Surface tension colloid stability

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

Energy can also be stored in other ways on a microscopic scale, e.g., by electrical charges being forced near each other in colloidal systems and by emulsion drops being distorted from the spherical shape. In this case, the surface tension gives them stabilizing surfactant layers on dispersed particles being pressed into each other. 

Several nonionic surfactants, such as the alcohol ethoxylates, can also be used as wetting agents. These molecules consist of a short hydrophobic chain (mostly Cjo) which is also branched a mediumpolyethylene oxide (PEO) mostly consisting of six EO units or fewer is used in this case. The above molecules also reduce the dynamic surface tension within a short time (<20 s) and have reasonably low cmc-values. In all cases, the minimum amount of wetting agent should be used in order to avoid interference with the dispersant that needs to be added to maintain colloid stability during dispersion and storage. 

When a relatively water-insoluble vinyl monomer, such as styrene, is emulsified in water with the aid of anionic soap and adequate agitation, three phases result (see Fig. 6.17) (1) aqueous phase in which a small amount of both monomer and emulsifier are dissolved (i.e., they exist in molecular dispersed state) (2) emulsified monomer droplets which are supercolloidal in size (> 10,000 A), stability being imparted by the reduction of surface tension and the presence of repulsive forces since a negative charge overcoats each monomer droplet (3) submicroscopic (colloidal) micelles which are saturated with monomer. This three-phase emulsion represents the initial state for emulsion polymerization. 

Donnan has investigated the problem of stability of colloidal solutions from the thermodynamic standpoint, starting with the idea of an effective negative surface tension His investigation, which is an extension of that given in Vol I of this book, will be found m the Zeitsch phystk Chem, 46, 197, 1903... 

Micellar dispersions, which contain micelles along with individual surfactant molecules, are the typical examples of lyophilic colloidal systems. Micelles are the associates of surfactant molecules with the degree of association, represented by aggregation number, i.e. the number of molecules in associate, of 20 to 100 and even more [1,13,14]. When such micelles are formed in a polar solvent (e.g. water), the hydrocarbon chains of surfactant molecules combine into a compact hydrocarbon core, while the hydrated polar groups facing aqueous phase make the hydrophilic shell. Due to the hydrophilic nature of the outer shell that screens hydrocarbon core from contact with water, the surface tension at the micelle - dispersion medium interface is lowered to the values othermodynamic stability of micellar systems with respect to macroscopic surfactant phases. 

The effective film elasticity is especially important in emulsions in which the interfacial tension is small and can not ensure the stability of surfaces against the deformation due to random causes. The Gibbs effect is a thermodynamicfactorof colloid stability (this emphasizes only the nature of the effect, and one need not assume that this factor can ensure high stability of disperse systems). 

As we shall see, this is the case in the destruction of emulsions by the coalescence of emulsion droplets. In more complex systems, which we shall not discuss, colloid stability is controlled by changes in both surface tension and area ... 

In the preceding chapters we have seen how one can in a general way understand colloid formation and stability in terms of the variation of free energy with the separation between the surfaces of two particles. When this free energy is measured with respect to the state in which the two surfaces are in contact, it may be identified with the surface or interfacial tension (see Figure 2.4). A major contribution to this surface tension arises from the van der Waals attractive forces between the particles. It turns out, however, that the surface tension, and hence the force between the two surfaces, is also strongly influenced by the adsorption of molecules at the surfaces. 

In the formation of polymer colloid particles in the size region of 1 pm, de-wetting of the particles can also occur as the monomer is used up and the surface tension of the aqueous phase rises adsorption of the particles at the air-water interface in this case can lead to a particle-stabilized foam. Particles of this sort can also be spread on a Langmuir trough and will form a hexagonally close-packed monolayer [76]. 

It is hence essential to be able to measure and modify the surface free energy of the solid if criteria other than trial and error are used to predict the wettability (and the colloidal stability, for that matter) of a solid drug, since the surface tension of the liquid is readily measurable by any well-establi.shed method (Wilhelmy plate, du Nouy ring, drop shape analysis, etc. see. e.g. Ref. 37). In the case of solids, only indirect methods are available to estimate Ys and Ysl here we shall give details on the simplest one. contact angle measurements, and the reader is referred to other chapters of this volume for details on other methodologies. The technique is based on Young s equation (37) ... 

As will be seen in later sections concerned with Uquid-fluid systems, this equation normally employed to determine the amount of adsorbed material at an interface as a function of interfacial tension, solid surfaces, it is difficult or impossible to determine a directly. It is, however, relatively easy to determine the amount of adsorbed material directly and use that information to calculate a value of the interfacial tension. Such exercises are of great theoretical importance in understanding why and how molecules are adsorbed at an interface, and of even greater practical importance for understanding how such adsorption affects the characteristics of the interface and its interaction with its surroundings, especially in the context of colloidal stability and wetting phenomena. 

Details are given of the synthesis of nanosize PS particles by miniemnlsion polymerisation. The effect of varying the surfactant concentration on interfacial tension and colloidal stability was examined. Surface tensiometry was nsed to monitor the aqneons phase surfactant concentration via a calibration curve. TEM was used to confirm particle diameters and to measure the particle size distributions. 8 refs. 

---