Isentropic compressibility and transport properties of CTAB-alkanol-hydrocarbon-water microemulsion systems

Colloids and Surfaces A: Physicochemical and Engineering Aspects 136 (1998) 35-41

ELSEVIER

COLLOIDS AND SURFACES

A

Isentropic compressibility and transport properties of CTAB-alkanol-hydrocarbon-water microemulsion systems S.K. Mehta a.,, Kawaljit a

Deparonent ofChemisoT, Panjab University, Chandigarh 160 014, bldia

Received 16 April 1997: received in revised form 12 June 1997; accepted 12 June 1997

Abstract

Ultrasonic velocity, density, conductivity and viscosity of microemulsions containing CTAB, pentanol, benzene/toluene/ethyl benzene/p-xylene and water/NaCl solution have been measured at 303.15 K. An attempt has been made to compute the density Pm and isentropic compressibility Ks.m of the micellar phase from the experimental p and u data. The conductivity shows non-monotonic variation with increased addition of water. Unlike conductivity, the dynamic viscosity increases rapidly as volume fraction of water ~ increases. The viscosity results have been analyzed in terms of the Krieger equation. © 1998 Elsevier Science B.V.

Kerwords: Ultrasonic velocity; Conductivity: Viscosity: CTAB; Pentanol I. Introduction

Systems of oil, water and amphiphile exhibit many interesting properties which have made them the object of a great deal of study. In the absence of the amphiphile, it is well known that oil and water do not mix. The addition of only a small amount of an amphiphile, typically a few percent by weight, causes oil and water to form one isotropic phase. The smaller the amount needed to solublize the oil and water, the better the amphiphile is said to be. Although isotropic, the disordered mixture of oil, water and amphiphile does not appear to be homogeneous. Rather the fluid is thought to be structured, consisting of coherent regions of oil and of water which are separated by the amphiphile. The picture is reasonable because the structure of the amphiphile is such that its polar head * Corresponding author. 0927-7757/98/$19.00 :t:) 1998 Elsevier Science B.V. All rights reserved. PH S0927-7757(97 )00321-X

prefers to be surrounded by water, whereas its aliphatic tail prefers the oil. Thus the molecule tends to create an oil-water interface. In most amphiphile formulations for preparing microemulsion, a cosurfactant such as alcohol is generally used in combination with the primary amphiphile [1-3]. The most important fundamental role of alcohol [4] is probably the ability to destroy the liquid crystalline and/or gel structure which disturb the formation of a microemulsion. In general, the factors that affect the transition between different types of systems include temperature, salinity and molecular structure of the surfactant, the nature of the oil, and the oil-water ratio etc. Since the discovery of microemulsions, many different techniques have been used to investigate their structures. Among these, ultrasonic has proved to be a very useful tool for obtaining information on the dynamics of liquid systems [5]. In the analysis of volumetric properties like density

36

S.K. Mehta, Kawaljit / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 35-41

and isentropic compressibility, Ks provide useful information both on the compressibility and on the structural conditions of the micellar phase at different ~b. Similarly, the transport properties such as static shear viscosity and electrical conductivity enable one to characterize the role of the interdroplets connectivity due to attractive interactions. In an earlier report [6] we presented the results of conductivity, viscosity and ultrasonic velocity of Tween 20-propanol-water-oil microemulsions throughout the single phase region as a function of volume fraction of water ~b. A large electrical conductivity transition was observed in the system indicating percolation phenomenon. The analysis of the results allows one to determine the value of percolation threshold 4~c for various systems. In this paper, detailed measurement of volumetric (p.KO and transport (qoa) pioperties of samples formulated by CTAB, pentanol, oil (benzene/ toluene/ethylbenzene/p-xyiene) and water as a function of ~ throughout the single phase region is presented. Close scrutiny of the results indicate that the microemulsion containing C'rAB as surfactant does not show percolation phenomenon and the conductivity changes with ~b in a more complex fashion. It is not easy to interpret them [7], at least at first, because of their complex shapes.

2. Experimental High grade benzene (Sisco Chem), toluene (Fluka Buchs), ethylbenzene (Fluka Buchs), pxylene (Fluka Buchs), pentanol (Fluka Buchs), CTAB (Fluka Buchs) and triply distilled water have been used to prepare the microemulsion samples. The emulsifier (CTAB and pentanol ) was mixed with the oil in the proportion (by weight) 0.10 CTAB:0.11 pentanol:0.77 oil. Water was then added to give the desired final composition. The prepared microemulsions were highly stable under the operating conditions. The ratio of number of moles of alcohol to the number of moles of surfactant (na/ns)was kept as 3.65. Tile densities of the solutions were measured with a high precision Anton Paar vibrating tube densimeter. For the accurate ultrasonic velocity

measurements, the ultrasonic time intervalometer (UTI-101) from Innovative instruments based on the pulse-echo-overlap technique (PET) was used. A transducer of frequency 2 MHz was used for all the measurements. The conductivity a was measured with a digital conductivity meter (model NDC 732) of Naina Electronics (Cell constant=0.53 m-X). Measurements of viscosity were carried out using a suspended level dilution Ubbelohde capillary viscometer. The viscometer, was placed in a water bath thermostated at 30_0.01°C. The dynamic viscosity r/was obtained from the kinematic viscosity data by multiplying it with the density of the studied sample. All the samples were characterized by the volume fraction of water ~b and the experiments were performed at 30.0 +_0.01~C. Three types of variations were made with changes of oil, salinity and surfactant concentration in the microemulsion.

3. Analysis of results and the discussion 3.1. Density studies

Fig. 1 displays the dependence of density p versus ~b at constant temperature for microemulsion containing benzene as oil. There is constant increase in the density as ~b increases. To account for the ~bdependence of p, let us consider 1010 .--,..

g80

~",= 950

'

,

'

•

~

'

d'~

""

'

: ~ a20

,3aO 8600

,J

0.07

i

J

0.14

0.21

i

0.28

0.35

'0

Fig. i. Variation of density p and Pm with 4 for CTAB+ pentanol + benzene+ water: o, p; O. p,,.

37

S.K. Mehta. Kawaljit / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 35--41

the role of the micellar phase. Assuming the additivity of volumes of micellar and oil phase [8], one can attempt to evaluate the micellar density by the relation p=~raPm-t-q~op o,

(1)

where Po and Prn are the densities of the oil and micellar phases respectively. The micellar phase is assumed to comprise surfactant, cosurfactant and water. ~bo is the volume fraction of oil phase whereas ~bm=(1-4)o) represents the volume fraction of miceUar phase. Eq. (1) considers that the microemulsions are made by two separated noninteracting regions, one containing droplets and/or connected droplets and the other the oil phase. The validity of Eq. ( 1 ) can be checked from plot of ( P - ~ o Po) versus ~ which shows linear trend. The calculated micellar densities lrom the Eq. ( l ) at different ~ are also shown in Fig. 1. It indicates that as ~ increases, Pm increases, indicating an increased water-like character of the reversed micelles. Similar observation has also been reported by different group~ [9-11].

4. Ultrasonic velocity The graphical representation of the velocity data is shown in Fig. 2. An inspection of this figure

1300

I

I

I

1290 A sU~

shows that there is no evidence for an ultrasonic velocity discontinuity over the entire investigative ~b range. A slight decrease is observed in u up to =0.13, followed by a rapid increase at high ~b values. We can use the ultrasonic velocity [12] u data to characterize the compressional elasticity of the present microemulsions. Ultrasonic velocity is related to the density p and to the isentropic compressibility Ks of the system by the relation (2)

Ks = 1/u2p.

To get some structural information, we analyzed the systems in terms of the isentropic compressibility Ks (Fig. 3) which is much more sensitive than the velocity to structural changes. Using the same argument as in the case of density, we can compute micellar isentropic compressibility by using the Wood relation [8,13-15] (3)

Ks=~mKs.m+~oKs,o,

where Ks.o and Ks.,, are the isentropic compressibility of oil and micellar phase, respectively. We can use Eq. (3) to compute the Ks,m values for each 4. Fig. 3 displays a comparison between Ks and Ks.,, data over the whole volume fraction range. A slight maxima is seen at low ~b in Ks,m but as ~b increases a sharp decrease is observed unlike Ks data which shows little decrease as q~ increases. Up to ~b= 0.2, the micellar compressibility is more than Ks and a reverse trend is being observed for ~b> 0.2 indicatin~ possibly the forma-

/ I

800

,

,

,

r--

760

1280

o°,~.

v'=~ 7:.;0 t:l..

E

.m~w

_~ 1.270

ID,.-

v3 61~0 1260 6,10

12500

I 0.07

n 0.14

i 0.21

' 0.28

0.35 6i'00

_

"%

I

0.07

i

._

0.14

I

0.21

--.,

"~

0 o o . ~,,

0.35

~0

Fig. 2. Variation of ultrasonic velocity u with ~b for CTAB+pentanol+oil+water: V, benzene: O, toluene: o, ethyl benzene: 0. p-xylene.

Fig. 3. Variation of K~ and Ks. m with ~b for CTAB+ pentanol + benzene+ water: o, Ks: O, Ks.re.

S.K. Mehta. Kawaljit /' Colloids Surfaces A." Phj,sicochem. Eng. Aspects 136 ( 19981 35-41

38

=320

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:l.B

,

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.-,

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,.

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.........

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,~.

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02

0.2

0.3

0.,

%-

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lJ"

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.-. 2c.;6

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0.2

0.4

Fig. 4. Variation of ultrasonic velocity u with $ tbr CTAB+ pentanol + toluene + water: V, 1,5 g CTAB; O, 2.0 g CTAB: o, 2.5 g CTAB.

Fig. 5. Variation of conductance cr with $ for CTAB+ pentanol + oil + water: V, benzene" O, toluene: o, ethylbenzene; 0 , p-xylene.

tion of reverse micelles. The overall trend being the same, the ultrasonic velocity increases with change of oil in the order benzene
[17] and Baker et al. [18] for the S-type systems. In these cases, the conductivity keeps low and undergoes non-monotonic variation (Fig. 5). We can explain the results in terms of three distinct regions of water concentrations: (a) <~bs, i.e. below the maximum; {b) between Ss and $c and (c) greater than $c. Here ~]~ represents the concentration of water from where conductance starts decreasing and t/J¢as the critical water concentration at which the conductivity tends to increase. It has been proposed by Baker et al. [18] that below t]J~ there is an excess of surfactant compared to the amount of water. Therefore mainly hydrated surfactant-alcohol aggregates ar,~ present in this region. The addition of water results in the increase in the dissociation of surfactant which is responsible for the rise in conductivity in this region. Similar conclusions have been drawn by Shah and Hamlin [19] and Boned et al. [20]. At Ss there is formation of microemulsion, i.e. the transformation, of surfactant-alcohol aggregates in the form of micelles with a definite water core. The decrease in conductivity beyond $' must be due to the gradual replacement of the hydrated surfactant-alcohol aggregates with microemulsion droplets. The near constant value of the conductivity between ~/~ and 4~~ after the initial drop, with its numerical value depending on the CTAB concentration indicates that within this region more and

5. Conductivity studies

Recently Clausse and coworkers [16], in their extensive study on microemulsions using conductivity technique, have distinguished between two main type of systems which they designated as S and U. The type S system was characterized by a phase diagram in which the w/o and o/w areas were disjoined and separated by a composition zone over which viscous and turbid systems were formed. In contrast, with the type U system, the "'oil-rich'" and "water-rich" regions merge into each other so as to form a unique domain. The behavior was reflected in the electrical conductivity and viscosity behavior of the systems [7] i.e. the U type systems show percolation phenomena, whereas in S type, the conductivity changes in a more complex fashion. In the present investigation, the conductivity behaviour of the microemulsions follows the trend described by Clausse et al. [16], Eicke and Denss

S.K. Mehta. Kawaljit / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998,1 35-41 20

I

I

i

I

I

39 I

15

~:.0

rl (.,*,I.

,,

5

O0

0.1.

0.2 %0

0.3

Fig. 6. Variation of conductance a with ¢, for CTAB+ pentanoi + toluene + water: V, 1.5 g CTAB; e , 2.0 g CTAB: o. 2.5 g CTAB.

('0

I

I

0.07 0.14 0"1

I

0.28 0.35

Fig. 8. Variation of dynamic viscosity ~l with 4' for CTAB + pentanol + oil + water: V, benzene; 0 , toluene; o, ethylbenzene; 0 , p-xylene.

4

We analyzed our measurements in terms of the empirical law due to Krieger [21].

3

ll-- l]o( l --4,m/4,p) -r,

,"0

't'

•

0.4

(4)

where ~l Slo

C,

4,m 4,p

06

I

o.1

I

0.2

I

0.3

o., v and 4,p

Fig. 7. Variation of conductance a with ,/J Ibr (;TAB+ pentanol+toluene+ NaC! solution: V. I M NaCI: @, 0.1 M NaCI: 'i), 0.01 M NaCI.

more droplets are being formed, but these remain fairly well apart from each other. The change of oil, surfactant concentration and salinity does not effect the overall behavior of conductance in these microemulsions ( Figs. 5-7).

6. Viscosity studies The variation of the dynamic viscosity !! as a function of the 4, is shown in Fig. 8. It is observed that as 4' increases, there is regular increase in the viscosity of the microemulsion.

static shear viscosity; viscosity of continuous phase; volume fraction of micellar phase; packing fraction [22] ( 4 , p = 0 . 7 5 for compact cubic arrangement of spheres and 4 , p = 0 . 6 5 for random arrangement of spheres); free parameters.

The Krieger equation has been applied after estimating the values of 4,p and v by computational analysis. It has been possible to fit our data when 4,p = 0.65 and v = 0.6. A comparison of these computed parameters with the earlier studies on various systems shows a reasonable agreement, e.g. Abilion et al. [22] have obtained 4,p=0.65 and v= 0.5 for microemulsion comprising of CTAB, dodecane, Brine and butanol, whereas values of 0.65 and 0.4, respectively, were evaluated by Cametti et al. [23] on a A O T + d o d e c a n e + w a t e r system. Recently, Leaver and Olsson [24] have used 4, = 0.63 and v=0.5 for non-ionic microemulsion comprised of Ct2Es, water and decane. As expected, both theoretical and experimental static shear viscosity obey the Krieger equation for very dilute

40

S.K. Mehta, Kawaljit / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 35-41

I

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conditions of the micellar phase in CTABalkanol-oil-water microemulsions. These exhibit a trend towards an enhanced water-like character of the dispersed phase. Fig. 10 displays some similarity among the three properties--u, a and ~1. The rapid change in each property starts at around 4, =0.11. The ultrasonic velocity passes through a minima, conductance keeps on decreasing and the viscosity starts increasing rapidly. This may be due to the fact that all the surfactant-water-oil aggregates are replaced by water droplets at around ~b= 0.11, which are responsible for the observed trends.

Fig. 9. Variation of dynamic viscosity Pl with 4~ for CTAB+pentanol+toluene+water: V, i.5g CTAB;Q, 2.0g CTAB; o, 2.5 g CTAB.

o 1.300 - lO,

11

u

4

:t 2~,.',

5 2

:L260 j 0

0 0

0,I

0.2

~o

0.3

0.4

Fig. 10. Variation of conductance 104 tr (Sin-t), ultrasonic velocity u (ms-t), dynamic viscosity tl (cP) with ,k for CTAB + pentanol + benzene + water: V, tr; 0 , u; o, tl.

dispersions. The experimental static shear viscosity deviate fi'om the viscosity obtained from the Krieger equation at high 4, values. The oil polarity and effect of surfactant concentration have a similar influence on viscosity, as was observed in their conductance behavior (Figs. 8 and 9). 7. Conclusion It may be inferred that Pm and Ks. m results provide some useful information on the structural

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S.K. Mehta, Kawaljit / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 35--41 [18] R.C. Baker, A.T. Florence, R.H. Ottewill, T.H.F. Tadros, J. Colloid Interface Sci. 100 (1984) 332. [19] D.O. Shah, R.M. Hamlin, Jr, Science 171 (1971) 483. [20] C. Boned, M. Clausse, B. Lagourette, J. Peyrelasse, V.E.R. McClean, R.J. Sheppard, J. Phys. Chem. 84 (1980) 1520. [21] I.M. Krieger, Adv. Colloid Sci. 3 (1972) 11 !.

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[22] O. Abillon, B.P. Binks, C. Otero, D. Langevin, R. Ober, J. Phys. Chem. 98 (1988J 4411. [23] C. Cametti, P. Codastefano, G.D. Arrigo, P. Tartaglia, J. Rouch, S.H. Chen, Plays. Rev. A. 42 (1990) 3421. [24] M.S. Leaver, U. Olsson, Am. Chem. Soc, 10 (1994) 3449.