`This article is protected by copyright and is provided by the University of Wisconsin-
`Madison under license from John Wiley & Sons.All rights reserved.
`
`
`
`
`
`J. Pharm. Pharmacol., 1964, 16, 323-331
`
`Received October 14, 1963
`
`Thesolubilisation and inactivation of preservatives
`by non-ionic detergents*
`
`W. P. EVANS
`
`A novel potentiometric method, which depends on the pH changes which occur
`when anacidic material is solubilised, has been used to determine the solubilisation
`of the weakly acidic preservative, p-hydroxybenzoic acid, in a non-ionic detergent.
`From the pH changesobserved, the partition coefficient, Km, for the distribution of
`the un-ionised acid between the micellar and the aqueous phase hasbeen calculated
`and found to be 2:8 x 10%.
`Specific interaction between the acid and the detergent.
`to form a complex is shown not to be important.
`It is suggested that the arguments
`against specific interaction apply generally to other preservatives and non-ionic
`detergents.
`
`ON-IONIC detergents are used to an increasing extent as solubilising
`Nand emulsifying agents in cosmetic and pharmaceutical systems. They
`-
`have some disadvantages, the main one being the inactivation of preserva-
`tives such as p-hydroxybenzoic acid and benzoic acid or their esters,
`which are commonly employed in such systems (de Navarre, 1953, 1956).
`The cause of the inactivation has not been established, although
`complex formation between the preservative and the non-ionic detergent
`is often postulated. Higuchi & Lach (1954) state that Carbowax, a
`polyethylene glycol, forms complexes with benzoic, salicylic andp-hydroxy-
`benzoic acid by reaction of the phenolic or carboxylic hydrogen with the
`ether oxygen of the glycol. Several authors have suggested that similar
`interactions also occur with non-ionic surface-active agents. The following
`formula has been proposed, but no quantitative data have been given, for
`a complex between phenol and a non-ionic detergent.
`
`|
`
`H
`'
`RO(CH,CH,O)a (CHsCH,O)sH
`
`a |
`
`_
`
`C,H,O
`
`_| a
`
`Protonation of the ether oxygen is, however, extremely improbable at
`the non-ionic detergent concentration and pH values of most cosmetic
`preparations; this is proved by the results described in this paper. Further-
`more,it is unlikely that such interaction is the whole or even part of the
`Inactivation, since inactivation may occur with preservatives of widely
`different types (Wedderburn, 1958). A more probable explanation is
`solubilisation of the preservative in the non-ionic micelles. Solubilisation
`of phenolic bactericides by micellar soap solutions is well known andit
`has been shown by many investigators that solubilisation results in a
`decrease of bactericidal activity (Alexander, 1946, 1949).
`From the Unilever Research Laboratory, Unilever Ltd., Port Sunlight, Cheshire.
`* Part of a paper presented at a conference of ‘“‘“Group Rech. Prod. Superf. Actf.
`5° Coll.”’, in Paris, 1959
`
`323
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`W. P. EVANS
`
`The normal method of estimating the amount solubilised by anionic
`and cationic detergents is to add increasing amounts of the solubilisate
`to the detergent solution and to equilibrate. When excess solubilisate is
`present the solutions are turbid and the amount solubilised can then
`readily be determined. Unfortunately, it is impossible in many cases to
`determine solubilisation by non-ionic detergents by this method because
`their cloud points are depressed by many solubilisates;
`the observed
`turbidity in such cases is not due to excess solubilisate but to “salted out”
`detergent or coacervate.
`Apart from being inapplicable to many non-ionic detergents the equili-
`bration solubilisation method is generally unsatisfactory even with anionic
`and cationic detergents since the results are difficult to interpret because
`measurements, apart from one or two exceptions (McBain, 1940), have
`always been made in saturated systems. The present paper describes a
`simple titration method which overcomesthis difficulty, and which allows
`determination of solubilisation in unsaturated systems;
`it can be used
`with solubilisates containing a weak acidic group and depends on pH
`changes which result from preferential solubilisation of the un-ionised
`acid.
`It has been used in the present study to determine the solubilisation
`of p-hydroxybenzoic acid (a common cosmetic preservative) by the
`non-ionic detergent octyl phenol condensed with 8-5 moles of ethylene
`oxide.
`
`TITRATIONS
`
`Experimental
`
`p-Hydroxybenzoic acid solutions (100 ml, either 0-01 or 0:03 mM) con-
`taining varying concentrations of the non-ionic detergent (from 0-03 to
`0-20 M) were titrated potentiometrically with 0-1 N sodium hydroxide
`using calomel and glass electrodes. Preliminary titrations with acetic
`and hydrochloric acids showed that the titration curves of the two acids
`were unaffected by the detergent, proving thatit did notaffect the potentials
`of the electrodes.
`
`CLOUD POINTS
`
`Cloud points were determined by the usual method of heating a 1%
`solution of the detergent alone or with additive until a faint turbidity
`appeared. The temperatures at which turbidity first appeared were taken
`as the cloud point of the detergent. These were reproducible to -+-0-2°.
`
`Results and discussion
`
`Fig. 1 shows a plot of the apparent “solubility” at 25° of p-hydroxy-
`benzoic acid in varying concentrations of the detergent determined by
`the conventional turbidity method.
`Up to a detergent concentration of about 3-5% (w/v), the apparent
`solubility of the acid in the detergent solution is less than its solubility
`in pure water. That the turbidity is due, not to excess solubilisate, but
`to a salted-out mixture of detergent plus solubilisate was confirmed by
`324
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`INACTIVATION OF PRESERVATIVES BY NON-IONIC DETERGENTS
`
`id(molar)
`
`aciApparentsolubilityofp-hydroxybenzoic
`
`
`
`
`
` 0
`
`8
`6
`4
`2
`Conc. non-ionic detergent (% w/v)
`Fic. 1. Apparent solubility of p-hydroxybenzoic acid (molar) at 25° in varying
`concentrations of octyl phenol/8-5 moles ethylene oxide.
`
`10
`
`allowing the solutions to stand for some time, when twolayers separate
`out, one layer being detergent rich, the other being water rich, but both
`layers containing the solubilisate. The separation of non-ionic detergents
`in this way is related to the cloud point phenomenon; a solution of the
`
`
`
`Cloudpoints°C
`
`30 20
`
`0.05
`
`0-10
`
`Conc. p-hydroxybenzoic acid (moles/litre)
`Fic. 2. Cloud points of various concentrations of octyl phenol/8-5 moles ethylene
`oxide in presence of p-hydroxybenzoic acid. © 10% (w/v) octyl phenol/8-5 moles
`ethylene oxide.
`/\ 5% (w/v) octyl phenol/8-5 moles ethylene oxide. 1 1% (w/v)
`octyl phenol/8-5 moles ethylene oxide.
`\7 0-1% (w/v) octyl phenol/8:5 moles
`ethylene oxide.
`
`325
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`
`W. P. EVANS
`
`detergent becomes cloudy when heated above a certain temperature—
`“the cloud point”. The effect of p-hydroxybenzoic acid on the cloud
`point of different concentrations of the detergent is shown in Fig. 2.
`It is often stated that addition of organic additives such as phenol or
`hydroxybenzoic acid, results in precipitation of a complex of the non-
`ionic detergent and its additive. This is unlikely, however, since pre-
`cipitation also occurs with additives such as benzene or sodium sulphate,
`with which it is difficult to visualise the formation of such a complex.
`The precipitate which separates in these instances is not a detergent-
`additive complex, but the separation of a detergent-rich layer containing
`the dissolved additive,
`i.c., a coacervate. For example, a 20% (w/v)
`solution of octyl phenol/8-5 mole ethylene oxide saturated with p-hydroxy-
`benzoic acid separates into two layers at 25°. The analysis of the two
`layers is as follows:
`
` Cloud point Surface tension | Acid in each
`
`°C
`_ dynes/em at 25° |
`layer %
`Aqueous layer
`..
`.
`>98
`36-0
`0-68
`
`oe —_— i — ,..Non-ionic layer 19-0
`
`
`
`
`
`
`
`
`
`The aqueous phase contains virtually no detergent as shown by the
`high “cloud point” and only 0-68% (w/v) of acid, while the detergent
`phase contains 19-0% (w/v) of the acid.
`[The solubility of the acid in
`pure water is 0-63°% (w/v) and in the anhydrous detergent 26% (w/v]).
`
`10
`
`0
`
`4
`
`8
`mi 0-IN NaOH
`
`12
`
`16
`
`20
`
`Fic. 3. Titration of 100 ml of 0-01 Mm p-hydroxybenzoic acid containing varying
`concentrations of octyl phenol/8-5 moles ethylene oxide with 0-IN NaOH.
`YV/ Acid
`alone. © Acid containing 5% (w/v) ethanol.
`x Acid containing 0-03m octyl
`phenol/8:5 moles ethylene oxide. © Acid containing 0-10m octyl phenol/8-5 moles
`ethylene oxide.
`/\ Acid containing 0:20m octyl phenol/8°5 moles ethylene oxide.
`326
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`INACTIVATION OF PRESERVATIVES BY NON-IONIC DETERGENTS
`
`With sodium sulphate as the additive, separation into two phases again
`occurs, with the sodium sulphate dissolved in both phases, but, unlike
`the p-hydroxybenzoic acid,
`it is preferentially soluble in the aqueous
`phase. Further unpublished results confirm that with most additives
`the detergent-additive precipitate is not a definite chemical complex but
`a coacervate of varying stoichiometry.
`Typical titration curves of p-hydroxybenzoic acid in the presence of a
`non-ionic detergent (below the cloud point) are given in Fig. 3.
`The titration curves given in Fig. 3 show a pH shift to higher values
`in the presence of the detergent, therefore either the free concentration of
`the un-ionised acid has decreased as a result of some interaction with the
`detergent, or the detergent has changed the dielectric constant of the
`solution or the electrode potentials because it has been adsorbed on the
`glass electrode.* The titration curves of acetic acid and hydrochloric
`acid, however, were not affected by the presence of the detergent, so that
`the pH changes observed are not due to dielectric changes or to changes
`in the electrode potentials, but must be due to a decrease in the concen-
`tration of un-ionised acid. Furthermore,
`the normaltitration results
`observed with acetic and hydrochloric acids show that protonation of the
`ether oxygen of the detergent as suggested by various authors does not
`take place, or, at least, the extent of protonation is too small to be detected
`by pH changes.
`The titration results of p-hydroxybenzoic acid can be interpreted by
`assuming a decrease in the concentration of the un-ionised acid due either
`to complex formation of the type suggested by Higuchi & Lach (1954)
`or to solubilisation of the un-ionised acid; the results have therefore been
`used to calculate constants (Ke and Km), assuming:
`(1) Formation of a 1:1 complex between non-micellar detergent(i.e...
`single molecules) and the un-ionised acid (Ke).
`(2) Solubilisation of the un-ionised acid in the detergent micelles (Km).
`The equilibria considered here are:
`HAwater = Ht + A-
`I
`HAnteetles
`
`The constants obtained are given in Table 1, from which it is evident.
`that the “constant”, Ke, for a 1:1 complex shows considerable drift;
`interaction of the acid with monomeric detergent molecules is therefore
`unlikely.
`Furthermore, any such interaction should, at constant acid concen-
`tration, result
`in pH shifts which are independent of total detergent
`concentration provided this is above the critical micelle concentration
`(about 1-7 x 10-* m for the detergent used here) since the concentration of
`the monomeric detergent molecules is constant above the cmc. Further
`evidence against complex formation was obtained by titrating aqueous
`
`* Since this work was completed (Paris, 1959), Donbrow & Rhodes (1963) have
`published titration curves showing similar pH shifts, but no quantitative treatment
`was given.
`
`327
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`
`W. P. EVANS
`
`solutions of p-hydroxybenzoic acid containing polyethylene glycol 400,
`a polymer of 9 ethylene oxide units, identical with the hydrophilic part
`of the non-ionic detergent, but, unlike the latter, not aggregating to form
`micelles; no pH shifts were observed.
`
`TABLE 1.
`
`THE pH, CONCENTRATION OF THE VARIOUS SPECIES, AND SOLUBILISATION
`AND COMPLEXING CONSTANTS (KmKc) FOR VARIOUS CONCENTRATIONS OF
`P-HYDROXYBENZOIC ACID AND OCTYL PHENOL/8°5 MOLES ETHYLENE OXIDE
`
`
`
`[HAk
`[Detergent]
`[A-]
`[HAlaq
`{HA]m
`Ke x 10¢* | Km x 1073
`
`
`
`
`
`0-02970
`3:97
`0:001056
`0-009902
`0-003557
`0-005289
`1-2
`|
`2:8
`0-009710
`4:48
`0-002824
`1-2
`|
`2-8
`0-009346
`5:73
`00626
`0-21
`=|
`25
`0-02670
`4:59
`0-01030
`1:3
`!
`2-7
`0:02577
`5-44
`0-01322
`0-19
`\
`29
`0-009524
`5:20
`0:004568
`0-35
`=|
`29
`0-02770
`4:96
`0-007109
`0-20
`=|
`26
`0-009616
`5-05
`0-003695
`0-3
`{
`2:9
`0-009854
`414
`0-001488
`1-1
`2-8
`0-02670
`5-22
`0-01028
`0-19
`i
`2:8
`
`
`0-02718
`512
`
`
`
`2:8 0-008718 0-001807 0-01666 019 |
`
`\y
`
`
`
`: :4™~. :
`= Concentration acid anion HO -
`Aq}
`+ COO- (moles/litre).
`\aZ
`t
`(HAlt = Total concentration of acid in system (moles/litre).
`
`
`
`
`[HA]laqg = Concentration un-ionised acid HO - x » - COOHin water phase (moles/fitre).
`[HA]m = Concentration un-ionised acid HO- <> - COOHin micelle (moles/litre).
`
`Detergent] = Concentration of detergent (moles/litre).
`* Calculations assuming 1:2 or 1:3 complexes also gave unsatisfactory constants, Ke.
`
`The results can, however, be interpreted satisfactorily by assuming
`solubilisation of the p-hydroxybenzoic acid in the detergent micelles,
`without postulating any specific interaction. McBain & Hutchinson
`(1955) point out that many instances of solubilisation can be quantitatively
`described as a partition of solute between two immiscible phases. When
`the solubilisate carries a polar group, as in the present study,
`it may
`orient itself on the surface of the micelle instead of passing into the
`hydrocarbon interior, but even in this case the solubilisation may be
`approximately characterised by a simple distribution coefficient.
`In the
`present work the solubilisation is also treated as a distribution phenomena
`of the un-ionised acid between the aqueous phase and the micellar phase
`and a distribution constant, Km, calculated. The distribution constant
`is defined as:
`
`Ko = [HA]miceite mole acid/mole detergent
`™ ~~ [HA]water
`mole acid/mole water
`
`That solubilisation and not complex formation is the correct inter-
`pretation of the pH shifts is further supported by the fact that pH shifts
`are also observed when long-chain fatty acids are titrated in the presence
`of anionic detergents (e.g., lauric acid in the presence of sodium dodecyl
`sulphate). Again there is no evidence of specific interaction to form
`complexes but from the results (to be published) similar partition coeffi-
`cients for solubilisation can be calculated.
`(From the data on page 326
`328
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`INACTIVATION OF PRESERVATIVES BY NON-IONIC DETERGENTS
`
`a partition coefficient was also calculated and found to be 2:3 x 10%, in
`good agreement with that calculated from the potentiometric data).
`The anions of the acid would not be expected to be solubilised to the
`same extent, and this is proved by the fact that there are no pHshifts
`when the phenolic group of the p-hydroxybenzoic acid is titrated. A
`number of authors (e.g., Maclay, 1956) have stated that solubilisation
`results in a decrease in the hydrophilic character of the non-ionic micelles,
`and any solubilised material would therefore decrease the cloud point
`whereas the mono-sodium salt, which is not solubilised,
`increases the
`cloud point (Fig. 4).
`
`Cloudpoints°C
`
`Ss
`
`30 20
`
`0-05
`
`010
`
`Conc. additives (moles/litre)
`
`Fic. 4. Cloud points of 1% (w/v) octyl phenol/8-5 moles ethylene oxide in presence
`of various additives.
`/\ p-hydroxybenzoic acid, EF] monosodium salt of p-hydroxy-
`benzoic acid, © benzoic acid, \7 sodium benzoate, x phenol, < sodium phenate.
`
`The value of the distribution constant, Km, obtained in this work
`(Km = 2:8 x 10%)
`is constant over
`the whole concentration range
`examined, andis of the same orderas those values reportedin the literature
`for solubilisation by anionic and cationic detergents (McBain & Hut-
`chinson, 1955). Few data are available for solubilisation by non-ionic
`detergents; Moore & Bell’s results (1957) on the solubilities of phenyl
`ethyl alcohol and benzaldehyde in hexadecyl/14 moles ethylene oxide and
`in hexadecyl/24 moles ethylene oxide respectively as determined by the
`conventional turbidity method are given in Table 2 (in Moore and Bell’s
`study, saturation of the solution by the solubilisate was apparently reached
`before the cloud point was reduced sufficiently to precipitate the deter-
`gents).
`
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`W. P. EVANS
`
`Moore and Bell’s results have been used to calculate the relevant
`distribution constants, Km, which are of the same orderas those obtained
`in the present study.
`
`TABLE 2.
`
`sovupiLity (g/100 ml) OF PHENYLETHYL ALCOHOL AND BENZALDEHYDE
`IN HEXADECYL/24 MOLES ETHYLENE OXIDE AND HEXADECYL/14 MOLES
`ETHYLENE OXIDE RESPECTIVELY AT 25° (Moore & Bell, 1957)
`
`Solubility of phenylethy!
`Solubility of benzaldehyde
`alcohol in hexadecyl/24
`in hexadecyl/14
`
`moles ethylene oxide moles ethylene oxide |
`
`
`Concentration of detergent %
`
`100
`25
`| 50
`10-0
`25
`50
`|
`——
`———
`———|
`4:13
`Total wt solubilised (g) ..
`5-04
`8-28
`,
`3-24
`
`0-33
`Wtin aqueous phase (g)
`1-60
`3-8
`Wtin micellar phase (g)
`1-64
`
`..
`54
`Km x 10-3
`..
`2-7
`
`
`oe
`
`Apart from the exception noted (McBain, 1940), all previously reported.
`solubilisation data have been obtained by examination of systems con-
`taining excess insoluble phases, where both the micellar phase and the
`aqueous phase are saturated with the solubilisate. By subtracting the
`solubility of the solubilisate in pure water from its solubility in the deter-
`gent solution, the amount of solubilisate dissolved in the micelles can be
`obtained. This type of calculation, however, assumes that the concentra-
`tion of the solubilisate in the aqueous phase in a saturated detergent
`solution is the same asits solubility in pure water; this may not always
`be true, e.g., potassium laurate even below the CMC increases the water
`solubility of phenol five times. Furthermore, due to deviation from ideal
`behaviour, the distribution constant of a solute between two immiscible
`phases changes as the two phases become saturated. That the results
`obtained in the present paper are of the same order as those calculated
`from Moore and Bell’s work must therefore be regarded as somewhat
`fortuitous.
`
`TABLE 3. MOLAR CONCENTRATION OF UN-IONISED P-HYDROXYBENZOIC ACID IN THE
`WATER PHASE IN THE PRESENCE AND ABSENCE OF 5:8% (w/v) DETERGENT
`AT VARIOUS pH’s USING 0:1% (w/v) TOTAL CONCENTRATION OF p-HYDROXY-~
`BENZOIC ACID
`
`0:27
`
`[HA]water in
`Total (%) (w/v) acid required in presence of
`presence of
`5-8 % (w/v) detergent to be equivalent to
`pH
`[HA]water
`5-8 % detergent
`0-1 % (w/v) acid in pure water
`
`35
`0-00662
`0°00118
`0.56
`40
`0-00559
`5
`0-49
`45
`0:00376
`0:36
`5-0
`0-00220
`
`All the results obtained in this study can be adequately explained by
`solubilisation of the un-ionised p-hydroxybenzoic acid in the non-ionic
`micelles, without postulating specific interaction or formation of com-
`plexes as has been done by most authors. Since the un-ionised acid
`(Simon, 1952), is generally regarded as being the active preservative, and.
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`
`since only the acid in the aqueous phaseis effective, it is reasonable to
`assume, at least in the case of p-hydroxybenzoic acid and the detergent
`used, that solubilisation and not complex formation is the cause of the
`inactivation by non-ionic detergents. Knowing the total concentration
`of p-hydroxybenzoic acid,
`its dissociation constant, Ke, (2°95 x 10-5),
`the concentration of the detergent, and the partitition coefficient, Km, it
`is easy to calculate the amounts of the acid dissolved in the aqueous
`and the micellar phases at varying pH, detergent, and total acid concen-
`tration. The results of such a calculation, assuming a total concentration
`of 0-1% (w/v) p-hydroxybenzoic acid and 5-8°% (w/v) detergent are given
`in Table 3.
`Thus if 0-1% (w/v) is the optimum total concentration of acid needed
`to give the required preservative effect in water at pH 4-0, the concen-
`tration of the un-ionised acid (the ‘active’ species) is 0-00559 m. Addition
`of 5-8% of the detergent reduces this concentration of un-ionised acid
`to 000114 m (Column 3, Table 3), and Column 4 showsthat at pH 4-0,
`0-49% (w/v) of acid would be required in presence of 5:8% (w/v) of the
`detergent to give the optimum molar concentration of the un-ionised acid
`in the water phase—.e., five times as much acid would be required to give
`the same concentration of the un-ionised acid in the aqueous phase in
`the presence of the detergent as in the absence of the non-ionic detergent.
`Acknowledgements. The author wishes to acknowledge many valuable
`discussions with Dr. T. G. Jones of this Department.
`
`References
`
`Trans. Far. Soc., 45, 528-536.
`Agar, A. & Alexander, A. E. (1949).
`Alexander, A. E. & Trim, A. R. (1946). Proc. Roy. Soc. (Lond.), B, 133, 200-234.
`Donbrow, M. & Rhodes, C. T. (1963).
`J. Pharm. Pharmacal., 15, 233-238.
`Higuchi, T. & Lach, J. L. (1954).
`J. Amer. pharm. Ass. Sci. Ed., 43, 465-470.
`Kushner, L. N., Hubbard, W. D. & Dean,A. S. (1957).
`J. phys. Chem., 61, 371-373.
`Maclay, W. N. (1956).
`J. Coll. Sci., 11, 273-285.
`McBain, J. W. & O’Connor, J. J. (1940).
`J. Amer. chem. Soc., 62, 2855-2859.
`McBain, M. E. L. & Hutchinson, E. (1955). Solubilisation and Related Phenomena,
`Ist ed., New York: Academic Press.
`Moore, C. D. & Bell, M. (1957). Soap, Perfum. Cosm., 30, 69-76.
`Navarre de, M. G. (1954).
`1st World Congress on Surface Active Agents, Vol. 2,
`pp. 741-742. Paris: Chambre Syndicate Tramagras.
`Navarre de, M. G. (1956).
`J. Soc. cos. Chem., 7, 427-430.
`Simon, E. W. (1952). New Phyrol., 51-2, 163-197.
`Wedderburn, D. L. (1958).
`J. Soc. cos. Chem., 9, 210-228.
`
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