Solid-liquid interface, detergents

The surface tension of the system can also be changed by grinding with liquid, thus decreasing interfacial tension. This gives rise to a variety of parameters since, by adding suitable chemicals (electrolytes or surface-active agents), one can modify the end-product properties. Conversely, the size of crystals formed from a supersaturated solution of a substance is related to the surface tension (at the solid-liquid interface). Thus, to obtain fine crystals, a suitable detergent is added, and thus, finer crystals are obtained. 

Synthetic surfactants and polymers are probably most often used to modify the characteristics of a solid surface, i.e., they function at the solid - liquid interface, such as in the processes of detergency, lubrication, or the formation of adhesive bonds. The performance of modem FT - IR spectrometers is such that many new applications to the characterization of the solid - liquid interface, particularly in kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated total reflectance (ATR) techniques have been applied to "wet" interfaces, even the air - water interface, and have figured prominently in recent studies of "self -assembled" mono - and multilayers. 

Since these interfaces are usually constructed of charged detergents a diffuse electrical double layer is produced and the interfacial boundary can be characterized by a surface potential. Consequently, electrostatic as well as hydrophilic and hydrophobic interactions of the interfacial system can be designed. In this report we will review our achievements in organizing photosensitized electron transfer reactions in different microenvironments such as bilayer membranes and water-in-oil microemulsions.In addition, a novel solid-liquid interface, provided by colloidal Si02 particles in an aqueous medium will be discussed as a means of controlling photosensitized electron transfer reactions. 

For optimum surfactant adsorption at solid/liquid interfaces, mixed micelles have more efficient packing, which in turn contributes to better detergency by lowering the CMC and interfacial tension. 

Probably the most commonly encountered solid-liquid interfaces involve solids in contact with aqueous solutions, and these are the only systems that will be considered here, with emphasis on aspects relevant to detergency applications. 

This chapter will mainly describe and tentatively explain how a polymer and a surfactant can influence each other s adsorption profile at different solid-liquid interfaces. All the studies that are reported here have been realized in water. Nonaqueous systems are not discussed here, as they are not widely applicable in industrial processes and are even totally irrelevant in detergency science. 

The technological, environmental, and biological importance of adsorption from solution onto a solid surface can hardly be overestimated. The impact of such phenomena on our everyday lives is evident in such areas as foods and food science, agriculture, cosmetics, pharmaceuticals, mineral ore froth flotation, cleaning and detergency, the extraction of petroleum resources, lubrication, surface protection, and the use of paints and inks. Each of these applications, and many more, would be difficult if not impossible in the absence of the effects of adsorbed surfactants and stabilizers at the solid-liquid interface. 

The adsorption of surface-active materials onto a solid surface from solution is an important process in many situations, including those in which we may want to remove unwanted materials from a system (detergency), change the wetting characteristics of a surface (waterproofing), control the triboelectric properties of a surface (static control), or stabilize a finely divided solid system in a liquid where stability may otherwise be absent (dispersion stabilization). In these and many other related applications of surfactants or amphiphilic materials, the ability of the surface-active molecule to situate itself at the solid-liquid interface and produce the desired effect is controlled by the chemical natures of the components of the system the solid, the surfactant, and the solvent. The following discussions summarize some of the factors related to chemical structures that significantly affect the mechanisms of surfactant adsorption and the orientation with which adsorption occurs. 

Surfactants that effectively adsorb at the solid-water and dirt-water interfaces make the best detergents. Consider a dirt (oil, mineral, carbon, etc.) particle adhering to a solid surface immersed in a liquid. The work of adhesion (AG) between the dirt and solid is,... 

The adsorption mechanisms of surfactant at interfaces have been extensively studied in order to understand their performance in many processes such as dispersion, coating, emulsification, foaming and detergency. These interfaces are liquid-gas (foaming), liquid-liquid (emulsification) and liquid-solid (dispersion, coating and detergency). 

In this chapter, we will discuss how the chemical and physical properties of substances at interfaces differ from those in the bulk. For quantitative description, quantities like surface tension and surface energy have to be introduced. With the help of these quantities, phenomena known from everyday life like the lotus effect can be explained. However, perhaps you are more interested to learn how detergents clean Then have a look at Sect. 16.3 which deals with the adsorption on liquid surfaces. The next section covers the adsorption on solid surfaces and the variation of the extent of coverage with pressure or concentration of the substance to be adsorbed. Langmuir s isotherm, the simplest description of such an adsorptiOTi process, is deduced by kinetic interpretation of the adsorption equilibrium. Alternatively, it can be derived by introducing the chemical potential of free and occupied sites and cmisideiing the equilibrium condition. In the last part of the chapter, some important applications such as surface measurement and adsorption chromatography are discussed. 

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