Amphiphile hydrocarbon

Amphiphile hydrocarbon chain length increases Aqueous phase salinity increases Aqueous phase electrolyte valence increases (Na" Oil phase length (hydrocarbon) increases... 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Another important class of materials which can be successfiilly described by mesoscopic and contimiiim models are amphiphilic systems. Amphiphilic molecules consist of two distinct entities that like different enviromnents. Lipid molecules, for instance, comprise a polar head that likes an aqueous enviromnent and one or two hydrocarbon tails that are strongly hydrophobic. Since the two entities are chemically joined together they cannot separate into macroscopically large phases. If these amphiphiles are added to a binary mixture (say, water and oil) they greatly promote the dispersion of one component into the other. At low amphiphile... 

Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

There has been considerable interest in the simulation of lipid bilayers due to their biological importance. Early calculations on amphiphilic assemblies were limited by the computing power available, and so relatively simple models were employed. One of the most important of these is the mean field approach of Marcelja [Marcelja 1973, 1974], in which the interaction of a single hydrocarbon chain with its neighbours is represented by two additional contributions to the energy function. The energy of a chain in the mean field is given by ... 

Carboxylate groups are hydrophilic ( water loving ) and tend to confer water sol ubility on species that contain them Long hydrocarbon chains are lipophilic ( fat loving ) and tend to associate with other hydrocarbon chains Sodium stearate is an example of an amphiphilic substance both hydrophilic and lipophilic groups occur within the same molecule... 

Monolayers at the Air—Water Interface. Molecules that form monolayers at the water—air interface are called amphiphiles or surfactants (qv). Such molecules are insoluble in water. One end is hydrophilic, and therefore is preferentially immersed in the water the other end is hydrophobic, and preferentially resides in the air, or in a nonpolar solvent. A classic example of an amphiphile is stearic acid, C H COOH, wherein the long hydrocarbon... 

The first synthesis of amphiphilic porphyrin molecules involved replacement of the phenyl rings in TPP with pyridine rings, quaternized with C2QH 2Br to produce tetra(3-eicosylpyridinium)porphyrin bromide (3) (36). The pyridinium nitrogen is highly hydrophilic the long C2Q hydrocarbon serves as the hydrophobic part. Tetra[4-oxy(2-docosanoic acid)]phenyl-porphyrin (4) has also been used for films (37). 

The conditions for surfactants to be useful to form Hquid crystals exist when the cross-sectional areas of the polar group and the hydrocarbon chain are similar. This means that double-chain surfactants are eminently suited, and lecithin (qv) is a natural choice. Combiaations of a monochain ionic surfactant with a long-chain carboxyHc acid or alcohol yield lamellar Hquid crystals at low concentrations, but suffer the disadvantage of the alcohol being too soluble ia the oil phase. A combination of long-chain carboxyHc acid plus an amine of equal chain length suffers less from this problem because of extensive ionisa tion of both amphiphiles. 

For gas absorption, the water or other solvent must be treated to remove the captured pollutant from the solution. The effluent from the column may be recycled into the system and used again. This is usually the case if the solvent is costly (e.g., hydrocarbon oils, caustic solutions, amphiphilic block copolymer). Initially, the recycle stream may go to a treatment system to remove the pollutants or the reaction product. Make-up solvent may then be added before the liquid stream reenters the column. 

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

A microemulsion (p.E) is a thermodynamically stable, transparent (in the visible) droplet type dispersion of water (W) and oil (O a saturated or unsaturated hydrocarbon) stabilized by a surfactant (S) and a cosurfactant (CoS a short amphiphile compound such as an alcohol or an amine) [67]. Sometimes the oil is a water-insoluble organic compound which is also a reactant and the water may contain mineral acids or salts. Because of the small dispersion size, a large amount of surfactant is required to stabilize microemulsions. The droplets are very small (about 100-1000 A [68]), about 100 times smaller than those of a typical emulsion. The existence of giant microemulsions (dispersion size about 6000 A) has been demonstrated [58]. 

The most commonly used amphiphiles to build L-B hlms for tribological applications are the straight chain hydrocarbon compounds with simple functional groups such as the fatty acids, including stearic acids, arachidic acids, and behenic acids [32], but other amphiphilic molecules, e.g., 2,4-heneicosanedione and 2-docosylamina-5-nitropyridine, are also applied in some cases. There are two major systems of self-assembled monolayers, namely the alkylsilance derivatives (e.g., OTS, octadecyltrichlorosilane) on hydroxylated surfaces and the alkanethiols on metal substrates, which have been investigated extensively to examine their properties as solid lubricants and protective surface films [31 ]. 

Traditional amphiphiles contain a hydrophilic head group and the hydrophobic hydrocarbon chain(s). The molecules are spread at molecular areas greater (-2-10 times) than that to which they will be compressed. The record of surface pressure (II) versus molecular area (A) at constant temperature as the barrier is moved forward to compress the monolayer is known as an isotherm, which is analogous to P-V isotherms for bulk substances. H-A isotherm data provide information on the molecular packing, the monolayer stability as de-... 

By small-angle neutron scattering experiments on water/AOT/hydrocarbon microemulsions containing various additives, the change of the radius of the miceUar core with the addition of small quantities of additives has been investigated. The results are consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water/surfactant interfacial region, decreasing the micellar radius, whereas toluene remains predominantly in the bulk hydrocarbon phase. The effect of n-alcohols on the stability of microemulsions has also been reported [119],... 

Surface active agents (surfactants) are active (adsorb) at surfaces and reduce surface tensions. Surfactants work because they are amphiphilic they have opposing solubility tendencies in one molecule, such as a hydrocarbon chain and a polar end. Because of this disparity in solubility, they tend to form concentration gradients at dissimilar phase interfaces. Surfactant additives are classified according to the interface at which they are active. 

It is essential to state that the heavy fractions such as asphaltene and preasphaltene do contain large numbers of polar molecules (23.24). These polar molecules behave exactly as surfactants or amphiphiles (asphaltene usually contains a long-chain substituent (25)). We again have to emphasize that it is almost not possible to create a colloidal micelle from pure hydrocarbon and water without any surfactant. Hence, we conclude to say that asphaltene or asphaltene-like molecules (as-phaltics) will participate in a manner according to membrane-mimetic chemistry. 

Israelachvilli JN, Mitchell DJ, and Ninham BW. 1976. Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers. Journal of the Chemical Society, Faraday Transactions 2 Molecular and Chemical Physics 72 1525-1568. 

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