Langmuir 1993.9,956-961
956
Physical Stability and Microstructure of Concentrated Dispersions of Lamellar Liquid-Crystalline Droplets Containing Nonionic Surfactants in Aqueous Electrolyte Solution F. J. Schepers,+W. K. Toet, and J. C. van de Pas' Unilever Research Laboratorium Vlaardingen, Olivier van Noortlaan 120, 3133 A T Vlaardingen, The Netherlands Received October 12, 1992. I n Final Form: January 25, 1993 The formation and physical properties of aqueous dispersions of lamellar liquid-crystalline droplets, containingtwo nonionic surfactants, have been investigated. The formation of the "lamellar phase droplets" is determined by the geometric packing constraints of the surfactants used. Physically stable lamellar systems are obtained if, considering both (types of) surfactant molecules, the surfactant headgroup area is smaller than or equal to twice the all-transcross-sectionalarea of the hydrophobicalkyl chain. Microscopic observations, the rheological behavior of the systems (the fact that they are strongly shear thinning), and the observation that their viscosity increases strongly with the volume fraction of the lamellar phase all indicate that the physically stable systems are dispersionsof spherical droplets, each consistingof concentric hydrophobic layers alternated with water layers. The water layer thickness, and therefore the volume fraction of the lamellar droplets, is determined by the interaction forces between the lamellar layers and can be influenced by changingthe solvent quality for the hydrophilic headgroups of the nonionic surfactant. The effects of electrolyte concentration, chemical composition of the surfactant headgroup, and the temperature can be rationalized using this approach.
Introduction Liquid detergent products containing suspended solid materials (e.g., phosphate or Zeolite 4A particles) have been on the market for many years. To combine good pourability and good physical stability against sedimentation of particles, a concentrated dispersion of lamellar liquid-crystalline droplets can be used as the suspending base of these products. The rheology, physical stability, and microstructure of such lamellar dispersions consisting of water, anionic surfactant, nonionic surfactant, and electrolyte have been investigated previously.'v2 Physical stability of a sample is meant to indicate that the system shows no macroscopically visible phase separation during (at least) 1-weekstorage a t room temperature. Physically stable lamellar dispersions (i.e., those with a volume fraction of lamellar phase (qbam)higher than 0.6) were only obtained for certain anionic surfactanthonionic surfactant ratios and in the presence of large amounts of electrolyte.lv2 Lamellar systems are generally found if the surfactant headgroup area is smaller than twice the all-trans crosssectional area of the alkyl chains of the ~ u r f a c t a n t sThe .~~~ average headgroup area of both anionic and nonionic surfactants is influenced by electrolyte. At high electrolyte concentration the anionic surfactant headgroups are small enough to obtain the required average headgroup size of the surfactant mixture. In systems containing anionic and nonionic surfactantsat a sufficiently high electrolyte level the thickness of the water layers of the lamellar phase depends on the solvent quality of the continuous electrolyte phase for the ethoxylated headgroup of the nonionic ~~
* To whom correspondence should be addressed.
+ Present address: Unilever Research US.-Edgewater Laboratory 45 River Rd., Edgewater, NJ 07020. (1)Jurgens, A. Tenside, Surfactants, Deterg. 1989,26, 222. (2) Van de Pas,J. C . Tenside, Surfactants, Deterg. 1991, 28, 158. (3) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (4) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L. J . Chem. SOC.,Faraday Trans. I 1983, 79, 975.
surfactant.2 In a good solvent, the nonionic surfactant headgroup is well hydrated and physically stable lamellar dispersions with thick water layers are found, while in a poor solvent, physically unstable lamellar systems with thin water layers are found. At relatively low electrolyte levels, the electrostatic interaction forces between the anionic surfactant headgroups must also be taken into account. The presence of nonionic surfactant headgroups makes it difficult to calculate the area of the anionic surfactant headgroups under these conditions. To avoid these complications, we have investigated the phase behavior of systems containing two nonionic surfactants in water. By mixing nonionic surfactants (with large headgroups) with nonionic cosurfactants (with small headgroups), we can adjust the average areas of the surfactant headgroups. Because electrostatic interactions are absent, it will be easier to check whether the formation of the lamellar dispersion is also governed by the surfactant headgroup area. For the "all-nonionic surfactant" systems discussed in this paper we have investigated the effects of electrolyte concentration, storage temperature, and chemical composition of the surfactant headgroup on the phase behavior and physical stability of the lamellar dispersions. Experimental Section
Materials. Two types of nonionic surfactant have been used in this study: a C13-15alcohol ethoxylate with on average 7 mol of oxyethylene (Clbl&O, Synperonic A7, ICI) and fatty alcohol polyglucosides with a low degree of polymerization, obtained from different suppliersas concentratedaqueous solutions.APG 23-1 (A.E.Staley MFG Co., UK) is a C12-13alcohol condensed with (on average)1.4 glucose units;APG Type BW 2761 ZA (Has AG, Marl, BRD) has a degree of polymerization of 1.1-1.3, APG6OO (Henkel,KGaA, Diieseldorf, BRD) consists of a C12-16 alcohol with on average 1.4 glucose units. As cosurfactants we employed 1-dodecanol (p.a.; Merck, Darmstadt, BRD), 1,2dodecanediol (p.a.; Aldrich Chemie, Steinheim, BRD), and glycerol alkyl ether. The glycerol alkyl ether was prepared by reacting a C S - alcohol ~~ (Dobanol-91, Shell) with 1-chloro-2.3-
0'743-746319312409-0956$04.00/0 Q 1993 American Chemical Society
Langmuir, Vol. 9, No. 4,1993 967
Dispersions of Lamellar Liquid-Crystalline Droplets epoxypropane using an antimony pentachloride ~ a t a l y s t . A ~ glycerol ether synthesized from pure n-decanol was supplied by Unichema (Gouda, The Netherlands); sodium citrate dihydrate (p.a.; Merck, Darmstadt, BRD) was selected as the electrolyte in our systems. All materials were used without further purification. The surfactant headgroup areas were, with the exception of C1+1&0,calculated from surfacetension measurements6using the Gibbs adsorption isotherm.' Construction of Phase/Physical Stability Diagrams. Several phase/physical Stability diagrams with four components (water,nonionicsurfactant, nonionic cosurfactant,sodiumcitrate) have been constructed. All compositions consist of (on a weight basis) 80 parte water, 20 parts surfactant, and, in addition, various amounts of sodium citrate dihydrate. The systems with ethoxylated nonionic surfactant were prepared by mixing first the surfactant and cosurfactant at 50 "C. The mixing ratio (weight parte) of these two surfactants was varied between 200 and 1 0 10. Each of these surfactant premixes was then dispersed into water at room temperature. Sodium citrate was added stepwise in portions of 2-5 % After each step the sample was stored for at least 1week to assess the physical stabilityand phase structure and to measure the conductivity and viscosity. The systems with APG were prepared by mixing the supplied APG solution with water or electrolyte solution before addition of the second surfactant. Characterization of the Phase Structure. For a preliminary characterization of the phase structure, samples were examined with a Zeiw Universal light microscope using polarized light. Some systems were also investigated by electron microscopy. Twosample preparation techniques have been used freeze fracture with replication and the thin film technique.* The latter experiments have been carried out in collaboration with the University of Maastricht. A measure for the din, can be obtained by conductivity measurements using the procedure described by Br~ggeman.~ The conductivities of the samples have been measured at 25 "C. Furthermore 'simulated" continuous phases of the samples were made by mixing all ingredients without the surfactants. The conductivities of these electrolyte solutions were also measured. An estimate for the conductivity of the lamellar droplets was obtained by centrifuging samples with various electrolyte concentrations for 16 h at 4oooOg and measuring the conductivity of the lamellar top layer. Values between 0.03 and 0.3 mS/cm were obtained in this way. An average value of 0.1 mS/cm has been used for the conductivity of the lamellar phase in the present work. The dlnmof the samples without sodium citrate was obtained by measuring the volume of the phases obtained after centrifugation of the sample at 4oooOg for 16 h. Viscosity Measurements. Most of the physically stable samples were characterized by measuring the viscosity at a shear rate of 21 s-I in'a H a d e Rotovisco RV20, at 25 "C. This shear rate has been chosen because it is in the range where pouring processes take place.1° Equilibrium values were registered after a 30-min measuring time. I t is very difficult to measure viscosities of physicallyunstable samples since their viscosity often depends on time effects and their shear history. The influence of the storage temperature on the rheological properties was investigated using a Bohlin CS rheometer. The shear stress was increased stepwise from 1to 45 Pa, resulting in a range of shear rates varying between about 0.2 and 800 s-I.
.
Results and Discussion Variation of the Cosurfactant. The phase/physical stability diagrams of the four-component systems water, (5) Farkaa, L.; Morgos,J.; Sally, P.; Rusznak, I.; Bartha, B.; Veress, G. J . Am. 081 Chem. SOC.1981,58, 650. (6) Hall, P. J. Personal Communication. (7) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley: New York, 1978; Chapter 2.111. (8) Frederik, P. M.; Stuart, M. C. A.; Bomans, P. H. H.; Busing, W. M. J . Microsc. 1989, 153, 81. (9) Bruggeman, D. A. G. Ann. Phys. 1935, 24,636. (10) Barnes, H. A.; Hutton, J. F.;Waiters, K. An Introduction to Rheology; Elsevier Science: Amsterdam, 1989; p 13.
Na-citrntr.ZH,O Ipartal
IPMI)
Iwr)
Figure 1. Phase/physical stability diagram of Cla-lsE0/ 1-dodecanol/water/sodium citrate systems at room temperature. The composition is the following(parts by weight): 20 surfactant (mixture),80 water, and variable amounts of electrolyte. Drawn lines represent phase boundaries. The dashed line represents the physical stability border. L = isotropic phase, 2L = two isotropic phases, LAM = lamellar droplet phase, and C = crystals. Figures above dots represent &,,, of a particular system. Nccitmw ZHP
'Ol
S n ZL+IAM cyll-=-L ~ L A M0.30 +C 6
0,30
.
r O,EE
4
L+IAM+C
L+IAM\
1
2
\
0
rtabl area L+UM
0.6 mole frmlon
0.4
0.2
Figure 2. Phase/physical stability diagram of C13-15E0/ 1,2-dodecanediol/water/sodium citrate systems at room temperature. For further explanation see legend to Figure 1.
.
Na-citratr.ZH.0
0.30
0.40 8
0.m
0.38 8
L+IAM
0
0:2
0.4
O h mole fraction
Figure 3. PhaWphysical stability diagram of Cl+l&O15E0 model system a t different storage temperatures: ( 0 )a t 25 O C with 4lam = 0.81, (H) a t 38 "C with #lam = 0.87, and (A)a t 52 "C with @lam = 0.71. At 52 "C the system is flocculated. The composition in parts by weight is the following: 14 C13-15EO, 6 glycerol decyl ether, 80 water, and 5 sodium citrate.
test whether this is also true for our APG-containing systems, the viscosity and #lam of two systems containing either APG or C1g15E0,in combination with glycerol n-decyl ether, were compared at different temperatures. The viscosity as a function of shear rate and the conductivity were measured after 1month of storage at 25,37, and 52 "C. The results are plotted in Figures 9 and 10. To eliminate the effect of the continuous phase, the results are expressed as relative viscosities (i.e., viscosity of the (26) Kahlweit, M.; Strey, P.; Haase, D. J. Phys. Chem. 1985,89,163171.
Dispersions of Lamellar Liquid- Crystalline Droplets dispersion divided by the viscosity of the continuous phase). The relative viscosity of both systems decreases stronglywith increasing shear rate. For the APG/glycerol ether system (Figure 9)the relative viscosity in the higher shear rate range increases with temperature, which is probably due to an increase of the 41- with temperature. Figure 10 illustrates that for the C1&30 system a temperature increase from 25 to 37 "C hardly influences the relative viscosity, despite the increase in 4 b . At 52 "C dehydration of the poly(oxyethy1ene) headgroup of results in a strong decrease in 41- and the c13-&0 strong flocculation of the droplets. Because of the fast phase separation, the viscosity could only be measured at high shear rates. The relatively high viscosity suggests that strong aggregation of droplets has taken place. It appears that an increase in either temperature or electrolyte concentration has similar effects on the phase behavior of each system. The effects of electrolyte and temperature on C1slbEO-containingsamples is to cause dehydration of the ethoxylated headgroup, while the effect on APG-containing samples is to increase the volume fraction of the lamellar phase. This difference in behavior between APG and C1&30is probably caused by the difference in sensitivity to dehydration by electrolyte between the glucosideheadgroup of the APG and the poly(oxyethylene) headgroup of the C13-1&0.
Conclusions The formation of lamellar dispersions consisting of nonionic surfactants is determined by the geometric packing constraints of the surfactants. Physically stable lamellar systems are obtained if, considering all surfactant molecules, the average surface area per hydrophilic headgroup is roughly equal to twice the all-trans crosssectional area of the hydrophobic alkyl chain. Light and electron microscopic results, the rheological behavior of the systems (stronglyshear thinning), and the fact that their viscosity strongly increases with 41- all
Langmuir, Vol. 9,No. 4, 1993 961 indicate that the physically stable systems are dispersions of spherical droplets, each consisting of concentric hydrophobic layers alternated with water layers. The water layer thickness (and therefore the volume fraction) of the lamellar droplets is determined by the interaction forces between the lamellar layers and can be influenced by changing the solvent quality for the hydrophilic headgroups of the nonionic surfactant. The physical stability of the systems is mainly determined by the resistance of the surfactant headgroup toward dehydration at a high electrolyteconcentration and/or elevated storage temperature. The effects of storage temperature and electrolyte concentration on the phase behavior of these samples are similar: both can influence the solvent quality for the nonionic surfactant headgroups.
Acknowledgment. The authors wish to thank Dr. P. J. Hall (Unilever Research Laboratory, Port Sunlight, U.K.) for measurements and calculations of surface areas per headgroup of the nonionic cosurfactants, A. Visser (Unilever Research Laboratorium Vlaardingen, The Netherlands) for the synthesis of the glycerol Cs-11 ether, and E. C. Royers (Unilever Research Laboratorium, Vlaardingen, The Netherlands) for the measurement of the X-ray diffraction patterns. We further wish to thank Dr. I. Heertje (Unilever Research Laboratorium, Vlaardingen, The Netherlands) and Dr.P. M. Frederik (University of Maastricht, the Netherlands) for their contribution to the electron microscopicstudies. Prof. Dr. J. B. F. N. Engberta (University of Groningen, Groningen, The Netherlands) is thanked for useful comments on this work. Two practical consequences of the present work have been the subject of patent applications: (i) the use of "allnonionic surfactant" structured liquid detergents (WO 91/ 00331,priority date 26/06/89,Unilever) and (ii) the use of APGs to prepare physically stable, structured liquid detergents at a high electrolyteconcentration (EP 359308, priority date 16/09/88,Unilever).
