Chapter 14
Rheology of Sucrose Ester Aqueous Systems
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C. Gallegos, J. Muñoz, A. Guerrero, M. Berjano Departamento de Ingenierĭa Quĭmica, Universidad de Sevilla, c/ Prof. Garcĭa González, s/n 41012 Sevilla, Spain
The overall objective of this research was to study the influence that sucrose ester concentration and temperature exert on the viscous and viscoelastic behavior of micellar and lamellar liquid crystal phases occurring in aqueous systems containing a sucrose ester. Four sucrose esters prepared from different fatty acids were used. The relationship between Newtonian specific viscosity and temperature for sucrose laurate systems was adequately described by a modified Goodwin model. Entangled micellar systems and liquid crystalline phases showed non-Newtonian viscous behavior, although a limiting viscosity at low shear rates was always found. The flow curves at different temperatures were superposed by calculating a shift-factor from the Ellis model. The development of lamellar liquid crystals coincided with the occurrence of measurable normal stresses.
Sucrose esters are nonionic surfactants manufactured from sugar and vegetable oil. They are known to be useful for various applications (/). Because of their low toxicity, sucrose esters are widely used as food emulsifiers (2-5) and in the cosmetic (4) and pharmaceutical industries (5). The properties of sucrose esters depend dramatically on the degree of esterification of the sucrose (6). This influences their applications. Sucrose esters may form different micellar and liquid crystalline phases in the presence of aqueous and nonaqueous solvents (7). The existence and extent of these phases depend on surfactant concentration, temperature and the hydrophilic-lipophilic balance (HLB) of the sucrose esters. The different association structures that develop in the above mentioned binary systems significantly affect their rheological properties. This paper is concerned with the viscous and viscoelastic behavior of aqueous systems containing a high H L B sucrose ester (sucrose laurate, palmitate, oleate or stéarate). Specific subjects that will be discussed are: a) Influence of sucrose ester concentration and temperature on the viscous behavior of globular micelles, entangled micelles and liquid crystalline 0097-6156/94/0578-0217$08.00/0 © 1994 American Chemical Society Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
218
STRUCTURE AND FLOW IN SURFACTANT SOLUTIONS
phases; b) Linear viscoelastic behavior of an entangled micellar phase; and c) Non linear viscoelasticity (normal stresses) of a lamellar liquid-crystalline phase. Experimental Sucrose esters (SE) were supplied by Mitsubishi Kasei Corporation (Japan) and used as received. The four sucrose esters investigated were those of lauric (L-1695), palmitic (P-1570), stearic (S-1570) and oleic (O-1570) acids. Thefirsttwo digits correspond to the HLB value of the emulsifier. The second two digits refer to the minimum purity (%wt) of the fatty acid used. Aqueous systems of each surfactant up to 50%wt SE were prepared. Rheological measurements were carried out in a Haake Rotovisco RV-20 rheometer (Germany), using a CV-20N measuring head. Steady flow measurements were conducted using different Mooney-Ewart sensor systems. Their radii ratios were always less than 1.1. Shear rate was varied between 0.1 and 300 s". All the samples were presheared at 300 s" for 10 minutes. Oscillatory flow measurements were carried out, at 50°C, only on samples containing sucrose stéarate, up to a maximum concentration of 15%wt SE. Normal stress measurements were conducted, at 50°C, using a cone and plate sensor system (4°, 45 mm) on samples with a sucrose stéarate concentration ranging between 10 and 45%wt surfactant. All the samples were presheared at 300 s" for 10 minutes. 1
1
1
Results and Discussion Non-entangled Micellar Systems. The experimental results demonstrate that aqueous binary systems, with sucrose laurate contents up to 45%wt, show Newtonian viscous behavior over the whole temperature range studied. This behavior has been related to the presence of globular micelles. Although the absolute viscosity decreases as temperature increases, the specific viscosity, η , is related to temperature according to equation 1 for surfactant concentrations up to 45%wt. 5 ρ
In
isp /IspiTref)]
\_
3
10 k
1
1 T
T
refJ
1
1
6
- 10 k
2
τ
2
(1)
where T = 298K. This equation may be derived from the activated diffusive relaxation model, described by Goodwin (8), and has been successfully used to describe the temperature dependence of anionic surfactant solutions (9). This model establishes that, in disperse systems at a very low shear gradient, the relative viscosity is defined by a linear superposition of stresses, some due to hydrodynamic interactions and others to the structural relaxation of the uniform distribution of micelles in the quiescent micellar dispersion. This equation is: ref
ïlrei =q i+Jexp(E/kT) re
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
(2)
14. GALLEGOS ET AL.
219
Rheology of Sucrose Ester Aqueous Systems
The contribution due to the hydrodynamic interactions is expressed as follows 1
q^O-kpd.)- '*'
(3)
where k is the crowding factor, [η] the intrinsic viscosity, and φ the volume fraction of the micelles. The parameter J is defined as p
J = /z/167ir| a
3
(4)
w
where h is Planck's constant divided by 2π, η is the dynamic viscosity of the continuous phase and a is the mean micellar radius. As previously reported for anionic surfactant solutions (9), if the activation energy for long-range diffusive motion is expressed by: λ ν
E =E - ^
(5)
0
where the parameters E and A do not change with temperature within the range studied, then the Goodwin equation becomes: 0
J " ^el-qrel 1 hrel(T f)-qrelJ re
=
(Vfi U J|_
T
J Li _ f T J {k J T ref
2
1_ T*r
(6)
Since experimental viscosity values fit equation 1 fairly well, the value of q i may be expected to be unity. Then equation 6 yields equation 1, where kj = ( E / k) -10" and k = (A / k) · 10" . The values of these parameters are shown in Table I. A dramatic increase in the values of these parameters is observed for concentrations higher than 25%wt sucrose laurate. This increase coincides with a significant change in the slope of the reduced viscosity as a function of the sucrose ester concentration (JO). This change in slope has been related by other authors (77) to micelle overlapping. The fit of specific viscosity values for different surfactant concentrations to equation 1 is shown in Figure 1. For sucrose ester concentrations of 20%wt or greater, a concentration dependent maximum is observed in the viscosity versus temperature curve. This relationship between specific viscosity and temperature can be related to an re
3
0
6
2
TABLE I. Values of Goodwin's Parameters for Sucrose Laurate Newtonian Aqueous Systems k (K2)
Concentration (%wt) 10 15 20 25 30 35 40 45
2
11.7 11.3 13.5 12.2 16.8 19.4 27.1 28.4
1.91 1.82 2.12 1.92 2.60 2.98 4.09 4.22
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
220
STRUCTURE AND FLOW IN SURFACTANT SOLUTIONS
increase in micellar size as temperaturerises,as previously reported for other nonionic surfactants, such as polyoxyethylene alkyl ethers (72). This micellar growth is associated with the progressive breakdown of hydrogen bridge-bonds between water molecules and surfactant hydroxyl groups with temperature. The presence of a maximum specific viscosity, which appears at lower temperatures as surfactant concentration increases, results from a balance between two different processes: an increase in specific viscosity with temperature due to a larger micelle size, and a decrease in specific viscosity due to the fact that the increased thermal motion tends to disorient the particles and, consequently, weakens the strength of the micelle-micelle attractive interaction. An increase in surfactant concentration implies a progressive decrease in micellar hydration, which means that the preferential contribution of micellar growth to the specific viscosity is gradually displaced towards lower temperatures. Entangled Micellar Systems and Lamellar Liquid-crystalline Phases. Subsequent increases in concentration produce a dramatic change in the viscous behavior of the sucrose laurate system. Similar complex viscous behavior may also be found using other SE (sucrose oleate, sucrose palmitate, sucrose stéarate) but at much lower surfactant concentrations. For instance, systems containing 1 and 2%wt sucrose stéarate show a power-law decrease in viscosity with shear rate, although a tendency to a constant viscosity region (η*,) occurs at high shear rate. At higher sucrose stéarate concentrations the flow curve shows constant viscosity values at low shear rates, up to a critical value that depends on the surfactant concentration. Above this critical shear rate a shear-thinning behavior is observed. These results fit the Carreau model A (73) fairly well, as is observed in Figure 2. 2
5-=l/[H-( Y) ]
s
(7)
tl
Ίο where γ is the shear rate, η is the limiting viscosity at low shear rates and t is a characteristic time of the system. In Table II limiting viscosities η , critical shear rates γ , and the values of the parameter S (related to the slope of the shear-thinning region) are shown as a function of sucrose stéarate concentration. It can be observed that the limiting viscosity remains constant up to 10%wt SE. Between 10 and 35%wt SE, a nearly linear increase in the limiting viscosity occurs. Moreover, the critical shear rate, reciprocal of parameter ^ in equation 5, rises sharply with surfactant concentration up to 10%wt. Then the rate of increase slows so that a smooth maximum is reached around 30-35%wt SE. Finally, no significant differences are observed in the parameter S with increasing surfactant concentration below 35%wt sucrose stéarate. However, around that and at higher concentrations a decrease in S is noticed. Oscillatory shear experiments were also carried out on these samples. First, strain sweep tests were carried out to search for the linear viscoelastic domain. There is a significant linear viscoelastic range for systems having less than 15%wt SE. Moreover, the sample containing l%wt SE exhibited dramatically lower values of G' and G". The results obtained in frequency sweep tests show that, at low frequency, G' and G" become proportional to the frequency squared and to the frequency, 0
t
0
0
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
14. GALLEGOS ET AL.
Rheology of Sucrose Ester Aqueous Systems
221
Specific Viscosity 70 60
0 15% Δ 20% 025% #30% V35%
Goodwin model
50 40 30 20 10 275
-O285
295
305
315
325
335
Temperature (K)
Figure 1. Specific viscosity values versus temperature for sucrose laurate aqueous solutions at different concentrations.
Viscosity / mPa.s 10,000
1,000 ο
