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surface tension, its determination, surfactant, HLB scale


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Surface & Interfacial Phenomenon:

Surface & Interfacial Phenomenon Ankit Agrawal Department of Pharmaceutics CDIP Indore

Interfacial tension:

Interfacial tension Interfacial tension is the force of attraction between the molecules at the interface of two fluids. The SI units for interfacial tension are milli - Newtons per meter ( mN /m). These are equivalent to the former units of dynes per centimeter (dyne/cm).

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Surface tension is a phenomenon in which the surface of the liquid is in contact with the gas. Surface tension is defined as the force acting at right angles to a line of unit length present in the surface. This is denoted by ‘γ’. W = F × d where W = work; F = force; d = distance. Therefore, (γ) Gamma = F/d SI units for the surface tension are newton /meter or dyne/cm. Surface tension The surface tension of a liquid is the energy required to increase the surface area which increases the stability of the solution and increases the solubility rate.

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Walking on water:  Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface. Floating a needle:  A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink. Surface tension disinfectants:  Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them. Soaps and detergents:  These help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas. Surface tension and droplets:  Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. Examples of surface tension

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Water molecules want to cling to each other. At the surface, however, there are fewer water molecules to cling to since there is air above (thus, no water molecules). This results in a stronger bond between those molecules that actually do come in contact with one another, and a layer of strongly bonded water (see diagram). This surface layer (held together by surface tension) creates a considerable barrier between the atmosphere and the water. In fact, other than mercury, water has the greatest surface tension of any liquid. Within a body of a liquid, a molecule will not experience a net force because the forces by the neighboring molecules all cancel out (diagram). Surface tension at a molecular level

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Cont…….. However for a molecule on the surface of the liquid, there will be a net inward force since there will be no attractive force acting from above.   This inward net force causes the molecules on the surface to contract and to resist being stretched or broken. Thus the surface is under tension, which is probably where the name "surface tension" came from. Due to the surface tension, small objects will "float" on the surface of a fluid, as long as the object cannot break through and separate the top layer of water molecules. When an object is on the surface of the fluid, the surface under tension will behave like an elastic membrane.

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Surface Tension of Water at Different Temperatures

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Measurement of Surface Tension 1. CAPILLARY RISE METHOD: This is the oldest method used for surface tension determination. A consequence of the surface tension appearance at the liquid/gas interface is moving up of the liquid into a thin tube, that is capillary, which is usually made of glass. This phenomenon was applied for determination of the liquid surface tension. For this purpose, a thin circular capillary is dipped into the tested liquid. If the interaction forces of the liquid with the capillary walls (adhesion) are stronger than those between the liquid molecules (cohesion), the liquid wets the walls and rises in the capillary to a defined level and the meniscus is hemispherically concave. In the opposite situation the forces cause decrease of the liquid level in the capillary below that in the chamber and the meniscus is semispherically convex. Both cases are illustrated in Fig…

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If the cross-section area of the capillary is circular and its radius is sufficiently small, then the meniscus is semispherical. Along the perimeter of the meniscus there acts a force due to the surface tension presence. If the cross-section area of the capillary is circular and its radius is sufficiently small, then the meniscus is semispherical. Along the perimeter of the meniscus there acts a force due to the surface tension presence. …………. (1) Where: r – the capillary radius, g – the liquid surface tension, q – the wetting contact angle.

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The force f1 in Eq.(1) is equilibrated by the mass of the liquid raised in the capillary to the height h, that is the gravity force f2. In the case of non-wetting liquid – it is lowered to a distance – h. ………………. (2) where: d – the liquid density (g/cm3) (actually the difference between the liquid and the gas densities), g – the acceleration of gravity. In equilibrium (the liquid does not moves in the capillary) f1 = f2 , and hence ……………… (3) ……………… (4) If the liquid completely wets the capillary walls the contact angle θ = 0, and cos θ = 1. In such a case the surface tension can be determined from Eq. (5). ……………… (5)

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2. DROP VOLUME METHOD – STALAGMOMETRIC METHOD 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

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3. WILHELMY PLATE METHOD This method was elaborated by Ludwig Wilhelmy . In this method a thin plate (often made of platinum or glass) is used to measure equilibrium surface or interfacial tension at air-liquid or liquid-liquid interfaces. The plate is oriented perpendicularly to the interface and the force exerted on it is measured. The principle of method is illustrated in Fig. The plate should be cleaned thoroughly (in the case of platinum – in a burner flame) and it is attached to a scale or balance by means of a thin metal wire. The plate is moved towards the surface until the meniscus connects with it. The force acting on the plate due to its wetting is measured by a tensiometer or microbalance. To determine the surface tension g the Wilhelmy equation is applied.

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If the plate has a width l and its weight is W plate, then the force F needed to detach it from the liquid surface equals: F = Wtotal = Wplate + 2 l g cos θ Multiplying by 2 is needed because the surface tension acts on both sides of the plate, whose thickness is neglected. If the liquid wets completely the plate, then cos θ = 1 and the surface tension is expressed by Eq. The accuracy of this method reaches 0.1%, for the liquids wetting the plate completely.

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4. THE RING METHOD – THE TENSIOMETRIC METHOD (DU NOUY RING TENSIOMETER) Instead of a plate a platinum ring can be used, which is submerged in the liquid. As the ring is pulled out of the liquid, the force required to detach it from the liquid surface is precisely measured. This force is related to the liquid surface tension. The platinum ring should be very clean without blemishes or scratches because they can greatly alter the accuracy of the results. Usually the correction for buoyancy must be introduced. The total force needed to detach the ring W total equals the ring weight Wr and the surface tension multiplied by 2 because it acts on the two circumferences of the ring (inside and outside ones). Where: R – the ring radius. It is assumed here that the inner and outer radii of the ring are equal because the wire the ring is made of is very thin.

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4. MAXIMUM BUBBLE PRESSURE METHOD This method is also called the bubble pressure method. In this method air gas bubble is blown at constant rate through a capillary which is submerged in the tested liquid. The scheme of the apparatus proposed by Rebinder is shown in Fig. The pressure inside the gas bubble is increasing. Its shape from the very beginning is spherical but its radius is decreasing. This causes the pressure increase inside it and the pressure is maximal when the bubble has a hemispherical size. At this moment the bubble radius equals to the radius of the capillary, inner if the liquid wets the tip of the capillary and outer if it does not wet it.

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Fig. shows the changes in the bubble radius with each step of the bubble formation. Then the maximum pressure difference Pmax is described by the Laplace equation.

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If the capillary tip is dipped into the liquid to a depth h from the liquid surface, then the correction reducing Pmax should be introduced. The correction is due to the additional hydrostatic pressure caused by the liquid layer of thickness h, the pressure that the detaching bubble has to overcome, Pmax – Ph. For very accurate determination of the surface tension, other corrections are needed. They can be found in special tables. The accuracy of this method is about several tenth of percent and it is applied both for surface and interface tensions measurement.

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SPREADING COEFFICIENT Spreading coefficient (S) is the difference between work of adhesion and work of cohesion. Work of Cohesion ( Wc ) may be defined as the surface free energy increased by separating a column of pure liquid into two halves: Surface free energy increase = γ dA Wc = γL (dA + dA) = 2 γLdA S = Wa – Wc = ( γ L + γ S – γ LS) - 2 γ L = γ S – γ L – γ LS S = γ S – ( γ L + γ LS) γL - Surface tension of spreading liquid γ S - Surface tension of sublayer liquid γ LS - Interfacial tension

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Work of Adhesion ( Wa ) may be defined as the surface free energy increased by separating a column of two immiscible liquids at its boundary into two sections As two sections of immiscible liquids are already separated by a boundary, the energy requirement will be less by an amount γLS dA Wa = γ LdA + γ S dA - γ LS dA Here the columns are of cross sectional area 1cm2 Wa = γ L + γ S - γ LS Here the column is of cross sectional area 1cm2 Wc = 2 γ L

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Spreading occurs when spreading coefficient S is positive i.e , γ S > ( γ L+ γ LS) When spreading coefficient S is negative ie , (γ L+ γ LS) > γ S Spreading liquid forms globules or floating lens. That is spreading will not take place. When free energy of the spreading liquid and the interfacial tension with the sub layer is less than that of sublayer the spreading becomes spontaneous in an attempt to reduce free energy of sublayer . There may be saturation of the liquid with the other and there may be change in the surface tension of the sublayer liquid. In that case the spreading coefficient may become negative after saturation, the spreading liquid coalesces and form a lens on the surface of the sublayer . In the case of a DUPLEX FILM if S become negative after saturation, it forms a monolayer and excess liquid remains as lens on the surface

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Surfactants are materials that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. In the general sense, any material that affects the interfacial surface tension, can be considered a surfactant, but in the practical sense, surfactants may act as wetting agents, emulsifiers, foaming agents, and dispersants. Surfactants is a Amphiphilic compound that has: a hydrophobic group, such as a hydrocarbon chain, that has no affinity for aqueous solvents. a hydrophilic group that has an affinity for water. A molecular or ion that possesses this type of structure is termed amphipathic ( amphiphilic ).

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Surfactants is a Amphiphilic compound that: Is soluble in at least one phase of system. Forms oriented monolayers at phase interface. Exhibits equilibrium concentrations at phase interfaces higher than those in the bulk solution & forms micelles at specific concentration. Exhibits characters- detergency, foaming, wetting, emulsifying, solubilizing & dispersing. Tail or hydrophobic group this group is usually hydrocarbon (alkyl) chain. Head or hydrophilic group can be neutral or charged. Their surface activity arises from adsorption at the solution air interface – the means by which the hydrophobic region of the molecule ‘escapes’ from the hostile aqueous environment by protruding into the vapour phase above. Adsorption at the interface between aqueous and nonaqueous solutions occurs in such a way that the hydrophobic group is in the solution in the nonaqueous phase, leaving the hydrophilic group in contact with the aqueous solution.

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Classification of Surfactants: Dependent on the molecular composition and the nature of dissociation of their polar head groups the surfactants are classified as ionic (cationic, anionic, amphiphilic ) or nonionic. (a) Low molecular mass surfactants Nonionic: sorbitan esters, polysorbates , poloxamer Ionic: SLS, QAC Amphoteric : N- dodecyl - N,Ndimethylbetaine (b) Polymeric surfactants Synthetic Natural

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PROPERTIES OF SURFACTANT Wetting Emulsification Detergency Solubalization Micellization Wetting : Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting ( wettability ) is determined by a force balance between adhesive and cohesive forces.

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b. Emulsification: Emulsification is the process by which a system comprising of two immiscible liquids (usually oil and water), one of which is dispersed as small droplets within the other, is produced.

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c. Detergency • Detergents are surfactants used for removal of dirt. • Detergency involves Wetting of the dirt particles Removing the insoluble dirt as a deflocculated particle or as a emulsion (oil soluble material). • Washing d. Solublisation Process of preparing clear solution, Microemulsion , Swollen micelle Phenolic compound such as cresol, thymol , chlorocresol chloroxylenol stabilized to form clear solution used as disinfection. Low solubility of steroids in water is major problem in ophthalmic formulation. So use of non ionic surfactant produce a clear solution which are stable to sterilization. Polysorbate used in preparation of aqueous injection of water insoluble vitamins A,D,E & K.

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MICELLIZATION Micelle Formation: In dilute aqueous solution, amphiphiles tend to concentrate on surface. As concentration increased- surface become more crowded-till no more space in surface layer. Then forced to remain in aqueous solution and causes disruption of the hydrogen bonding between water molecules. To minimize this disruption amphiphile molecules tend to aggregate into multiple molecular structures. It disrupts water-water attractions. So driving force for formation of micelles-entropy gain from disruption of the water structure. As concentration of surfactant increased there is alteration in physical properties of solution. Self-association of the amphiphile into small aggregates called micelles.

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Concentration of surfactant at which micelles first appear in solution is called as Critical Micellar Concentration (CMC). Reason for micelle formation is the attainment of a minimum free energy state. Driving force for the formation of micelles is the increase of entropy that occurs when the hydrophobic regions of the surfactant are removed from water and the ordered structure of the water molecules around this region of the molecule is lost. Most micelles are spherical and contain between 60 and 100 surfactant molecules.

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Factors affecting CMC and Micellar size Nature of hydrophilic group Nature of hydrophobic group Nature of counter ion Effects of electrolyte Effect of temperature Effect of pressure

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Nature of hydrophobic group Increase in length of the HC results in: • decrease in CMC, which for compounds with identical polar head groups is expressed by the linear equation: log [CMC] = A – Bm where m is the number of carbon atoms in the chain and A and B are constants for a homologous series. • corresponding increase in micellar size. • Branching of HC increases CMC • Unsaturation of HC increases CMC

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Nature of hydrophilic group Non-ionic surfactants generally have very much lower CMC values and higher aggregation numbers than their ionic counterparts with similar hydrocarbon chains. An increase in the ethylene oxide chain length of a non-ionic surfactant makes the molecule more hydrophilic and the CMC increases. Nature of Counter ion Micellar size increases for a particular cationic surfactant as the counter ion is changed according to the series Cl − < Br− < I−, and for a particular anionic surfactant according to Na+ < K+ < Cs+. Ionic surfactants with organic counterions (e.g. maleates ) have lower CMCs and higher aggregation numbers than those with inorganic counter ions. Effect of Pressure •Increase in CMC with pressure up to 150 Mpa followed by a CMC decreases at high pressure.

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Addition of electrolyte Electrolyte addition to solutions of ionic surfactants decreases the CMC and increases the micellar size. This is because the electrolyte reduces the forces of repulsion between the charged head groups at the micelle surface, so allowing the micelle to grow. At high electrolyte concentration the micelles of ionic surfactants may become non-spherical. Aqueous solutions of many nonionic surfactants become turbid at a characteristic temperature called the cloud point. At temperatures up to the cloud point there is an increase in micellar size and a corresponding decrease in CMC. Temperature has a comparatively small effect on the micellar properties of ionic surfactants. Effect of Temperature

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Hydrophile-lipophile balance (HLB scale) Hydrophile-lipophile balance:surfactants contain both hydrophilic groups and lipophilic groups with one or the other being more predominant, the hydrophile-lipophile balance (HLB) number is used as a measure of the ratio of these groups. It is a value between 0-40 defining the affinity of a surfactant for water or oil. HLB value of nonionic surfactants ranges from 0-20. HLB numbers >10 have an affinity for water (hydrophilic) and number <10 have an affinity of oil ( lipophilic ). Calculation of HLB Values of Surfactant Mixtures: The HLB values of the surfactant mixtures were calculated according to the following equation: where C1,C2,C3 are the percent of component proportion and HLB1,HLB2,HLB3 are the HLB values for the each component.

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Polyhydric Alcohol Fatty Acid Esters ( Ex. Glyceryl monostearate ) HLB = 20 ( 1 – S / A ) 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 oxyethylene groups HLB =E / 5

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