K. V. Schubert

Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n‐alkane, and n‐alkyl polyglycol ether (CiEj) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with well‐structured mixtures (exhibiting nonwetting middle phases and well‐expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the...

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.

Small‐angle neutron scattering from microemulsions near the disorder line in water/formamide–octane‐CiEj systems

Small‐angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n‐octane, and a sufficient amount of n‐alkyl polyglycol ether (CiEj). By changing the surfactant chain length in the order C8E3, C6E2, and C4E1 in pure water, as well as for given C8E3 and C6E2 by increasing the relative amount of formamide in the water/formamide mixture, the amphiphilic strength (the amphiphilicity) is reduced. We observe that the characteristic scattering peak becomes weaker and its position moves into q=0 showing that the microstructure becomes disordered. However, a q−2 and a q−4 behavior for large q in film and bulk contrast, respectively, is still observed indicating the persistence of internal interfaces. As the microstructure is further weakened, a disorder line is passed at which the behavior of the real space correlation function changes from a damped oscillatory behavior to a monotonically decreasing one. We determine that this line has be...

Small‐angle neutron scattering of D2O–C12E5 mixtures and microemulsions with n‐octane: Direct analysis by Fourier transformation

Spherical, cylindrical, and lamellar microstructures were examined by (SANS) in the binary system water (D2O)/pentaethylene glycol mono‐perdeutero‐n‐dodecyl ether (C12D25E5) and in the ternary system (D2O)/pentaethylene glycol mono‐n‐dodecyl ether (C12E5)/perdeuterated n‐octane (C8D18). A model‐independent picture of the structures emerges from Fourier transformation of the measured SANS spectra. The data analysis makes no a priori assumptions about the type of structure. Rather, the pair distribution functions obtained are so unique that an unambiguous assignment of the local geometry is possible. The different structures for a given mixture are obtained by changing temperature. For water‐rich samples the sequence of spherical, cylindrical, and planar structures is obtained with increasing temperature. For oil‐rich samples the same sequence occurs as a function of decreasing temperature. As an important additional result, the scattering length density profile normal to the interface is obtained. The inte...

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.

The Lifshitz line in binary systems: Structures in water/C4E1 mixtures

In a recent publication we compared phase behavior and scattering data obtained from SANS in water, n‐alkane, n‐alkyl polyglycol ether (CiEj) mixtures. By analyzing the scattering spectra using the Teubner–Strey formula, it is possible to determine a measure for the amphiphilic strength of each system called the amphiphilicity factor, fa. It was demonstrated that the amphiphilicity factor is constant on a variety of surfaces within the three‐dimensional space of composition and temperature. For instance, the Lifshitz surface, where fa=0, depends on composition and temperature and may be a precursor for the Lα phase. Here we show that the same sequence exists in the binary system water/C4E1, where the surfaces become lines in the two‐dimensional composition‐temperature planes. Variations in amphiphile concentration and solution temperature allow one to reach almost the entire accessible amphiphilicity scale (∞≳fa≳−1) within this simple binary mixture. The occurrence of the Lifshitz surface at high C4E1 con...

Effect of Formamide on Microemulsions

We studied the effect of formamide on microemulsions containing water, n—alkanes, and n—alkyl polyglycol ethers (CiEj). The water/formamide ratio is an ideal “tuning” parameter for these systems because formamide allows to modify the hydrophobic effect. Therefore, non—wetting→wetting transitions can be induced, tricritical points can be reached, and, using SANS, it is possible to detect a crossing of a disorder line.