Physios of , Micelles and 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 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 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 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 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 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 . For this reason amphi­ philes are often called 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 (, 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 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 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. The full lines in Fig. ionic micellar solution as a 4 represent theoretical calculations of function of the amphiphile the scattered neutron intensity perform­ concentration at various ionic ed by assuming that the micelles are strengths. hard spheres and by using the mean spherical approximation. When the tem­ perature is increased, it is found that the fit to the experimental data requires the introduction of an attractive potential among the non-ionic micelles. This fin- contains also hydrodynamic effects (be­ cause the movement of one particle through the fluid generates a velocity field which affects the motion of neigh­ Experimental bouring particles). At low ionic strength Nuclear Physicists V(r) is dominated by the electrostatic There are vacancies for experimental physicists to join the repulsion term. At increasing ionic Nuclear Structure Division at Daresbury Laboratory, an strength, the Coulomb potential is establishment of the Science and Engineering Research Council screened more and more effectively by which operates major national facilities for scientific research. The group is involved in carrying out and supporting a research the free ions, and both the excluded programme on the Nuclear Structure Facility (NSF), a large tandem volume and attractive interaction be­ accelerator which is operating at up to 20 MV on terminal come important. The fit to the experi­ mental data allows one to determine the Senior Scientific Officer micellar radius (through Do) and the rele­ vant parameters describing V(r) Ref. DL/911 The successful applicant will be required to organise and direct (through kD). It should be added that at work necessary for the operation, maintenance and development of high ionic strength (above 0.5 mol NaCl) major equipment for charged particle identification and detection micelles grow into elongated structures. used in programmes aimed at studies of nuclear collision dynamics, spectroscopy, and nuclei far from stability. Candidates will be Presently it is not clear whether such a expected to carry out research in conjunction with university users growth can be described by an indepen­ on the NSF and to play an active role in initiating and developing dent particle model or whether interac­ new programmes. Other duties will involve the overall coordination tions play an essential role. and scheduling of the scientific programme and the assessment of future needs for instrumentation and other facilities on the NSF Fig. 4 shows small-angle neutron Applicants (male or female) should have a good honours degree scattering results obtained with an (or equivalent qualification) in physics and several years aqueous solution of non-ionic micelles. postgraduate experience in experimental nuclear research. A Ph.D qualification in nuclear physics and a period of relevant Such micelles behave at room tempera­ post-doctoral experience would be an advantage. ture and low concentration like hard spheres. The interesting feature of Fig. 4 is that the micelles remain globular even Higher Scientific Officer at concentrations which are close to the Ref. DL/913 boundary between the isotropic micellar The successful applicant will be required to liaise with university research teams using the NSF Candidates will also be expected to phase and the liquid-crystalline hexago­ collaborate in nuclear research programmes and to play an active nal phase. This behaviour is in contrast role in initiating and developing new programmes. Other duties will to the one found with ionic micelles involve work on the design and development of major equipment through to commissioning, operation and maintenance. As a which show a progressive growth with member of the in-house team of scientists he/she will be expected Fig. 4 — Scattered neutron intensity from a to provide expertise in the methods of experimental nuclear physics to aid and direct the technical and scientific support staff in non-ionic micellar solution as a function of operating the NSF and for diagnosing problems as they arise. the modulus of the scattering vector at Applicants (male or female) should have a good honours degree various amphiphile concentrations, from M. (or equivalent qualification) in an appropriate discipline with a Corti et al., Chem. Phys. Lett. 109 (1984) period of relevant post-graduate experience. A PhD degree in 579. nuclear physics and a period of post-doctoral experience in experimental nuclear research would be an advantage. The salary scales according to qualifications and experience for the Higher Scientific Officer and Senior Scientific Officer posts are £7,435 to £10,039 per annum and £9,329 to £12,050 per annum respectively. These salary scales are currently under review. There is a non-contributory superannuation scheme, a generous leave allowance and a flexible working hours scheme. CLOSING DATE: 30th June 1985. For further information please write or telephone Dr J. S. Lilley on Warrington (0925) 65000, Ext. 558. Application forms may be obtained from and should be returned quoting the appropriate reference number to: The Personnel Officer Science & Engineering Research Council, Daresbury, Warrington, WA4 4AD.

11 centration (and in temperature) because diate value for butanol because some of the radius of curvature of the interface the alcohol molecules are intercalated between the amphiphile layer and the with SDS molecules at the interface and continuous phase is fixed by the nature some move freely in the continuous of the amphiphile monomer and cannot phase. be shifted too much from the optimum Microemulsions present a large varie­ value. For instance, if we start with a ty of phenomena of great interest for water-in-oil and we add statistical physics. The microemulsion water, the droplet should swell to ac­ droplets may show an attractive pair- commodate the extra water, but the potential and exhibit critical phenomena new droplet may now be unstable, characteristic of a gas-liquid transition, because of the unfavourable bending similar to the non-ionic micelles already energy, and the system may undergo discussed. Water-in-oil microemulsions phase separation by segregating a show, at increasing droplet concentra­ water-rich phase. In order to modify the tion, an enhancement of the electric bending energy of the interface, thus in­ conductivity which strongly suggest the creasing the stability range of the micro­ existence of a percolation threshold. , an alcohol (typically butanol Furthermore, the interfacial tension bet­ or pentanol) is often added to the water- ween two coexisting microemulsion oil-amphiphile system. phases may be as low as 10-3 dyne/cm Fig. 5 — Light scattering measurement of the osmotic compressibility of non-ionic It is important to note that the micro­ (five orders of magnitude smaller than micellar solutions as a function of the tem­ emulsion structure does not always con­ the water-oil interfacial tension). At pre­ perature distance, ε = (Tc-T)/Tc, from the sist of droplets. In certain cases, as for sent, the understanding of the connec­ critical point. The slope of the plots repre­ instance in those region of the phase tions between all these phenomena is sents the critical exponent γ which appears diagram near the transition between rather limited. to depend on the nature of the amphiphile. water-in-oil and oil-in-water droplets, the It is almost needless to say that micro­ structure of the system seems to be bet­ can be extremely important ding is consistent with the existence of a ter described by a bicontinuous random for applications. Most of the early work consolute curve at high temperature partition of oil and water which are was aimed at increasing the efficiency (see Fig. 2) with a lower consolute point separated by a fluctuating amphiphile- of oil extraction from oil wells. Indeed, (critical point). The critical behaviour of alcohol interface. there are many situations in which a non-ionic micellar solutions has been The experimental study of microemul­ considerable fraction of the oil remains much investigated in recent years for sion structure has been undertaken with trapped in the microporosities of the two main reasons : a variety of scattering and NMR techni­ well and can be recovered only by pro­ i) since the micelle is a spontaneous ques. As an example, in Fig. 6 are shown moting, through additives, the forma­ aggregate it may change size and shape some results obtained with self-diffu­ tion of stable phases containing both on approaching the critical point, thus sion NMR. It is clearly seen that state 3 water and oil. introducing a new degree of freedom in corresponds to oil in water droplets (the the critical behaviour, diffusion coefficient D of oil [toluene] SUGGESTED READING ii) the observed critical exponents γ and amphiphile [SDS] molecules is the 1. Tanford C., The (see Fig. 5) and v do not coincide with same and is much lower than the diffu­ (Wiley) 1980. those derived from the renormalization sion coefficient of water) and that state 2. Micellization, Solubilization, and Micro­ group calculation and measured for all emulsions, Ed. K.L. Mittal (Plenum Press) 5 corresponds to water in oil droplets. In 1977. critical fluids and critical binary mix­ the intermediate cases the system does 3. Surfactants in Solution, Eds. K.L. Mittal, tures. The origin of this departure from not show a well-defined droplet struc­ and B. Lindman (Plenum Press) 1984. the law of universality of critical pheno­ ture. Note also that D is always low for 4. Physics of Amphiphiles Micelles, Vesicles mena is presently unknown. the amphiphile, which is constrained to and Microemulsions, Eds. V. Degiorgio and M. Corti (North-Holland) 1985. Microemulsions stay at interfaces, and takes an interme­ So far I have discussed binary sys­ Fig. 6 — NMR measurement of self-diffusion coefficients in a SDS-butanol-toluene-water tems (amphiphile plus water) or pseudo­ microemulsion, from B. Lindman et al., J. Interface Sci. 83 (1981) 569. binary systems (amphiphile plus salty water). A new phenomenology arises when we add to the amphiphile solution a water-insoluble compound (oil). Under appropriate selection of the amphiphile and of the oil, it is possible to prepare a clear, isotropic, non-viscous, thermody­ namically stable phase containing large amounts of both water and oil. This phase is called a microemulsion. The simplest possible structure of a micro­ emulsion consists of water (oil) droplets (typically, the radius is 100 A) coated with an amphiphile layer, immersed in a sea of oil (water). The droplet structure may have a limited stability range in con­ 12