Capítulo 3 3
3.6. Conclusiones
2.3.1 General Aspects
Surfactants are surface active and amphiphilic molecules. Amphiphilic derives from the two Greek words “amphi” (both sides) and “philia” (loves)
towards oil and water
group and the hydrophobic tail, the (Fig. 2.10).
Figure
Typically, surfactants are classified by
with a charged head group are either anionic, cationic or zwitterionic. Surfactants without a charge at the head group are called nonionic surfactants
92, 93].
rfactants are surface active and amphiphilic molecules. Amphiphilic derives from reek words “amphi” (both sides) and “philia” (loves), describing the affinity towards oil and water. These properties are the result of the hydrophopilic head
surfactants are classified by the charge of their head group.
with a charged head group are either anionic, cationic or zwitterionic. Surfactants without a charge at the head group are called nonionic surfactants
Table 2.3: Classification of surfactants.
rfactants are surface active and amphiphilic molecules. Amphiphilic derives from , describing the affinity the hydrophopilic head parts of a surfactant
urfactant monomer molecule.
the charge of their head group. Surfactants with a charged head group are either anionic, cationic or zwitterionic. Surfactants without a charge at the head group are called nonionic surfactants (Tab. 2.3) [56,
Example
When surfactants are dissolved in water, the hydrophopic chain disrupts the hydrogen bonding structure of water, yielding an increased free energy of the system. Therefore, surfactants adsorb to the air/water surface and align the head group into the water and the tail towards the air to minimize the contact area with the water, resulting in a reduced free energy of the system [56]. After exceeding a well defined concentration, the critical micellar concentration (cmc), the surfactant starts to self-assembly into micelles. Self-assembling and micellization are primarily entropy driven processes [94-96]. However, the assembling of the surfactants accompanies with a loss of freedom and for ionic surfactants electrostatic repulsion of approximated similarly charged head groups increases, resulting in an increase of the free energy of the system and opposing the micellization. Hence, the cmc depends on the balance between forces, favouring the micellization (van der Waals and hydrophobic forces) and the opposing forces (kinetic energy of the molecules and electrostatic repulsion) [97]. This particular concentration can be determined from the kink of the plot of physical properties of the solution as a function of the surfactant concentration (Fig. 2.11) [56, 92, 94, 98]. Above the cmc the physical properties (except the solubility) change only slightly with increasing surfactant concentration because the added surfactant monomers are consumed in the micelle formation.
Concentration Osmotic pressure
cmc
Surface tension Self diffusion
Conductivity Solubilization
Figure 2.11: Schematic representation of the development of concentration dependent physical properties of an amphipile dissolved in water.
The cmc depends strongly on the charge on the surfactant head group, the chain length, the degree of alkyl chain branching, the temperature, the valency of the counterions and the presence of cosolutes like electrolytes or alcohols [94, 98-103].
The shape of the formed micelles also depends strongly on the geometry of the surfactant monomers (Tab. 2.4) [56]. The packing parameter
P
enables an estimation of the micelle shape (equation 2.5). It depends on the volumeν
of the single surfactant hydrocarbon chain, the cross-sectional areaa
of the head group and the lengthl
of the fully extended hydrocarbon chain [56, 92, 93, 98, 104, 105].l a P v
= *
(2.5)The values
ν
andl
can be estimated by the approximations made by Tanford depending on the number of carbon atoms,n
C (equations 2.6, 2.7) [106].9
CIf the surfactant is sufficiently soluble, the formation of liquid crystal phases occurs with increasing surfactant concentration [92, 94]. Depending on the micelle shape, which depends on the surfactant structure, different liquid crystalline phases can be observed (Tab. 2.5).
Table 2.5: Correlation of micellar shape and type of formed liquid crystal by increasing the surfactant concentration.
Micelle shape Liquid crystal
spherical cubic
rod hexagonal
disc lamellar
Lyotropic liquid crystals consist of an amphiphile and a solvent and combine characteristics of liquids and crystals [95]. They have a certain order which can reach from atomic scale to longer length scales and are less viscose than crystals.
Some liquid crystals are optically anisotropic, like for example the hexagonal and the lamellar one, the most important ones in applications [92, 95]. Therefore, both are showing a characteristic texture under the polarizing optical microscope [107-110].
The lamellar mesophase is built up by surfactants arranged in double layers which are separated by a water phase (Fig. 2.12a). The alkyl chains and the water phases are in a liquid like disordered state [95, 111-113]. In the hexagonal mesophase the amphiphiles are assembled in parallel cylindric micelles which are packed in a hexagonal order (Fig. 2.12b). For the hexagonal mesophase two alternative types are distinguished, the normal (H1) and the reversed one (H2). In the normal one the water is the continuous phase and in the reversed one the alkylchains are the continuous phase. The hexagonal phases have a higher viscosity than the lamellar phase [92, 95, 109, 110, 112, 113].
Figure 2.12: Schematic illustration of (a) a lamellar liquid crystalline phase and (b) a normal hexagonal liquid crystalline phase.
2.3.3 Solubility
The solubility of ionic surfactants depends strongly on temperature. For low temperatures the solubility of surfactant is low and increases with increasing temperature. At a certain temperature the solubility increases abrupt. At this point the solubility curve and the cmc curve are equal, it is called the Krafft temperature (Fig. 2.13) [56, 92, 93, 95]. The Krafft temperature is determined by the energetic relationship between the solid crystalline state (melting point) and the heat of hydration of the system [114].
Concentration
Temperature
cmc
TKr Solubility
curve
Hydrated solid Micelles
Monomers
Figure 2.13: Binary phase diagram of a surfactant solution in water in the region of the Krafft temperature.
A decrease of the Krafft temperature can be achieved by hindering the crystal packing of the monomers, for example by using highly hydrated polar head groups or counterions [56, 92, 93, 115]. Recently, Collins concept of matching water affinities was applied to regulate the Krafft temperature [116-118]. It was shown that the combination of large polarisable head groups with a counterion with the same water affinity results in an increase of the Krafft temperature. In contrast, the combination of the head group with a small less polarisable counterion delivers an amphiphile with a higher solubility and a decreased Krafft temperature [119].
The temperature dependent behaviour of a nonionic surfactant is different. The characteristic feature of nonionic surfactants is the phase separation with increasing temperature. The characteristic point is called the cloud point and depends strongly on the chain length and the number of polar groups like ethoxy groups in the hydrocarbon chain [92, 120, 121]. The driving forces for the
solubilisation of a nonionic surfactant like an alkyletherglycol are the hydration of the hydrophilic head group and the formation of hydrogen bonds between the ether units and the water. The strength of the H-bonds is strongly temperature dependent and decreases with increasing temperature, resulting in phase separation above the cloud temperature [56]. However, this behaviour can be influenced strongly by the addition of cosolutes [122, 123].