Physical Pharmacy Interfacial phenomena Khalid T Maaroof MSc. Pharmaceutical sciences School of pharmacy Pharmaceutics department 1
Introduction The boundary between two phases is generally described as an interface. When one of the phases is a gas or a vapor, the term surface is frequently used. When boundary is between two liquids or a liquid and a solid the term interfacial phase is used 2
Real-life examples Phases (interface) Gas Gas Gas liquid (g/l) Gas - Solid (g/s) Liquid liquid (l/l) Liquid - solid (l/s) Solid - Solid (s/s) Pharmaceutical Dosage Form none Aerosols Tablets &Capsules Emulsions & lotions Suspensions Powder particles inside a capsule 3
The reason behind all these?!
Interfacial tension and surface tension Molecules in the bulk liquid are surrounded in all directions by other molecules for which they have an equal attraction (only cohesive forces). Molecules at the surface can only develop cohesive forces with other molecules that are below and adjacent to them; and can develop adhesive forces with molecules of the other phase. 5
Interfacial tension and surface tension This imbalance in the molecular attraction will lead to an inward force toward the bulk that pulls the molecules of the interface together and contracts the surface, resulting in a surface tension. Surface tension is the force per unit length that must be applied parallel to the surface to counterbalance the net inward pull. It has the units of dynes/cm or N/m. Interfacial tension is the force per unit length existing at the interface between two immiscible phases. 6
Interfacial tension and surface tension Free energy In the bulk of the liquid: each molecule is pulled equally in all directions by neighboring liquid molecules (net force of Zero) The surface layer of a liquid possesses additional energy as compared to the bulk liquid. This energy increases when the surface of the liquid increases and is therefore called surface free energy. Why a drops of liquids are usually round?
Interfacial tension and surface tension Usually the term surface tension is used when the interface is between a liquid and a gas, or a solid and a gas. While interfacial tension is used when the interface is between two liquids or two solids or a solid and a liquid. Question: Why surface tension of a liquid is more than its interfacial tension against another liquid? For example Mercury has surface tension of 476 dynes/cm while its interfacial tension against water is 375 dynes/cm 8
Measurement of Surface and Interfacial Tensions The choice of a particular method often depends on whether surface or interfacial tension is to be determined, the accuracy and convenience desired, the size of sample available, and whether the effect of time on surface tension is to be studied. Common methods Capillary rise method Ring method Drop weight and drop count method 9
Capillary rise method When a capillary tube is placed in a liquid contained in a beaker, the liquid rises up in the tube to a certain distance. By measuring this rise in the capillary, it is possible to determine the surface tension of the liquid using the formula: Where that formula came from? This method cannot be used to obtain interfacial tensions. 10
Example 11
The DuNoüy Ring Method The force necessary to detach a platinum iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The surface tension is given by the formula: 12
Surface active agents - SAAs 13
Lets first know what is adsorption Certain molecules and ions, when dispersed in the liquid, move of their own accord to the interface. the surface free energy and the surface tension of the system are automatically reduced. Molecules and ions that are adsorbed at interfaces are termed surface-active agents or surfactants. An alternative term is amphiphile,
Surface active agents (SAAs) Are solutes that are preferentially adsorbed at the surface or interface of liquid and reduce the surface or interfacial tension and therefore termed Surface active agent.. Surface active agents consists of 2 parts; A lipophilic (hydrophobic) group consisting of a long carbon chain which has little affinity for aqueous solvents A hydrophilic (or lipophobic) group consisting of polar group such as COOH, OH, which has high affinity for polar solvents These molecules are referred to as amphiphilic or amphipathic
Surface active agents (SAAs) It is the amphiphilic nature of surface-active agents that causes them to be absorbed at interfaces, Thus, in an aqueous dispersion of amphiphile, the polar group is able to associate with the water molecules. The nonpolar portion is rejected, As a result, the amphiphile is adsorbed at the interface.
Surface active agents (SAAs) For the amphiphile to be concentrated at the interface, it must be balanced with the proper amount of water- and oil-soluble groups. If the molecule is too hydrophilic, it remains within the body of the aqueous phase and exerts no effect at the interface. if it is too lipophilic, it dissolves completely in the oil phase and little appears at the interface
Structural Classification of SAAs Depending on their ionization in aqueous solution surfactants can be classified as Non-ionic SAA Ionic SAA Anionic SAA Cationic SAA Ampholytic SAA
Micelles and the Critical Micelle Concentration When present in a liquid medium at low concentrations, the amphiphiles exist separately. As the concentration is increased, aggregation occurs. These aggregates, which may contain 50 or more monomers, are called micelles The concentration of monomer at which micelles form is termed the critical micelle concentration(cmc).
Micelles and the Critical Micelle Concentration spherical micelle in aqueous media, reversed micelle in nonaqueous media, laminar micelle, formed at higher amphiphile concentration, in aqueous media.
Micelles and the Critical Micelle Concentration
Micelles and the Critical Micelle Concentration The surface tension decreases up to the CMC (CMC): the concentration at which micelles form in the solution. Beyond the CMC, the line becomes horizontal because further additions of surfactant are no longer being accompanied by a decrease in surface tension
Factor affecting CMC 1. Molecular structure of surfactant in polar solvents: Polarity of Surfactant molecules Length of hydrocarbon chain CMC CMC But in non-polar solvents: Polarity of Surfactant molecules Length of hydrocarbon chain CMC CMC 2. Effect of additives: for example addition of electrolytes (in case ionic SAAs WHY?), alcohols, and hydrocarbons can affect CMC (decrease)
Micellar solubilization Micellar solubilization is the ability of the micelles to increase the solubility of materials that are normally insoluble, or only slightly soluble, in the dispersion medium used. 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 as the number of micelles increases the extent of solubilization increases.
Wetting Adsorption at solid surfaces is involved in the phenomena of wetting and detergency. When a liquid comes into contact with the solid, the forces of attraction between the liquid and the solid phases begin to play a significant role. In this case, the behavior of the liquid will depend on the balance between the forces of attraction of molecules in the liquid and the forces of attraction between the liquid and the solid phases.
Wetting In the case of mercury and glass, attractive forces between molecules of mercury and glass are much smaller than the forces of attraction between molecules of mercury themselves (cohesion). As a result, mercury will come together as a single spherical drop.
Wetting Contact angle The contact angle is the angle between a liquid droplet and the surface over which it spreads. the contact angle between a liquid and a solid may be 0 signifying complete wetting, or may approach 180, at which wetting is insignificant.
Wetting wetting agents A wetting agent is a surfactant that, when dissolved in water, lowers the advancing contact angle, aids in displacing an air phase at the surface, and replaces it with a liquid phase. The most important action of a wetting agent is to lower the contact angle between the surface and the wetting liquid. the displacement of dirt and debris by the use of detergents in the washing of wounds is an application of wetting agents the application of medicinal lotions and sprays to the surface of the skin and mucous membranes.
Homework We discussed use of SAAs as solubilizing agents and wetting agents. Any other applications
Questions?