Selection of a Surfactant

An excellent article by Bernhardt [69] tabulates dispersion systems for hundreds of ceramic powders. These dispersion systems consist of a solvent and surfactant with a range of useful concentrations listed. The solvents are both aqueous and nonaqueous and the surfactants are ionic, nonionic, and ionic polymers. This is the most extensive table of established dispersion systems available in the literature today. 

For organic solvents, a system based on the relative strengths of the hydrophilic and lipophillic (i.e., hydrophobic) portions of the surfactant, 

TABLE 9.13 Adsorption Kinetics for Poly (Acrylic Add) (Mw 5,000) onto BaTiOs (2.9 m gm ) from Aqueous Solution  

A surfactant HLB number can be estimated roughly by mixing a small portion of the surfactant with water and observing the nature of the mixture. On the basis of Table 9.15 an approximate HLB munber can be assigned for these observations. 

The use of the HLB number to select a surfactant (or mixture of surfactants) is achieved by matching the surfactant HLB number to that of the material being dispersed. Unfortunately, little information is available on the HLB munber for ceramic powder surfaces. What data there exists is given in Table 9.16. For ceramic systems, the HLB of the surfactant is usually optimized by experiments with various surfactants. 

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Chemically, the preparation of a "stable" foam or emulsion requires the use of a surfactant to aid in dispersion of the internal phase and prevent the collapse of the foam (or emulsion) into separate bulk phases. The selection of a surfactant is made on the basis of severity of conditions to be encountered, the gas to be entrained (N2, C02, LPG, CH, or air), the continuous phase liquid (water, alcohol, or oil), and half-life of foam stability desired. 

Since that time an enormous number of surfactants covering a wide range of chemical and physicochemical properties have been developed for quite universal as well as specific tasks in domestic and industrial applications. The criteria for selection of a surfactant for industrial production is directly connected with the feasibility of large-scale production. This is determined by several factors including availability and costs of raw materials, cost of manufacture, and performance of the finished products. In addition to these aspects, environmental considerations likewise play an increasingly important role. 

Angus (23) and Couch (24). The selection of a surfactant to optimize the physical characteristics of the formulation is acconplished by several methods reviewed in depth (25) (26). 

Method development in MLC requires first the selection of a surfactant/organic solvent system. The second step concerns the optimization of the selectivity. The separation can be improved by varying only one factor, or modifying one after optimizing the other (i.e., proton, surfactant, and organic solvent concentration). However, in operations like this, the best separation conditions can be easily missed. Reliable optimal conditions can be obtained only when all factors are simultaneously taken into account. This requires the use... 

One of the factors that determines foam propagation and foam-flood economics is surfactant loss in the reservoir, most importantly adsorption at the solid—liquid interface. Adsorption levels of foaming surfactants, mostly those suitable for high salinity conditions, cover a wide range and lead to vastly different distances of foam propagation. Therefore, selection of a surfactant with minimal adsorption levels for the reservoir conditions of interest is crucial. 

The relationship between the chemical structure of a surfactant and its resultant surface-active properties is quite complex. Many of the following chapters will point out general rules relating the two types of information. In the end, it will usually be firsthand experience that leads to a final decision on the selection of a surfactant for a given end use. That experience can be made less painful, however, by the application of rules of thumb and chemical common sense. 

For evaluating the surfactants for the abovementioned applications, one approach is to select the suitable surfactants based on the hit and trial methodology. By this way, each candidate surfactant would have to be evaluated by actually making the given formulation and checking the formulation for various criteria. This is a sure short method of selection of a surfactant for a given formulation. However, this is not a sdentilic approach. 

Because of the cost and the time factors involved, oil displacement studies are always preceded by certain test tube screening procedures. Specifically, the interfacial tension (IFT) of less than 0.01 dyne/cm is recognized to be the necessary but not the sufficient criterion for selection of a surfactant system. Many investigators (10-15) have shown that ultralow IFT of less than 0.001 dyne/cm can be achieved with less than 0.1 wt. % surfactant solution. Since this low surfactant concentration system is several hundred times more dilute than the ones used in a typical surfactant-polymer flooding process, the economics dictates that the oil displacement by such low surfactant concentration solution should be explored. Moreover, it should be established that the... 

For a series of anionic surfactants with the same ionic head group, the lifetime of a micelle decreases with decreasing alkyl chain length of the hydrophobic component. Branching of the alkyl chain could also play lui important role in the lifetime of a micelle. It is, therefore, important to carry out dynamic surface tension measurements when selecting a surfactant as an adjuvant as this may play an important role in spray retention. However, these above measurements should not be taken in isolation as other factors may also play an important role, e.g. solubilization which may require larger micelles. The selection of a surfactant as an adjuvant requires knowledge of the factors involved. 

The amphiphilic nature of non-ionic surfactants may be expressed in terms of the balance between the hydrophobic and hydrophilic portions of the molecule. An empirical scale of HLB (hydrophile-lipophile balance) numbers was devised by Griffin which is useful in the selection of a surfactant mixture for the emulsification of a particular oil (see Chapter 8). Although applied mainly to nonionic surfactants, the HLB system may also be used for ionic surfactants. For... 

The findings of this study clearly vindicate the HLB approach to the selection of a surfactant for the preparation of suspensions of drug compounds. 

Selection of a surfactant with good surface properties, i.e., effective and efficient reduction of surface and interfacial tensions and effective adsorption. When adsorption of the surfactant makes the surface of the liquid or solid more hydrophilic, the interfacial tension between the aqueous solution and the second phase will be decreased and it will be easier to increase the area of that interface. 

If the cost of the surfactant is signihcant compared to that of other components of a system, the least expensive material producing the desired effect will usually be preferred, all other things being equal. Economics, however, cannot be the only factor, since the final performance of the system will be of crucial importance. To make a rational selection of a surfactant, without resorting to an expensive and time-consuming trial-and-error approach, the formulator must have some knowledge of... 

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