Surfactant Structures and Sources

FIGURE 33. When adsorption occurs at the interface, the adsorbed molecules will have a preferred orientation that tends to minimize tmfavorable interactions between the aqueous phase and the surfactant molectrlar sections. 

In order to understand the relationship between the surface activity of a given material and its chemical structure, it is important to understand the chemistry of the individual chemical components that in concert produce the observed phenomena. The following discussion will introduce some of the structural aspects of surfactants, ranging from basic raw materials and sources to the chemical group combinations that result in the observed surface activity. Since 

The chemical reactions that produce most surfactants are rather simple, understandable to anyone surviving the first year of organic chemistry. The challenge to the producer hes in the implementation of those reactions on a scale of thousands of kilograms, reproducibly, with high yield and high purity (or at least known levels and types of impurity), and at the lowest cost possible. With very few exceptions, there will always be a necessity to balance the best surfactant activity in a given application with the cost of the material that can be borne by the added value of the final product or process. 

---

The physico-chemical theory of surface activity is a vast field and no more than broad principles can be touched on here major reference sources exist for those who require more detail of the relationship between chemical structure and the various surfactant properties such as wetting, detergency and emulsification-solubilisation [32-36]. 

Through a co-assembling route, mesostructured lamellar molybdenum sulfides are formed hydrothermally at about 85 °C using cationic surfactant molecules as the templates. The reaction temperature and the pH value of the reaction system are important factors that affect the formation of the mesostructured compounds. The amount of the template and that of the S source are less critical in the synthesis of the compounds. For the three as-synthesized mesostructured materials, the interlayer distance increases linearly with the chain length of the surfactant. Infrared and X-ray photoelectron spectroscopy reveals that the individual inorganic layers for the three compounds are essentially the same both in composition and in structure. The formal oxidation state of the molybdenum in the materials is +4 whereas there exist S2 anions and a small amount of (S-S)2 ligands in the mesostructures. The successful synthesis of MoS-L materials indicates that mesostructured compounds can be extended to transition metal sulfides which may exhibit physico-chemical properties more diverse than non-transition metal sulfides because of the ease of the valence variation for a transition metal. 

Ionic compounds can also gelate solvents, perhaps one of the nicest examples being that of dicationic gemini surfactants in which tartrate is used as the counterion and source of chirality [158], because it shows the very important role of chirality on the property of the salt. When either d- or L-tartrate dianions and dimers of cetyltrimethylammonium cations are combined, stable gels are formed in chlorinated solvents, but neither the mixture of enantiomers nor the meso tartrate form a gel. The structure of the gelator... 

Among the many varieties of mesoporous materials several stand out as most extensively studied. The initial 3 structures [3,4] remain dominant although MCM-50 has attracted least attention and found little practical value. It has an unstable structure collapsing upon calcination but still exhibits considerable microporosity, which is intriguing and possibly deserving closer attention. However, if a post treatment by adding a reactive silica source, e.g. TEOS, to the as-synthesized MCM-50 is done prior to air calculations, then a uniform mesopore structure (after calcination to remove the surfactant) with retention of the lamellar-like XRD is observed. MCM-41 and MCM-48 type structures and SBA-15 are considered here as representative of the entire class and to reflect the expected range of characteristics and behavior. 

Vegetable oils and natural fats are traditional raw materials for the production of soaps and other surfactants. Coconut oil, palm and palm kernel oil, rape oil, cotton oil, tall oil, as well as the fats of animal origin (tallow oil, wool wax), present renewable raw sources. Linear paraffins and olefins (with terminal or internal double bond), higher synthetic alcohols, and benzene are fossil sources for surfactant production which are obtained from oil, natural gas and coal. Other auxiliary materials are required to construct amphiphilic surfactant structure, such as ethylene oxide, sulphur trioxide, phosphorous pentaoxide, chloroacetic acid, maleic anhydride, ethanolamine, and others. 

CMK-2 [114] is an ordered mesoporous carbon obtained from sucrose as a source of carbon and SBA-1 silica as template [Fig. 34b)]. Electron diffraction showed that this carbon is composed of c s iirtercoimected with two different types of pores (meso- and micropores). CMK-3 [115] is an ordered mesoporous carbon that was synthesized using SBA-15 mesoporous silica as template and sucrose as carbon source. The structure of CMK-3 was the faithful replica of the mesoporous silica template, as revealed by XRD and TEM. This carbon had a hi BET surface area (1500 n g % and a pore size around 4.5 nm. The systematic control of pore wall thickness of hexagonal mesoporous silica templates (by varying the ratio of surfactants CwTAB and CmEOg, Fig. 35) afforded a close control of the... 

Similar to most chemical systems of interest, the characterization of colloidal solutions requires the determination of the size, shape, structure, and stability of the particles present. This information is especially important for the understanding and utilization of organized assemblies of surfactants, in particular microemulsions, because the physical properties of the particles usually depend strongly on the thermodynamic conditions such as overall composition, temperature, and external force fields. This dependence is mainly due to the sensitivity to conditions of the monomer-aggregate equilibrium of the surfactant, which is responsible for the existence of the particles, and to the delicate balance of forces that maintain their integrity. For microemulsions, an additional complication arises from the compartmentalization of the systems, which is a source of possible phase transitions but is also a reason for most of their practical applications. 

Of the different animal species, pig skin is usually considered as the closest in structure to human skin, although pig skin s thickness is much greater. However, the difficulty of manipulating pigs in the laboratories has limited their use as live animal models and made them an ideal source for studies on excised animal skin. Rabbit, rats, and mice are more favored for laboratory tests despite the problems linked to the hairs for topical application of surfactant solutions, and increased penetration rate of the substances along the hair follicles. 

---