Phase behaviour

A precise knowledge of the phase boundaries of the systems is essential for the use of microemul-sions in decontamination processes. In this section the basics for the understanding of the phase behaviour of microemulsion systems are explained and the procedure for gaining these informations is illuminated. Usually phase diagrams are taken from pseudo-binary systems, where the ratio of two components is kept constant. This correlates with a cut through the phase tetrahedron, in our case usually at constant oil to water ratio ofα=0.5. The phase diagram is recorded by preparing a series of samples with increasing content of the third component, here the surfactant amountγ was varied in the range of 0.025≤γ≤0.40. Depending on the used surfactant, the overall sample size varied between 1 – 2 g for pure sugar surfactants and 5 g for the technical grade surfactants.

All components except the co-surfactant were weighed into a scaled glass tube with screw cap and homogenized according to the procedures described in section 4.2.

Recording phase boundaries

The determination of the phase boundaries is the most time consuming step in the inverstigation of the microemulsion behaviour based on sugar surfactants. The phase inversion can not be achieved by a simple temperature variation experiment as in the case of the C_iE_j surfactants [11, 12, 13, 16]. Hence, the phase behaviour is tuned by stepwise titration with the co-surfactant, changing the curvature of the amphiphilic film. In most cases the chosen cut through the phase tetrahedron is the

"Kahlweit-fish" (see e.g. figure 4.1).

As co-surfactants usually a short or medium chain alcohol is used[13, 16, 17]. The influence of the co-surfactant on the internal film is described in section 3.2.2 and shown in figure 3.7. Starting at δ=0, usually the alcohol content is increased in steps of∆δ=δ_new−δ_old=0.05 until the upper phase boundaries are reached. That is when the three or one phase region (3 or 1) became the 2. The added mass of alcohol is calculated by (4.1), with m_w+s+o being the overall weight of the

sample without the alcohol (usuallym_w+s+o=5 g for SL 55).

∆m_alcohol= δ_new∗m_w+s+o

1−δ_new −δ_old∗m_w+s+o

1−δ_old (4.1)

With the specific density of the alcohol the related volume determined and added to the sample using a microliter pipette. After each co-surfactant addition the sample is homogenized with a vortex mixer and afterwards equilibrated in a thermostated water bath until complete phase separation appeared.

This step is the most time consuming in the recording of the phase behaviour, as some samples take several days for the phase separation, especially when polymers are involved.

Here, the samples are visually inspected in transmitted and scattered light. The samples are kept at a constant temperature of 20^◦C by a thermostated water bath. After an adequate equilibration time, the phases are distinguished as follows: The bicontinuous microemulsion phase1is an optical isotropic and clear solution in transmitted light. Scattered light is in the systems with greater length scales of a pale blue. This phase remains clear upon shaking or stirring. In contrast the phase separated samples are of turbid, milky appearance during shearing or mixing. After phase separation the two and three phase systems can easily be differentiated. In the polymer containing systems the equilibration time is minimum 8 hours, in some systems up to several days due to a meta stable phase. Here, one has to be careful not to obtain wrong-three phase results. To distinguish the two 2-phase systems 2 and 2, the microemulsion phase has to be identified. This can be done by observing the scattering of coherent light, a simple laser pointer is sufficient for that task. The internally structured phase scatters the laser light. In some systems an optical matching appears.

Here, the phase boundaries are hard to detect due to the similar refractive indices of the phases.

Carefully shearing or mixing (by soft tapping the glass tube) leads to the formation of cords at the contact surface of the two phases. With the same method systems with two or more phases with one dominant phase, as they appear close to theX-point, can be distinguished from single phase systems. Soft tapping leads to cords, bicontinuous systems stay completely clear.

Lamellar phases (L_α) are optically anisotropic phases. They can be identified due to their static birefringence using crossed polarizers[13, 15, 16].

Pure versus technical surfactants

As the technical grade sugar surfactants are usually a mixture of sugar surfactants with varying headgroup size (glucoside or maltoside) and different chain length, the observed phase diagrams

differ from those of pure sugar surfactants.

The comparision of Glucopon 220, which is a mixture with approximately C_8···10G_1.3, and n-octyl-α-glycoside (C₈G₁) is shown in figure 4.1. Here, the usually observed behaviour can be explained. The pure surfactants show smaller3and1phase regions, often with extended lamellar phases. In some cases the1region is completely suppressed by the formation of the lamellar phases, as the identical chain length of the surfactant tends to form the more ordered structure. Additional higher amounts of cosurfactant are needed to change the curvature of the amphiphilic film. The technical grade surfactants with their variety of different surfactant structures are able to stabilize larger amounts of oil and water (broader3and1) and lower ˜γvalues. Often only small lamellar regions appear at higherγvalues. On the other hand, the properties of the technical grade surfactants may vary from batch to batch, slightly changing the borders of the phase diagrams.

0.05 0.10

0.05 0.10 0.15 0.20 0.25

γ

δ

pentanol

water/cyclohexaneα=0.5 C_8···10G_1.3

b b b b b b

b b b b b b

C₈G₁

Figure 4.1: Cut through the phase tetrahedrons of the system water/ cyclohexane / surfactant / n-pentanol with pure sugar surfactant n-octyl-α-glycoside (C₈G₁) in blue and the technical grade surfac-tant Glucopon 220 C_8···10G_1.3 in black at a constant oil-water ratio of α = 0.5 and 293 K. Further explanation see text.

Determination of the X-point according to Kunieda

The influence of the polymers on a microemulsion system can easily be tested by observing the change in the bicontinuous (microemulsion) phase in a three phase system[4, 6]. Preparing the samples in small glass cuvettes this method works fine utilizing temperature sensitive surfactants, where the phase transition can be achieved by a simple change of the temperature. For sugar sur-factants a co-surfactant is needed to tune the phase behaviour and therefore a fourth component has to be added stepwise in small amounts. This makes larger sample volumes necessary to reduce the addition failures. Close to the X-point the determination of the different phases was inaccurate.

0,075 0,080 0,085 0,090 0,095 0,100 0,105 0,4

0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8

Volume water rich phase

Volume oil rich phase

Linear fits

V/mL

Figure 4.2: Determination of V_{mid d l e}in the system Water/Lanol 99/Simulsol SL55/pentanol as the connection point of the linear fits of the changes in the volumes of the upper (oil rich) and the lower (water rich) phase in the three phase region upon increasingδ. Example shown for a sample containing 5% F108 atγ=0.05.

Hence, we used the method derived by KUNIEDA et al. [5, 18, 14]to determine the location of the X-point. Following the blue line in the three phase region (see 3.7, section 3.2.2), the lower and the upper phase are in an equilibrium with the middle phase. The more surfactant is added, the more oil and water can be stabilized in the microemulsion (middle) phase, so this phase is growing while the other two are reduced. The equilibrium state is reached, when the volume of the upper

(oil-rich) and the lower (water rich) phase are equal. Therefore the change of the volumes was ob-served upon increasing the cosurfactant contentδ, as shown exemplary forγ=0.05 in the system water/lanol 99/SL55 (5% F108)/pentanol in figure 4.2. As the equilibrium point often is located in between theδsteps, it was identified by linear fits. Hence, the resulting volume fraction of the middle phase in the three phase body V_middle/V_total was measured as a function of the surfactant contentγ. When the three phase body evolves and the volume fractionV_middleincreases with rising γuntil the X-point is reached atV_middle/V_total=1. As shown in Figure 4.3, the ˜γvalue can be easily determined by linear fitting. The X-point in the example was calculated to ˜γ=0.152, which fits well to the recorded phase diagram shown in figure 5.5.

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,0

0,2 0,4 0,6 0,8 1,0

V middle /V total

Figure 4.3: Determination of the X-point (˜γ) by linear fitting the swelling of the middle phase V_middle/V_total plotted versus γ. The example is taken from the system Water / Lanol 99 / Simulsol SL55 (5% F108)/pentanol.