M. Kahlweit

How to study microemulsions.

The application of various experimental techniques for studying the properties of ternary mixtures H 2 O-oil-nonionic amphiphile is demonstrated. Based on detailed knowledge of the phase behavior, in particular, the positions and extensions of the three-phase bodies, the origin of the ultralow interfacial tensions and strong opalescence of the solutions is elucidated. Within the three-phase body one finds a maximum of the velocity of phase separation of the three phases that is correlated to a minimum of the viscosity and a jump of the electric conductivity. The properties of the homogeneous microemulsions are measured along a path through the phase prism at constant amphiphile concentration but varying oil/water ratio. The dynamics of the microstructure is studied by viscosity, dynamic light scattering, and electric field and pressure jump relaxation, as well as by ultrasonic absorption. The characteristic dimensions of the structure are determined by SANS and SAXS, NMR self-diffusion, electric conductivity, and freeze fracture electron microscopy.

A new purification technique for alkyl polyglycol ethers and miscibility gaps for water-CiEj

Abstract We report here a new procedure to purify nonionic amphiphiles (alkyl polyglycol ethers, C i E j ). The method utilizes the well-known phase behavior of ternary systems of water, oil, and nonionic amphiphiles to extract simultaneously water- and oil-soluble impurities. The three-phase extraction technique (denoted as 3PHEX) is easy to apply and demonstrated to yield reproducible miscibility gaps of binary water-nonionic amphiphile mixtures. The critical temperatures and compositions for amphiphiles of varying chain length, ranging from C 6 E 3 to C 12 E 6 , are given.

Kinetics of formation of association colloids

Abstract The present state of the theory of the kinetics of micellization is summarized. Consideration of ionic micelles as charged colloidal particles is proposed. At low counterion concentrations the electrostatic repulsion prevents coagulation of the submicellar aggregates, so that the micelles can grow by incorporation of monomers only: N1-1 + N1 → Ni. At high counterion concentrations, however, this reaction path is bypassed by a reversible coagulation of submicellar aggregates: Nk + N1 → N1; k + l = i. With nonionic systems, the electrostatic repulsion being absent, both reaction paths compete right from the CMC on. The analysis of the time constants and the amplitudes of the relaxation process in aqueous solutions is presented, making use of the prediction of the DLVO theory. The results are in good agreement with the experiments.

The phase behavior of systems of the type H2O-oil—nonionic surfactant—electrolyte

In a recent publication we had suggested study of “simple” systems of the type H2O — Oil — Nonionic Surfactant — Electrolyte as model for the pseudoquaternary systems relevant in tertiary oil recovery, in order to make qualitative predictions for the phase behavior of such systems. In this paper the first results of such studies are presented. It is shown in detail how the formation of the three-phase triangle arises from the change of the phase diagram of such systems with temperature. It turns out that, within a limited temperature range, the phase diagrams can show two critical points, which are responsible for the ultra-low interfacial tensions between neighbouring phases of the three-phase triangle. Furthermore, a simple recipe is proposed which permits to predict whether or not a system — with given oil — will show a three-phase triangle with rising temperature or addition of an appropriate electrolyte. The predictions are compared with experiments using short chain polyglycolethers (CiEj) as surfactants, which have the advantage that these surfactants do not form anisotropic phases.

Electric conductivity in microemulsions

Microemulsions exhibit a rather sudden change of electric conductivity upon varying either composition at fixed temperature, or temperature at fixed mean composition. In literature this has been interpreted as a percolationlike process. In this paper we question this interpretation, and suggest instead that the change of conductivity is caused by a dispersion inversion that takes place in a plane through the symmetry axes of the three‐phase body, irrespective of the volume fraction of the conducting phase.

Tricritical points in H2O–oil–amphiphile mixtures.

Quaternary mixtures of water (A), an oil (B), a nonionic amphiphile (C), and an appropriately chosen fourth component offer an opportunity for searching for tricritical points (tcp) at atmospheric pressure. It is shown that for reaching a tcp, one has to couple an A–B–C mixture that shows the phase sequence 2_→3→2 with rising temperature, with a second ternary mixture that shows a 2_→2 transition, the bar denoting in which of the two phases the amphiphile is mainly dissolved. With weakly structured solutions, that is, with short‐chain amphiphiles as (C) this can be done by either adding an oil with a lower carbon number, or by adding a nonaqueous polar protic solvent such as formamide. With strongly structured solutions, that is, with long‐chain amphiphiles, one has to add a short‐chain amphiphile for destroying the structure as a prerequisite for reaching a tcp. Insofar, our earlier presumption that with long‐chain amphiphiles, a tcp may also be reached, either by increasing their amphiphilicity or by lowering the carbon number of the oil, does not seem to apply. Experience shows that in A–B–C’ mixtures with sufficiently short‐chain amphiphiles as C’ that separate into three phases: the amphiphile‐rich middle phase always wets the A/B interface. If a short‐chain amphiphile is added to an A–B–C mixture with a nonwetting middle phase one will, therefore, inevitably find a nonwetting→wetting transition as one approaches a tcp.

Wetting in water–oil–nonionic amphiphile mixtures

In a three‐phase equilibrium of H2O –oil–amphiphile mixtures, the middle amphiphile rich phase may or may not wet the water/oil interface. For nonwetting middle phases, theory predicts a nonwetting→wetting transition upon approaching either one of the two critical endpoints. With respect to an experimental confirmation of this prediction, the situation appears to be controversial. In this paper, we have, therefore, studied the wetting behavior of the middle phase as it depends on the amphiphilicity of nonionic amphiphiles. We find that in mixtures with short‐chain amphiphiles, the middle phase wets the water/oil interface in the entire three‐phase interval, whereas with long‐chain amphiphiles it (apparently) never wets. For medium‐chain amphiphiles, however, one does find a nonwetting→wetting transition. On the basis of this result, we suggest that there exist four cases for the wetting behavior as a consequence of the dependence of the relations between the three interfacial tensions on amphiphilicity. The wetting behavior can be correlated with the evolution of the three‐phase bodies from a tricritical point. Upon increasing amphiphilicity, their characteristic properties pass through maxima in the range of medium‐chain amphiphiles, coinciding with the transition from always wetting to never wetting.

Phase behavior of the ternary system H2O-oil-polypropyleneglycol (PPG)

This paper reports on studies of the phase behavior of systems of the type H2O-oil-amphiphilue-electrolyte with a simple polar polymer as nonionic amphiphile. It turns out that the system behaves similar to systems with monomeric nonionic amphiphiles which is due to the fact that both the binary system H2O-polar polymer and oil-polar polymer show the same characteristic features as the corresponding systems with monomeric amphiphiles. The results suggest the tailoring of polydetergents by varying both their amphiphilicity and their degree of polymerization in order to obtain highly efficient amphiphiles for the preparation of socalled microemulsions.

On the stability of microemulsions. II.

In literature, the stability of microemulsions has been attributed to the existence of a natural curvature as well as a bending energy of the saturated amphiphilic monolayers separating the polar and the nonpolar solvents. Alternatively, it has been suggested recently seeing the reason for the stability in the steep rise of the interfacial tension as soon as the interfacial layers become unsaturated. In this paper, we compare the predictions of the latter model with respect to the stability limits in ternary water–oil–nonionic amphiphile mixtures at temperatures below and above the three‐phase interval. The agreement between theory and experiment is reasonable.

Über das Phasenverhalten ternärer systeme des Typs-H2O-Öl-Nichtionisches Tensid

ZusammenfassungDie Arbeit behandelt die Veränderung des Phasendiagramms ternärer Systeme des Typs H2O-Öl-Nichtionisches Tensid mit der Temperatur, insbesondere die Frage nach den Ursachen für den Zerfall solcher Mischungen in drei Phasen. Es wird gezeigt, daß das Phasenverhalten im wesentlichen durch das Zusammenspiel zwischen der unteren Mischungslücke zwischen Öl und Tensid mit der oberen Mischungslücke zwischen H2O und Tensid bestimmt wird. Aus systematischen Untersuchungen mit Dodekan, Oktan, Cyclohexan und Toluol als Ölen und „einfachen“ Alkylpolyäthylenglykoläthern CiEj als Tensiden werden zehn qualitative Regeln über die Abhängigkeit der Lage und Breite des Temperaturintervalls des Dreiphasengebietes von der Hydrophobie des Öls und der Amphiphilität des Tensids hergeleitet. Die Regeln erleichtern die Auswahl des geeigneten Tensids für verfahrenstechnische Anwendungen. Schließlich wird das Phasenverhalten mit Ölmischungen bzw. Tensidmischungen behandelt.AbstractThis paper deals with the change of the phase diagram of ternary systems of the type H2O-oil-nonionic surfactant with temperature, in particular, with the question regarding the reasons for the separation of such mixtures into three phases. It is shown that the phase behavior is mainly determined by the interplay between the lower miscibility gap between oil and surfactant with the upper miscibility gap between H2O and surfactant. From systematic investigations with dodecane, octane, cyclohexane and toluene as oils with „simple“ alkylpolyethyleneglycolethers CiEj as surfactants, ten qualitative rules are derived concerning the dependence of the position and the width of the three-phase temperature interval on the hydrophobicity of the oil and the amphiphilicity of the surfactant. The rules permit the choice of the appropriate surfactant for application in chemical engineering. Finally, the phase behavior with mixtures of oils and mixtures of surfactants, resp., is studied.