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Physics of Amphiphiles, Micelles and Microemulsions

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Physics of Amphiphiles, Micelles and Microemulsions Physios of Amphiphiles, Micelles and Microemulsions Vittorio Degiorgio, Pavia (Dipartimento di Elettronica, Università di Pavia) Advanced research in condensed- forces. The choice among all the possi­ Micellar Properties matter physics and statistical physics ble shapes (globular, rodlike, or disclike The most remarkable property of am­ was, until recent times, confined to sys­ micelle, vesicle) is primarily determined phiphiles in aqueous solution is that they tems of simple molecules forming well- by geometric packing constraints, but self-associate to form structures in defined simple phases. However, most the transition from one shape to the which the hydrophobic tails are in the of the molecules and states of matter we other may be induced by changing the middle, avoiding contact with water as meet in everyday life or find in biological temperature or concentration or by ad­ much as possible, and the hydrophilic systems or use in industrial processes ding a third component. Furthermore, groups are at the surface. The simplest are very complex, and their understan­ because of the existence of attractive in- possible aggregate is a globular micelle, ding requires specific experiments and termicellar interactions, micellar solu­ formed by 20-100 monomers which as­ new concepts. In the last twenty years, tions and microemulsions may present semble their hydrophobic tails in a physicists have become more and more critical consolute points which strongly hydrocarbon core and expose the polar interested in such fields as polymers and influence the properties of the system (or ionic) heads to water (Fig. 1). An ob­ biological macromolecules, liquid crys­ over a wide temperature (and concen­ vious geometric constraint to the shape tals, gels, membranes and, more recent­ tration) region. The interparticle interac­ of amphiphile aggregates is that any ly, micelles and microemulsions. All tions may generate a large variety of point inside the hydrophobic core can­ these areas of research are now truly in­ structural phases, including liquid crys­ not have a distance from the surface terdisciplinary, and show a very stimula­ tals, gels and viscoelastic solutions. larger than a monomer length (usually ≡ ting intermingling of fundamental and Over recent years research on amphi­ 1.5 - 2.5 nm). This means that the applied aspects. phile solutions has flourished. The ex­ growth of micelles is restricted to one The word amphiphile describes the tensive application of light and neutron (rod) or two dimensions (disc). Some presence in the same molecule of both scattering and of nuclear magnetic reso­ amphiphiles, like biological phospholi­ hydrophilic and hydrophobic parts. The nance has produced a wealth of impor­ pids, may form aggregates, called vesi­ hydrophilic part (sometimes called the tant experimental results. Moreover, the cles, consisting of a spherically concen­ head of the amphiphile) can be charged utilization of advanced statistical-me­ tric bilayer (or several). or polar; the hydrophobic groups are chanics methods is leading to signifi­ From the point of view of applications, usually hydrocarbons consisting of one cant theoretical contributions. the most important property of micelles or two linear chains. It is well known that when a hydrocarbon is in contact with water the network of hydrogen bonds Fig. 1 — Various amphiphile structures in aqueous solution, from J.N. Israelachvili, Ref. 4. between water molecules reconstructs itself to avoid the region occupied by hydrocarbon. This constraint on the local structure of water decreases the entropy of the water near the hydrocar­ bon and results in a larger free energy for the total system. The hydrophobic ef­ fect, therefore, arises because of the self-attraction of water for itself, which tends to squeeze the hydrocarbon out, and not because of the repulsion bet­ ween water and hydrocarbon. The opposition of properties within an amphiphilic molecule gives rise to the formation of monolayers at air-water in­ terfaces. Since the partition coefficient between surface and bulk may be very large in aqueous amphiphile solutions, extremely low amphiphile concentra­ tions may reduce very effectively the surface tension. For this reason amphi­ philes are often called surfactants or sur­ face-active compounds. Amphiphile solutions present many challenging problems. Aggregation is a multiple equilibrium process controlled by intermolecular and interaggregate 9 In region V the system is in a liquid- crystalline phase, the hexagonal phase, consisting of rod-shaped micelles of in­ definite length packed in a hexagonal array and separated by a continuous water region. Other liquid-crystalline phases (lamellar, cubic,...) may appear in different concentration-temperature re­ gions. A very interesting feature of am­ phiphile solutions is that lyotropic nema­ tic phases have been found in some sys­ tems. These phases consist of aligned rod- or disc-like micelles, and can usually be oriented in magnetic fields. Region IV is a two-phase region, typical of non­ ionic micellar solutions, in which both phases are micellar. The behaviour of the system near the consolute curve is discussed below. Interacting Brownian Particles : Critical Phenomena The most direct way of determining the aggregate size and shape is by using Fig. 2 — Typical phase diagram of a water-amphiphile system. light- or neutron-scattering techniques. However, the information obtained is that it is possible to solubilize within When an amphiphile is dissolved in through a scattering experiment is the hydrocarbon core, compounds water, micelle formation occurs only if usually a combination of single-particle which are insoluble or sparingly soluble the temperature T is above a critical properties and collective properties. If in water. Clearly, this is relevant to many value To (critical micelle temperature) an appropriate model for the pair-inter- industrial (detergents, cosmetics, emul­ and the amphiphile concentration c is action potential V(r) is available, one can sion polymerization) and biological (cho­ above co (critical micelle concentration). derive from the experiment not only mi­ lesterol solubilization) processes. Vesi­ A qualitative phase diagram is shown in cellar parameters, but also the relevant cles are useful as membrane models, Fig. 2. Region I corresponds to an parameters describing V(r). and may be helpful as drug carriers aqueous solution of monomeric am­ Since micellar solutions may show a (drugs are solubilized in the inner water phiphile. In this region the amphiphile wide variety of sizes and shapes and of region). concentration is too low thermodynami­ interaction potential, they represent a In order to understand how the size cally to favour micelle formation which very good testing ground for theoretical and shape of the aggregate are chosen implies the association of typically 100 calculations (or numerical simulations) in a specific amphiphile solution, it is monomers and is a highly co-operative on systems of interacting Brownian par­ essential to realize that the hydrophilic process. Micelle formation is not a true ticles. Some recent experiments serve headgroups do not completely cover the phase separation process, so that, to illustrate this aspect. micellar surface, but leave an energeti­ strictly speaking, there is not a critical Fig. 3 shows the collective diffusion cally unfavourable interface between concentration, but a narrow concentra­ coefficient D of sodium dodecyl sulfate the hydrocarbon core and water. The tion range below which no micelles exist (SDS) micelles in aqueous solutions at tendency to reduce this interface is and above which virtually all added am­ various NaCl concentrations, as measu­ counterbalanced by the electrostatic phiphile enters the micellar state. Region red by quasielastic light scattering. The repulsion of the head groups, so that one II corresponds to an aqueous solution of SDS micelles have an aggregation num­ can define an optimal surface area a micelles in dynamic equilibrium with ber around 100, and an electric charge which an amphiphile molecule requires monomers. The monomer concentra­ about 40 e. Their shape is globular, with at the micellar surface. This area a is tion c1 is in this region ≡ c0. The mi­ a hydrodynamic radius of 25 nm. The usually much larger than the cross-sec­ celles are globular at low concentration, dependence of D on the micelle concen­ tion of a surfactant chain. From geome­ but they often become rodlike as the tration c can be explained, at least at tric considerations it follows that the concentration is increased much above NaCI concentrations below 0.5 mol, by parameter determining the aggregate c0. Region III corresponds to an aqueous assuming that the micelle size and shape p = v/al, where v is the volume of solution of monomers co-existing with shape does not depend appreciably on c the surfactant chain. For p ≤ 1/3 only precipitated hydrated solid amphiphile. It and that the c-dependence of D is only globular micelles exist, for 1/3 < p ≤ 1/2 is believed that at the phase boundary due to intermicellar interactions. For di­ rodlike micelles are formed, for 1/2 ≤ p between regions II and III a conforma­ lute solutions, < 1 disclike micelles may exist and for p tional change of the hydrocarbon chain D = Do[1+Kd (c-c0)], = 1 an infinite bilayer is obtained. By of the amphiphile occurs : for T > T0, changing temperature or amphiphile the chain is flexible, whereas, for T < where Do is the translational diffusion concentration, or by adding electrolytes, T 0, the chain is rigid. Micelles will not coefficient of the SDS micelle and kD is a it is possible to modify the optimal head- form below T 0, because rigid molecules "dynamic second virial coefficient" rela­ group area a, and thus to induce transi­ will not easily pack into a micellar struc­ ted to the interparticle interaction poten­ tions in the aggregate structure. ture. tial V(r) by an integral expression which 10 Fig. 3 — Quasielastic light concentration in preparation for the sys­ scattering measurement of tem comprising the rodlike structures of the diffusion coefficient in an the hexagonal phase.
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