Controlling surfactant self-assembly

doi:10.1016/j.cocis.2004.06.008

Current Opinion in Colloid & Interface Science

Volume 9, Issues 3–4, November 2004, Pages 201-212

https://doi.org/10.1016/j.cocis.2004.06.008 Get rights and content

Abstract

Spontaneous self-assembly of surfactants in water to form vesicles has been reviewed, starting from the hypothesis that monomer solubility rather then packing parameters based on geometrical considerations could be utilized as the criterion to control this process. After a brief review of current theoretical considerations, several groups of surfactants have been evaluated. Surfactants were grouped based on criteria affecting monomer solubility: (i) changes in solution composition and temperature, (ii) enhanced attractive interactions between monomers, and (iii) changes in monomer structure. Main objectives were finding systems that result in stable, monodisperse, and small unilamellar vesicles with applicability in drug delivery, and identifying inexpensive processes to produce these systems. Several surfactant systems have been found that could meet these requirements, certain phospholipid surfactants, cationic/anionic mixtures, surfactants carrying sugar head groups, block copolymers, and mixtures between diblock copolymers and conventional surfactants. These examples clearly support above hypothesis that monomer solubility can be utilized as the criterion to control surfactant self-assembly. In addition, three approaches involving simple processing of surfactant systems have been discussed as potential drug carrier systems, utilizing (i) preformed vesicles as a matrix, (ii) inverted emulsions, and (iii) stabilized core-shell micelles.

Introduction

The formation of particulate systems with defined particle size and shape is of eminent interest in medical and technical applications. For example, particles utilized as carriers in drug delivery applications generally should be in the nanometer range, uniform in size and, preferably, spherical in shape to enhance their ability to cross cell membranes and reduce the risk of undesired clearance from the body through liver and spleen. On the other hand, changes in the rheological behavior of complex fluids strongly depend on the orientation of particles with high aspect ratio, i.e., rods and fibers, relative to the flow direction. Surfactants are an important class of molecules whose ability to self-assemble in water into a large variety of morphologically different structures makes them prime candidates for particulate systems. The phase behavior of many surfactant systems has been studied in detail and reported in numerous articles and reviews [1^•]. In general, with increasing concentration surfactants assemble to form micelles above the system-specific critical micelle concentration (cmc), followed by the formation of various mesophases and, finally, crystal formation. In some cases, micelle-to-vesicle transformations have been observed as a part of the phase sequence. This morphological transformation can be triggered by a large variety of parameters, which have been thoroughly reviewed very recently [2^•]. However, controlling the self-assembly of surfactants with the goal to form well-defined structures in a predictable and reproducible way remains a major challenge. The main factors that have to be taken into considerations are (i) molecular size and geometry, (ii) intermolecular interactions, (iii) mixtures of surfactants, (iv) addition of co-solutes, (v) condition of the bulk solution (i.e., salt concentration, temperature, pH, metal ions), and (vi) application of external forces (i.e., shear, sonication).

This review will focus on spontaneous vesicle formation in water via monomer assembly from solution and transformation from micelles. Emphasis will be on factors that allow controlling the size, shape, and monodispersity of vesicles in a reliable and reproducible way. From an industrial point of view, the cost of preparation of a particulate system is an important factor as well. Another point of interest will be how “spontaneous” vesicles really form in these systems or how much processing is needed to achieve defined and reproducible products.

Section snippets

Theoretical predictions of vesicle formation

Surfactant self-assembly in aqueous solution is widely explained using the concept of molecular packing parameter, introduced by Israelachvili, Mitchell, and Ninham. The IMN packing parameter, v_o/a_el_o, is based on geometrical considerations, where v_o and l_o are the volume and the length of the surfactant tail and a_e is the equilibrium area per molecule at the aggregate surface [3]. For 1/2<v_o/a_el_o≤1, for example, vesicles and lamellae are expected to form. While this concept is convenient to

Monomer solubility—a hands-on measure to control self-assembly?

Despite this impressive development in simulation technologies, the question remains whether a more practical, hands-on approach exists to predict the self-assembly of surfactants to form vesicles. Above the cmc, surfactants aggregate to micelles. These are highly dynamic aggregates with residence times of a single molecule within a micelle between microseconds (dodecyl C₁₂ chains) and milliseconds (hexadecyl C₁₆ chains). One can imagine that reducing these exchange kinetics would allow

Vesicle formation based on changes in solution composition

A very detailed study of the kinetics of micelle-to-vesicle transformation in aqueous lecithin–bile salt mixtures revealed a strong dependence on the total amphiphile concentration and, even more pronounced, on ionic strength [12^•]. Upon dilution, mixed micelles of both amphiphiles change composition because of the higher water solubility of the bile salt, causing a transition from a spherical to disklike intermediate micelles, growth of these metastable micelles up to the critical radius r*

Vesicle formation based on attractive intermolecular interactions

There are essentially three important groups of attractive intermolecular interactions that will reduce monomer mobility and, therefore, promote formation of stable bilayers and vesicles: hydrogen bonds, metal ion coordination, and electrostatic attraction. The directional order within these three interactions decreases, from one-directional hydrogen bonds to multi-directional metal ions (depending on the respective coordination number) to non-directional or isotropic electrostatic attraction.

Vesicle formation based on changes in monomer structure

Solubility as well as exchange kinetics of monomers in micellar solution will be affected by molecular weight and structure of the monomers. There are several options to change weight and structure, such as using diblock or triblock copolymers, dimeric (gemini) surfactants, which essentially consist of two single-tail surfactants connected in the head group region via a spacer, and bolaform amphiphiles, i.e., molecules in which the hydrophobic moiety carries two head groups, one at each end of

Carrier formation based on novel processes

Several surfactant systems discussed so far produced very encouraging results regarding the development of formulations that are able to either spontaneously or after little processing self-assemble into monodisperse vesicles suitable for drug delivery applications. However, none of the systems seems to be an instant “problem solver”. In this remaining section, therefore, three approaches will be presented that allow formation of well-defined surfactant-based carriers after some level of

Conclusions

Several surfactant systems have been identified that would meet the criteria required for drug delivery applications, i.e., the formation of stable, monodisperse, and unilamellar vesicles based on biocompatible components. These systems included phospholipid-based surfactants such as lecithin–bile salt mixtures and mixtures of sodium dimyristoylphosphatidylglycerol (DMPG) and sodium dilauroylphosphatidylglycerol (DLPG); solutions of dimyristoylphosphatidylcholine (DMPC) or

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Controlling surfactant self-assembly

Abstract

Introduction

Section snippets

Theoretical predictions of vesicle formation

Monomer solubility—a hands-on measure to control self-assembly?

Vesicle formation based on changes in solution composition

Vesicle formation based on attractive intermolecular interactions

Vesicle formation based on changes in monomer structure

Carrier formation based on novel processes

Conclusions

Curr. Opin. Colloid Interface Sci.

Biophys. J.

Biochim. Biophys. Acta

Chem. Phys. Lett.

Biochim. Biophys. Acta

Adv. Colloid Interface Sci.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Adv. Colloid Interface Sci.

Colloids Surf. A Physicochem. Eng. Asp.

Curr. Opin. Colloid Interface Sci.

The role of steric constraints and intermolecular interactions in the formation of surfactant phases

Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers

J. Chem. Soc., Faraday Trans. 2

Molecular packing parameter and surfactant self-assembly: the neglected role of the surfactant tail

Langmuir

Theoretical model and phase behavior for binary surfactant mixtures

Langmuir

The energetics of forming equilibrated bilayer vesicles

Langmuir

Molecular dynamics simulation of the formation, structure, and dynamics of small phospholipids vesicles

J. Am. Chem. Soc.

The mechanism of vesicle fusion as revealed by molecular dynamics simulations

J. Am. Chem. Soc.

Molecular dynamics simulation of the spontaneous formation of a small DPPC vesicle in water in atomistic detail

J. Am. Chem. Soc.

Dissipative particle dynamics study of spontaneous vesicle formation of amphiphilic molecules

J. Chem. Phys.

Self-assembly and self-organization: important processes—but can we predict them?

J. Dispers. Sci. Technol.

Various bilayer organizations in a single-tail nonionic surfactant: unilamellar vesicles, multilamellar vesicles, and flat-stacked lamellae

Langmuir

Shape fluctuations of large unilamellar lipid vesicles by laser light scattering: influence of the small-scale structure

Langmuir

Vesicle–micelle transition and the stability of the vesicle dispersion in mixtures of tetradecyldimethylamine oxide hemihydrochloride and sodium naphthalenesulfonate

J. Phys. Chem., B

Spontaneous vesicles formed in aqueous mixtures of two cationic amphiphiles

Langmuir

Aggregation of the aqueous dodecyltrimethylammonium bromide–didodecyldimethylammonium bromide system at low concentration

Colloid Polym. Sci.

Sterically stabilized vesicles

Angew. Chem., Int. Ed. Engl.

Sponge–vesicle transformation in binary mixtures of ionized phospholipids bilayers

Langmuir

Molecular recognition and hydrogen-bonded amphiphiles

Top. Curr. Chem.

Reversible vesicle formation by changing pH

J. Phys. Chem.

Encapsulating vesicles and colloids from cochleate cylinders

Langmuir