J. Phys. Chem. 1995, 99, 1281-1284
1281
How To Prepare Microemulsions at Prescribed Temperature, Oil, and Brine M. Kahlweit Max-Planck-Institutf i r Biophysikalische Chemie, Postfach 2841, 0-37018 Gottingen, Germany Received: July 8, 1994; In Final Form: October 13, 1994@ In the first part of this paper we recall the difference between nonionic and ionic amphiphiles with respect to the effect of thermal energy on their distribution between water and oil and the consequences that result therefrom for the preparation of microemulsions if temperature, the composition of the oil, and that of the brine are prescribed. In the second part we demonstrate how to monitor the position of the three-phase interval of ionic microemulsions on the temperature scale by means of an “lipophilicity scan”, that is, by using a sufficiently lipophilic double-tailed ionic amphiphile and varying its effective lipophilicity by adding either a less lipophilic single-tailed or a more lipophilic double-tailed ionic co-surfactant.
1. Introduction
Nonionic
Microemulsions are thermodynamically stable, macroscopically homogeneous, and optically isotropic colloidal dispersions of either “oil” domains in water (olw) or water domains in oil (wlo), stabilized by interfacial layers of amphiphiles. The type of dispersion, that is, whether olw or wlo, is determined by the distribution of the amphiphiles between water and oil. If an amphiphile is more soluble in water than in oil, one finds olw; if it is more soluble in oil than in water, one finds wlo emulsions. When applying microemulsions in industry or research, in general, the temperature Texpof application, the composition of the oil, and that of the brine are prescribed. Wanted is an efficient amphiphile for preparing the desired type of emulsion at Texp with that particular oil and brine. Experiment tells that-within a well-defined temperature interval AT-mixtures of water, oil, and amphiphiles separate into three coexisting liquid phases, namely, the amphiphile-rich microemulsion in equilibrium with a water-rich and an oil-rich excess phase. The width of AT and its mean temperature T depend sensitively but systematically on the carbon number k of the oil, as well as on the lipophilicity of the amphiphile, that is, the hydrophilicity of its head group, and the carbon number of its hydrocarbon tail(s). Upon increasing the mass fraction y of the amphiphile in the mixture, the volume fractions of the two excess phases s h r i n k until, at sufficiently high y , the microemulsion remains as the only phase.’ Experiment, furthermore, tells that the amount of amphiphile required to prepare a single-phase microemulsion, as well as the interfacial tension between the water-rich and the oil-rich excess phase, passes through deep minima at T, so that-if Texp,oil, and brine are prescribed-the task is to chose the amphiphile such that the Flies near Texp.In this paper we briefly summarize the recipes how to systematically search for appropriate amphiphiles. They are based on many experiments made by American, Australian, European, Israelian, and Japanese groups during the past decades. Because, however, the recipes are scattered over many publications, it seems useful to summarize them in the following section.
11. Difference between Nonionic and Ionic Amphiphiles The separation into three liquid phases is the consequence of a rapid inversion of the distribution of the amphiphile (C) between oil (B) and water (A).2 Defining the distribution @
Abstract published in Advance ACS Absrracrs, January 1, 1995. 0022-365419512099-128 1$09.0010
1
-Kc
Ionic
1
2
1
-
KC
Figure 1. Temperature dependence of the distribution of nonionic (top) and ionic (bottom) amphiphiles between oil and water in salt-free mixtures (schematic).
coefficient (at mean compositions above the cmc) by
K, = c,/c, experiment tells that at ambient temperatures both nonionic and ionic amphiphiles are much more soluble in water than in oil, Le., KC e 1. However, as to the temperature dependence of KC, there exists an important difference between nonionic and ionic amphiphiles: With rising T, one finds with nonionic KC >> 1 within a rather amphiphiles an inversion KC *: 1 narrow temperature interval as shown schematically in the top of Figure 1, so that with nonionics a separation into three liquid phases can be enforced by raising the thermal energy to the corresponding level, with the consequence that, for preparing nonionic microemulsions, adding a fourth component is not necessary. Evidently, the following applies: (i) The thermal energy required for traversing the three-phase (temperature) interval AT in mixtures with nonionic amphiphiles
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0 1995 American Chemical Society
1282 J. Phys. Chem., Vol. 99, No. 4, 1995
Nonionic
-€
Ionic
Kahlweit cusps in Figure 2 move to the left. At fixed 6, the three-phase temperature interval AT hence rises with ionics but drops with nonionics as the lipophilicity of the amphiphile is increased. At low brine concentrations, mixtures with ionics as well as such with nonionics separate into two phases with the amphiphile being mainly dissolved in the lower aqueous phase (2 = Winsor I), being an olw emulsion. As E is increased at fixed T (horizontal arrows in Figure 2), the aqueous olw dispersion separates at a critical end point into a concentrated and a diluted dispersion. With further increasing E, the concentrated dispersion takes up oil, passes through a bicontinuous spongelike structure, and inverts to a w/o dispersion that eventually merges with the upper oil-rich phase at a second critical end point. At high E, the mixtures separate into two phases with the amphiphiles being mainly dissolved in the oil-rich phase (2 = Winsor 11), being a wlo dispersion. Hence, if E is increased -at fixed T, both mixtures show the phase sequence 2. 3 2. However, if Tis raised at fixed E (vertical arrows in Figure 2), mixtures with ionics (provided, they are initially in state 5) show 3 2, whereas such with nonionics the phase sequen_ce 5 show 2 3 2. Alcohols are not required for preparing microemulsions. The use of alcohols has historic reasons: The first experiments on microemulsions were primarily performed with single-tailed ionic amphiphiles such as sodium dodecyl sulfate (SDS). Experiment, however, tells that single-tailed ionic amphiphiles are, in general, too hydrophilic to be salted out even into oils of low carbon numbers such as aromatic oils. An example is the classic mixture
- -
T
t -E Figure 2. Salting out of amphiphiles: T--E cusps for nonionic (top) and ionic (bottom) amphiphiles (schematic). increases with increasing carbon number k of the oil (at fixed amphiphile) and decreases with increasing lipophilicity of the amphiphile (at fixed oil), that is, either decreasing hydrophilicity of its head group or increasing chain length of its tail. Hence, in a T-k plot, AT shapes a cusp that ascends and widens as k is increased (see, e.g., Figure 4 in ref 3), whereas, if plotted vs the lipophilicity of the amphiphile, the cusp descends and shrinks as the lipophilicity is increased (Figure 5 in ref 3). With ionic amphiphiles, on the other hand, one finds Kc to decrease even further with rising T so that KC
