КОЛЛОИДНЫЙ ЖУРНАЛ, 2007, том 69, № 2, с. 277-283
УДК 541.18
STUDIES ON THE MIDDLE-PHASE MICROEMULSION OF LAURIC-N-METHYLGLUCAMIDE
© 2007 Xiao-Deng Yang, Yan-Hong Gao, Jin-Ling Chai, Zhong-Ni Wang, Cheng-Kuan Qin
Department of Chemistry, Shandong Normal University Jinan, 250014 China E-mail: jlchai99@sina.com Поступила в редакцию 18.01.2006 г.
The middle-phase behaviour and the solubilization power of quaternary system lauric-N-methylglucamide/al-
cohol/alkane/water have been studied. A series of phase inversions of Winsor I (2) —*- III (3) —*- II (2) were observed from the fishlike phase diagram. The compositions of the hydrophile-lipophile balanced interfacial layer in the middle of the middle-phase body were calculated by the HLB plane equation. The effects of different alkanes, alcohols and concentration of NaCl solution on the phase behaviour and solubilization power were investigated, which indicates that alkanes with short hydrocarbon chains and alcohols with long alkyl chains have large solubilization power. The concentration of NaCl solution has small influence on solubilization power, whereas, the larger the NaCl concentration, the smaller the weight fraction of alcohol needed to balance the hydrophile-lipophile layer.
INTRODUCTION
Microemulsion is thermodynamically stable, isotropic mixtures of water, oil, alcohol and surfactant and is used in a variety of industrial applications (e.g. enhanced oil recovery, pharmaceutics, cosmetics, nano-particle synthesis and chemical engineering) [1, 2], and its phase behaviour has been under study for several decades. The more commonly used non-ionic surfactants to produce microemulsions are the ethylene oxide-based compounds (CE) [3-8]. In microemulsion systems containing QEj surfactants, a partial dehydration of the oxyethylene chain of CiEj molecules may be caused and phase inversion may be promoted by temperature, this sensitivity to temperature can be a disadvantage in certain fields of application.
N-alkyl glucamides (NAGAs) are biodegradable, less sensitive to temperature and easy to synthesize from renewable raw materials such as reducing sugars and fatty acid esters [1, 9-11]. They are an interesting class of non-ionic green surfactants and have received attention in recent years [11-17]. NAGAs/acid/chloro-carbon/water microemulsion system was studied and applied to surfactant-enhanced aquifer remediation [11, 12]; many chlorocarbons and organic acids in the microemulsion system were observed to generate classical Winsor I —- III —► II phase behaviour. General properties, such as (dynamic) surface tension and critical micelle concentration etc., of glucamide surfactants, possessing two n-alkyl chains and two glucamide head-groups have been studied [16]. The adsorption kinetics of octanoyl-N-methylglucamine at air/solution interface by means of maximal bubble pressure method was also studied [17].
Surfactant systems, which form microemulsions containing equal amounts of oil and water, are termed balanced. The microstructure of these systems is typically bicontinuous and consists of a monolayer of surfactant of near zero mean curvature separating oil domains from water domains. In this paper, the detailed microstructure of the balanced interfacial layer in the middle of the middle-phase region of lauric-V-methyl-glucamide (MEGA-12) microemulsion systems have been studied.
EXPERIMENTAL
Materials and Apparatus
Lauric-N-methylglucamide, C12H25CON(CH3)CH2(CHOH)4CH2OH, was synthesized from N-methylglucamine and lauric acid methyl ester ourselves, and was recrystallized twice with alcohol/acetone mixture. The other materials used in this study are all of A.R. grade and are used without further purification. Water was doubly distilled.
An FA1104 electron balance, a 501 super thermostat and an ultra centrifuge were used in this experiment.
Methods
Samples were prepared by weighing MEGA-12 into Teflon-sealed glass tubes, equal water and oil were weighed into tubes, and the total volume of the solution was fixed at 10 ml. The weight fractions of MEGA-12 in aqueous solutions were ranged from 0.0046 to 0.0722. n-Butanol was injected into the tubes and all samples were allowed to equilibrate at 40°C in a water
Fig. 1. Fishlike phase diagram for the quaternary systems MEGA-12/ra-octane/ra-hexanol/5% NaCl solution (O) and MEGA-12/ra-dodecane/ra-butanol/5% NaCl solution (■) at 40°C and a = 0.5.
bath for one to two weeks. The phase behaviour of the NAGAs based microemulsion is less sensitive to temperature and a little higher temperature (40°C) than room temperature was chosen in this study in favour of the phase separation. Phase equilibrium was determined by visual inspections of a large number of samples with different overall compositions.
RESULTS AND DISCUSSION
Fishlike Phase Diagram
The phase behavior in a cut through the multidimensional phase diagram defined by a constant temperature, pressure and water-to-oil ratio was studied. The methodology used here was introduced by Kahlweit and his coworkers [18] for a quaternary system surfactant (S)/alco-hol (A)/oil (O)/water (W). The variables are defined as the weight fraction of oil in oil/water mixture, a = O/(O + W), the total weight fraction of the surfactant and alcohol in the quaternary system, y = (A + S)/(A + S + O + W), and the weight fraction of alcohol in surfactant/alcohol mixture, 5 = A/(A + S). Making a two-dimensional phase diagram for a quaternary mixture requires that three of the five variables T, P, a, y, and 5 be held constant. If T (40°C) and a (0.5) are constants (with pressure P constant at ambient), y is plotted horizontally, and 5 is plotted vertically, a two-dimensional y-5 phase diagram can be obtained [19]. The y-5 phase diagram is usually called fishlike phase diagram.
The fishlike phase diagram for quaternary system MEGA-12/alcohol/alkane/5% NaCl solution at 40°C and a = 0.5 is shown in Fig. 1. It can be seen from Fig. 1 that increasing 5 at constant y causes a series of phase
inversions Winsor I (2) —- III (3) —► II (2). When the concentration (weight ratio) of alcohol is low, an oil-in-
water microemulsion in contact with excess oil (2) exists; as alcohol concentration increases, the phase inverts into water-in-oil microemulsion in contact with
excess water (2) via a middle-phase microemulsion in contact with excess oil and water (3).
The phase inversion in Fig. 1 is mainly caused by the dissolve of alcohol in the interfacial layer and oil phase. Alcohol can act both as a cosurfactant and as a cosolvent in microemulsion systems [20]. As a cosur-factant, alcohol is incorporated into the interfacial layer, changing the curvature of the amphiphile layer from positive values (i.e., oil on the concave side of the interfacial layer) toward negative values (i.e., water on the concave side of the interfacial layer). Accordingly, alcohol is expected to cause a transition from oil-in-water droplet microemulsion (2) to water-in-oil droplet microemulsion (2). As a hydrophobic cosolvent, alcohol partitions predominantly into the oil and causes the oil to become more polar. As a consequence the surfactant will also partition more and more into the oil phase where it can support a water-in-oil droplet microemulsion. So alcohol as a hydrophobic cosolvent will also
promote a series of phase inversions 2 —► 3 —► 2.
The middle-phase region in Fig. 1 is shifted to the higher 5 at lower y, which shows that much higher proportions of alcohol are necessary to reach middle-phase region at lower y It is a direct consequence of the competition between the incorporation of alcohol molecules into the interfacial layer and its solubility in the bulk oil phase. When alcohol is added to the system, part of the alcohol is dissolved in the oil and not available as cosurfactant. This preferential extraction of it from the amphiphilic mixture, which is composed of surfactant and alcohol, leads to a decreasing lipophilic-ity of the interfacial layer. The fraction of alcohol extracted in the oil phase increases with decreasing y In order to compensate for this dissolved alcohol, a higher fraction of alcohol is needed to obtain a balanced interfacial layer.
Calculation of Related Parameters
The hydrophile-lipophile property of the microemulsion system is just balanced in the midst of the middle-phase region [21, 22]. The midst line of the middle-phase region (Fig. 1) can be characterized by
the locus of 5, the mid-point value of 5 in the middle-phase region for given values of a and y The HLB plane equation [19] can be expressed as
ô = As + FaI i- 1
F =
AOSs - S0As 1 - S0 - A0 '
5 1.0
O 1.0
1- 1
0.4 0.2 0
Fig. 2. Plots of 8 vs --1 for the 8 and Y values in the middle
of the middle-phase body at 40°C and a = 0.5 for the systems MEGA-12/ra-dodecane/ra-butanol/5% NaCl solution (1) and MEGA-12/ra-octane/ra-hexanol/5% NaCl solution (2).
Fig. 3. Volume fractions, O, of the middle-phase at the midpoint of the middle-phase region as a function of Y at 40°C and a = 0.5 for the systems MEGA-12/ra-octane/ra-hex-anol/5% NaCl solution (1) and MEGA-12/ra-dode-cane/ra-butanol/5% NaCl solution (2).
where SO and A° are, respectively, solubilities of surfactant and alcohol in oil, SS and AS denote the weight fractions of surfactant and alcohol in the surfactant plus alcohol mixtures in the balanced interfacial layer, respectively. It can be seen from Fig. 2 that a plot of 8 vs
1 - 1 is indeed a straight line. Y
The "fish head" (point B) is the start point of the middle-phase microemulsion and the "fish tail" (point E) is the intersection between three and single-phase regions. The composition of these two points cannot be determined directly with acceptable accuracy, as the volumes of the middle phase become zero at the "head" (YB, 8b) and the volumes of the two excess phases become zero at the "tail" (ye, 8e). The volume fractions,
O, of the middle phase at the mid-point 8 are measured for a serie
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