Langmuir 1996,11, 1122-1126
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Effect of NaCl on a Nonionic Surfactant Microemulsion System S. Ajith and Animesh Kumar Rakshit" Department of Chemistry, Faculty of Science, M. S. University of Baroda, Baroda 390 002, India Received July 25, 1994. I n Final Form: November 9, 1994@ Various types of microstructure and macroscopic phase changes of the pseudoternary system alkane/ propanollwater, studied in presence of different amounts of sodium chloride at different temperatures, are presented and discussed in this paper. Viscosity and conductance studies reveal that the system at 80% surfactant (S), where S indicates Brij 35 and 1-propanol together, probably exists as a bicontinuous microemulsion at all water fractions even in the presence of NaCl. In the presence of NaC1, Pseudoternary phase diagrams of the system changes with change of temperature. Increase of the concentration of NaCl brings down the Winsor I I11 transition temperature and at 2.5 M NaCl the system exists as Winsor I11 along the entire range of temperature studied (30-70 OC). However a Winsor I11 I1transition was not observed in this case in this temperature range and also within the NaCl concentration range studied. The cloud point of the microemulsions and the contact angle of these systems with a Teflon surface were also determined and discussed. The contact angle values indicate similar structure over the whole range of the oillwater ratio. Brij 35
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Introduction Microemulsions are generally comprised of four components, i.e. oil, water, surfactant, and cosurfactant. The amount and nature of all these components have profound influence on physicochemical characteristics of microem~lsion.l-~The dependency of the system on so many variables makes for interesting and intriguing properties and, a very complicated and difficult elucidation of the structure. Analysis of various parameters which affect the microstructural transition in a microemulsion has attracted the attention of many researcher^.^,^ The properties of interfacial surfactant films which get affected by a change in temperature and/or electrolyte concentration are responsible for the microstructural changes in microemulsions containing both ionic and nonionic surfactants.6-10 Hence in continuation of our earlier wherein we looked at the effect of polymerslZa and NaC112bon a sodium dodecyl sulfate containing microemulsion, we thought it would be interesting to study the effect ofNaCl
* Author to whom correspondence should be sent. *Abstract published in Advance A C S Abstracts, February 1, 1995. (1)Leung, R.; Shah,D. 0.;O'Connell, J. P.; J.Colloid Interface Sci. 1986,114,286. (2)Cazabat, A. M.;Langevin, D.; Meunier, D.; Pouchelon,A.; Adu. Colloid Interface Sci., 1982,16, 75. (3)Belloq, A. M.;Gazeon, 0.;Prog. Colloid Polym. Sci., 1988,76, 203. (4)de Gennes, P.G.; Taupin, C.; J.Phys. Chem. 1986,86, 2294.
( 5 ) Weber, R.; Leser, M. E.; Farago,B. Prog. Colloid Polym. Sci. 1990,
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R I GA __,
(6)Mitchell, D.J.;Ninham, B. W.; J.Chem. SOC.Faraday Trans. 2 1981,77,601. (7)Kellay, A.; Meunier, J.; Binks, B. P.; Phys. Rev. Lett., 1993,70, 1485. (8)Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn Jones, E., Eds.; Kluwer Academic Publishers: Netherlands, 1990;p 557. (9) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1983,87, 5032. (10)Kahlweit, M.; Strey, R.; Busse, G. J.Phys. Chem. 1991,95,5344. (ll)Ajith, S.; Rakshit, A. K. J. Surf. Sci. Technol. 1992,8 , 365. (12) a)John, A. C.; Rakshit, A. K.; J.ColEoid Interface Sci. 1993,156, 202. b) John, A. C.; Rakshit, A. K. Langmuir 1994,10,2084. (13) Ajith, S.;John, A. C.; Rakshit,A. K. PureAppl. Chem. 1994,66, 513.
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at various temperatures on a nonionic surfactant, Brij 35, containing microemulsion, the microemulsion system being alkanemrij 35 propanollwater, where the alkanes were heptane and nonane. The Brij 35-1-propanol ratio was always 1:2 by weight.
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Materials and Methods Brij 35 ( C ~ ~ H ~ ~ ( O C H Z C was H ~ )from ~~O Merck H ) and was used as received. 1-Propanol, heptane, and nonane were obtained from SDs and were used after purificationby standard methods.14 NaCl (Analar,Merck) was dried at 150 "C for more than 2 h and then cooled in a desiccator before use. Doubly distilled water S cm-l) was used. (conductance 3 x Phase diagrams were determined by a simple titration technique described ear1ier.l1Jza Conductance was measured with a Mullard conductivity Bridge (0.1 M NaCl solution was used as the aqueous phase for conductivity measurements). Viscosity was determined with an Ubbelohde viscometer. Adiabatic compressibility was computed by the relation ,f3 = l/(eu2), where e and u are the density of the medium and velocity of sound through the medium, respectively. The sound velocity was deter~ninedll-'~ using a multifrequency interferometer (Mx3, Mittal Enterprises, New Delhi, India). The cloud point for each composition was determined by taking the microemulsion in a standard joint test tube and sealing this with Teflon tape. These samples were then kept in a temperature-controlled water bath, and the temperature was increased at regular intervals till clouding appeared. Then a second series of experiment was done within a narrower range of temperature. The declouding temperature was also noted. The average of the clouding and declouding temperatures is the reported cloud point. The error in cloud point is h0.5 "C. The contact angle ofthe microemulsion samples with a Teflon surface was determined at room temperature (-35 "C) with the help of a contact ( 8 )meter developed at the Department of Colour Chemistry, University of Leeds, U.K., and obtained as a gift. The Teflon tape (Samson) was washed with chromic acid, doubly distilled water, and acetone and then dried at room temperature before use. At least 10 readings were taken for each system over the various parts of the surface, and the average of these values is reported (*2").
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Results and Discussion Figure 1is a representative phase diagram ofthe system alkane (O)/Srij 3 5 propanol (Wwater (W). The one-
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(14)Vogel's Text Book ofpractical Organic Chemistry, Longman and Co. (P)Ltd.; New York, 1978.
0743-746319512411-1122$09.00/00 1995 American Chemical Society
Effect of NaCl on a Microemulsion
Langmuir, Vol. 11, No. 4, 1995 1123
i w b
0
75
50
OP
25
0.4
I
08
I
I
0.4
Ob
I
0 W
Figure 1. Pseudoternary phase diagram of heptane (0)Brij 35 1 propanol @)/water(W) at 40 "C.
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Figure 3. Plot of specific conductance k6'* vs volume fraction of water (&) for (a) heptane, (b) nonane, and (c) heptane nonane (1:1 v/v); s = 80% w/w: 0 , 30 "c;W, 40 "c;A, 50 "c;
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x, 60 "C; V, 70 "C.
@W
Figure 2. Plot of specific conductance (k)vs volume fraction of water (&) for (a) heptane and (b) nonane systems; S = 80% w/w: 0, 30 "c;0,40 "c;A, 50 "c;V, 60 "c;x, 70 "c.
phase microemulsion was obtained at higher surfactant concentration. Hence physicochemical properties of onephase microemulsions were studied at high (80%) surfactant concentration. (This indicates 26.67% of Brij 35 and double that amount of 1-propanol,the sum being 80%). The composition of various microemulsions studied are shown by an asterisk in Figure 1. The one-phase microemulsion area was a function of temperature, which has been discussed earlier.g In Figure 2 the variation of specific conductance with water volume fraction ofvarious alkandBrij35 propanoY water microemulsions a t 80% surfactant concentration are presented. Conductance of the system increases smoothly with an increase in water volume fraction, which is equal to the water volume divided by the sum of the water and oil volumes. No percolation, i.e. a sudden and sharp increase in conductance, was observed. This indicates either a single structured form throughout or a molecularly dispersed solution. Ninham et al.15J6reported similar conductance plot for a tetradecane/DDAB (dodecyldimethylammonium bromide) water system and suggested the existence of a bicontinuous structure throughout for their system. Further it was suggestedll that the specific conductance of the microemulsion (k)is
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(15)Chen, S. J.; Evans,D. F.; Ninham, B. W. J. Phys. Chem. 1980, 88, 1631.
(16)Hyde, S. T.; Ninham, B. W.; Zemb, T. J.Phys. Chem. 1989,93, 1464. (17)Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Nature 1979,281, 60.
r
I
6
12
I
18
% WEIGHT OF WATER Figure 4. Plot of viscosity vs water (% w/w) for (a) heptane and (b) nonane; S = 80% w/w: 0,30"C; 0,40"C; A, 50 "C; V, 60 "C; x, 70 "C.
proportional to the volume fraction of water, i.e. k = (#w - &It where &, the percolation threshold, indicates the concentration of water only above which percolation can be seen. It has also been mentioned that if the system is bicontinuous, then t = 815 or k5I8 = A(#w- &I, where A is a proportionality constant. Thus a plot of k5I8versus #w should meet the X axis a t A&. Figure 3 shows that &,is somewhat negative. This indicates the absence of 4: in this system. We hence assume that the structure is similar over the entire range of the oil-water ratio. In Figure 4 the variation of absolute viscosity over the entire range of the oil-water ratio for both heptane and nonane systems is shown. It is observed that absolute viscosity steadily increased with an increase of the water percentage, indicating a regular change in structure. For discontinuous systems, this regular variation is not generally observed. For bicontinuous systems, the widening of the water conduits is the reason for the continuous increase of the microemulsion viscosity. It was reported earlier15J6that a tetradecane/DDABlwater microemulsion retains bicontinuity due to the poor penetration of oil molecules into the interfacial surfactant film. Highly penetrating oils need a more curved surface, which induces spherical structure to the aggregate.16Low penetration leads t o a bicontinuous structure. Heptane and nonane are reasonably hydrophobic and hence cannot penetrate effectively to the highly hydrophilic interfacial layer consisting of Brij 35 and propanol. This probably
Ajith and Rakshit
1124 Langmuir, Vol. 11, No. 4, 1995
b)
Cbl
OS
t
M
80
30
50
fro
70
TEMPERAT M E
(e
m
C)
Figure 6. Phase prism representation of the (a) heptane and (b) nonane ternary systems at various temperatures; W = 1M aqueous NaC1: solid, 14; striped, 24 (Ln); open, 34 (L/L/L).
Figure 6. Plots of volume fraction vs temperature of various Winsor formation for (a) heptane and (b) nonane systems at different NaCl concentrations; S = 30% w/w.
leads to the formation of a bicontinuous structure. Steric strain associated with the spherical arrangement of a large number of surfactant molecules around a small water or oil domain also dictates a somewhat continuous structure rather than discrete structures. These observations in conductance and viscosity properties of our systems are similar to the observations for those systems which have been proved bicontinuous by self-diffusion or scattering studies. Hence we suggest that the microstructure of our systems is probably bicontinuous. Effect of NaCl. It was reported earlier by usll that temperature (up to 70 "C) does not have much effect on the present nonionic surfactant microemulsion system. The only change observed was in the one-phase microemulsion area. In Figure 5, we present the phase prism representation of the same system where water was replaced by 1M NaCl solution a t various temperatures. It is obvious from the diagram that the system became temperature sensitive in presence of the electrolyte, and Winsor (WII, 111, and IV phases were observed. It is to be noted that all three phases coincide a t one particular point in the phase diagram. Nonionic surfactant becomes temperature sensitive in the vicinity of the cloud point where a water-surfactant miscibility gap appears. Brij 35 is a highly hydrophilic surfactant and its cloud point is above 100 "C. Hence temperature, up to 70 "C, plays little role in its properties. But addition of an electrolyte tends to "salt out" the amphiphile as well as the oi1.18 That is the hydrophilicity of the amphiphile seems to decrease and the system becomes sensitive to temperature. The WIII phase, where the microemulsion is in equilibrium with excess water and oil, was found to be present a t 40 "C for the heptane system. It was suggested that a proper hydrophile-lipophile balance is required for the formation of a WIII system.19 The effect of increasing electrolyte concentration and temperature are similar in this nonionic surfactant system. Nonane is relatively more
hydrophobic than heptane. Hence, for nonane system a somewhat higher temperature was required for the formation of the WIII system, which is quite obvious in Figure 5. It was also observed that the formation of WIII was a t the cost of the WI area, and a plot of the WIII area vs temperature in both cases was linear, though not parallel. In Figure 6a,b we present the change in the volume fraction of different phases as a function of temperature a t various NaCl concentrations for a system having 30% Brij 35 1-propanol. It was observed that with an increase of temperature the system gradually changes from WI (microemulsion in equilibrium with oil) to WIII. The WI WIII transition temperature was lowered as the NaCl concentration was increased, and a t high NaCl concentration only WIII was present over the entire temperature range studied. In absence of NaC1, the system existed only as WI. It was somewhat surprising to find that the rise of temperature and increase of salinity did not induce a WIII- WII transition. Once WIII was formed,the middle phase microemulsion volume remained more or less invarient to these parameters. According to the work of Ninham et a1.,6,20which was later modified by Aveyard et al.,Bthe ratio of the cross sectional areas of the solvated tail group (At) and head group (Ah) actually determine the formation of various Winsor forms. When A & , > 1,a WII system is formed, and WI will be obtained when AJAh < 1. At = Ah will result in the WIII or WIV, i.e. a bicontinuous system. In the present case due to large number, i.e. 23, of oxyethylene groups and the relatively small lipophilic chain in Brij 35, the solvated cross sectional area of the hydrophilic group (Ah) is much higher than the solvated cross sectional areas ofthe tail (At). A rise in temperature and increase in electrolyte concentration bring down Ah due to dehydration of the head groups and thus facilitates a WI WIII transition. However due to poor penetration of alkane molecules to the surfactant layer, At did not WII increase more than Ah, thus preventing a WIII
(18)Ahsan, J.;Aveyard,R.;Binks, B. P. Colloids S u ~1991,52,339. . (19)Shinoda, K.;Kunieda, H.; Arai, T.; Saijo, H. J. Phys. Chem. 1984,88, 5186.
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(20)Israelachvili, J.N.;Mitchell, D. J.;Ninham, B. W. J.Chem. SOC.
Faraday Trans. 2 1978,72,1525.
Langmuir, Vol. 11, No. 4, 1995 1125
Effect of NaCl on a Microemulsion
"
I
I
t
1
h
b
'
2o
o'6
N a C l i 2 C O N0C E H I R A T 1 ~ " ~ ~ ~
1.2
Figure 7. Plots of viscosity vs NaCl concentration (M), (a) heptane (8%)and (b) nonane (8%);S = 80% w/w:0,30"C;0, 40 "c; A, 50 "c.
transition in this case, even a t 80 "C and 2.5 M NaC1. However, when the cosurfactantlsurfactant weight ratio was changed to 9 the WIII WII transition was observed.21 However, we did not follow the system in detail under that condition. Figure 7 shows the variation of viscosity values of the microemulsion (alkane (8%)lSrij 35 propanol (80%)/ water (12%)by weight) with increasing NaCl concentration. Addition of NaCl into the system increases the viscosity. However further addition of NaCl within the studied range did not have much effect on the viscosity value. It was also observed that variation of the viscosity of the 80% surfactant system with changes in the water weight percentage is similar to the system in which NaCl was absentell (No figure is shown.) Thus, we can conclude that the microstructure of the one-phase microemulsion a t 80% surfactant concentration is the same in the presence or absence of NaCl. It was suggested to be bicontinuous in the absence of NaC1, and it is known that the middle phase microemulsion in WIII which is observed in presence of 1M NaCl is also bicontinuous in nature.22 When the adiabatic compressibility of the system was plotted against NaCl concentration, the graph passed through a minimum. This minimum was found to be drifting toward the lower NaCl concentration with a rise of temperature. A low NaCl concentration might help make the water structure more compact. This caused the initial decrease of ,B. But a high NaCl concentration disrupts the structure with a large intake of water molecules for solvation; thus compressibility increases (Figure 8). In Table 1, ,B values of the system a t different compositions and at various temperatures are tabulated. p changes linearly with a rise of temperature as well as the oil percentage in the system. These results indicate that though the presence of NaCl changes the absolute values of ,B, the mode of variation with different parameters remained more or less the same. The cloud point is a characteristic property of all nonionic surfactants. We have already reported13 that this particular system exhibits the clouding phenomenon only in the presence of an electrolyte. Cloud point values of the system a t 80% surfactant concentration with variation of the NaCl concentration, oillwater ratio, and alkane phase are tabulated in Table 2. It is clear from the values that the cloud point of microemulsion media is a multivariable dependent phenomenon. This can be exhibited through a three-dimensional profile (Figure 9). It was also observed that a change in the alkane phase did not deviate the mode of variation of the cloud point
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(21) Ajith, S.; Rakshit, A. K. Unpublished data. (22) Taupin, C.; Ober, R.; Cotton, J. P.; Auvray, L. In Progress in
Microemulsions; Martellucci, S., Chester, A. N., Eds.; Plenum Press: New York, 1989.
6.0
0
1.2
0.6
1.2
46
FMCl CONCENTRATION
(M?
Figure 8. Plots of adiabatic compressibility vs NaCl concentration (M); (a) heptane (8%)and (b) nonane (8%);S = 80% w/w: 0, 30 "c;W, 40 "c; A, 50 "c. Table 1. Adiabatic Compressibility (B) Values of the System Alkane (O)/Brij 35 Propanol (S)/1M NaCl (W) and the Contact Angle ( 0 ) Data of These Microemulsions with Teflon at 35 "C
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composition S W 0
alkane
heptane
80 80 80 80
18 12 8 2
2 8 12 18
0 80 80 80 80 80 80 0
100 20 18 12 8 2 0
0 0 2 8 12 18 20 100
nonane
0
e at 35 oc (&2')
151 102 96 93 83 81 80 spreads
,8 x 30°C 6.11 6.70 7.24 7.96
5.77 6.56 6.84 7.59
loll cm2/dyn
40°C 6.27 6.98 7.68 8.50
50°C 6.67 7.21 8.10 9.04
6.14 6.82 7.16 8.18
6.37 7.16 7.56 8.71
Table 2. Cloud Points of the System Alkane (O)/Brij 35 + Propanol (S)/Water(W)at 8W0 (w/w) Surfactant (Brij 35 + Propanol) Concentration cloud point ("C)at different alkane
system S/O/W
0.8
heptane
80/2/18 80/4/16 80/6/14 8018/12 80/10/10 8011218
nonane
80/2/18 90 80/4/16 80/6/14 84 80/8/12 86 80/10/10 80/12/8
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heptane nonane (1:l)
82 85 92 84
NaCl concentrations (M) 1.0 1.2 1.5 1.8 2.1 68 54 35 26 20 58 46 34 26 22 53 48 36 28 22 60 50 41 33 24 71 59 47 39 28 67 62 48 42 67 62 65 70 81
62 52 54 59 68 94
48 39 40 47 82
36 30 31 34 44 60
30 26 26 32 36 54
51
80/2/18 80/4/16 8016/14
73 66 68
54 51 58
42 40 42
32 31 32
26 33 28
80/8/12 80/10/10 80/12/8
72
61 56
45 56 75
36
33 42 48
46
67
with NaCl concentration and oil percentage in the system. The cloud point shows a linear decrease with a n increase of NaCl concentration but passes through a minimum when plotted against the oil percentage in the system.13 In Table 1the contact angle values of microemulsions (with various nonane-water ratios) on Teflon are presented. From the data it is quite clear that the contact
1126 Langmuir, Vol. 11, No. 4, 1995
Ajith and Rakshit indicates that over the range studied a gross change of the structure does not happen though some minor variations are possible. It can be concluded that though the increase of temperature and also the presence of NaCl cause drastic changes on the pseudoternary phase diagram as well as on the absolute magnitudes of the physical properties, they did not enforce any microstructural changes in the single-phase microemulsion formed a t high surfactant concentration within the range of temperature and NaCl concentration variation studied.
Figure 9. Three-dimensionalrepresentation of the variation of the cloud point with nonane weight percentage and NaCl concentration of the aqueous phase for the studied microemulsion system; S = 80% w/w. angle values are not much different from each other, though there is some spread over the whole range. This
Acknowledgment. Thanks are due to the Institute of Reservoir studies, ONGC, Ahmedabad, India, for financial assistance and also for permission to publish this paper. The authors also thank Ms. E. I. Tessy for some experimental help. LA9405915
