Langmuir 2000, 16, 3119-3123
`
`3119
`
`Fluorescence and Light-Scattering Studies of the
`Aggregation of Cationic Surfactants in Aqueous Solution:
`Effects of Headgroup Structure
`
`Laura T. Okano, Frank H. Quina, and Omar A. El Seoud*
`
`Instituto de Quı´mica, Universidade de Sa˜ o Paulo, C.P. 26077, 05513-970 Sa˜ o Paulo, S.P.,
`Brazil
`
`Received August 23, 1999. In Final Form: December 7, 1999
`
`Aggregation of the following cationic surfactants has been studied in aqueous solution: cetyltrimethyl-
`ammonium chloride, CMe3ACl; cetyldimethylphenylammonium chloride, CMe2PhACl; cetyldimethylben-
`zylammonium chloride, CMe2BzACl; cetyldimethyl-2-phenylethylammonium chloride, CMe2PhEtACl; and
`cetyldimethyl-3-phenylpropylammonium chloride, CMe2PhPrACl. Critical micelle concentrations, cmc’s,
`were obtained from surface tension and conductance measurements; data of the latter were also employed
`to determine the degree of the surfactant counterion dissociation, R. Static and quasi-elastic light-scattering
`measurements were employed to obtain micellar weight-average molecular weights, aggregation numbers,
`Nagg, micellar hydrodynamic radii, Rh, and interfacial area/surfactant headgroup. The latter area was also
`obtained from surface tension measurements. Finally, time-resolved fluorescence decay measurements
`with pyrene as probe were employed to obtain Nagg. The structure of micelles of CMe2PhACl is different
`from that of the other phenyl-group-bearing surfactants because its aromatic ring cannot fold back on the
`micellar interface. Increasing the number of the methylene segments in the headgroup results in an
`increase in R and interfacial surface area/headgroup and a decrease in the cmc, Nagg, and Rh. There is good
`agreement between micellar properties obtained by the different techniques employed.
`
`Introduction
`Changes of the molecular structure of surfactants have
`important consequences for the physicochemical proper-
`ties of their solutions, and hence on applications of the
`organized assemblies formed.1-3 For aqueous micelles,
`increasing the length of the surfactant hydrophobic tail
`results in a decrease of the degree of micelle counterion
`dissociation, R, and the critical micelle concentration, cmc,
`and an increase of the micellar aggregation number, Nagg,
`and the Kraft point (i.e., the temperature above which
`the solubility of the surfactant in water noticeably
`increases).1-3 For cationic surfactants, a change of the
`counterion from chloride ion to bromide ion decreases the
`charge density at the micelle interface and alters micellar
`effects on reaction rates and equilibria.1-4 Effects of
`variation of the structure of the surfactant headgroup on
`micellar properties have been much less studied, although
`this structural modification produces some interesting
`consequences. For example, rates of reactions that are
`sensitive to changes of solvent polarity (e.g., the sponta-
`neous decarboxylation of the 6-nitrobenzisoxazole-3-
`carboxylate ion) or to desolvation of the attacking nu-
`cleophile (e.g., elimination by the E2 pathway, SN2 and
`nucleophilic aromatic substitution reactions) are enhanced
`by an increase of the hydrophobic character of the
`headgroup, e.g., upon going from trimethyl- to tri-n-
`butylammonium. The reason suggested is that large
`headgroups partially exclude water from the interface,
`resulting in a less polar reaction environment.5,6 This
`conclusion is supported by recent determinations of the
`
`* Corresponding author. Fax: +55-11-818-3874. E-mail: elseoud@
`iq.usp.br.
`(1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York,
`1982.
`(2) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry,
`Pharmacy, and Biology; Chapman and Hall: London, 1984.
`(3) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213.
`(4) El Seoud, O. A. Adv. Colloid Interface Sci. 1989, 30, 1.
`
`microscopic polarity of interfacial water of cationic sur-
`factant micelles.7
`The present study is a part of our interest in effects of
`the structure of the surfactant headgroup on the aggre-
`gation and catalytic properties of organized assemblies.8-10
`We have studied micelles of the cationic surfactants shown
`in Chart 1 by four independent techniques, namely, surface
`tension, conductance, static and quasi-elastic light scat-
`tering, LS, and fluorescence.
`We turned to cationic surfactants because the synthesis
`of homologous series with variable headgroup structure
`is much easier than that with anionic surfactants.
`Previously, extensive work has been carried out on cationic
`surfactants with the general structure RN+R¢R¢¢R¢¢¢
`X-,
`where X- ) halide ion, R ) octyl to octadecyl, and R¢, R ¢¢,
`and R¢¢¢ generally represent identical alkyl groups, e.g.,
`trimethyl to tri-n-pentyl. Alternatively, a number of
`studies have employed R¢ and R¢¢ ) methyl and R¢¢¢ )
`alkyl group (e.g., ethyl to n-octyl, or 2-hydroxyethyl), or
`R¢ ) methyl and R¢¢ and R¢¢¢ ) alkyl group (e.g., methyl
`to n-butyl, or 2-hydroxyethyl).1,2,8,11 Of the surfactants
`bearing a phenyl headgroup (see Chart 1), only CMe2-
`
`(5) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys.
`Chem. 1989, 93, 1490. (b) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J.
`Phys. Chem. 1989, 93, 1497. (c) Bonan, C.; Germani, R.; Ponti, P. P.;
`Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem.
`1990, 94, 5331. (e) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.;
`Bunton, C. A. J. Phys. Org. Chem. 1991, 4, 71.
`(6) Broxton, T. J.; Christie, J. R.; Theodoridis, D. J. Phys. Org. Chem.
`1993, 6, 535.
`(7) (a) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999,
`1, 1957. (b) Novaki, L. P.; El Seoud, O. A. Langmuir 2000, 16, 35.
`(8) (a) Okano, L. T.; El Seoud, O. A.; Halstead, T. Colloid Polym. Sci.
`1997, 275, 138. (b) El Seoud, O. A.; Bla´sko, A.; Bunton, C. A. Ber. Bunsen-
`Ges. Phys. Chem. 1995, 99, 1214.
`(9) Bazito, R. C.; El Seoud, O. A.; Barlow, G. K.; Halstead, T. Ber.
`Bunsen-Ges. Phys. Chem. 1997, 101, 1933.
`(10) Possidonio, S.; Siviero, F.; El Seoud, O. A. J. Phys. Org. Chem.
`1999, 12, 325.
`(11) Zana, R. Colloids Surf. A 1997, 123-124, 27 and references
`therein.
`
`10.1021/la9911382 CCC: $19.00 © 2000 American Chemical Society
`Published on Web 02/23/2000
`
`Downloaded via PENNSYLVANIA STATE UNIV on February 27, 2020 at 19:33:44 (UTC).
`
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`

`3120 Langmuir, Vol. 16, No. 7, 2000
`
`Chart 1
`
`BzACl has been studied in detail because it is a major
`component of “benzalkonium chloride”, a product widely
`used as an antiseptic in pharmaceutical preparations.12
`Except for our previous NMR investigation of the ag-
`gregates of the cationic surfactants shown in Chart 1,8
`and of Zana’s and Lang’s work on the formation of water-
`in-oil microemulsions by CMe2PhACl, CMe2BzACl, and
`CMe2PhEtACl,13 there appear to be no published data on
`the properties (e.g., R, Nagg) of aqueous micelles of CMe2-
`PhACl, CMe2PhEtACl, and CMe2PhPrACl, respectively.
`In the present study, cmc’s and interfacial areas/
`headgroup were obtained from surface tension measure-
`ments, whereas R and cmc (in the absence of added
`electrolyte) were obtained from conductance measure-
`ments. Static and quasi-elastic LS measurements provided
`micellar weight-average molecular weights, Mw, and
`hydrodynamic radii, Rh, respectively, from which Nagg and
`area/headgroup were calculated. Values of Nagg indepen-
`dently obtained from fluorescence measurements with
`micelle-solubilized pyrene as probe were in good agree-
`ment with those obtained by static LS.
`
`Experimental Section
`Materials. The reagents, obtained from Aldrich and Merck,
`were purified as described elsewhere.14 The surfactants were
`available from a previous study.8 Before use, they were dried
`under reduced pressure over P2O5 to constant weight. All solutions
`were prepared in ultrapure water (Millipore Milli-Q).
`Apparatus. Surface tensions of aqueous solutions were
`measured with a Lauda TE 1C digital De No¨uy tensiometer,
`equipped with a thermostated solution compartment. To prevent
`water evaporation, the latter was fitted with a glass cover. At
`25 °C, measured cmc values for CMe3ACl and CMe2BzACl agreed
`with those reported elsewhere.15 Conductance measurements
`were carried out with a Fisher Accumet-50 pH meter, equipped
`with a Fisher conductivity cell (cell constant ) 1.0 cm-1) and
`interfaced to a microcomputer. Static and quasi-elastic LS
`measurements were performed with a Malvern 4700 MW system,
`equipped with a 60 mW He/Ne laser light source. Refractive
`index increments were obtained with a Wyatt Optilab 903
`interferometric refractometer equipped with a He/Ne laser light
`source.
`Steady-state emission measurements were performed at 35
`°C with a Hitachi F4500 fluorescence spectrometer. Pyrene
`fluorescence decay curves were collected at 35 °C by the single-
`photon-counting technique, using an Edinburgh Analytical
`Instruments Model FL-900 Lifetime Spectrometer (H2 flashlamp
`gas, 337 nm excitation; 390 nm emission).
`Measurements and Calculations. Surface Tensions. Solu-
`tions were thermostated in the compartment for 15 min before
`
`(12) Gump, W. In Othmer Encyclopedia of Chemical Technology;
`Wiley-Interscience: New York, 1979; Vol. 7, p 815.
`(13) (a) Verrall, E. E.; Milioto, S.; Zana, R. J. Phys. Chem. 1988, 92,
`3939. (b) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1990, 94, 381. (c)
`Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J.
`J. Phys. Chem. 1990, 94, 387.
`(14) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
`Chemicals, 3rd. ed.; Pergamon Press: New York, 1988.
`(15) Mukerjee, K.; Mysels, K. J. Critical Micelle Concentrations of
`Aqueous Surfactant Systems; NSRDS-NBS 36; U.S. Government
`Printing Office: Washington, D.C., 1971.
`
`Okano et al.
`
`their surface tension was measured. Measurements were re-
`peated until four successive readings gave a standard deviation
`of 0.12 mN m-1. Cmc’s were determined from plots of surface
`tension (mN m-1) versus log [surfactant]. The maximum area/
`surfactant headgroup at the micellar interface, A in Å2, was
`calculated from the following equations, as given elsewhere16
`
`¡R4NCl )
`
`1
`
`d(cid:240)
`
`(1)
`
`(2)
`
`4.606RT(d(log[R4NCl] + log f())
`
`
`A ) 1020
`N(¡R4NCl)
`
`where (cid:240) is the surface pressure, ¡R4NCl is the maximum surface
`excess concentration, f( is the mean activity coefficient of the
`surfactant, and N is Avogadro’s number.
`Conductance Measurements. Surfactant solutions were ther-
`mostated for 15 min before conductance was measured. Plots of
`specific conductance versus [surfactant] exhibited two straight
`lines intersecting at the cmc. Values of R were calculated from
`specific conductance results by the methods of Evans, eq 3,17 and
`Frahm, eq 418
`
`1000S2 ) (Nagg-(cid:226))2
`3/4 (1000S1 - (cid:236)Cl-) + R(cid:236)Cl-
`Nagg
`
`R ) S2/S1
`
`(3)
`
`(4)
`
`where S2, S1, (cid:236)Cl-, and (cid:226) refer to the slope of the linear portion
`above the cmc, the slope of the linear portion below the cmc, the
`equivalent conductance of Cl- at infinite dilution, and the number
`of counterions attached to the micelle, respectively.
`LS Measurements. Nagg was calculated from the aggregate
`molecular weight, Mw, and the molecular weight of the monomer.
`Static LS measurements were used to construct the Zimm plot,
`and Mw was obtained from the equation19
`
`K[CR4NCl]
`Rı
`
`) 1
`Mw
`
`+ 2BCR4NCl
`
`(5)
`
`where CR4NCl is the surfactant concentration in g/cm3, K and Rı
`are constants that depend on the wavelength of incident light,
`the intensities and angles of incident and scattered light, the
`refractive index of the solvent, the refractive index increment of
`the solution, and the distance from sample to the photomultiplier,
`respectively, and B is the second virial coefficient. Equation 6
`was used to calculate A from Rh
`19
`
`2
`
`A ) 4(cid:240)Rh
`Nagg
`
`(6)
`
`The latter was calculated from quasi-elastic LS measurements
`by the Stokes-Einstein expression (valid in the limit of infinite
`dilution), eq 7
`
`Rh ) kT
`6(cid:240)ŁD0
`
`(7)
`
`where k, T, Ł, and D0 refer to the Boltzmann constant, the absolute
`temperature, the shear viscosity of the solvent (cP), and the
`micellar translational diffusion coefficient, respectively. The
`latter coefficient is strongly dependent on intermicellar interac-
`
`(16) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.;
`Wiley: New York, 1989.
`(17) Evans, H. C. J. Chem. Soc. 1956, 579.
`(18) Frahm, J.; Diekmann, S.; Haase, A. Ber. Bunsen-Ges. Phys. Chem.
`1980, 84, 566.
`(19) Hiemenz, P. C. Principles of Colloid and Surface Chemistry,
`2nd ed.; Marcel Dekker: New York, 1986.
`
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`
`

`

`Aggregation of Cationic Surfactants
`
`Langmuir, Vol. 16, No. 7, 2000 3121
`
`Table 1. Critical Micelle Concentrations, cmc’s, and Degree of Micelle Dissociation, r, for the Surfactants Studied, at 25
`(cid:176)C a
`
`104 cmc, M,
`103 cmc, M,
`R, Evan’s method
`R, Frahm’s method
`electrolyte solutionc
`aqueous solutionb
`surfactant
`0.18
`0.32
`0.25
`1.05
`CMe3ACl
`CMe2PhACl
`0.20
`0.37
`0.20
`1.26
`CMe2BzACl
`0.22
`0.40
`0.20
`0.40
`CMe2PhEtACl
`0.22
`0.41
`0.20
`0.19
`CMe2PhPrACl
`0.24
`0.42
`0.06
`0.10
`a Literature cmc’s, determined by conductance measurement, are 0.0013 and 0.0014 M for CMe3ACl and 0.0004 and 0.0005 M for
`CMe2BzACl.23 b Determined by surface tension and/or conductance measurements. Results of both techniques were in good agreement.
`c At 35 °C, in the presence of 0.5, 0.08, 0.045, and 0.04 M NaCl for CMe3ACl, CMe2BzACl, CMe2PhEtACl, and CMe2PhPrACl, respectively,
`and 0.15 M LiCl for CMe2PhACl. d Frahm’s or Evan’s method.5b,24
`
`R, literatured
`0.37, 0.38, 0.25, 0.26
`
`0.33
`
`tions,20 which can be suppressed by addition of a strong electrolyte
`to the micellar solution. The appropriate [electrolyte] was taken
`to be the concentration at which D0 became invariant with
`[surfactant]. NaCl was used for all surfactants except CMe2-
`PhACl, which required the use of LiCl in order to avoid
`precipitation. Since solutions of CMe2PhACl became turbid in
`the presence of 0.15 M LiCl at 25 °C, all LS and fluorescence
`measurements were carried out at 35 °C.
`Fluorescence Measurements. The method is based on analyzing
`the fluorescence decay curves of a probe (pyrene) in the presence
`of variable concentrations of a micelle-bound suppressor, cetyl-
`pyridinium chloride. Aliquots of an aqueous supressor stock
`solution, whose concentration was verified by UV-vis spectros-
`copy ((cid:15) ) 4200 M-1 cm-1 at 260 nm),21 were added to 4.5 (cid:2) 10-6
`M solutions of pyrene in 0.040 M air-equilibrated cationic
`surfactant containing the same concentration of salt used in the
`quasi-elastic LS measurements.
`Fluorescence decays in the absence of quencher were analyzed
`utilizing the standard single-exponential decay routines of the
`FL-900 operating software. The micelle-quenching module of
`Edinburgh Instruments Level 2 analysis software was employed
`to fit the corresponding decay curves in the presence of quencher.
`In all cases, the observed fits were consistent with the Infelta-
`Tachiya equation for a nonmobile quencher, given by22
`F(t) ) F(0) exp[-t/(cid:244)° - Ænæ{ 1 - exp (-kqt)}]
`
`(8)
`
`where F(0) is the initial pyrene fluorescence intensity at time t
`) 0, (cid:244)° is the pyrene fluorescence lifetime in the absence of
`quencher, kq is the rate constant for intramicellar quenching,
`and Ænæ
`is the average number of bound quenchers per micelle.
`Nagg values were calculated from the relationship
`Ænæ Csurf
`[CPyCl]tot
`
`Nagg )
`
`(9)
`
`where Csurf is the analytical concentration of micellized surfactant
`and [CPyCl]tot is the total concentration of added quencher.
`
`Results
`Electrolyte Type and Concentration Employed in
`LS Measurements. Intermicellar interactions consist of
`two opposing forces, repulsive and attractive.16 At low
`electrolyte (i.e., NaCl and/or LiCl) concentrations, the
`micellar interactions are predominantly repulsive and the
`micellar diffusion coefficient increases with [surfactant].
`This electrostatic repulsion depends on the ionic strength
`of the medium, Rh, and the mean micellar charge Q
`
`(20) (a) Evans, D. F.; Mukerjee, S.; Mitchell, D. J.; Ninham, B. W.
`J. Colloid Interface Sci. 1983, 93, 184. (b) Dorshow, R. B.; Bunton, C.
`A.; Nicoli, D. F. J. Phys. Chem. 1983, 87, 1409. Frenot, M. P.; Ne´ry, H.;
`Canet, D. J. Phys. Chem. 1984, 88, 2884.
`(21) Soldi, V.; Erismann, N. M.; Quina, F. H. J. Am. Chem. Soc.
`1988, 110, 5137.
`(22) (a) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199.
`(b) Barzykin, A. V.; Tachiya, M. Heterogen. Chem. Rev. 1996, 3, 105.
`(c) Kalyanasundaram, K. Photochemistry in Microheterogeneous Sys-
`tems; Academic Press: Orlando, FL, 1987. (d) Malliaris A. Int. Rev.
`Phys. Chem. 1988, 7, 95.
`
`()RNagg). The slope of the D versus [surfactant] plot
`decreases as a function of increasing the initial electrolyte
`concentration because of the attractive term, whose
`strength is determined by the Hamaker coefficient and
`Rh. At a certain [electrolyte] there is a net balance between
`intermicellar attractions and repulsions, resulting in a
`slope of zero for the micellar diffusion coefficient vs
`[surfactant] plot.20 The appropriate electrolyte concentra-
`tions at which D0 becomes invariant with [surfactant] were
`found to be 0.5, 0.15, 0.08, 0.045, and 0.04 M for CMe3ACl,
`CMe2PhACl, CMe2BzACl, CMe2PhEtACl, and CMe2-
`PhPrACl, respectively. The dependence of these “critical”
`salt concentrations on the surfactant headgroup is due to
`differences in R and the Hamaker constant.20b
`Critical Micelle Concentrations and Degrees of
`Micellar Counterion Dissociation. Table 1 collects the
`cmc and R values for the surfactants studied, at 25 °C,
`along with corresponding literature values, where avail-
`able. This table also lists cmc’s determined at the
`electrolyte concentrations employed in the LS measure-
`ments. As expected, addition of electrolytes decreases the
`cmc’s and practically eliminates their dependence on
`headgroup structure (for CMe2PhACl, CMe2BzACl, and
`CMe2PhEtACl) because of the compression of the Gouy-
`Chapman electrical double layer. The difference between
`R calculated by Evan’s and Frahm’s methods is due to the
`fact that the former takes into account the contribution
`of the micelle (as a macroion) to the measured conductivity.
`LS and Fluorescence Measurements. Micellar ag-
`gregation numbers, calculated from static LS and fluo-
`rescence measurements, and hydrodynamic radii calcu-
`lated from quasi-elastic LS measurements are shown in
`Table 2. This table also compares the areas/surfactant
`headgroup’s calculated from LS and surface tension
`measurements. As shown in Tables 1 and 2, the values
`of the cmc, R, Nagg, and area/headgroup at the micellar
`interface agree with available literature values for CMe3-
`ACl and CMe2BzACl.
`
`Discussion
`Effects of Headgroup Structure on the Properties
`of Cationic Micelles. Interpretation of the consequences
`of increasing the surfactant headgroup size on the micellar
`
`(23) (a) Paredes, S.; Tribout, M.; Sepu´ lveda, L. J. Phys. Chem. 1984,
`88, 1871. (b) Fouda, A. A. S.; Madkour, L. H.; Evans, D. F. Indian J.
`Chem. 1986, 25A, 1102. (c) Weinert, C. H. S. W. S. Afr. J. Chem. 1978,
`31, 81. (d) Treiner, C.; Makaysse, A. Langmuir 1992, 8, 794.
`(24) (a) Sepulveda, L.; Corteˆs, J. J. Phys. Chem. 1985, 89, 5322. (b)
`Fabre, H.; Kamenka N.; Khan, A.; Lindblom, G.; Lindman, B.; Tiddy,
`G. J. T. J. Phys. Chem. 1980, 84, 3428. (c) Treiner, C.; Makaysse, A.
`Langmuir 1992, 8, 794.
`(25) (a) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504.
`(b) Roelants, E.; Gelade, E.; Van Der Auweraer, Croonen, Y.; De Schryver,
`F. C. J. Colloid Interface Sci. 1983, 96, 288.
`(26) (a) Blois, D. W.; Swarbrick, J. J. Colloid Interface Sci. 1971, 36,
`226. (b) Singh, L. R.; Babadur, P.; Srivastava, S. N. Colloid Polym. Sci.
`1975, 253, 769.
`
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`
`

`

`3122 Langmuir, Vol. 16, No. 7, 2000
`
`Okano et al.
`
`Table 2. Micellar Aggregation Numbers, Nagg, Hydrodynamic Radii, Rh, and Areas/Headgroup, A, at 35 (cid:176)C
`surfactant
`A, Å2, surface tension
`A, Å2, LS
`Nagg, LSa
`Nagg, flourescenceb
`Rh, Åc
`118 ( 4
`138 ( 6
`24.2 ( 0.8
`52 ( 5
`62 ( 4
`CMe3ACld
`112 ( 3
`110 ( 9
`24.8 ( 0.5
`56 ( 1
`69 ( 3
`CMe2PhACl
`102 ( 4
`90 ( 7
`24.2 ( 0.4
`62 ( 2
`72 ( 2
`CMe2BzACle
`94 ( 4
`85 ( 6
`23.4 ( 0.9
`69 ( 1
`73 ( 4
`CMe2PhEtACl
`85 ( 6
`82 ( 4
`22.5 ( 0.2
`71 ( 2
`75 ( 1
`CMe2PhPrACl
`a In the presence of LiCl for CMe2PhACl and NaCl for all other surfactants; see text for details. b In the presence of the same salt
`concentrations employed in the light-scattering measurements. c From quasi-elastic LS measurements. d For this surfactant, literature
`Nagg vary from 8425a to 115,25b Rh vary from 23.425a to 27 Å,20 and area/headgroups vary from 8025b to 84 Å2.25a e For this surfactant, literature
`area/headgroups vary from 74.526a to 85 Å2.26b
`
`Figure 1. Schematic representation of limiting conformations of the phenyl group of CDPhACl and CDPhPrACl at the micellar
`interface. Structure A for CMe2ACl indicates that all segments of the cetyl group come into some contact with water. B is energetically
`more favorable than C, whereas E is more favorable than F. See text for discussion.
`
`properties is complex because the interfacial free energy
`of the micelle is affected by steric and electrostatic
`interactions between neighboring headgroups, by changes
`in the micellar surface charge density, and also by changes
`in the packing of the alkyl tails.27 Additionally, long alkyl
`groups at the ammonium center may fold back onto the
`micellar surface, as well as into the micellar core, and
`thereby influence R and the intermicellar interactions.
`Therefore, the overall micellar structure depends on a
`balance of interactions of the headgroups with themselves
`and the counterions, and of the long alkyl tails in the
`micellar core.27-29 Thus, for example, several studies of
`the aggregation of cationic surfactants of the general
`structure RN+(R¢) 3Br-, where R ) octyl to tetradecyl and
`R¢ ) methyl to n-butyl, have shown that, for a given R,
`an increase in the length of R¢ results in a decrease in the
`cmc and Nagg and an increase in R.30 Similar conclusions
`have been reported for a series of 1,20-bis(trialkyl-
`ammonium) eicosane dibromide surfactants.31
`
`We discuss our results with the aid of Figure 1, which
`presents a schematic representation of the limiting
`conformations of the surfactant monomers in the aqueous
`micelle. CMe3ACl has been included because the other
`surfactants are derived from it by substitution of one of
`the methyl headgroups by a phenyl or a phenylalkyl group.
`Structure A was drawn to convey the notion that all
`segments of the cetyl chain come into some contact with
`water;12a,29 for simplicity, the cetyl chains of the other
`structures were drawn in the stretched, all-trans con-
`formation. The figure also shows the limiting conforma-
`tions of the two monomers in which the phenyl group is
`attached to the quaternary ammonium ion directly or via
`the n-propyl tether. On the basis of our previous 1H NMR
`data, structure B represents the most probable conforma-
`tion of the aromatic ring of CMe2PhACl. This (average)
`position of the phenyl group with respect to the interface
`is similar to that suggested for the benzenesulfonate
`headgroup of micellized dodecylbenzenesulfonate.8,32 Three
`limiting conformations are depicted for CMe2PhPrACl.
`
`(27) (a) Chacahty, C.; Warr, G. G.; Jansson, M.; Puyong, L. J. Phys.
`Chem. 1991, 95, 3830. (b) Blackmore, E. S.; Tiddy, G. J. T. J. Chem.
`Soc., Faraday Trans. 2 1988, 84, 1115. (c) Buckingham, S. A.; Garvey,
`C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236.
`(28) Bacaloglu, R.; Bla´sko, A.; Bunton, C. A.; Cerichelli, G.; Shirazi,
`A. Langmuir 1991, 7, 1107.
`(29) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183
`and references therein.
`
`(30) Jacobs, P. T.; Anacker, E. W. J. Phys. Chem. 1973, 43, 105. (b)
`Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (c) Lianos, P.; Zana, R.
`J. Colloid Interface Sci. 1982, 88, 594. (d) Lianos, P.; Lang, J.; Zana,
`R. J. Colloid Interface Sci. 1983, 91, 726. (e) Malliaris, A.; Paleos, C.
`M. J. Colloid Interface Sci. 1984, 101, 354.
`(31) Yasuda, M.; Ikeda, K.; Esumi, K.; Meguro, K. J. Langmuir 1990,
`6, 949.
`(32) El Seoud, O. A. J. Mol. Liq. 1997, 72, 85.
`
`Opiant Exhibit 2316
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`Page 4
`
`

`

`Aggregation of Cationic Surfactants
`
`Langmuir, Vol. 16, No. 7, 2000 3123
`
`In D, the 3-phenylpropyl group stretches out into the
`aqueous pseudophase, perpendicular to the micelle in-
`terface. In E this group lies more or less parallel to the
`micellar surface, the driving force being the interaction
`between the aromatic ring and the quaternary ammonium
`ion.5b,29 In F this group folds back into the micellar interior,
`by analogy to the suggested structure of micellized dodecyl-
`tri-n-butylammonium bromide in water27b,c and of do-
`decyldimethyl-n-butyl- ammonium bromide in water-in-
`oil microemulsions.13a
`The change in Gibbs free energy of micellization ¢G°m
`can be derived from27
`
`¢G°m ) (2 - R)RT ln [cmc]
`
`(10)
`
`Calculated ¢G°m are (in kJ/mol, using R obtained by
`Frahm’s method, no added electrolyte) -28.6, -27.0,
`-30.8, -33.8, and -36.1 for CMe3ACl, CMe2PhACl, CMe2-
`BzACl, CMe2PhEtACl, and CMe2PhPrACl, respectively.
`For CMe3ACl, ¢G°m agrees with that reported elsewhere
`(-27.6 kJ/mol).23a The fact that ¢G°m (CMe2PhACl) >
`¢G°m (CMe3ACl) is opposite to the behavior observed upon
`incorporation of the phenyl group in 4-(alkylphenyl)-
`trimethylammonium halides; in the latter, the decrease
`in the cmc provoked by the phenyl group is equivalent to
`increasing the hydrophobic tail by ca. four methylene
`groups.33 Regular packing of the (rigid) phenyl headgroup
`of CMe2PhACl (Figure 1, conformer B) and its hydrophobic
`hydration probably results in a more hydrated, i.e., more
`polar, interface, and hence a smaller j¢G°mj. Indeed, the
`microscopic polarity of interfacial water, determined by
`the solvatochromic probe 1-methyl-8-oxyquinolinium
`betaine, decreases upon going from CMe2PhACl to
`CMe3ACl.34 The contribution per interfacial CH2 group
`decreases from 3.8 kJ/mol/CH2 upon going from CMe2-
`PhACl to CMe2BzACl to 2.3 kJ/mol/CH2 upon going from
`CMe2PhEtACl to CMe2PhPrACl. This may reflect the
`increased contact of the phenylalkyl group with water as
`the length of the “tether” is increased, e.g., as in conformer
`E, Figure 1.
`There are several reasons for choosing conformer E as
`the most probable one for CMe2PhPrACl. If D were
`preferred, the properties of the resultant micelle should
`not be very different from those of a CMe3ACl micelle.
`Thus, with the 3-phenylpropyl group pointing away from
`the interface, its steric hindrance to the approach of the
`counterion would not be sizably different from that of a
`methyl group. One would expect, therefore, similar
`interfacial charge densities (hence similar R) and similar
`areas/headgroup and Nagg for both surfactants. Tables 1
`and 2 show that this is not the case. It is also possible to
`calculate Rh for the surfactants employed, either from
`geometric considerations as outlined elsewhere30b,35 or by
`determination of the length of the surfactant monomer
`(C16H33N+(CH3)2(CH2)mC6H5 Cl-) in the energy-minimized
`conformation (in the gas phase), e.g., by employing the
`PM3 Hamiltonian (implemented in the MOPAC93 R2
`program package). The micellar radii, calculated for the
`extended conformation are 26.6, 28.2, 28.6, and 30.6 Å,
`for m ) 0, 1, 2, and 3, respectively. These radii are
`significantly larger than the corresponding experimental
`values, even when allowance is made for the fact that the
`optimum surfactant chain length in the micelle is less
`than its fully extended conformation.3,11 Energetic and
`geometric considerations imply that conformer F is
`unlikely. If the energy-minimization calculation (by the
`
`(33) Wisniewski, M. J. Colloid Interface Sci. 1989, 128, 115.
`(34) Novaki, L. P., personal communication.
`(35) (a) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (b) Tanford, C.
`J. Phys. Chem. 1974, 78, 2649.
`
`PM3 method) is started with the phenylpropyl group
`bending inward, as shown in conformer F, then the lowest
`energy conformation obtained approximates that of E. On
`the other hand,
`if a constraint is imposed on the
`conformation of the phenylpropyl group, to maintain it as
`in conformation F, then the calculated surface area/
`headgroup is much larger than the one measured. Thus,
`conformers D and F are inconsistent with the micellar
`radii and areas/headgroup measured by quasi-elastic LS.
`The conclusion that they are unlikely is also in agreement
`with the results of our previous 1H NMR study of the
`aggregation of this surfactant in D2O.8 The gradual
`decrease in Nagg and Rh and the increase in the surface
`area/headgroup upon going from CMe2PhACl to CMe2-
`PhPrACl indicate that CMe2BzACl and CMe2PhEtACl
`assume conformations similar to E, the interaction
`between the phenyl group and the quaternary ammonium
`ion increasing as a function of increasing m. This interac-
`tion screens the quaternary ammonium ion (R increases)
`and decreases the intermicellar attractive interactions.
`The last conclusion is in agreement with the observation
`that the [electrolyte] required to suppress the intermicellar
`interactions in the LS experiments decreases with in-
`creasing m.
`Finally, there is good agreement between the aggrega-
`tion numbers derived from static LS measurements and
`from fluorescence decay measurements. This is interesting
`in light of the fact that the first technique is noninvasive
`whereas the second one employs a relatively voluminous
`probe. This agreement indicates that pyrene is probably
`adsorbed in the interfacial region, as discussed elsewhere
`for this and other polycyclic aromatic compounds.5b,36
`
`Conclusions
`Aggregation of a homologous series of cationic surfac-
`tants with the structure (C16H33N+(CH3)2(CH2)mC6H5 Cl-),
`m ) 0, 1, 2, and 3, has been studied by several techniques:
`surface tension, conductance, static and quasi-elastic LS,
`and fluorescence. The properties of the anilinium type
`micelles, m ) 0, are different from those with m g 1 because
`the phenyl group cannot fold back onto the micellar
`surface. This latter conformation seems to be preferable
`for the other three phenyl-containing surfactants, its
`population increasing with m. An increase in the surfac-
`tant headgroup volume leads to a decrease in the cmc and
`Nagg and an increase in R and the interfacial area per
`surfactant headgroup. The good agreement between
`micellar weight-average molecular weights obtained from
`LS and fluorescence measurements indicates that pyrene
`is solubilized in the interfacial region.
`
`Acknowledgment. We thank FAPESP for financial
`support and the CNPq and CENPES for a graduate
`fellowship to L.T.O. and for research productivity fellow-
`ships to F.H.Q. and O.A.E.S., and G. A. Marson for drawing
`Figure 1.
`
`Supporting Information Available: Representative
`experimental data, showing determination of cmc for CMe2-
`PhEtACl in water at 25 °C by surface tension; determination of
`cmc and R for CMe2BzACl in water at 25 °C by conductance
`measurement; dependence of the diffusion coefficient of CMe2-
`PhPrACl (measured by quasi-elastic light scattering) on [NaCl];
`determination of the aggregate molecular weight, Mw, for CMe2-
`PhPrACl by the Zimm plot (4 pages). Ordering and access
`information is given on any current masthead page.
`LA9911382
`
`(36) (a) Hoshino, T.; Imamura, Y. Bull. Chem. Soc. Jpn. 1990, 63,
`502. (b) Chung, J. J.; Kang, J.-B.; Lee, K. H.; Seo, B. I. Bull. Korean
`Chem. Soc. 1994, 15, 198.
`
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`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 5
`
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