GR1010076B - Method for the determination of interfacial tension between liquids and study of the liquidgas and liquid-liquid interface stability - Google Patents

Abstract

The invention relates to the determination (a) of the interfacial tension (static and dynamic tension) between fluids, which tension allows the determination of foaming or emulsifying ability of these last. It also relates to the study (b) of the stability of liquid / gas and liquid / liquid interfaces to make possible the evaluation of the stability of the formed foam or emulsion, respectively. To achieve this, electrical measurements are made in the fluid to be measured and equations (Young-Laplace and Laplace) are solved for the first case while a specific part of the conductivity (or resistance) curve with regard to the time is evaluated for the second case. The ability to simultaneously determine the foam / emulsion creation capacity and their stability will lead to a significant improvement in both, the production process and the production of final products in various industries.

Description

DESCRIPTION

Title : Method for determining interfacial tension between fluids and studying the stability of liquid/gas and liquid/liquid interfaces

The present invention enables accurate determination of static and dynamic interfacial tension between fluids. In addition, it allows the study of the stability of liquid/liquid interfaces located in emulsions, as well as liquid/gas interfaces located in foams, by applying electrical conductivity measurements. Emulsions and foams find applications in a range of products and processes such as food, shaving products, cosmetics, drugs, fire extinguishers, detergents, flotation process for metal separation, oil extraction, etc. In some cases, the presence of foams and emulsions is beneficial while in others it is undesirable, so the study of their ability to form and their stability is considered important. The measurement of the interfacial tension (static and dynamic) between fluids and the study of the stability of liquid/liquid and liquid/gas interfaces through electrical measurements carried out with the present method, allows the simultaneous determination of the ability to form and the stability of emulsions/foams, with significantly lower cost and greater accuracy than existing methods.

Technical field of the invention

The invention relates to the simultaneous determination of a) the ability to form a foam or emulsion and b) the stability of the foam or emulsion prepared. This simultaneous determination is achieved by means of electrical measurements (ie measurements of the electrical resistance or conversely of the electrical conductivity) of a liquid bridge between two electrodes. The measurement of electrical conductivity allows, through the solution of Young-Laplace and Laplace equations, to calculate the interfacial tension between fluids, i.e. the bridge fluid and the air or another fluid surrounding the bridge, and by extension to determine the capacity formation of a foam or emulsion, as the lower the interfacial tension the more easily a foam or emulsion can form. At the same time, it becomes possible to determine the stability of liquid/gas (found in foams) and liquid/liquid (found in emulsions) interfaces.

Level of Prior Inventions

Foams (liquid/gas interfaces) and emulsions (liquid/liquid interfaces) occur in many products and production processes. In some cases, the presence of a stable foam or emulsion is desirable and in others undesirable. For this reason, the simultaneous determination of the ability to form a foam or emulsion and its stability is considered particularly important.

[0003] The interfacial tension (static and dynamic) is a property of the fluid that is directly linked to the possibility of foaming and emulsification respectively. Its reduction, through the addition of surfactants/emulsifiers, contributes to the formation of foams and emulsions, while its increase destroys them [1, 2]:

Most methods for measuring interfacial tension (static and dynamic), such as Wilhelmy plate, du Nouy ring and drop weight, are not suitable for surfactant solutions and require a relatively large amount of fluid. Other methods such as those of the hanging drop and the resting drop are based on optical measurements, which, however, due to unwanted shadows and the difficulty of determining a fixed optical recording angle can lead to incorrect measurements. Finally, methods based on pressure measurements (e.g. maximum bubble pressure) are easily affected by noise disturbances [3, 4].

[0005] The proposed method, which is based on precise electrical (conductivity) measurements of a bridge that forms a quantity of the fluid under study between two electrodes, has been experimentally shown to be suitable for measurements of the interfacial tension of solutions of surfactants. Furthermore, because it is based on electrical measurements, the disadvantages of optical measurements and pressure measurements are avoided, while a very small amount of liquid (about 10-20μl) is required to perform the measurement.

Foams are gas/liquid structures in which gas bubbles are separated from each other by a liquid film (liquid/gas interface). The destruction of the foams occurs through the breakdown of this interface caused by drainage. A foam becomes more stable when the liquid/gas interface is more stable, otherwise it becomes more unstable [2, 5]. Therefore, the study of the stability of liquid-gas interfaces is important in determining foam stability.

Emulsions are heterogeneous systems of one liquid dispersed in another in the form of droplets. The stability of emulsions depends on the surfactant's ability to prevent droplet incorporation. This is achieved by creating stable interfaces between liquids. Therefore, in this case too, the study of the stability of liquid/liquid interfaces is important to determine the stability of the emulsion.

To date very few techniques have been developed to study the stability of liquid/gas and liquid/liquid interfaces, such as interferometry, infrared spectroscopy and X-ray reflectivity. However, these methods are complex and based on mathematical models that assume that the interfaces have specific structures, leading to incorrect results [6].

[0009] In the present invention, in addition to the measurement of interfacial tension, it is also possible to study the stability of liquid/gas (in the case of foams) and liquid/liquid (in the case of emulsions) interfaces. As in the case of determining the interfacial tension, the study of the stability of the interfaces is carried out through electrical (conductivity) measurements of a bridge, which forms a quantity of the fluid being studied between two electrodes. Consequently, the determination of interfacial stability does not rely on the assumption that the interfaces have specific structures, so it is more accurate. The difference between this measurement and the interfacial tension measurement is the creation of a gas bubble within the liquid bridge, in the case of measuring liquid/gas interfaces, or a drop of liquid in the case of liquid/liquid interfaces measurement.

Collectively, the proposed invention allows the simultaneous determination of the interfacial tension (ability to form foam or emulsion) and the study of the stability of the liquid/gas and liquid/liquid interfaces (foam or emulsion stability respectively). No other method has been identified that has this capability.

Purpose of the invention

[0011] The method simultaneously determines, at low cost and with great accuracy, the ability to form and the stability of a foam or emulsion. It is intended to be usable by the following groups of users:

A. Metallurgy-Recovery of Metals (flotation method) and Oil Extraction Industries.

With the present invention, greater control of the production process is made possible in the metallurgical and oil recovery industries. Particularly in metallurgy and specifically in the process of flotation, which is applied for the recovery of metals, the role of foam is particularly important. Very stable foams increase the recovery rate, but less stable foams improve recovery selectivity [7]. Determining the foamability and stability of the foam to be formed from the liquid to be measured before introducing it into the flotation cells improves the control of the whole process. Similar to the oil extraction industries, the use of foam over other materials for extraction is gaining ground [8]. The ability to determine foamability and foam stability becomes important for mining control and can be achieved with the present invention.

[0013] B. Cosmetics Industries

In these industries, the need to simultaneously determine the ability to form and foam or emulsion stability is important due to the wide variety of products. The benefit of using the method by the present industries is particularly important, as the measurements of the foaming and emulsifying ability and the stability of foams and emulsions can be performed simultaneously by a single device, thus reducing the time and cost of analysis.

[0014] C. Food industries

Knowing the ability to form and the stability of foam or emulsion is particularly important, both in final products such as sparkling wines where foam stability is a quality criterion, and in the production process to avoid unpleasant consequences such as unexpected foaming of tanks. This simultaneous determination can be achieved with the present methodology.

Summary of the invention

The invention relates to a method for the simultaneous determination of a) the ability to form foam or emulsion and b) their stability, by means of electrical measurements. In particular, in the first case, the method allows the calculation of the interfacial tension (static and dynamic) through measurements of the electrical conductivity of a liquid bridge formed between two electrodes by the liquid to be measured, when the distance between the electrodes is changed or the volume is reduced fluid of the bridge and solving the Young-Laplace and Laplace equations. In the second case, an air bubble is formed inside the liquid bridge in the case of studying the stability of foams, or a drop of another liquid in the case of studying the stability of emulsions, and the electrical conductivity of the bridge is measured during the reduction of the amount of liquid in the bridge. The stability of the film formed between the bridge liquid and the bubble or drop during the reduction of the amount of bridge liquid is directly related to the stability of the liquid or emulsion respectively.

Description of the figures

Figure 1. General presentation of the methodology of the invention.

Figure 2. Change of the shape of the liquid bridge during the increase of the distance between the electrodes.

Figure 3. Illustration of the time variation (t-s) of the electrical conductivity (C-mS) during the variation of the distance of the electrodes and by extension the shape of the liquid bridge. The points of the curve (conductivity values) that will be used to determine the dynamic interfacial tension have been marked with a red circle and the corresponding points of the static interfacial tension with a blue circle.

Figure 4. Variation of liquid bridge volume to determine gas/liquid interface stability and consequently foam stability.

Figure 5. Time variation (t-s) of the electrical conductivity (C-mS) upon changing the shape of the liquid bridge (by reducing the volume of the liquid bridge), in the case of studying the stability of liquid/gas interfaces.

Detailed description of the invention

[0016] For the application of the methodology the following are required:

A. Necessary equipment (Figure 1): Two vertical electrodes (Figure 1-B) between which the liquid bridge is formed (Figure 1-D), an instrument for adjusting and measuring the distance between the electrodes (Figure 1-A), two support bearings for each of the electrodes so that the bridge fluid can contact the surrounding fluid (Figure 1-T), a syringe (Figure 1-C) (which can be adapted to the upper electrode) for introducing air or liquid (Figure 1-H) into the bridge to form a gas/liquid and liquid/liquid interface for the surface and interfacial tension measurements, a pump (Figure 1-E) connected to the lower electrode and drawing in a quantity of liquid representing the drain conditions, a frequency generator (Figure 1-G) to generate AC alternating current driven into the liquid bridge, and an instrument/technology to measure the electrical conductivity (Figure 1-F) of the liquid bridge. In the present case, the technology described in the patent EP 3005942 A1 (2016) is proposed as a technology for measuring the electrical conductivity of the liquid bridge, due to the greater sensitivity it provides compared to the other methodologies.

B. The temperature of the chamber where the measurement is carried out must be constant (+- 0.5 °C) and the humidity 85-90% to reduce the possibility of evaporation or, on the contrary, condensation of water vapor in the liquid bridge.

Liquid and Electrode Properties

[0017] The liquid of the bridge should be conductive (different conductivity from the fluid that surrounds it, in the case of determining the interfacial tension) and the electrodes metallic (preferably) cylindrical copper.

Description of the method

A. Determination of interfacial tension (static and dynamic) between fluids:

To determine the interfacial tension (static and dynamic) between fluids, electrical measurements are made (electrical resistance is measured or, conversely, electrical conductivity) during an increase in the distance between the electrodes (Figure 2 and 3) or a decrease in the volume of the fluid bridge. The liquid bridge is surrounded by air in the case of measuring the interfacial tension of a liquid (determination of foaming ability), while for the determination of the interfacial tension between two liquid phases (determination of emulsification ability) the liquid bridge is immersed in a second liquid. In the latter case, the two liquid phases must have distinct electrical conductivity values.

Changing the distance of the electrodes or reducing the volume of the liquid results in a change in the shape of the liquid bridge and consequently in a change in its conductivity, which is measured experimentally ("experimental" values). Then by solving the Young-Laplace equation (1.1):

for each change in electrode spacing or reduction in the amount of liquid bridge, the shape of the liquid bridge is determined. For each shape of the liquid bridge, through the solution of the Laplace equation (1.2):

the "theoretical" value of the conductivity is calculated and a corresponding graph is formed. The points of identification of the "theoretical" values, with the experimental measurements, allows the precise determination of the interfacial tension (c) of the liquid of the bridge through the equation (1.3).

Bo=pgR<2>/γ (1.3)

The lower the interfacial tension of the measured fluid, the more easily a foam or emulsion will form. It is noted that the "experimental" conductivity values marked in the curve of Figure 3 in a red circle are used to determine the dynamic interfacial tension, while those marked in a blue circle are used to determine the static one.

The steps followed to measure the interfacial tension are as follows:

STEP 1: Creating a liquid bridge (amount of liquid 10-20μl) between the two vertical electrodes (and immersing it in another liquid of distinct conductivity in the case of determining the emulsification capacity).

STEP 2: Generate an alternating electrical signal, of the desired type/amplitude/frequency (10-100kHz) via a frequency generator, which will flow through the liquid bridge.

STEP 3: Gradual increase of the distance between the electrodes or specific reduction of the amount of fluid of the bridge, until the rupture of the bridge.

STEP 4: Record the change in electrical conductivity ("experimental values").

STEP 5: Determination of the shape of the liquid bridge for each change in electrode spacing or reduction in the amount of liquid in the bridge, by solving the Young-Laplace equation (1.1).

STEP 6: Solving the Laplace equation (1.2), for each shape of the liquid bridge and calculating the "theoretical" conductivity values.

STEP 7: Identification of the "theoretical" conductivity with the "experimental" measurements and determination of the interfacial tension (static and dynamic) of the liquid of the bridge, through the determination of the variable Bo, i.e. through the solution of equation (1.3).

B. Study of the stability of liquid/gas and liquid/liquid interfaces:

In the present invention it is possible to study the stability of liquid/gas (in the case of foams) and liquid/liquid (in the case of emulsions) interfaces through electrical (conductivity) measurements of a bridge that forms a quantity of the liquid being studied between two vertical electrodes. The difference with the case of interfacial tension measurement is the creation of a gas bubble within the liquid bridge in the case of measuring liquid/gas interfaces or a droplet of another liquid (discrete conductivity) in the case of measuring liquid/liquid interfaces (Figure 1).

More specifically, a bubble or droplet is created within the liquid bridge and the electrical conductivity of the bridge is measured during the decrease in liquid volume (Figure 4). During this time, the values of the electrical conductivity decrease continuously at a high rate, as is also reflected in section A of the curve in Figure 5. Subsequently, while the volume of the liquid bridge continues to decrease, the interface acquires its smallest thickness and the rate of decrease of electrical conductivity values decreases significantly (section B of the curve). The longer the B portion of the electrical conductivity versus time curve, the more stable the interface.

The steps followed to determine the interfacial tension are the following:

STEP 1: Create a liquid bridge (10-20μl) between the two vertical electrodes and create an air bubble (air/liquid interface) or a drop of another liquid (liquid/liquid interface), within the liquid bridge.

STEP 2: Generate an alternating electrical signal, of the desired type/amplitude/frequency (10-1000kHz) via a frequency generator, which will flow through the liquid bridge.

STEP 3: Gradually reduce the amount of bridge fluid, with a specific rate of drainage.

STEP 4: Record the change in electrical conductivity and graph it against time.

STEP 5: Locate and measure the B portion of the conductance versus time curve (Figure 5).

REFERENCES

[1]: Abdolahi F., Moosavian M. A. and Vatani A., 2005, The Mechanism of Action of Antifoams, Journal of Applied Sciences, 5, 1122-1129.

[2]: Georgiou E., 2010, Development of a prototype technique for monitoring individual liquid films during foam drainage, Master Thesis, Department of Chemistry, Aristotle University of Thessaloniki.

[3]: Drelich J., Fang Ch., White C.L., 2002, Measurement of interfacial tension in fluid-fluid systems, Encyclopedia of Science and Colloid Science, 3152-3166.

[4]: Michalentzakis D., 2012, Dynamic surface tension and dilatational surface viscoelasticity of aqueous SDS solutions, Master Thesis, Department of Chemistry, Aristotle University of Thessaloniki.

[5]: C. Stubenrauch, B.R. Blomqvist, 2007, Foam films, foams and surface rheology of nonionic surfactants: amphiphilic block copolymers compared with low molecular weight surfactants, in: T.F. Tardos (Ed.), Colloid Stability: The Role of Surface Forces I, Wiley, Weinheim.

[6]: M. Kostoglou, E. Georgiou, T.D. Karapantsios., 2011, A new device for assessing film stability in foams: Experiment and theory, Colloids and Surfaces A: Physicochemical and Engineering Aspects 382, 64-73.

[7]: Morar, Sameer & Hatfield, D & Barbian, N & Bradshaw, Dee & Cilliers, J & Triffett, B, 2006, A comparison of flotation froth stability measurements and their use in the prediction of concentrate grade: IMPC 2006 - Proceedings of 23rd International Mineral Processing Congress.

[8]: http://news.rice.edu/2014/08/12/foam-favorable-for-oil-extraction/. assessed 1/1/2019

Claims (7)

1. A method performed by applying electrical conductivity measurements to a fluid-surrounded liquid bridge for the simultaneous determination of (a) the interfacial tension (static and dynamic) and (b) the stability of the gas/liquid and liquid/ liquid, which is characterized by the following processes: • Formation of a liquid bridge between two electrodes • Contact of the liquid bridge, with the fluid that surrounds it • Changing the distance of the electrodes or reducing the volume of the bridge fluid • Recording of the electrical conductivity during the change of the distance or the reduction of the volume of the liquid of the bridge • Solving the Young -Laplace and Laplace equations. • Determination, through the electrical conductivity time graph, of point B, that is, the point where the rate of decrease of electrical conductivity values decreases significantly. 2. Method according to claim 1, where the fluid surrounding the bridge is air or a liquid, different from the liquid of the bridge, and when determining the interfacial tension (static and dynamic), by extension, the ability to foam and emulsify is also determined of the bridge fluid. 3. A method according to claim 1, wherein the surrounding fluid of the bridge is air and an air bubble is created inside the liquid bridge and in determining the stability of the gas/liquid interfaces, the stability of the foam that would form is also determined the fluid of the bridge. 4. Method according to claim 1, where the surrounding fluid of the bridge is liquid and inside the bridge, a drop of liquid similar to that surrounding the bridge is created and the stability of the liquid/liquid interfaces and by extension the stability of the emulsion that would form the bridge fluid. 5. A method according to claim 1, in which any fluid that is conductive can be used. 6. Method, according to claim 1, in which any conductive materials that allow the formation of a liquid bridge between them can be used as electrodes. 7. Arrangement for carrying out the method of claims 1 to 6 (method claims), which includes: a. Two cylindrical electrodes (Figure 1-B) (diameter 2-5mm), between which the liquid bridge (Figure 1-D) is formed, as mentioned in all the above claims. b. A conductive fluid (10-20µl) to form the liquid bridge as mentioned in all the above claims (Figure 1-D). c. a second fluid, which surrounds the liquid bridge as described in claim 1 and enters the bridge in the case of claims 3 and 4 (Figure 1-H) and as stated in claim 5, of distinct conductivity from the first, i.e. the liquid of the bridge, for the determination of interfacial tension between two liquid phases and the stability of liquid/liquid interfaces. d. Support bearings for each of the cylindrical electrodes so that the bridge fluid can contact the surrounding fluid, as described in all preceding claims (Figure 1-Θ). e. Instrument for micrometric adjustment and measurement of the distance between the electrodes (Figure 1-A), which varies from 3-6mm, to determine the interfacial tension and the stability of liquid/liquid and liquid gas interfaces. f. A syringe (Figure 1-C) for introducing air or liquid into the liquid bridge as described in claims 1, 3, 4, 5, 6. g. A pump (Figure 1-E) to reproduce the drainage conditions for all the preceding claims. h. A frequency generator (Figure 1-G) for producing an alternating electrical signal of desired type/amplitude/frequency (10-1000kHz), which electrically excites the liquid bridge as recited in all preceding claims. i. Instrument for measuring electrical resistance (conductivity) (Figure 1-F), as described in all preceding claims.