Surfactant

Surfactants in pharmaceuticals

1. Surfactants M r . K . K . Mali Assistant Professor , YSPM’s Yashoda Technical Campus, Satara

2. contents Introduction of surfactant Classification of Surfactant Properties of surfactant Phase behaviour of Surfactant

3. Introduction Surface active agents Lowers surface tension of water Any material that makes a surface contribution to the free energy of the surface phase of two component system. 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

4. Introduction 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.

5. Introduction Surfactants - behavior

6. Surface and Interfacial Tension; Surface and Interfacial Free Energy ‘‘surface’’ is usually reserved for the region between a condensed phase (liquid or solid) and a gas phase or vacuum. ‘‘interface’’ is normally applied to the region between two condensed phases. liquid–gas interface, molecules of the liquid in the boundary can only develop attractive cohesive forces with molecules situated below and adjacent to them. They can develop attractive adhesive forces with molecules of the gaseous phase. At the gas–liquid interface, these adhesive forces are quite small. The net effect is that molecules at the surface of the liquid have potential energies greater than those of similar molecules in the interior of the liquid and experience an inward force toward the bulk of liquid. This force pulls the molecules of the interface together and the surface contracts. Thus, the surface of a liquid behaves as if it were in a state of tension—the surface tension (  )—due to the contracting force acting in all directions in the plane of the surface.

7. Surface and Interfacial Tension; Surface and Interfacial Free Energy In order to extend the surface of a liquid it is necessary to bring molecules from the interior to the surface against the inward pull. The work required to increase the surface area by unit area is termed the surface free energy . At the interface between two condensed phases, the dissimilar molecules in the adjacent layers facing each other have potential energies greater than those of similar molecules in the respective bulk phases. This is due to the fact that cohesive forces between like molecules tend to be stronger than adhesive forces between dissimilar molecules. The interfacial tension is the force per unit length existing at the interface between two immiscible or partially miscible condensed phases. The interfacial free energy is the work required to increase the interface by unit area.

8. Surfactant Classification Depending on their charge characteristics the surface-active molecules may be Anionic: SLS Cationic: QAC Zwitterionic ( ampholytic ): N- dodecyl -N,N- dimethylbetaine Nonionic: Sorbitan esters, Polysorbates , Poloxamer

9. surfactant classification Examples

10. Griffin's scale of HLB

11. Properties of surfactant Wetting Emulsification Detergency Solubalization Micellization

12. Properties of surfactant Wetting

13. Properties of surfactant Emulsifying Agent

14. Properties of surfactant 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

15. Properties of surfactant 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.

16. Properties of surfactant Solublisation Micelle used in targeted drug delivery system . Micelle is used to encapsulate antibiotic and anti-cancer drug. Amphotericin is encapsulated in a deaggregated form in micelle of monomethoxy poly Phospholipids formed by solvent evaporation Complex cisplatin block copolymer is used in cancer therapy. Drug activity & absorption Low concentration of surfactant increases absorption due to the enhanced the contact of drug with absorbing membrane. Concentration above the CMC either produce additional effect or cause decrease absorption because drug held within the micelle so that the concentration available for absorption is reduced.

17. 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.

18. Micellization Cross section of micelle core Shear surface Structure of Micelles

19. Micellization Micellization As concentration of surfactant increased there is alteration in physical properties of solution. Self-association of the amphiphile into small aggregates called micelles. Concentration of surfactant at which micelles first appear in solution is called as 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.

20. Micellization Critical Micelle Concentration Minimum concentration at which surfactants molecules begin to form micelles

21. Micellization Critical Micelle Concentration Extremely dil. Dil . solution Solution at CMC Above CMC

22. Micellization

23. Micellization Micelles S tructure Critical packing parameter The shape of the micelle formed by a particular surfactant is influenced to a large extent by the geometry of the surfactant molecule, as can be seen if we consider the packing of space-filling models of the surfactants. The dimensionless parameter of use in these considerations is called the critical packing parameter (CPP) and is defined as where v is the volume of one chain, a is the cross-sectional area of the surfactant head group lc is the extended length of the surfactant alkyl chain

24. Micellization Micelles S tructure Critical packing parameter Structure of the aggregate that will be formed in solution. Consideration of the packing of molecules into spheres shows that when CPP ≤ 1/3, which is the case for surfactants having a single hydrophobic chain and a simple ionic or large nonionic head group, a spherical micelle will be formed. Ionic Micelle

25. Micellization Micelles S tructure Critical packing parameter It is easily seen that if we double volume (v) by adding a second alkyl chain then the value of CPP will exceed 1/3 and nonspherical structures such as bilayers (CPP = 1) will form in solution, from which vesicles are formed .

26. Micellization Micelles S tructure Critical packing parameter The ‘effective’ cross-sectional area of the surfactant molecule is strongly influenced by the interaction forces between adjacent head groups in the micelle surface. These forces are decreased by addition of electrolyte, leading to a decrease of a, an increase of the CPP, and a change of shape of the aggregate,

27. Micellization Micelles S tructure ionic surfactants consists of: Hydrophobic core composed of the hydrocarbon chains of the surfactant molecule a Stern layer surrounding the core, which is a concentric shell of hydrophilic head groups with (1 – ) N counterions , where is the degree of ionisation and N is the aggregation number (number of molecules in the micelle). For most ionic micelles the degree of ionisation is between 0.2 and 0.3; that is, 70–80% of the counterions may be considered to be bound to the micelles a Gouy –Chapman electrical double layer surrounding the Stern layer, which is a diffuse layer containing the N counterions required to neutralise the charge on the kinetic micelle. The thickness of the double layer is dependent on the ionic strength of the solution and is greatly compressed in the presence of electrolyte.

28. Micellization Micelles S tructure ionic surfactants When concentration increased converted spherical to cylindrical

29. Micellization Micelles S tructure Nonionic surfactants are larger than their ionic counterparts and may sometimes be elongated into an ellipsoid or rod-like structure. attributable to the removal of electrical work which must be done when a monomer of an ionic surfactant is added to an existing charged micelle. nonionic micelles are frequently asymmetric due to its size. have a hydrophobic core formed from the hydrocarbon chains of the surfactant molecules surrounded by a shell (the palisade layer) composed of the oxyethylene chains of the surfactant and entrapping a considerable number of water molecules, which is highly hydrated.

30. Micellization

31. Micellization Importance Micelles make insoluble material soluble in water. The structure of the micelles can affect the viscosity of the solution. Micelles are reservoirs of surfactants.

32. Micellization Nature of hydrophilic group Nature of hydrophobic group Nature of counter ion Effects of electrolyte Effect of temperature Effect of pressure Factors Affecting CMC and Micellar size

33. Micellization Factors Affecting CMC and Micellar size 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

34. Micellization Factors Affecting CMC and Micellar size Nature of hydrophobic group

35. Micellization Factors Affecting CMC and Micellar size 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.

36. Micellization Factors Affecting CMC and Micellar size Type of Counterion Micellar size increases for a particular cationic surfactant as the counterion 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 counterions .

37. Micellization Factors Affecting CMC and Micellar size 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.

38. Micellization Factors Affecting CMC and Micellar size Effect of Temperature Aqueous solutions of many non-ionic 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.

39. Micellization Factors Affecting CMC and Micellar size Effect of Pressure Increase in CMC with pressure upto 150 Mpa followed by a CMC decreases at high pressure.

40. Micellization Thermodynamics of Micelle Formation Mass action model Micelles are in equilibrium with unassociated surfactant. For nonionic surfactant , n [the aggregation number] molecules of the monomeric un ionised surfactant (S) react in a single step to form a micelle, M nS  M Equilibrium constant for micelle formation Km is given as Term in bracket refers molar concentration of species.

41. Micellization Thermodynamics of Micelle Formation Mass action model When n is large – the free energy of micellization at the CMC is G m represents the standard free energy change for transferring 1 mol of S from the aqueous solution into its micellar form.

42. Micellization Thermodynamics of Micelle Formation Mass action model For ionised surfactant , ionic micelle derived from an anionic surfactant is formed by the association of n surfactant ions S - and of [n-p] firmly bound counterions (X + ) as follows: nS - + (n-p)X +  M -p Equilibrium constant for micelle formation is given by When aggregation number n is large, the free energy of micellization at or near the CMC reduces to

43. Micellization Thermodynamics of Micelle Formation Phase –Separation model considers micelles as a separate phase at CMC. Hence  s and  m are defined as the chemical potentials [per mol] of the free surfactant in the aqueous phase and of the associated surfactant in the micellar phase respectively At equilibrium  s =  m If activity coefficients are ignored,  s is related to the surfactants standard state  s by

44. Micellization Thermodynamics of Micelle Formation Phase –Separation model On the other hand, micellar material is in standard state, and  m =  m The standard free energy of micellization is The analogous approach for an ionized surfactant yields

45. Phase behaviour Equilibrium phase structures As the concentration of a surfactant solution is increased, different structures encount ered . At concentrations well above the CMC, a more ordered structuring of the solution occurs. Two main types of liquid crystalline phases Middle phase, M, exhibiting a hexagonal array of indefinitely long, mutually parallel rods; and the neat phase, G, with a lamellar structure. The liquid crystalline hexagonal phase, like the micellar phase, can exist either in a normal or reverse orientation. The order of phase structures formed upon increasing surfactant concentration generally follows a well defined sequence with a ‘‘mirror plane’’ through the lamellar phase in such a way that normal phase structures can be considered to be ‘‘oil-in-water’’ and the reverse structures to be ‘‘water-in-oil.

46. Phase behaviour Modified phase structures Vesicular forms of surfactants are generally formed by dispersing lamellar phases in an excess of water (or non-aqueous polar solvents such as ethylene glycol or dimethylformamide ) or, in the case of reversed vesicles, in an excess of oil. With most surfactants, vesicles are non-equilibrium structures that will eventually re-equilibrate back into the lamellar phases from which they originated. Vesicles are structural analogs of liposomes ; they are approximately spherical structures and have the ability to ‘‘ solubilize ’’ both lipid soluble and water soluble agents.

47. Phase behaviour Formation of liquid crystals and vesicles Lyotropic liquid crystals The liquid crystalline phases that occur on increasing the concentration of surfactant solutions are referred to as lyotropic liquid crystals.

48. Phase behaviour Formation of liquid crystals and vesicles Lyotropic liquid crystals Increase of concentration of a surfactant solution frequently causes a transition from the typical spherical micellar structure to a more elongated or rod-like micelle. Further increase in concentration may cause the orientation and close packing of the elongated micelles into hexagonal arrays; this is a liquid crystalline state termed the middle phase or hexagonal phase. With some surfactants, further increase of concentration results in the separation of a second liquid crystalline state – the neat phase or lamellar phase. In some surfactant systems another liquid crystalline state, the cubic phase, occurs between the middle and neat phases

49. Phase behaviour Formation of liquid crystals and vesicles Lyotropic liquid crystals The lyotropic liquid crystals are anisotropic, that is, their physical properties vary with direction of measurement. The middle phase, for example, will only flow in a direction parallel to the long axis of the arrays. It is rigid in the other two directions. The neat phase is more fl uid and behaves as a solid only in the direction perpendicular to that of the layers. Plane- polarised light is rotated when travelling along any axis except the long axis in the middle phase and a direction perpendicular to the layers in the neat phase.

50. Phase behaviour Formation of liquid crystals and vesicles Thermotropic liquid crystals Thermotropic liquid crystals are produced when certain substances, for example the esters of cholesterol, are heated. The arrangement of the elongated molecules in thermotropic liquid crystals is generally recognisable as one of three principal types Nematic liquid crystals: Groups of molecules orientate spontaneously with their long axes parallel, but they are not ordered into layers. Because the molecules have freedom of rotation about their long axis, the nematic liquid crystals are quite mobile and are readily orientated by electric or magnetic fields.

51. Phase behaviour Formation of liquid crystals and vesicles Thermotropic liquid crystals Smectic liquid crystals: Groups of molecules are arranged with their long axes parallel, and are also arranged into distinct layers. As a result of their two-dimensional order the smectic liquid crystals are viscous and are not orientated by magnetic fields.

52. Phase behaviour Formation of liquid crystals and vesicles Thermotropic liquid crystals Cholesteric (or chiral nematic ) liquid crystals: Are formed by several cholesteryl esters. Can be visualised as a stack of very thin two-dimensional nematic -like layers in which the elongated molecules lie parallel to each other in the plane of the layer. The orientation of the long axes in each layer is displaced from that in the adjacent layer and this displacement is cumulative through successive layers so that the overall displacement traces out a helical path through the layers. The helical path causes very pronounced rotation of polarised light, which can be as much as 50 rotations per millimeter. The pitch of the helix (the distance required for one complete rotation) is very sensitive to small changes in temperature and pressure and dramatic colour changes can result from variations in these properties. The cholesteric phase has a characteristic iridescent appearance when illuminated by white light due to circular dichroism .

53. Phase behaviour Formation of liquid crystals and vesicles Vesicles Vesicles are formed by phospholipids and other surfactants having two hydrophobic chains. There are several types: Liposomes Liposomes are formed by naturally occurring phospholipids such as lecithin ( phosphatidyl choline ). They can be multilamellar (composed of several bimolecular lipid lamellae separated by aqueous layers) or unilamellar (formed by sonication of solutions of multilamellar liposomes ). They may be used as drug carriers; water-soluble drugs can be entrapped in liposomes by intercalation in the aqueous layers, whereas lipid-soluble drugs can be solubilised within the hydrocarbon interiors of the lipid bilayers .

54. Phase behaviour Formation of liquid crystals and vesicles Surfactant vesicles and niosomes Formed by surfactants having two alkyl chains. Sonication can produce single-compartment vesicles. Vesicles formed by ionic surfactants are useful as membrane models. Vesicles formed from non-ionic surfactants are called niosomes and have potential use in drug delivery.

55. Phase behaviour Formation of liquid crystals and vesicles Monoolein vesicles Polar amphiphilic lipids such as glyceryl monooleate ( monoolein ) form bilayers , the nature of which depends on the temperature and concentration. phase formed at monoolein concentrations of 60–80% w/w is the bicontinuous cubic phase. The structure of this phase is unique and consists of a curved bicontinuous lipid bilayer extending in three dimensions, separating two networks of water channels with pores of about 5 nm diameter. On dilution, these structures coexist with excess water and there is the formation of dispersed cubic phase vesicles or cubosomes . Cubic phases have been shown to incorporate and deliver small molecule drugs and large proteins by oral and parenteral routes, in addition to local delivery in vaginal and periodontal cavities.

56. Phase behaviour Binary system Ternary system Cloud point Phases Emulsifier-oil-water Best Example

57. Phase behaviour Liquid crystalline phase

58. Phase behaviour Example of ternary system

59. Phase behaviour Ionic surfactant Factors Non-Ionic surfactant Nature of solublizate Chain length Effect of molecular structure Nature of oil phase Effect of temperature Effect of HLB

60. Phase behaviour Best example Tetrachloromethane Methyl- octanoate Caprylic acid

61. applications Surfactant as emulsifying agent. Surfactant for contact lenses cleaning. Surfactant in drug absorption from rectal suppositories. Surfactant as flocculating agent