surfactant-oil-water systems near the affinity inversion

J. DISPERSION SCIENCE AND TECHNOLOGY, 11(4) 397- 407 (1990)

SURFACTANT-OIL-WATER SYSTEMS NEAR THE AFFINITY INVERSION: PART VI: EMULSIONS WITH VISCOUS HYDROCARBONS

J.L. Salager, G. Lopéz-Castellanos and M. Miñana-Pérez Lab FIRP. Ingenieria Química, Universidad de los Andes Merida, Venezuela

ABSTRACT The phase behavior of surfactant-water-viscous oil systems is studied as well as the properties of the corresponding emulsions. The oil viscosity is increased up to 1000 cP by using different hydrocarbon mixtures. This change implies a variation in the Equivalent Alkane Carbon Number (EACN), which is compensated in order to maintain a comparable formulation. When the oil viscosity increases, the A+ region, which exhibits stable W/O emulsions with medium to high internal phase ratio, tends to shrink, and finally vanishes when the viscosity exceeds 50-100 cP; the inversion line on a formulation-WOR map exhibits a shift of its A+/C+ branch; the remaining of the inversion line, as well as the general phenomenology concerning the emulsion properties, still follows the patterns found with light hydrocarbons.

INTRODUCTION In most systems dealt with in the previous papers of this series, the oil phase was a low viscosity alkane or distillation cut [1-5]. In such cases, it was shown that the type of emulsion obtained by stirring a pre-equilibrated surfactant-oil-water systems is directly associated with the phase behavior when the water/oil ratio (WOR) is not too far away from unity.

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A general pattern was found on a formulation-WOR diagram in which the inversion locus and the optimum formulation line mark the limits of six regions, so-called A, B, and C with + or – superscripts depending upon the formulation [3]. Each region exhibits a characteristic set of emulsion properties, i.e., type, stability and viscosity [5,6]. The present study analyses the effect of the viscosity of the oil phase on the inversion line and on the emulsion properties.

PRODUCTS AND EXPERIMENTAL PROCEDURES Surfactants were used as received from the manufacturer. Sodium dodocyl sulfate is a > 95% pure grade product from Merck. TRS 10-80 and TRS 18 are two petroleum sulfonate sodium salts made by Witco Chemicals with activity 80 and 60% and equivalent molecular weight 425 and 500 respectively. SPAN and TWEEN are commercial sorbitan esters and ethoxylated esters provided by ICI Americas. The HLB scan was carried out either by using pure products or mixtures of them when an intermediate HLB value was sought. Alcohol is added as usual for the following practical reasons: to avoid the formation of liquid crystals, to speed up the equilibration, to reduce the time required to study the emulsion stability, and finally to eventually modify the formulation (n-pentanol). The kerosen is a commercial distillation cut (viscosity 1.3 cP at 25 ºC); the paraffinic oil is a pharmaceutical grade white oil (viscosity 38 cP at 25 ºC); the lubricant oil bass is made by Lagoven SA under the MVI-3000 specification (4105 cp at 25 ºC). The general experimental procedure to pre-equilibrate and emulsify the surfactant-oil-water systems has been detailed in previous papers [1,2]. All emulsions are produced by stirring 50 to 100 ml equilibrated systems with a Taurus turbine blender of the stem type (5000 rpm, 15 s) according to a standard procedure. The emulsion stability is estimated by the time required to obtain a 2/3 phase volume separation according to the technique previously described [1]. The emulsion conductivity is measured with an open pass platinum cell and the emulsion type is interpreted as previously [3]. The oil viscosity is measured at 25ºC with a Rheomat 30 Contraves viscometer at 10-100 s-1 in a MSO cell.

Emulsions with Viscous Hydrocarbons

COMPENSATING THE OIL CHANGE EFFECT ON THE GENERALIZED FORMULATION It is known that a formulation change can be obtained by scanning any of the physico-chemical variables, including the temperature. In most studies, only one formulation variable is scanned at the time, but equivalent formulation changes with other variables can be calculated by using the correlations for optimum formulation [7-9]. It is useful to define the generalized formulation as the deviation from optimum [3]. Such a deviation has been found to render the surfactant affinity difference SAD, i.e., a physico-chemical measurement of the hydrophile lipophile balance of the system [10]. The comparison of two systems in the same SAD state is needed if the effect of the generalized formulation on the phonomenology is to be hold constant, so that the influence of other variables may be ascertained. This remark is important because it is not possible to change the oil viscosity, i.e., a physical property, without changing the physicochemical formulation. Indeed, in order to change the oil viscosity, either the oil nature or the temperature must be changed. Since a change in temperature produces variations not only in the oil viscosity, but also of the water viscosity and of the overall affinity balance, it is more reasonable to change the nature of the oil. This may be achieved either by taking different MW oils in the some structural series, or by mixing two oils. Since pure hydrocarbons become highly viscous only near their freezing point, it is advisable to use oil mixtures or even mixtures of oil mixtures to avoid the formation of crystals in viscous oils. Figure 1 shows the viscosity of different mixtures of a kerosen cut and a lubricant oil base, both with a roughly equivalent balance of paraffinic, cycloparaffinic and aromatic species. The equivalent alkane carbon number (EACN) of the kerosene is deduced from comparison with an alkane scan [11,12], while the EACN of the different mixtures are calculated from the optimum salinity variation by using the correlation for optimum formulation [7] in a finite difference form, i.e.: D(Ln S) = 0.16 D(ACN) Where the D indicate a difference, LnS is the natural logarithm of the salinity of the aqueous phase (S in % wt NaCl), and 0.16 a characteristic of the sulfonate. There is a large uncertainty at high lube oil content cases because of the extremely extended three-phase region and the related inaccuracy in pinpointing the optimum.

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It is seen that the variation of EACN with composition is close to linearity with respect to the volume fraction, while the EACN most usual mixing rule exhibits linearity when molar fraction weighting factors are used [9,11]. Since the lubricant oil has a much higher MW than the kerosen, a linear mixing rule based on mole fraction would result in a viscosity curve with positive concavity when plotted against a volume fraction; this is indeed what Figure 1 shows at low lubricant oil content. However, above 50% the concavity turns downward; this is probably due to a fractionation of the oil phase. The very light color of the microemulsion compared with the dark brown color of the oil phase is another hint that the oil which is solubilized is the middle phase contains less lube oil than the excess oil phase. As a consequence it means that the lube oil phase EACN cannot be obtained by extrapolation, even from 90% lube oil overall composition. In any case the EACN of the lube oil is probably well over 25. Figure 1 indicates the concomitant change of viscosity and EACN to be taken into account, which is what is needed for our purpose. In order to compare systems in an equivalent SAD framework, the salinity of the aqueous phase is adjusted for each oil mixture so that the term (LnS - 0.16 EACN) remains constant. The formulation is changed through a surfactant s parameter scan [7] obtained by mixing two anionic surfactants: sodium dodecyl sulfate (SDS) and Witco TRS 10-80, with respective s values of - 4.3 and + 1.2. The aqueous phase volumetric fraction (fw is 0.5). The phase behavior at equilibrium is indicated in Figure 2 (a). Thanks to the selected compensating changes of oil EACN and salinity, optimum formulation is always obtained at about 45 wt % SDS. However, it is worth noting that the three-phase zone range increases considerably when both the oil EACN and the salinity increase. This is a associated to a decrease in solubilization or an increase of the tension at optimum, and it is known to correspond to a decrease in quality of the system, i.e., a lower affinity of the surfactant for both phases [10].

EMULSION INVERSION Each pre-equilibrated system is emulsified and its conductivity is measured as in previous studies [1-5]. The inversion line indicated in figure 2 (b) corresponds to the strong conductivity variation exhibited at inversion, typically from a few mS/cm to several mS/cm [1,3].

401

3

30

10

20

10

10

10

2

OIL VISCOSITY (CP)

OIL EACN


1

0 0

VOL % LUBRICANT IN MIXTURE 100

Fig. 1: Viscosity (at 25 ºC) and measured equivalent alkane carbon number (EACN) of different mixtures of kerosen and lubricant oil.

TOTAL SURFACTANT 0.02 M

3 % vol. Sec-BUTANOL 1.2 SURFACTANT PARAMETER s

mol % TRS in TRS/SDS Mixt.

2

3

2

STABLE

W/O

+ -

(a)

0

-4.3 0

20

1.3

3.1

40

60

5.2 10 18.5

80 410

A+

C+

UNSTABLE

O/W

UNSTABLE

0

SAD

100

WOR = 1

C+ W/O/W MULTIPLE

MOW A-

UNSTABLE

O/W STABLE

(b)

VOLUME % LUBRICANT IN MIXTURE

100 1100

OIL MIXTURE VISCOSITY (cP) AQUEOUS PHASE SALINITY % NaCl

0.6

1

1.8

5

8

12

Fig. 2: Properties of systems containing different oil mixtures, and different salinity of the aqueous phase to compensate for the EACN change. (a) phase behavior at equilibrium. (b) emulsion inversion map.

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A classical behavior for near unit WOR systems is observed up to an oil phase of 4 cP (25 % lube oil), i.e., the emulsion type is associated with the phase behavior and the inversion occurs near optimum formulation [3,13]. Above 75% lube oil no W/O emulsion is found whatever the formulation. This is the typical case of a formulation scan carried out at a water fraction fw , located at a value higher than the one corresponding to the A+/C+ branch of the inversion locus [3,10]. It has been shown that when the oil becomes heavier, this branch tends to shift toward lesser fw values (sea fig. 6 Ref. 3) and may be located at fw lower than 0.5, which is the present value. The vanishing of the W/O region above 75 % lube oil corresponds to the spreading of the C+ unstable emulsion zone; the conductivity value shows that multiple W/O/W emulsions are present in this zone. Between 25 and 70% lube oil the W/O emulsion A+ region becomes narrower and narrower as the oil viscosity increases. The W/O emulsion occurrence gets restricted to the 3- 2 boundary zone where the phase behavior favors this type and where the tension is still relatively low, because of the vicinity of the optimum formulation. Away from optimum formulation the tension becomes probably too high to allow a W/O emulsion occurence and a C+ type O/W emulsion is formed. This argument is also sustained by the observation that the tension at the 3- 2 boundary increases from left to right and that a higher stirring energy tends to increase both the width and the length of the W/O zone tongue. The presence of such a W/O region limited to the 3- 2 boundary zone was previously found when the inversion locus A+/C+ branch was strongly tilted (see fig. 3 Ref. 5).

INVERSION LOCUS ON A FORMULATION-WOR DIAGRAM The previous observations are easier to interpret on a bidimensional formulation-WOR diagram. Figure 3 shows the phase behavior and the inversion locus on such a diagram for a paraffinic oil of medium viscosity. The WOR variation is expressed as the water values fraction fw.


0.8 W/O Ln S (S % wt NaCl)

B+

0.4

403

PARAFINIC OIL 38 cP 1 % wt TRS 18 4 % vol Sec-BUTANOL

C+ 3 PHASE BEHAVIOR

O/W C+

0 B-

-0.4

W/O

C-

A-

O/W 6 5 Isostability contours

-0.8

2 2.5 3 4

0 WATER VOL. FRACTION fw

1

Fig. 3: Formulation-fw map for an anionic surfactant system showing the inversion locus and the isostability contours expressed as the logarithm of the time (in sec) for 2/3 phase Separation.

The inversion locus exhibits the classical step shape with tug specific features. First the extension of the A region (from fw = 0.2 at A-/B- to fw = 0.32 at A+/C+) is narrower than with less viscous oils. On the other hand there is a W/O region tongue in the neighbourhood of optimum formulation; this inversion locus ”bump” makes the A region a bit wider in the vicinity of optimum formulation. In any case an A+ W/O stable emulsion cannot be obtained with this system in the conditions of emulsification. With this exception, the iso-stability contours exhibit the typical pattern, i.e., a stable A- O/W emulsion zone and unstable emulsions in the three-phase region [14] and in the abnormal B- and C+ zones. Viscosity data higher than expected and a conductivity value lower than normal indicate the occurrence of a multiple W/O/W emulsion in the C+ zone near the W/O tongue.

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4

CALCULATED HLB

6 8

0.6 % wt SPANs, TWEEN, & MIXT. 4 % vol Sec-BUTANOL PARAFINIC OIL 38 cP 1 % wt NaCl +

B

C+

W/O

O/W 3f

10 3

12

B-

A-

C

-

O/W

14 7

16

6

5

3.5

Isostability contours 0 WATER VOL. FRACTION fw 1

Fig. 4: Formulation-fw nap for a nonionic surfactant system showing the inversion locus and the isostability contours expressed as the logarithm of the time (s) for 2/3 phase separation.

Figure 4 shows the formulation-WOR diagram for a nonionic surfactant system with the same paraffinic oil. Here the formulation change is obtained by mixing sorbitan ester compounds. In this case the extension of the A region is about the same than in the previous figure. However the inversion line “bump” is less pronounced but more extended outside the three-phase region. Similar results were obtained with other systems [15]. When the oil phase becomes extremely viscous, e.g., more than 200-500 cP the inversion line becomes a vertical line located at a fw value of about 0.2 - 0.3. In most case there is no visible “bump” anymore even in the neighbourhood of optimum formulation. With very viscous black hydrocarbons such as crude oils, the three-phase behavior cannot be detected easily, but the optimum formulation still can be located by the minimum of interfacial tension or the sharp drop in emulsion stability. Such a diagram with an extra heavy crude oil has been recently published [16].


SAD

h = 1 - 5 cP W/O

5 - 10 cP B+

+ B+ A+ C + 0

-

B-

0

A-

C-

O/W

fw

B-

A+

C+

A-

405

10 - 100 cP B+

C+

> 500 cP B+

C+

A+

B-

A-

B-

A-

1

Fig. 5: General trends of variation of the inversion locus an a formulation-fw map when the oil phase viscosity is changed.

CONCLUSION: GENERAL TRENDS The effect of the oil phase viscosity on the emulsion properties may be ascertained through its influence upon the inversion locus on a formulation-WOR map. It must be pointed out that, for a given stirring procedure, the lower A-/B- branch of the inversion locus remains at a fw value about 0.2 - 0.3 whatever the oil phase viscosity. As for as the upper A+/C+ branch of the locus is concerned, all the results indicate that it tends to be shifted toward lower fw values when the oil viscosity increases. The effect is very rapid, and is quite significant for a viscosity change as low as from 1 to 4 cP. It seems that the shift is of lesser extent near optimum formulation than away from it; as a consequence the A+/C+ breach tends to be tilted and a lagging A+ W/O zone may remain in the neighbourhood of the three-phase region. When the viscosity of the oil phase reaches a high value (say more than 500 cP), then the inversion locus becomes a vertical line and the A+ region no longer exists. Figure 5 sums up the effect of increasing the oil viscosity for a system with a narrow and non-tilted three-phase zone. As far as the emulsion properties are concerned, the consistency with the general phenomenology is maintained [5,6] with the exception of the A+ zone, where the W/O emulsion is not stable if the formulation is too near optimum and/or the fw is too low.

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On the basis of a dozen examples it may be thought that when the three-phase zone is more extended and/or more tilted, the A+/C+ branch of the locus seems to shift leftward at even lower values of the oil viscosity; indeed in extreme cases the inversion locus is a vertical line even for low viscosity hydrocarbons. As a final comment it is worth remarking that the increase in the oil viscosity has different consequences. On the one hand it produces an increase of the oil EACN, which results in a system at lower quality, with a higher tension and a wider three-phase zone. On the other hand the increase in viscosity makes more difficult the process of formation of a W/O emulsion, specially if the interfacial tension is high. At intermediate viscosity, only 2 systems with low enough tension (near optimum formulation), together with a sufficient oil content, i.e., a low fw, may produce a W/O emulsion in the B+ zone. Quite the contrary occurs with O/W emulsions which may be obtained in the Aregion with a wide variety of oil phases; indeed it is relatively easy to find a formulation that leads to a stable O/W emulsion even with the most viscous hydrocarbons [16].

ACKNOWLEDGEMENTS The authors wish to express their appreciation to Professor J. Andérez for his Comments, and to Ms F. Vejar and Ms M. Briceño who helped with some of the experiments. The Lab. FIRP research program is sponsored by the University Research Council CDCHT and the National Research Council CONICIT, Venezuela.

REFERENCES [1] Salager J.L., Loaiza-MaIdonado I., Miñana-Pérez M.. and Silva F., J. Dispersion Sci. Technol., 3, 279 (1982) [2] Salager J.L., Miñana-Pérez M., Andérez J., Grosso J., Rojas C. and Layrisse I., J. Dispersion Sci. Technol. 4, 161 (1983) [3] Salager J.L., Miñana-Pérez M., Perez-Sanchez H., Ramirez-Gouveia M., sad Rojas C., J. Dispersion Sci. Technol., 4, 313 (1983)


[4] Anton R., Castillo P., and Salager J.L., J. Dispersion Sci. Technol., 7, 319 (1986) [5] Miñana-Pérez M., Jarry P., Perez-Sanchez M., Ramirez-Gouveia M.. and Salager J.L., J. Dispersion Sci. Technol., 7, 331 (1986) [6] Jerry P., Miñana-Pérez M., and Salager J.L., in “Surfactants in Solution” . Mittal Ed., Vol. 6, 1689 , Plenum Press (1967) [7] Salager J.L., Morgan J., Schachter R.S., Wade W. H., and Vasquez E., Soc. Petrol. Eng. J., 19, 107 (1979) [8] Bourrel M., Salager J.L., Schechter A.S., and Wade H. W., J. Colloid Interface Sci., 75, 461 (1980) [9] Bourrel H. and Schechter R.S., “Microemulsions and related systems", M. Dekker (1988) [10] Salager J.L., in “Encyclopedia of Emulsion Technology”, P. Becher Ed., Vol. 3, 79, M. Dekker (1988) [11] Cash R.. Cayias J., Fournier G., MacAllister D., Schares T.. Schechter R.S.. and Wade W. H. J. Colloid Interface Sci., 59, 39 (1977) [12] Salager J.L., Bourrel H., Schechter R.S.. and Wade W.H., Bull. Cent. Rech. Exp. Prod.. 2, 399 (1978) [13] Salager J.L., Quintero L., Ramos E., and Anderez J., J. Colloid Interface Sci., 27, 288 (1980) [14] Anton R. E., and Salager J.L., J. Colloid Interface Sci., 111, 54 (1986) [15] Miñana-Pérez M.. Lopez-Castellanos G., and Salager J.L.. Preprints XIVth Congress of the Int. Fed. Soc. Coss. Chem., Vol. II, 629, Barcelona. Spain (1986) [16] Duran L., and Salager J.L.. Preprints 3rd Int. EOR Symp. SIREMCRU, Vol. II, 1231, Maracaibo. Venezuela (1989).

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