The optical properties of a water/perchloroethylene emulsion
If the refractive indices of the dispersed and the dispersing phase are different, light will be scattered by the droplets. As a result, an emulsion consisting of two colorless, transparent liquids such as water and perchloroethylene will have a milky white appearance. The intensity of the scattered light depends, among other things, upon the ratio of the refractive indices of the dispersed to the dispersing phase, upon the ratio of the droplet radii to the wavelength of the light, and upon the share by volume of the dispersed phase. The stronger the scatter, the less will be the permeability to light of the emulsion. The light scatter by the individual droplet can be described quantitatively by means of Mie's theory. Nevertheless, it is only in special cases that the state of an emulsion can be determined quantitatively by overall light scatter measurements. In cases where it is only intended to examine the breaking-up of an emulsion, it will often suffice to measure the transmittance as a function of time. The greater the number of droplets which settle out or separate by floating, the more transparent the emulsion.
An intact water/perchloroethylene emulsion will at first look milky white. The more the water separates by flotation, the more transparent will the emulsion become. If the emulsion is broken, we have practically the permeability of the pure, colorless, transparent perchloroethylene. We made use of this phenomenon when we examined the stability (see section headed "Life of a water/ perchloroethylene emulsion".
Vapour pressure of a water/perchloroethylene emulsion
It is most important to know the vapour pressure of the w/p emulsion as a function of the temperature. For one thing, the vapour pressure curve shows which external pressure is necessary to attain a given dyeing. Secondly, it indicates how the components of the emulsion-water, perchloroethylene and emulsifying additives-influence the total pressure.
Measuring arrangement
The vapour pressure is measured by means of the apparatus shown schematically in Fig. 7.5.: fill about 50 cm3 liquid into a glass flask of 100 cm3 volumes. After that, evacuate the glass flask and tube (e). Seal the feed tube (= exhaust tube) at (c). Connect the glass flask with a mercury manometer (d) by means of a capillary tube (b). If this system is introduced into a thermostat and heated to a given temperature, the pressure in the glass flask will increase. Apply counter-pressure to the manometer (d) via the tube (e) until both menisci are on the same level. The pressure in the glass flask will then be equal to the counter-pressure in the tube (e). This counter-pressure can be read very accurately from an indicator instrument. The mercury manometer (d) is therefore only used as a zero instrument. Since the stability of the solution is endangered by subnormal pressure, we carried out control measurements where the air remained in the glass flask. The air pressure in the glass flask at a certain temperature was calculated according to the laws for an ideal gas, and deducted from the measured total pressure. Depending upon the degree of filling of the flask, the expansion of the emulsion had to be taken into account. The expansion of the emulsion was equaled to the expansion of the 100% perchloroethylene (for the expansion coefficient, see the section headed "Conclusions for dyeing from water/perchloroethylene emulsions"). The measurement of the vapour pressure with and without air in the flask gave the same result.
Results of measurements
The vapour pressure curve of a w/p emulsion (see the section headed "How to prepare a water/perchloroethylene emulsion"), is shown in fig.7.5. The points of measurement are shown as crosses or circles. For comparison, we plotted the vapour pressure curve obtained by adding the vapour pressures of water and perchloroethylene. (We conducted our own measurements, since the values given in literature for the vapour pressure of perchloroethylene differ widely.) We found that within the limits of experiment, the vapour pressure of the emulsion is the sum total of the vapour pressures of water and perchloroethylene. This is easy to understand, as water and perchloroethylene are only very slightly miscible. (In perchloroethylene, 0.01 per cent by weight of water is soluble at 25°C, while in water 0.01 per cent by weight of perchloroethylene is soluble at 25°C.) On the other hand, it is also evident that the emulsifying additives do not exercise any appreciable influence on the vapour pressure of the emulsion. From the vapour pressure curve we get a boiling point of 37.8°C for the emulsion.
Fig. 7.4.
Fig. 7.5.
The dyeing temperature which can be attained when a water/perchloroethylene emulsion is used. The boiling point condition.
It is important to know for the practical dyeing from emulsions under which conditions a given dyeing temperature Tp can be attained. Since the boiling temperature of a liquid cannot be exceeded under the conditions of equilibrium, this means that the boiling temperature Tg of the emulsion must be above the necessary dyeing temperature, or at least equal to it:
TF ≤ Ts (5)
The boiling temperature Ts depends very strongly upon the pressure P exercised on the emulsion. As long as P is bigger than the vapour pressure p of the emulsion, boiling is prevented. The emulsion will only begin to boil if its own vapour pressure is equal to the pressure P exercised on it. At 87.8°C, the vapour pressure of the emulsion is 760 Torr which means that its boiling point at normal atmospheric pressure is 87.8X, If the pressure on the emulsion is increased, higher temperatures will be needed, until the vapour pressure of the emulsion equals the external pressure, which means that the boiling point is higher. Hence, while TF is a given value, Ts can be shifted.
The vapour pressure curve shows which pressure is needed under practical conditions so that a given temperature can just be attained without bringing the emulsion to the boil. All that is needed is to read the vapour pressure p (T) belonging to the temperature T, and we can be sure that for all pressures for which_
p (T) < P, (6)
the emulsion will not boil.
Hence, for a dyeing temperature Tf, the system must be guided in such a way that
p (TF) ≤ P. (7)
since the necessary condition (5) is only fulfilled if this is effected.
Stability condition
Although the condition (5), respectively (7), is necessary, it is by no means sufficient, if dyeing has to be effected from a stable system, we must be quite sure that the emulsion remains stable at the necessary dyeing temperature. To be more precise: the life т of the emulsion must be significantly longer than the time tp which is required for the dyeing, under the conditions which obtain during the dyeing process:
tF < τ (8)
The external conditions are the dyeing temperature Tp and the pressure P. Since dyeing time is independent of pressure, (8) can be expressed as follows:
tF (TF) < т (TF; P) (9)
tp (T) depends upon the fiber to be dyed, the dyestuff, and other additives.
No general prediction can be given. However, certain data are known by experience for different fibers and dye-stuffs.
τ (Т; P), i.e. the life of the emulsion as a function of temperature and pressure, is entirely unknown. As long as it has not been determined, (9) remains irrelevant for anyone engaged in practical work. Hence we set ourselves the task to determine τ (Т; P). In the following section we shall describe how we set about it, and the results we obtained.
Fig. 7.5. Vapour pressure curve of a water/perchloroethylene emulsion boiling point of the azeotropic mixture
Fig. 7.6. Vapour pressure curves of perchloroethylene and water
The life of a water/perchloroethylene emulsion.
Measuring arrangement.
The apparatus to examine the stability (Fig. 7.7) consists of a glass flask of 153 cm3 capacity. Fill 120 cm3 of the emulsion (a) into this flask. Enter the glass flask with the emulsion into an oil bath (b) which can be kept at a given temperature by means of a thermostat (±0.3°C). A glass tube is sealed into the flask. In this glass tube, a thermo-element (f) is fitted to measure the emulsion temperature. This temperature is recorded as a function of time by one of the recording units of a two-channel recorder. A definite pressure can be exercised on the emulsion (a) via the tube (i). In the excess pressure range, the pressure is adjusted by means of compressed nitrogen, while in the reduced pressure range a vacuum pump is used. A water reflux cooling arrangement provides for condensation of the vapour escaping from the emulsion.
To find out how the state of the emulsion will change at a given pressure and temperature, the light transmittance of the emulsion is measured as follows: Two further glass tubes are sealed into the glass flask. One tube contains a small electric bulb (g) fed by a stabilized source of current. The other tube contains a light wire (d). The intensity of the light incident on the entrance face of the light wire depends only on the light transmittance of the emulsion, provided the secondary conditions (cur rent supply of the electric bulb, position of the light wire etc.) are constant. The light absorbed by the light wire is transmitted to a photo-electric cell (e). The resulting photo-current is amplified and recorded by the second recording unit of the two-channel recorder which has already been mentioned above, as a function of time. We found the following relationship between the light intensity I and photo-voltage, or current, U :
U ~ I1,2
To keep the emulsion lightly moving, the flask is coupled to a 50 c.p.s. vibrator.
Measure of stability
A typical measurement result obtained with the apparatus described in the section headed "Measuring arrangement", page 38, is shown in Fig. 7. The emulsion was heated to 100 °C at a pressure of 4.1 atm., since preliminary trials had shown it is largely stable under these conditions. After the conditions had become constant, the pressure in the glass flask was reduced within a few minutes from 4.1 atm. to 1.2 atm., while the temperature of the oil bath remained at 100°C. As a result, the temperature in the emulsion dropped. At the same time, the light transmittance of the emulsion increased. After about 22 minutes, the final value of the light transmittance had been attained (total change = Δ). At that point, the emulsion had disintegrated. The glass flask contained a water-clear liquid (mainly perchloro-ethylene) with small quantities of water at the surface. The light transmittance curve, therefore, tells us very accurately the amount of time tΔ required at a given pressure and a given surrounding temperature (= temperature of the oil bath) to break up an emulsion completely. (Originally we had tried to determine by visual assessment the time required to break the emulsion. But we found it impossible to detect the beginning of the breaking-up. It was also difficult to find out when the final state had been reached.) This time tΔ can be considered a measure for the stability of the emulsion for a given pressure and a given environmental temperature, tΔ is identical with the life r. But experience showed that long periods may be required until a w/p emulsion breaks up completely. To work with shorter experimental times, we used as a measure of the stability, not the time tΔ, but the half-life 1/2 Δ. τ 1/2Δ-which will be represented by the symbol t1/2 for short- is the time which elapses until the change in light transmittance attains one-half of its final value, i.e. 1/2Δ. The final value of the light transmittance which represents the completely broken-up emulsion, is easily measured. (In trials which are continued until the emulsion is completely broken up, the final value of the light transmittance is obtained as a matter of course. In trials which are broken off prematurely, the final value can be obtained by breaking up the emulsion through reduced pressure or increased temperature.) In this manner, Δ and 1/2Δ can be determined. t1/2 can then be read from the transmittance curve. We may mention that in equation (8) or (9), 2 t1/2 can be substituted for т with sufficient accuracy.
Temperature in the emulsion
If the pressure in the glass flask is reduced to such an extent that the emulsion begins to boil, the emulsion temperature will drop below the temperature of the surrounding medium. Fig. 7.8. shows that the emulsion temperature attains a minimum, and will assume a constant value after some time. The difference between the temperature of the surroundings and the minimum emulsion temperature ΔT is used as a measure of the temperature drop.
Measurement results
The stability of the emulsion and the temperature drop in the emulsion were determined as a function of the pressure by experiments. In these experiments, the temperature of the surrounding medium was maintained at 100oC. The results are shown in Fig. 7.8.
Fig.7.7. Apparatus to measure the stability of emulsions, a emulsion, b oil bath, с thermometer, d light wire, e photo cell, f thermo-element, g bulb, h pressure tubing and manometer
Fig.7.8. Change of light transmittance and temperature in the emulsion when the temperature is reduced. Ambient temperature = 100°C
This illustration shows that the emulsion is unstable in the pressure range in which T is different from zero, i.e. in the boiling range. Dyeing would be impossible within the time at our disposal. The emulsion is largely stable outside this boiling range, where dyeing would be possible. The next problem requiring clarification was whether similar conditions would obtain at other environmental temperatures. For this reason, we also conducted experiments at SOX and at 120°C. In all three cases, the stability curves present a similar appearance. There are three main stability ranges at each temperature: In the range of relatively low pressures, the stability is very low, while in the range of relatively high pressures the stability is high. In addition, there is an intermediate range in which the transition from low to high stability is rather sudden.