Motion of surfactant molecules

Molecular Motion of Surfactant Molecules at the Air—Water Interface... 

The strong adsorption of such materials at surfaces or interfaces in the form of an orientated monomolecular layer (or monolayer) is termed surface activity. Surface-active materials (or surfactants) consist of molecules containing both polar and non-polar parts (amphiphilic). Surface activity is a dynamic phenomenon, since the final state of a surface or interface represents a balance between this tendency towards adsorption and the tendency towards complete mixing due to the thermal motion of the molecules. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

In two-phase systems, however, where surfactant and water can partition between a fluid and a liquid phase, significant pressure effects occur. These effects were studied for AOT in ethane and propane by means of the absorption probe pyridine N-oxide and a fluorescence probe, ANS (8-anilino-l-naphthalenesulfonic acid) [20]. The UV absorbance of pyridine A-oxide is related to the interior polarity of reverse micelles, whereas the fluorescence behavior of ANS is an indicator of the freedom of motion of water molecules within reverse micelle water pools. In contrast to the blue-shift behavior of pyridine N-oxide, the emission maximum of ANS increases ( red shift ) as polarity and water motion around the molecule increase. At low pressures the interior polarity, degree of water motion, and absorbance intensity are all low for AOT reverse micelles in the fluid phase because only small amounts of surfactant and water are in solution. As pressure increases, polarity, intensity, and water motion all increase rapidly as large amounts of surfactant and water partition to the fluid phase. The data indicate that the surfactant partitions ahead of the water thus there is a constant increase in size and fluidity of the reverse micelle water pools up to the one-phase point. An example of such behavior is shown in Fig. 4 for AOT in propane with a total fVo of 40. The change in the ANS emission maximum suggests a continuous increase in water mobility, which is due to increasing fVo in the propane phase, up to the one-phase point at 200 bar. 

Hydrocarbon, typically an alkyl chain of 12-16 carbons, constitutes the major of surfactant molecules, offering H and nuclei for NMR studies. Being /=l/2 nuclei, their relaxation is due mainly to dipole-dipole interactions. The easy access and the relatively high sensitivity (particularly important in early continuous-wave NMR studies) has made H NMR (in particular bandwidth measurements) studies particular popular. Micellar growth can easily be studied in a qualitative manner by monitoring the bandwidths of aliphatic (-CH2-) protons in the spectrum. A quantitative analysis is, however, complicated by the coupling of the various protons, with locally different motional characteristics, along the alkyl chain. 

The slow motion is a combination of droplet tumbling and the lateral diffusion of surfactant molecules within the surfactant film. For spherical aggregates, this motion is described by a Lorentzian spectral density function,... 

Alteration of the surface tension of the DEG-l-OP-10 system with temperature is greatly influenced by the surfactant concentration (Fig. 2.2). At llkg/m of OP-10, the surface tension does not depend on temperature, and at high surfactant concentrations even increases. The OP-10 associations with DEG-1 must be of increased solubility and must easily desorb from the boundary surface in the course of raising the temperature. Thus, temperature increase may result both in decrease of the surface tension on account of increase of the thermal motion of the molecules eind in its increase due to the desorption of surfactant molecules, and this increase has a direct dependence on the sxufactant concentration. Superposition of these factors for certain surfactant concentrations can bring about independence of the system siuface tension on temperature, confirmed by experiment. In the case of siuTactant desorption, the system entropy increased due... 

A very different kind of surfactant molecule obtains if the carboxylate head group of an alkanoic acid is replaced by one that is both hydrophobic and lipophobic, like a perfluoroalkyl group. Such diblock molecules, F(CF2) (CH2) H(FmHn), have been shown to form normal and reverse micelles in perfluoroalkanes and alkanes, respectively [68,69]. On the bases of the well-known antipathy between hydrocarbons and perfluorocarbons, the disparity between cross-sectional areas and volumes of CH (18.5 and 10.22 A ) [70, 71] and CF (28.3 A and 16.04 A ) [70, 71] groups, and the tendency of long perfluoroalkyl chains to adopt helical conformations, it is expected and found that organized assemblies of FmH/i molecules exhibit some strange properties. For instance, the alkyl portions of many of these molecules melt before their perfluoroalkyl portions [72], and the allowed motions resemble closely those of -alkanes in their rotator phases [73, 74]. 

The above processes are also influenced by the change in the system temperature. Raising the temperature results in increase of the polymerization reaction rate and at the same time in decrease of the tendency of surfactant molecules to aggregate because of the increase of thermal motion that follows from the Debye equation ... 

The notion of hydrophobic interaction was well developed by Tanford [9]. When a nonpolar solute is dissolved in water, some hydrogen bonds are disrupted. The solute tends to locally distort the water structure and to restrict the motion of water molecules. Thus, a large entropy increase in the water molecules is associated with the removal of the nonpolar solute from aqueous solution [9]. This entropy increase is responsible for the surface activity and micelle formation of surfactant molecules. 

Micellar solutions are sometimes called ordered media [12]. The chemical order in a micellar solution seems to be greater than in a classical solution. Equation 2.9 shows ftiat the micellization of surfactant molecules obeys the second principle of thermodynamics. It seems that the surfectant hydrocarbon chains have a much higher freedom of motion inside the micelle core than in the water bulk [13]. The micelle structure minimizes the molecule energy. The large entropy increai of water molecules associated with the removal of nonpolar surfactant tails from the aqueous solution (hydrophobic effect) is the main micelle driving force. Electrostatic forces tend to separate the polar heads that bear the same charge. The whole micelle is an equilibrium between these forces. This equilibrium is very sensitive to any chemical additive or parameter that can act on any of the forces, such as salts, polar or nonpolar solutes, temperature and/or pressure. 

On the basis of literature data and the tests performed, it can be stated that the effective action of the mixtures with SML/ESMIS ratios 3 7 and 5 5 results from the most favorable packing of surfactant molecules in the interfacial film at these ratios. As a result, when the stability of the adsorbed film is the highest, it leads to the highest load-carrying capacity as well as the largest decrease in motion resistance and wear. 

In view of these investigation results, it has been assumed in this chapter that an appropriate sorbitan-ester/ethoxylated-sorbitan-ester composition will also be capable of producing stable adsorbed films at the lubricant/friction-pair material interface. It is predicted that an optimal arrangement of surfactant molecules in the surface phase can result in a very effective reduction in resistance to motion and in wear and seizure prevention. 

The dynamic processes that can occur can be summarized as (a) motion of interfacial molecules, (b) exchange, and (c) coalescence/fission of drops. Motion of interfacial molecules refers to the mobile hydrophobic chains of surfactant molecules adsorbed at the interface. Ahlnas et al. [102] determined that this occurs on the picosecond timescale. Exchange processes can be of surfactant, cosurfactant, or dispersed solvent. For a dispersed phase of water, the exchange between immobilized or bound water (in a layer associated with the polar surfactant head groups) and free water was determined to occur on the millisecond timescale by NMR [103]. 

The first mechanism is due to interfacial turbulence, which may occur as a result of mass transfer. In many cases the interface shows unsteady motions streams of one phase are ejected and penetrate into the second phase, shredding small droplets (Figure 14.1). Localised reductions in interfacial tension are caused by the non-uniform adsorption of the surfactant at the oil/water interface [14] or by mass transfer of surfactant molecules across the interface [15, 16]. With two phases that are not in chemical equilibrium, convection currents may form, conveying liquid rich in surfactants towards areas of liquid deficient of surfactant [17, 18]. These convection currents may give rise to local fluctuations in interfacial tension, causing oscillation of the interface. Such disturbances may amplify themselves, leading to violent interfadai perturbations and eventual disintegration of the interface, when liquid droplets of one phase are thrown into the other [19]. 

Chachaty and co-workers [8.20, 8.37, 8.38] were first to describe correlated internal motions in alkyl chains of surfactant molecules that form lyotropic liquid crystals. The last section described an extension of the master equation method of Wittebort and Szabo [8.4] to treat spin relaxation of deuterons on a chain undergoing trans-gauche jump rotations in liquid crystals. This method was also followed by Chachaty et al. to deal with spin relaxation of nuclei in surfactants. However, they assumed that the conformational changes occur by trans-gauche isomerization about one bond at a time. In their spectral density calculations (see Section 8.3.1), they used a transition rate matrix that was constructed from the jump rate Wi, W2, and Ws about each bond. Since W3 is much smaller than Wi and W2, the time scale of internal motions was practically governed by Wi and W2 of each C-C bond. Since... 

Motion of Hydrophobic or Hydrophilic Chains of Surfactant Molecules... 

H NMR spectroscopy using D2O as a probe is widely used to study the phase behavior of more concentrated surfactant systems [7, 41] but provides less information for very dilute systems, in which the residual quadrupole coupling resulting from the anisotropic motion of water molecules next to the bilayers is smaller than the line width. In favorable cases, when the domains of a lamellar phase are large and free of defects which lead to curved bilayers, quadrupole... 

The structure of these globular aggregates is characterized by a micellar core formed by the hydrophilic heads of the surfactant molecules and a surrounding hydrophobic layer constituted by their opportunely arranged alkyl chains whereas their dynamics are characterized by conformational motions of heads and alkyl chains, frequent exchange of surfactant monomers between bulk solvent and micelle, and structural collapse of the aggregate leading to its dissolution, and vice versa [2-7]. 

---