University of Groningen

Surfactant structure and the thermodynamics of micelle formationBijma, Koos

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Publication date:1995

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Summary

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SUMMARY

Surfactants are molecules which possess a dualistic character. They combine a water-preferring(hydrophil ic) and a water-rejecting (hydrophobic) part in one molecule. This dualistic character isthe basis of their surface activity. When surfactants are dissolved in water, the molecules maygroup together to form aggregates. In these self-assemblies the hydrophobic hydrocarbon tailstend to assemble, whi le thei r hydrophi l ic headgroups (and counter ions) remain hydrated. A widevariety of aggregates (e g micelles or vesicles) may be formed, depending on surfactantstructure, surfactant concentration, and temperature. This thesis deals with one of these typesof aggregates, namely micelles. Micelles are important in industry and biology as a result oftheir solubil izing function: matter can be transported in aqueous solutions after it has beensolubil ized by the hydrocarbon interiors of micelles.

The size, shape and stabil ity of micelles is strongly influenced by the molecular archi-tecture of the surfactant. Surfactant monomers aggregate into spherical micelles above thecrit ical micelle concentration (CMC) At higher surfactant concentrations, worm-like micelles areoften formed. Highly concentrated surfactant solutions may exhibit lyotropic l iquid-crystall inebehavior. In the present study, the formation of various aggregates or phases has been exami-ned using a variety of techniques: (i) conductometry (CMC and percentage of counterions boundto the micellar surface), (i i) microcalorimetry (CMC and information on interactions broken up orcreated in the course of micelle formation or micellar growth), (i i i) NMR (micellar growth andcounterion orientation/position), (iv) UVtulS-spectroscopy (Krafft temperature and micellarcatalysis), (v) optical polarization microscopy and differential scanning calorimetry (l iquid-crystal-l ine behavior), and (vi) theoretical thermodynamic models (prediction of micellar parameters).

This thesis deals with members of a familv of cationic surfactants, namelv thealky lpyr id in ium sur factants:

Chart 1. Structure of alkylpyridinium surfactants.

The hydrophobic par t of the molecule consists of an a lky l chain ( typ ical ly n=12), and thehydrophi l ic par t consis ts of a pyr id in ium headgroup and counter ions (X ) . This thesis main lyfocuses on the influence of counterion structure on the aggregation behavior of alkylpyridiniumsurfactants, both in dilute and concentrated aqueous solution.

Surfactant chemist ry is in t roduced in Chapter 1. This chapter focuses on micel les, e i therspherical or worm-like Relationships between surfactant structure and the size, shape andstabi l i ty of micel les in dr lu te aqueous solut ion and the type of lyot ropic mesophase formed in

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Summary

concentrated solutions are discussed. Furthermore. attention is paid to the drivino force formicelle formation.

Chapter 2 describes the influence of counterions on the properties of micelles formed by1-methyl-4-n-dodecylpyridinium surfactants. The crit ical micelle concentration, which is related tothe Gibbs energy of micelle formation, increases upon increasing size of the hydratedcounterion. Next to electrostatic interactions, also specific counterion effects are important withrespect to micelle size, shape and stabil ity. Factors which are to be considered are the positionof the counterion at the micellar surface and the orientation of the counterion and rtssubstituents with resoect to the surfactant monomers in a micelle. The crit ical micelle concentra-tion decreases upon increasing counterion hydrophobicity. 'Tilt ing' of the (aromatic) counterionwith respect to the surfactant monomer in a micelle and introduction of o-methoxy substituentsin aromatic counterions (i.e. the counterion becomes 'unsymmetric') results in higher crit icalmicelle concentrations and lower degrees of counterion binding. Introduction o-hydroxy and p-chloro substituents in benzoate counterions, on the other hand, stabil izes the micelles to amarked extend. Now long, worm-like micelles are formed in dilute aqueous solution.

Fundamental issues of microcalorimetry are described in Chapter 3. The main conclusionwhich emerge from our studies are: (i) Surfactant solutions are thermodynamically nonideawhich can be demonstrated using microcalorimetry; (i i) measured enthalpies of micelle formatjondepend on surfactant concentration in the syringe and are independent of the stirring speed, (i i i)measured enthalp ies of micel l izat ion are not equal to s tandard enthalp ies of micel le format ion.

Structure/property relationships, with respect to the enthalpy of micelle formation, areexplored in Chapter 4. The enthalpy of micelle formation, Àr,"H, becomes more exothermicupon increasing chain length of the a lky l chain, and decreasing headgroup s ize of the sur factantmonomers. Ar,"H is , within error l imit, unaffected by the degree of branching of the alkyl chainof the surfactant. Analysis of the influence of counterions on measured enthalpies of micelleformation appeared to be more subtle. When only electrostatic interactions between counterionand headgroup are present, AmicH becomes more exothermic upon increasing counterion size.p-Methyl substitution in aromatic counterions of surfactants results in more exothermic enthal-pies of micelle formation due to enhanced London-dispersion rnteractions between counterionsand surfactant monomers in the micelle. p-Hydroxy substituents on the aromatic ring of thecounterion also result in more exothermic enthalpres of micell ization, when compared to cationicsurfactants with unsubstituted aromatic counterions, which is attributed to counteri-on(substituent)-water interactions in the Stern layer. Tilt ing of the counterion with respect to thesurfactant headgroup or an unsymmetric nature of the counterion leads to less exothermicenthalp ies of micel l izat ion.

Long, worm- l ike micel les are formed in d i lu te aqueous solut ion of 1-methyl -4-n-dodecylpyridinium surfactants containing either salicylate or p-chlorobenzoate counterions.Enthalpies of micell ization are more exothermic for these surfactants than for suíactant formingspher ica l micel les.

The same Chapter descr ibes a study of heat capaci t ies of micel le format ion, which arerelated to hydrophobic hydration, as a function of counterron structure. No trend was found withrespect to this structural variable.

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Summary

aid to the driving force for

rerties of micelles formed byentration, which is related tosing size of the hydrated,n eÍfects are impoftant withconsidered are the positionof the counterion and itsre crit ical micelle concentra-of the (aromatic) counterionn of o-methoxy substituentsc') results in higher crit icaltroduction o-hydroxy and p-stabil izes the micelles to a

lueous solut ion.tpter 3, The main conclus ionhermodynamical ly nonideal ,rthalpies of micelle formationent of the stirring speed; (i i i)t lpies of micelle formation>y of micelle formation, arebecomes more exothermic

jgroup size of the surfactantbranching of the alkyl chain

rsured enthalpies of micelleractions between counterionI increasing counterion size.in more exothermic enthal-

tctions between counterionsrn the aromatic ring of the, when compared to cationicis attributed to counteri-runterion with respect to then leads to less exothermic

solution of 1-methyl-4-n-rhlorobenzoate counterions.s than for surfactant forming

nicelle formation, which arere. No trend was found with

Enthalpies of rnicelle formation can also be calculated using thermodynamic modelsThese values were calculated using the phase-separation model, mass-action model and Pois-son-Boltzmann model. Neither of the three model gave satisfactory agreements. Trends werepredicted correctly, however, with respect to alkyl chain lengths and variations in branching ofthe a lky l chain.

Chapter 6 deals with the challenging phenomenon of micellar growth. Spherical micellesgrow to Íorm worm-like micelles at lower concentration upon: (i) Increasing counterion size; (i i)increasing counterion hydrophobicity; (i i i) increasing counterion (substituent)- water interactionsin the Stern layer.

Alkylpyridinium surfactants containing salicylate or p-chlorobenzoate as counterions formextremely long, worm-like micelles, which form entangled networks. Solutions of these surfac-tants are viscoelastic. This unusual unidirectional growth is hampered upon: (i) decreasing alkylchain length and (i i) increasing headgroup hydrophobicity.It is proposed that the orientation and microenvironment of the substituent in the aromatic coun-terion is of decisive importance for the observed properties.

At higher surfactant concentrations intermicellar interactions become important andliquid-crystall ine phases are formed. Chapter 7 describes a preliminary study to the influence ofthe counterion on the l iquid-crystall ine behavior of alkylpyridinium surfactants. lt is shown thatthe type and stabil ity of the mesophases formed depend on the structure of the counterion.

Chapter 8 deals with micellar catalysis of the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate. Changes in catalytic efficiencies with the molecular structureof the micelle-forming surfactant are interpreted mainly in terms of init ial state hydrogen bondingat the micellar binding sites.

ln the f ina l chaoter the main f ind inos and conclus ions are summarized and d iscussed.

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