Molecular structure of surfactants

Hydrophilic molecules are composed of ions (such as sulphonate, sulphate, carboxylate, phosphate and quaternary ammonium), polar groups (such as primary amines, amine oxides, sulphoxides and phosphine oxide) and non-polar groups with electronegative atoms (such as oxygen atom in ethers, aldehydes, amides, esters and ketones and nitrogen atoms in amides, nitroalkanes and amines). These molecules associate with the hydrogen bonding network in water. 

Hydrophilic head Soap (sodium salt of fatty acids) 

Hydrophobic tail Alkylbenzene sulphonate Phosphatidyl choline (phospholipids) Alkyl secondary amines 

111 Polymeric alkyl phenol ethoxy-lates Silicone polymeric surfactants Polyester surfactants 

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The solubility characteristics of surfactants (in water) is one of the most studied phenomena. Even though the molecular structures of surfactants are rather simple, their solubility in water is rather complex as compared to other amphiphiles such as long-chain alcohols, etc., in that it is dependent on the alkyl group. This is easily seen since the alkyl groups will behave mostly as alkanes. The hydrophobic alkyl part exhibits solubility in water, which has been related to a surface tension model of the cavity (see Appendix B). However, it is found additionally that the solubility... 

The adsorption of surfactants at the liquid-solid surface is affected by the nature of the solid surface (surface charge, polarity and non-polarity), the molecular structure of surfactant molecules (head group charge and characteristics of hydrophobic tail) and the characteristics of the liquid phase (pH, electrolyte concentration, presence of additives and temperature). 

The molecular structure of surfactants controls not only the concentration of the surfactants at the interface and the resulting reduction in surface/interfacial tensions, but also affects the orientation of the molecules at the interface. The hydrophilic group is either ionic in nature or highly polar. Based on the nature of the polar group, surfactants can be classified as anionic, cationic, non-ionic or amphoteric. Among these types, anionic and non-ionic surfactants are preferably employed in enhanced oil recovery processes (EOR) due to their low adsorption on reservoir rocks. Therefore, these surfactants are briefly described. 

Tsujii and Tokiwa [148] concluded from the shift in the melting point of calf thymus DMA that lauroylprolylprolylglycine has a larger influence on the tertiary structure of the DNA than does SDS. The results showed that the amino acid sequence in polypeptide surfactants could play an important role in the interaction with the DNA. They concluded that the primary factor governing the interaction with DNA is the molecular structure of surfactants, rather than their surface activity or ionic nature. 

The maximum amount of surfactant, F , that can be delivered at interfaces (water-air or water-oil) depends on the ability of molecules to pack and is an important parameter for practical applications, such as detergency applications, in determining such properties as foaming and emulsification. The maximum value of the surface excess concentration is commonly called the effectiveness of adsorption of the surfactant. Extensive studies have been devoted to compare various molecular structures of surfactants for their effectiveness in reducing the interfacial tension. Table 1 provides values of r , in mol/m, and the area per molecule at the interface at maximum adsorption aZ, in A, for representative surfactants. 

The word surfactant is an acronym for surface-active agents. It stands for molecules that tend to adsorb at interfaces when they are in solution. The molecular structure of surfactants is characterized by a polar group connected to a typically long nonpolar hydrocarbon chain. The polar group, frequently referred to as the head of the molecule, is also known as the hydrophilic group because it is compatible with water (hydro). The nonpolar part, frequently referred to as the tail of the molecule, is also known as the hydrophobic (water hating) or lipophilic group because it is compatible with lipids. The relative size of (balance between) the hydrophilic to the lipophilic (frequently abbreviated and known as HLB) determines whether a surfactant will be predominantly water soluble or oil soluble. 

Measurements of the bulk solution properties (e.g., surface tension, electrical conductivity, fluorescence, and light scattering intensity) as a function of surfactant concentration can be used to determine the CMC. As shown schematically in Figure 2.2, the point at which the sudden change in surface tension occurs is taken as the CMC of the aqueous surfactant solutions. How to establish the correlation between the CMC data and the molecular structure of surfactants is of great importance in the selection of optimum surfactants for the effective stabilization of various emulsion polymerization systems. This subject will be the focus of the following discussion. 

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 7.9.1 The molecular structure of the anionic surfactant sodium lauryl sulfate.

These assumptions have been expanded upon (Shah and Capps, 1968 Lucassen-Reynders, 1973 Rakshit and Zografi, 1980), especially in regard to the application of the ideal mixing relationship in gaseous films (Pagano and Gershfeld, 1972). It has been pointed out that water may contribute to the energetics of film compression if the molecular structures of the surfactants are sufficiently different (Lucassen-Reynders, 1973). It must be noted that this treatment assumes that the compression process is reversible and the monolayer is truly stable thermodynamically. It must therefore be applied with considerable reservation in view of the hysteresis that is often found for II j A isotherms. 

Lundquist and the Stenhagens concentrated their efforts on the physical aspects of monolayer chemistry and did not elaborate then-work much in the direction of structural variation of the surfactant molecules. Their results show clearly, however, that the response of chiral monolayers to changes in surface pressure and temperature is sharply dependent on both the molecular structure of the surfactant and the optical purity of the sample. The Stenhagens were keenly aware of the possible application of the monolayer technique to stereochemical and other structural problems (72) however, they failed to exploit the full potential suggested by their initial results and, instead, pursued the field of mass spectrometry, to which they made substantial contributions. 

The force-area curves for racemic and (5 )-(+>2-tetracosanyl acetate were shown in Figures 17 and 18, respectively, while those of methyl esters of racemic and (5 )-(+)-2-methylhexacosanoic acid are found in Figs. 21 and 22, respectively. All these curves were obtained under identical experimental conditions at thevarious temperatures indicated in the figures. Simple inspection shows that the force-area curves of the two racemic samples are very similar, as are those for both optically pure samples. Lundquist suggested that this is merely a result of the very similar shapes and molecular structures of these chiral surfactants. Apart from the chain length, the only structural difference is limited to a reversal of the positions of the carbonyl group and ester oxygen. 

CED values can be determined from surface tension measurements, (2) the effects of particular molecular components of surfactant molecules on surface tension and CED can be addressed, and (3) the emulsion type and stability can be evaluated based on either molecular structure surface tension and/or CED. 

J. Weiss, J.N. Coupland, D. Brathwaite, and D.J. McClements Influence of Molecular Structure of Hydrocarbon Emulsion Droplets on Their Solubilization in Nonionic Surfactant Micelles. Colloids Surfaces A 121, 53 (1997). 

The molecular structure of the surfactant influences the form of the aggregate, and there are some geometrical empirical rules (Israelachvili et al, 1977, Israelachvili, 1992) illustrated in Figure 9.2, based on the geometrical parameters of the surfactant molecule. In particular the volume V occupied by the surfactant, the head area... 

Discuss the observed differences in permeability between (a) the two alcohols at the same film pressure and (b) the two 18-carbon surfactants at different pressures. In your comments include comparisons of the molecular structure of the surfactants and the efficiencies of these monolayers in retarding evaporation. 

Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2],... 

Scheme 4.6 Molecular structures of the Triton surfactants and Tween surfactants used for C02 absorption. Reproduced from Ref. [83] by permission of The Royal Society of Chemistry...

Finally, one word about the lattice theories of microemulsions [30 36]. In these models the space is divided into cells in which either water or oil can be found. This reduces the problepi to a kind of lattice gas, for which there is a rich literature in statistical mechanics that could be extended to microemulsions. A predictive treatment of both droplet and bicontinuous microemulsions was developed recently by Nagarajan and Ruckenstein [37], which, in contrast to the previous theoretical approaches, takes into account the molecular structures of the surfactant, cosurfactant, and hydrocarbon molecules. The treatment is similar to that employed by Nagarajan and Ruckenstein for solubilization [38]. 

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