J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2743-2751 Surface Chemistry and Microemulsion Formation in Systems containing Dialkylphthalate Esters as Oils Robert Aveyard, Bernard P. Binks, P. D. 1. Fletcher* and Paul A. Kingston School of Chemistry, University of Hull, Hull, UK HU6 7RX Alan R. Pitt Kodak Ltd., Headstone Drive, Harrow, Middlesex, UK HA I 4TY The surface chemical behaviour of phthalate esters in oil-water mixtures, both in the presence and absence of a conventional surfactant, has been investigated. We have estimated the surface activity of phthalate esters and the extent to which this type of moderately polar oil can be coadsorbed into concentrated monolayers of conven- tional surfactants at the oil/water interface. In the absence of surfactant, phthalate oils are adsorbed from dilute solution in heptane at the heptane/water interface, exhibiting a surface activity typical of non-aromatic diesters and similar to that of alcohols.However, the surface chemical and microemulsion phase behaviour in mixtures of phthalate oils and aqueous NaCl solutions in the presence of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) indicates a low extent of penetration of the phthalate oils into AOT monolayers. This is interesting since the phthalates have similar surface activities (from hydrocarbon oils) to alcohols which are very effective cosurfactants capable of strong coadsorption into surfactant monolayers. The main effects of substituting phthalate oils for alkanes in mixtures of oil-AOT-aqueous NaCl are (i) to increase the critical aggre- gate concentration of surfactant in the oil phase, (ii) to increase the NaCl concentration required for micro- emulsion phase inversion and (iii) to increase the magnitude of the minimum interfacial tension obtained by varying the salt concentration. It is concluded that phthalate oils do not act as strongly adsorbed cosurfactants in AOT-water-oil systems and the extent of phthalate oil penetration of AOT monolayers is less than for alkanes.Surface chemistry and microemulsion formation in oil-water-surfactant mixtures have been widely investigated in recent years. The behaviour of such systems is understood qualitatively for mixtures containing water and apolar oils such as alkanes.’-5 Additionally, mixtures of surfactant-apolar oil-polar solvents other than water have been investi- The effect of varying the oil structure has received attention and systems containing oils consisting of mixtures of an apolar hydrocarbon and an alcohol have been widely Inin~estigated.~.~ these latter mixtures, the alcohol (commonly termed a ‘cosurfactant ’ is generally distributed between the surfactant film at the oil/water interface and the bulk oil and water solvents.Oil species containing polar groups other than hydroxy groups have been less widely studied (e.g. ref. 10-14) although the interaction of such mod- erately polar oils with surfactants is important in a number of technologies including photographic emulsions, foods con-taining triglyceride oils and perfume oil delivery systems.The effect of oil molecular structure on microemulsion for- mation (and associated properties including the oil/water interfacial tension) is thought to be a consequence of the extent of penetration of the surfactant monolayer by the oi1.8,15.16 Apolar oils such as alkanes can penetrate and swell the tail region of surfactant monolayers and the extent to which this occurs is an important factor determining the phase behaviour and monolayer properties such as the bending elasticity.I7 Alcohol oils can penetrate further into the monolayer such that the hydroxy group is located in the plane of the surfactant headgroups. In this study we attempt to determine whether moderately polar oils such as esters (the subject of this work) penetrate concentrated surfactant monolayers and, if so, whether they mix with the surfactant chains only (like alkanes) or with both surfactant chain and headgroups (like alcohols).We have investigated the behaviour of mixtures containing the anionic twin-tailed surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), aqueous solutions of NaCl and dialkyl phthalate esters as the oil component. These oils are inter- esting since they may offer an alternative to environmentally harmful materials currently used and they are of particular interest in the photographic industry as solvents for certain dye materials. We first consider the adsorption of phthalate oils from dilute solution in heptane onto the oil/water interface to establish the surface activity in the absence of surfactant. Secondly, we describe the adsorption of AOT at the phthal- ate oil/water interface and the evolution of low tensions with aqueous phase NaCl concentration. Third, we discuss equi- librium microemulsion phase compositions for AOT-water- phthalate oil-NaC1 mixtures.Finally, some preliminary experiments concerning the interaction of the phthalate oils with AOT monolayers at both air/water and alkane/water interfaces are described. Experimental Materials AOT was obtained from-Sigma and was used without further purification. Water was purified by reverse osmosis and by using a Milli-Q reagent wa,ter system. Heavy water (D,O) was obtained from either Aldrich or MSD Isotopes. The air/ water surface tension at 25°C of the samples used was 71.8 0.1 mN m-’, in good agreement with the literature value of 71.9 kO.1 mN m-1.18 Heptane (Fisons HPLC grade), diisooctyl phthalate (DOP) (Janssen Chimica, 98%) and di-n-butyl phthalate (DBP) (Janssen Chimica, 99%),di-n-heptyl phthalate (Lancaster) and all other chain length phthalate esters used (Eastman Kodak) were passed over an alumina column prior to use to remove polar impurities. The phthalate oils (general structure shown below) showed only a single spot by thin-layer chromatography.The absence of surface-active impurities was checked further by observing emulsions prepared by shaking the oils with pure water. Oil Table 1 Properties of di-ester phthalate oils at 25 "C phthalate ester chain group oil/air tension/mN m -density/g cm -ethyl 37.3 1.1137 butyl 33.5 1 .O423 hexyl 31.8 0.9993 heptyl 31.5 0.9859 isooct yl 31.3 0.9803 nonyl 29.2 0.9653 decyl 30.0 0.9552 samples showing stable emulsions (indicating the presence of surface-active impurities) were rejected.Methods As shown in Table 1, phthalate oils have densities similar to that of water. Low oil-water density differences can cause dif- ficulties in tension measurements. Surface and interfacial ten- sions were determined using one of three methods. Systems with tension values ,>10 mN m-l and density difference values >O.l g were measured by the du Nouy ring method using a Kruss K12 (maximum pull) instrument.Lower tension systems were measured by the spinning drop technique using a Kruss Site 04 instrument. Measurements of systems showing a low density difference ( <0.1 g cm- 3, were made using a sessile drop apparatus constructed in this laboratory. In this method, developed by Padday and Pitt," a drop of the more dense liquid is placed on a solid surface with which it has a contact angle of ca. 180" when immersed in the second liquid. The height of the drop passes through a maximum value with increasing drop volume and this maximum height is recorded. Details of the method of obtaining the tension can be found in ref. 19 and 20. The solid surface used in this work was polished glass on which the oil drops under water showed a contact angle of 170-180".The accuracy of the tension values was estimated to be ca. 5% for systems with a density difference of ca. 0.05 g and it was checked that literature tension values could be reproduced within the estimated accuracy. Ancillary density measurements were made using a Paar DMA 55 instrument. Equilibration of phases was achieved by shaking samples of known composition in stoppered glass tubes followed by centrifugation at ca. 1500 rpm in a Denley BR401 ther-mostatted centrifuge for 1 h. The AOT concentrations in equilibrated phases were determined using a two-phase titra- tion method" with Hyanine 1666 as titrant. Water concen- trations were measured by Karl Fischer titration using an automated Baird and Tatlock AF3 titrator.Chloride ion con- centrations were determined by Mohr titration.22 Results and Discussion Properties of the Pure Phthalate Oils Table 1 shows the measured oil/air surface tensions and den- sities for a range of dialkyl phthalate oils. The oil density matches that of water at a chain length of about six carbon atoms. The density difference between oil and water can be increased by the use of D20 for which the densityI8 at 25 "C is 1.1044 g ~m-~. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Adsorption of Phthalate oils from Dilute Solution in Heptane onto the Heptanewater Interface Adsorption of two of the dialkyl phthalates (DBP and DOP) was measured in order to compare the surface activity of the phthalate oils with that of previously studied species such as alcohols (which can operate as cosurfactants) and with other moderately polar materials such as non-aromatic esters.Adsorption was determined by measuring the heptane solution/water interfacial tension for pre-equilibrated mix- tures prepared by shaking water with heptane containing dif- ferent quantities of either DBP or DOP. It was necessary to check whether the phthalate oils are distributed into the water phase to a significant degree. This was done by measuring the oil/water tension for samples pre- pared by equilibrating water with different volumes of oil containing a constant initial concentration of phthalate oil. If distribution occurs, it leads to a depletion of the oil phase concentration for low oil :water volume ratios and a con- comitant increase in the equilibrium tension.Thus, if the tension is observed to be independent of the oil: water volume ratio it can be concluded that loss to the water phase in this system is insignificant. Fig. 1 shows a plot of tension us. heptane :water volume ratio of a constant initial oil phase concentration of DBP of 7.5 mmol dm-3 (=0.107 mol%). No significant variation of tension is observed and it can be esti- mated that the distribution coefficient P of DBP between heptane and water (defined as P = [DBP],,,,,/[DBP],,,,,,,, , concentrations in mol dm-3) must be 50.2. Data for DOP (not shown) show a similar independence, and hence the equilibrium phthalate concentration can be reliably equated with the (known) initial value for both DBP and DOB under the conditions of the experiments.Fig. 2 shows the variation of the heptane/water tension with ln(phtha1ate mole fraction) for both DBP and DOP; the heptane :water volume ratio was 20 : 1. The solid lines show the best-fit polynomial of third order in each case. If it is assumed that the phthalate solutions in heptane behave ideally in the concentration ranges studied, the area occupied per phthalate molecule A can be obtained as a function of bulk concentration using the appropriate form of the Gibbs equation, A = -kT/(dy/dlnx) (1) where y is the tension, k is Boltzmann's constant, T is the absolute temperature and x is the mole fraction of solute.The gradients of the plots (dyldlnx) were obtained by differentia- 0 5 10 15 20 n-heptane : water volume ratio Fig. 1 Variation of heptane/water tension with heptane :water ratio for a constant initial concentration of 0.107 mol% DBP in heptane at 25 "C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 r I E z E--. C .-In 0)c (0.-50 C .-t In [mole fraction (%)I 52 I 1 (b) II E z E--. C .-0 cn 0)* -([1.-0 't c .-In[mole fraction (%)I Fig. 2 Heptane/water interfacial tension vs. ln[mole fraction (%)Iof (a)DBP and (b) DOP in heptane at 25 "C tion of the best-fit polynomial equations. The values of surface pressure Il (i.e. the lowering of surface tension caused by adsorption) and A derived from the data of Fig.2 were plotted according to the Schofield-Rideal surface equation23 of state (Fig. 3), which is n(A-A,) = /3kT (2) where A, is the limiting area at high surface pressure and /3 is a constant reflecting intermolecular interactions within the adsorbed monolayer. Eqn. (2) reduces to the Volmer surface equation of state when /3 = 1. This corresponds to a two-dimensional surface gas-like film in which the adsorbed mol- ecules show only short-range excluded area interactions, i.e. they behave as 'hard-disks'. Values of /3 < 1 signify the pres- ence of attractive interactions within the film.24 The tendency for the phthalate oils to be adsorbed can be quantified by estimating the standard Gibbs energies of adsorption (A, po) derived from the limiting low concentration slopes of plots of surface pressure us.solute mole fraction. A, po = RT ln(x/ll) (3) Aapovalues quoted here refer to standard states of unit mole fraction for the solutions and II = 1 mN m- ' for the surface. Values of A,, /3 and Aapo for the two phthalate oils are shown in Table 2. The values of the limiting areas A, appear reasonable for species with two alkyl tails. For DOP, /3 z 1, showing that Table 2 Parameters for the Schofield-Rideal surface equation of state for the adsorption of phthalate oils from dilute solution in heptane to the heptane/water interface at 25 "C oil A,/nm B Aa po/kJ mol -DBP 0.87 0.77 -23.9 DOP 0.77 1.08 -25.2 2745 1.6-1.4-1.2-1.0-E r I z E 0.8-\ h k! 1 Fv -0.6 -0.4 -0.2 //I I I I I 0 1 2 3 4 5 6 area occupied per phthalate molecule at the heptane/water interface/nm2 Fig.3 Variation of l/n us. A for DBP (0)and DOP (0)mono-layers at the heptane/water interface at 25 "C this molecule follows approximately the Volmer surface equa- tion of state. For DBP, /3 < l, indicating that there is a sig- nificantly longer range intermolecular attraction between adsorbed short-chain phthalate molecules in the monolayer. The values of standard Gibbs energies of adsorption are similar for both phthalate chain lengths and can be compared with the following values for adsorption from dilute solution in alkanes for a range of polar oils.(i) The standard Gibbs energy of adsorption for linear alcohols adsorbing from alkane solution to the alkane/water interface is constant ( & 0.5 kJ mol- I) for different temperatures and chain lengths of the alkanes and alcohok2' For the puuposes of this dis- cussion, we quote an average value representative of alcohols of -23.5 kJ mol-'. (ii) The value for methyl dodecanoate adsorbing from dilute solution in octane to the octane/water interface26 at 30°C is -16.4 kJ mol-'. (iii) Diethyl esters of the dicarboxylic acids of structure (CH,),(CO,H), for n = 2, 3 and 8 at 20 and 30°C give a constant value for the standard Gibbs energy of adsorption from solution in octane26 of -25.8 kJ mol-'. It can be seen that phthalate diester surface activity is greater than that of methyl dodecanoate and is comparable to that of non-aromatic diesters and (coincidentally) similar to that of n-alcohols.Note that these data provide a compari- son of the relative extents of adsorption into dilute films from solution in apolar alkanes in the absence of surfactant. The relative adsorptions into a concentrated film of a convention- al surfactant from the pure polar liquids can be expected to be very different. AOT Adsorption at the Phthalate Oilwater Interface In this section we compare the adsorption of AOT at the phthalate oil/water interface with that determined previously at the alkane/water interface. Plots of phthalate oil/water interfacial tension us. equilibrium AOT concentration in either the aqueous or oil phases are shown in Fig.4 and 5 for J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 .; E.. 15 C tl 5 10.-f0 .-E5 0 In( [AOT] .&no1 dm-3 .-5 -16 -14 -12 -10 -8 -6 4 In( [AOT]oi,/mol dm-3) Fig. 4 Variation of y with ln([AOT]/mol dm-3) with DBP as oil. (a) and (b)refer to the equilibrium AOT concentrations in the water and oil phases, respectively; [NaCl] = 0 (O), 0.5 (0)and 1(@) mol dm-3. The arrows in (b)mark the values for c.a.c.,,, . .-0 rn Q)c -16 -14 -12 -10 -8 -6 -4 L.-In([AOT]oi,/mol dm-3) Fig. 5 Variation of y with ln([AOTJ/mol dm-3) with DOP as oil. (a) and (6) refer to the equilibrium AOT concentrations in the water and oil phases, respectively; [NaCl] = 0 (O), 0.5 (0)and 1 (0)mol dm-’. The arrows shown in (b)mark the values of c.a.c.,,, .DBP and DOP, respectively. The break points mark the criti- cal aggregation concentrations (c.a.c.) in the aqueous and oil phases. Values of c.a.c. for the different NaCl concentrations and oils are collected in Table 3. For water, the slopes of the plots for concentrations below the c.a.c. can be analysed using the Gibbs equation to yield areas per AOT molecule at the oil/water interface. For AOT (a 1 :1 electrolyte) in water containing a 1 :1 inert electrolyte such as NaCI, the appropriate form of the Gibbs equation is: A = -kT[x + 2(8 lnf*/a In mD),,,]/(dy/d In mD) (4) where mD and m, are the respective molarities of the sur- factant anion and added salt (NaCI in this case) andf, is the mean ionic activity coefficient of the surfactant.The factor x = 1 + [mD/(mD + m$] varies between two (for zero added salt) and one (for [NaCl] % [AOT]).27 It has been shown previously that the activity coefficient term is eqn. (4) is negli- gibly small when the total ionic strength is small and when [NaCl] % [AOT].7 Thus the term involvingf, is negligible for all systems considered here. The limiting values of A for AOT (obtained at concentrations close to the c.a.c. where the monolayers become ‘close-packed’) are given in Table 3. The variation of the limiting areas with aqueous-phase NaCl concentration can be compared with the behaviour of AOT-alkane-water-NaC1 systems. For alkane systems, the areas decrease with increasing [NaCl] until a certain NaCl concentration is attained after which the areas remain con- stant. According to the simple geometrical ideas discussed by Mitchell and NinhamYZ8 and developed further by Aveyard et aLY8the area at high [NaCl] should be governed (perhaps mainly) by the size of the surfactant tail region including any penetrating oil solvent.Thus, a high limiting area at high salt concentrations for a particular oil implies that oil penetrates the surfactant monolayer to a high degree. Fig. 6 shows the limiting areas of AOT us. [NaCI] for DBP and DOP as oil phases. Although only three area values have been measured, the areas initially fall with increasing [NaCl], but attain con- stant values at high [NaCI], as seen for alkane systems.The limiting values of 0.9 and 0.7 nm2 for interfaces involving DBP and DOP, respectively, can be compared with areas of 0.73 nm2 determined for heptane and 0.61 nm2 for hexa- decane.’ Thus, increasing the chain length of the phthalate oils leads to a smaller area consistent with decreased pen- etration of the AOT monolayer by the longer chains of the oil molecules. Fig. 4(b) and 5(b) show the variation of the oil/water tension with the oil-phase concentrations of AOT. No plots of this type are shown for zero NaCl concentration as no AOT was detected in the oil phase. The plots involving oil- phase concentrations show less-marked break points at the c.a.c. than the corresponding water-phase concentration plots.The oil-phase concentrations which are present in equi- librium with the c.a.c. in water (c.a.c.,,,,,) are designated c.a.c.,,i, and increase with increasing aqueous-phase NaCl concentration (Table 3). If the activity coefficient of AOT in the oil phase remains invariant with AOT concentration [i.e. (a Inf,/a In mD),,,, = Table 3 Summary of parameters for the adsorption of AOT from dilute solution at the phthalate oil/water interface at 25 “C ’ ~ ~~ oil [NaCl]/mol dm - c.a.c.,,,,,/mmol dm - c.a.c.,i,/mmol dm - A/nmz DBP 0 2.5 0 1.23. 0.5 0.075 0.75 0.88 1.o 0.055 18 0.90 DOP 0 3.0 0 1.68 0.5 0.075 1.o 0.70 1 .o 0.045 3.O 0.70 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90I1.4 , I I 1 I 0 0.2 0.4 0.6 0.8 1 .o [NaCI]/mol dm-3 Fig. 6 Variation of the limiting areas per AOT molecule with WaCl] for DBP (a) as oiland DOP (0) 01, the slopes of the oil-phase plots should be the same as those of the water-phase plot because the area per AOT derived from each data set must have the same value.At the c.a.c. in each phase, the oil-phase slope is CQ. 0.5 that of the water slope for both oils at 0.5 mol dmP3 NaCl. The ratio of slopes is 0.2 for 1 mol dm-3 NaCl for both oils. It can be seen that the oil plot slopes are much lower than for the cor- responding water plots and hence [from eqn. (4)] the value of (aIn f*/a In mD), for the oil phase must be significantly less than zero.Such a decrease in the activity coeficient of the surfactant in the oil phase with increasing concentration is consistent with a progressive aggregation of the surfactant in the oil at concentration less than the c.a.c.,,. The non-ideal behaviour in the oil phase increases with increasing NaCl concentration. This type of pre-c.a.c. aggregation has been observed previously for AOT in systems containing non-aqueous polar solvents with toluene as the oil.' We now consider the variation of the system behaviour with aqueous phase NaCl concentration for AOT concentra- tions above the c.a.c. in mixtures containing comparable volumes of oil and water. With alkanes as the oil the behav- iour is as follows. At low salt concentrations such systems form two phases consisting of an oil-in-water (o/w) micro- emulsion phase in equilibrium with an excess oil phase (a Winsor I system).At high salt concentrations a Winsor I1 system consisting of a water-in-oil (w/o) microemulsion coexisting with an excess aqueous phase is formed. At inter- mediate salt concentrations a three phase Winsor 111 system is formed consisting of a surfactant-rich phase coexisting with excess water and oil phases. The phase progression from Winsor I to Winsor I11 to Winsor I1 is called microemulsion phase inversion and corresponds to a progression in aggre- gated surfactant monolayer curvature from positive in o/w microemulsions, through zero in the Winsor 111 range, to negative in w/o microemulsions. The tendency of the sur-factant monolayer to curve depends on the effective geometry of the surfactant in the monolayer which, in turn, depends on surfactant molecular structure, solvent penetration into the head and tail regions and the strength of electrostatic inter- actions between adjacent surfactant headgroups.Phase inver- sion can be achieved by the variation of any solution parameter which alters the tendency of the monolayer to curve. For example, the addition of salt drives the phase pro- gression towards the Winsor I1 system by screening the elec- trostatic repulsion between headgroups and thus shrinking their effective size. For surfactant concentrations greater than the c.a.c., the interfacial tension of the plane oil/water interface separating the bulk oil and water phases (7,) passes through a minimum value when the system is driven through a phase inversion. The minimum tension occurs within the Winsor I11 range and hence is found under conditions when the preferred sur- factant monolayer curvature is close to zero.The variation of yc with aqueous-phase [NaCl] for DBP and DOP as oil is shown in Fig. 7. The tension passes through a minimum in each case at a salt concentration of ca. 0.5 mol dm-3 NaCl. In comparison, the [NaCl] required for phase inversion of AOT with n-alkanes as oil varies from cu. 0.05 mol dm-3 NaCl for heptane to ca. 0.1 mol dm-3 NaCl for tetradecane.8 This variation for alkanes has been rational- ised on the basis that short-chain alkanes penetrate and swell the tail regions of surfactant monolayers to a greater extent than long-chain alkanes.Hence, both dialkyl phthalates behave like very long chain length (non-penetrating) alkanes in terms of the [NaCl] required to achieve phase inversion. Phase Compositions of Equilibrated AOT-W a t er-Dial kylpbthala te Oil Mixtures Equilibrium distributions of AOT between water and phthal- ate oil phases (DBP and DOP) for AOT concentrations less than the c.a.c. are shown in Fig. 8. No results are shown for zero [NaCl] since 5 mol dm-3 AOT (the detection n [NaCl]/mol dm-3 0.2 (b) [NaCl]/mol dm-3 Fig. 7 Variation of log(y,/mN me')with aqueous phase [NaCl] for (a)DBP and (b) DOP as oil J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 h mI E 0.1 0-zE-.. m i=09 v 0)-0.0' c 11 0.1 1 10 100 log( [AOT]oi,/mmol dm-3) Fig. 8 Equilibrium distributions of [AOT] between water and the phthalate oils for AOT concentrations less than the c.a.c. Note the logarithmic scales. The solid line shows a slope of unity. (a)DBP and (0)DOP (0.5 mol dm-3 NaCl), (W) DBP and (0)DOP (1 mol dm -NaCl). limit of the surfactant titration method21 was found in either of the equilibrated phthalate oil phases. The distribution of a solute may be described by the dis- tribution or partition coefficient P equal to the ratio of equi- librium solute activities in the oil and water phases, eqn. (9, P = Cs~ecieslwaterfwater/C~~e~iesloilf,il (5) where f refers to the activity coefficient and the subscripts refer to the phase. The equilibrium concentration of the species in oil is directly proportional to the concentration in water only if the ratio of activity coefficients in both phases remains constant over the range of concentrations tested.For such a situation, a plot of log[spe~ies]~,,,~ isus. log[specie~]~~, linear and of unit slope (see Fig. 8). For the aqueous phases in the present work, the concentrations of AOT are all < mol dm-3 and hence f,,,,, should be constant for a particular NaCl concentration. If it is assumed that all the deviations from the unit slope behaviour in the data of Fig. 8 are due to changes only inLi, (as for the interpretation of the oil/water tension data), then the results can be used to give a crude estimate of the decrease in activity coefficient of AOT in the oil as the AOT concentration is increased up to the c.a.c.in the oil. It can be seen that the curves for the different oils overlap, but that the 1.0 mol dm-3 NaCl data deviate more from ideal behaviour than the 0.5 rnol dme3 data. For both DBP and DOP as oil, the activity coefficient of AOT decreases ca. five-fold for 0.5 mol dm-3 NaCl and ca. 100-fold for 1.0 mol dm-3 NaCl as the concentration is increased in the c.a.c. in the oil phase. This variation suggests that AOT undergoes a progressive aggregation in the oil phase reaching aggregation numbers in the order of five and 100 for the two different salt concentrations before attaining the c.a.c.at which microemulsion aggregates are formed. Thus, the pre- c.a.c. partitioning behaviour strengthens the qualitative con- clusion concerning non-ideality drawn from the interfacial tension data. For AOT concentrations much larger than the c.a.c., the equilibrium distributions of total (monomer plus aggregated) AOT between the water, oil and third phase (where present) as a function of aqueous phase [NaCl] for both phthalate oils are shown in Fig. 9. For zero [salt], virtually all the AOT IOOr [NaCl]/mol dm-3 2ot t I In 0.5 1.o 1.5 :0 [NaCl]/mol dm -3 Fig. 9 Equilibrium distribution of total AOT between water (O),oil (0)and third phase (0)(where present) as a function of aqueous phase WaCl] for (a) DBP ; (b)DOP is in the aqueous phase, hereas it is all in the oil phase for salt concentrations 21 mol dmV3.Over a range of NaCl concentrations in the region of 0.5 mol dm-3, the AOT is located mainly in a third phase. Thus it can be seen that the post c.a.c. AOT distribution follows the pattern expected for the Winsor 1-111-11 phase progression and that micro-emulsion phase inversion is centred around 0.5 mol dm-, NaCl for both phthalate oils, as indicated also by the varia- tion of yc with [NaCl]. The compositions of the equilibrim surfactant-rich third phases were measured as a function of [NaCl] over the Winsor I11 range. The volume fractions of AOT and water were obtained by Hyamine and Karl Fischer titration, respectively. The oil volume fraction was then estimated by difference.For DBP as oil, the oil volume fraction remains virtually constant at 0.1 over the range of salt concentration from the Winsor-I11 transition to the Winsor 111-11 bound-ary. Over the same salt concentration range, the water volume fraction decreases from 0.73 to 0.30. For DOP as oil, the water volume fraction decreases from 0.55 to 0.20, whilst the oil volume fraction increases from 0.08 to 0.45. Hence DBP is not taken up into third phases to any great extent whereas DOP is extensively solubilised at salt Concentrations close to the Winsor 111-11 boundary. For both oils, the sur- factant volume fractions in the third phases are relatively high (in the range 0.4-0.6). These composition changes in three-phase systems are significantly different to those in systems containing non-polar There are a number of different possible microstructures for surfactant-rich phases of Winsor I11 systems which all have the average surfactant monolayer curvature is ca.zero. Possible microstructures include lamellar liquid-crystalline phases, bicontinuous microemulsions or L, phases.30 All samples of third phases prepared with either DBP or DOP were observed to be optically isotropic and hence are not J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 lamellar phases. For bicontinuous microemulsions, the phase composition is related to the microstructural repeat distance 5 according to eqn. (6),31,32 < = constant 4oil~,,,,,/[surfactant]A (6) where $oil and are the volume fractions of oil and water, respectively, [surfactant] is in units of the number of molecules per unit volume and A is the area occupied per surfactant molecule at the interface.Depending on the exact model assumed for the microstructure, the numerical con- stant can take values of 4,33634 or 5.82.35For the purposes of this discussion we have assumed a value of 6. For both DBP and DOP the volume fraction of AOT ($AOT) in the third phase is of comparable magnitude to $oil and $,at,,. In order to proceed further with the approximate estimation of 5 the oil and water volume fractions have each been taken to include half 4AOT, i.e. 4Later = +water + &$AOT and 6bil = 4oil + $$AoT. If the values of 5 and A are constant throughout the range of Winsor I11 microemulsions, then a plot of [surfactant] us.the product ($bil 4kater)should be linear with the slope equal to (6/5A).The plots for DBP and DOP (Fig. 10) are approximately linear. Taking average values of A for AOT over the range of [NaCl] of 0.9 nm2 in systems with DBP and 0.7 nm2 in the case of DOP (see Table 3), 5 values of ca. 1 and 5 nm are obtained for the DBP and DOP systems, respectively. Owing to the crude approximations used, these values should be viewed only as an indication of the order of magnitude of 5. It has been shown the~retically~~ and experimentally for a variety of microemulsion systems37 that the minimum post- c.a.c. interfacial tension y, (min) scales approximately with the inverse square of the characteristic size of the third-phase microemulsions.To a zeroth-order approximation : y,(min) = k~/r~ (7) Using eqn. (7) and the values of y,(min) (0.67 and 0.48 mN m-' for the DBP and DOP systems, respectively), the values I I I I I0.5 $Later 4bil Fig. 10 Variation of [AOT] with ($Li,$kater) for third phases with DBP (0)and DOP (0)as oil of t are estimated to be 2.5 and 3.0 nm. Thus, the values of the minimum tensions are broadly consistent with the magni- tudes of the calculated repeat distances in the third phases. The partitioning of chloride ion between the bulk aqueous phase and the dispersed water of the third phases was mea- sured by Mohr titration of both the aqueous and third phases.The results can be conveniently represented in terms of the ratio of chloride ion concentration per unit volume of dispersed water in the third phase to that in the aqueous phase (Pdwlaq{[Cl-] in the dispersed water/[Cl-] in the = aqueous phase)). For both DBP and DOP as oil, the value of Pdwlaqdecreases from ca. 0.7 at 0.55 mol dm-3 NaCl to ca. 0.5 at 1 mol dm-3 NaCl. This behaviour, observed previously for AOT in systems with alkanes as oil3* and for a range of other ionic microemulsion is caused by the proximity of the charged surfactant interface which repels the C1- ions and hence causes a reduction in the concentration of NaCl in the dispersed water. For Winsor I1 systems formed at high [NaCl], the extent of water solubilisation in the w/o microemulsion phase is expected to decrease as the preferred monolayer curvature becomes increasingly negative with increasing salt concentra- tion.Fig. 11 shows the variation of R (defined as [water]/ [AOT] in the equilibrium w/o microemulsion phases of the Winsor I1 systems) with [NaCl]. As expected, the water solu- bilisation (and hence the microemulsion droplet size) decreases with increasing [NaCl]. Overall, the amount of water solubilised is rather low, corresponding to little more than the hydration requirements of the AOT molecule.42 In this case, the aggregates formed are probably better con-sidered as hydrated reversed micelles rather than w/o micro- emulsions. Uptake of Phtbalate oils into AOT Monolayers When added to the surface of an aqueous surfactant solution, a small quantity of a non-spreading oil will form a lens in I 12 10 R 8 6 I I ,4 1.o 1.5 2.0 2.5 3.0 [NaCl]/mol dm-3 Fig.11 Variation of molar ratio of solubilised water to AOT (R) vs. [NaClJ for Winsor I1 microemulsions with DBP (0)and DOP (0)as oil J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 neptane/water tensions for the addition of DBP and DOP to Winsor I and Winsor I1 systems containing AOT (2.3 mmol dmP3), water, NaCl and heptane at 25 "C [phthalate oil]/mmol dm-3 DBP Winsor I 0 1.9 9.4 Winsor I1 0 1.9 9.4 DOP Winsor I 0 1.3 Winsor I1 6.3 0 1.3 6.3 equilibrium with a mixed monolayer containing both oil and s~rfactant.~~?~~The adsorption of oil into the monolayer results in a decrease in the surface tension by an amount Ay.The surface excess concentration of the oil rOi,in the mixed monolayer is given by the following equation. roil= Ay/kT (8) Eqn. (8) is accurate insofar as Ay is equal to the differential quantity (dAy/d In uoiJ in the limit of the activity of the oil aoiltending to unity. This assumption is true to within ca. 15% for alkane Values of Ay for dialkyl phthalate oils of chain lengths ranging from ethyl to dodecyl were measured for aqueous AOT solutions above the c.m.c. in the absence of NaCl. Ay was found to be <0.1 mmol dm-3 m-' (equal to the accu- racy of the measurements) in every case indicating that little oil penetration of AOT monolayers occurs under these condi- tions.A similar lack of penetration by long-chain alkanes into AOT monolayers has been noted previ~usly.~~The experiment reported here therefore shows that the extent of penetration is not increased to any large extent for the mod- erately polar phthalate oils. Possible coadsorption and cosurfactant properties of the phthalate oils DBP and DOP were examined in the following way. As shown in ref. 8, the addition of strongly adsorbing consurfactant species such as alcohols to oil-water mixtures containing an AOT concentration in excess of the c.a.c. can result in a minimum oil/water interfacial tension under appropriate conditions. For an initial Winsor I system, addi- tion of a cosurfactant which increases the negative curvature of the monolayer will yield a tension minimum whereas a cosurfactant favouring increased postive curvature will cause phase inversion only for an initial Winsor I1 system.The possible cosurfactant effect of DBP and DOP was tested by measuring the heptane/water tension for different phthalate oil concentrations in heptane-water-NaC1-AOT mixtures forming either a Winsor I or Winsor I1 system in the absence of phthalate oil. The results are shown in Table 4; the addition of both phthalate oils to the heptane systems always results in an increase in tension for both Winsor I and I1 systems consistent with the fact that the curves of yc us. [NaCl] for DBP and DOP are shifted to higher tensions relative to the curve for heptane.It can be concluded that DBP and DOP do not act as strongly adsorbing cosurfac- tants in the AOT-alkane system. The lack of penetration of concentrated AOT monolayers by the phthalate oils is con-sistent with the observation that a relatively high NaCl con- centration (compared with the concentrations required for alkanes as oils) is required to phase invert the phthalate oil systems. [NaCl]/mol dm- yJmN m-' 0.0 174 0.35 0.0174 0.39 0.0174 0.48 0.0685 0.14 0.0685 0.16 0.0685 0.25 0.0174 0.35 0.0174 0.37 0.0174 0.35 0.0685 0.14 0.0685 0.21 0.0685 0.32 Conclusions The main conclusions of this study can be summarised as follows. (i) Phthalate oils are adsorbed at the heptane/water interface from dilute solution in heptane.The surface activity for dilute monolayers is typical of diesters and similar to that of alcohols. (ii) AOT is adsorbed at the phthalate oil/water interface to yield saturated monolayers of similar surface con- centrations those found for alkane oils. (iii) At concentrations below the c.a.c., AOT partitions from water to phthalate oils to a significant extent at high salt concentrations. (iv) AOT aggregates progressively in solution in the phthalate oils at concentrations below that required for micromulsion forma- tion. (v) Systems containing AOT and dialkyl phthalates undergo the Winsor 1-111-11 microemulsion phase inversion progression with the addition of NaCl. Phase inversion is centred at ca. 0.5 mol dm-3 NaCl for both DBP and DOP. (vi) The characteristic domain size in the isotropic third phases and the maximum water solubilisation in w/o micro- emulsions are both relatively small (and the minimum oil/ water tension is correspondingly high) for systems with phthalate oils as compared with short chain-length alkane systems. (vii) Phthalate oils appear to show little tendency to penetrate concentrated AOT monolayers at both the oil/ water and air/water interfaces.This is interesting since the phthalates have similar surface activities (from dilute solution in alkane oils) as alcohols and the latter are very effective cosurfactants. We wish to thank Kodak Ltd. (Harrow) and the SERC for the provision of a CASE Studentship to P.A.K. References 1 K.Shinoda and S. Friberg, in Emulsions and Solubilisation, Wiley ,New York, 1986. 2 R. Aveyard, Chem. Ind., 1987,474. 3 D. Langevin, Acc. Chem. Res., 1988,21,255. 4 M. Kahlweit, R. Strey, P. Firman, D. Haase, J. Jen and R. Scho-macker, Langmuir, 1988,4,499. 5 M. Kahlweit, R. Strey, R. Schomacker and D. Haase, Langmuir, 1989,5, 305. 6 A. Martino and E. W. Kaler, J. Phys. Chem., 1990,94, 1627. 7 R. Aveyard, B. P. Binks, P. D. I. Fletcher, A. J. Kirk and P. Swansbury, Langmuir, 1993,9,523. 8 R. Aveyard, B. P. Blinks and J. Mead, J. Chem. SOC., Faraday Trans. I, 1986,82,1755. 9 M. Kahlweit, R. Strey and G. Busse, J. Phys. Chem., 1991, 95, 5344. 10 S. E. Friberg and L. Gan-Zuo, J. SOC. Cosmet. Chem., 1983, 34, 73. 11 H.Kunieda, H. Asaoka and K. Shinoda, J. Phys. Chem., 1988, 92, 185. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2751 12 13 14 15 J. Alander and T. Warnheim, J. Am. Oil Chem. SOC., 1989, 66, 1656. K. Wormuth and E. W. Kaler, J. Phys. Chem., 1989,93,4855. M. El-Nokaly, G. Hiler, Sr. and J. McGrady in Microemulsions and Emulsions in Foods, ed. M. El-Nokaly and D. Cornell, ACS Symposium Series 448, Washington DC, 1991, ch. 3. S. Mukherjee, C. A. Miller and T. Fort, J. Colloid Interface Sci., 29 30 31 32 0. Abillon, B. P. Binks, C. Otero, D. Langevin and R. Ober, J. Phys. Chem., 1988,92,4411. H. Kellay, Y. Hendrikx, J. Meunier, B. P. Binks and L. T. Lee, J. Phys. 11, 1993,3, 1747. L. Auvray, J. P. Cotton, R. Ober and C. Taupin, J. Phys. (Paris), 1984,45,913.B. P. Binks, J. Meunier and D. Langevin, Prog. Colloid Polym. 1983,91, 223. Sci., 1989,79,208. 16 B. W. Ninham, S. J. Chen and D. Fennel1 Evans, J. Phys. Chem., 33 P. Debye, H. R. Anderson Jr. and H. Brumberger, J. Appl. Phys., 1984,88,5855. 1957,28,679. 17 B. P. Binks, H. Kellay and J. Meunier, Europhys. Lett., 1991, 16, 34 J. Jouffroy, P. Levinson and P. G. de Gennes, J. Phys. (Paris), 53. 1982,43,1241. 18 Solute-Solvent Interactions, ed. J. F. Coetzee and C. D. Ritchie, Marcel Dekker, New York, 1969, p. 393. 35 36 Y. Talmon and S. Prager, J. Chem. Phys., 1978,69,2984. M. Kahlweit and H. Reiss, Langmuir, 1991,7, 2928. 19 20 21 22 23 24 25 J. F. Padday and A. Pitt, Proc. R. SOC. London, A, Math. Phys. Sci., 1972,329,421.P. A. Kingston, Ph.D. Thesis, University of Hull, to be published. V. W. Reid, G. F. Longman and E. Heinerth, Tenside, 1967, 4, 292. A. I. Vogel, in A textbook of Quantitative Inorganic Analysis, Longman, London, 5th edn. 1989. R. K. Schofield and E. K. Rideal, Proc. R. SOC.London, A Math. Phys. Sci., 1925, 109, 57. R. Aveyard and D. A. Haydon, in An Introduction to the Prin- ciples of Surface Chemistry, Cambridge University Press, Cam- bridge, 1973, ch. 1. R. Aveyard and B. J. Briscoe, J. Chem. SOC.,Faraday Trans. I, 37 38 39 40 41 42 43 44 R. Aveyard, B. P. Binks, S. Clark, P. D. I. Fletcher, H. Giddings, P. A. Kingston and A. Pitt, Colloids Surf., 1991,59,97. P. D. 1. Fletcher, J. Chem. SOC.,Faraday Trans. 1, 1986,82,2651. P. L. de Bruyn, J. Th. G. Overbeek and G. J. Verhoeckx, J. Colloid Interface Sci., 1989, 127, 244. S. Levine and K. Robinson, J. Phys. Chem., 1972,76,876. J. Biais, M. Barthe, M. Bourrel, B. Clin and P. Lalanne, J. Colloid Interface Sci., 1986, 109, 576. C. A. Martin and L. J. Magid, J. Phys. Chem., 1981,85,3938. R. Aveyard, P. Cooper and P. D. I. Fletcher, J. Chem. SOC., Faraday Trans., 1990,86,3623. J. R. Lu, R. K. Thomas, R. Aveyard, B. P. Binks, P. Cooper, P. D. I. Fletcher, A. Sokolowski and J. Penfold, J. Phys. Chem., 1992,%, 10971. 1972,68,478. 26 27 R. Aveyard and J. Chapman, Can. J. Chem., 1975,53,916. D. G. Hall, B. A. Pethica and K. Shinoda, Bull. Chem. SOC.Jpn., 1975,48,324. 28 J. D. Mitchell and B. W. Ninham, J. Chem. SOC.,Faraday Trans. 2, 1981, 77, 601. Paper 41029576; Received 18th May, 1994