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be acceptable, or at least useful. This was particularly true of the former, and it is likely that the latter would be considerably improved by the water gas shift reaction that would be present in actual use. The activities of alumina-supported oxide catalysts (Shelef et al., 1968) for these reactions were very similar to those reported here for Fey, but they were (understandably) too susceptible to poisons to be useful. There were other important differences: the CO NO reaction was faster than the CO + O2 with oxides, whereas the reverse was true with Fey, and N 2 0 was the dominant product with the oxide (as with Cry) whereas only N2 was formed over Fey. Perhaps different chemistries are operative with the two systems. It might be supposed that N 2 0 is reacted completely (much faster than NO) in these experiments, but the data of Figure 5 , and of Table IV, make this seem unlikely. More questions than answers have been derived from this work, but they are interesting and possibly tractable ones.
+
1980, 19, 304-309
etries are based. Literature Cited Bielanski, A., Lagan, J. M., Rev. Roum. Chim., 22, 191 (1977). Boudart, M., Garten, R. L., Delgass, W. N., J . Phys. Chem., 73,2970 (1969). Deeba, M., Ph.D. Dissertation, University of Wisconsin-Milwaukee, 1979. Delgass. W. N., Garten, R. L., Boudart, M. J., J. Chem. phys., 50,4603 (1969). Hall, W. K., Chem. Eng., Prog., Symp. Ser. AiChE, 83, 68 (1967). Hall, W. K., Massoth, F. E., J . Catal., 34, 41 (1974). Jacobs, P.A., UyttemOeven, J. B., Beyer, H. K., J. Chem. Soc., Faraday Trans. I . 73, 1755 (1977). Jacobs, P. A., Beyer. H. K., J . Phys. Chem., 83, 1174 (1979). Kuhl, G. H., ACS Symp. Ser., 40, 96 (1977). Landau, J. L., Peterson, E. E., J. Chromafogr. Sci., 12,362 (1974). Massoth. F. E., CHEMTECH, 285 (1972). Minachev, H. M., Antoshin, G. V., Yusifov, Y. A,, Shlpiro, E. S.,ACS Symp. Ser., 40, 559 (1977). Mlzumoto, M., Yamazoe, M., Selyama, T., J. Catal., 59, 319 (1979). Seiyama, T., Arakawa, T., Mats&, T., Takita, Y., Yamazoe, N., J. Catai., 48, l(1977);Chem. Led. (Jpn), 781 (1975). Shelef, M., Otto,K., Gandi. J. Catal., 12, 361 (1968). Uytterhoeven, J. E.,Chrlstner, L. C., Hall, W. K., J. Phys. Chem., 69, 2117
(1965).
Received for review November 30, 1979 Accepted May 12, 1980
Acknowledgment
We are grateful to the General Motors Research Laboratories for their generous support of this work. Special thanks are due to Drs. N. M. Potter and A. Wims of GM for supplying the analytical data on which our stoichiom-
This work was presented at the 178th National Meeting of the American Chemical Society, Washington, D.C., Sept 1979, Division of Industrial and Engineering Chemistry.
II. 53rd Colloid and Surface Science Symposium Rolla, Missouri, June 1979 (Continued from March and June 1980 issues)
pH-Sensitive Microemulsions Gunilla Gillberg' Celanese Research Company, Summit, New Jersey 0790 1
Lelf Erlksson The Swedish Institute for Surface Chemistry, Stockholm, Sweden
The influence of pH on the solubilization capacity of mixtures of nonionic and ampholytic surfactants was investigated at different temperatures. The temperature at which a minimum amount of surfactant will give an isotropic solution of equal amounts of oil and water was strongly pH sensitive. The regions of isotropic solutions as well as type of microemulsionsformed at constant temperatures were also strongly affected by small changes in pH. The effects are due to the variation in the ionization degree of the ampholytic surfactant with pH and thereby the changes in the magnitude of the repulsion energy in the surfactant layer. The pH sensitivity of these microemulsions allows them to be broken and separated without dilution or change in temperature.
Introduction The possibility of obtaining enhanced oil recovery from oil wells and tar sands by flooding with microemulsions has led to a rapid growth in the research concerning microemulsions. However, since no generally accepted definition of microemulsions exists (Prince, 1977), the concept is being used for systems which are different from each 0196-4321/80/1219-0304$01.OO/O
other from a thermodynamic point of view. Many of the properties which make microemulsions attractive for technical applications are, however, only pertinent to the thermodynamically stable microemulsions, viz. the high stability and the spontaneous formation. The microemulsions contain large amounts of two immiscible liquids simultaneously, e.g., oil and water. This is accomplished 0 1980 American Chemical
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by the use of carefully adjusted ratios of surfactants and co-surfactants which idlow one of the liquids to be dispersed in the other in the form of microdroplets with a size less than 100 nm. However, although we talk about O / W and w/o microemulsioiis and the interface between the oil and water, the thermodynamically stable microemulsions are one-phase systems by definition. The swollen micelles are therefore best described as forming a pseudophase. The need of surfactants and co-surfactants is generally high in the microemulsions due to the small size of the dispersed phase and thereby the large interfacial area. The so-called surfactant phase (Friberg and Lapczynska, 1975) which is commonly observed in systems of nonionic surfactants, oil, and water at the HLB temperature (Shinoda and Friberg, 1975) allows large amounts of oil and water to be solubilized simultaneously by comparatively low amounts of surfactant.. The HLB temperature corresponds to the temperature a t which the hydrophilic-lipophilic properties of the surfactants balance for a given hydrocarbon-water system (Shinoda, 1967). The strong hydration forces between the hydrophilic moiety of the surfactant and the water which exist at temperatures t)elow the HLB temperature lead to oil-swollen hydrophilic micelles to be energetically favorable. The hydrophilic interactions decrease with an increase in temperature, and above the HLB temperature the lipophilic interactions are dominating and waterswollen lipophilic micelles are obtained. The tendency of the system to form normal or inverse micelles will be the same at the HLB temperature and a more or less layered structure should be favored. The structure of the surfactant phase has not been established, however. Shinoda and co-workers have shown that the solubilization capacity of a given nonionic surfactant is maximum closer to the HLB temperature and also that the maximum amount of water or oil which can be solubilized decreases with increasing HLB temperature (Shinoda and Ogawa, 1967; Saito and Shinoda, 1967). Friberg and co-workers showed that the same trend holds for the surfactant phase which is displaced to higher surfactant concentrations with increasing ethylene oxide chain lengths of a monodisperse nonionic surfactant (Friberg et al., 1977) and that combinations of nonionic surfactants with an increasing difference in the hydrophilic part cause a gradual increase in the HLB temperature and lead to a higher demand for surfactants for all kinds of solubilization. However, an equimolar mixture of tri- and pentaethylene glycol dodecyl ether gave almost identical solubilization behavior to the pure tetraethylene glycol dodecyl ether (Friberg et al., 1976). Robbins has developed a theory which quantitatively predicts the phase behavior in microemulsions (Robbins, 1974,1977). The theory treats the interface between the oil and the water as an oriented duplex film of surfactant molecules whose heads and chains act as separate uniform liquid phases with water dissolved in the heads and oil in the chains. The direction and degree of the curvature are determined by a lateral stress gradient in the interface resulting from differences in interactions between the heads and chains, respectively. A w/o microemulsion is obtained if the head-head interaction is greater than the chain-chain interaction. The stress gradient is expressed in terms of physically measurable quantities-the surfactant volume, interfacial tension, and compressibility. Both the surfactant volume and the compressibility are split into separate contributions by the heads and the chains. The head compressibility thus depends on both the water solubility and the ionic repulsion of the hydro-
Hexadecane
Figure 1. Characteristics of ternary diagrams: corners, pure components; sides, binary mixtures; point A, binary mixture of water and surfactant (67% and 33%); point B, ternary mixture of water, surfactant, and hexadecane (50%, 25%, and 25%); line A-B-hexadecane (-), constant ratio of water to surfactant equal to 2 to 1 and increasing concentrations of hexadecane from 0% at point A, 25% a t point B, and 100% at corner hexadecane; lines parallel to side (- - -), constant concentration of the component, the corner of which is opposite to the side parallel to the line, e.g., 50% HzO in all points along line 50-B-50.
philic group. The theory also shows that it is possible to have a positive interfacial tension and yet a spontaneous water absorption. Robbins also shows that the theory in combination with idealized ternary diagrams explains well the phase behaviors observed by Shinoda and co-workers for microemulsions of nonionic surfactants as a function of temperature, salinity, and oil aromaticity (Robbins, 1977). The temperature range of existence for the surfactant phase is very low. We showed in a previous paper (Gillberg et al., 1978) that the addition of small amounts (less than 1%) of an ionic surfactant to a monodisperse nonionic surfactant led to a pronounced extension of the solubility area to lower surfactant concentrations and to an improved temperature stability. A rational explanation of these effects was that the repulsion energy between the charged surfactants was enough to counteract the bending energy which causes the transition from the surfactant phase to an inverse micellar phase in the case of the pure nonionic surfactant system. The addition of an ampholytic surfactant (dodecyl-@-alanine)at its isoionic point had no effect, while its sodium hydroxide salt and its hydrochloric acid salt displayed the effects of the additions of ionic surfactants. Since a change in pH will cause a shift in the charge of an ampholytic surfactant, we decided to investigate this phenomenon more closely. The possibility of making or breaking a microemulsion by the addition of small amounts of acid or base should be attractive for their use as extraction media (Robbins and Brownawell, 1972). Ternary Diagrams Systems of three components are best described graphically by the totally symmetric equilateral triangle diagram (Figure 1). Each corner of the equilateral diagram represents a pure component (water, surfactant, or hexadecane). Each side corresponds to binary mixtures of the two components which limit the side under consideration. Point A in Figure 1thus is a mixture of water and surfactant. The concentrations can be calculated by means of the lever rule or read directly from the percent numbers along this side. Thus, the composition of point A is 33% surfactant and 67% water. The point describing the composition of the system will move along line A-Bhexadecane, if we add successive amounts of hexadecane to point A. It is easily proven geometrically that the ratio
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of water to surfactant is constant along this line. Thus, point B contains 25% hexadecane, 25% surfactant, and 50% water. Lines which are parallel to the side opposite the corner of a pure component are characterized by a constant concentration of this component. Every point along the line parallel to the water-surfactant side and passing through point B will contain 25% hexadecane. Thus, the concentrations of the three components are described by a point within the diagram are obtained by drawing in the three lines parallel to the sides and reading the percent numbers from the correct side scale. Experimental Section Materials. Monodisperse nonionic surfactants, tetraethylene glycol dodecyl ether (TEGDE) and diethylene glycol dodecyl ether (DEGDE) were obtained from Nikkol. The ampholytic surfactant, dodecyl-@-alanine(C12H25NH-CH2CH2-COOH, C12-A)was synthesized from n-dodecylamine (Kebo, puris) and @-propiolactone(Merk zur Synthese) (Gresham et al., 1951). Its sodium hydroxide salt sodium dodecyl-@-alaninate(ClzHz5-NH-CH2CHzCOO-Na+, C12A-Na+)and its hydrochloric acid salt, dodecyl-Balaninium chloride (C12Hw-NH2+-CH2CHz-C00H C1-, C12A+C1-)were prepared by precipitation with ethanolic solutions of sodium ethylate and concentrated hydrochloric acid, respectively, and recrystallized once. Tetradecyl-P-propiobetaine (C14H,-N+(CHJ2-CH2CH2COO-, C14-PB) was synthesized from N,N-dimethyl-ntetradecylamine (K & K) and @-propiolactone(Gresham et al., 1951). n-Decanol (Kebo, puris) and n-hexadecane (Fluka p.a.) were used without further purification. Double-distilled water was used. pH Determinations. Preliminary investigations showed that pH adjustments via the water solution gave too large a variation in the final pH of the microemulsions. Pre-made mixtures of the dodecyl-P-alanine and its salts in various weight ratios were added to water alone or water, hexadecane, and nonionic surfactants, and the pH was measured by glass electrode. The pH was not determined in general for the microemulsions. HLB Temperature. Surfactants in a total concentration of 5-20 wt % and hexadecane and water in a weight ratio of 1:l were weighed into ampules, which were then sealed. The samples were homogenized and then placed in a thermostated bath, the temperature of which was raised in increments of 2.5 "C in the temperature range of 15 to 80 "C. Each temperature was kept a t least for 30 min. The solubility areas were marked as the beginning and the end of the temperature range where the solutions became transparent. The HLB temperature was determined as the temperature at which a minimal amount of surfactant mixture gave an isotropic solution. Solubility Regions. The regions of the isotropic solutions at a given temperature were determined by weighing the surfactant mixture and water or hexadecane into an ampule and then adding the third component (hexadecane and water, respectively) stepwise. The samples were closed and homogenized after each addition and immediately returned t o the thermostat. The solubility area was marked as the beginning and the end of the concentration range where the mixtures became transparent. A few samples close to the solubility borders were sealed and stored for 48 h. Results The pH's measured for mixtures of dodecyl-Palanine and its salt in water and in systems also containing the nonionic surfactant and hexadecane are given in Figure 2. The pH was also measured in all the different solubility regions with their large variations in the relative amounts
weigy fraction
14r
'
0.2
c,$l+cI-
0.6 '
l!O ' C12*
06
0:2 C,.$Na'
Figure 2. pH as a function of the weight fraction dodecyl-@-alanine (C,,A) and its acid salt (Cl2AtC1-) or basic salt (Ci2A-Nat): X, 5% of C12Aand Cl2A'Cl- or C12A-Nat in water; 0 , 5 % of a surfactant mixture consisting of 6 parts of TEGDE and 1 part of ClzA and its salts in an equal mixture of hexadecane and water.
I
Y
5
10
Surfactant
15
(z)
eol
5
10
Surfactant
15
(z)
Figure 3. Minimum surfactant requirements for obtaining isotropic solutions containing equal amounts of hexadecane and water as a function of temperature. A. Surfactant, TEGDE; shadowed area = isotropic solution. B. Surfactant, TEGDE and dodecyl-8-alanine (C12A)or its acidic salt (Cl2A+C1-)or its basic salt (C&Nat) in a weight ratio of 199:l;pH 5.7, CI2A(- - -); pH 2.0, Cl2A+C1pH 12.0, CI2A-Nat (-). (-a);
of hexadecane, water, and surfactant mixture for one of the investigated systems. A nonsystematic variation of f0.2 pH unit around the theoretical pH was observed. The addition of small amounts of dodecyl-@-alanine (0.5% calculated on the nonionic surfactant) to the pure nonionic surfactant, TEGDE, has no effect on the temperature dependence or region of existence of the isotropic solutions containing water and hexadecane in a weight ratio of 1:l and with various amounts of surfactant (Figures 3A and B). However, additions of its basic or acid salt reduce considerably minimal amount.9 of surfactant needed to give an isotropic solution (Figure 3). Similar reductions were observed with anionic and cationic surfactants (Gillberg et al., 1978). The very low concentrations of ampholytic surfactant in these investigations leave the systems a very low buffering capacity. Therefore, investigations with a higher relative amount of ampholytic surfactant were performed. The solubility areas were determined a t a ratio of TEGDE to ampholytic surfactant of 6:l. The results a t three different pH values are given in Figure 4A. The surfactant mixture which has no net charge a t the isoionic point of pH 5.7 shows an expected increase in HLB temperature due to the stronger hydrophilicity of the alanine group than that of the tetraethylene glycol group. Further increases in the HLB temperature are obtained when some of the ampholytic surfactant molecules are transferred into charge species by a pH change from the isoionic point. The high temperatures needed for isotropic solutions to be formed make these systems of little practical interest. A decrease in HLB temperature at a retained high level
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
c:
C
A
BO
9
g
L
e
n
E 4c
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E L
E
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e 20
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(z)
-
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5
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Figure 4. Minimum surfactant requirements for obtaining isotropic solutions containing equal amounts of hexadecane and water as a function of temperature and pH. A. Surfactant, TEGDE and dodecyl-@-alanine(C12A)with its salts in a weight ratio of 6 : l . pH 5.7 (111111); pH 4.1 (-); pH 7.8 I - - - ) . B. Surfactant, TEGDE, DEGDE, and C12Ain a weight ratio of 4 3 : l ; pH 4.5 (-); pH 4.8 (IIIIII); pH 7.8 (- - - -); pH 8.6 (---). C. Surfactant, TEGDE, decanol, and C12Ain a weight ratio of 6:l:l; pH 3.8 (- -); pH 4.1 (---); pH 4.5 (- - -1; pH 4.8 pH 5.7 (-). D. Surfactant, TEGDE, decanol, and C12A in a weight ratio of 6 : l : l ; pH 7.8 ( 1 1 1 1 1 1 ) ; pH 8.3 ( - - - ) ; pH 8.9 (111111);
(-.e-).
of ampholytic surfactant can be obtained by decreasing the HLB temperature of the nonionic surfactant. This can be accomplished by choosing a monodisperse nonionic surfactant with a shorter ethylene oxide chain length or by adding a more hydrophobic nonionic surfactant to the TEGDE. Figure 4B shows the major solubility areas obtained when a mixture of TEGDE and DEGDE, with 3 ethylene glycol units ox1 average, is used as the nonionic surfactant. (Very narrow solubility areas, observed at lower temperatures than those shown, are omitted for clarity in Figures 4B-D). The expected decrease in HLB temperature is observed. An HLB temperature of 48 "C was thus observed at pH 7.8 when TEGDE was used as the nonionic surfactant (Figure 4A), while one of approximately 28 "C was observed when a mixture of TEGDE and DEGDE was used (Figure 4B). The lower HLB temperature also leads to the isotropic solutions being formed at lower amounts of surfactants. Decreaging or increasing the pH has the same effect on the HLB temperature. Figures 4C and 4D show the major solubility areas when a hydrophobic co-surfactant l-decanol is added to the TEGDE. More pronounced changes in the HLB temperature as a function of pH are obtained with this surfactant mixture than in the system with DEGDE as the hydrophobic additive. The optimum solubility area at a constant temperature was observed at the HL:B temperature in the case of small additives of ionic surfactant (Gillberg et al., 1978). However, the pH-sensitive systems in Figures 4C and 4D show a large variation in HLB temperatures and also, in many
307
of the cases, a too high HLB temperature for economical technical use. The solubility areas were studied therefore at constant temperatures of 25 and 35 " C . Figures 5A-C show representative examples of these studies. Two solubility areas are observed at 35 "C for the surfactant mixture at the isoionic point of the ampholytic surfactant (Figure 5A, pH 5.7). Water is soluble in the hexadecanesurfactants solutions on the right side in the diagram (the so-called L2 area). No isotropic solutions extending to the water corner are observed a t 35 "C which is above the HLB temperature of the system. Instead, an isolated solubility area, a surfactant phase, exists which shows a narrow solubility channel reaching toward the water corner. A t pH 8.7, a very small surfactant phase exists but the dominating solubility area is a large solubility region coalesced with the hydrocarbon-surfactants solution. A slight decrease in pH (Figure 5B) yields a surfactant phase channelling toward the water corner while a t pH 3.8 two surfactant phases were observed along with a solution sector extending from the water corner. The narrow surfactant phase existing at surfactant concentrations of approximately 10% was not time-stable but separated into one or two isotropic solutions and a liquid crystalline phase at storage. This surfactant phase formed spontaneously when an appropriate mixture was allowed to reach 35 "C, either from above or below. Figure 5C shows the solubility areas obtained a t pH 8.7 at 25 and 35 "C. The diagram showing the minimum surfactant requirements for obtaining isotropic solutions containing water and hexadecane in equal amounts for this surfactant mixture is given in Figure 5D for comparison. Three solubility areas are observed a t 25 "C, viz. one isotropic solution sector extending from the water corner, a small isolated surfactant phase and the L, area. The surfactant phase is shifted toward lower surfactant concentrations at an increase in the temperature. The lower narrow solution area in Figure 5D shows how this surfactant phase will move with temperature. The large solubility area a t 35 "C in Figure 5C corresponds to the upper solubility area in Figure 5D, and it is evident that 35 "C is not the optimum temperature for this isotropic solution with regard to its need of surfactants.
Discussion The effect of addition of different amounts and types of ionic surfactants to a monodisperse nonionic surfactant was reported in a previous paper (Gillberg et al., 1978). The present investigation uses different types of mixtures of nonionic surfactants and the relative amount of ionic surfactant is varied by changes in pH. A knowledge of the weight ratio of neutral surfactants to ionic surfactants is essential for an interpretation of obtained results, and the calculated weight ratios at the different pH values are given in Table I. The addition of the dodecyl-@-alanine,i.e., a neutral surfactant with a higher hydrophilic-lipophilic balance (HLB) than the pure TEGDE, shifts the HLB temperature from 30 to 40 "C but does not increase the minimal amount of surfactant necessary to form an isotropic solution of equal amounts of water and hexadecane (Figures 3A and 4A). This indicates that the dodecyl alanine has a stabilizing effect on the micelles. This is further confirmed by the fact that the addition of decanol, which would give very poor solubilization capacity if added to the pure TEGDE, causes an extension of the solubility area to lower surfactant concentrations a t the resulting lower HLB temperature of 33 "C (Figure 4C). The highly heterogeneous surfactant consisting of TEGDE, CI2-A,and decanol thus can form microemulsions at lower surfactant con-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 Hexadecane
Hexadecane
Id0 Water
Surfactant
Water
Surfactant
Hexadecane D
6,
f -
L
E a
P
E 40-
e
20-
5
is
10
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(z)
Figure 5. Regions of isotropic solutions in a system of water, hexadecane, and a surfactant consisting of TEGDE, decanol, and C12Ain a weight ratio of 6:l:l. A. Temperature, 35 "C; pH 5.7 pH 8.7 (---). B. Temperature, 35 "C; pH 3.8 (-1; pH 4.8 (- --). C. pH 8.7, temperature, 25 "C (-); 35 "C (---). D. Minimum surfactant requirements for obtaining isotropic solutions containing equal amounts of hexadecane and water at pH 8.7. (e-);
Table I. Weight Ratios of Nonionic Surfactants Including Neutral Ampholytic Surfactant to Ionic Surfactants and Corresponding pH Values TEGDE 6 6 4 4 4 4 6 6 6 6 6 6 6 6 6
DEGDE
decanol
3 3 3 3
1 1 1 1 1 1
1 1 1
C,,A 0.8 0.95 0.90 0.95 0.95 0.85 0.70 0.80 0.90 0.95 1.00 0.95 0.90 0.85 0.80
centrations than the pure TEGDE. Introducing ionic surfactants in this surfactant mixture by shifting the pH from the isoionic point causes similar increases in the HLB temperature and the shifts in the minimal amount of surfactant needed for forming isotropic solutions as observed in the systems of TEGDE and sodium dodecyl sulfate (see Figure 1, Gillberg et al., 1978). The systems where the nonionic surfactant is a mixture of TEGDE and DEGDE, which should have an HLB temperature and solubilization capacity equal to that of pure triethylene glycol dodecyl ether, show a much lesser response to the ionization of the ampholytic surfactant (Figure 4B) than the systems containing decanol (Figures 4C and 4D). This difference indicates that a specific in-
C,,A+Cl0.2 0.05 0.10 0.05
C,,A-Na+
0.05 0.15 0.30 0.20 0.10 0.05 0.05 0.10 0.15 0.20
wt ratio nonionic:ionic 34:l 139: 1 79: 1 159:l 159: 1 52: 1 26:l 39: 1 79:l 159:l 8: 0 159: 1 79:l 52:l 39: 1
PH 4.1 7.8 4.5 4.8 7.8 8.7 3.8 4.1 4.5 4.8 5.7 7.8 8.3 8.7 8.9
teraction takes place between the OH group of the decanol molecules and the amine group in the dodecyl-P-alanine. If this is so, we would expect a different behavior as in the case of a quaternary ammonium compound. The internally ionized tetradecyl-P-propiobetaine(C14-PB)has the same charge distribution as the internal salt of the dodecyl-Palanine and therefore was chosen as the quaternary ammonium compound. The addition of 0.5% of C14-PBto TEGDE had no effect on the solubility area of an equal mixture of hexadecane and water. (The HLB temperature was 30 "C and the minimal amount of surfactant was 12%.) The addition of CI4-PBto TEGDE to a weight ratio of 1:6 gave the expected increase in HLB temperature (60 O C ) but no shift to a higher surfactant concentration. The
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 309 Hexadecane
600
e -
Water
Surfactant
F i g u r e 7. Regions o f isotropic solutions in systems o f water, hexadecane, a n d a surfactant: surfactant, TEGDE a t 30 " C (---); TEGDE a n d sodium dodecyl sulfate in a weight r a t i o o f 199:l a t 35 F i g u r e 6. Minimum surfactant requirements for obtaining isotropic solutions containing equal amounts o f hexadecane a n d water as a f u n c t i o n o f temperature: surfactant, TEGDE a n d tetradecyl propiobetaine in a weight r a t i o o f 6 1 (-); TEGDE, decanol, a n d tetradecyl propiobetaine in a weight r a t i o o f 6:l:l (-),
addition of decanol to yield a surfactant mixture of 6 parts of TEGDE, 1part of C14-PB,and 1part of decanol extends the solubility area strongly to lower surfactant concentrations (Figure 6). These results are very surprising, since the extension of the solubility areas to lower surfactant concentrations as in the case of small additions of ionic surfactant could be explained by the repulsion forces between the surfactant ions being strong enough to counteract the bending forces. This would prevent a transition of the surfactant phase to an inverse micellar solution (Gillberg et al., 1978; Robbins, 1979). Since the propiobetaine has no net charge, similar low additions of C14-PB had no effect. In the systems with high levels of C14-PB, the molar ratio will be 5 mol of TEGDE, 2 mol of decanol and 1 mol of CI4-PB. The distance between the C14-PB molecules thus will be relatively low, on the order of 1-2 nm. Local repulsion between the carboxyl groups therefore might be possible. The pH sensitivity of the microemulsions containing ampholytic surfactants should allow easy breaking of them without dilution or changes in temperature. An inspection of Figure 4C thus shows that a microemulsion will spontaneously form a t 35 " C and a surfactant concentration larger than 7% if the pH is 4.8, while no isotropic solutions exist if the pH is reduced to 3.8. A knowledge of the extension of the regions of isotropic solutions is necessary for application purposes. The phase diagrams in Figure 5B confirm that a change in pH will transfer a microemulsion into a multiphase area. (The very narrow surfactant phase at pH 3.8 was not time stable.) The number of phases formed and their chemical composition has to be determined before the practicality of the system can be assessed.
oc
(-1.
The most drastic changes in the extension of the solubility areas are observed in the very small additions of ionic surfactant to the nonionic surfactant (Figure 7 ) . Since small additions of ampholytic surfactant to TEGDE had no effect on the solubility area while the salts had the typical behavior of ionic surfactants (Figure 3B), we can expect similar phase diagrams to be obtained as shown in Figure 7. Low concentrations of ampholytic surfactant thus should produce a higher pH sensitivity than the higher ones. However, the disadvantage with these systems is that they will be equally sensitive to unintentional pH changes, i.e., due to dissolution of carbon dioxide. Acknowledgment Mrs. D. Cabarle is heartily thanked for her linguistic criticism of a preprint of this paper. Literature Cited Friberg, S., Lapczynska, I . , Prog. Colloid Polym. Sci., 56, 16 (1975). Friberg, S., Lapczynska, I., Gillberg, G., J. CoikM Interface Sci., 56, 19 (1976). Friberg, S.,Buraczewska, I., Ravey, J. C., Micellization, Solubilization, Microemulsions [Proc. Int. Symp.], 2, 901 (1977). Gillberg, G.. Eriksson, L.,Friberg, S.,in "Emulsions, Latices, and Dispersions", p 201, P. Becher and M. N. Yudenfreund, Ed., Marcel Dekker, New York, 1978. Gresham, T. L., Jansen, J. E., Shaver, F. W., Bankert, R. A,. Fiedorek, F. T., J . Am. Chem. SOC.,73, 3168 (1951). Prince, L. M., in "Microemuisions, Theory and Practice", p xi, L. M. Prince, Ed., Academic Press, New York. 1977. Robbins, M. L.. preprint for the Symposium on Interfacial Phenomena in Oil Recovery, AIChE National Meeting, Tulsa, Okia., March 1974. Robbins, M. L., Micellization, Solubilhtbn, Microemukions [Proc. Int. Symp.], 2, 713 (1977). Robbins. M. L., Brownaweii, D. W., U S . Patent 3641 181 (1972). Saito, H.,Shinoda, K.. J . Colloid Interface Sci., 24, 10 (1967). Shinoda, K., J. Colloid Interface Sci., 24, 4 (1967). Shinoda. K.. Friberg, S.,Adv. Colloid Interface Sci., 4, 281 (1975). Shinoda, K., Ogawa, T., J . Colloid Interface Sci.. 24, 56 (1967).
Received for review N o v e m b e r 5, 1979 Accepted April 21, 1980 Presented at t h e 53rd Colloid and Surface Science S y m p o s i u m , Mo., June 1979.
Rolla,
