Langmuir 1989,5, 1344-1346
1344
Rheological Properties of Lamellar Lyotropic Liquid Crystals S. Paasch, F. Schambil,* and M. J. Schwuger Henkel KGaA, 0-4000 Diisseldorf - 1, FRG Received January 27, 1989. I n Final Form: May 18, 1989
Lamellar lyotropic liquid crystals formed by nonionic surfactants and water are plastic fluids with very high yield values (up to 90 Pa). Depending on the pretreatment, orientation phenomena are observed, which are due to anisotropic rheological properties.
Introduction Surfactants are of great importance in both the chemical industrial and the consumer sectors. Thus they are used in detergents and cleaning agents and as emulsifiers in food, pharmaceuticals, and cosmetic products, as well as in many products for treating surfaces. Many applications make use of the surface activity of these substances in diluted solutions (e.g., washing, degreasing metal surfaces). On the other hand, in some applications, and especially in the formulation of fluid products, surfactants are used in high concentrations. In such cases, the rheological properties of these mixtures are of interest for the user. Whereas surfactants aggregate to micelles in low concentrations, in high concentrations they often form lyotropic liquid crystals with water, alcohols, fats, and hydrocarbons. In aqueous systems, hexagonal and lamellar structures in particular are often encountered, as well as cubic and nematic phases.'-3 Whereas the hexagonal liquid crystals are highly viscous phases that give rise to production and application problems, the lamellar phases are of low viscosity and are used in many chemical industrial products and market articles in the detergent and cleaning agent sector. In particular, lamellar structures often occur in the continuous phase of dispersed systems such as emulsions and suspensions and strongly influence their rheological properties. Despite their great importance, the rheological properties of lamellar lyotropic liquid crystals have not yet been systematically studied as a function of their composition. Investigations of the shear viscosity and its temperature dependence are reported in ref 4. In this paper, the results of studies on the flow behavior of lamellar liquid crystals formed by nonionic alkyl polyethylene glycol ethers with the general formula C,.jEO (i,number of C atoms in the alkyl chain; j , number of ethylene oxide units) and water are presented.
Theoretical Principles The flow behavior of a fluid can be described by flow or viscosity curves. On a flow curve, the shear rate y of a sample is plotted against the shear stress 7. If the flow curve passes through the orign of the coordinate system and a linear relationship between shear stress and shear (1) McBain, J. W.; Langdon, G . M. J. Chem. SOC. 1925,127, 852. (2) Luzatti, V.; Mustacchi, H.; Skoulios, H.; Husson, F. Acta Crystallogr. 1960, 13, 660. (3) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. 1988, 100, 117. (4) Oswald, P.; Allain, M. J. Colloid Interface Sci. 1988, 126, 45.
rate is observed, the fluid is said to be Newtonian. For a Newtonian fluid, the constant of proportionality between y and 7 is the viscosity q , which is independent of the shear rate. Non-newtonian fluids, on the other hand, show no linear relationship between y and 7. A special group of non-Newtonian fluids is made up of fluids with a yield value. A yield value is the lowest shear stress necessary to produce viscous flow. Substances that have a yield value are known as plastic fluids. The viscosity of non-Newtonian fluids is dependent on the shear rate and is also termed apparent viscosity qaPp. For fluids with.a yield value, the apparent viscosity approaches m as y 0.
-
Experimental Section Alkyl polyethylene glycol ethers form lamellar liquid crystals with water;5the existence areas of these phases in the binary systems can be extended? Comparative studies can be made by varying the alkyl chain length and the degree of ethoxylation of the surfactant. In the context of this paper, studieswere carried out on lamellar liquid crystals of the surfactants
CI0*3EO,ClO.4EO,CI2*3EO,Cl2.4EO, and CI2.5EO. Alkyl and EO chain pure alkyl polyethylene glycol ethers were obtained from Nikko Chemicals Co. of Japan.and used without further purification. The liquid crystals were produced by stirringthe surfactantswith double distilled water under reduced pressure to avoid the introduction of air bubbles into the samples. After several days of equilibration, the flow properties of the liquid crystals were determined with the aid of a shear stress controlled rheometer (Carri-Med, Great Britain) with a cone plate measuring system at 20 "C. Measurements were made while increasing and decreasing the shear stress (ascending and descending curves),and subsequently the same samples were measured again.
Results and Discussion The rheological measurements of lamellar liquid crystals indicate that the phases studied are, without exception, plastic systems. As an example, flow curves obtained from measurements of CI2-4EO,60% by weight, are reproduced in Figure 1. The first measurement on the ascending curve indicates a clearly higher yield value than is given by the descending curve. When the measurements are repeated, the amount of the yield value on the ascending curve corresponds to that on the first descending curve. This suggests that an orientation of the anisotropic phases took place in the shear field when they were first subjected to shear stress. The lower yield value (5) Mitchell, D. J.; Tiddy, G . J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. SOC.,Faraday T r a m . 1 1983, 79,975. (6) Andersson, B.; Olafsson, G . Colloid Polym. Sci. 1987, 265, 318.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 6,1989 1345
Lamellar Lyotropic Liquid Crystals shear stress [N/m2]
1
15.0 17,5 \
, 5
10
,
I
15
20
I
25
30
35
4/
,
,
40
45 50 shear rate [I /B]
Figure 1. Flow curves of C1,.4E0 (60 wt %)-water: (a) first
measuremenG (b) repeat measurement.
w
1
3
5
4
6
7
8
9 IO shaar rate [1 /a]
Figure 4. Shear viscosities of various liquid crystals in the system C12.4EO-water (repeat measurements):(a) 50%; (b) 65%; (c) 70%; (d) 75% by weight.
[pal
D 0 I
0,02
0,W
0,06 0,08
0,l
0,12
,
I
70
-
60
-
0,2 mole fraction Clo.4E0
0,M
0,W
0,16
Figure 2. Yield values of the system Clo-4EO-water. M-
viscosity [Pa.s]
I
c
3
4
5
n EO
Figure 5. Yield values given by the ascending curves as a function of the length of the alkyl chain and the degree of ethoxylation (concentration 10 mol %).
3
4
5
6
7
I
D
8
10 shear rate [1 /a] 9
Figure 3. Shear viscosities of various liquid crystals in the system CI2.4EO-water (first measurements): (a) 50%; (b) 65%; (c) 70%; (d) 75% by weight.
obtained when the measurements were repeated corresponds to that of the aligned sample. Figure 2 shows the yield value given by the ascending and descending curves for C,,.4EO as a function of the surfactant concentration. When first subjected to shear stress, the samples gave yield values up to 80 Pa; when the measurements were repeated, these values sank below 20 Pa. The yield value increases with the surfactant content of the lamellar phase, in order to fall rapidly into the two-phase region L, + L2. Before the first measurements are taken, the orientation of the anisotropic phases is not defined. Therefore, their rheological properties vary distinctly. This is not the case with the oriented samples; there is scarcely any recognizable concentration dependence; the yield values are around 10 Pa. Similar results are obtained for the shear viscosity as a function of concentration. For the freshly produced samples, not previously subjected to shear stress, the viscosity increases systematically as the concentration of the
surfactant is increased (Figure 3). After being stressed, the samples show distinctly smaller variations in viscosity when the measurements are repeated (Figure 4). This also suggests that the viscous flow of the oriented samples occurs by sliding of layers whose properties are little affected by the composition. Figure 5 shows the yield values of samples of the same composition (10 mol %), obtained from the ascending curve, as a function of EO and C chain length. As the EO number increases, the yield value increases by about 10 Pa in the case of C,, ethoxylates and about 5 Pa in the case of Cl0 surfactants. The value for the C,, polyethylene glycol ethers is higher than the values of the CI2 ethoxylates; this is probably a result of the higher proportion of EO chains in the molecule. The same relationship holds true at different concentrations. In contrast, the yield values of the oriented samples at a surfactant concentration of 10 mol ?& show no dependence of the type of surfactant. The liquid crystals formed from the various surfactants have all roughly the same value of approximately 10 Pa. This is surprising, since the lengths of the polyethylene glycol and alkyl chains apparently have no influence on the plastic properties of the oriented liquid crystals.
Conclusions The investigations of the rheological properties of the lamellar lyotropic liquid crystals have shown that these
Langmuir 1989,5, 1346-1350
1346
substances exhibit a complex flow behavior that depends on the anisotropic structure of these mesophases. The orientation of the lamellar phases especially depends on the mechanical pretreatment of the samples. These aspects must be considered for the handling of
such fluids in technological processes and their use in consumer products. Registry No. ClO-3EO, 4669-23-2; Clo*4EO, 5703-94-6; C,,.3EO, 3055-94-5; C,,.4EO, 5274-68-0;C,,.5EO, 3055-95-6.
Aggregation of Silica Using Cationic Surfactant: A Neutron-Scattering Study K. Wong,+ B. Cabane,* R. Duplessix,§ and P. Somasundaran*pt Henry Krumb School of Mines, Columbia University, New York, New York 10027, DPC-SCM-UA331, CEN, Saclay, Gif sur Yvette, France, and Institut Charles Sadron, Strasbourg, France Received September 12, 1988. In Final Form: May 19, 1989 Neutron scattering is used to look at the structure of aggregates made of silica spheres flocculated with cationic surfactants. With single-chain surfactants, the structures show no local order, but on a certain length scale self-similar behavior is observed and is characterized by an apparent fractal dimension. It is not affected by changes in the surfactant’s chain length, but it increases as a function of the surfactant concentration. With a double-chain surfactant, reordering to a liquidlike structure and redispersion are observed. We consider the main features of this system, namely, charge neutralization and the heterogeneity of the adsorbed surfactant layer, and discuss the implications for the formation and structure of these aggregates.
Introduction Aggregation is a phenomenon of interest to scientists in a multitude of areas ranging from molecular biologists studying subunit association of proteins to mineral engineers selectively separating ore (Villar, 1975; Colombo, 1968). I t is also of principal importance in a number of industrial processes, among which are clarification of various liquids ranging from waste waters (Peters, 1986) to champagne, enhanced oil recovery (Somasundaran, 1985), and the synthesis of monodisperse particles (Vanderhoff, 1973). Despite such a multitude of applications, basic experimental information is lacking on the structure of aggregates and how the structure is determined by different chemicals which induce aggregation. We make use of small-angle neutron scattering (SANS) to record the interference patterns produced by the floc samples. These patterns result from the particular spatial arrangement of silica spheres in the sample; in principle, different arrangements produce different patterns. Neutrons are especially appropriate in these studies primarily due to the large dimensions which can be observed (currently to about 5000 &much larger than the spheres used) but also because they easily penetrate dense matter such as aggregates of silica. Recently, we have found two general classes of structures of silica aggregates (Figure 1); the first class consists of objects which have a liquidlike structure. These show a short-range organization with a full coordination shell at distances on the order of the particle diameter; they are formed when the surface charges are not neu-
’Columbia University. Gif sur Yvette.
* Institut Charles Sadron. 0743-7463/89/2405-1346$01.50/0
tralized by the flocculant, such as in the case of flocculation with polymers of low cationicity. The resultant structure represents an equilibrium between the attractive forces engendered by the bridging polymer and the repulsive ones arising from the residual charges on the spheres. The second class of silica aggregates consists of fractal structures. These, by contrast, show voids at all scales down to the first coordination shell, which is incomplete, and exist at all other distance scales up to textural dimensions. This is the case for aggregates flocculated by highly charged organic and inorganic polymers and multivalent salts: the surface charge of the particles is effectively neutralized by the adsorbing species, the particles stick on contact, and the final structure of the aggregate is determined by the kinetics of growth (Witten, 1981; Jullien, 1987). In particular, the positions of neighboring particles are fiied at contact, and the number of nearest neighbors is much lower than in the case of structures with short-range order. Thus we have studied the structure of aggregates made by (1)flocculation with organic and inorganic polymers and (2) coagulation by multivalent salts (Wong, 1988). We now report on the structure of silica aggregates induced by cationic surfactants.
Experimental Section Alkyltrimethylammonium bromide Surfactants (SIGMA) with single-chain lengths from 10 to 16 carbons, CloTAB-C,,TAJ3, and a 16-carbon double-chain acetate, (C,,),TOAc, were prepared in H,O at concentrationsof 0.02-0.05 mol dm-3. The monodisperse silica spheres (ca. 380- and 200-A diammeter) were prepared by precipitation of sodium silicate with sulfuric acid; in basic solutions,this silica is highly charged. To make aggregates, surfactant was added to a dilute suspension of silica in a D,O/H,O solution, and the mixture was shaken 0 1989 American Chemical Society
