Arenediazonium Salts: New Probes of the Compositions of

Aug 31, 2000 - Arenediazonium Salts: New Probes of the Compositions of Association Colloids. 7. Average Hydration Numbers and Cl- Concentrations in th...
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Arenediazonium Salts: New Probes of the Compositions of Association Colloids. 7. Average Hydration Numbers and Cl- Concentrations in the Surfactant Film of Nonionic C12E5/Octane/Water Macroemulsions: Temperature and NaCl Concentration Effects† Jihu Yao and Laurence S. Romsted* Department of Chemistry, Wright and Rieman Laboratories, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received April 10, 2000. In Final Form: July 13, 2000 A surface-active arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, was used as a chemical trapping probe of the concentrations of weakly basic nucleophiles in the surfactant film between the aqueous and oil domains of opaque relatively long-lived C12E5/octane/water macroemulsions as a function of temperature and added NaCl. The results demonstrate that the chemical trapping method “sees” the composition of the oligooxyethylene or interfacial layer on the aqueous side of the surfactant film. Product yields from trapping of the available nucleophiles, H2O, the terminal OH group of C12E5, and Cl-, were used to estimate average hydration numbers of all aggregates present in the macroemulsions as a function of increasing temperature and added NaCl. At 19.9 °C, the average hydration number in the macroemulsions is 2.5, compared to a 70% larger value of 4.2 in C12E6 micelles at 20 °C obtained previously by chemical trapping. Average hydration numbers decrease with increasing temperature and with added NaCl at constant temperature as macroemulsion structure changes from oil-in-water type through the balanced point to water-in-oil type. These results are consistent with the oriented wedge theory of macroemulsion stability. Chemical trapping estimates of Cl- concentrations within the interfacial layer of the surfactant film of the macroemulsions show that the molarity of Cl- in the aqueous region of the interfacial layer is always about 10% greater than the Cl- molarity in the aqueous domain. Thus, Cl-, and probably Na+, move freely between the bulk aqueous domain and the surfactant film of the macroemulsions, contradicting an assumption of adsorption/depletion model for the effect of lyotropic salts on the hydration of the interfacial layer. These results demonstrate the potential of chemical trapping for probing surfactant film compositions of opaque macroemulsions.

Introduction Macroemulsions are mixtures of oil, water, and surfactant that often contain other components such as salts.1,2 Macroemulsions differ from transparent, thermodynamically stable solutions of micelles and microemulsions composed of the same components because macroemulsions are opaque thermodynamically unstable mixtures of immiscible phases, although they may have substantial kinetic stabilities. Macroemulsions tend to be either oil-in-water (O/W) or water-in-oil (W/O) type in which water or oil, respectively, is the continuous phase. Several approaches, e.g., Bancroft’s rule3 and Griffin’s hydrophilic-lipophilic balance, HLB, scale4 have been used, with varying degrees of success, to correlate surfactant structure and properties with macroemulsion stability.5 More recently, Shinoda’s phase inversion temperature (PIT),1 now called the balanced temperature or point,5 approach also takes the nature of the oil, temperature, and salt into account. In this approach, the * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: 732-445-3639. FAX: 732-445-5312. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. (1) Shinoda, K.; Friberg, S. Emulsions and Solubilization; John Wiley & Sons: New York, 1986. (2) Sjoblom, J. Emulsions-A Fundamental and Practical Approach; Kluwer: Amsterdam, 1992; Vol. 363. (3) Bancroft, W. D. J. Phys. Chem. 1915, 19, 275. (4) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 249. (5) Kabalnov, A.; Wennerstrom, H. Langmuir 1996, 12, 276-292.

stabilities of nonionic macroemulsions composed of surfactants of the monoalkyl oligooxyethylene, CmEn, type are correlated with the properties of the corresponding thermodynamically stable phases of the same component concentrations. Because macroemulsions are opaque, models for their structures and compositions, especially that of the surfactant film between the oil and water domains, are difficult to test experimentally. The chemical trapping method, based on the dediazoniation chemistry of 4-hexadecyl-2,6-dimethylarenediazonium ion, 16-ArN2+, bound the interfacial region of surfactant aggregates, provides a novel way to “see” the compositions of the oligooxyethylene or interfacial layer of the surfactant film between the oil and aqueous domains. The method already has been used to estimate the hydration numbers of nonionic and mixed nonionic micelles6-8 and the interfacial anion concentrations of aqueous zwitterionic9,10 micelles and of cationic micelles and microemulsions.11-14 16-ArN2+ is trapped by weakly (6) Bunton, C. A.; Romsted, L. S. Organic Reactivity in Microemulsions; Kumar, P., Ed.; Marcel Dekker: New York, 1999; pp 457-482. (7) Romsted, L. S.; Yao, J. Langmuir 1999, 15, 326-336. (8) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425-2432. (9) Cuccovia, I. M.; Romsted, L. S.; Chaimovich, H. J. Colloid Interface Sci. 1999, 220, 96-102. (10) Jain, M. K.; Rogers, J.; Yu, B.-Z.; Yao, J.; Romsted, L. S.; Berg, O. G. Biochemistry 1997, 36, 14512-14530. (11) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59-71. (12) Yao, J.; Romsted, L. S. J. Am. Chem. Soc. 1994, 116, 1177911786. (13) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362-8367.

10.1021/la000533l CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000

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basic nucleophiles, such as H2O, terminal OH groups of CmEn, and Cl- in the experiments described here, that are present in the interfacial layer, and the product yields are used to estimate their interfacial concentrations over wide ranges of component concentrations. Indeed, the chemical trapping method is applicable to many nucleophiles commonly found in biomembranes and surfactant-based commercial products.6 In 1996, Kabalnov and Weers measured the rate of breakdown of the “milky” layer of macroemulsions prepared from octane/H2O/C12E5 as a function of temperature and of added NaCl at constant temperature.15 The experiments were designed as a test of the “oriented wedge” theory for the kinetic stability of macroemulsions.5 These macroemulsions are generally long-lived between 5 and 50 °C, and their half-lives for breakdown vary from about 103 s at 50 °C to 106 s at lower temperatures, except at temperatures between 31 and 34 °C. In this temperature range the macroemulsions become quite unstable, and the half-life for breakdown shows a sharp minimum at 32.7 °C with a half-life of about 102 s, a maximum decrease in stability of about 104 s. Addition of NaCl from 0 to 20 wt %, at 19.9 °C produces the same basic pattern, i.e., long-lived macroemulsions with half-lives g106 s, except between about 8 and 10 wt % NaCl where a sharp minimum occurs at 9.25 wt % with a half-life of about 102 s. Macroemulsions with minimal stability are formed from thermodynamically stable phases in which the spontaneous curvature of the surfactant aggregates is about zero; i.e., the boundary between the aqueous and oil domains is essentially flat. This characteristic temperature or salt concentration for the system is called the balanced point, which depends on surfactant structure, oil type, and salt concentration.1,5 The oriented wedge theory gives an excellent interpretation of these observations. Changes in macroemulsion stability with increasing temperature or added salt are generally attributed to changes in spontaneous curvature of the surfactant aggregates in the thermodynamically stable phases.5,16,17 For example, at low temperatures the thermodynamically stable octane/H2O/C12E5 system is biphasic with an upper phase of excess oil containing monomeric C12E517,18 in equilibrium with a lower aqueous phase containing surfactant micelles with positive curvature; i.e., the surfactant headgroups on the surface of octane containing spheriodal micelles are in contact with bulk water and the surfactant tails are in the centers of the micelles. This biphasic system is in the Winsor I19 region in the phase diagram for these components. Mixing these phases produces relatively long-lived O/W macroemulsions. As the temperature increases, the system becomes triphasic with a microemulsion phase, in which the spontaneous curvature of the aggregates approaches zero, layered between an upper oil phase and a lower aqueous phase. The triphasic system is in the Winsor III region in the phase diagram. Mixing these phases gives relatively shortlived macroemulsions. At higher temperatures the system again becomes biphasic with the upper oil phase containing reverse micelles with negative spontaneous curvature; i.e., the headgroups of the surfactant are buried in a water droplet and the surfactant tails are dissolved in the (14) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351-8361. (15) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 1931-1935. (16) Strey, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 402-410. (17) Strey, R. Colloid Polym. Sci. 1994, 272, 1005-1019. (18) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. Phys. Chem. Chem. Phys. 1999, 1, 4299-4306. (19) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth: London, 1954.

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Figure 1. Cartoon showing the composition of the oil and water microdomains in the immediate vicinity of a small section of the surfactant film of a nonionic microemulsion or macroemulsion composed of octane/H2O/C12E5 and NaCl and the location of chemical trapping reagent 16-ArN2+. The reactive diazonio group is assumed to be in the interfacial layer within in the surfactant film. The dashed line represents the boundary between the oil domain and the hydrated interfacial layer. No attempt is made to represent the actual distributions or concentrations of components.

surrounding bulk oil in contact with a lower excess aqueous phase containing a small amount of monomeric surfactant. This biphasic system is in the Winsor II region of the phase diagram. Mixing these phases produces relatively long-lived W/O macroemulsions. In thermodynamically stable phases, nonionic surfactants form monomolecular films between the oil and water domains. In macroemulsions, surfactants are also assumed to layer the boundaries between oil and water domains. Figure 1 is a cartoon of a small cross section of the surfactant film for both micro and macro systems. This representation has the same basic features as the swollen block-copolymer brush model of the surfactant film between the oil and water domains.20 The drawing illustrates an interface with zero spontaneous curvature, i.e., the surfactant film at the balanced point in a Winsor III microemulsion phase. At low temperatures in the Winsor I region, the surfactant aggregates have positive spontaneous curvature and the boundary between the oil domain and aqueous interfacial layer is curved toward the oil domain. Conversely, at high temperatures reverse aggregates have a negative spontaneous curvature and the boundary is curved toward the aqueous domain. This change in spontaneous curvature, positive to zero to negative, with increasing temperature is attributed to decreasing hydration and tighter packing of the oligooxyethylene chains.5,15 (20) Kabalnov, A.; Olsson, U.; Wennerstrom, H. J. Phys. Chem. 1995, 99, 6220-6230.

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At 19.9 °C, increasing the NaCl concentration induces the same pattern of phase changes and changes in macroemulsion stability as increasing the temperature.15 These transitions have been interpreted by using a adsorption/depletion model for specific anion effects on the structure of nonionic microemulsions.20 Added “lyotropic” salts, e.g., NaCl, NaBr, and Na2SO4, are assumed to dehydrate the interfacial layer. Added “hydrotropic” salts, e.g., NaI and NaSCN, have the opposite effect because they absorb to and swell the surfactant film. The goals of the chemical trapping experiments in macroemulsions described here were to answer three questions concerning the chemical trapping method and the oriented wedge and adsorption/depletion models. First, does the method provide estimates of the concentrations of terminal OH groups of C12E5, H2O, and interfacial Clconcentrations in C12E5/octane/water macroemulsions under the identical conditions used by Kabalnov and Weers?15 Second, does increasing the temperature and adding NaCl decrease the average hydration number of these macroemulsions as required by the oriented wedge theory?5 Third, does added NaCl dehydrate the interfacial layer of the surfactant film without penetrating it as assumed in adsorption/depletion theory?20 Rationale of the Chemical Trapping Method The chemical trapping probe, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, is insoluble in water but dissolves readily in surfactant solutions and in macroemulsions. 16-ArN2+ orients at the interfacial layer of surfactant aggregates with its hydrocarbon tail buried in the hydrophobic domain and its headgroup located in the oligooxyethylene or interfacial layer, Figure 1.7 The ensemble of aggregate-bound arenediazonium ions is assumed to distribute with the C12E5 surfactant throughout the totality of the coexisting aggregate structures present in the macroemulsions, from large droplets to small micelles. Dediazoniation of 16-ArN2+ is a thermalinduced, spontaneous, rate-determining loss of N2, to give a highly reactive aryl cation that is trapped by weakly basic nucleophiles within the interfacial layers of the aggregates.14,21 Scheme 1 illustrates the basic mechanism for the trapping of the weakly basic nucleophiles in bulk solution using 1-ArN2+ and in the interfacial layer using 16-ArN2+ as shown in Figure 1. Product yields from trapping of 1-Ar+ by nucleophiles in bulk solution and of 16-Ar+ by nucleophiles in the interfacial layers in the aggregates are proportional to the concentrations of nucleophiles in bulk solution and the interfacial layers, respectively. Two pieces of information are needed to estimate interfacial Cl- concentrations and average hydration numbers in interfacial layers of macroemulsions. (a) The (21) Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic Compounds; VCH Publishers: Weinheim, 1994.

first is quantitative measurements of product yields from dediazoniation reactions in the macroemulsions, Scheme 1. Products are separated and analyzed by high-performance liquid chromatography (HPLC) using a UV detector calibrated with independently prepared samples of the products or structurally related analogues. The macroemulsions are diluted with sufficient EtOH or i-PrOH to give homogeneous solutions prior to HPLC analysis. (b) The second is selectivities of the dediazoniation reaction toward the terminal OH group of C12E5 compared to water and toward Cl- compared to water in macroemulsions. These selectivities are defined by eqs 1 and 2, respectively

SWROH )

SWCl )

{% 16-ArOH} H2Om ) {% 16-ArOH} ROHm {% 16-ArOH} NW (1) {% 16-ArOH} NROH

{% 16-ArCl} H2Om {% 16-ArCl} NW ) Cl {% 16-ArOH} {% 16-ArOH} NCl m

(2)

where % 16-ArOR, % 16ArCl, and % 16-ArOH represent the ether, halide, and phenol product yields, respectively, from reaction 16-ArN2+ with the terminal OH groups of C12E5, Cl-, and H2O in the interfacial layers of the macroemulsions. H2Om, Clm, and ROHm represent the molarities and NW, NCl, and NROH, represent the moles of H2O, Cl-, and terminal OH groups, respectively, that trap 16-Ar+ in the interfacial layer of the macroemulsions (see below). Values for SWROH and SWCl cannot be determined in aggregated systems because both the selectivities and interfacial concentrations are unknown within the interfacial layers of aggregates. The selectivities are obtained independently in reference solutions that mimic the compositions of the interfacial layers of the aggregates (see Results). SWROH is given by eq 3

SWROH )

{% 1-ArOR} [H2OW] {% 1-ArOH} [ROHW]

(3)

where % 1-ArOR and % 1-ArOH are product yields from reaction with a water-soluble alcohol that models the terminal OH group of C12E5 and H2O, respectively, and [ROHW] and [H2OW] are the molarities of the model alcohol and H2O in solution. Note: here and throughout the text square brackets represent concentration in moles per liter of total solution volume in the absence of surfactant or concentration in moles per liter of aqueous phase volume in macroemulsions. SWCl is given by eq 4

SWCl )

{% 1-ArCl} [H2OW] {% 1-ArOH} [ClW]

(4)

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where % 1-ArCl and % 1-ArOH are product yields from reaction with Cl- and H2O, respectively, and [ClW] and [H2OW] are the measured concentrations of Cl- and H2O in solution. Average hydration numbers determined by chemical trapping are defined as the molar ratio of H2O to ethylene oxide units, NW:NE, of the oligooxyethylene chain of C12E5 in the interfacial layer, first equality in eq 5. The second equality in eq 5 shows that this ratio is equal to

average hydration number ) NW {% 16-ArOH} NROH ) SWROH (5) NE {% 16-ArOR} NE the product of the selectivity of the dediazoniation reaction, the measured product yield ratio from reaction with water and the terminal OH groups of C12E5, and the average molar ratio of terminal OH groups, NROH, to ethylene oxide units, NE, throughout the interfacial regions of the ensemble of the aggregates in the macroemulsion. As noted above, SWROH is determined in aqueous solutions by using eq 3 in the absence of macroemulsions (see below). We assume that the selectivity of the reaction toward model alcohols and water in homogeneous solution is the same as that of the terminal OH groups and water in the interfacial layers in the macroemulsions. The molar ratio, NROH/NE, is obtained from the uniform distribution model.7,8 In the uniform distribution model, components in the oligoethylene layer, i.e., terminal OH groups, ethylene oxide units, H2O, and Cl-, are assumed to be uniformly distributed throughout the interfacial region. Therefore, the concentrations of these nucleophiles in the vicinity of the headgroup of 16-ArN2+ within the interfacial layer are the same as their concentrations throughout the interfacial layers of the macroemulsion and the molar ratio of terminal OH groups to ethylene oxide units is equal-to-their ratio in the surfactant, i.e., NROH/NE ) 1/5. Recently, Burauer et al. estimated the mass fractions of a number of CmEn monomers in the oil phase in microemulsions.18 Using their value for C12E5 in octane, we estimated that as much as 36.2% of C12E5 could be in the octane, assuming that the distribution of C12E5 is the same in macroemulsions as in microemulsions. However, chemical trapping estimates of average hydration numbers should be unaffected because they depend on product yield ratios and molar ratios of terminal OH groups to ethylene oxide units in the interfacial layer, which are independent of the distribution of C12E5 between the oil and water domains and the surfactant film. Earlier work shows that in aqueous solutions of oligooxyethylene glycols, EOH, specifically tetraethylene and hexaethylene glycols and their mixtures, the selectivity of the 1-ArN2+ (the short-chain water-soluble analogue of 16-ArN2+, z )1, Scheme 1) is constant, SWEOH ) 0.6, and independent of oligooxyethylene glycol chain length, water/ EOH molar ratios, and temperature.8 In both dilute and concentrated solutions of 1-butanol, SWBuOH ) 0.3, about half the value for glycols.14 Here we determine SWROH in aqueous ethylene glycol monomethyl ether, C1E1, and, for comparison, aqueous MeOH solutions in the presence and absence of added NaCl. The selectivity toward C1E1 is used to estimate average hydration numbers of the macroemulsions. The selectivity of the chemical trapping reaction toward Cl- versus H2O, SWCl, has been determined in aqueous tetramethylammonium chloride, TMACl, solutions.11,14,22 (22) Cuccovia, I. M.; da Silva, I. N.; Chaimovich, H.; Romsted, L. S. Langmuir 1997, 13, 647-652.

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Experimental values of SWCl, eq 2 are not constant, but decrease gradually from about 7.8 to 4.6 as the concentration of TMACl increases from 0.5 to 3.5 M. To estimate the interfacial Cl- concentrations in macroemulsions, we assume that the selectivity of the chemical trapping reaction of 16-ArN2+ toward Cl- compared to H2O in the macroemulsions at a specific Clm is the same as that of the trapping reaction of 1-ArN2+ toward Cl- compared to H2O in an aqueous NaCl solution of the same concentration, i.e., Clm ) [ClW]. In practice, eq 6 is used to fit plots of % 1-ArCl versus aqueous [ClW],11,14,22 where A and B are empirical parameters obtained from the fit

% 1-ArCl ) A([ClW])B

(6)

Equation 7, which is an exponential form of eq 6, is used to estimate Clm from measured % 16-ArCl values

Clm ) exp((ln{% 16-ArCl} - ln{A})/B)

(7)

obtained from chemical trapping reactions in the macroemulsions, using the same values of the parameters A and B obtained from fitting the 1-ArCl versus [NaCl] data. This calculation is equivalent to assuming that when % 1-ArCl in aqueous NaCl solutions equals % 16-ArCl in a macroemulsion, [ClW] in the aqueous NaCl solutions in units of moles per liter of solution volume equals Clm in the interfacial layers of the macroemulsions in units of moles per liter of aqueous volume in the interfacial layer. In brief, when yields are the same, concentrations are the same. Results Determination of SWROH and SWCl: Chemical Trapping in Alcohol, Water, and NaCl Solutions. Dediazoniations of 1-ArN2+ were carried out in aqueous alcohol, ROH, where ROH is either MeOH or C1E1 as a function of increasing [NaCl], at different ROH/H2O molar ratios, and at two temperatures, 19.9 and 40 °C. The purpose of these experiments was to determine the effect of solution composition and temperature on the selectivity of reaction of 1-ArN2+ toward Cl- and ROH versus H2O. C1E1 was selected as a model for the oligooxyethylene chains of C12E5, but it has a limited solubility in aqueous NaCl solutions having molar ratios greater than 1:10 C1E1:H2O. MeOH was selected for comparison with C1E1 because it is much more soluble in aqueous NaCl. The maximum [NaCl] used is just below its solubility limit in each ROH solution. The yields of the corresponding products, 1-ArOH, from reaction with H2O, 1-ArCl, from reaction with Cl-, 1-ArOE1C1, from reaction with C1E1, and 1-ArOMe, from reaction with MeOH, were obtained from their corresponding HPLC peak areas by using separate calibration curves for each compound. Observed product yields, expressed as percent of initial [1-ArN2+], are given in the Supporting Information. Total product yields vary between 80 and 100% and average about 90%, and following past practice,7,8 we used normalized product yields to estimate Clm and average hydration numbers. Figure 2 shows the effect of added NaCl on SWROH, eq 3. At [NaCl] ) 0, the selectivities toward the two alcohols are different, ca. 0.9 and 0.98 for MeOH and 0.6 for C1El, which is the same value obtained in tetraethylene and hexaethylene glycol/H2O solutions.8 Increasing the temperature has a negligible effect on SWROH in aqueous MeOH, but SWROH does depend modestly on the molar ratios of MeOH to H2O. Added NaCl produces a gradual

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Figure 2. Selectivities, SWROH, for chemical trapping of alcohols, ROH (MeOH and C1E1), versus H2O as a function of NaCl molarity at different ROH/H2O molar ratios and temperatures: (O) C1E1/H2O (NROH:NW ) 1:10), 19.9 °C; (3) MeOH/H2O (NROH:NW ) 1.2:1), 19.9 °C; (2) MeOH/H2O (NROH:NW 1.2:1), 40.0 °C; (0) MeOH/H2O (NROH:NW ) 1:2.1), 19.9 °C; (9) MeOH/ H2O (NROH:NW ) 1:2.1), 40.0 °C. Solid lines are linear leastsquares fits.

Figure 3. Normalized percent yields of 1-ArCl versus [NaCl] in water and in three different alcohol/H2O mixtures at 19.9 °C from dediazoniation of 1-ArN2+ (1.1 × 10-4 to 1.1 × 10-3 M): (b) H2O; (O) C1E1/H2O (NROH:NW ) 1:10); (3) MeOH/H2O (NROH: NW ) 1.2:1); (0) MeOH/H2O (NROH:NW ) 1:2.1). Solid lines are fitted to each data set by using eq 6 and best-fit values of parameters A and B (see text).

linear increase in SWROH, with the slope being slightly greater for MeOH than that for C1E1. Figure 3 shows plots of % 1-ArCl versus [NaCl] (M) in water and in the different ROH/H2O mixtures at 19.9 °C. At a given [NaCl], the larger the ROH/H2O ratio, the greater % 1-ArCl. The yield of 1-ArCl also depends on ROH type. Note that a 1:10 mole ratio of C1E1/H2O gives 1-ArCl yields that are about the same as 1:2 MeOH/H2O mole ratios. The dependence on alcohol type and concentration disappears, Figure 4, when the Cl- concentrations are adjusted by using an empirical excluded volume parameter, f, eq 8.

[Cl-]corr ) [NaCl]corr )

NNaCl VT - fVROH

(8)

[Cl-]corr is the corrected molarity of Cl- (and NaCl), i.e., the Cl- concentration in the region of the solution not

Figure 4. Normalized percent yields of 1-ArCl (same yield data as in Figure 3) versus Cl- concentrations corrected to the % 1-ArCl yield curve in H2O in the absence of ROH, [Cl-]corr, eq 8 (see text) in water and in three different alcohol/H2O mixtures at 19.9 °C: (b) H2O; (O) C1E1/H2O (NROH:NW ) 1:10); (3) MeOH/H2O (NROH:NW ) 1.2:1); (0) MeOH/H2O (NROH:NW ) 1:2.1). The solid line is fitted to all the data by using eq 6 with A ) 12.0 and B ) 0.743 (see text).

occupied by the hydrophobic portions of the alcohols. [Cl-]corr equals the total moles of NaCl in solution, NNaCl, divided by VT - fVROH, where VT is the total solution volume, VROH is the total volume of alcohol present, and f is the fraction of the alcohol molar volume composed of hydrocarbon. Figure 4 shows that a value of f can be selected that makes the separate curves in Figure 3 converge on the % 1-ArCl yield curve in H2O. The f values that give the tightest convergence are approximately 0.6 for NROH/NW ) 1:2.1 and 0.70 for NROH/NW ) 1.2:1 with MeOH and 0.75 for NROH/NW ) 1:10 with C1E1. For comparison, we estimated values for the fraction of the molecular volume of MeOH and C1E1 occupied by their hydrocarbon parts from the effective packing radii of the CH3-, -CH2-, -O-, and -OH groups.23 The volume fraction of CH3- in MeOH is 0.72, and the combined volume fractions of the CH3- and -CH2- groups of C1E1 are 0.78. These volume fractions are reasonably close to the f values obtained by the curve fitting described above, given that the changes in molecular packing or partial molar volumes of the aqueous alcohol solutions with composition are ignored. These qualitative comparisons support our assumptions (a) that the parameters A and B describe the relationship between % 1-ArCl and [Cl-]corr within the aqueous region of ROH/H2O solutions, i.e., not including the hydrocarbon volume, across the entire % 1-ArCl versus [NaCl] profile in Figure 4 and (b) that the same values of parameters A and B describe the relationship between % 16-ArCl and Clm in the aqueous region of the interfacial layer of the macroemulsions, i.e., not including the hydrocarbon volume of the oligooxyethylene chains of C12E5. The bestfit values of the parameters based on eq 6 are A ) 12.0 and B ) 0.743. Values of Clm from % 16-ArCl yields in macroemulsions were obtained by using eq 7 with the same values of parameters A and B. The Clm values obtained by this approach are in units of moles per liter of interfacial layer including the H2O, the ethylene oxide oxygens, and NaCl, but excluding the volume of oligooxyethylene methylenes. (23) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991.

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Table 1. Normalized Product Yields from Dediazoniation of about 2 × 10-4 M 16-ArN2+ in C12E5/Octane/H2O Macroemulsions with Increasing Weight Percent of NaCl at 19.9 °C NaCl (wt %)a

[NaCl] (M)b

0 0 2.2 4.4 5.8 6.1 7.0 11.7 13.0 15.6 17.0 18.2 20.7 22.9

0 0 0.4 0.8 1.0 1.1 1.2 2.1 2.4 2.9 3.2 3.4 4.0 4.4

normalized product yields % 16-ArOH % 16-ArOE5C12 % 16-ArCl 95.4 95.4 87.8 83.0 81.1 80.7 79.2 72.2 70.7 66.0 64.3 62.4 58.3 53.6

4.6 4.6 4.4 4.5 4.3 4.4 4.4 4.9 4.7 5.2 5.2 5.3 5.6 5.9

0 0 7.8 12.5 14.6 14.9 16.4 22.9 24.6 28.8 30.5 32.3 36.1 40.5

Table 2. Normalized Product Yields from Dediazoniation of about 2 × 10-4 M 16-ArN2+ in C12E5/Octane/H2O Macroemulsions from 25 to 45 °C normalized product yields T (°C)

%16-ArOH

%16-ArOE5C12

45

94.8 94.7 94.9 94.8 94.8 94.9 95.2 95.1 95.2 95.4

5.2 5.3 5.1 5.2 5.2 5.1 4.8 4.9 4.8 4.6

40 35 30 25

a NaCl concentration in weight percent in the aqueous solution used to prepare the macroemulsion. b Converted from concentration in NaCl wt % by using the standard densities of aqueous NaCl solutions.

Average Hydration Numbers and Interfacial ClConcentrations: Chemical Trapping in Macroemulsions. Macroemulsions of C12E5/octane/H2O are “milky” and breakdown much more slowly than the rate of dediazoniation, except in the vicinity of the balanced point.15 In the comparisons that follow, we assume that the half-life for dediazoniation in the macroemulsions is the same as that in micelles at the same temperature based on the extraordinary insensitivity of dediazoniation rates to solvent polarity and composition.14 The half-life for dediazoniation of 16-ArN2+ in 0.02 M C12E6 micelles at 18 °C is 5.5 × 104 s; significantly shorter than the halflive (or lifetime, τ15) for macroemulsion breakdown, g106 s, at 19.9 °C.15 The half-life for dediazoniation of 16-ArN2+ in C12E6 micelles has not been measured at 45 °C. However, the half-life for dediazoniation at 40 °C is about 3 × 103 s, showing that the half-life for dediazoniation should still be about 5-0 times smaller than the 104 s half-life for macroemulsion breakdown at 45 °C. The half-lives for macroemulsion breakdown in the O/W and W/O regions in the presence of added NaCl at 19.9 °C are always g106 s, much slower than the half-life for dediazoniation. At the balanced point, the half-life for macroemulsion breakdown is about 102 s, faster than the half-life for dediazoniation under the conditions used in these experiments. For these reasons experiments were carried out at temperatures and NaCl concentrations on either side of the balanced point so that product yields primarily reflect the compositions of the interfacial layers of the surfactant films of the aggregates in relatively long-lived macroemulsions. Tables 1 and 2 list normalized product yields from dediazoniation of 16-ArN2+ in the macroemulsions as a function of added NaCl at 19.9 °C and in the absence of NaCl as function of temperature. The macroemulsions were prepared by mixing 1.00 g of an aqueous solution (weighted amounts of H2O and NaCl) with 0.736 g of octane containing 4.33 wt % of C12E5. The volume ratio of the two phases was about 1:1. The reported weight percents of NaCl are based on the amount of NaCl in the aqueous stock solution and NOT on the total weight of the macroemulsion. Similarly, reported NaCl molarities are based on the concentration of NaCl in the aqueous stock solution and not the total solution volume because, after the oil and water phases are mixed, the surfactant film of the macroemulsions takes up only about 1-2% of the

Figure 5. Average hydration numbers of C12E5/octane/H2O macroemulsions as a function of the weight percent of added NaCl at 19.9 °C: (0) SWROH ) 0.6 ) constant; (O) SWROH ) 0.66([NaCl] (M)) + 0.598 (see Figure 2 and text). Curves through data were drawn to aid the eye. The vertical line marks the balanced point at NaCl wt % 9.25 (see text).

water and NaCl based on our estimated average hydration numbers and Clm values. Thus, the calculated concentrations NaCl and H2O in the aqueous domain are essentially the same as the concentrations of NaCl and H2O in the aqueous solution prior to mixing. Complete data sets are in the Supporting Information. Increasing [NaCl] up to 22.9 wt % (4.4 M), Table 1, results in a large increase % 16-ArCl (from 0 to 40.5%), a small increase in % 16ArOE5C12 (from 4.6 to 5.9%), and a substantial decrease in % 16-ArOH (from 95.4 to 53.6%). Increasing the temperature from 25 to 45 °C, Table 2, produces a small increase in % 16-ArOH and a small, but reproducible, decrease in % 16-ArOE5C12. Average Hydration Numbers. Figure 5 shows plots of the average hydration numbers of the macroemulsions as function of the weight percent of added NaCl obtained by using eq 5. Values for the product yields ratio, % 16ArOH/% 16-ArOE5C12 are obtained from the data in Table 1 and NROH/NE ) 5 (see Rationale). The selectivity the chemical trapping reaction toward C12E5 terminal -OH groups versus H2O was treated two ways. First, SWROH was assumed to depend on Cl- concentration and, second, it was treated as a constant. Figure 2 shows that SWROH increases linearly with added NaCl in C1E1/H2O solutions and the solid line is described by eq 9. In the first approach, a separate value of SWROH was calculated for each NaCl weight percent ([NaCl] (M)) in Table 1 by assuming that

SWROH ) 0.066 × ([NaCl] (M)) + 0.598,

r ) 0.987 (9)

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Figure 6. Average hydration numbers of C12E5/octane/H2O macroemulsions as a function of increasing temperature in the absence of added NaCl. Line through data was drawn to aid the eye. The vertical line marks the balanced point (PIT) at 32.7 °C (see text).

Figure 7. Change in interfacial Cl- concentration, Clm, as a function of the concentration of NaCl in the aqueous stock solution (see text). Line is the least-squares fit: Clm ) 1.1‚ ([NaCl] (M)) + 0.091, correlation coefficient ) 0.998. Vertical line marks the balanced point at 1.6 M NaCl.

SWROH in the macroemulsion interfacial layers is equal to SWROH in aqueous C1E1 solutions when Clm ) [Cl-]corr ) [NaCl] (M) in the aqueous phase of the macroemulsions. That is, when the aqueous and interfacial Cl- concentrations are the same, then SWROH in the interfacial layers of the macroemulsions is the same as in an aqueous solution of C1E1 and NaCl of equivalent concentration. SWROH values at each [Cl-]corr were substituted into eq 5 to obtain the average hydration numbers. Data points up to about 1.4 M NaCl are obtained by interpolation. Values above 1.4 M NaCl are obtained by extrapolation. In the second approach, the average hydration number is calculated by assuming that SWROH ) 0.6, Figure 5. Both methods show that the average hydration number decreases with increasing NaCl, consistent with the decrease in total H2O with added NaCl. When SWROH ) 0.6, the decrease average hydration number is linear through the balanced point at 9.25 wt % NaCl (vertical line). When SWROH is assumed to vary with [Cl-]corr, the average hydration number is essentially constant in the O/W region but decrease linearly in the W/O region above the balanced point. Figure 6 shows that the average hydration number obtained by using eq 5 decreases linearly from about 2.5 to about 2.2 as the temperature increases from 25 to 45 °C through the balanced temperature at 32.7 °C. The product yields used to calculate the % 16-ArOH/% 16ArOE5C12 ratio at each temperature are listed in Table 2 (data at 19.9 °C are in Table 1 at NaCl ) 0 wt %), and as above, NROH/NE ) 5 and SWROH ) 0.6. SWROH is assumed to be independent of temperature as it is for MeOH, Figure 2. Interfacial Concentration of Cl-, Clm. Calculated values of Clm in the macroemulsions increase linearly with added [NaCl] (M) through the balanced point at 1.2 M NaCl, Figure 7. Values of Clm were calculated from % 16-ArCl yields by using eq 7, with A ) 12.0 and B ) 0.743. This calculation is the direct application of the assumption that when yields are the same, concentrations are the same. Specifically, when the normalized % 16-ArCl yields in the macroemulsions (Table 1) equal the % 1-ArCl yields in aqueous NaCl (Figure 4), then Clm ) [Cl-]corr. Note that Clm is about 10% greater than [NaCl] (M); i.e., Clm ) 1.1 × [NaCl] + 0.091, r ) 0.998. These results show that Cland probably NaCl concentrations (the method provides no information on Na+ distributions), in the aqueous and

interfacial layers of nonionic macroemulsions, are the same within 10%. Discussion The chemical trapping method works in macroemulsions. The product yields in Tables 1 and 2 demonstrate that 16-ArN2+ is trapped by all three nucleophiles: H2O, the terminal OH group of C12E5, and Cl-. Each set of product yields is obtained from average peak areas from duplicate or triplicate chromatography injections. The yields from duplicate experiments in Tables 1 and 2 show that the percent yields are generally the same within (0.1%. Figures 5 and 6 show that the average hydration number decreases with temperature and added NaCl as predicted by the oriented wedge theory.5 However, Figure 7 shows that Clm is essentially the same as [ClW] in the aqueous domain and that added NaCl penetrates the interfacial layer as well as reducing the concentration of water between the oligooxyethylene chains. Basic Assumptions and Current Limitations. The assumptions and limitations associated with using the chemical trapping probe have been presented in detail elsewhere.7,11,14 Only those relevant to chemical trapping in macroemulsions are discussed here. Probe Location. 16-ArN2+ is located in the surfactant film of the macroemulsions, Figure 1. The orientation of 16-ArN2+ in the macroemulsions cannot be observed, but as a surface-active ion its average location should be with its tail in the oil domain and its cationic headgroup in the hydrated interfacial layer adjacent to the octane/oligooxyethylene boundary.7 The aryl cation intermediate, 16-Ar+, which is also a surface-active and almost structurally and electronically identical to 16-ArN2+,24,25 should be in the same average location. Thus, the distribution of nucleophiles within the immediate vicinity of 16-Ar+ should be the same as the distribution of nucleophiles within the immediate vicinity of 16-ArN2+.14 Selectivity Assumption. The results in Figure 2 and earlier work show that the selectivity of the chemical trapping reaction toward terminal OH groups of oligooxyethylene chains compared to H2O is independent of ROH/ H2O ratios and temperature.8 This observation is the basis (24) Glaser, R.; Horan, C. J.; Zollinger, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 2210-2213. (25) Glaser, R.; Horan, C. J. J. Org. Chem. 1995, 60, 7518-7528.

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for our assumption that the selectivity of reaction of 16ArN2+ toward terminal OH groups of C12E5 at macroemulsion surfaces is also independent of the degree of hydration of the interfacial layer and temperature and that it has the same selectivity as the trapping reaction of 1-ArN2+ in aqueous oligooxyethylene glycol solutions. The effect of added NaCl on SWROH is accounted for by making SWROH a linear function of [NaCl] as it is in the reference solutions, i.e., when Clm ) [ClW], SWROH in the macroemulsions is the same as in the C1E1/H2O/NaCl solution of that [NaCl]. SWCl also changes with [NaCl], and as in previous work,11,14 we correct for this dependence by assuming that when % 16-ArCl in the macroemulsion ) % 1-ArCl in aqueous NaCl, then Clm ) [ClW]. Structural Perturbations by Probe/Ion and Molecule Activities. In these experiments, the molar ratio of C12E5 to 16-ArN2+ in the macroemulsions is 10:1, but the probe should not significantly perturb interfacial structure in terms of measured hydration numbers and interfacial Clconcentrations for several reasons. First, the results reported here are consistent with this assumption because the average hydration numbers of the macroemulsions are similar to those in micelles (see below). Second, 16ArN2+ also gives high yields of 16-ArCl in cationic micelles,11 which are known to have high local counterion concentrations, and in formally charge neutral zwitterionic micelles and vesicles in the presence of added salts.9,10 Third, the electric field created by positive charge on 16ArN2+ could affect the distribution of ions and molecules in its immediate vicinity in the interfacial region of the emulsions, for example, by altering the distribution of Cl- ions in the interfacial region. Two conditions suggest that this problem is minimal. (a) Charge effects on ion distributions are greatest when the ionic strength is low, i.e., 98%), and octane (>99%) were purchased from Fluka and used as received. Preparations of 4-n-hexadecyl-2,6-dimethylbenzenediazonium tetrafluoroborate, 16-ArN2BF4, 2,4,6-trimethylbenzenediazonium tetrafluoroborate, 1-ArN2BF4, 4-n-hexadecyl2,6-dimethylphenol, 16-ArOH, and 16-ArCl are described elsewhere.14 Ethylene glycol monomethyl ether (methoxyethanol), C1E1 (>99%), was purchased from Fluka and used as received. 2,4,6-Trimethylphenol (99%), 1-ArOH, MeI, CDCl3, NaCl, and MeOH are from Aldrich. 2,4,6-Trimethylchlorobenzene, 1-ArCl, was a gift from Dr. John Lorand. All water used in preparation of solutions was distilled, passed over activated carbon and a deionizing resin, and redistilled. 2,4,6-Trimethylanisole (1-ArOMe). Pure liquid 1-ArOMe was obtained in 65% yield by reaction of 2,4,6-trimethylphenol with MeI by using a previously described procedure for preparing

Langmuir, Vol. 16, No. 23, 2000 8779 1-ArOBu.14 1H NMR (CDCl3) δ (ppm): 2.35 (9H, s, ArCH3), 3.82 (3H, s, ArOCH3), 6.90 (2H, s, ArH). Macroemulsion Preparations. C12E5/octane/H2O macroemulsions were prepared, in the presence and absence of NaCl, by the procedure of Kabalnov and Weers.15 C12E5/octane mixtures, 0.736 g (4.33 wt % C12E5), and an aqueous solution, 1.00 g, containing the needed [NaCl], were placed into 10-mL volumetric flasks and thermally equilibrated in a water bath at a constant temperature, (0.1 °C, emulsified by using a vortex mixer, and returned to the water bath. Dediazoniation Reactions. Dediazoniation was initiated by adding freshly prepared, ice cold, MeOH stock solutions of 16-ArN2BF4 or 1-ArN2BF4 into thermally equilibrated mixtures containing all other reagents. Final arenediazonium ion concentrations are listed in relevant table headings and figure captions. The reactions ran for at least 5 half-lives (>97% conversion) at all temperatures (5t1/2 ≈ 3.2 days at 19.9 °C, 10t1/2 ≈ 6 h at 40 °C). Cyclohexane (50 µL) was layered on top of each dediazoniation reaction of 1-ArN2+ in salt solutions to prevent loss of 1-ArCl by vaporization into the headspace of the volumetric flask.14 Product Yields. Prior to HPLC analysis, the macroemulsions containing the dediazoniation products were diluted with sufficient EtOH or i-PrOH to give homogeneous solutions. Product mixtures from 1-ArN2+ dediazoniations were diluted with sufficient MeOH prior to HPLC analysis to dissolve the cyclohexane. Dediazoniation products of 16-ArN2+ were separated by HPLC by using a mobile phase of 64% MeOH/36% i-PrOH and flow rate of 0.8 mL/min. Typical retention times for the products in minutes are as follows: 16-ArOH, 7.6; 16-ArE5C12, 21.4; 16ArCl, 18.2. Dediazoniation products of 1-ArN2+ were separated by HPLC by using a mobile phase of 80% MeOH/20% water and flow rate of 0.8 mL/min. Typical retention times for the products in minutes are as follows: 1-ArOH, 5.7; 1-ArE1C1, 9.4; 1-ArOMe, 11.0; 1-ArCl, 17.2. Absorbance changes were monitored at 220 nm. Concentrations of products from dediazoniation of 16-ArN2+ or 1-ArN2+ were obtained from their HPLC peak areas by using calibration curves obtained with standard solutions. 16-ArOE5C12 and 1-ArOE1C1 were not prepared independently. Their peak areas were obtained by using the calibration curves for 16ArOE6C12 and 1-ArOMe.8 HPLC peak areas are average values from duplicate or triplicate injections of each sample solution. Observed percent yields of products are given by their concentrations divided by the concentration of added 16-ArN2BF4 or 1-ArN2BF4. Total yields varied from 80 to 100% and averaged about 90%. Normalized percent yields of each product were obtained by dividing their observed percent yields by the total observed yield of all products. The results in Table 2 and Figure 6 illustrate the reproducibility of the method, which is on the order of 1-2% of the measured yield. Experimental details are in the Supporting Information.

Acknowledgment. L.S.R. appreciates helpful conversations with Alexy Kabalnov, Barbara McKernan, Ronald Sauers, and Jianbing Zhang and the financial support of the NSF Organic Dynamic Division (CHE9985774) and the Center for Advanced Food Technology at Rutgers, The State University of New Jersey (CAFT Publication No. D-10535-2-00). Supporting Information Available: Six tables containing complete data sets for chemical trapping results in the macroemulsions and in the aqueous reference solutions shown in Figures 2-6 and Tables 1 and 2. Included are HPLC peak areas, measured, total and normalized product yields, HPLC calibration curves, and fitting equations. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. LA000533L