Hydrophilic groups, of surfactants

Proteins solubilized in aqueous solution interact more or less with hydrophilic groups of surfactants at the oil-water interface. Therefore, the type of hydrophilic group is strongly influenced by the protein extraction efficiency. Anionic and cationic surfactants interact with charged protein surfaces more strongly than non-ionic surfactants. This feature also means that the non-ionic surfactants are favourable for protein stabilization in water droplets because of the not-so-hard interaction between the protein and the surfactant. In protein extraction, such an electrostatic interaction between proteins and surfactants is the main driving force in protein transfer. 

Depending on the nature of the hydrophilic groups of surfactants, they can be divided into anionic, nonionic, cationic, and amphoteric surfactants. The last-mentioned class only plays a minor role with respect to domestic and industrial applications and practically no methods for the environmental analysis of amphoteric surfactants have been published so far. 

This means that the mobility of water molecules as well as host molecules comes to be restricted with a decrease in This might lead to the suppression of the side reactions and the stability of the substrate or product. Furthermore, the electrostatic force can be controlled by changing the hydrophilic groups of surfactants. This is important because the localization of the ionic host molecules depends on the electrostatic force of the interface. 

Figure 1 is a schematic representation of precipitate flotation with small bubbles and surfactant ions having opposite charge to the precipitate surface. The hydrophilic groups of surfactant ions are orientated toward the surface of the precipitate, and hence the precipitate is made effectively hydrophobic. Small bubbles are easily trapped on the surface of the precipitate and flotation is achieved. An excess of surfactant serves to form a stable foam layer that prevents the redispersion of the floated precipitate into the bulk solution (see Figure 2). 

Considering the emulsification of oil in water, the strategy is to match the hydrophobic and hydrophilic groups of surfactant, respectively, with the oily and continuous aqueous phases. This can be achieved by manipulating the molecular structure of surfactant or by adjusting the composition of one or both phases. When the choice of surfactant is limited, one can adopt the concept of solubility parameter to effectively modify the recipe. As a first approximation, the solubility parameter of a mixture (finux) can be estimated by the following equation ... 

Figure 5.1 Amphiphilic nature of surfactants. We typically represent the hydrophilic groups of surfactants as spheres and the hydrophobic hydrocarbon chains as stalks (lines) the stalks are mobile...

As esters of sulfuric acid, the hydrophilic group of alcohol sulfates and alcohol ether sulfates is the sulfate ion, which is linked to the hydrophobic tail through a C-O-S bond. This bond gives the molecule a relative instability as this linkage is prone to hydrolysis in acidic media. This establishes a basic difference from other key anionic surfactants such as alkyl and alkylbenzene-sulfonates, which have a C-S bond, completely stable in all normal conditions of use. The chemical structure of these sulfate molecules partially limits their conditions of use and their application areas but nevertheless they are found undoubtedly in the widest range of application types among anionic surfactants. 

The hydrophobic group is usually a hydrocarbon chain and is generally called the tail, while the hydrophilic group, called the head, is an ionic group and very polar. Depending on the nature of the hydrophilic group, the surfactants are classified as ... 

Hydrophilic ends, of surfactants, 22 725 Hydrophilic libers, 9 158 11 168 Hydrophilic flavor compounds, 11 551 Hydrophilic fumed silica, 22 368 Hydrophilic head group, 24 137 Hydrophilic-hydrophobic block copolymers, 20 485... 

Optimum hydrophilic- lipophilic Weight percentage of hydrophilic group in surfactant molecule, Whvdro HLB = 5... 

Fig. 4.1 Emulsified particle demonstrating hydrophilic and hydrophobic end groups of surfactant...

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

The hydrophile-lipophile balance (HLB) system is the measure of the surfactant s polarity as well as other physical properties of surfactants and the emulsifying materials. The more lipophilic the surfactant is, the lower the HLB values will be. Table 4.5 empirically classifies and compares surfactants according to their optimum use. Table 4.6 shows the HLB values for a selected group of surfactants. The HLB value of the surfactant or surfactant mixture should be matched with that of the oil or the mixture of oils to ensure a stable emulsion. The required HLB values of a... 

Considering a system of two immiscible phases (e.g., heptane and water), a surface-active molecule that is adsorbed at the interface between the two liquids will tend to orient itself with its hydrophilic end toward the more polar liquid (water), and its hydrophobic end toward the less polar liquid (heptane). Thus the surfactant molecules replace water and/or heptane molecules of the original interface. The interaction across the interface is then between the hydrophilic group of the surfactant and the water molecules on one side of the interface, and between the hydrophobic group of surfactant and heptane on the other side of the interface. These interactions are much stronger than the original interactions between the unlike molecules of heptane and water therefore the interfaciai... 

Surfactant molecules may be classified based on the nature of the hydrophilic group within the molecule. The four main groups of surfactants are defined as follows ... 

A wide variety of this group of surfactants is commercially available. They include some of the Tritons (alkyl phenol additives, as far as they are non-ionic). Tweens (sorbitan fatty acid ester polyoxyethylene ethers). Spans (sorbitan fatty acid esters) and alkyl polyoxyethylenes (C Ej, where n and x stand for the number of CH2- or CH3-. and CHjO-groups in the hydrophobic and hydrophilic parts of the molecule, respectively). Given our interest in the fundamentals we shall emphasize only the last-mentioned group, and only when n and x are sharply defined emd the two moieties are either linear or branched in a defined way. Unless specified otherwise, there is an OH-group at the end of the E-chain. Notwithstanding the non-ionic nature of these molecules, micelles sometimes appear to cany a (low) charge probably caused by preferential uptake of ionic species. 

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