Journal of Inclusion Phenomena and Macrocyclic Chemistry 45: 83–89, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

83

Solubilization of Synthetic Perfumes by Nonionic Surfactants and by SulfoalkylEther β-CDs

QI QU∗, EDWARD TUCKER and SHERILL D. CHRISTIANDepartment of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, U.S.A.

(Received: 11 March 2002; in final form: 9 July 2002)

Key words: perfume, non-ionic surfactants, head-space chromatography

Abstract

Head-space gas chromatography has been used to study the solubilization of synthetic perfumes by nonionic surfactants andby sulfoalkyl ether β-cyclodextrins. This study shows that the addition of nonionic surfactants reduces the vapor pressureof these perfumes due to reduction of the free perfume concentration in water caused by partition of the solute betweensurfactant micelles and water. The relationship between solubilization capacity of nonionic surfactants and characteristicsof both the surfactant and the synthetic perfumes was evaluated. Similar vapor pressure reduction also observed in perfumesin sulfoalkyl ether β-cyclodextrins. The stability of the inclusion complexes depends on the structures of both perfumemolecules and substitution group on sulfoalkyl ether β-cyclodextrins. The results also indicated that in some cases theremay be more than one perfume molecule included in a β-CD molecule while there is no indication of complexes formedbetween more than one β-CD molecule and a single perfume molecule.

Introduction

Cyclodextrins are known by their ability to include a widerange of guest molecules to form noncovalently bondedinclusion complexes. The stability of these inclusion com-plexes in solution is characterized by the stability constant ordissociation constant. The stability constant can be definedas the equilibrium constant of the formation of the inclusioncomplex in the form of

K = [Complex][CD] · [Guest] , (1)

where [Complex], [CD] and [Guest] represent the concen-tration of cyclodextrin complex, empty cyclodextrin and free(un-included) guest molecules, respectively.

In many industrial applications of cyclodextrins such ascosmetic or pharmaceutical formulation, the final effects ofthe addition of cyclodextrin depend on the stability of theinclusion complexes as well as the ability to release activeingredients at desired conditions. It is believed that the in-clusion complexes with stability constants around 2000 M−1

are preferred for practical applications [1].Several experimental methods have been developed to

study cyclodextrin inclusion complexes in solution includingmaximum solubility method, conductometry, microcalor-imetry, potentiometry, circular dichroism, NMR, fluores-cence, UV/Visible spectrometry, HPLC and head-space gaschromatography. Solubility method is the simplest and mostused method for slightly water soluble guest molecules

∗ Present address BJ Services, U.S.A., 16610 Aldine-Westfield Road,Houston, TX 77032, U.S.A.

while UV/Visible spectroscopy [2, 3] and HPLC [4] arealso widely used. Head-space GC has been used in studyof solute-surfactant association equilibria of volatile organicsolutes [5, 6] and it can be used for study of volatile organicinclusion complexes with cyclodextrins. However, there arenot many publications on head-space GC in this area [7].

Head-space gas chromatography is a technique to de-termine physical properties of condensed phase by analyzinggas phase, into which some part of the components of thecondensed phase are partitioned with the establishment ofequilibrium [8]. If volatile components are present in anequilibrium in solution, the analysis of equilibrium vaporpermits the determination of equilibrium constants in solu-tion. Association constants of volatile organic compoundscan be obtained by head-space gas chromatography [9, 10].

When a complex is in solution or liquid form, head-spaceGC can give direct measurements of volatility reduction byinclusion complexation between cyclodextrin and volatileorganic substances. A head-space GC experiment has shownthat the addition of cyclodextrin to an aqueous solution ofaroma components has depressed the vapor pressure of thesearoma components and increased the retention time of aromafrom evaporation. Because the vanishing rate of aroma com-ponents is proportional to the concentration of free aromacomponents in solution, head-space gas chromatography canprovide direct measurement of the free aroma component.

Synthetic perfumes are widely used in various commer-cial products. If solubility of the perfumes in the mediumis insufficient to provide desired odor intensity, a solubilizermust be used to increase the solubilization of the perfume.Nonionic surfactants usually serve as such a solubilizer. Eth-

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Figure 1. Schematic diagram of head-space GC technique.

oxylated fatty alcohols based on C12 to C18 alcohols andseven to nine ethoxyl units have proved very useful [11]. Sol-ubilization of synthetic perfumes by anionic and/or nonionicsurfactant systems has been systematically studied by Abe’sresearch group [12–14]. The solubilization behavior of syn-thetic perfumes has been characterized by their hydrophobicproperty as well as the synergistic effect of anionic/nonionicsurfactant mixtures.

Cyclodextrins can be excellent solubilizers for variousflavor components to improve their solubility, their stabilityto heat, to oxygen and to light, and reduce their volatility.However, systematic information on cyclodextrin inclusioncomplexation of synthetic perfumes have not been reported[15]. The effectiveness of cyclodextrin complexation to atargeted ingredient depends on the association constant ofthe CD inclusion complex and competition from other in-gredients in the formulation. This work studied the inclusioncomplexation between sulfoalkyl ether β-cyclodextrins andsynthetic perfumes and the association constants of the in-clusion complexes are strongly influenced by the structureof the perfumes and of the CD derivatives.

Head-space GC for inclusion complexation study

The head-space GC technique is based on the equilibriumdistribution of the substance between the coexisting phases.For many dilute liquid-gas systems a direct proportional-ity between the concentration of a given component in theequilibrium phases can be established.

Cg = KC1.

Here K is distribution coefficient, and Cg and C1 representthe concentrations of a given substance in gas phase andliquid phase, respectively.

One of the techniques to measure K is gas chromato-graphy. The head-space GC analysis used in present study isschematically illustrated in Figure 1.

The investigated solution with known concentration ofthe synthetic perfumes C0

1 is placed in a fixed volume con-tainer. The container is kept at constant temperature toestablish the liquid-gas phase equilibrium. The quantitative

representation of head-space GC is accomplished by stand-ard calibration of peak area of GC measurement against C0

1 .However, due the analysis of a large number of samplesover a wide range of concentrations in current study, thecalculation of sample concentration in the liquid phase wasdone according to a calibration. This method has been usedalmost exclusively in solubilization study of volatile sub-stance by surfactant micelle solutions and by cyclodextrinsolutions. The absolute concentration of synthetic perfumesin vapor phase can be estimated, provided that a relationshipbetween peak area of GC measurement and concentrationof the perfumes has been established [16]. In the presenceof surfactant micelles or cyclodextrins, the vapor pressureof the perfumes was decreased and the peak area vs. per-fume concentration is interpreted in terms of the fractionof perfume bound to the micellar aggregates or inclusioncomplexed by cyclodextrins.

The computation of the fraction of free perfume andbound perfume can be illustrated by Figure 2. The horizontalseparation between two curves at any ordinate represents A

and B. B corresponds to the perfume fraction solubilizing inthe micellar phase or included by cyclodextrin cavity, whileA is the fraction remaining in the aqueous phase as free per-fume. By knowing the fractions of perfume in both aqueousphase and the bonded phase, a distribution constant can becalculated.

Assuming that a partition-like interaction is respons-ible for the perfume distribution between surfactant micellarpseudophase and water phase, the overall amount of per-fume in solution is divided into perfume incorporated intomicellar phase and associated with water phase. The wa-ter and micellar pseudophases are interspersed but remainchemically distinct, with characteristic properties. The con-centration based partitioning of perfume between these twopseudophases can be defined as

CTot.

Cw

= Cw + Cmic.

Cw

= 1 + Cmic.

Cw

. (2)

where Ctot. is the total concentration of perfume in solution,Cw and Cmic. are the concentration of perfume in “water”pseudophase and in micellar pseudophase, respectively. Thepartition coefficient is defined as

Kmw =(

Perfume Concentration in Micellar PhaseConcentration of Surfactant in Micellar Phase

)Perfume Concentration in Water Phase

.

(3)Then Equation (2) can be modified to

Ctot.

Cw

= 1 + Kmw · [Surf.], (4)

where [Surf.] is total concentration of surfactant and whentotal concentrations are well above the CMC, [Surf.] �CMC. The total concentration of perfume is known and thefree perfume in water phase can be measured by head-spaceGC experimentally. The partition coefficient, Kmw , thencan be estimated from the slope of the plot of Ctot./Cw vs.[Surf.].

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Figure 2. Peak area plot for a perfume in solution in the absence of sur-factant or cyclodextrin (a) and in the presence surfactant or cyclodextrin(b).

In the case of cyclodextrin inclusion complexation, theassociation constant, K , of the inclusion complex canbe represented by the following derivations, assuming 1:1complexation:

[CD]t = [CD]f + [CD-S]

[S]t = [S]f + [CD-S]

K = [CD-S][CD]f · [S]f (5)

[CD]t = 1

K

( [S]t[S]f − 1

)+ ([S]t − [S]f ), (6)

where [CD]t , [CD]f and [CD-S] are the total cyclodextrinconcentration, free cyclodextrin concentration and cyclodex-trin inclusion complex concentration, respectively. [S]t and[S]f are the total perfume concentration and free perfumeconcentration in the aqueous phase. By utilizing nonlin-ear least square regression, the association constant can becomputed.

Experimental

All studies were performed using a HP5890 gas chroma-tograph equipped with a flame ionization detector. Theanalyses were done on a 10 m × 0.53 mm id capillarycolumn with a stationary phase of polydimethylsiloxane.Helium was used as carrier gas at various flow rates de-pending on the particular perfume analyzed. A HP 2310integrator was used to record the signals. Carrier gas flowrate and experimental temperature range were adjusted tomeet the most appropriated experimental condition for eachperfume.

Sulfoalkyl ether β-cyclodextrins were synthesized andcharacterized as described previously [20]. Sulfoethyl etherβ-CD has an average degree of substitution of 3.36 andcontain 1.19 water per CD; sulfopropyl ether β-CD has anaverage degree of substitution of 1.6 and with 4.62 water perCD and the average degree of substitution of sulfobutyl etherβ-CD is 2.46 with 4.53 water per CD. The average molecularweights for sulfoethyl, sulfopropyl and sulfobutyl ether β-CDs are 1593.2, 1448.6 and 1605.2, respectively. Benzylalcohol (>99%), benzyl acetate (>99%), phenethyl alcohol(>99%), phenethyl acetate (>99%) and ethyl phenylacetate(>99%) were purchased from Aldrich and used as received.Polyethylene glycol mono-p-nonylphenyl ether with 10, 15and 20 oxylethylate units and C16POE20 were used as re-ceived from TCI, Japan. All the solutions were prepared withdeionized water. A sample solution (10 mL) was placed inan EPA certified 40 ml sample vial, sealed airtight with asilicon septum, and placed in a water bath at 20 ◦C for 24hours to achieve equilibrium before injection. A gas-tightsyringe was used for the sample injection and all sampleswere injected manually. A 20 µl sample was injected intogas chromatograph each time. Table 1 illustrates all thestructures and physical constants of synthetic perfumes andnonionic surfactants.

Results and discussion

All data on nonionic surfactant micellar solutions were ana-lyzed according to Equation (3) to calculate the partitioncoefficient of perfumes by plotting total perfume concentra-tion over free perfume concentration as a function of totalsurfactant concentrations. A clear linear relationship was ob-tained in each system. The linear curve gave an intercept of 1within the experimental error, which supports the partition-like interaction model. From the slope of each straight line,the value of the partition coefficient of each synthetic per-fume between water pseudophase and surfactant micellarpseudophase can be calculated (Table 2).

The calculated Kmw indicates that in each surfactantsolution the partition coefficients are in the order of ben-zyl alcohol < phenethyl alcohol < benzyl acetate < ethylphenylacetate < phenethyl acetate. For a homologous seriesof synthetic perfumes, the perfume with higher hydrophobi-city has higher partition coefficient in surfactant micelles.Generally speaking, the alcohol is more hydrophilic or lesshydrophobic than acetate while molecules with longer hy-drocarbon chains are more hydrophobic than those withshort hydrocarbon chains. Benzyl alcohol is the most polarperfume and has the highest water solubility in the absenceof surfactant which means it has the lowest hydrophobicity.Thus it has the lowest partition coefficient in each surfactantsolution. On the other hand, phenethyl acetate has the lowestwater solubility or highest hydrophobicity and the highestpartition coefficient.

For the apolar esters, such as benzyl acetate, ethylphenylacetate and phenethyl acetate, the partition coeffi-cients in different surfactant micelles are approximately the

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Table 1. Structure, molecular weight and abbreviation of nonionic surfactants and syntheticperfumes

Structure Name MW Abbreviation

C9H19C6H4(OCH2CH2)10OH Polyoxyethylenated p- 660.9 APE (n = 10)

nonylphenol (n = 10)

C9H19C6H4(OCH2CH2)15OH Polyoxyethylenated p- 881.1 APE (n = 15)

nonylphenol (n = 15)

C9H19C6H4(OCH2CH2)20OH Polyoxyethylenated p- 1101.4 APE (n = 20)

nonylphenol (n = 20)

C16H33(OCH2CH2)20OH Polyoxyethylenated 1123.5 AE (n = 20)

straight-chain alcohol

C6H5CH2OH Benzyl alcohol 108.14 BA

C6H5CH2CH2OH Phenethyl alcohol 122.17 BAC

C6H5CH2OCOCH3 Benzyl acetate 150.18 PEA

C6H5CH2CH2OCOCH3 Phenethyl acetate 164.20 PEC

C6H5CH2COOCH2CH3 Ethyl phenylacetate 164.20 EPC

Table 2. Kmw of synthetic perfumes in nonionic surfactants

APE APE APE AE

(n = 10) (n = 15) (n = 20) (n = 20)

M−1 M−1 M−1 M−1

Benzyl alcohol 8.0 ± 0.8 14.6 ± 0.9 16.3 ± 0.5 19.3 ± 0.6

Benzyl acetate 29.4 ± 1.0 33.5 ± 1.9 30.2 ± 3.4 35.6 ± 1.4

Phenethyl alcohol 11.4 ± 0.5 14.3 ± 0.8 22.2 ± 1.3 23.1 ± 1.4

Phenethyl acetate 64.7 ± 1.1 57.8 ± 1.7 63.0 ± 3.0 57.5 ± 3.8

Ethyl phenylacetate 55.7 ± 2.3 51.9 ± 1.1 51.2 ± 1.7 47.0 ± 3.5

Figure 3. Schematic diagram of polar solute solubilization in nonionicsurfactant.

same within experimental error. This suggests that each sur-factant has similar solubilization capacity to these esters.This fits well with a solubilization model illustrated in Fig-ure 3. For a surfactant containing polyoxyethylene chain asits head group, the micelles could consist of two parts, aninner core of hydrocarbon tails and an outer shell of hy-drated polyoxyethylene. Polar organic solutes such as benzylalcohol or phenol, which have strong affinity to the poly-oxyethylene group, may be incorporated at least partly intothe outer shell. This type of solubilization is classified as in-clusion into the polyoxyethylene exterior of micelles ratherthan as adsorption on the surface of micelles [17].

Apolar solutes such as esters are mainly solubilized in-side the micellar core or deeply within the palisade layer.The incorporation site of a solubilized solute depends on itsrelative hydrophobic and hydrophilic tendencies. Because ofthe hydrocarbon tails of the nonionic surfactants used, thesolubilization capacities of these surfactants should be in thesame range for an apolar solute. Also, partition coefficientsof apolar esters in these surfactants are in the same range.It was reported that solubilization of benzene by nonionicsurfactants is only slightly decreased when the EO number inthe nonionic surfactants varies from 9 to 40. Particularly, atrelatively low surfactant concentration there is no significantdifference in the benzene solubilization in various nonionicsurfactants with different EO numbers [10]. Our results arein agreement with the above observation and the solubil-ization of apolar solutes being independent of the EO unitnumber of the nonionic surfactants.

For benzyl alcohol or phenethyl alcohol in the abovesurfactant solutions, the partition coefficients increase inthe order of APE (n = 10) < APE (n = 15) < APE(n = 20) < C16POE20. This is in agreement with the pre-diction from the above solubilization model. Polar solutessuch as benzyl alcohol are solubilized within the palisadelayer of EO units surrounding the hydrophobic core. Thesolubilization of polar solutes increases with the EO unitnumber in the nonionic surfactants. The partition coefficientfor each synthetic perfume is plotted against the surfactanttype (Figure 4).

Head-space GC measurement of the same synthetic per-fumes in sulfoalkyl ether β-CDs was conducted. By assum-ing 1:1 inclusion complexation, the stability constant, K ,can be estimated from the parameter P1 that is obtained fromnonlinear least square regression in each plot of syntheticperfumes in sulfoalkyl ether β-CDs by P1 = 1/K . Table 3lists all the calculated Ks.

The stability constant of inclusion complexes formedbetween synthetic perfumes and each sulfoalkyl ether β-CD is in the order of benzyl alcohol < phenethyl alcohol< benzyl acetate < ethyl phenylacetate < phenethyl acetate,

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Table 3. Stability constants of inclusion complexes between syntheticperfumes and sulfoalkyl ether β-CDs

Sulfoethyl Sulfopropyl Sulfobutyl

ether β-CD ether β-CD ether β-CD

(M−1) (M−1) (M−1)

Benzyl alcohol 13 12 12

Benzyl acetate 92 158 124

Phenethyl alcohol 24 43 37

Phenethyl acetate 106 397 262

Ethyl phenylacetate 195 206 238

Figure 4. Partition coefficients of synthetic perfumes in nonionic surfact-ants.

except for the sulfoethyl ether β-CD. This order is similar tothat of partition coefficients in nonionic surfactants.

The binding between cyclodextrins and guest perfumesdepends on both hydrophobicity of the guest molecule andthe geometric accommodation of the guest molecule intothe CD cavity. When the hydrophobicity of various guestmolecules differs greatly and their sizes and geometric struc-tures are similar, the stability constants of the inclusioncomplexes are mainly determined by the hydrophobicity ofthe guest molecules, such as is the case with benzyl al-cohol compared to benzyl acetate and phenethyl alcoholcompared to phenethyl acetate. The more hydrophobic theguest molecule, the more stable the inclusion complex be-comes. When the geometric structure and size of the guestmolecules have significant differences and their hydrophobi-city is similar, the geometric fit of the guest molecule intothe CD cavity becomes the main factor in determining thestability of the inclusion complexes. Among the three esters,benzyl acetate forms the least stable inclusion complex andphenethyacetate forms the most stable complex with eachof the sulfoalkyl ether β-CDs, except with sulfoethyl etherβ-CD.

The trend is not as obvious as that for different perfumesin the same sulfoalkyl ether β-CD. Most of the perfumesform the more stable inclusion complex with sulfopropyl β-CD, followed by that with sulfobutyl ether β-CD and withsulfoethyl ether β-CD. A low average substitution degreeof sulfopropyl ether β-CD could be the reason. It was sug-gested that the high substitution degree of β-CD derivatives

Figure 5. Benzyl alcohol in surfactants and sulfoalkyl β-CDs.

Figure 6. Benzyl acetate in surfactants and sulfoalkyl β-CDs.

may block the accessibility of the guest molecule to the CDcavity [19]. Another factor influencing stability constant isthe size of the substituent group. A bulky substituent grouplimits the accessibility to CD cavity. Combination of severalfactors such as hydrophobicity of the substituent group, aver-age substitution degree, geometric feature of the substituentgroup, and positional distribution of the substitution must beconsidered [20].

Figure 7. Ethyl phenylacetate in surfactants and sulfoalkyl β-CDs.

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Figure 8. Phenethyl alcohol in surfactants and sulfoalkyl β-CDs.

Figure 9. Phenethyl acetate in surfactants and sulfoalkyl β-CDs.

Average substitution degree is of particular importancein this case. When the guest molecule is small enough, suchas benzyl alcohol, there is no preference among all threesulfoalkyl ether β-CD derivatives. Binding constants of theinclusion complexes between benzyl alcohol and sulfoalkylether β-CDs are almost the same. As the guest’s molecu-lar size becomes large, a more stable inclusion complexwas formed as the β-CD derivatives have a low averagesubstitution degree.

Figures 5 to 9 summarized the concentrations of the freesynthetic perfumes in the presence of nonionic surfactants orsulfoalkyl ether β-CDs. One observation is that there is anupward deviation from a linear relationship based on Equa-tion (7) for perfume complexation with CD derivatives. Thisis particularly apparent for the phenethyl alcohol and phen-ethyl acetate while it is slightly deviated in the case of benzylalcohol, benzyl acetate and ethylphenyl acetate. However,there is no evidence to indicate the downward deviation fromthe 1:1 inclusion assumption. The deviation may suggest theratio of CD molecule to perfume molecule in their complex-ation. Job plot analysis could be an effective way to confirmthe complex stoichiometry. However, due to the limitation ofthe head space GC method, the maximum molar fraction of

β-CDs that can be added to the liquid phase is less than 0.5;otherwise, the gas phase has too little perfume to be detectedby head space GC. Unfortunately, it is not possible to havea clear indication to predict the complexation stoichiometrythrough Job plot analysis.

Conclusion

Head-space gas chromatography has been used to study thesolubilization of synthetic perfumes by nonionic surfact-ants and by sulfoalkyl ether β-cyclodextrins. This study hasshown that both nonionic surfactants and cyclodextrin de-rivatives reduce the vapor pressure of these perfumes. Thesolubilization capacity of nonionic surfactants depends onproperties of both the surfactant and synthetic perfumes.Sulfoalkyl ether β-CDs can form stable inclusion complexeswith a wide range of guest molecules. The sulfoalkyl etherβ-CDs synthesized in this work have the combined charac-teristics of low average substitution degree and high watersolubility, which will confer great advantages in their pos-sible industrial applications as compared with other β-CDderivatives.

Head-space GC is an effective way to characterize theinclusion complexes formed between sulfoalkyl ether β-CDs and synthetic perfumes. The stability of the inclusioncomplexes depends on both hydrophobicity of the guest mo-lecules and the size of the substituent on β-CD and degree ofthe substitution, while the degree of substitution is especiallyimportant. The results from the head-space GC on solubiliz-ation of synthetic perfumes in nonionic surfactants supportthe partition-like model.

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