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Water Behavior in Nonionic Surfactant Systems I: Subzero Temperature Behavior of Water in Nonionic Microemulsions Studied by DSC
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
178, 60–68 (1996)
0093
Water Behavior in Nonionic Surfactant Systems I: Subzero Temperature Behavior of Water in Nonionic Microemulsions Studied by DSC N. GARTI,* ,1 A. ASERIN,* S. EZRAHI,* ,2 I. TIUNOVA,*
AND
G. BERKOVIC †
*Casali Institute of Applied Chemistry, School of Applied Science and Technology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel; and †Department of Materials & Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel Received February 9, 1995; accepted August 4, 1995
is an essential participant in biomolecular processes (3). Water functions not just as an inert solvent; rather, it plays a significant part in the mechanisms of these processes (4), such as solvation of polar groups and nonspecific effects of the medium (3). In this context microemulsions may well serve as model systems for the investigation of water in confined spaces, such as molecular crevices (1) and interstices left by the packing of globular proteins (3), since microemulsions may be envisaged as tiny droplets of water (or oil) dispersed in a continuous oil (or aqueous) medium (1). For example, membrane proteins, which are immersed in a lipophilic membrane while many water molecules are very close to the membrane surfaces (5), may be linked with water–oil interfaces, typical of microemulsions. When describing the state of water in relation to any surface, a distinction is usually made between ‘‘bulk’’ and ‘‘bound’’ water. Bulk or free water is assumed to have physicochemical properties not much different from those of pure water. Thus, it should, for example, have a heat of fusion similar to that of ordinary water, freeze at approximately 07C, and have heat capacities nearly the same as ice below this temperature and very close to those of water above this temperature (6). Bound water, known also as ‘‘hydration shell’’, ‘‘solvent shell’’, ‘‘hydration water’’ (7), or ‘‘vicinal water’’ (1), may be defined by an operational definition or by a more precise and independent one. Definitions of the first kind refer to the water detected by a certain technique as having been influenced by the surface of the substrate in contact with the water. The presence of a nearby surface may alter thermodynamic properties (among others, the melting point, enthalpy, and heat capacity) and hydrodynamic and kinetic properties of the water. Thus, water whose properties differ detectably from those of bulk water in the same system, as a result of the presence of a surface, may be defined as bound water (6). One such rather common operational definition derives from the low-temperature behavior of the water. Water in which the thermodynamic properties have been sufficiently modified so as to remain unfrozen at subzero temperatures
Subzero temperature differential scanning calorimetry (DSC) has been applied to a model nonionic microemulsion system, water/pentanol/dodecane/C12 (EO)8 , in order to determine the relative concentrations of bound and free water. On the basis of data thus acquired, conclusions are drawn relating to the low-temperature behavior of the systems studied, the surfactant hydration and the role of pentanol. It is shown that the surfactant becomes saturated with water at a ratio of three molecules of ‘‘interphasal’’ water per ethylene oxide group. Free water can only be detected by DSC at higher water/surfactant ratios. Apparently nonfreezable water is not formed in the model system, at least not before the inversion from W/O to O/W microemulsion has occurred. The thickness of the bound water layer in the nonionic model system ˚ ; i.e., two has been evaluated by several methods to be ca. 5 A monolayers of interphasal water are closely associated with the surfactant. It is further demonstrated that although pentanol enhances water solubilization, and is present at the interface, its interaction with water or surfactant is not revealed by DSC. q 1996 Academic Press, Inc.
Key Words: nonionic microemulsion; differential scanning calorimetry (DSC); low-temperature behavior; bound water; free water; hydration.
INTRODUCTION
Water molecules are sensitive to the presence of adjacent interfaces, be they inorganic, such as clays and ion exchangers, or organic, like biomembranes and proteins (1). It has been amply shown that liquid water departs considerably from its average bulk behavior when near solute molecules (2), macromolecules, colloidal particles, and solid or liquid interfaces. Naturally, the investigation of the interaction between water and such interfaces may help us to gain some insight into the behavior of water in living organisms. Water 1
To whom correspondence should be addressed. Part of the results presented in this paper were included in S.E.’s doctoral thesis in Applied Chemistry at The Hebrew University of Jerusalem, Israel. 2
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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may be called bound water, nonfreezing, or nonfreezable water (6). In such cases a definite amount of water, unable to freeze into normal ice, is readily determined experimentally (6) with practically the same results obtained by different methods (7). This water is clearly distinguishable from bulk water. However, the significance of this particular definition may not always be clear-cut, as, a priori, there need not be any relation between properties of hydration measured at low and room temperatures (6). This point will be elaborated later on. A technique-independent definition considers bound water as that water associated with a surface of a substrate, at a level of hydration beyond which further addition of water leaves the system unchanged (except for a dilution effect). The same endpoint of the hydration process is obtained by various techniques (7). The hydration process has been intensively investigated, especially for proteins and other biomolecular compounds. However, the following picture (6, 7) may also be adequate for any surface interaction of water. Thus, on a molecular level, as water is added incrementally to a water–substrate system, the nature of the hydration process is gradually altered. The process begins with the formation of isolated, disordered, and sharply localized adsorbates on active sites. As the amount of water increases, it forms mobile, hydrogenbonded clusters, first by interaction with polar surface groups and then by condensing onto weakly interacting segments of the substrate surface. The process is completed when the surface is covered with a monolayer of water. More layers of water may be piled on the hydrated substrate, but their properties will be progressively changed until they approach those of bulk water. It should be emphasized that the degree of order and mobility and the strength of the bonding in water interfacial interactions are lower than what might be inferred from the somewhat misleading name bound water. This distinction between bound and bulk water is important since it helps us to understand biochemical interactions. It is also relevant to the development of industrial microemulsions for which the evaporation rates of water incorporated in their microstructure are of significance. For example, fire-resistant hydraulic fluids based on microemulsions (which we are developing (8)) consist of water, additive-based mineral oil, nonionic surfactant, and an alcohol. Water is prone to evaporation in open hydraulic systems. This loss of water leads to a decrease in the fire-resistance capability of the hydraulic fluid, or even to the microemulsion breakdown. As bound water should evaporate less than free water (9), the problem of assessing the relative amount of bound water in microemulsions is certainly pertinent to our research. The nature of water–substrate interactions and the extent to which water is bound have been investigated using spectroscopic methods, such as NMR (10, 11), IR (12–14), or ESR (10), and calorimetric methods, which may be divided
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into ambient (15–17) and low-temperature measurements (10, 18–22). Our experimental formulations, which include many major components and additives, are evidently too complicated to be treated by any of the above-mentioned methods. Instead, we chose a model system and examined it by low-temperature differential scanning calorimetry. To the best of our knowledge, nonionic microemulsions have not as yet been investigated by this technique. EXPERIMENTAL
Materials We used the following chemicals without further purification: highly purified (by HPLC, minimum 99%) octaethylene glycol mono-n-dodecyl ether [C12 (EO)8 ] from Nikko Chemicals Co., Japan; 1-pentanol, n-dodecane, and n-hexadecane from Aldrich Chemical Company, Inc., U.S.A. (purity ca. 99% or above); D2O from E. Merck, Germany (minimum 99.8%). Double-distilled water was used for the preparation of the samples. Phase Diagram The model system chosen was n-dodecane/C12 (EO)8 /1pentanol/water. Although it is more usual to use alcohols with ionic surfactants, we found that pentanol enhances the water solubilization in our nonionic model system. It functions as a cosurfactant or a cosolvent. The experiments performed for the determination of the phase diagram were conducted at 27 { 0.27C. Predetermined amounts of C12 (EO)8 were mixed with dodecane and pentanol at fixed weight ratios and then titrated with water in order to detect phase transitions characteristic of the system. The detailed procedure is described elsewhere (23). Since the behavior of four components is to be depicted on a pseudo-ternary phase diagram, the relative concentration of two of the components must be kept constant for all the experiments. Although phase diagrams of such quaternary systems are generally based on constant ratios of surfactantto-water or cosurfactant-to-surfactant, we preferred to use a fixed weight ratio of oil-to-cosurfactant, which is also quite common, particularly among solubilization researchers (see our paper (24) and references therein); it was kept fixed at unity. The phase diagram is given in Fig. 1. The main feature of the phase diagram is a continuous monophasic area extending from the water apex to the opposite edge, bordered by a two-phase region on one side, and by a liquid crystalline domain on the other. This is an example of a U-type microemulsion (25). If we draw a line from the water corner to the midpoint of the side that connects the pentanol/dodecane and the surfactant apexes, it will represent system compositions for which the initial concentration of C12 (EO)8 is 50 wt%. This
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Thermal Analysis We followed the method utilized by Senatra et al. (1, 18, 19, 26–31), in which the endothermic scanning mode was applied and the peaks representing various states of water were identified and analyzed. Senatra et al. differentiate between three types of water (28):
FIG. 1. Phase diagram for the system n-dodecane/C12 (EO)8 /1-pentanol/water (277C; weight ratio dodecane/pentanol Å 1). No attempt was made to further identify the liquid crystals or any other phase within the LC area. The dashed line represents the water dilution line along which the weight ratio of dodecane:pentanol:C12 (EO)8 is kept constant at 1:1:2, respectively.
is the line marked as ‘‘50%’’ in Fig. 1. It can be seen that the amount of water contained in the monophasic systems represented along this line may vary continuously from very low to quite high values. All compositions chosen by us for the DSC investigation lie along this line. Calorimetric Measurements A Mettler TA 4000 thermal analysis system, equipped with a TC 11 TA processor and a DSC 30 low-temperature cell, was used. The DSC measurements were carried out as follows: microemulsion samples (5–15 mg) were weighed, using a Mettler M3 microbalance, in standard 40 m aluminium pans and immediately sealed by a press. All DSC measurements were performed in the endothermic scanning mode (i.e., the controlled heating of previously frozen samples) thus circumventing possible complications from supercooling when measurements are performed in the cooling (exothermic) mode. The samples were rapidly cooled by liquid nitrogen at a predetermined rate from ambient to 01007C (sometimes to 0307C) and then heated at a constant scanning rate (usually 37C/min) back to ambient temperatures. The same results were obtained when the samples were kept at the minimum temperature for 1 h. An empty pan was used as a reference. The calorimeter measured and recorded the heat flow rate of the sample as a function of temperature, while the sample underwent the aforementioned cooling and heating procedure. The instrument also determined the total heat transferred in the observed thermal processes. The enthalpy changes associated with thermal transitions were evaluated by integrating the area of each pertinent DSC peak. DSC temperatures reported here were reproducible to {0.57C and have absolute values calibrated to {17C.
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1. ‘‘Free’’ water, which is water with properties that are similar to those of bulk water. It melts at 07C. 2. ‘‘Interphasal’’ water, which is the water confined within the region separating the aqueous core from the oleic dispersing phase. This region is considered to be a finite in extent and not just an imaginary line which serves as a border between the two phases. The interphasal water melts at ca. 0107C. This complicates the study of dodecane-containing systems, such as our model system, as dodecane itself also melts at 0107C, and thus its fusion peak overlaps the peak of interphasal water. However, the existence of this type of water was clearly shown for our system, using the following considerations: —The heat of fusion, measured at 0107C was higher than that required for the known amount of dodecane in the system. By subtracting the enthalpy change due to the dodecane, the contribution of the interphasal water is readily calculated. —The endothermic peak at 0107C remained in the absence of dodecane, or when hexadecane was substituted for dodecane. —Analysis of microemulsions in which D2O was used instead of water, and with all other components and compositions being the same, showed a typical shift of the D2O endotherms toward higher temperatures. 3. ‘‘Bound’’ water, which is bound to hydrophilic groups and counterions, and which melts at temperatures lower than 0107C. We calculated the concentration of the interphasal water by the equation WI Å
DHI (exp.) 0 DHD fD 1 100, DH I
[1]
where WI is the interphasal water concentration (in wt%), DHI (exp.) is the measured enthalpy change for the 0107C peak (it includes contributions of interphasal water and dodecane), DHD is the heat of fusion of pure dodecane which is 191.6 J/g for our experimental conditions, fD is the dodecane weight fraction, and DHI is the heat of fusion of interphasal water. It is not exactly known which of the polymorphic forms of ice will be assumed by the interphasal water at its freezing point. Some researchers (32) neglect the polymorphism of ice and use the crystallization enthalpy of water, without introducing an appreciable error. We followed Senatra et al. (19) and used DHI Å 312.38 J/g.
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In a similar way, the concentration of free water was calculated, using the equation WF Å
DHF (exp.) 1 100, DH F
[2]
where WF is the free water concentration (in wt%), DHF (exp.) is the measured enthalpy change for the 07C peak, and DHF is the heat of fusion of pure water, measured by us, and equals 323.72 J/g for our experimental conditions. RESULTS AND DISCUSSION
As may be seen from the phase diagram (Fig. 1), proceeding along the 50% dilution line, [(pentanol / dodecane)/ C12 (EO)8 Å 1], by adding small amounts of water, may be continued without passing through a liquid crystalline mesophase or a two-phase region until more than 85 wt% water is included in the monophasic area of the diagram. The reasoning underlying the investigation of the thermal behavior of microemulsion along this line is clear. In the following, the various endothermic peaks in typical thermograms for samples along the dilution line will be identified. The separation between free and bound water, characteristic of DSC, enables us to trace the behavior of both these types of water in relation to two parameters, temperature of fusion and relative concentrations, as a function of total water concentration. Simultaneously, the endothermic peak of pentanol will be examined at various total water concentrations. DSC results will then be utilized to describe the interphasal water layer in terms of a molecular model and to evaluate its thickness. Typical Thermograms Fig. 2 shows a typical thermogram for our system. The peak at 078.17C was identified as belonging to pentanol and
FIG. 2. Thermogram for the system dodecane (13.7%)/pentanol (13.7%)/C12 (EO)8 (27.6%)/water (45.0%). The nonconstant baseline above about 307C is probably due to water evaporation.
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FIG. 3. Thermogram for the system hexadecane (18.6%)/pentanol (18.6%)/C12 (EO)8 (37.2%)/water (25.6%). The nonconstant baseline above about 307C is probably due to water evaporation.
the identification was corroborated by comparing this peak with that of pure pentanol. The peak at 02.77C is assigned to free water. The peak at 010.47C is related to both dodecane and interphasal water. This identification was confirmed by using D2O in place of H2O (data not shown), and by using hexadecane instead of dodecane as shown in Fig. 3 (a new peak appears at 15.37C, the melting of hexadecane, while the 010.47C peak remains). We were not able to detect another thermal event related to water, which would give rise to a peak at subzero temperature, down to 01007C. Variation of Water Peak Temperatures with the Water Content We plotted the temperatures of the midpoints of the peaks related to water vs water content (see Figs. 4, 5). In spite of the rather appreciable scatter (although only within a 37C interval) both plots showed the same pattern: a gradual increase of the temperature to a less negative value and then a more or less constant temperature was obtained. Similar behavior was observed for the freezable bound water peak of poly(4-hydroxystyrene) when the phase transition temperature was plotted vs the sorbed water content (32). We observed the same behavior (albeit less pronounced) for the free water peak. In Fig. 4, the plateau begins at a total water content of close to 30 wt%, which is the point where free water begins to appear. The average fusion temperature of the interphasal water at the plateau is 010.0 { 0.37C. In Fig. 5 the transition to the plateau occurs at a total water content between 50 and 55 wt%. As will be shown, in this region there is an inversion to O/W microemulsions. Thus we may distinguish between two types of free water:
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FIG. 4. Variation of the interphasal water fusion temperature with the total water concentration.
—water confined to the core of the aggregate though not bound to the surfactant (such water appears only at total water content between 30 and 50 wt% and —water which constitutes the continuous phase after the inversion. The average transition temperature of the free water at the plateau is 01.6 { 0.27C, which suggests that free water in microemulsions is not entirely free, or pure. The detection
FIG. 5. Variation of the free water fusion temperature with the total water concentration.
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FIG. 6. Variation of free and interphasal water content with the total concentration of water. Concentrations are calculated in weight percentage, relative to the weights of the microemulsion samples. X stands for free water and L stands for interphasal water.
of bulk-like water by DSC may be the basis for the identification of W/O and O/W microemulsions in a convenient way, using small amounts of samples. Determination of Interphasal and Free Water Content From the intensities of the DSC peaks, the content of interphasal and free water in the microemulsions the compositions of which lie along the 50% dilution line (fixed weight ratio C12 (EO)8 )/pentanol/dodecane), can be plotted. The results are shown in Fig. 6. It can be seen from Fig. 6 that the concentration of the interphasal water increases until it reaches a constant value, which we have evaluated from the experimental data to be 28.4 { 1.0 wt%. We may say that when the total concentration of water equals 28 wt%, virtually all the water in the microemulsion system is of the interphasal type. At 30 wt% water (based on total sample weight) free water begins to appear and its fraction increases steadily, while the concentration of the interphasal water remains constant at 28.4 { 1.0 wt% until somewhere between 45 and 48 wt% water. At this point the decrease in concentration of the interphasal water is clearly seen. We have shown, independently, by NMR techniques that an inversion to an O/W microemulsion occurs at 50–55 wt% water (D. Waysbort et al., in preparation). As was suggested by one of the referees, the decrease in interphasal water content may also be linked (partially at least) with a corresponding decrease in the available total surface area due to microstructural changes.
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W0 at the Beginning of Free Water Formation The molar ratio of total water/surfactant (referred to hereafter as W0 ) was evaluated at the beginning of free water formation in two ways. First, the beginning of the plateau of Fig. 6 coincides approximately with the first appearance of free water. The total concentration of water is then 28.4 { 1.0 wt% and thus W0 is 23.8 { 1.2. A similar molar ratio is obtained from the intersection of the free water curve with the abscissa in Fig. 6. This curve is a straight line ( r 2 Å 0.998) and the intersection occurs at total water concentration of 30.4 { 1.0 wt%, which corresponds to W0 Å 26.2 { 1.2. The agreement between these two values of W0 is good, as they were calculated from independent data. Taking into consideration all the measured values of W0 , it may be shown that on the average Ç25 water molecules are bound per surfactant molecule before free water begins to form. This means 25/8 à 3 molecules of water are then bound per ethylene oxide (EO) group. The same molar ratio was determined in our laboratory for the system polyalkylene oxide– modified polydimethylsiloxane–water (I. Shaul and N. Garti, in preparation). As far as we know, no other nonionic quaternary microemulsion systems have been investigated by such a technique. However, it will be instructive to compare these systems with similar binary systems. Our results are in good agreement with those of Carlstro¨m and Halle (33), obtained by a oxygen-17 magnetic relaxation study. They concluded that the head-group shell of C12 (EO)8 aggregates in water contain less than five and possibly only two to three water molecules per EO group. Schulz and Puig (34) have shown by low-temperature DSC that for aqueous dispersions of C12 (EO)23 and C16 (EO)20 there are 3.07 molecules of interfacial water per EO group, in excellent agreement with our results. It may prima facie be inferred that the water– surfactant interaction is independent of the length of the hydrophilic head group of the ethoxylated surfactant and that the presence of pentanol (and dodecane) does not affect the interaction between water and the EO groups of nonionic surfactants. These conclusions are, however, preliminary. The dependence of water–surfactant interactions upon the length of surfactant head group will be demonstrated in relation to the problem of nonfreezable water. Is There Nonfreezable Water? We discussed two types of water in our microemulsion system: free water (peak at 07C) and interphasal water (peak at 0107C). However, in theory nonfreezable bound water might also be present. This type of water cannot directly be seen by DSC. By subtracting the amounts of the free and interphasal water determined from the total water in each sample, the existence (or absence) of nonfreezable water may be inferred. For all microemulsion compositions we checked (up to 70% water content) the sum of the free and
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interphasal water was equal to, or within experimental error ( {5 wt%) of the total water content. Thus, there is no evidence for nonfreezable water. A possible small discrepancy exists for the very high water content microemulsions, which could have at most 5% nonfreezable water. Although there is no clear evidence for nonfreezable water in this microemulsion, nonfreezable water has unequivocably been observed in related systems, such as the binary water–C12 (EO)23 (Brij 35) and water–C16 (EO)20 (Brij 58) systems, where some water is assumed to be trapped in the helicoidal structure of the hydrophilic chain (34). In such a case we assume that the space available for the trapped water is very restricted. Thus, the water–surfactant interaction may be strong enough to prevent the water from freezing. The short head group of our surfactant, C12 (EO)8 , does not form such coils (35), and thus, no nonfreezable water is expected. Possibility of Phase Separation Study of water hydration in microemulsions by subzero temperature DSC experiments could be criticized on the ground that cooling the microemulsion system to at least 0307C may bring about phase separation, including the possibility of separation of an almost pure water phase (10, 36). For our system, DSC demonstrates that until W0 Å 26.2 no free water is detected; i.e., the freezing–thawing process imposed on the interphasal water clearly does not cause phase separation. At higher water concentrations, free water begins to appear. Whether it remains free in the microemulsion or segregates—fully or partly—as a separate phase, the concentration of the interphasal water (relative to the constant total weight of the microemulsion) is not altered, as is evidenced by the plateau. Thus, the possibility of phase separation does not distort the measured concentration ratio of free to interphasal water. Pentanol Interaction with the Other Constituents In principle, the cosurfactant may also interact with the other constituents. This point has been investigated in the same way as the water–surfactant interaction. We determined the concentrations of pentanol from the measured enthalpy change associated with the pentanol peak and the enthalpy change associated with pure pentanol. The derived concentrations were compared to the actual concentrations. Within experimental error, the concentrations of the pentanol introduced into the microemulsion samples are equal to those detected by assuming that all the pentanol freezes at the temperature corresponding to the fusion of pure pentanol. These results suggest that as pentanol appears free, it is unlikely to be involved in interactions with other components of the system which can be revealed by DSC measurements. It should be mentioned that the behavior of the surfactant is different. Its distinct fusion peak at 29.57C (measured on
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a sample of pure C12 (EO)8 ) is considerably reduced or even disappears when the system contains water, as a result of their mutual interaction. Thus, the interphasal water peak at 0107C is due to water–C12 (EO)8 interaction, presumably hydrogen bonding between the water protons and the etheric oxygen atoms of the ethylene oxide (EO) groups of the surfactant. Butanol behaves like C12 (EO)8 . When water– butanol binary mixtures and butanol-containing four-component microemulsions were tested by DSC, the peak of the alcohol was reduced or even disappeared. This suggests that the alcohol–water interaction may occur in the bulk (as it is related to the water–alcohol mutual solubility) rather than at the interface. Evaluation of the Thickness of the Interphasal Water Layer The DSC results, which show that there are three water molecules per EO group, may be used to assess the thickness of the interphasal water. The exact location of the water molecules around the head-group chain is not known. Using molecular modeling, we may postulate a tetrahedrally coordinated structure where each oxygen atom of an ethylene oxide group is hydrogen bonded to two water molecules, and a third water molecule is hydrogen bonded to both of the two other water molecules, forming an octagonal ring. A similar network of structured water for the polyethylene oxide–water system was suggested (37). Of the several possibilities analyzed by the researchers (37), regarding the binding of water to EO, one was the possibility that on the average three water molecules can bind per one EO group. However, it should be remembered that such an ice-like structure may be distorted due to the stereochemical constraint imposed by the presence of the surfactant. Our model (see Fig. 7) leads to an effective thickness of ˚. the two monolayers of interphasal water of ca. 4.5 A ˚ thick water bilayer It was also possible to derive a Ç5-A around the surfactant from the freezing point data as well. Most cells in organic tissues freeze at temperatures near 0107C, even when the external medium contains ice (38). This freezing process is due to seeding of the supercooled water occluded in the cells by nuclei of external ice. At approximately 0107C these ice crystals can penetrate into small pores imbedded in the cell membrane, whereas above this temperature the ice just melts, while trying to pass through these pores. By applying a modified form of the Kelvin equation (which relates the ice–water surface tension to the freezing temperature and the radius of cylindrical capillaries through which water presumably permeates) the following relationship was derived: DT à 52/a, where DT is the freezing point depression and a is the pore radius in ˚ . Thus, in 5 A ˚ pores, freezing will occur at 0107C in A accordance with the experimentally determined pore size (38). A quite similar equation relates the thickness of the
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FIG. 7. A possible configuration of hydrated EO groups. Filled spheres, O atoms; blank spheres, H atoms; grey spheres, C atoms.
˚ ) on mineral surfaces with unfrozen water layer, du , (in A u, the temperature (in degrees) below 07C: u à 50/du . It was found that at temperatures between 06 and 0107C, du (calculated as the ratio of the volume of unfrozen water to the specific surface area of the solid substrate) lies in the ˚ (39). Thickness within 4.5–6 A ˚ per two range of 5–8 A layers of bound water was determined for proteins (7) and clays (40, 41). The similarity in behavior has led to the suggestion (39) that the same empirical relation is obeyed for biological and mineral systems. Microemulsions might also be associated with this alleged relation. In our case, the equation y Å 50/ DT is valid for DT Å 0107C and y Å 5 ˚ . For the reverse micellar system of water/AOT/i-octane A or cyclohexane, the bound water layer thickness lies between ˚ (42, 43). Thus, for various substrates, a depression 3 and 5 A of 107C in the freezing point of water is associated with ca. ˚ thickness of the bound water layer enveloping these 5 A substrates. On the basis of a power law suggested by Gilpin (44) the interaction of solid surfaces with interfacial water has been expressed in a more quantitative way in terms of an attractive force field. Thus, the chemical potential dmw and the freezing point depression DT of interphasal water decrease with the distance l0 from the water–substrate interface (i.e., the thickness of water layer adhered to the substrate) according to the power law (44) a dmw Å 0al 0 0 ,
[3]
where a and a are adjustable parameters ( a is usually taken as 2). On the basis of the power law and thermodynamic considerations the following equation was derived for planar bilayers (45): DTmin Å 2 a/1 (l0 /L) a . DTmax
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TABLE 1 Calculations of the Thickness of the Interphasal Water Layer Water content (wt%) Parameters
30
˚) Bilayer thickness (A ˚) Water layer thickness, L (A DTmin (7C) DTmax (7C) ˚) l0 at a Å 2 (A
57.1 15.1 03.2 010.3 3.0
40 66.7 24.7 02.7 09.6 4.6
50 80.0 38.0 02.2 010.4 6.2
DTmin is the highest melting point of ice; DTmax is the maximum depression of the freezing point of water in the presence of the bilayers. DTmin and DTmax are determined from the positions of the endothermic peaks of free and interphasal water, respectively, on the thermograms. In order to interpret L for our case, we use data obtained by SAXS [O. Regev et al., submitted], that one-dimensional swelling occurs along the 50% dilution line between 20 and 60 wt% of water. Thus, the microemulsion structure in this region may be depicted as a layered one. By applying Eq. [4] to our microemulsion system, L is the thickness of the (total) water layers, alternating with surfactant (together with oil and alcohol) layers. L may be evaluated from SAXS measurements: by plotting 1/d (where d is the periodicity) vs the volume fraction of water, fw , we get a straight line from which the bilayer thickness is determined, while the extrapolation of the plot to fw Å 0 gives d 0 , the thickness of the surfactant layer. Thus, L Å d 0 d 0rl0 is the thickness of the bound (interphasal) water layer. The results are shown in Table 1 for three water concentrations (30, 40, and 50 wt%) along the dilution line 50%. The calculated thickness ˚ , in reasonable agreement with the results derived is 3–6 A from the molecular model and the freezing point data. SUMMARY AND CONCLUSIONS
We investigated the state of water in the system n-dodecane/1-pentanol/C12 (EO)8 /water by DSC. Following Senatra et al. we determined the relative concentration of free and interphasal water, when the total water concentration was varied along the 50% dilution line on the pseudo-ternary phase diagram of the system. With the help of these data we were able to draw the following conclusions: (i) The system does not contain nonfreezable water. Thus, the water–surfactant interaction is rather weak, as expected of uncharged nonionic surfactant. However, it is possible that after the inversion to the O/W microemulsion some nonfreezable water is formed ( £5%), perhaps through interaction with the OH group of the surfactant. (ii) No trapped water is detected in our system. Interpha-
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sal water should be regarded as weakly interacting bound water. (iii) Water introduced into the system until a total concentration of 30 wt% was reached interacted with the EO surfactant groups. All of this water is found as interphasal water. This amounts to three molecules of water per EO group which results in a double layer of bound water surrounding the surfactant, distributed on both sides of the headgroup chain. (iv) When working in the endothermic mode, supercooling and hysteresis phenomena do not interfere with interphasal water formation. (v) If some phase separation of the microemulsion system does occur upon cooling, it does not noticeably affect the hydration behavior of the surfactant. (vi) Although pentanol enhances water solubilization, no evidence for interaction with water or surfactant can be found in DSC measurements. ACKNOWLEDGMENT The authors thank Dr. E. Wachtel of the Weizmann Institute of Science, Rehovot, Israel, for critical reading of the manuscript and helpful discussions.
REFERENCES 1. Senatra, D., Lendinara, L., and Giri, M. G., Can. J. Phys. 68, 1041 (1990). 2. Derbyshire, W., in ‘‘Water—A Comprehensive Treatise’’ (F. Franks, Ed.), Vol. 7, p. 339–469. Plenum, New York, 1982. 3. Finney, J. L., and Savage, H. F. J., in ‘‘The Chemical Physics of Solvation. Part C: Solvation Phenomena in Specific Physical, Chemical and Biological Systems’’ (R. R. Dogonadza, E. Kalman, A. A. Kornyshev, J. Ulstrup, Eds.), p. 603. Elsevier Scientific, Amsterdam, 1988. 4. Tait, M. J., and Franks, F., Nature 30, 91 (1971). 5. Edsall, J. T., and McKenzie, H. A., Adv. Biophys. 16, 53 (1983). 6. Kuntz, I. D. Jr., and Kauzmann, W., Adv. Protein Chem. 28, 239 (1974). 7. Rupley, J. A., and Careri, G., Adv. Protein Chem. 41, 37 (1991). 8. Garti, N., Feldenkriez, R., Aserin, A., Ezrahi, S., and Shapira, D., Lubrication Eng. 49, 404 (1993). 9. Pathananthan, K., and Johari, G. P., J. Chem. Soc. Faraday Trans. 90, 1143 (1994). 10. Hauser, H., Haering, G., Pande, A., and Luisi, P. L., J. Phys. Chem. 93, 7869 (1989). 11. Senatra, D., Lendinara, L., and Giri, M. G., Prog. Colloid Polym. Sci. 84, 122 (1991). 12. Giammona, G., Goffredi, F., Turco Liveri, V., and Vassallo, G., J. Colloid Interface Sci. 154, 411 (1992). 13. Jain, T. K., Varshney, M., and Maitra, A., J. Phys. Chem. 93, 7409 (1989). 14. Onori, G., and Santucci, A., J. Phys. Chem. 97, 5430 (1993). 15. Goto, A., Yoshioka, H., Kishimoto, H., and Fujita, T., Thermochim. Acta. 63, 139 (1990). 16. Goto, A., Yoshioka, H., Kishimoto, H., and Fujita, T., Langmuir 8, 441 (1992). 17. Goto, A., Harada, S., Fujita, T., Miwa, Y., Yoshioka, H., and Kishimoto, H., Langmuir 9, 86 (1993).
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18. Senatra, D., Guarini, G. G. T., Gabrielli, G., and Zoppi, M., J. Phys. 45, 1159 (1984). 19. Senatra, D., Gabrielli, G., and Guarini, G. G. T., Europhys. Lett. 2, 455 (1986). 20. Boned, C., Peyrelasse, J., and Moha-Ouchane, M., J. Phys. Chem. 90, 634 (1986). 21. Quist, P.-O., and Halle, B., J. Chem. Soc. Faraday Trans. I 84, 1033 (1988). 22. Rosano, H. L., Nixon, A. L., and Cavallo, J. L., J. Phys. Chem. 93, 4536 (1989). 23. Garti, N., Aserin, A., and Ezrahi, S., in ‘‘Proceedings of the National Conference on Tribology’’ (E. Assenova and K. Dannev, Eds.), p. 107. Sofia, Bulgaria, 1994. 24. Garti, N., Aserin, A., Ezrahi, S., and Wachtel, E., J. Colloid Interface Sci. 169, 428 (1995). 25. Clausse, M., Nicolas-Morgantini, L., and Zradba, A., in ‘‘Microemulsion Systems’’ (H. L. Rosano and M. Clausse, Eds.), Surfactant Science Series, Vol. 24, p. 15. Marcel Dekker, New York, 1987. 26. Senatra, D., Guarini, G. G., Gabrielli, G., and Zoppi, M., in ‘‘Macroand Microemulsions: Theory and Applications’’ (D. O. Shah, Ed.), ACS Symposium Series No. 272, p. 133. Am. Chem. Soc., Washington, DC, 1985. 27. Senatra, D., Guarini, G. G., and Garbielli, G., in ‘‘Physics of Amphiphiles, Micelles, Vesicles and Microemulsions’’ (V. Degiorgio and M. Corti, Eds.), p. 802. North Holland, Amsterdam, 1985. 28. Senatra, D., Gabrielli, G., Caminati, G., and Shou, Z., IEEE Trans. Electr. Insul. 23, 579 (1988). 29. Senatra, D., and Zhou, Z., Prog. Colloid Polym. Sci. 76, 106 (1988).
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30. Senatra, D., Gabrielli, G., and Guarini, G. G. T., in ‘‘Progress in Microemulsions’’ (S. Martellucci and A. N. Chester, Eds.), Ettore Majorana International Sci. Ser., Physical Sciences, Vol. 41, p. 207. Plenum, New York/London, 1989. 31. Senatra, D., Gabrielli, G., Caminati, G., and Guarini, G. G. T., in ‘‘Surfactants in Solution’’ (K. L. Mittal, Ed.), Vol. 10, p. 147–158. Plenum, New York, 1989. 32. Nakamura, K., Hatakeyama, T., and Hatakeyama, H., Polymer 24, 871 (1983). 33. Carlstro¨m, G., and Halle, B., J. Chem. Soc. Faraday Trans. I 85, 1049 (1989). 34. Schulz, P. C., and Puig, J. E., Colloids Surf. A 71, 83 (1993). 35. Ro¨sch, M., in ‘‘Non-ionic Surfactants’’ (M. J. Schick, Ed.), Surfactant Science Series, Vol. 1, p. 753. Marcel Dekker, New York, 1966. 36. Luisi, P. L., and Magid, L. J., CRC Crit. Rev. Biochem. 20, 409 (1986). 37. Kjellander, R., and Florin, E., J. Chem. Soc. Faraday Trans. I 77, 2053 (1981). 38. Mazur, P., Ann. N. Y. Acad. Sci. 125, 658 (1965). 39. Banin, A., and Anderson, D. M., Nature 255, 261 (1975). 40. Yariv, S., in ‘‘Modern Approaches to Wettability—Theory and Applications’’ (M. E. Schrader and G. I. Loeb, Eds.), p. 279. Plenum, New York, 1992. 41. Israelachvili, J. N., and Pashley, R. M., Nature 306, 249 (1983). 42. Maitra, A., J. Phys. Chem. 88, 5122 (1984). 43. Kotlarchyk, M., Huang, J. S., and Chen, S. H., J. Phys. Chem. 89, 4382 (1985). 44. Gilpin, R. R., J. Colloid Interface Sci. 68, 235 (1979). 45. Ter-Minassian-Saraga, L., and Madelmont, G., J. Colloid Interface Sci. 81, 369 (1981).
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0093
Water Behavior in Nonionic Surfactant Systems I: Subzero Temperature Behavior of Water in Nonionic Microemulsions Studied by DSC N. GARTI,* ,1 A. ASERIN,* S. EZRAHI,* ,2 I. TIUNOVA,*
AND
G. BERKOVIC †
*Casali Institute of Applied Chemistry, School of Applied Science and Technology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel; and †Department of Materials & Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel Received February 9, 1995; accepted August 4, 1995
is an essential participant in biomolecular processes (3). Water functions not just as an inert solvent; rather, it plays a significant part in the mechanisms of these processes (4), such as solvation of polar groups and nonspecific effects of the medium (3). In this context microemulsions may well serve as model systems for the investigation of water in confined spaces, such as molecular crevices (1) and interstices left by the packing of globular proteins (3), since microemulsions may be envisaged as tiny droplets of water (or oil) dispersed in a continuous oil (or aqueous) medium (1). For example, membrane proteins, which are immersed in a lipophilic membrane while many water molecules are very close to the membrane surfaces (5), may be linked with water–oil interfaces, typical of microemulsions. When describing the state of water in relation to any surface, a distinction is usually made between ‘‘bulk’’ and ‘‘bound’’ water. Bulk or free water is assumed to have physicochemical properties not much different from those of pure water. Thus, it should, for example, have a heat of fusion similar to that of ordinary water, freeze at approximately 07C, and have heat capacities nearly the same as ice below this temperature and very close to those of water above this temperature (6). Bound water, known also as ‘‘hydration shell’’, ‘‘solvent shell’’, ‘‘hydration water’’ (7), or ‘‘vicinal water’’ (1), may be defined by an operational definition or by a more precise and independent one. Definitions of the first kind refer to the water detected by a certain technique as having been influenced by the surface of the substrate in contact with the water. The presence of a nearby surface may alter thermodynamic properties (among others, the melting point, enthalpy, and heat capacity) and hydrodynamic and kinetic properties of the water. Thus, water whose properties differ detectably from those of bulk water in the same system, as a result of the presence of a surface, may be defined as bound water (6). One such rather common operational definition derives from the low-temperature behavior of the water. Water in which the thermodynamic properties have been sufficiently modified so as to remain unfrozen at subzero temperatures
Subzero temperature differential scanning calorimetry (DSC) has been applied to a model nonionic microemulsion system, water/pentanol/dodecane/C12 (EO)8 , in order to determine the relative concentrations of bound and free water. On the basis of data thus acquired, conclusions are drawn relating to the low-temperature behavior of the systems studied, the surfactant hydration and the role of pentanol. It is shown that the surfactant becomes saturated with water at a ratio of three molecules of ‘‘interphasal’’ water per ethylene oxide group. Free water can only be detected by DSC at higher water/surfactant ratios. Apparently nonfreezable water is not formed in the model system, at least not before the inversion from W/O to O/W microemulsion has occurred. The thickness of the bound water layer in the nonionic model system ˚ ; i.e., two has been evaluated by several methods to be ca. 5 A monolayers of interphasal water are closely associated with the surfactant. It is further demonstrated that although pentanol enhances water solubilization, and is present at the interface, its interaction with water or surfactant is not revealed by DSC. q 1996 Academic Press, Inc.
Key Words: nonionic microemulsion; differential scanning calorimetry (DSC); low-temperature behavior; bound water; free water; hydration.
INTRODUCTION
Water molecules are sensitive to the presence of adjacent interfaces, be they inorganic, such as clays and ion exchangers, or organic, like biomembranes and proteins (1). It has been amply shown that liquid water departs considerably from its average bulk behavior when near solute molecules (2), macromolecules, colloidal particles, and solid or liquid interfaces. Naturally, the investigation of the interaction between water and such interfaces may help us to gain some insight into the behavior of water in living organisms. Water 1
To whom correspondence should be addressed. Part of the results presented in this paper were included in S.E.’s doctoral thesis in Applied Chemistry at The Hebrew University of Jerusalem, Israel. 2
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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may be called bound water, nonfreezing, or nonfreezable water (6). In such cases a definite amount of water, unable to freeze into normal ice, is readily determined experimentally (6) with practically the same results obtained by different methods (7). This water is clearly distinguishable from bulk water. However, the significance of this particular definition may not always be clear-cut, as, a priori, there need not be any relation between properties of hydration measured at low and room temperatures (6). This point will be elaborated later on. A technique-independent definition considers bound water as that water associated with a surface of a substrate, at a level of hydration beyond which further addition of water leaves the system unchanged (except for a dilution effect). The same endpoint of the hydration process is obtained by various techniques (7). The hydration process has been intensively investigated, especially for proteins and other biomolecular compounds. However, the following picture (6, 7) may also be adequate for any surface interaction of water. Thus, on a molecular level, as water is added incrementally to a water–substrate system, the nature of the hydration process is gradually altered. The process begins with the formation of isolated, disordered, and sharply localized adsorbates on active sites. As the amount of water increases, it forms mobile, hydrogenbonded clusters, first by interaction with polar surface groups and then by condensing onto weakly interacting segments of the substrate surface. The process is completed when the surface is covered with a monolayer of water. More layers of water may be piled on the hydrated substrate, but their properties will be progressively changed until they approach those of bulk water. It should be emphasized that the degree of order and mobility and the strength of the bonding in water interfacial interactions are lower than what might be inferred from the somewhat misleading name bound water. This distinction between bound and bulk water is important since it helps us to understand biochemical interactions. It is also relevant to the development of industrial microemulsions for which the evaporation rates of water incorporated in their microstructure are of significance. For example, fire-resistant hydraulic fluids based on microemulsions (which we are developing (8)) consist of water, additive-based mineral oil, nonionic surfactant, and an alcohol. Water is prone to evaporation in open hydraulic systems. This loss of water leads to a decrease in the fire-resistance capability of the hydraulic fluid, or even to the microemulsion breakdown. As bound water should evaporate less than free water (9), the problem of assessing the relative amount of bound water in microemulsions is certainly pertinent to our research. The nature of water–substrate interactions and the extent to which water is bound have been investigated using spectroscopic methods, such as NMR (10, 11), IR (12–14), or ESR (10), and calorimetric methods, which may be divided
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into ambient (15–17) and low-temperature measurements (10, 18–22). Our experimental formulations, which include many major components and additives, are evidently too complicated to be treated by any of the above-mentioned methods. Instead, we chose a model system and examined it by low-temperature differential scanning calorimetry. To the best of our knowledge, nonionic microemulsions have not as yet been investigated by this technique. EXPERIMENTAL
Materials We used the following chemicals without further purification: highly purified (by HPLC, minimum 99%) octaethylene glycol mono-n-dodecyl ether [C12 (EO)8 ] from Nikko Chemicals Co., Japan; 1-pentanol, n-dodecane, and n-hexadecane from Aldrich Chemical Company, Inc., U.S.A. (purity ca. 99% or above); D2O from E. Merck, Germany (minimum 99.8%). Double-distilled water was used for the preparation of the samples. Phase Diagram The model system chosen was n-dodecane/C12 (EO)8 /1pentanol/water. Although it is more usual to use alcohols with ionic surfactants, we found that pentanol enhances the water solubilization in our nonionic model system. It functions as a cosurfactant or a cosolvent. The experiments performed for the determination of the phase diagram were conducted at 27 { 0.27C. Predetermined amounts of C12 (EO)8 were mixed with dodecane and pentanol at fixed weight ratios and then titrated with water in order to detect phase transitions characteristic of the system. The detailed procedure is described elsewhere (23). Since the behavior of four components is to be depicted on a pseudo-ternary phase diagram, the relative concentration of two of the components must be kept constant for all the experiments. Although phase diagrams of such quaternary systems are generally based on constant ratios of surfactantto-water or cosurfactant-to-surfactant, we preferred to use a fixed weight ratio of oil-to-cosurfactant, which is also quite common, particularly among solubilization researchers (see our paper (24) and references therein); it was kept fixed at unity. The phase diagram is given in Fig. 1. The main feature of the phase diagram is a continuous monophasic area extending from the water apex to the opposite edge, bordered by a two-phase region on one side, and by a liquid crystalline domain on the other. This is an example of a U-type microemulsion (25). If we draw a line from the water corner to the midpoint of the side that connects the pentanol/dodecane and the surfactant apexes, it will represent system compositions for which the initial concentration of C12 (EO)8 is 50 wt%. This
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Thermal Analysis We followed the method utilized by Senatra et al. (1, 18, 19, 26–31), in which the endothermic scanning mode was applied and the peaks representing various states of water were identified and analyzed. Senatra et al. differentiate between three types of water (28):
FIG. 1. Phase diagram for the system n-dodecane/C12 (EO)8 /1-pentanol/water (277C; weight ratio dodecane/pentanol Å 1). No attempt was made to further identify the liquid crystals or any other phase within the LC area. The dashed line represents the water dilution line along which the weight ratio of dodecane:pentanol:C12 (EO)8 is kept constant at 1:1:2, respectively.
is the line marked as ‘‘50%’’ in Fig. 1. It can be seen that the amount of water contained in the monophasic systems represented along this line may vary continuously from very low to quite high values. All compositions chosen by us for the DSC investigation lie along this line. Calorimetric Measurements A Mettler TA 4000 thermal analysis system, equipped with a TC 11 TA processor and a DSC 30 low-temperature cell, was used. The DSC measurements were carried out as follows: microemulsion samples (5–15 mg) were weighed, using a Mettler M3 microbalance, in standard 40 m aluminium pans and immediately sealed by a press. All DSC measurements were performed in the endothermic scanning mode (i.e., the controlled heating of previously frozen samples) thus circumventing possible complications from supercooling when measurements are performed in the cooling (exothermic) mode. The samples were rapidly cooled by liquid nitrogen at a predetermined rate from ambient to 01007C (sometimes to 0307C) and then heated at a constant scanning rate (usually 37C/min) back to ambient temperatures. The same results were obtained when the samples were kept at the minimum temperature for 1 h. An empty pan was used as a reference. The calorimeter measured and recorded the heat flow rate of the sample as a function of temperature, while the sample underwent the aforementioned cooling and heating procedure. The instrument also determined the total heat transferred in the observed thermal processes. The enthalpy changes associated with thermal transitions were evaluated by integrating the area of each pertinent DSC peak. DSC temperatures reported here were reproducible to {0.57C and have absolute values calibrated to {17C.
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1. ‘‘Free’’ water, which is water with properties that are similar to those of bulk water. It melts at 07C. 2. ‘‘Interphasal’’ water, which is the water confined within the region separating the aqueous core from the oleic dispersing phase. This region is considered to be a finite in extent and not just an imaginary line which serves as a border between the two phases. The interphasal water melts at ca. 0107C. This complicates the study of dodecane-containing systems, such as our model system, as dodecane itself also melts at 0107C, and thus its fusion peak overlaps the peak of interphasal water. However, the existence of this type of water was clearly shown for our system, using the following considerations: —The heat of fusion, measured at 0107C was higher than that required for the known amount of dodecane in the system. By subtracting the enthalpy change due to the dodecane, the contribution of the interphasal water is readily calculated. —The endothermic peak at 0107C remained in the absence of dodecane, or when hexadecane was substituted for dodecane. —Analysis of microemulsions in which D2O was used instead of water, and with all other components and compositions being the same, showed a typical shift of the D2O endotherms toward higher temperatures. 3. ‘‘Bound’’ water, which is bound to hydrophilic groups and counterions, and which melts at temperatures lower than 0107C. We calculated the concentration of the interphasal water by the equation WI Å
DHI (exp.) 0 DHD fD 1 100, DH I
[1]
where WI is the interphasal water concentration (in wt%), DHI (exp.) is the measured enthalpy change for the 0107C peak (it includes contributions of interphasal water and dodecane), DHD is the heat of fusion of pure dodecane which is 191.6 J/g for our experimental conditions, fD is the dodecane weight fraction, and DHI is the heat of fusion of interphasal water. It is not exactly known which of the polymorphic forms of ice will be assumed by the interphasal water at its freezing point. Some researchers (32) neglect the polymorphism of ice and use the crystallization enthalpy of water, without introducing an appreciable error. We followed Senatra et al. (19) and used DHI Å 312.38 J/g.
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In a similar way, the concentration of free water was calculated, using the equation WF Å
DHF (exp.) 1 100, DH F
[2]
where WF is the free water concentration (in wt%), DHF (exp.) is the measured enthalpy change for the 07C peak, and DHF is the heat of fusion of pure water, measured by us, and equals 323.72 J/g for our experimental conditions. RESULTS AND DISCUSSION
As may be seen from the phase diagram (Fig. 1), proceeding along the 50% dilution line, [(pentanol / dodecane)/ C12 (EO)8 Å 1], by adding small amounts of water, may be continued without passing through a liquid crystalline mesophase or a two-phase region until more than 85 wt% water is included in the monophasic area of the diagram. The reasoning underlying the investigation of the thermal behavior of microemulsion along this line is clear. In the following, the various endothermic peaks in typical thermograms for samples along the dilution line will be identified. The separation between free and bound water, characteristic of DSC, enables us to trace the behavior of both these types of water in relation to two parameters, temperature of fusion and relative concentrations, as a function of total water concentration. Simultaneously, the endothermic peak of pentanol will be examined at various total water concentrations. DSC results will then be utilized to describe the interphasal water layer in terms of a molecular model and to evaluate its thickness. Typical Thermograms Fig. 2 shows a typical thermogram for our system. The peak at 078.17C was identified as belonging to pentanol and
FIG. 2. Thermogram for the system dodecane (13.7%)/pentanol (13.7%)/C12 (EO)8 (27.6%)/water (45.0%). The nonconstant baseline above about 307C is probably due to water evaporation.
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FIG. 3. Thermogram for the system hexadecane (18.6%)/pentanol (18.6%)/C12 (EO)8 (37.2%)/water (25.6%). The nonconstant baseline above about 307C is probably due to water evaporation.
the identification was corroborated by comparing this peak with that of pure pentanol. The peak at 02.77C is assigned to free water. The peak at 010.47C is related to both dodecane and interphasal water. This identification was confirmed by using D2O in place of H2O (data not shown), and by using hexadecane instead of dodecane as shown in Fig. 3 (a new peak appears at 15.37C, the melting of hexadecane, while the 010.47C peak remains). We were not able to detect another thermal event related to water, which would give rise to a peak at subzero temperature, down to 01007C. Variation of Water Peak Temperatures with the Water Content We plotted the temperatures of the midpoints of the peaks related to water vs water content (see Figs. 4, 5). In spite of the rather appreciable scatter (although only within a 37C interval) both plots showed the same pattern: a gradual increase of the temperature to a less negative value and then a more or less constant temperature was obtained. Similar behavior was observed for the freezable bound water peak of poly(4-hydroxystyrene) when the phase transition temperature was plotted vs the sorbed water content (32). We observed the same behavior (albeit less pronounced) for the free water peak. In Fig. 4, the plateau begins at a total water content of close to 30 wt%, which is the point where free water begins to appear. The average fusion temperature of the interphasal water at the plateau is 010.0 { 0.37C. In Fig. 5 the transition to the plateau occurs at a total water content between 50 and 55 wt%. As will be shown, in this region there is an inversion to O/W microemulsions. Thus we may distinguish between two types of free water:
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FIG. 4. Variation of the interphasal water fusion temperature with the total water concentration.
—water confined to the core of the aggregate though not bound to the surfactant (such water appears only at total water content between 30 and 50 wt% and —water which constitutes the continuous phase after the inversion. The average transition temperature of the free water at the plateau is 01.6 { 0.27C, which suggests that free water in microemulsions is not entirely free, or pure. The detection
FIG. 5. Variation of the free water fusion temperature with the total water concentration.
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FIG. 6. Variation of free and interphasal water content with the total concentration of water. Concentrations are calculated in weight percentage, relative to the weights of the microemulsion samples. X stands for free water and L stands for interphasal water.
of bulk-like water by DSC may be the basis for the identification of W/O and O/W microemulsions in a convenient way, using small amounts of samples. Determination of Interphasal and Free Water Content From the intensities of the DSC peaks, the content of interphasal and free water in the microemulsions the compositions of which lie along the 50% dilution line (fixed weight ratio C12 (EO)8 )/pentanol/dodecane), can be plotted. The results are shown in Fig. 6. It can be seen from Fig. 6 that the concentration of the interphasal water increases until it reaches a constant value, which we have evaluated from the experimental data to be 28.4 { 1.0 wt%. We may say that when the total concentration of water equals 28 wt%, virtually all the water in the microemulsion system is of the interphasal type. At 30 wt% water (based on total sample weight) free water begins to appear and its fraction increases steadily, while the concentration of the interphasal water remains constant at 28.4 { 1.0 wt% until somewhere between 45 and 48 wt% water. At this point the decrease in concentration of the interphasal water is clearly seen. We have shown, independently, by NMR techniques that an inversion to an O/W microemulsion occurs at 50–55 wt% water (D. Waysbort et al., in preparation). As was suggested by one of the referees, the decrease in interphasal water content may also be linked (partially at least) with a corresponding decrease in the available total surface area due to microstructural changes.
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W0 at the Beginning of Free Water Formation The molar ratio of total water/surfactant (referred to hereafter as W0 ) was evaluated at the beginning of free water formation in two ways. First, the beginning of the plateau of Fig. 6 coincides approximately with the first appearance of free water. The total concentration of water is then 28.4 { 1.0 wt% and thus W0 is 23.8 { 1.2. A similar molar ratio is obtained from the intersection of the free water curve with the abscissa in Fig. 6. This curve is a straight line ( r 2 Å 0.998) and the intersection occurs at total water concentration of 30.4 { 1.0 wt%, which corresponds to W0 Å 26.2 { 1.2. The agreement between these two values of W0 is good, as they were calculated from independent data. Taking into consideration all the measured values of W0 , it may be shown that on the average Ç25 water molecules are bound per surfactant molecule before free water begins to form. This means 25/8 à 3 molecules of water are then bound per ethylene oxide (EO) group. The same molar ratio was determined in our laboratory for the system polyalkylene oxide– modified polydimethylsiloxane–water (I. Shaul and N. Garti, in preparation). As far as we know, no other nonionic quaternary microemulsion systems have been investigated by such a technique. However, it will be instructive to compare these systems with similar binary systems. Our results are in good agreement with those of Carlstro¨m and Halle (33), obtained by a oxygen-17 magnetic relaxation study. They concluded that the head-group shell of C12 (EO)8 aggregates in water contain less than five and possibly only two to three water molecules per EO group. Schulz and Puig (34) have shown by low-temperature DSC that for aqueous dispersions of C12 (EO)23 and C16 (EO)20 there are 3.07 molecules of interfacial water per EO group, in excellent agreement with our results. It may prima facie be inferred that the water– surfactant interaction is independent of the length of the hydrophilic head group of the ethoxylated surfactant and that the presence of pentanol (and dodecane) does not affect the interaction between water and the EO groups of nonionic surfactants. These conclusions are, however, preliminary. The dependence of water–surfactant interactions upon the length of surfactant head group will be demonstrated in relation to the problem of nonfreezable water. Is There Nonfreezable Water? We discussed two types of water in our microemulsion system: free water (peak at 07C) and interphasal water (peak at 0107C). However, in theory nonfreezable bound water might also be present. This type of water cannot directly be seen by DSC. By subtracting the amounts of the free and interphasal water determined from the total water in each sample, the existence (or absence) of nonfreezable water may be inferred. For all microemulsion compositions we checked (up to 70% water content) the sum of the free and
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interphasal water was equal to, or within experimental error ( {5 wt%) of the total water content. Thus, there is no evidence for nonfreezable water. A possible small discrepancy exists for the very high water content microemulsions, which could have at most 5% nonfreezable water. Although there is no clear evidence for nonfreezable water in this microemulsion, nonfreezable water has unequivocably been observed in related systems, such as the binary water–C12 (EO)23 (Brij 35) and water–C16 (EO)20 (Brij 58) systems, where some water is assumed to be trapped in the helicoidal structure of the hydrophilic chain (34). In such a case we assume that the space available for the trapped water is very restricted. Thus, the water–surfactant interaction may be strong enough to prevent the water from freezing. The short head group of our surfactant, C12 (EO)8 , does not form such coils (35), and thus, no nonfreezable water is expected. Possibility of Phase Separation Study of water hydration in microemulsions by subzero temperature DSC experiments could be criticized on the ground that cooling the microemulsion system to at least 0307C may bring about phase separation, including the possibility of separation of an almost pure water phase (10, 36). For our system, DSC demonstrates that until W0 Å 26.2 no free water is detected; i.e., the freezing–thawing process imposed on the interphasal water clearly does not cause phase separation. At higher water concentrations, free water begins to appear. Whether it remains free in the microemulsion or segregates—fully or partly—as a separate phase, the concentration of the interphasal water (relative to the constant total weight of the microemulsion) is not altered, as is evidenced by the plateau. Thus, the possibility of phase separation does not distort the measured concentration ratio of free to interphasal water. Pentanol Interaction with the Other Constituents In principle, the cosurfactant may also interact with the other constituents. This point has been investigated in the same way as the water–surfactant interaction. We determined the concentrations of pentanol from the measured enthalpy change associated with the pentanol peak and the enthalpy change associated with pure pentanol. The derived concentrations were compared to the actual concentrations. Within experimental error, the concentrations of the pentanol introduced into the microemulsion samples are equal to those detected by assuming that all the pentanol freezes at the temperature corresponding to the fusion of pure pentanol. These results suggest that as pentanol appears free, it is unlikely to be involved in interactions with other components of the system which can be revealed by DSC measurements. It should be mentioned that the behavior of the surfactant is different. Its distinct fusion peak at 29.57C (measured on
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a sample of pure C12 (EO)8 ) is considerably reduced or even disappears when the system contains water, as a result of their mutual interaction. Thus, the interphasal water peak at 0107C is due to water–C12 (EO)8 interaction, presumably hydrogen bonding between the water protons and the etheric oxygen atoms of the ethylene oxide (EO) groups of the surfactant. Butanol behaves like C12 (EO)8 . When water– butanol binary mixtures and butanol-containing four-component microemulsions were tested by DSC, the peak of the alcohol was reduced or even disappeared. This suggests that the alcohol–water interaction may occur in the bulk (as it is related to the water–alcohol mutual solubility) rather than at the interface. Evaluation of the Thickness of the Interphasal Water Layer The DSC results, which show that there are three water molecules per EO group, may be used to assess the thickness of the interphasal water. The exact location of the water molecules around the head-group chain is not known. Using molecular modeling, we may postulate a tetrahedrally coordinated structure where each oxygen atom of an ethylene oxide group is hydrogen bonded to two water molecules, and a third water molecule is hydrogen bonded to both of the two other water molecules, forming an octagonal ring. A similar network of structured water for the polyethylene oxide–water system was suggested (37). Of the several possibilities analyzed by the researchers (37), regarding the binding of water to EO, one was the possibility that on the average three water molecules can bind per one EO group. However, it should be remembered that such an ice-like structure may be distorted due to the stereochemical constraint imposed by the presence of the surfactant. Our model (see Fig. 7) leads to an effective thickness of ˚. the two monolayers of interphasal water of ca. 4.5 A ˚ thick water bilayer It was also possible to derive a Ç5-A around the surfactant from the freezing point data as well. Most cells in organic tissues freeze at temperatures near 0107C, even when the external medium contains ice (38). This freezing process is due to seeding of the supercooled water occluded in the cells by nuclei of external ice. At approximately 0107C these ice crystals can penetrate into small pores imbedded in the cell membrane, whereas above this temperature the ice just melts, while trying to pass through these pores. By applying a modified form of the Kelvin equation (which relates the ice–water surface tension to the freezing temperature and the radius of cylindrical capillaries through which water presumably permeates) the following relationship was derived: DT à 52/a, where DT is the freezing point depression and a is the pore radius in ˚ . Thus, in 5 A ˚ pores, freezing will occur at 0107C in A accordance with the experimentally determined pore size (38). A quite similar equation relates the thickness of the
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FIG. 7. A possible configuration of hydrated EO groups. Filled spheres, O atoms; blank spheres, H atoms; grey spheres, C atoms.
˚ ) on mineral surfaces with unfrozen water layer, du , (in A u, the temperature (in degrees) below 07C: u à 50/du . It was found that at temperatures between 06 and 0107C, du (calculated as the ratio of the volume of unfrozen water to the specific surface area of the solid substrate) lies in the ˚ (39). Thickness within 4.5–6 A ˚ per two range of 5–8 A layers of bound water was determined for proteins (7) and clays (40, 41). The similarity in behavior has led to the suggestion (39) that the same empirical relation is obeyed for biological and mineral systems. Microemulsions might also be associated with this alleged relation. In our case, the equation y Å 50/ DT is valid for DT Å 0107C and y Å 5 ˚ . For the reverse micellar system of water/AOT/i-octane A or cyclohexane, the bound water layer thickness lies between ˚ (42, 43). Thus, for various substrates, a depression 3 and 5 A of 107C in the freezing point of water is associated with ca. ˚ thickness of the bound water layer enveloping these 5 A substrates. On the basis of a power law suggested by Gilpin (44) the interaction of solid surfaces with interfacial water has been expressed in a more quantitative way in terms of an attractive force field. Thus, the chemical potential dmw and the freezing point depression DT of interphasal water decrease with the distance l0 from the water–substrate interface (i.e., the thickness of water layer adhered to the substrate) according to the power law (44) a dmw Å 0al 0 0 ,
[3]
where a and a are adjustable parameters ( a is usually taken as 2). On the basis of the power law and thermodynamic considerations the following equation was derived for planar bilayers (45): DTmin Å 2 a/1 (l0 /L) a . DTmax
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[4]
WATER BEHAVIOR IN NONIONIC SURFACTANT SYSTEMS
TABLE 1 Calculations of the Thickness of the Interphasal Water Layer Water content (wt%) Parameters
30
˚) Bilayer thickness (A ˚) Water layer thickness, L (A DTmin (7C) DTmax (7C) ˚) l0 at a Å 2 (A
57.1 15.1 03.2 010.3 3.0
40 66.7 24.7 02.7 09.6 4.6
50 80.0 38.0 02.2 010.4 6.2
DTmin is the highest melting point of ice; DTmax is the maximum depression of the freezing point of water in the presence of the bilayers. DTmin and DTmax are determined from the positions of the endothermic peaks of free and interphasal water, respectively, on the thermograms. In order to interpret L for our case, we use data obtained by SAXS [O. Regev et al., submitted], that one-dimensional swelling occurs along the 50% dilution line between 20 and 60 wt% of water. Thus, the microemulsion structure in this region may be depicted as a layered one. By applying Eq. [4] to our microemulsion system, L is the thickness of the (total) water layers, alternating with surfactant (together with oil and alcohol) layers. L may be evaluated from SAXS measurements: by plotting 1/d (where d is the periodicity) vs the volume fraction of water, fw , we get a straight line from which the bilayer thickness is determined, while the extrapolation of the plot to fw Å 0 gives d 0 , the thickness of the surfactant layer. Thus, L Å d 0 d 0rl0 is the thickness of the bound (interphasal) water layer. The results are shown in Table 1 for three water concentrations (30, 40, and 50 wt%) along the dilution line 50%. The calculated thickness ˚ , in reasonable agreement with the results derived is 3–6 A from the molecular model and the freezing point data. SUMMARY AND CONCLUSIONS
We investigated the state of water in the system n-dodecane/1-pentanol/C12 (EO)8 /water by DSC. Following Senatra et al. we determined the relative concentration of free and interphasal water, when the total water concentration was varied along the 50% dilution line on the pseudo-ternary phase diagram of the system. With the help of these data we were able to draw the following conclusions: (i) The system does not contain nonfreezable water. Thus, the water–surfactant interaction is rather weak, as expected of uncharged nonionic surfactant. However, it is possible that after the inversion to the O/W microemulsion some nonfreezable water is formed ( £5%), perhaps through interaction with the OH group of the surfactant. (ii) No trapped water is detected in our system. Interpha-
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sal water should be regarded as weakly interacting bound water. (iii) Water introduced into the system until a total concentration of 30 wt% was reached interacted with the EO surfactant groups. All of this water is found as interphasal water. This amounts to three molecules of water per EO group which results in a double layer of bound water surrounding the surfactant, distributed on both sides of the headgroup chain. (iv) When working in the endothermic mode, supercooling and hysteresis phenomena do not interfere with interphasal water formation. (v) If some phase separation of the microemulsion system does occur upon cooling, it does not noticeably affect the hydration behavior of the surfactant. (vi) Although pentanol enhances water solubilization, no evidence for interaction with water or surfactant can be found in DSC measurements. ACKNOWLEDGMENT The authors thank Dr. E. Wachtel of the Weizmann Institute of Science, Rehovot, Israel, for critical reading of the manuscript and helpful discussions.
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