Electrically conductive microemulsions based on perfluoropolyethers

Abstract: Microemulsions of the water-in-oil type (w/o) capable of effecting ionic electric transfer and material interphase transfer, wherein the oily phase consists of a perfluoropoly­ ether having perfluoroalkyl end groups or functional end groups of the hydrophilic type, said microemulsions being obtained by using perfluorinated surfactants, in particular having a per­ fluoroalkylpolyether structure, and/or an alcohol as co­ surfactant.

Description: The fluorinated fluids having a perfluoropolyether structure (PFPE) exhibit utmost important characteristics such as : high chemical and thermal stability; complete immiscibility with water and hydrocarbons; high gas solubility. However, their structure and the properties of high water-repellency and very high resistivity do not allow the use thereof in electrochemical processes or in separation processes, as they do not permit either the transfer of substance from solutions or the electric transfer. Conversely, it could be very useful to have available systems based on perfluoropolyethers capable of providing substance transfer and ionic electric transfer, for use, for example, as membranes in electrochemical processes or in separation processes. Liquid systems consisting of water-in-hydrogenated oil microemulsions capable of conducting electricity are known in the scientific and patent literature. Their formation and their existence, however, are generally not considered to be predictable. It has now surprisingly been found that it is possible to impart the electric conductivity and material transfer properties to liquids having a perfluoropolyether structure if particular microemulsions of water in the perfluoropolyether liquid are prepared. Accordingly, the object of the present invention is the use of a microemulsion of the water-in-oil type having an electric conductivity (for ionic transfer) of at least 10 µSiemens.cm⁻¹ and consisting of a liquid, limpid or opalescent, macroscopically monophase substance which is obtainable by mixing (a) an aqueous liquid which optionally contains one or more electrolytes; (b) a liquid compound with perfluoropolyether structure and having (i) perfluoroalkyl end groups and an average molecular weight of from 500 to 10,000 or (ii) from 0.1 to 4 functional end groups for each chain, selected from carboxylic, polyoxyalkylene-OH, alcoholic, aminic, amidic, quaternary ammonium and ester groups (preferably functional groups of the hydrophilic type, such as the carboxylic group and the polyoxyalkylene-OH group, most preferably carboxylic groups), and an average molecular weight of from 1,500 to 10,000; (c) a fluorinated surfactant, preferably of the ionic type and preferably having a perfluoropolyether structure; and, optionally; (d) a (hydrogenated) C₁-C₁₂ (preferably C₁-C₆) alcohol and/or a fluorinated alcohol (co-surfactant); in electrochemical processes or in separation processes. "Microemulsion" means a mixture macroscopically consisting of a single limpid or opalescent, optically isotropic phase comprising two immiscible liquids and at least a surfactant. The microemulsions form spontaneously and their stability is of the thermodynamic type. Whenever used herein, the term "microemulsion" means also the systems, wherein the orientation of the molecules at the interphase leads to the formation of not optically isotropic systems, characterized by birefringence and probably consisting of oriented structures of the liquid-crystalline type (liquid crystals). The microemulsions used according to the present invention are mixtures macroscopically consisting of only one limpid or opalescent phase. They can be optically isotropic or birefractive and are characterized in that they are conductive, their conductivity being of at least 10 µS.cm⁻¹ and preferably higher than 100 µS.cm⁻¹. Since the composition of the microemulsions used according to the present invention is of type w/o, they must contain PFPE as "continuous phase": therefore it is preferable that the PFPE phase is present in excess (based on the volume) with respect to the aqueous phase. Both the existence of microemulsions of type w/o and the conductivity characteristics are not predictable "a priori": in general, by consequence, the microemulsions used in the present invention can be preferably described as the conductive portion of the single-phase areas which are present in the right-hand half of a water/surfactant / PFPE ternary diagram representable as is shown in figure 1. In figure 1, the bisecting line of the angle opposed to the water-PFPE base side is characterized by a constant W/PFPE ratio equal to 1. In principle, however, the presence of single-phase conductive areas of the w/o type also having a W/PFPE ratio higher than 1 cannot be excluded, since the existence of such systems cannot be predicted. The fact that the use of the microemulsions of the water-in-perfluoropolyether type is within the scope of the present invention can be easily ascertained by those skilled in the art by a simple electric conductivity measurement, as illustrated above. Perfluoropolyethers (PFPE) suitable for providing the microemulsions for the present invention are : a) PFPE having an average molecular weight ranging from 500 to 10,000 and preferably from 600 to 6,000, with perfluoroalkyl end groups, and belonging to one or more of the following classes : 1) with a random distribution of the perfluorooxyalkylene units, where R f and R' f , like or different from each other, are -CF₃, -C₂F₅or-C₃F₇, and m , n , p have such average values as to meet the above average molecular weight requirements. 2) R f O(CF₂CF₂O) n (CF₂O) m R' f with a random distribution of the perfluorooxyalkylene units, where R f and R' f , like or different from each other, are -CF₃ or -C₂F₅,and m and n have such average values as to meet the above molecular weight requirements. 3) with a random distribution of the perfluorooxyalkylene units, where R f and R' f , like or different from each other, are -CF₃, -C₂F₅ or -C₃F₇, and m , n , p , q have such average values as to meet the above molecular weight requirements. 4) where R f or R' f , like or different from each other, are -C₂F₅ or -C₃F₇ and n has such a value as to meet the above molecular weight requirements. 5) R f O(CF₂CF₂O) n R' f where R f and R' f , like or different from each other, are -CF₃ or -C₂F₅, and n has such an average value as to meet the above molecular weight requirements. 6) R f O(CF₂CF₂CF₂O) n R' f where R f and R' f , like or different from each other, are -CF₃ or -C₂F₅ or -C₃F₇, and n has such an average value as to meet the above molecular weight requirements. b) PFPE belonging to the above-described classes, having an average molecular weight ranging from 1,500 to 10,000 and preferably lower than 6,000, characterized in that they contain on the average from 0.1 to 2 and preferably from 0.3 to 1 non-perfluoroalkyl functional end groups for each polymeric chain. c) Perfluoropolyethers as are described in IT-A-20,346 A/86 in the name of the Applicant hereof, having functional groups along the perfluoropolyether chain and end groups of the perfluoroalkyl or functional type. The terms "non-perfluoroalkyl functional end groups" and "functional groups in the chain", for example, refer to carboxylate, carboxylic, alcoholic, polyoxyalkylene-OH, aminic, quaternary ammonium, amidic, hydroxylic and ester groups. The functional end groups or the functional groups in the chain which are best suited are those of the hydrophilic type and in particular the carboxylic groups. The functional end groups or the functional groups in the chain, of the abovesaid type, can be linked to the perfluoropolyether chain through a linking group consisting of a divalent non-fluorinated radical of the alkylene or arylene type, containing up to 20 carbon atoms, preferably from 1 to 8 carbon atoms. As belonging to the perfluoropolyethers to be utilized according to the present invention are to be considered also the ones of classes 1, 2 and 3, containing peroxy bridges in the chain and having acid end groups, obtained as raw products in the photo-oxidation process utilized for the synthesis of the aforesaid PFPE. Perfluoropolyethers of class 1) are known in commerce under the trade-mark Fomblin® Y or Galden® and the ones of class 2) under the trade-mark Fomblin® Z, all of them being produced by Montedison. Products of class 4) known on the market are the Krytox (Du Pont) products. Those of class 5) are described in US-A-4,523,039. Those of class 6) are described in EP-A-148,482. The ones of class 3) are prepared according to US-A-3,665,041. Other suitable perfluoropolyethers are the ones described by Lagow et al. in US-A-4,523,039 or in J. Am. Chem. Soc. 1985 , 107, 1197-1201. The fluorinated surfactants contained in the microemulsions used according to this invention may be either ionic or non-ionic. In particular,the following can be cited: a) salts of perfluoroalkylcarboxylic acids having 5 to 11 carbon atoms; b) salts of the perfluorosulphonic acids having 5 to 11 carbon atoms; c) non-ionic surfactants described in EP-A-51526, consisting of a perfluoroalkyl chain having a polyoxyalkylene hydrophilic head; d) salts of the mono- and di-carboxylic acids derived from perfluoropolyethers, preferably having average molecular weights not higher than 1000; e) non-ionic surfactants consisting of a perfluoropolyether chain bound to a polyoxyalkylene chain; f) perfluorinated cationic surfactants such as those containing 1,2 or 3 perfluoropolyether hydrophobic chains. The surfactants of the ionic type are preferred. Furthermore, the system can contain one or more co-surfactants belonging to one of the following classes : hydrogenated alcohols having 1 to 12 carbon atoms; alcohols comprising a perfluoropolyether chain; partially fluorinated alcohols. The aqueous liquid may be composed of water or of an aqueous solution of inorganic electrolytes (salts, acids or alkalis). The conductive systems are prepared by mixing the individual components and can be identified, for example, by measuring the variation in the specific conductivity (X) of the oil/surfactant/co-surfactant system upon variation of the composition due to addition of the aqueous solution. In practice, a sample containing a surfactant (and optionally a co-surfactant) in PFPE is titrated with small portions of the aqueous phase, measuring X after each addition. By operating in this manner, the possible presence of a compositive range corresponding to significant X values is ascertained. Once the composition corresponding to a sufficiently high X value has been identified, the conductive microemulsion can be prepared simply by mixing the individual components taken in any order. The following examples are to be considered as merely illustrative and not limitative of the possible embodiments of the present invention. Example 1 3.5 g of ammonium salt of a monocarboxylic acid having a perfluoropolyether structure and belonging to class 1, having an average equivalent weight of 694 and a broad distribution of the molecular weights, were dissolved in 8 ml of PFPE with perfluoroalkyl end groups, belonging to class 1, having an average molecular weight equal to 800, in the presence of 0.3 ml of a perfluoropolyether structure alcohol of class 1, with end group -CH₂OH and an average equivalent weight of 600. The resulting mixture was limpid at 20°C and exhibited a specific electric conductivity of 7.8 µS.cm⁻¹ (probably due to traces of H₂O present in the surfactant). By means of small additions of 0.1 M HNO₃ solution, in particular 50 µliters per step, the behaviour shown in figure 2 was obtained. There was observed a fast increase of X up to a maximum of 184.1 µS.cm⁻¹ for W = 2.2% by weight; by increasing the aqueous phase amount, the conductivity decreased to values lower than 1 µS.cm⁻¹ for W = 4% by weight and it was no longer measurable above 4.5% in the aqueous phase. However, the system was capable of solubilizing HNO₃ solutions up to 10% by weight at T = 20°C. Analogous systems prepared with the surfactant in the form of acid instead of ammonium salt were capable of solubilizing a lower amount of aqueous phase. The microemulsions so obtained did not show electrical conductivity. Example 2 5.5072 g of ammonium salt of a monocarboxylic acid with a perfluoropolyether structure belonging to class 1, having an equivalent weight of 692 and a narrow molecular weight distribution, were dissolved in 10.2862 g of PFPE having perfluoroalkyl end groups, belonging to class 1, and having an average molecular weight equal to 800. Following the procedure described in the preceding example and with an equilibration time of 4 minutes per step, the conductivity trend on increasing the water content at T = 22°C was measured. Bidistilled water having a conductivity of about 1 µS.cm⁻¹ was used. In the resulting w/o microemulsion, a rapid conductivity increase up to a maximum of 2.32 mS.cm⁻¹ for W = 2.77 - 3.07% by weight was observed; by increasing the water amount, the conductivity decreased down to values lower than 1 µS.cm⁻¹ for W > 11.3% by weight. The microemulsion, limpid at 22°C, was capable, however, to solubilize H₂O up to 15% by weight without any variation in the macroscopic characteristics of the system. Example 3 6.0309 g of ammonium salt of a monocarboxylic acid with perfluoropolyether structure belonging to class 1, having an average equivalent weight of 694 and a broad molecular weight distribution, were dissolved in 11.2348 g of PFPE belonging to class 1 and having perfluoroalkyl end groups and an average molecular weight equal to 800. At 26°C the system was turbid, but by addition of 1.25 ml of bidistilled water (W = 6.75% by weight), a limpid microemulsion having a conductivity = 7.2 mS.cm⁻¹ was obtained. By continuing the addition of water, the sample became turbid and the viscosity increased; the system containing 3.30 ml of water ( W = 16.05% by weight) was an opalescent gel having a specific conductivity equal to 3.06 mS.cm⁻¹. This gel was slightly birefractive when observed between two crossed polarizers. Example 4 This example illustrates the behaviour of a microemulsion (microE) containing : an ammonium salt of a monocarboxylic acid having a perfluoropolyether structure, belonging to class 1, and having an average equivalent weight of 694 with a broad distribution of the molecular weights; a PFPE belonging to class 1, having an average molecular weight equal to 800 and perfluoroalkyl end groups; perfluorinated alcohol H(CF₂)₆CH₂OH as a co-surfactant; the aqueous liquid consisting of a solution of electrolyte (HNO₃ or KNO₃). The maximum conductivity of the systems at 20°C, at two different concentrations for each electrolyte is reported in Table 1. By raising the water phase concentration, the conductivity decreased from the indicated values down to zero. The maximum amount of solubilizable water at 20°C without variation in the macroscopic properties of the microemulsions (microE) is reported in Table 2. Type of microE Maximum amount of solubilizable W (% by weight) a 15.2 b 25.0 c 17.2 d 23.0 When instead of the ammonium salt of the surfactant, the monocarboxylic acid was utilized, no system capable of a significant conductivity was ever obtained. Example 5 A sample containing: 9.5751 g of ammonium salt of a monocarboxylic acid having perfluoropolyether structure, belonging to class 1, with a narrow molecular weight distribution, having an equivalent weight of 520, plus 6.4839 g of PFPE with perfluoroalkyl end groups, belonging to class 1, having an average molecular weight of 800, plus 4.1970 g of an alcohol having a perfluoropolyether structure of class 1 with end group -CH₂OH, having an average molecular weight of 678, plus 1.5 ml of bidistilled water (10.2% by weight), exhibited a specific conductivity equal to 3.34 mS.cm⁻¹, and appeared in the form of a limpid and optically isotropic phase. By continuing the addition of water up to 30% by weight, a conductivity decrease down to 21.5 µS.cm⁻¹ and a viscosity increase of the limpid system were observed. Example 6 A sample containing : 8.6186 g of ammonium salt of a monocarboxylic acid with perfluoropolyether structure belonging to class 1, having a narrow molecular weight distribution and an average equivalent weight of 847, 13.2122 g of PFPE with perfluoroalkyl end groups belonging to class 1, having an average molecular weight of 1500, and 0.6 ml of water (2.67% by weight), exhibited a specific conductivity equal to -l 414 µS.cm⁻¹. The system was a limpid, highly viscous liquid. Example 7 To 16.992 g of a raw perfluoropolyether belonging to class 1, having an average equivalent weight of 7,000 and an average viscosimetric molecular weight equal to 4,000, containing peroxy bridges and acid end groups, neutralized with 0.3 ml of an ammonia solution at 30% by weight of NH₃, there were added 3.74 ml of tert-butyl alcohol under gentle stirring for a few minutes. The resulting product was limpid at 20°C and exhibited a specific conductivity of about 16 µS.cm⁻¹. After addition of small amounts of bidistilled water, usually 100 µliters per step, a specific conductivity increase up to a maximum value of 1.3 mS.cm⁻¹, corresponding to a water amount equal to 19% by weight, was observed. By increasing the aqueous phase amount, the specific conductivity decreased almost symmetrically with respect to its increase, down to a X value of 700 µS.cm⁻¹ in relation to 25% by weight of water. Beyond this percentage, the system was no longer able to solubilize water. Example 8 16.992 g of a raw perfluoropolyether belonging to class 1, having an average equivalent weight of 7,000 and an average viscosimetric molecular weight of 4,000, containing peroxy bridges and acid end groups, were neutralized with 0.3 ml of an ammonia solution at 30% by weight of NH₃ and were dissolved in 3.74 ml of t-butanol under gentle stirring. A mixture consisting only of a limpid phase at 20°C, which exhibited a specific conductivity of about 16 µS.cm⁻¹ was obtained. By adding small amounts of a 10⁻² M HNO₃ solution, generally 100 µliters per step, it was possible to observe an increase of the specific conductivity up to a maximum value of 1.76 mS.cm⁻¹, corresponding to a water phase amount of about 23% by weight. By increasing the water phase amount, the specific conductivity decreased to a value of 900 µS.cm⁻¹ at 28.6% by weight of aqueous solution. Beyond this percentage, the system was no longer able to solubilize the aqueous phase.

Claims: Use of a microemulsion of the water-in-oil type having an electric conductivity (for ionic transfer) of at least 10 µSiemens.cm⁻¹ and consisting of a liquid, limpid or opalescent, macroscopically monophase substance which is obtainable by mixing (a) an aqueous liquid which optionally contains one or more electrolytes; (b) a liquid compound with perfluoropolyether structure and having (i) perfluoroalkyl end groups and an average molecular weight of from 500 to 10,000 or (ii) from 0.1 to 4 functional end groups for each chain, selected from carboxylic, polyoxyalkylene-OH, alcoholic, aminic, amidic, quaternary ammonium and ester groups, and an average molecular weight of from 1,500 to 10,000; (c) a fluorinated surfactant, preferably of the ionic type; and, optionally, (d) a (hydrogenated) C₁-C₁₂ alcohol and/or a fluorinated alcohol; in electrochemical processes or in separation processes. Use according to claim 1, wherein the fluorinated surfactant is selected from: a) salts of a perfluoroalkylcarboxylic acid having 5 to 11 carbon atoms; b) salts of a perfluorosulphonic acid having 5 to 11 carbon atoms; c) salts of a mono- or di-carboxylic acid derived from perfluoropolyethers; d) cationic perfluorinated surfactants or surfactants derived from perfluoropolyethers having 1, 2 or 3 hydrophobic chains. Use according to claim 1, wherein the fluorinated surfactant is of the non-ionic type, consisting of a perfluoroalkyl chain and a polyoxyalkylene hydrophilic head. Use according to any one of claims 1 to 3, wherein the perfluoropolyether has functional groups along the chain.

Inventors: CHITTOFRATI ALBA, LENTI DARIA, CHITTOFRATI, ALBA, LENTI, DARIA
Applicants: AUSIMONT SRL, AUSIMONT S.R.L.
Application year: 1988
Main classification: C08J3
IPC codes: B01F17, H01B1, C08L71, C08J3, B01J13