# Surfactant phases

## Colloidal electrolytes - Evidence of a new phase

 Equivalent conductivity at 25 C of undecyl, dodecyl, and tetradecyl sulfonic acid over a wide concentration range. McBain, 1950, Fig. 17.1, p. 243. Deviation of specific conductivity from that for a fully dissociated electrolyte. McBain, 1950, Fig. 17.2, p. 243.
J.W, McBain, 1950ish

## Osmotic coefficient - More evidence of a new phase

 The variation of the osmotic coefficient with concentration for potassium chloride and for potassium laurate. The straight line is the Debye-Huckel-Onsager slope. McBain, 1950, p. 248

## Thermodynamic evidence of a new phase

Solution surface tension versus sodium dodecyl sulfate (SDS) concentration. (Note the logarithmic scale.)

Evans, Fig. 1.1, p. 6

\begin{align}  & d\sigma =-\Gamma _{2}d\mu _{2} \\ & \Gamma _{2}=-\frac{d\sigma }{d\mu _{2}}\simeq -\frac{1}{kT}\left( \frac{d\sigma }{d\ln c_{2}} \right)_{T} \\  \end{align}

The two linear regions represent concentration ranges where the surface excess is constant, but different.

WARNING! - This data may be more interesting than you think.

## Colligative evidence of micelle formation

Morrison, Fig. 13.7

## Micellization - a dynamic equilibrium

• If micellization is an equilibrium reaction:
• $KB\leftrightarrow B_{K}$
• The law of mass action gives:
• $\frac{\left[ B \right]^{K}}{\left[ B_{K} \right]}=\text{constant}$
• The value of K can be quite large; for SDS it is about 64. An equilibrium can look like a phase change (The argument of phase change vs kinetics is an old one largely settle in favor of kinetics.)
• Adsorption at the air/liquid interface and absorption into a micelle are competing processes. However, once the surface is saturated, any addition of surfactant produces more (not usually larger) micelles.

## Inverse micelles

• The lipophilic groups are on the outside, the hydrophilic groups are on the inside.
• Since the hydrophilic groups are typically the salts of strong acids or bases, the core of the inverse micelle is highly polar, somewhat like a room temperature molten salt.
• The micelles are typically large, 10's of nm.
• Molecules that form this structure usually have a small optimal interface area or a large chain volume to length ratio. Double chains, nonionic heads, and cis unsaturated chains are also common. When inverse micelles form, the solution changes from appearing as oil droplets in water to water droplets in oil.
• Ex: Phospholipid

Phospholipid molecules, composed of fatty acid “tails” and a phosphate “head,” form an inverse micelle in a nonaqueous solution. The phosphate group converts one end of the lipid molecule into a polar, or hydrophilic, group, leaving the preferentially attracted nonpolar, or hydrophobic, end of the molecule to react with the nonaqueous solution.

## The Krafft temperature

Evans, Fig. 1.3

Krafft temperature is the temperature at which surfactant solubility equals the critical micelle concentration. Above the Krafft temperature surfactants form micellar dispersions; below the Krafft temperature the surfactant crystallizes out of solution as hydrated crystals. (Evans, Fig. 1.3, p. 9)

## Solubilization

• Simple solvent mixtures do not abruptly change the solubility of another component (see lower curve on the RHS).
• However, surfactants abruptly change the solubility of a component, even a nonsoluble one, above their critical micelle concentration.

 Solubility of 2-nitrodiphenylamine in aqueous solutions of potassium laurate. Solubilization of dye in solutions of Triton X-100 versus solvent action by acetone. Morrison, Fig. 13.14 McBain, 1950, Fig. 17.24, p. 264

### Solubilization in digestion

Solubilization in micelles is not only an important industrial application, but also a recurrent process in physiological functions. As demonstrated in the picture, fat digestion in the animal gastrointestinal tract is achieved through micellization. Once ingested, fats are emulsified by mechanical action in the upper part of the small intestine. Then, pancreatic enzymes break down triglyceride fats into fatty acids and lipids. At the same time, bile salts (such as cholate and glycocholate) are released from the gall-bladder. These act as surfactants, which mix with the fatty acids and lipids to form composite micelles. Lower into the intestine, fatty acids and lipids are adsorbed on the membrane on the intestine and ultimately form triglycerides by re-esterification. Bile salts are more polar and therefore are adsorbed further down in the intestine. They ultimately flow into the liver where they're recycled.

## Biosurfactants

Surfactin is a very interesting biosurfactant: it has very impressive surfactant power and is important biologically as an antibiotic because of this action.

As an example of its surface active properties, it can lower the water's surface tension from 72 mN/m to 27 mN/m at a concentration as low as 20 µM (from this paper, also cited in Arima K et al., Biochem Biophys Res Commun 1968, 31, 488). This can be compared to SDS, a very common surfactant, that typically reduced water's surface tension from 72 mN/m to 40 mN/m.

The mechanism of action (how it enters the lipid bilayer) for this antibiotic is disputed, although after looking in the three main theories, the one that makes the most physical sense to me is the mixed micelle formation. Essentially, surfactin will insert itself into the top portion of the lipid bilayer of a cell or viral envelope. As more and more of these very large head groups are incorporated, the cells outer layer becomes more mixed than it usually is. At high concentration of surfactin, the bilayer becomes overrun with the surfactant and is solubilized. This is an example of the detergent properties of surfactants. It is important to note that the bilayer is not normally homogeneous, and can handle a certain amount of disruption; it is not until very high concentration of surfactin that the membrane dissolves. The fact that it dissolves the bilayer in such a non-specific way (it requires no extracellular signals or membrane bound tags, etc) makes this antibiotic interesting because it will have very little resistance generation: the bacteria or virus would have very little chance of spontaneous or directed evolution to escape the detergent action, short of randomly rearranging its entire membrane structure.

This is not a "good" antibiotic target for clinical research for several reasons: it requires a very high concentration of the molecule to have antibacterial properties, and it is completely non-specific and will dissolve any membrane (especially, red blood cell membranes, which may perhaps be due to their structural instability upon aging).

This is an interesting piece from the Biophysical Journal on the how it interacts with synthesized membranes (which explains more physically how the antimicrobial properties come about and how it enters all types of cells indiscriminately cells): Interactions of surfactin with membrane models

And this one shows a way to study the interfacial tension of a biological surfactant: Molecular Dynamics Simulation of Surfactin Molecules at the Water-Hexane Interface

--BPappas 21:30, 2 November 2008 (UTC)

An interesting property of surfactin - and possibly other biological surfactants - is that their activity can be modulated not only by pH, as described in the class notes, but probably also by other signature components of the environment they are supposed to work in. A study by Straight et al. (http://www1.qiagen.com/support/references/ReferenceListPrint.aspx?a=221&as=1&q=) describes the activity of surfactin in the context of two different fungi (Bacillus subtilis and Streptomyces coelicolor) trying to grow hyphae (long, branching filaments in fungi, see Wikipedia article) in the air. In one case (B. subtilis), surfactin works as expected and reduces the surface tension between the fungal cells and the air, thus allowing the organism to increase its surface-to-volume ratio easily, grow a hypha and stay healthy. However, when this same surfactin interacts with another bacteria, the effect is the opposite: "Our experiments reveal that surfactin acts antagonistically by arresting S. coelicolor aerial development", claim the authors of the 2006 study. This demonstrates several things: i) the activity of lipopeptides can be subtly modulated to go beyond the basic expected behavior of a surfactant; ii) organisms can use this as a competitive advantage when colonizing an environment; iii) simple things in Nature aren't always that simple!

Note that hyphae are sometimes collectively called "mycelium" or "mycelia", which is surprising close to the word micelles we have been studying lately. However, "mycelium" comes from the Greek root for "mushroom", as opposed to "micelle", which comes from the Latin word for "grain".

Biologically interesting functions can be immobilized by attaching the (hydrophilic) moiety to a hydrocarbon chain and making it surface active. For example:

 DNA surfactant immobilized via its hydrophobic tails. Its DNA head strand is capable of binding to its complementary strand tagged with a rhodamine fluorescence label). Lu et al. Current Contents in Colloid & Interface Sci, 12, 60, 2008.

## Particles as surfactants

Binks, Current Contents in Colloid & Interface Sci., 21, 68, 2002.

An analysis of the interfacial energy required to remove a partially partially wetted particle into either the oil or water phases (contact angle measured through the water phase) give the following two equations:

\begin{align}  & \Delta F_{1}=\pi r^{2}\sigma _{12}\left( 1-\cos \theta \right)^{2} \\ & \Delta F_{2}=\pi r^{2}\sigma _{12}\left( 1+\cos \theta \right)^{2} \\  \end{align}

 Calculated for a 10 nm radius particle at the O/W interface with an interfacial tension of 36 mN/m. Contact angle measured through the oil phase. Morrison, Fig. 22.4

## "Typical" soap phase diagram

 Phase diagram for sodium stearate-water. McBain, 1950, Fig. 17.23, p. 263. “Soaps are remarkable in that their aqueous solutions, depending upon the temperature and concentration, exist as different phases, some being ordinary isotropic, clear, transparent, fluid solutions, colloidal except in extreme dilution; and other being liquid crystalline, anisotropic, plastic, and translucent. Among the latter are middle soap, soap boiler’s neat soap, superneat, superwaxy, subneat, and neat soap phases..”

## Surfactant self assembly

When concentrations of surfactants reach critical levels, several unusual properties are observed that indicated that changes have occurred in the liquid's structure. Above a critical concentration, which varies depending on the surfactant, the osmotic pressure and surface tension remain constant while self-diffusion decreases and light-scattering increases. These observations are consistent with a transition from single surfactants to organized structure of the surfactants.

This self-assembly in surfactant micelles leads to a variety of different structures. These are nicely described in a picture in the lecture. Initially, surfactants gather in micelles with the non-polar portion directed towards the center (assuming we are discussing an aqueous solution). Many of these structures are studied in water because it is the most effective solvent to use for observing their formation.

Have these structures ever been directly observed? Is there any experimental evidence for bicontinous structure? From a quick literature search, it seems that they are only found in simulations.

source: Holmberg et al.

 (a) Spherical micelles with an interior composed of the hydrocarbon chains and a surface of the polar head groups (pictured as spheres) facing water. (b) Cylindrical micelles with an interior composed of the hydrocarbon chain and a surface of the polar head groups facing water. The cross-section of the hydrocarbon core is similar to that of micelles. The micellar length is highly variable so these micelles are polydisperse. (c) Surfactant bilayers which build up lamellar liquid crystals have for surfactant-water systems a hydrocarbon core with a thickness of ca. 80% of the length of two extended alkyl chains. (d) Reverse or inverted micelles have a water core surrounded by the surfactant polar head groups. (e) A bicontinuous structure with the surfactant molecules aggregated into connected films characterized by two curvatures of opposite sign. (f) Vesicles are build from bilayers similar to those of the lamellar phase and are characterized by two distinct water compartments.(Description from Holmberg et al. Fig. 2.1, p. 40; Diagrams from Evans et al., Fig. 1.6, pp 14-15).

## Kinetics in surfactant systems

• Transitions can be between all phases.
• Molecule entering micelles - microseconds
• Micelles to molecules - order of a minute.
• Micelles to vesicles – order of hours
Gradzielski, Current Contents in Colloid & Interface Sci, 8, 337, 2003.

## 3 Component Surfactant phase diagrams

 Morrison, Fig. 13.21, p. 272. Phase diagram of the E100P70E100–butanol–water system at 25oC. The tie-lines are represented by full straight lines. L1 denotes the water-rich (micellar) solutions region, I1 the normal (“oil”-in-water) micellar cubic liquid crystalline region, H1 the normal hexagonal liquid crystalline region, Lalpha the lamellar liquid crystalline region, and L2 the alkanol-rich solution region.

## Liquid crystal phases

 Chiral – having a mirror image. Sometimes causes neighboring molecules to align at a slight angle, leading to a helix in space with a well-defined pitch much longer that the size of a molecule. And can form chiral nematic phases. Smetic A – director is parallel to the layer normal Smetic C – director makes an angle to the layer normal (that is, layers of tilted molecules) Columnar (or discotic) – Molecules form disk-like phases.

Phase Positional order Orientational order
Liquid None None
Nematic None Yes
Smectic One-dimensional Yes
Columnar Two-dimensional Yes
Crystalline Three-dimensional Yes

Here are some 3D images of what layers of the different mesophases of liquid crystals look like. Left to right: Nematic, Smectic (A and C) and Chiral.

My question is related to the fact that these images make the molecules look symmetrical when I thought these were surfactant molecules like all the
others we have studied. I can sort of see how the nematic and smectic could work but am having difficulty visualizing how the actual molecules
would look in the chiral phase.


de Gennes won his Nobel Prize for his study of polymers AND liquid crystals. Check out the history of Liquid Crystals on a Nobel Prize site: [1]

The wikipedia page of Liquid Crystals is quite expansive. At least much fuller than it was 2 years ago. Check it out if you want more detail about anything listed here and more! [2]

## Critical packing shapes

 $\frac{v}{a_{0}l_{c}}$ where $\nu$ is the volume of the micelle, $a_{0}$ is the optimum area for the head groups and $l_{c}$ is the critical chain length. $M=\frac{4\pi l_{c}^{3}}{3v}$ $l_{c}$ is a length approximately the extension of the hydrocarbon before it loses a liquid-like state, and M is the mean aggregation number.
Israelachvili, 1991, p. 381

Holmberg p. 278

## Surfactant - Protein interactions

Missing reference!!!!

• The effect of gelatin on the surface tension of solutions of SDS (circles) and Triton-X100 (triangles). Without gelatin (filled) and with gelatin (open).
• For Triton X-100: no protein/surfactant interaction.
• For SDS: a plateau corresponding to a protein/surfactant association.

## Biogenic monolayers - Lung surfactant

Surface tension is greatly reduced by compression on exhaled breath, regulating capillary pressure, promoting involuntary inhaling. Premature babies lack lung surfactant. The layers are strong – maximum spreading pressures of 68-72 mN/m compared to stearic acid of about 40 mN/m. The collapsed films relax slowly on expansion.

 Fluorescence microscopy a monolayer of phospholipids containing Rh-DPPC at different surface pressures (in units of nMm-1) during continuous compression at 0.03 nm2 per molecule-min. The non-fluorescent area correspond liquid condensed domains, fluorescent areas correspond to liquid expanded domains. Highly fluorescent spots are collapsed structures.* Engelskirchen. Current Contents in Colloid & Interface Sci, 12, 68, 2008.

## Commercial Applications of Nano-Micelles

Micelles are being used in cosmetics, vitamins, pharmaceuticals and food products. Recently, companies have been producing nano-micelles, which have a diameter of around 30 nm, to encapsulate various products. These nanomicelles result in clear solutions (since 30 nm is smaller than the wavelength of light) and are more bioavailable than larger particles such as liposomes and micro-emulsions. In contrast to other food delivery particles, they are stable in gastric acid. The hydrophilic casing allows them to solubilize in water and the bloodstream very easily, while this same casing protects the product in the core from oxidation.

Micellar Vitamin K is used to provide Vitamin K to patients with acute liver failure. A healthy liver naturally takes vitamin K from food and turns it into mixed micelles made up of bile salts and pancreatic lipolytic products, but failed livers cannot do this. [4]