Quaternary N-alkylaldonamide–brine–decane–alcohol systems. Part I: phase behaviour and microemulsions

Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 311–320

Quaternary N-alkylaldonamide–brine–decane–alcohol systems. Part I: phase behaviour and microemulsions F. Bastogne, C. David * Universite´ Libre de Bruxelles, Chimie des polyme`res et des syste`mes organise´s, CP 206/1, Boulevard du triomphe, 1050 Brussels, Belgium Received 8 August 1997; accepted 4 February 1998

Abstract N-alkylaldonamides belong to a family of surfactants which differ by the length of their hydrophilic and hydrophobic part and by the branching of their polar head. The purpose of this work was to search for microemulsions stabilized by these molecules in ternary N-alkylaldonamide–water–decane systems and to study the corresponding phase diagrams and the microstructure. This paper first describes a new methodology to determine the conditions required for the formation of microemulsions. This is based on the study of binary systems and their miscibility gap and simplifies the determination of the phase behaviour. N-alkylaldonamide–water and N-alkylaldonamide–decane phase diagrams did not show any critical temperature in the temperature interval investigated and the interplay of the miscibility gaps in the phase prism could not lead to a triphasic equilibrium. N-alkylaldonamide could not stabilize microemulsion in a ternary N-alkylaldonamide–water–decane system because it was too miscible with water, but not soluble enough in decane. The critical temperatures were modified by addition of a lyotropic salt (NaCl ) in water and of alcohol in decane so that the microemulsion in the triphasic equilibrium was found. It was, thus, shown that N-alkylaldonamides formed microemulsions in the quaternary N-alkylaldonamide–brine–decane–alcohol system. In the second part of this paper, corresponding pseudo-ternary phase diagrams, keeping the water-to-decane ratio constant, were established and compared. They present different kinds of equilibrium and a characteristic ‘‘fish shape’’ phase separation. The effect of the nature of the surfactant and of the alcohol on the phase separation was analysed. The branching of the polar head of the surfactant and the length of the alcohol have a large influence on the extent of the three-phase body in the phase diagram. These phase diagrams are the important basis of the study of the microstrucure of the microemulsion phases. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Microemulsion; N-Alkylaldonamide; Phase diagram

1. Introduction A broad series of N-substituted aldonamides have been synthesized in our laboratory [1,2]. The relationship between the chemical structure of these compounds and their ability to form thermo* Corresponding author. Tel: 0032 2 650 5406; Fax: 0032 2 650 3418. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 28 6 - 6

tropic and lyotropic phases has been well established [3,4]. After studying the behaviour of pure aldonamides and their binary mixtures with water, our present interest concerns mixtures of N-alkylaldonamide–water–hydrocarbon and the formation of microemulsions in such systems. A microemulsion is defined as a thermodynamically stable, optically isotropic dispersion of aqueous and hydrocarbon phases stabilized by the presence of

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a surfactant. In recent years, microemulsions have been extensively studied because of their physicochemical properties (solubilizing power, ultralow interfacial tension) and wide range of applications. The purpose of our work is the study of microemulsions and particularly of their microstructure. It is thus, at first, important to determine the conditions which lead to the formation of these microemulsions and to establish the phase diagrams of the corresponding systems. This first step of our study is the object of this paper. In a second part of the work, the microstructure of the microemulsion lying in the monophasic area of the phase diagram will be investigated by NMR and spectroscopic fluorescence. These topics will be developed in forthcoming papers. The results reported here concerns two surfactants that differ by the length of their hydrophobic part and the branching of their polar head:

N-decylisosaccharinamide (IsoN ): 10

N-octylribonamide (C N ):The hydrocarbon 5 8 phase is always decane (C10). The problem is to determine if these surfactants could form microemulsions in a ternary system Nalkylaldonamide–water–decane, and to search the required conditions to find this kind of equilibrium. Our approach consists of studying the corresponding binary systems and their critical points. Indeed, as shown by Kalwheit [5,6 ], the phase behaviour of multicomponent mixture is essentially determined by the features of corresponding binary mixtures. The origin of the separation of a ternary mixture into different phases arises from the interplay between the miscibility gaps of binary mixtures. It is, thus, possible to anticipate the existence of microemulsion phases without establishment of complicated ternary phase diagrams. Therefore, the first part of our paper involves the study of the binary mixtures N-alkylaldonami-

de–water and N-alkylaldonamide–decane and their critical temperatures in order to determine the conditions of formation of microemulsions. This method allowed us to show that N-alkylaldonamides form microemulsions only in a quaternary mixture N-alkylaldonamide–brine–decane– alcohol. We were, thus, able to go further with the second part of this work, the establishment of pseudo-ternary phase diagrams.

2. Experimental part 2.1. Synthesis N-octylribonamide was synthesized at room temperature by addition of stoechiometric amounts of N-octylamine to a methanol solution of (+)-ribonic acid-c-lactone. The crude product was recrystallized twice in methanol. N-decylisosaccharinamide was prepared by reaction at room temperature of stoechiometric amounts of N-decylamine and -c-glucoisosaccharinolactone dissolved in methanol. -c-glucoisosaccharinolactone was obtained by alkaline degradation of maltose. The crude N-decylisosaccharinamide was recrystallized twice in ethylacetate and twice in acetonitrile. 2.2. Phase diagrams 2.2.1. Binary and pseudo-binary diagrams Mixtures were prepared by weight in sealed tubes which were rapidly warmed to the isotropic phase. Then they were vigorously shaken and allowed to cool in a thermostated bath. The temperature of the bath was progressively raised and the phase behaviour of the mixtures was observed (temperature±0.5°C ) 2.2.2. Ternary and pseudo-ternary phase diagrams Phase diagrams were established by a progressive dilution method. The ratio of the mixture brine– decane to surfactant was kept constant. The experiments was started with compositions corresponding to equal-spaced points on the axis connecting the brine–decane and surfactant vertices. Tubes containing known amounts of each compounds were

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sealed, warmed to the isotropic phase, shaken and allowed to cool in different thermostated baths (25, 40, 60 and 80°C ). Alcohol was added in steps to each composition and the resulting phase(s) (number and volume) observed after attainment of equilibrium. Using this method, samples containing an increasing amount of alcohol were obtained so that entire phase diagram (except the extremely alcohol-rich region) was mapped.

3. Results and discussion The phase behaviour of a ternary system water–oil–surfactant can be represented in an upright phase prism. The Gibbs phase rule states that in this prism, in a well-defined temperature interval, the mixture may separate into three fluid phases, namely, the surfactant-rich microemulsion in equilibrium with a water-rich and an oil-rich excess phase [5]. To determine the required conditions for the formation of microemulsion with Nalkylaldonamide, this three-phase separation is searched for: firstly, because this kind of microemulsion possesses the most pronounced properties of solubilization and low interfacial tension; and, secondly, because three phases are easier to observe with the naked eye.

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The conditions to be verified to obtain the microemulsion in a triphasic equilibrium for a ternary system N-alkylaldonamide–water– hydrocarbon are summarized as follows: $ the binary mixture surfactant–water has an upper miscibility gap (with a lower critical temperature Tb); $ the binary mixture surfactant–hydrocarbon has a lower miscibility gap (with an upper critical temperature Ta); $ Ta is lower than Tb; $ the critical line connecting the critical points in the phase prism is broken. The study of the binary systems N-alkylaldonamide–water and N-alkylaldonamide–decane will show that these conditions are not met and that triphasic equilibrium appears only after addition of NaCl and alcohol.

3.1. Ternary system N-alkylaldonamide–water–decane Fig. 1 shows the N-decylisosaccharinamide– water (IsoN –H O) and N-octylribonamide– 10 2 water (C N –H O) phase diagrams. Above the 5 8 2 melting point of the mixture, the systems present mesomorphic phases and are monophasic over the

Fig. 1. IsoN –H O (a) and C N –H O (b) phase diagram. C=crystal, I=isotropic, H=hexagonal, L=lamellar, Cu=cubic, S= 10 2 5 8 2 smectic.

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entire temperature/concentration interval investigated. These systems do not show any miscibility gap and the critical temperature, if it exists, is higher than 135°C. For the systems N-decylisosaccharinamide– decane (IsoN –C ) and N-octylribonamide– 10 10 decane (C N –C ), Fig. 2 shows that above the 5 8 10 melting point of the mixture, they separate into two phases in the entire temperature/concentration interval observed. The critical points of these mixtures, thus, lie above 135°C. From these binary diagrams, it appears that the ternary systems IsoN –H O–C and C N – 10 2 10 5 8 H 0–C cannot form microemulsions in triphasic 2 10 equilibrium in the temperature interval investigated. Preliminary experiments with ternary mixtures have, indeed, shown that these systems remain biphasic with surfactant dissolved in the lower aqueous phase. This is because over the temperature interval studied, N-alkylaldonamides are much more miscible with water than they are with decane. To meet the above conditions required to form microemulsions, Ta and Tb have to be lowered in the studied temperature interval. The miscibility gaps could be in the appropriate position to interplay in the phase prism so that the system separates into three phases.

3.2. Pseudo-binary systems N-alkylaldonamide–brine To decrease the critical temperature Tb, the lyotropic salt sodium chloride, which is known to reduce the mutual solubility between water and non-ionic surfactants [7–9], was added. Fig. 3 shows the water-rich side of the phase diagram IsoN –brine that have been determined for 10 different salt concentration in water. The minimum concentration of salt in water needed for the appearance of a miscibility gap is 5% (weight %). The critical temperature decreases from 80°C with a salt concentration of 5% to 51°C with a NaCl concentration of 10%. In the case of C N and for the same salt 5 8 concentrations (Fig. 4), the system separates into two phases above the melting point of the mixture, indicating that the critical temperature of the miscibility gap lies below the melting point. Although the hydrocarbon part of C N is 5 8 shorter, this surfactant appears less soluble in brine than IsoN since for the same salt concentration, 10 the critical temperature of C N is lower. Indeed, 5 8 for non-ionic surfactants, a more hydrophilic compound corresponds to a higher critical temperature [10]. But, the present N-alkylaldonamides also differ in their hydrophilic part. The linear polar

Fig. 2. IsoN –C (a) and C N –C (b) phase diagram. C=crystal, I=isotropic, 2w=2 phases. 10 10 5 8 10

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In conclusion, addition of NaCl to the binary systems N-alkylaldonamide–water decreases the mutual solubility between water and surfactant and the pseudo-binary systems show a miscibility gap with a lower critical temperature Tb.

3.3. Pseudo-binary systems N-alkylaldonamide–decane–alcohol

Fig. 3. Pseudo-binary diagram IsoN –H O–NaCl for different 10 2 NaCl concentrations. C=crystal, I=isotropic, w=phase.

Fig. 4. Pseudo-binary diagram C N –H O–NaCl with 5% NaCl 5 8 2 in water. C=crystal, I=isotropic, 2w=2 phases.

head of C N seems, thus, more efficient for 5 8 decreasing the water solubility than the branched head of IsoN . 10

To increase the solubility of the surfactant in decane, the first possibility is to lengthen its hydrocarbon part; but this also means an increase of the melting point of the mixture and less favourable experimental conditions. To solve this problem, the hydrophobicity of decane is decreased by adding an alcohol. Because alcohols distribute between the water-rich and the oil-rich phase it will affect both the N-alkylaldonamide–water and the N-alkylaldonamide–decane phase diagrams. The distribution coefficient of alcohol depends only weakly on temperature, but is sensitive to their carbon number for a given hydrocarbon [11]. Medium and long chain alcohols (carbon number ≥4) show a rather wide miscibility gap with water and, thus, dissolve mainly in the oil-rich phase, reducing the effective carbon number of the oil. To increase the solubility of N-alkylaldonamide in decane, two different alcohols are used: $ butanol (C OH ) which is the shortest alcohol 4 that is not totally miscible with water (saturated solution: 7.9 g C OH/100 ml H O); 4 2 $ hexanol (C OH ) which is almost immiscible 6 with water (saturated solution: 0.59 g C OH/100 ml H O). 6 2 Fig. 5 shows the pseudo-binary phase diagrams IsoN –C –C OH and IsoN –C –C OH with a 10 10 4 10 10 6 constant concentration (10%) of alcohol in decane. Above the melting point of the mixture, the solution is monophasic in the temperature interval investigated. The miscibility gap with its upper critical temperature Ta, thus, lies below the melting curve. The same behaviour is observed in the same mixtures with C N as surfactant. 5 8 Addition of alcohol to the N-alkylaldonamide– decane system increases the surfactant solubility in decane and decreases the critical temperature Ta. Because this always lies below the melting

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Fig. 5. Pseudo-binary diagram IsoN –C –C OH (C /C OH=9) (a) and IsoN –C –C OH (C /C OH=9) (b). C=crystal, I= 10 10 4 10 4 10 10 6 10 6 isotropic, w=phase.

point of the mixture, it is not possible to compare the efficiency of the two alcohols. 3.4. Quaternary systems Nalkylaldonamide–brine–decane–alcohol The study of the binary phase diagrams has shown that N-alkylaldonamide–water–decane systems do not separate in triphasic equilibrium because the surfactant is too miscible with water and not miscible enough with decane, so that both the phase diagrams N-alkylaldonamide–water and N-alkylaldonamide–decane do not show any critical temperature. By addition of NaCl and alcohol, the solubility of the surfactant is, respectively, decreased in water and increased in decane. The N-alkylaldonamide–water system has a lower critical temperature and the N-alkylaldonamide–decane system has a upper critical temperature. Unfortunately, in most of the cases, these critical temperatures lie below the melting point of the mixture, so it was not possible to verify the second condition (Ta lower than Tb). To ensure that N-alkylaldonamide–water– decane systems form a three-phase body, some mixtures in the phase prism are studied. Since one property of the microemulsion is the solubilization

of equal amounts of water and hydrocarbon, mixtures with a constant water-to-decane ratio are investigated. For known concentrations of salt, surfactant and alcohol, the temperatures of appearance and disappearance of the triphasic equilibrium are determined. Without detailing all the results, we can note that: $ the upper and lower temperatures of the threephase interval decrease with increasing salt concentration; $ the triphasic equilibrium appears at room temperature in all cases for a minimum salt concentration of 8% in water; $ the upper temperature of the three-phase interval decreases when IsoN is replaced by 10 C N , indicating that in the quaternary mix5 8 ture, IsoN is more hydrophilic. 10 By addition of NaCl and alcohol to the Nalkylaldonamide–water–decane system, the conditions to find triphasic area and microemulsions have, thus, been determined. Therefore, more complete phase diagrams can be established in order to study the phase behaviour of these systems. As NaCl is almost insoluble in the organic phase, the mixture water–NaCl is considered as a pseudo-component with a constant salt weight concentration of 8% in water. At constant temper-

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Fig. 6. Section through the phase tetrahedron of a quaternary system surfactant–water–hydrocarbon–alcohol keeping the water-tohydrocarbon ratio equal to 1.

ature, the phase diagram of a quaternary mixture can be presented in a tetrahedron. To avoid complex three-dimensional representation, we chose to establish pseudo-ternary phase diagrams, keeping the water-to-decane ratio constant ( Fig. 6). Figs. 7 and 8 represent these sections for IsoN –brine–C –C OH and IsoN –brine– 10 10 4 10 C –C OH systems at 25°C. For both systems 10 6 C N –brine–C –C OH and C N –brine–C – 5 8 10 4 5 8 10 C OH (Figs. 9 and 10), it is necessary to raise the 6

temperature to 60°C to observe the entire threephase region. At lower temperature, the phase separation is partially obscured by a non-solubilized surfactant region. All the diagrams present a characteristic ‘‘fish’’ shape for different equilibria. At low surfactant concentration and increasing alcohol concentration, we meet successively: $ 2=Winsor I equilibrium ( lower microemulsion phase in equilibrium with an exces oil phase);

Fig. 7. Section through the phase tetrahedron IsoN –H O 10 2 (NaCl 8%)–C –C OH keeping the H O (NaCl 8%)/C ratio= 10 4 2 10 1 (T=25°C ). CR+L=crystal+liquid.

Fig. 8. Section through the phase tetrahedron IsoN –H O 10 2 (NaCl 8%)–C –C OH keeping the H O (NaCl 8%)/C ratio= 10 6 2 10 1 (T=25°C ). CR+L=crystal+liquid.

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Fig. 9. Section through the phase tetrahedron C N –H O 5 8 2 (NaCl 8%)–C –C OH keeping the H O (NaCl 8%)/C10 10 4 2 ratio=1 (T=60°C ). CR+L=crystal+liquid.

Fig. 10. Section through the phase tetrahedron C N –H O 5 8 2 (NaCl 8%)–C –C OH keeping the H O (NaCl 8%)/C ratio= 10 6 2 10 1 (T=60°C ). CR+L=crystal+liquid. $

$

3=Winsor III equilibrium present in the ‘‘body of the fish’’ (surfactant-rich microemulsion phase coexisting with both water and oil phases); 2=Winsor II equilibrium (upper microemulsion phase in equilibrium with an excess water phase) [12]. Increasing surfactant concentration leads to an

increase of the middle-phase volume at the expense of the oil and water phases and gives rise to the monophasic system. Winsor I and Winsor II microemulsions are characterized by oil-in-water and water-in-oil droplets, respectively [13–15]. The oil-in-water structure evolves continuously to a water-in-oil structure through the bicontinuous structure of the Winsor III microemulsion [16,17]. The microstructure of the monophasic solution can vary with concentration. The ‘‘tail of the fish’’, which corresponds to the intermediate phase of the triphasic Winsor III equilibrium that has solubilized the entire water and oil phases, is generally associated to a bicontinuous structure [18,19]. This microemulsion can evolve to a water-in-oil or a oil-inwater microemulsion depending on the path taken through the phase diagram [20]. The addition of a large amount of alcohol can also lead to weakly organized structure [21]. This kind of phase separation is frequently observed by replacing the effect of alcohol by an increase in the temperature or salt addition [22–24]. To compare these different systems showing the same type of phase separation, the following parameters, given in Table 1, are measured: $ CA =minimum alcohol concentration required L for the appearance of the triphasic equilibrium; $ CA =maximum alcohol concentration U required for the disappearance of the triphasic equilibrium; $ c*=minimum surfactant concentration required to homogenize equal amount of water and oil. Some of these values were measured at 60°C, but we have verified that a variation from 25°C to 60°C did not modify the three-phase interval, but only the solubilization area of the surfactant. Influence of the nature of the surfactant Calculation of the HLB [25] of the two surfactants from the weight contribution of their hydrophilic and hydrophobic part (11.8 for C N and 5 8 11.2 for IsoN ) shows that C N is slightly more 10 5 8 hydrophilic than IsoN . However, in the phase 10 diagram the pseudo-component water+NaCl has to be considered. Addition of salt modifies interactions between water and surfactant, leading to

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F. Bastogne, C. David / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 311–320 Table 1 Characteristic parameters (CA , CA and c*) of the three-phase intervals for different quaternary systems L U Quaternary systems

CA (wt.%) L

CA (wt.%) U

c* (wt.%)

IsoN –H O (NaCl 8%)–C –C OH 10 2 10 4 IsoN –H O (NaCl 8%)–C –C OH 10 2 10 6 C N –H O (NaCl 8%)–C –C OH 5 8 2 10 4 C N –H O (NaCl 8%)–C –C OH 5 8 2 10 6

6a 4a 7b 4b

20a 9a 23a 9b

22a 14a 33a 26b

aMeasured at 25°C. bMeasured at 60°C.

a relative decrease of their HLB. From the critical temperatures and the saturation concentrations of the surfactants, we can say that this lowering in HLB is greater in the case of C N , which becomes 5 8 less hydrophilic than IsoN . Therefore, CA 10 L should be greater for IsoN than C N . Table 1 10 5 8 shows that CA and CA are nearly identical for L U the two surfactants in systems containing the same alcohol. Addition of alcohol decreases the relative hydrophobicity of decane, but it also acts on surfactant in the interface, modifying the hydrophobicity of the surfactant–alcohol mixture. Since the three-phase interval appears and disappears for the same alcohol concentration for both surfactant, and since IsoN is more hydrophilic than 10 C N , it seems that alcohol is more efficient at 5 8 increasing the solubility of IsoN in decane. 10 Both salt and alcohol affect solvation of surfactant. In the case of IsoN , more salt is needed to 10 break water–surfactant interactions, while less alcohol is needed to favour surfactant–alcohol interactions. So, it seems that when the polar head of N-alkylaldonamide is branched, like in N-decylisosaccharinamide, hydrogen interactions with alcohol or water in the solvatation shell are more important than with a linear polar head. Influence of the nature of the alcohol With the same surfactant, the three-phase interval is smaller when the system contains hexanol instead of butanol. Alcohol can be distributed between aqueous and hydrocarbon phases and at the interface. Hexanol, almost insoluble in water, is mixed with the hydrocarbon phase and decreases the effective carbon number of the decane. With butanol (the shortest alcohol not totally miscible with water), a part of the alcohol is dissolved in

the water phase where it increases the miscibility between water and surfactant. This part is no longer available to decrease the hydrophobicity of decane, but, in addition, it decreases the hydrophobicity of the surfactant. Thus, more butanol than hexanol is needed to achieve the three-phase equilibrium when surfactant is equally soluble in the aqueous and the hydrocarbon phases. In the same way, more butanol is needed for the disappearance of the three-phase interval when the surfactant becomes insoluble with water.

4. Conclusions In this work, we have shown that N-alkylaldonamides are able to form microemulsions in water– decane mixtures. To determine favourable conditions for the formation of these microstructures, we developed a methodology that does not require tedious establishment of phase diagrams. From binary diagrams N-alkylaldonamide–water and Nalkylaldonamide–decane and their critical points, it appears that the surfactant is too soluble with water, and not soluble enough with decane, to form microemulsions in a triphasic equilibrium. The hydrophilicity of N-alkylaldonamide is modified by addition of sodium chloride in water, while addition of alcohol lead to a variation of the hydrophobicity of both decane and surfactant. N-alkylaldonamide can, therefore, form microemulsions in the quaternary system N-alkylaldonamide–brine–decane–alcohol. The different pseudo-ternary phase diagrams of these systems, keeping the water-to-decane ratio constant, show all the same patterns of phase separation with four kinds of equilibrium. The principal difference

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between these systems lies in the extent of the three-phase region and this results from the conformation of the polar head of the surfactant. The phase diagrams present a wide monophasic area. The microstucture of the mixture in this part of the diagram is important for some applications of microemulsions and will be the subject of forthcoming papers.

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