Self-association of 1,10-decanedicarboxylates in aqueous solution

Self-Association of 1 ,lO-Decanedicarboxylates in Aqueous Solution R O L F O. SKOLD ~ AND MATS A. R. TUNIUS Metalworking Chemistry Group, Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden Received January 30, 1991; accepted January 7, 1992 Two separate transition concentrations are observed in surface tension and pH data for aqueous solutions of 1,10-decanedicarboxylic acid neutralized with either triethanolamine or sodium hydroxide. This is taken as an indication of the occurrence of a two-stage micellization process. The first transition is believed to be caused by dimer formation, while the second irregularity in surface tension and p H vs concentration curves probably corresponds to the formation of loosely associated micelles. Aggregational transitions are less distinct in conductivity-concentration data, indicating rather weak m o n o m e r interactions. With sodium counterions, surface tension data and pH data indicate the possible existence of a third aggregational transition. Substantial differences in cross-sectional areas at the air-water interface for the difunctional surfactant were observed for different cations and ionic strengths but corresponding values for CMCs are not m u c h affected. It is concluded that the disodium c~,c0-dodecanedioate assumes a folded conformation at the air-water interface. Data for the corresponding triethanolamine salt imply a similar but more tightly folded conformation for the surfactant hydrocarbon chains, and conductivity data suggest that diamine salt micelles of the dicarboxylic acid are sparesly ionized. © 1992 AcademicPress,Inc. INTRODUCTION

The present work has been conducted as an initial part of a program for the study of adsorption of a,w-difunctional surfactants from aqueous solution onto metal oxide surfaces. Increased basic knowledge regarding chemical and physical factors that may affect adsorption is needed for the successful application of environmentally more acceptable water-based chemical systems in the industry as replacements for mineral oil and solvent-based products. Typical examples are chemical systems where surfactants serve as key components of lubricants, corrosion inhibitors, and cleaners in the metalworking industry. Bolaform surfactants like 1,10-decanedicarboxylic acid are used in technical applications, e.g., in waterbased metalworking fluids, where they display interesting performance with regard to cotTo whom correspondence should be sent: Berol Nobel AB, Surface Chemistry Division, S-444 85 Stenungsund, Sweden.

rosion inhibition and lubrication combined with low foaming and low oil emulsifying properties. Acid-base equilibria play an important role and are often decisive for the extent of adsorption of ionic molecules onto solid substrates from aqueous solution, due to the effect of surface charge on electrostatic interactions. This effect is particularly dominant in affecting adsorption of ionic surface-active solutes at low concentrations. For weakly protolytic acidic and basic adsorbents, protolysis equilibria of the adsorbing species themselves as well as acid-base reactions at the hydrated surface of the adsorbate are important. In addition, with surface-active adsorbents of the same description, monomers and aggregated molecules seem to display different acid or base strengths, interpreted as a result of an energetic drive to avoid incorporating charged functional groups into the crowded surface region of micellar aggregates ( 1 - 3 ) . This effect is, e.g., observed as an anomalous increase in 183

Journal*fColloid and lmetfaceScience,Vol. 152,No. 1, August1992

0021-9797/92 $5.00 Copyright© 1992by AcademicPress,Inc. All rightsof reproductionin any formreserved.

184

SKOLD AND TUNIUS

pH at concentrations above the critical micellization concentration (CMC) with solutions of weakly acidic surfactants preferring to form miceUes in their acidic, nonionic form (4). The indicated mechanism can be used for the determination of CMC and is substantiated in the present paper by repeating an experiment in a solution of high ionic strength provided by 0.5 molal NaC1 and observing that the pH effect is substantially reduced as compared to the low ionic strength condition. Danielsson (5) made a comprehensive study of long-chain dipotassium a,~0-alkanedioates in aqueous solution and reported anomalies in the pH-concentration curves at concentrations far below the "true" CMC. He suggested that these were caused by premicellar aggregation into dimers of partially proton dissociated dicarboxylic acid monomers. Elworthy (6) found two discontinuities in diffusion and conductometric data for the dipotassium salt of~,w-octadecanedioate but did not report similar findings with the a,w-tetradecane and a,w-decane analogues. Observations indicating the existence of more than one aggregational transition treated as consecutive CMCs were reported by Meguro et al. (7) on surface-active a,w-disulfates in aqueous solution and a two-stage micellization process was suggested based on results of studies including surface tension, conductivity, static light scattering, 13C NMR chemical shifts, and solubilization properties.

(BDHA; >98% (glc)) was produced by BASF. All chemicals were used as received.

Instruments and Methods

Surface tension was measured with a ring tensiometer from Krfiss. Efficiency of cleaning procedures for platinum ring and glass vessels were tested by standardization versus pure demineralized and distilled water prior to each series of measurements. No further corrections of the observed surface tension values were made. Solutions were always prepared on the day before use and left under continuous stirring overnight. Equilibrium surface tension values on each solution were ensured by repeating measurements until a value constant to within +_0.1 m N / m was obtained in at least three successive measurements. Experiments were always performed in order of increasing concentrations. Vessels were cleaned by rinsing three times with the solution to be tested prior to the actual measurements. The same solutions and order of measurements as described above for surface tension measurements were also applied to the measurement of pH. The four-digit (2 decimals) instrument used was a Metrohm Type #632 pH-meter equipped with a glass combination electrode (Broadley James Corporation). Solutions were always well stirred at a constant stirring rate by means of a magnetic stirring bar, and the glass sample container was always of the same type and size in order to conserve flow geometries and provide optimal condiEXPERIMENTAL tions to reduce electrode polarization as much as possible. The electrode was rinsed with disChemicals tilled water between each measurement and The sample of 1,10-decanedicarboxylic acid the last drop of water was removed from the (DDD; >99%; m.p. 128-130°C; b.p. 245°C tip of the electrode by touching the drop with at 10 Tort) used was obtained from Aldrich a soft tissue paper. The electrode was stanChemic & Co., K.G. Sodium hydroxide dardized versus stirred buffer solutions at pH (>99%) was made by E. Merck. Nonanoic 5 and 8 between each measurement. Adjustacid (NA; >99% (acidimetric assay); >95% ments of instrument settings were rarely (GLC)) and N,N,N-tri-(hydroxyethyl)amine needed and never by more than + 0.05 pH (TEA; >99%; m.p. 20-22°C; b.p. 206-207°C units. Stable pH readings were always obtained at 15 Torr), was received from Fluka AG. within a few minutes of insertion into solution. Conductivity was measured by use of a N-Butyl- N, N-di-(2-hydroxyethyl)amine Journal of Colloid and Interlace Science, Vol. 152, No. 1, August 1992

ASSOCIATION OF 1,10-DECANEDICARBOXYLATES

4-digit Philips PW 9527 conductometer equipped with platinum black electrodes, pH and surface tension measurements were performed at ambient temperature (22 +_ I°C), while conductivity values were obtained in a thermostatted glass vessel at 25 + 0.1 °C. Measurements were always done in order of increasing concentrations. Electrodes were cleaned between each successive measurement by wiping with soft tissue paper. Stable conductivity values were always obtained instantaneously if thermal equilibrium was assured for the system. Discontinuities in pH-concentration curves together with surface tension vs concentration data were used to determine aggregational transitions. In two instances, conductivity data were also obtained in an attempt to further establish transition points. Semilogaritmic plots were judged to offer the most distinct presentation of break points and concentrations are therefore given on a logarithmic scale in all figures of the present paper. The notation " C M C " is used for observed aggregational transitions even i f " t r u e " micelle formation is not involved. Data from surface tension measurements may be conceived as being a result of a tran-

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FIG. 1. Surface tension versus the logarithm of the molal concentration of nonanoic acid in a mixture composed of the relative concentrations of [ NA ] : [ D E G B ] : [ B D H A ] = 1.5: l: 1 ( N A = n o n a n o i c acid, D E G B = diethyleneglycol m o n o b u t y l ether, and B D H A = N-butyl-N,N-di-(2-hydroxyethyl) a m i n e ) .

185

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FiG. 2 pH. versus the logarithm of the molal concentration of nonanoic acid in a mixture composed of the relative concentrations of [ N A ] : [ D E G B ] : [ B D H A ] = 1.5: 1:1 ( N A = nonanoic acid, D E G B = diethyleneglycol m o n o b u t y l ether and B D H A = N-butyl-N,N-di-(2-hydroxyethyl ) a m i n e ) .

sition process appearing strictly in the surfactant film adsorbed at the air-liquid interface, while trends in pH data more unambiguously should indicate changes in bulk liquid conditions; combined surface tension and pH data are believed to be more reliable. Concentration notations refer to molality of the dicarboxylic acid. Aqueous solutions of the different salts were prepared by mixing components into water, starting with the alkaline compounds, followed by slow addition of an equinormal amount of the investigated acid. The method employed for the preparation of the dicarboxylic acid salts does not entirely ensure removal of possible impurities and stoichiometric acid-base relations. It is believed that this is of marginal significance for the usefulness of reported results, particularly for data at concentrations exceeding incipient dimer formation. RESULTS AND DISCUSSION

Comparison of CMC Values Obtained by Surface Tension and pH Measurements CMC for nonanoic acid/BDHA/DEGB. Surface tension and pH measurements were performed on a solution consisting of a mix.hmrnal qfColloid and lmel:/aceScience. Vol. 152, No. 1, August 1992

186

SKOLD AND TUNIUS

ture having molal proportions of [NA]: [ BDHA ] : [ DEGB ] = 1.5:1:1. This composition is identical to a solution used earlier in an ellipsometric and electrochemical adsorption study (8) and serves as a reference example of a system containing a monofunctional anionic surfactant. Results of surface tension and pH measurements are shown in Figs. 1 and 2, respectively, pH at low concentrations is comparable to pH obtained at low concentrations with the close to equinormal mixtures of the difunctional acid and TEA discussed below, despite the relatively high carboxylic acid to amine molar concentration ratio in this mixture. This is due to the relative high pKa for the BDHA compared to TEA, 8.9 (9) and 7.8 (10), respectively. An effect of the relatively high base strength of this amine is the lower buffering capacity at the pH values observed at concentrations above CMC, which in turn will serve to enhance pH effects caused by aggregation. It is obvious from a comparison of the trends in the experimental values displayed in

the semilogarithmic plots of Figs. 1 and 2 that observed changes in surface tension and solution pH have a c o m m o n origin. Within experimental uncertainties, the linear parts before and below CMC in the graph of surface tension vs log m in Fig. 1 coincide at the same value for CMC, 0.005 molal, as in Fig. 2. C M C for 1,10-decanedicarboxylic acid/ TEA. In Figs. 3 and 4 results of surface tension and pH measurements of solutions of close to equimolar mixtures of D D D and TEA are shown. Closely the same concentration values, 0.017 and 0.015 molal, respectively, are identified in Figs. 3 and 4 at the first point of discontinuity. For CMC 2 the indicated value is 0.22 molal with both methods. Different forms of aggregation was earlier suggested by Meguro et al. (7) in a study including surface tension, conductivity, static light scattering, ~3C N M R chemical shifts, and solubilization properties of aqueous solutions of disodium dodecyl disulfate. The same authors also observed discontinuities in the surface tension and conductivity data and sug-

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Concentration [molal] FIG. 3. Surfacetension versusthe logarithm of the molal concentrationof dicarboxylicacid in a mixture composedofequinormal concentrationsof DDD and TEA (DDD - 1,10-decanedicarboxylicacid and TEA = N,N,N-tri-(2-hydroxyethyl) amine). Journal o(Co[[oid and Interface Science Vol. 152, No. 1, August 1992

ASSOCIATION OF 1,10-DECANEDICARBOXYLATES 7,0

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Concentration [molal] FIG. 4 pH. versus the logarithm of the molal concentration of dicarboxylic acid in a mixture composed ofequinormal concentrations of DDD and TEA ( DDD = 1,10-decanedicarboxylicacid and TEA = N,N,Ntri- (2-hydroxyethyl)amine).

gested that the surfactants are lying flat at the air-water interface at the lower transition point. In the same work it was further suggested that a condition termed as premicellar aggregation involving several aggregate sizes is prevailing at concentrations between CMC 1 and C M C 2, while a wicket-like conformation was assumed for concentrations above the second C M C both in bulk solution and at the air-water interface. Interestingly, Johnson and Fleming ( 11 ), in a study of the apparent molal volumes of aqueous solutions of bolaform cationic molecules, observed extensive association even of compounds with too-short alkylene chains to form micelles. In order to confirm surface tension and p H data, conductometric data were also produced. The data are shown graphically in Fig. 5 as a plot of equivalent conductivity vs log m, and somewhat lower values, 0.013 molal and 0.19, m a y be determined from the points of intersection of the linear portions of the graph with the curved intermediate region. A p H of 7.25 was observed in a separate

experiment at a concentration of 2.5 molal (equaling a total organic content of 66.5% w / w ) and this value coincides excellently with the extrapolated straight line above CMC 2 in the semilogaritmic plot of Fig. 4. The corresponding surface tension value at 2.5 molal was 53.3 m N / m and also does not indicate the existence of further transitions in the state of surfactant association.

CMC for 1,10-decanedicarboxylic acid/ Na +. The effect of a strong electrolyte as the counterion in place of the protonated TEA was 53.3 m N / m and also does not indicate the existence of further transitions in the state of surfactant association. Results of surface tension and p H measurements are given graphically in Fig. 6 and Fig. 7, respectively. Discontinuities in the drawn lines are more striking in the graph based on p H data. A difference seems to exist in the value for C M C 1 from p H and surface tension data, 0.012 and 0.020 molal, respectively. It should be noted, however, that CMC 1 is difficult to determine accurately from the Journal qfColloid and Interlace Science, Vol. 152,No. 1, August1992

188

SKOLD AND TUNIUS 140 -

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Concentration [molal] FIG. 5. Equivalent conductivity versus the logarithm of the molal concentration of dicarboxylic acid in a mixture composed of equinormal concentrations of DDD and TEA (DDD = 1,10-decanedicarboxylic acid and TEA = N,N,N-tri- (2-hydroxyethyl) amine).

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Concentration [molal] FIG. 6. Surface tension versus the logarithm of the molal concentration of dicarboxylic acid in a solution of DDD neutralized with NaOH (DDD = 1,10-decanedicarboxylic acid).

Journal of Colloid and Interface Science Vol. 152, No. I, August 1992

ASSOCIATION OF 1,10-DECANED1CARBOXYLATES

189

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Concentration [molal] FIG. 7 pH. versus the logarithm of the molal concentration of dicarboxylicacid in a solution of DDD neutralized with NaOH (DDD = 1,10-decanedicarboxylicacid).

latter data. The initial rise in surface tension is indicative of a negative adsorption of solutes to the air-water interface and is typical for highly hydrated species in solution (12). A comparison between surface tension data for the TEA and sodium neutralized solutions in Figs. 3 and 6, respectively, indicates that the effect may be attributed largely to the Na +-ions. An excellent agreement is observed for CMC 2, both methods give the value 0.13 molal. Conductivity measurements were extended over the same concentration range as the pH and surface tension experiments and two points of deviation from the linear portions of the equivalent conductivity vs log m curves were identified as shown in Fig. 8. The points of intersection coincide well with CMC 1 and CMC 2 as determined from pH and surface tension data. The values obtained were 0.013 and 0.23 molal, respectively, which is in fair agreement with data from pH and surface tension measurements. Both surface tension and pH-concentration curves in Figs. 6 and 7 indicate the possible

existence of a third aggregational transition at a concentration of about 0.4 molal but inadequate solubilities prevented measurements at higher concentrations. The fact that the surface tension continues to decrease at concentrations above the second CMC is at least evidence for substantially increasing surfactant activity in solution causing changes in surface pressure even at concentrations above CMC 2. Interestingly, from pH data published by Danielsson (5) on aqueous solutions of dipotassium tridecanedioate it is also possible to determine three separate transition points at 0.013 molal, 0.11 molal, and 0.65 molal at 40°C. Danielsson found that conductivity and freezing point data did not demonstrate any association of monomers of dipotassium tridecanedioate, even if the possibility was not ruled out; instead, from results ofdecanol solubilization and specific volume measurements, a single CMC around 1.2 molal was determined. Diffusion and conductivity data for dipotassium octadecanedioate in aqueous solution are interpreted by Elworthy (6) to indicate two Journal ()/'Colloid and Interface Science,

V o l . 152, N o . 1, A u g u s t

1992

190

SKOLD AND TUNIUS 1~40

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Concentration [molal] FIG. 8. Equivalentconductivityversus the logarithm of the molal concentration of dicarboxylicacid in a solution of DDD neutralizedwith NaOH (DDD = 1,10-decanedicarboxylicacid). aggregational transitions at 25°C. A somewhat lower concentration value was obtained for the first transition, 0.011 molal, as compared to ca. 0.013 molal for the disodium dodecanedioate in the present work. A second transition was found at 0.031 molal for dipotassium octadecanedioate. Not surprisingly, this is substantially lower than our observed values for disodium dodecanedioate above. The first transition was interpreted as caused by dimerization, while the transition occurring at higher concentration was believed to reflect the formation of sperical micelles. It is also interesting to note that Elworthy (6) failed to observe any transitions in the conductivity data for dipotassium tetradecanedioate in the concentration range between 0.003 and 0.1 molal. Irregularities in pH-concentration curves for dilute aqueous solutions of dipotassium hexadecanedioate and dipotassium octadecanedioate were explained by Danielsson (5) in a similar fashion as Elsworthy and the formation of dimers constituted by diacid and acid salt ( H 2 A - H A - ) was suggested. It was also sugJournal ~f Coltoid and lnter/h~J Sciemx~. Vol. 152, No. 1, August 1992

gested that discontinuities in pH-concentration curves observed at somewhat higher concentrations of dipotassium hexadecanedioate and dipotassium octadecanedioate were caused by higher aggregates of partially proton dissociated acids. Danielsson's (5) CMC value for dipotassium octadecanedioate at 60°C from results ofdecanol solubilization, pH, and conductivity measurements was 0.04 molal. This value is in reasonable agreement with E1worth's result at 25°C, 0.031 molal. Despite the somewhat higher pH level for experiments done with the sodium dicarboxylate solutions in the present work, a higher and more pronounced pH response caused by surfactant aggregation is observed than with the TEA neutralized acid solution. This is explained largely by the lack of buffering capacity at the pH levels where the discontinuities occur as compared to the TEA containing solutions, which have an optimal buffering capacity at pH 7.8 as well as a good buffering capacity in the range between pH 4.9 and 7.8. Another contribution to the relatively marked pH increase at concentrations above CMC 1 and

ASSOCIATION OF 1,10-DECANEDICARBOXYLATES CMC 2, comes from the fact that it is more unfavorable to adsorb a small and highly hydrated electrolyte like the sodium ion at the micellar surface than a large, polarizable counterion like the protonated amine (12). This would lead to an even greater tendency for aggregation of the protonated acid and hence a higher p H in the bulk solution with sodium counterions.

CMC for 1,10-decanedicarboxylic acid/ TEA in 0.5 molal NaCl. A separate study was done in order to study the influence of high ionic strengths on experimental p H effects of aggregation and 0.5 molal NaC1 was added to the solution of D D D and TEA. The results of surface tension and p H measurements are displayed in Figs. 9 and 10, respectively. Even if observed surface tension changes were small, the transition point marked by a break in the linear portions of the graph before and after the point was taken as CMC. The single aggregation transition was thus determined to occur at a carboxylate concentration of about 0.008 molal from the data plotted in Fig. 9.

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As would be expected for an ionic surfactant with sodium counterions, this value is not very different from CMC in solutions of low ionic strength. p H effects marking CMC are substantially suppressed in the high ionic strength environment, as seen in Fig. 10. The expanded p H scale in the figure should be noted. Despite the moderate p H effects observed, a transition m a y be observed in the concentration range connecting the linear portions of the graph below and above the transition. The value obtained from p H data is estimated to be around 0.012 molal. Insufficient solubility prevented measurements at higher concentrations.

Estimation of Area of Adsorbed Molecules at the Air- Water Interface from Gibbs Equation The surface area requirement for solutes as calculated from Gibbs equation using surface tension data, is an often used measure of the extent of close-packing of molecules at the air-

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Concentration [molal] FIG. 9. Surfacetension versus the logarithm of the molal concentration ofdicarboxylic acid in a mixture composed of closeto equinormal concentrations of DDD and TEA in the presenceof 0.5 molal NaC1(DDD - 1,10-decanedicarboxylicacid and TEA = N,N,N-tri-(2-hydroxyethyl) amine). J) rnal r f Colloid and lnte;Tface Science, Vol. 152, No. 1, August 1992

192

SKOLD AND TUNIUS 6,65 -

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Concentration [molal] FIG. 10. pH versus the logarithm of the molal concentration of dicarboxylic acid in a mixture composed of close to equinormal concentrations of DDD and TEA in the presence of 0.5 molal NaCl (DDD = 1,10decanedicarboxylic acid and TEA = N, N, N-tri-( 2-hydroxyethyl) amine).

water interface (12). Such data m a y also offer insights into the mode of aggregation of the same molecules in solution and suggest the structure of layers adsorbed at a solid-liquid interface. Gibbs equation is usually written in the form of a surface excess of solute ( m o l / m 2 ) , noted by the symbol r2, according to r2 = - ( d v / d l n

c)/(nRT),

[1]

where n is a factor assuming different values depending on type of surfactant. Thus, for a nonionic surfactant or an ionic surfactant in a high ionic strength m e d i u m n = 1, whereas for a solution of a 1:1 electrolyte n = 2, and for a 2:1 electrolyte n -- 3. It is expected that the tendency for the carboxylates to aggregate in nonionic form or by forming strong dipoledipole bonds with organic counterions, as discussed above, will lead to a situation closer to that of a nonionic surfactant. Surface excess values are converted into molecular surface area values (A2/molecule), Journal of Colloid and Interface Science,

Vol. 152, No.

I, A u g u s t

1992

noted by the symbol ~, by use of the relation O" = ( r 2 * N A ) -1,

[2]

where NA is Avogadros number. Results of calculations based on the present data have been collected in Table I together with data for CMC as well as surface tensions and p H values at CMC. Selection of values for n in Eq. [ 1] is not always obvious, and two different values have been used in uncertain cases. The different values used are indicated in the notes of the table. The a-values for D D D / T E A believed to be most probable at C M C 1 and CMC 2, 90 and 50 Aa/molecule, respectively, are lower than the values given by Meguro et al. (7) for disodium dodecyl disulfate at the corresponding transition points, 171 and 118 AZ/molecule, respectively. This m a y be explained by the difference in the nature of both the anion and the cation. More similar values are thus seen for the disodium dodecanedioate in the present work, 378 and 105 A2/molecule, even if the former value is rather uncertain.

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CMC (molal)"

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refers to c o n c e n t r a t i o n of carboxylate g r o u p s ( n o r m a l i t y in m o l a l units). i.e., a is c a l c u l a t e d for a c o m p l e t e l y dissociated m o n o v a l e n t electrolyte. i.e., a 2:1 electrolyte is considered. due to the high ionic strength of the solution. i.e., n o n i o n i c c o n d i t i o n s are considered.

Hexadecanedicarboxylic acid/K Hexadecanedicarboxylic acid/K N o n a n o i c acid/N,N-di(2-hydroxyethyl)N-butyl-amine/dietyleneglycol monobutyl ether

Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / N a Decanedicarboxylic a c i d / t r i e t h a n o l a m i n e / 0 . 5 m o l a l NaCI Undecanedicarboxylic acid/K Undecanedicarboxylic acid/K Undecanedicarboxylic acid/K Undecanedicarboxylic acid/K

Carboxylic acid/counterion

C M C 1 f r o m surface t e n s i o n m e a s u r e m e n t s CMC 1 from pH measurements CMC l from conductivity measurements C M C 2 f r o m surface t e n s i o n m e a s u r e m e n t s CMC 2 from pH measurements CMC 2 from conductivity measurements C M C 1 f r o m surface t e n s i o n m e a s u r e m e n t s CMC 1 from pH measurements CMC 1 from conductivity measurements C M C 2 f r o m surface tension m e a s u r e m e n t s CMC 2 from pH measurements CMC 2 from conductivity measurements C M C 3 f r o m p H a n d surface t e n s i o n meas. C M C 1 f r o m surface t e n s i o n m e a s u r e m e n t s CMC 1 from pH measurements C M C 1 d e t e r m i n e d p H data in Ref. (5) C M C 2 d e t e r m i n e d from p H d a t a in Ref. (5) C M C 3 d e t e r m i n e d from p H d a t a in Ref. (5) C M C f r o m d e c a n o l solubilization a n d specific v o l u m e d a t a in Ref. (5) C M C 1 f r o m c o n d u c t i v i t y m e a s u r e m e n t s i n Ref. (6) C M C 2 f r o m c o n d u c t i v i t y m e a s u r e m e n t s i n Ref. (6) C M C f r o m surface t e n s i o n m e a s u r e m e n t s CMC from pH measurements

Comments

CMC, Surface Area o f A d s o r b e d M o l e c u l e s at the A i r - W a t e r Interface, a n d V a l u e s for p H a n d Surface T e n s i o n at C M C for A q u e o u s Solutions of Salts of S o m e a , w - D i c a r b o x y l i c Acids a n d N o n a n o i c A c i d

TABLEI

© X ~

;>

,~

Z

Q >

194

SKOLD AND TUNIUS

The surface area data indicate a rather solutions. Thus, this is an expected but hardly tightly close packed conformation for the di- significant effect of reduced head-group recarboxylic acid at the air-water interface of pulsion under the influence of an increased the D D D / T E A mixture, particularly consid- solution ionic strength allowing a tighter foldering that two carboxylate groups are included ing and close-packing of the surfactant hydroin the surface area value. The surface area carbon chains. value associated with the second transition For D D D / T E A in 0.5 molal NaC1 the value concentration for the disodium salt, 105 A2/ 182 A2/molecule is noted for the single tranmolecule, is consistent with a folded confor- sition point. The value is intermediate between mation for the surfactant at the air-water in- the values observed at the lowest transition terface. A closest possible molecular packing concentration for D D D / T E A and D D D / N a + area for a monocarboxylic acid has been assuming completely dissociated 1:1-electroshown to be about 20 A2/molecule (13). For lytes, 60 and 252 A2/molecule, respectively. TEA-dodecanoate a surface area of 40 A2/ This is a result of amine cations being replaced molecule was determined (14); if strictly by sodium ions by simple mass-action at the nonionic conditions are assumed, a compar- same time as the diffuse double layer is comison with the insignificantly higher value ob- pressed under influence of the high ionic served in the present paper for D D D / T E A at strength of the medium. Further, as seen in the second transition point, 50 A2/molecule, Table I, corresponding CMC-values do not would leave little space for a folded confor- differ substantially as a result of a change in mation for the partially proton dissociated di- counterion or ionic strength. This is also concarboxylic acid at the air-water interface. The sistent with the finding of Rosen et al. (16) apparent degree of micellar ionization (15) that adsorption of an anionic surfactant at the was found to be a nonlinear function of con- liquid-air interface is considerably more afcentration and large differences were obtained fected by an increase in the concentration of depending on type ofcounterion. Typical val- a c o m m o n ion than is CMC. ues observed for the apparent degree of miElworthy (17) reported surface area values cellar ionization at concentrations between for 1,16-hexadecane disodium sulphate rangCMC 1 and CMC 2 are 0.9 and 0.75 for the ing from 98 to 86 A2/molecule with salt consodium and TEA counterions, respectively. At centrations increasing from 1.0 m M t o 1.0 M. concentrations above CMC 2 the correspond- Only one CMC was reported but data in the ing values are in the order of 0.75 and 0.2. As paper display decreasing surface tensions at point of reference in the calculations was used concentrations above t ~ given CMC. the concentration derivative of the specific In the present work, two different a-values conductivity at infinite dilution. By analogy, are ascribed to N A / D E G B / B D H A in Table these values seem to justify the assumption of I, depending on whether the situation is conclose to nonionic conditions for the surface of sidered to be one of a 1:1 electrolyte or of a the aggregates formed by the dicarboxylic closely interacting charged dipole pair treated acid-TEA salt at the air-water interface. like a nonionic surfactant. The values given The air-water interfacial area associated are 87 and 44 A2/molecule, respectively. The with the incipient third transition concentra- lower surface area value is of an order of magtion for the disodium salt, "CMC 3" in Table nitude expected for a straight chain surfactant I, is slightly lower than the value for the second of moderate surface activity (4). It should be transition, 98 and 105 AZ/molecule, respec- noted that the uncorrected surface tension tively. Apparent degree of micellar ionization value at CMC is low, 27 m N / m , and it is not was found from conductivity data to vary from clear to what extent the cosolvent and the 0.85 to 0.70 in the concentration range from somewhat hydrophobic counterion influences 0.1 to 0.4 molal for disodium dodecanedioate this value or are included in the air-water inJournal of Colloid and Inter'face Science, Vol. 152, No. 1, August 1992

ASSOCIATION

OF

1,10-DECANEDICARBOXYLATES

195

salt a more tightly folded surfactant conformation at the air-water interface seems more probable than the palisade-like structure that would have to be envoked as an alternative model at concentrations approaching CMC 2. A surface area value corresponding to only slightly ionized micelles, i.e., in the range 6070 A2/molecule, is implied from combined SUMMARY surface tension and conductivity data. In furThere is a scarcity of data available in the ther support of the more tighly folded conforliterature regarding the surfactant behavior of mation model, it is noted that maximum surc~,~o-dicarboxylic acid salts and no data have face pressure observed with the diamine salt been found regarding 1,10-decanedicarboxy- is ca. 20 m N - m -l , while the corresponding lates. In accord with findings of other workers value for the disodium salt is only half, 10 regarding dicarboxylates and other types of m N . m - l . bolaform ionic surfactants (5-7, 17, 23-25 ), The pH effect caused by micellization may more than one point of transition in surface be treated as an apparently different pKa for tension, conductivity, and pH data as a func- aggregated protolytes as compared to the sittion of concentration was also found in the uation in a monomer solution. According to present work. Conductivity data are judged to results in the present work, interactions bebe less unambigous as a result of rather weak tween the charged dipoles formed by the two surfactant association. It has also been pointed species of the ion pair of a carboxylic acid and out by McBain ( 18 ) that conductances of sur- an amine in aqueous solution enhances closefactant aggregates not necessarily deviate very packing at the air-water interface compared much from monomer conductances. to the aggregates formed by the more extenThe most prevalent models for the associ- sively dissociated disodium salt, but critical ation ofbolaform ionic surfactants suggest the concentrations for changes in state of surfacformation of small aggregates, most probably taut aggregation in solution are not greatly afdimers of partially dissociated diacid at low fected by the nature of the counterion. concentrations, followed by a transition to A rather weak aggregation of the difunchigher aggregational numbers at some higher tional surfactants up to the concentration of concentration (5-7). A wicket-like confor- the second transition point in surface tension mation for the surfactant at the air-water in- and pH data is also indicated by preliminary terface was suggested by Menger and Wrenn results of ongoing adsorption experiments with (19) for ~,w-difunctional cationic surfactants; TEA-neutralised 1,10-decanedicarboxylic acid this model was also adopted by Meguro et al on magnetite. This conclusion comes from the (7) in a study of disodium dodecyl disulfate. finding that, in contrast to what is generally An analogue conformation is conceivable for found for other surfactants, adsorption of this bolaform surfactants in loosly aggregated mi- compound does commence at concentrations cellar structures and this has also been sug- well past the concentration of the lower trangested by several authors (5-7, 20-22). sition observed in surface tension, pH and Surface tension, pH and conductivity data conductivity data for the D D D / T E A mixture in the present paper may be explained by a in the present work. The somewhat feeble similar model for the aqueous disodium o~,w- transitions in conductivity-concentration data dodecanedioate investigated. In addition, data noticed in the present paper as well as in the for the disodium salt indicate the possible ex- work of others (5, 14) on a,w-difunctional istence of a third point of transition for this carboxylates are also consistent with this decompound. For the corresponding diamine scription, even if conductivity data for bolaterfacial layer. It is likely that both dipolar and hydrophobic interactions between the carboxylic acid and the rather hydrophobic amine lead to even more pronounced nonionic micellar surface conditions than was observed for the dicarboxylic acid and TEA.

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form surfactants with stronger electrolyte headgroups i n some cases have been s h o w n (7, 2 3 - 2 5 ) to give good resolution of low conc e n t r a t i o n ( p r e m i c e l l a r ) transitions as well. ACKNOWLEDGMENTS This work was supported financiallyby Berol Nobel AB and the Swedish National Board for Technical Development (STU). REFERENCES 1. Wennerstr0m, H., and Lindman, B., Phys. Rep. 52, 1 (1979). 2. Pandit, N. K., and Strykowski, J. M., J. Pharm. Sci. 78, 767 (1989). 3. Bunton, C. A., and Minch, M. J., J. Phys. Chem. 78, 1490 (1974). 4. Tunius, M., and Sk61d, R., Colloids Surf 46, 297 (1990). 5. Danielsson, I., Acta Acad. Abo. Ser. B XX(15), 165 (1956). 6. Elworthy,P. H., J. Pharm. Pharmacol. 11, 557 (1959). 7. Meguro, K., Ikeda, K., Otsuji, A., Taya, M., Yasuda, M., and Esumi, K., J. Colloid Interface Sci. 118, 372 (1987). 8. Alwin,H., Bjorklund, R. B., Johansson, I., and Sk61d, R. 0., Langmuir 8, 571 (1992). 9. Golec, K., Hill, E. C., Kazemi, P., and Sk61d, R. O., Tribol. Int. 22, 375 (1989).

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10. Christensen, J. J., Hansen, L. D., and Izatt, R. M., "Handbook of Proton Ionization Heats and Related Thermodynamic Quantities." Wiley, New York, 1979. 11. Johnson, J. R., and Fleming, R., 3. Phys. Chem. 79, 2327 (1975). 12. Hiemenz, P. C., "Principles of Colloid and Surface Chemistry." Dekker, New York, 1976. 13. Tingle, E. D., Trans. FaradaySoc. 46, 93 (1950). 14. W~irnheim,T., and J/Snsson, A., ,L Colloid Interface Sci. 138, 314 (1990). 15. Lianos,P., and Lang, J., J. Colloidlnterface Sci. 96, 222 (1983). 16. Rosen, M. J., Dahanyake, M., and Cohen, A. W., Colloids Surf 5, 159 (1982). 17. Elworthy,P. H., J. Pharm. Pharmacol. 11,624 (1959). 18. McBain, J. W., "Frontiers in Colloid Science," Vol VIII, p. 127. Interscience, New York, 1950. 19. Menger, F. M., and Wrenn, S., J. Phys. Chem. 78, 1387 (1974). 20. Yiv, S., Kale, K. M., Lang, J., and Zana, R., J. Phys. Chem. 80, 2651 (1976). 21. Yiv, S., and Zana,. R., J. Colloid Interface Sci. 77, 449 (1980). 22. Zana, R., Yiv, S., and Kale, K. M., J. Colloid Interface Sci. 77, 456 (1980). 23. Ueno, M., Hikota, T., Mitama, T. T., and Meguro, K., J. Am. Oil Chem. Soc. 49, 250 (1972). 24. Ueno, M., Yamamoto, S., and Meguro, K., J. Am. Oil Chem. Soc. 51, 373 (1974). 25. Abid, H., Hamid, S. M., and Sherrington, D. C., J. Colloid Interface Sci. 120, 245 (1987).