measurements of the krafft point of surfactant molecular complexes: insights into the intricacies...

Measurements of the Krafft Point of Surfactant MolecularComplexes: Insights into the Intricacies of “Solubilization”

Hirotaka Hirata,* Akiko Ohira, and Nahoko Iimura

Niigata College of Pharmacy, 2-13-5 Kamishin’eicho Niigata 950-21, Japan

Received April 12, 1996. In Final Form: September 3, 1996X

Of the surfactant molecular complexes widely obtained by us in the system of quaternary ammoniumcationic surfactants such as CTAB (cetyltrimethylammonium bromide) and various aromatic substancesas additives, we have already reported several solution characteristics which reveal them to be novelsurfactant species. In the report it was disclosed that they had their own critical micelle concentrations(cmc’s) different from those of the mother surfactants and that the solubilization of such surfactants-solubilizates (additives)was just the dissolution of the surfactant complexes generated in the solubilizationprocesses. In this successive study through measurements of the Krafft point of the surfactant molecularcomplexes, it will again be clarified that the “solubilization” is not a special phenomenon but merely onewhich is recognized to be the dissolution of those surfactant complexes like that of any other crystallinematerial. The facts that the solubilization is obviously operative evenat sucha lowsurfactant concentrationas below its cmc and that the presence of surfactants far above their cmc suppresses the solubilizationto such an extent that it is much lower than that of the solubility of the solubilizates in pure water mightbeexplainedonlyby thenovel conceptwealreadypresented,bywhich theobsolete theory for thesolubilizationso far should be substituted.

Introduction

As one of themost conspicuous properties of surfactantsolutions, the phenomenon “solubilization” deserves to bepointedout first ofall. Solubilizationhas longbeenstudiedon the basis of the micellar aggregation theory, whichhad played an important role ridding researchers of ananomalous aspect of their colloidal solutions in the earlydays.1 Solubilization has, of course, been explained asone of themost remarkable effects of thosemicelles in themedium, the formation of which is accepted to be causedabove somecritical concentration called the criticalmicelleconcentration cmc.2-7 Today, the phenomenon “solubi-lization”, which stands for making insoluble or sparinglysolublematerials (solubilizates) soluble and the resultingsolutions clear and homogeneous in water, is generallybelieved to be due to the existence of surfactant micelles.The formation of micelles, above all, precedes then theprocess of making non-water-soluble materials homoge-neous follows it, in accordance with the observation thatbelow the cmc and before micelle formation no solubili-zation occurs.2-7 In these explanations the speciallydispersed phase containing the oleophilic solubilizates inthe micelle has been argued to be thermodynamicallystable.8In regard to the solubilization, apart from these

explanations and knowledge piled up so far, there are yetmany vague points and abstruse areas left to be solved.Under these circumstances we have recently discoveredthewide formation of the surfactantmolecular complexes

irrespective of their surfactant chemical nature.9-11

Especially quaternary ammonium cationic surfactantspecies such as CTABwere quite pertinent to provide thecomplexes in the crystalline state.10 By X-ray analysesthe well-grown crystals not only afforded us the detailedstructural knowledge of the surfactant molecular com-plexes but substantially proved the stable existence ofthe complexes, not with thoughts of interaction or com-plexationso farbetweensurfactantsandvariousmaterialsbut with firm bases as their stable isolation of thecomplexes.12

These crystalline surfactant molecular complexes al-ways arose from the homogeneously solubilized solutionsystems that were allowed to stand under cool conditionsfor a period of time after attainment of solubilizationequilibrium of any solubilizates in an appropriatelyadjusted surfactant solution.10,11 When the solutionswerewarmedup, the precipitated complex crystalswere easilydissolved again and all solution characteristics wererecovered and the original solubilized solutions instan-taneously realized.10,11 Thus the crystallization and thedissolution were perfectly reversible in the solubilizedsolution systems like those of any common crystallinematerial. The observations strongly suggested that thephenomenon so far referred to as “solubilization” was notspecial but simple and quite the same to what wasconventionally experienced as the dissolution of anysubstance.13

Successive studies for the solution behaviors of thesurfactant molecular complex species through electricconductivitymeasurements revealed that the specieswereall novel surfactant species which had their own char-acteristic cmc’s different from those of the mother sur-factants and they, moreover, showed different slopes inthe well-known linear relation for each complex homolo-gous series in the diagram of log(cmc) against carbonnumbers in the surfactant alkyl chain.13

X Abstract published in Advance ACS Abstracts, November 15,1996.

(1) McBain, J. W.; Hutchinson, E. Solubilization and RelatedPhenomena; Academic Press: New York, 1955.

(2) Preston, W. C. J. Phys. Colloid Chem. 1948, 52, 84.(3) Klevens, H. B. Chem. Rev. 1950, 47, 1.(4) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. Colloid

Surfactants; SomePhysico-Chemical Properties; Academic Press: NewYork, 1963.

(5) Shinoda, K. Solvent Properties of Surfactants Solutions; MarcelDekker, Inc.: New York, 1967.

(6) Elwothy, P. H.; Florence, A. T.; MacFarlane, C. B. SolubilizationbySurface-ActiveAgentsand itsApplication inChemistryandBiologicalSciences; Chapman and Hall: London, 1968.

(7) Mittal, K. L., Ed. Micellization, Solubilization, and Microemul-sions; Plenum: New York, 1977.

(8) MacBain, J. W. Advances in Colloid Science; IntersciencePublishers, Inc.: New York, 1942; Vol. 1.

(9) Hirata,H.;Kanda,Y.; Sakaiguchi, Y.Bull.Chem.Soc.Jpn.1989,62, 2461.

(10) Hirata, H.; Kanda, Y.; Ohashi, S.Colloid Polym. Sci. 1992, 270,781.

(11) Hirata, H.; Iimura, N. J. Colloid Interface Sci. 1993, 157, 297.(12) Kitamura, T.; Ohashi, Y.; Iimura, N.; Hirata, H. In preparation.(13) Hirata, H.; Yagi, Y.; Iimura, N. J. Colloid Interface Sci. 1995,

173, 151.

6044 Langmuir 1996, 12, 6044-6052

S0743-7463(96)00353-8 CCC: $12.00 © 1996 American Chemical Society

On the basis of these results we proceeded further tounderstandhow the situationwas correlated between thesolubilization by several surfactants with various solu-bilizates and thedissolution of the complex species yieldedfrom the same component systems. Through this surveyof the Krafft point detection of the surfactant complexeswe disclosed several new facts that each surfactantcomplex specieshas its owndifferent characteristicKrafftpoint from that of the mother surfactant species and thatthe temperature region of beginning of the dissolution ofthe surfactant complexes (Krafft point) correspondedverywell to the set up point of the solubilization by anysurfactant concerned.

Experimental SectionAll the surfactant molecular complexes were, on one hand,

produced from surfactants and solubilizates through a conven-tional treatment of solubilization in aqueous media. Thepreparation method is described in detail in the former re-ports.10,11 The molar composition ratios of the complexes wereeasily known from the elementary analysis, UV spectrometry,electric conductivity measurement, and most accurately by theX-ray crystallography.13 Of course, the results sufficientlycorresponded to each other. On the other hand, the specimensolutions of solubilizationwhichwere to be comparedwith thosefrom complexes were supplied through the conventional solu-bilization treatment after equilibrium achievement in a longperiod of time at a definite temperature of the measurement.UV spectra at a definite temperature were obtained by using

aUV-visible recording spectrophotometer (UV-160AShimadzuCorp.,Kyoto). Measurements of the electric conductivity (κ)werecarried out by using a conductivity meter (CM-30ET TOAElectronicsLtd., Tokyo). Beforemeasurements all the specimensolutions were carefully adopted to a definite temperature ofmeasurement to avoid a perturbation coming from nonequilib-rium states which might arise from some temperature shifts.

ResultsRegardingany ionic surfactant, a somewhat anomalous

dissolution behavior involved by a sharp rise of thesolubility with temperature was historically found byKrafft.14 Today the behavior is recognized as follows: Atlow temperature the solubility of the ionic surfactants isquite limited until the temperature reaches some criticalpoint. At the critical temperature an abrupt increase ofthe solubility occurs accompanied by a fundamentalchange in the dissolution state of the surfactant; abovethat point dissolved surfactant species commence ag-gregation, also known asmicelle formation, which causesa sudden increase of the solubility, i.e., at low temperaturemonomer solubility, which is very low, determines thetotal solubility in the system,while at higher temperaturewhen the monomer solubility has reached the cmc, themicelle solubility, which is far much higher than that ofmonomer, determines the system solubility.4,15,16 Con-sequently the Krafft phenomenon is often characterizedby a critical micelle temperature.17 On the basis of theseunderstandings, it is possible to deduce the solutioncharacteristics of the surfactant species. We conductedthereof the measurements of the Krafft point of severalcrystalline surfactantmolecular complexes in comparisonwith thesolubilizedsolutionsystemscomposedof thesamecomponents in order to get to a broader perspective viewand to see the connection between the solubilization andthe surfactantmolecular complex formation. Asdescribedbelow the Krafft points in each system corresponded well

between each system of complexes and of the correlatedsolubilization systems.In this study we dealt with systems of surfactant

molecular complexeswhichwere yielded fromquaternaryammoniumcationic surfactants suchasCTABandseveralphenolic or aromatic amines. Almost all of the complexesderived fromthose cationic surfactantswere of crystallinenature.10 Of course, these crystalline substances are notsimple cationic surfactant precipitates, which might becaused by the deprotonation of added phenols and by asubsequent shifting ofmediumpH,but crystals of genuinecomplexes which have crystallographically been provedby the X-ray structural analyses. The crystalline sur-factant molecular complexes dealt with in this report aresummarized inTable 1, where the composition, themolarcomposition ratios, and the cmc values of the complexspecies are gathered. It is interesting to note that all thevalues of the cmc of the complex species are considerablydepressed in comparison with that of the mother species(CTAB). The results correspondwell to accepted commonknowledge thatmost surfactantsalways tendtobeaffectedin the presence of a third material such as solubilizate orsome other coexisting material. Furthermore, a goodcoincidence in the behavior will be described below,regarding cmc’s and Krafft points in comparison of thesurfactant complex systems with those of the correspond-ing solubilized solutions composed of equimolar composi-tion to the complexes. However, such cmc depressionsand the Krafft point modifications in the solubilizedsolution systems should not be superficially recognizedby the effect of the coexistence of a third material withsurfactant but should properly be recognized by theinherent characteristics exhibited by those surfactantcomplexes. According to our opinionbasedonour findingsthe recognition for theseeffects tobe causedbyacoexistingthird material is not substantial, at least in so far as thesystems of the surfactant complex formation are con-cerned. Regarding the formation and the stable isolationof those surfactantmolecular complexes in detail, a seriesof our recent papers has demonstrated the whole pictureof the unambiguous existence of the complexes andmoreoverdeeplysuggestiveandnewaspectsof thesolutioncharacteristics.10The object of this article is to elucidate that (1) the

cationic surfactant species are novel surfactant specieswhichhave their inherent cmc’sandKrafft pointsdifferentfrom their mother surfactant (CTAB), based on theknowledge already presented,9-11 (2) the composite solu-tion systems (solubilization systems) composed of CTABandphenolsetc., as solubilizates, inequimolar compositionto the surfactant complexes are quite identical to eachother, and (3) depending on the above (1) and (2), theinterpretation upon the phenomenon “solubilization”based on the micelle theory which generally prevails todate is not substantial but apparent, in so far as our

(14) Krafft, F.; Wiglow, H. Chem. Ber. 1895, 28, 2566. Krafft, F.Chem. Ber. 1899, 32, 1596.

(15) Murray, R. C.; Hartley, G. S.Trans.Faraday Soc. 1935, 31, 183.(16) Tartar, H. V.; Wright, K. A. J. Am. Chem. Soc. 1939, 61, 539.(17) Mazer, N. A.; Benedek, G. B.; Carey,M. C. J.Phys.Chem. 1976,

80, 1075.

Table 1. Surfactant Molecular Complex MolarComposition Ratio and their cmc

surfactant complex(surfactant/additive)

molar compositionratio (surfactant/

additive)

cmc of thesurfactant complexspecies (mol/dm3)

CTAB/p-phenylphenol 2/1 0.40 × 10-3 a

CTAB/o-iodophenol 1/1 0.70 × 10-3 b

0.88 × 10-3 c

CTAB/2-naphthol 1/1 0.60 × 10-3 b

CTAB/indole 3/2 0.62 × 10-3 b

CTAB/R-naphthylamine 2/1 0.42 × 10-3 b

0.44 × 10-3 c

CTAB/diphenylamine 2/1 0.57 × 10-3 a

a At45°C,byUVspectroscopy. b At30°C,byelectric conductivity.c At 35 °C, by UV spectroscopy.

Krafft Point of Surfactant Molecular Complexes Langmuir, Vol. 12, No. 25, 1996 6045

systemsof themolecular complex formationare concerned.The argument originates from the observation that thosesurfactant complexes are simply precipitated in thecrystalline form at a cool condition from the solubilizedsolution systemsandonwarming theyareagaindissolvedinstantaneously causinghomogeneoussolubilizedsolutionsystems in which the solution characteristics are whollyrecovered. Thechangeof theprecipitation (crystallization)and the dissolution is always occurred, as ordinarily asobserved in any crystalline material, in a perfectlyreversible manner toward both directions with the tem-perature shifts. Thus, the cmc depression and the Krafftpoint modification observed everywhere in the followingdiagrams of the CTAB systems should not simply andsuperficially be understood to be caused by the additionof phenols etc., but these effects should be recognized tobe a result of an inherent nature of the surfactantcomplexesyielded in thesystemsasclarified incomparisonof the solubilization systems with the correspondingcomplex systems.Determinationof theKrafft point relied on the following

two methods: one was the measurement of electricconductivity (κ) and the other was spectroscopy.18 Bothmethods gave results which corresponded well to eachother. In each specimen solution over its cmc, thetemperature dependences of κare described by sigmoidal-shaped curves (Figures 1-5). In the low-temperatureregion rather low values of κ can be ascribed to asuppressed condition of occurrence of the charge carrierswhich come fromthedissolution of the surfactant complexspecies. A sudden rise of the curves with temperatureincrease followed by the suppressed κ region correspondsto an abrupt increase in the amount of the dissolved

species. An appropriately appointed temperaturewithinthe sharp rise region should designate the Krafft point ofthesurfactant complexspecies; i.e., above this temperaturesurfactant species are abruptly dissolved and commenceaggregation all at once to form micelles.18 In this studywe generally determined the Krafft points of the complexsurfactant species as the onset of the sharp rise in theκ-temperature behavior as illustrated by an arrow in thediagrams. The following slower increase or reversed sloperegion appears to be due to a transition of the aggregationstates. In fact, in thesystemofCTAB/o-iodophenol (Figure2) in which a remarkable heavy viscoelasticity is caused,suggesting the existence of enormously gigantic micellesin solution, the conductivity rise continues after a ratherlow κ region at a low temperature until the temperaturereaches a certain point at which the heavy viscoelasticityin solution involved in the giant micelles disappears,19-21

although the point is not seen on the diagram yet. It isvery important to note that in all those diagrams twoprofiles of the surfactant complexes and of the solubili-zation systems have a good correspondence to each other.The fact clearly proves the identity of both systems; i.e.,the solubilized solution systems are quite identical to thedissolution systems of the complex species in the samemolar composition.The absorbance-temperature diagrams over cmc con-

centration of each species also represent the same sig-moidal curve behavior as those of the κ-temperature.

(18) Ino, T. Nippon Kagaku Zasshi 1959, 80, 456.

(19) Shikata, T.; Sakaiguchi, Y.; Urakami, H.; Tamura, A.; Hirata,H. J. Colloid Interface Sci. 1987, 119, 291.

(20) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081.(21) Ulmius, J.; Wennerstrom, H.; Johansson, L. B. A.; Lindblom,

G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232.

Figure 1. Temperature dependence of κ in the systems ofsurfactant complex (CTAB/p-phenylphenol) solution (4) andthe same molarly composed solubilized solution (O) as thecomplex. Both systems which contain CTAB of 5 × 10-3 mol/dm3 corresponded well to each other, clarifying the identity ofthe systems. A sharp rise of κ with temperature is due to theKrafft phenomenon. Anarrow in thediagramshows theKrafftpoint of the complex surfactant species.

Figure 2. Temperature dependence of κ in the systems ofsurfactant complex CTAB/o-iodophenol solution (4) and thesamemolarly composed solubilized solution (O) as the complex.Both systems which contain CTAB of 5 × 10-3 mol/dm3

corresponded well to each other, clarifying the identity of thesystems. A clear change of dissolution of complex species isnoted to be caused by the Krafft phenomenon. An arrow in thediagram shows the Krafft point of the complex surfactantspecies.

6046 Langmuir, Vol. 12, No. 25, 1996 Hirata et al.

Much clearer detection of the Krafft points is achievedthrough the diagrams than through the κ-temperaturesas shownbyanarrow in eachdiagram. Absorbancesweremeasuredat thecharacteristicwavelengthof eachadditive(solubilizate) species. At the low-temperature regionbefore the Krafft point of the species, the limited very lowsolubility of the complexes led to very minute values ofabsorbance. Contrary, above the critical temperature theabruptly enhanced solubilities brought a steeply increas-ing absorbance as illustrated in Figures 6-10. In eachsystem of the complex species of a definite concentrationthe absorbance would reach some limited and expectedvalue which is specified by the sigmoidal curve as well asseen in κ vs temperature profiles. Also in these diagramsthe correspondence between both systems of complexspecies and of the solubilized solutions of the samemolarcomposition is quite satisfactory.In the absorbance-concentration diagrams at a given

temperature the difference of the dissolution behavior isvery remarkable in accordance with whether it is aboveor below a critical temperature as depicted in Figures11-13. All profiles in each diagram clearly consist of twolines with different slopes, and they intersect at a certainconcentration point. The concentration obviously corre-sponds to the cmc of the complex surfactant species. Theprominent absorbance at a higher temperature system is,of course, due to the higher level of dissolution of thespecies. In contrast, lower temperature profiles indicatea limited dissolution of very minute levels. The largedifference of the profiles depends simply on the temper-ature at which the measurements were carried out anddistinctly reflect the clear existence of the Krafft phe-nomenon. On this basis it can be explained that asolubilization which appears not to proceed at sometemperature just implies that at the temperature the

dissolution of the generated complexes composed of thesurfactants and the solubilizates is not possible becausethe temperature is too low. Thus for the lower temper-aturemeasurements in thesediagrams, the concentrationof the surfactant species plotted on the abscissa is justapparent, since in the systems the surfactant species canonly be dissolved at limited values at saturation. In thesecases the specimen solutions for the absorbance measure-ment has been filtered to take off the nondissolvedresiduals.In these diagrams we should take notice of significant

lower temperature behaviors inwhichwe can directly seean effect of suppressed solubilization, i.e., the effect oflimited solubility of solubilizates (additives) in the pres-ence of the surfactant compared with their rather highvalues by themselves in pure water at the same temper-ature. The effect is verydistinctive inCTAB/o-iodophenol(Figure 12) and /R-naphthylamine (Figure 13) systems.Thuswehaveherediscoveredayetunknownphenomenonof “reverse solubilization”. Of course, however, it is quiteready to recognize this apparently curious phenomenondue to the simple dissolution of the surfactant complexspecies associated with their Krafft phenomenon. It isalso important to note that the situation is quite similarto the above; the profiles of the same molarly composedsolubilization systemswhicharealwaysaccompaniedandcompared with those of complex species in each diagramareperfectly superposed onto those of dissolution systemsof the surfactant complexes. The fact obviously demon-strates that as for some materials as solubilizates thesystemof the solubilization is quite identical to thatwhichis brought about by dissolution of the preformed complexspecies between the same surfactants and the solubili-zates.In regard to Figures 11-13, especially to Figure 11 of

Figure 3. Temperature dependence of κ in the systems ofsurfactant complexCTAB/2-naphthol) solution (4) and thesamemolarly composed solubilized solution (O) as the complex. Bothsystems containing CTAB of 5 × 10-3 mol/dm3 correspondedwell to each other, proving them to be identical systems. Theκ behavior with temperature is in the same manner as theprevious figure. Anarrowshows theKrafft point of the complexsurfactant species.

Figure 4. Temperature dependence of κ in the systems ofsurfactant complex (CTAB/indole) solution (4) andsamemolarlycomposed solubilized solution (O) as the complex. Both systemscontaining 5 × 10-3 mol/dm3 CTAB corresponded well to eachother, proving them to be identical systems. The κ behaviorwith temperature is seen in the samemanner as the above. Anarrow shows theKrafft point of the complex surfactant species.


CTAB/p-phenylphenol, it should intensively be noted toa gradual increase in absorbances with increase of thesurfactantconcentration. Thegradual increase justbeginsat the origin of the diagrams, even in suchdilute conditionof the surfactant as below cmc. The fact that theabsorbance comes from the solubilizates proves that evenbelow cmc the surfactants like CTAB, etc., can solubilizethe solubilizates in severe contrast to the generallyaccepted views so far. The apparent inconsistency,however, canquite readilybeunderstoodas thedissolutionof once yielded surfactant molecular complexes. Theyielded surfactant molecular complex species are all newsurfactant species different fromtheirmother surfactantsas demonstrated in the previous report.13 They behavelike any common surfactant; they have their own char-acteristic cmc and they assume their own aggregationstates in themediumaccording to theambient conditions.13Other cases agreeable to such aspects can be referred toin the data already obtained as shown in Figure 14.13Throughout the diagrams verifying the identity of bothsystems of the surfactant complex solutions and thesolubilized solutions of the same molarity, the profilessimultaneously confirm that the clear κ increase whichexactly starts from the origin of the diagrams should beinterpreted to be caused by the dissolution of the newcomplex surfactant species, generated in the course of thesolubilization which has never been accepted so far insuch a low surfactant concentration range below the cmc.The aggregation states of those complex surfactant

species seem to be directly reflected in the slope changeof κ-concentration diagrams. In the low surfactantconcentration ranges below their cmc’s, the surfactantspecies are in the premicellar form; i.e., the aggregationstates consisted of small numbers of the complex surfac-tant species. Above their cmc’s theaggregation ismodifiedto the ordinalmicelles in a comparatively suddenmanner

at a definite concentration. The behavior is rationallyacceptedby the reason that the complex surfactant speciesare, in themselves, new different species as distinctlyillustrated, in thediagramsofFigure14,by the totalprofileof κ-concentration which is quite similar to their mothersurfactant, CTAB.With respect to each complex surfactant species the

estimatedKrafftpoints throughthese techniquesaregivenin Table 2 in which the value of the mother surfactant,CTAB, is put at the margin of the table as a reference.22In the table it is easy to see that all the values of theKrafft temperature are variouslymodified in comparisonwith that of the mother CTAB. It is worth whilerecognizing, however, that in our opinion the Kraffttemperature modification is not due to the addition of athird material such as phenol but due to one of inherentcharacteristics of the newly found complex surfactants,though themodificationseeminglyappearsdue toa simpleeffect by the material addition. The findings that thenewly yielded surfactant complexes are another differentnovel surfactant species from their mother, CTAB, havealreadybeendocumented inaprevious report.13 It shouldagainbe emphasized that each surfactant complex specieshas its own characteristic Krafft point showing that it isa different surfactant species from the mother species.Through these results thepoints aremuchmoredecisivelydeterminedasa temperatureof the sharp rise of the curvesof the absorbance-temperature diagrams preferably tothose of κ-temperature. Both values, however, haveacceptable agreement with each other.

(22) Adam,H.K.; Pankhurst, K. G. A.Trans.Faraday Soc. 1946, 42,523.

Figure 5. Temperature dependence of κ in the systems ofsurfactant complex (CTAB/R-naphthylamine) solution (4) andthe same molarly composed solubilized solution (O) as thecomplex. Both systems containing 5 × 10-3 mol/dm3 CTABcorresponded well to each other, proving them to be identicalsystems. The κ behavior is the same as the above. An arrowshows the Krafft point of the complex surfactant species.

Figure 6. Temperature dependence of absorbance in thesystemsof surfactant complex (CTAB/p-phenylphenol) solutionand the same molarly composed solubilized solution as thecomplex. Symbols respectively show that complex (4) andsolubilization (O) at 5.0×10-3mol/dm3CTAB. Theabsorbancewas measured at a wavelength of 325 nm in accordance withthe characteristic absorption of p-phenylphenol. A very sharprise of absorbance is due to the sudden enhancement ofdissolutionof speciesandcorresponds to theKrafftphenomenon.An arrow shows the Krafft temperature of the complexsurfactant species.


DiscussionThe crystalline surfactant molecular complexes in this

subjectwerealwaysprovided inhomogeneouslysolubilizedsolutionsystemscomposedof surfactantsandsolubilizatesinaqueousmedia, afteranattainmentof the solubilizationequilibrium.10,11 To date many of their structures havebeen identified thus establishing their stable existence ofthe complex species.12 The precipitated complex crystalswere easily dissolved again instantaneously by warming

the systems after recovering all their solution character-istics.10,11 The crystallization and the dissolution wereperfectly reversible like any common crystalline mate-rial.10,11 Theobservationgivesadefinitive clue toelucidatethe “solubilization”, that it is not a special phenomenonbut just a conventionally observableanddaily experiencedonewhich can readily be grasped as the dissolution of anycrystalline substance.10,11,13One of other remarkable features of the surfactant

complexes was that each of them was a novel species of

Figure 7. Temperature dependence of absorbance of CTAB/o-iodophenol systems in two different concentrations of CTABcontained in each system. Symbols show that complex (3) andsolubilization (y) at 1.5× 10-2 mol/dm3 CTAB and complex (4)andsolubilization (O) at 5.0×10-3mol/dm3CTAB, respectively.The absorbance wavelength at 279 nm was used as thecharacteristic absorption of o-iodophenol. In the diagram anarrow corresponding to the very sharp rise of absorbance showsthe Krafft point of the complex surfactant species.

Figure 8. Temperature dependence of absorbance of CTAB/2-naphthol systems. The absorbancewavelength is 327nm for2-naphthol. Symbols and the arrow are the same as those inFigure 7.

Figure 9. Temperature dependence of absorbance of CTAB/R-naphthylamine systems. The absorbancewavelength is 320nm forR-naphthylamine. Symbols and the arroware the sameas those in Figure 7.

Figure 10. Temperature dependence of absorbance of CTAB/diphenylamine systems at CTAB concentration of 5.0 × 10-3

mol/dm3. The absorbance wavelength is 279 nm for diphenyl-amine. Symbols and the arrow are the same as those in Figure6.


the surfactant; they showed distinctively different cmc’sof their own from the mother surfactants and satisfiedthe well-known linear relation between log(cmc) and thecarbon numbers of the surfactant alkyl chains in theirhomologous series.13,23,24 The knowledge offered throughthe results of κ measurements tempted us to developfurther the ideas for overthrowing and replacing theobsolete thoughts of “solubilization”which insist on an abinitio existence of micelles in the medium.The results obtained through measurements of Krafft

points by two different methods regarding the newsurfactant species of the surfactant molecular complexesbetween those cationic surfactants and several aromaticadditives provided a clear conclusion that they behavedlike any surfactant, showing a sharp increase of solubilityata certain temperaturepoint,whichshouldbedesignatedas their own Krafft point. At the same time it wasestablished that all of the behavior by two differentmethods ofKrafft pointmeasurements correspondedverywell between each system of surfactant complexes andsamemolarly composedsolubilizationsystem. Repeatedlythe results decisively prove that the phenomenon “solu-bilization” is just the dissolution of the precomposedsurfactant complexes which contain, of course, the sur-

factants and the solubilizates at a definite molar com-position. Figures 11-13 support the thoughts fromanother view point; the apparent effect that at a lowtemperature a surfactant has no ability to solubilize anysolubilizate is just due to the temperature being too lowto dissolve the once yielded surfactant complex species.Other important points to be emphasized are that the

surfactant complex speciesdealtwithherearequite stablein aqueous media. In figures 11-13, broken lines juststarting from the origin represent the absorbance valuesdue to singly dissolved solubilizates in purewater. In thepresence of surfactant, the absorbance values are all

(23) Klevens, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74.(24) Herman, K. W. J. Phys. Chem. 1962, 66, 295.

Figure 11. The difference of absorbances of CTAB/p-phen-ylphenol systemsat different temperatures. Symbols show thecomplex (3) and the solubilization (y) at 30 °C and the complex(4) and the solubilization (O) at 45 °C, respectively. Increasingabsorbance is in good accordance with increase of chromaticspecies in the systems, while one kept at low levels means thatthe species dissolution is limited at low concentration of thesaturation because of an extremely low solubility of the speciesbelow the Krafft temperature. In the latter case the concen-tration scaled on the abscissa is only apparent. Two brokenlines (b and 0) just starting from the origin illustrate theabsorbance of a single additive (p-phenylphenol) in pure waterat 45 and 30 °C, respectively. At both temperatures solubilitysaturation is observed. In the profiles of each temperature inthe low concentration region it should be noted that “reversesolubilization” occurs in the whole concentration range at 30°C and in a limited range at 45 °C, while at 45 °C solubilizationis normally promoted (see text).

Figure12. Thedifference of absorbance ofCTAB/o-iodophenolsystems at different temperatures. Symbols show the complex(3) and the solubilization (y) at 16 °C and the complex (4) andthe solubilization (O) at 35 °C, respectively. In the diagramthe same feature as Figure 11 is seen, reflecting the existenceof the Krafft phenomenon. A broken line denotes the singleadditive (o-iodophenol) absorbance,which is due to its solubilityin pure water at 16 °C. Note the clear occurrence of “reversesolubilization” associated with that shown in lower levelabsorbance at the same temperature.

Figure 13. The difference of absorbance of CTAB/diphenyl-amine systemsat different temperatures. All illustrations andexplanations are the very same as in Figure 12. In this systemalso the “reversesolubilization” is significantlyobservedat lowertemperature than the Krafft point of the surfactant complexspecies.


suppressed in the low-temperature systems. This re-markable effect means that apparently the solubilizatesare subjected to a condition of difficult solubilization, inother words, in the presence of surfactants “reversesolubilization” by surfactants occurs. The curious effect,of course, deeply correlates the decreased solubility of thecomplex surfactant species associated with that of thetemperature being below their Krafft points as statedabove. The effect, on onehand, suggests thehigh stabilityof the surfactant complex species in the medium. In thesurfactant complexes themother surfactants tightly holdthe component solubilizates like a mother holding herdaughter in her arms. Provided the stability of the

Figure 14. Diagrams showing dependence of κ on the surfactant concentration at 30 °C. It should be noted that even below cmcthe solubilization is operative, deduced from behaviors of both the complex solutions (4) and the solubilized solutions (O). Eachprofile corresponds to thesystemsof (a)CTAB/o-iodophenol, (b)CTAB/2-naphthol, (c)CTAB/o-cresol, and (d)CTAB/indole, respectively.A dotted line in each diagram shows the behavior of the single CTAB of which cmc is denoted by the kink point.

Table 2. Krafft Point of the Surfactant MolecularComplexesa

Krafft point (°C)

surfactant complex(surfactant/additive)

UVspectroscopy

electricconductivity (κ)

CTAB/p-phenylphenol 36.0 34.7CTAB/o-iodophenol 22.6 18.6CTAB/2-naphthol 22.7 19.7CTAB/indole 20.0 19.2CTAB/R-naphthylamine 24.0 23.5CTAB/diphenylamine 35.3a Reported Krafft point of CTAB is 24 °C (see ref 22).


complex species is very poor, i.e., the equilibrium pointsignificantly deviates to the dissociation side for thecomplex species, the liberated species (solubilizates)should increase the absorbance andultimately regain thebroken line levels. However, the fact appears not to bethat which is assumed, reflecting none of recognition ofliberatedmaterials in thesystem. Suchthermallyvariableeffects also suggest promising applications for variousfields such as recovery or removal of organic pollutantsin water, carrying out capture and release at the sametime just by changing temperature.25,26

The conceivable stability of the surfactant complexspecies in the aqueous medium, on the other hand, infershow the dissolved state of those species is. The stabilityof the surfactant complex is quite sufficient so that theyexist as a unit and conduct themselves in a firmly

connected form in the media not only statically butkinetically or dynamically. In this context we need totouch on interesting but somewhat complicated problemsto be solved in this stage, e.g., the problem of the positionof the solubilizates in micelles.27,28 On the basis of ourinformation obtained here, solubilizates always appearto be associated with the surfactants in the solubilizationsystem, being not separately operative but cooperative intheiranyphysicalperformance. Theargumentsdevelopedhere,moreover, seemtobedeeply related tovery importantproblems of material transport through living cell mem-branes and, of course, directly to the applicational areasinmany industrial fields. It looks likewearenowpassingthe turning point regarding the basic comprehension ofthe phenomenon “solubilization” not by commonplacethoughts but by a novel concept.

LA9603535(25) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F.

Langmuir 1990, 6, 230.(26) Scamehorn, J. F.; Harwell, J. H. Surfactant-Based Separation

Process; Marcel Dekker, Inc.: New York, 1989.

(27) Eriksson, J. C. Acta Chem. Scand. 1963, 17, 1478. Eriksson, J.C.; Gillberg, G. Acta Chem. Scand. 1996, 20, 2019.

(28) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620.


measurements of the krafft point of surfactant molecular complexes: insights into the intricacies...

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