to aggregate. Effect of electrolytes on the CMC of ionic surfactants The addition of salts to ionic surfactant solutions reduces the electric repulsion between the charged groups and lower CMC values

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100 Effect of Surfactant’s structure on CMC Branched chain systems and double bonds raise CMC Since the chains must come together inside the micelles Length of hydrocarbon chain and polarity of Surfactants Increase in chain length of hydrocarbon facilitate the transfer from aqueous phase to micellar form cause decrease in CMC Greater interaction with water will retard micelle formation thus ionized surfactants have higher CMC in polar solvents than nonionic Surfactants. In polar solvents: Polarity of Surfactant molecules CMC Length of hydrocarbon chain CMC In non-polar solvents: Polarity of Surfactant molecules CMC Length of hydrocarbon chain CMC

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101 Incompatibilities Involving Surfactants Nonionic surfactants have few incompatibilities with drugs and are preferred over ionic surfactants. even in formulations for external use, except when the germicidal properties of cationic and anionic surfactants are important. Nonionic surfactants form weak complexes with some preservatives as phenols, including esters of p­hydroxybenzoic acid (Parabenzes) and with acids like benzoic and salicylic via hydrogen bonds. This reduces the antibacterial activity of these compounds. Nonionic surfactants

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102 Ionic surfactants Ionic surfactants capable of reacting with compounds possessing ions of the opposite charge. These reactions may bind the surface active ions, sometimes with precipitation. The compounds which react with the surface active ions are also changed, and this may be harmful from the physiological or pharmacological point of view. Incompatibility of surface active quaternary ammonium compounds with bentonite, kaolin, talc, and other solids having cation exchange capacity.

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103 Anionic surfactants React with Cationic drugs (alkaloidal salts, local anesthetics, most sympathomimetic, cholinomimetic, adrenergic blocking, skeletal muscle relaxants, antihistamines, many tranquilizing and antidepressant agents) cause precipitation or the drugs lose potency or availability Drugs with carboxylic, sulfonic or phosphoric acid groups like salicylic and p­aminobenzoic acids interact with cationic surfactants. Cationic surfactants form complex with water soluble polymers containing negatively charged groups, as natural gums (acacia, tragacanth, agar, carrageenin), alginate, sodium carboxy methylcellulose, and Carbopol.

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