Adsorption of surfactants and polymers

Examples of the adsorption of dmgs and excipients on to solid surfaces are found in many aspects of dmg formulation, some of which, for example the adsorption of surfactants and polymers in the stabilisation of suspensions, are considered elsewhere in this book (see section 7.4). An interesting approach to the improvement of the dissolution rate of poorly water-soluble dmgs is to adsorb very small amounts of surfactant on to the drug surface. For example, the adsorption of Plutonic F127 onto the surface of the hydro-phobic dmg phenylbutazone significantly increased its dissolution rate when compared with untreated material."... 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

Thus, adsorption of small amounts of these surfactants can be monitored conveniently by SAXS because in the case of PS particles the main scattering intensity arises from the surface layer. In consequence, the scattering intensity of PS-particles covered by surfactant molecules strongly increases compared to the uncovered particles. Therefore PS latexes present ideal model systems for studying the adsorption of surfactants and polymers on colloidal particles from solution [53]. 

Silica is encountered in various industrial processes, and to control the performance of these processes, it is often necessary to modify its surface properties such as zeta-potential, suspension stabihty, hydrophobicity, and adsorption capacity. Surface properties of sihca are a function of its state of hydrolysis as well as pretreatment. Adsorption of surfactants and polymers can also lead to marked changes in its interfacial properties and yield desired performance. 

Surface properties can be adjusted by the adsorption of surfactants and polymers. Adsorption itself can essentially be considered to be preferential partitioning of the adsorbate into the into the interfacial region. It is the result of one or more contributing forces arising from electrostatic attraction, chemical reaction, hydrogen bonding, hydro-phobic interactions, and solvation effects. 

Measurement of zeta potential ( ) is valuable in determining the properties of dispersions. In addition, it has many other applications in various fields Electrode kinetics, electro-dialysis, corrosion, adsorption of surfactants and polymers, crystal growth, mineral flotation and particle sedimentation. 

In this section we describe the general classification of dispersing agents. The adsorption of surfactants and polymers at the solid/liquid interface was treated in Chapter 5. The various classes can be summarised as follows. 

Several interfacial aspects must be considered when dealing with agrochemical formulations (i) Both equilibrium and dynamic aspects of adsorption of surfactants at the air/liquid interface. These aspects determine spray formation (spray droplet spectrum), impaction and adhesion of droplets on leaf surfaces as well as the various wetting and spreading phenomena, (ii) Adsorption of surfactants at the oil/water interface which determines emulsion formation and their stability. This subject is also important when dealing with microemulsions, (ill) Adsorption of surfactants and polymers at the solid/liquid interface. This is important when dealing with dispersion of agrochemical powders in liquids, preparation of suspension concentrates and their stabilization. 

Several interfacial phenomena may be considered when dealing with colloidal dispersions (i) Charge separation and formation of electrical double layers, (ii) Wetting of powders and the role of surfactants, (iii) Adsorption of surfactants and polymers at the solid/liquid and liquid/liquid interfaces. 

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