第32巻 第3号 (1983) 135

総 説

The Effective Hydrophile-Lipophile Balance of

Nonionic Surfactants

Leszek MARSZALL

Pharmacy No.09068, Rynek 12 86-170 Nowe, Poland

The classical HLB (Hydrophile-Lipophile Balance) value of nonionic surfactants based on an original

molecular structure does not take into account several factors affecting the performance of surfactants

such as presence of additives, structural modifications of surfactant molecule, temperature , decomposition, etc. as is deduced from our own experiences and that of other authors. If these factors are taken into

account, the HLB value becomes a variable depending upon the physical and chemical conditions at the

time of the measurement. These effects may be described by assigning a more practical value , so-called effective HLB value which is defined as equivalent HLB of the reference surfactant which yields the same physical results as the one under investigation . The detailed knowledge of the relationship between the correctly estimated or calculated HLB and the physico-chemical properties appears to

facilitate the choice of the best surfactant for a particular purpose .

1 Introduction

The hydrophile-lipophile balance (HLB) value of nonionic surfactants is one of the important factors which predict the performance of non-i onics as emulsifiers, solubilizers, wetting agents, etc. The HLB value of an surfactant gives information on its hydrophilic-lipophilic balance, i.e. the balance between the size and strength of the hydrophilic (water-loving or polar) and the lipophilic (oil-loving or nonpolar) groups of the surfactant.

The HLB value of nonionic surfactants may be calculated or evaluated experimentally. The calculation methods are based on the knowledge of the molecular structure of the surfactant. This causes some problems regarding the use of HLB calculated in this way since industrially

produced nonionic surfactants are never com-pletely pure but are always composed of many homologous compounds. The theoretically cal-culated HLB value should therefore be applied with some precautions. The more known are two calculation methods: Griffin's1)2) and Da-vies'.3) The first method is based on the cal-culation of the percentage of hydrophilicity of surfactant molecule according to formula:

weight percent oxyethyiene content5 (1)

where MH is molecular weight of the hydro-

philic portion of the molecule, and M-molecular

weight of the total molecule.

In the second method, the HLB is treated

as constitutive and additive property of molecule

and is calculated from so-called group numbers

assigned to each group on the basis of equation:

HLBD=ƒ° (hydrophilic group numbers)

+ƒ° (lipophilic group numbers)+7 (2)

Although these methods are not the perfect,

these are the most widely used today of the

calculation methods by various authors, as indi-

cated the review of available literature in this

field. Also several analytical procedures have

been proposed for determining the HLB value

of nonionic surfactants. Detailed informations

about these analytical procedures may be ob-

tained from two reviews by Dobias4) and Muller5).

However, they all, with the exception of PIT

(Phase Inversion Temperature) system6) were

examined in isolation from the practical envi-

ronment so that the data are often of dubious

value.

1

136 油 化 学

To the best of this author knowledge, the

literature concerning the theory, determination

and application of HLB exceeds presently 700

publications, no taking into account may refer-

ences to HLB in textbooks and encyclopedias.

These works indicate that the technical useful-

ness of the HLB system is indisputable, although

in some instances its universal applicability was

critically evaluated by some authors5),7)•`9). The

authors besides critical evaluation on the the-

oretical backgrounds of the HLB system and

the various methods of the HLB determinations,

did not indicate better alternatives of choosing

surfactants based on their hydrophile-lipophile

ratio.

We maintained that the unapplicability of the

HLB selection for some systems can rusult from

two facts:

- the HLB value in final modeling of formu-

lations may not be taken as sole criterion of

surfactant selection, to say nothing of the cri-

terion of emulsion stability in classical methods-

phase separation-which is imperfect and sub-

jective, and-the surfactant has not a single unique value

for its HLB as was assumed in the beginning,

but this value is variable and conditioned by

several external and internal factors. These

assumptions lead to the effective HLB concept.

There are definite advantages in technology to

viewing HLB as a variable parameter as is

shown below. The concepts presented in this

work represent a personal understanding of

factors which the author considers important in

the determination of HLB value and its appli-

cation to final formulation. It is hoped that

this short discussion will prove helpful in com-

prehending these complex physico-chemical phe-

nomena and in applying them in a practical way.

2 Effective HLB value

The pioneering studies by Shinoda et al.6),10)

have indicated that the HLB may vary under

influence of several factors and this process may

be sensitively monitored by changes in PIT

(HLB-temperature) which is the temperature

at which hydrophile-lipophile property of sur-

factant at the interface just balances. Factors

such as types of oils and surfactants, the pre-

sence of some additives in water and or oil

phase11)•`18), and distribution of poly (oxyethylene)

chain19)•`21) have been investigated using PIT

procedure.

Also Heusch22),23) introduced to a classical

Griffin's equation, corrections taking into account

the various conditions at the air-water interface.

Rimlinger24),25) used so-called apparent HLB

which takes into account the interferences of

the medium on the relative hydrophilicity of

emulsifier and studied effect of such factors as

polarity and the molecular mass of the oil phases,

presence of electrolytes and some additives.

Thus, it is evident that for the correct deter-

mination of the real HLB value, several addi-

tional factors are owing to be also taken into

account in relation to former conception of the

HLB value based on an original molecular struc-

ture. Systematic investigations in the author's

laboratory indicate that they include factors of

primary significance such as temperature, addi-

tive and structural modification of surfactant

molecules, and of secondary significance, such

as decomposition of surfactant, pressure, prepa-

rative methods, etc. If these all factors are

taken into account, the HLB value becomes a

variable depending upon the physical and chem-

ical conditions at the time of the measurement,

or in the other words the HLB may assume a

multiplicity of values.

Accordingly, we propose so-called effective

HLB value, which is equivalent HLB of the

reference surfactant which yields the same

physical results as the one under investigation.

Such defined effective HLB reflects the changes

induced by mentioned factors. Criterion for

the HLB change of nonionic surfactants are

some properties conditioned by hydrophile-lipo-

phile balance and exhibited by surfactants in

investigated and standard conditions. The ef-

fective HLB is not yet one new parameter

characterizing the hydrophile-lipophile balance,

but indicates the real HLB value after consider-

ation of several internal and external factors

which practically till now were overlooked with

the exception of PIT method.

Of course, there are several other factors

affecting HLB, but they may be generally

treated as derivative of above mentioned factors

or changing under their influence. In the field

of emulsions, the problem nature of oil phase,

2

第32 巻 第3号 (1983) 137

well known as•gchemical matching•h of oil and

lipophile is also very important and resulting

the change of HLB (or the real balance of the

emulsifiers at the oil-water interface), may be

sensitively reflected and explained in the change

of CER (Cohesive Energy Ratio)26 or PIT27)

values. Problems in understanding the interac-

t ion of surfactant, oil and water were recently

discussed by Little28). On the other hand, the

concentration of emulsifiers limits the usefulness

of the HLB concept to formulations of reasonably

constant concentrations, since may form liquid

crystalline phases having a definite influence on

the behaviour of emulsions29)•`31).

HLB change would be expected to influence

of a number of physical properties of the system

both in its micellar and monomeric state. To

estimate the degree and direction of the HLB

change (reference HLB to effective HLB), in

simple and multicomponent formulation, we

verified several methods such as critical micelle

concentration (cmc)32),33), cloud point34),35), phe-

nol index35)•`37), phase inversion temperature35),

38) or emulsion inversion point (EIP)35),39). For

example, in systems comprising only polyoxy-

ethylated surfactants and water, the cloud point,

phenol index, cmc, and PIT values rise with

corresponding increase in the hydrophilicity of

surfactant molecule. The opposite trend is

exhibited by the EIP values. So, we assumed

that all of the mentioned factors which affect

these and other experimental parameters, will

affect an experimentally determined in standard

conditions, or calculated, HLB value of nonionic

surfactant. The importance of some factors

affecting the HLB of nonionic surfactants is

below briefly argumentated.

3 Additives

Recently we investigated the effects of several

additives such as alcohols39), glycols33),40), glycol

ethers32), polyethylene glycols41),42), formamide

derivatives43), and electrolytes44) on the HLB

value of nonionic surfactants using mentioned

methods. Early, Shinoda and Takeda12), and

Florence et al.45), investigated the HLB values

of nonionics in the presence of electrolytes

using cloud point and PIT procedures. This

last method was also used to studying the effect

of some other additives11)•`18).

From these investigations, it appears that

some of added compounds may in themselves

not show any surface activity, but their inter-

action with the surfactant or strong structural

changes of the solvent medium induced by

additives, may cause pronounced changes in

surfactant properties. A different explanations

of the mechanism of the additives effect on the

surfactant solutions were presented46), essentially

concerning the effects of organic additives46),47)

and electrolytes48,49) on the micelle formation.

It seems that in all of these explanations may

be found an one common factor. The net

result of the combination of a surfactant and

additives is an apparent change of the HLB (or

balance between hydration and cohesion con-

sidering the micellization process in the simplest

form) of the nonionic surfactant in comparison to the value exhibited in standard conditions,

e.g., in pure water. In the other words, the effective HLB of nonionic molecule in the

presence of additives is other than that calcu-lated or exhibited in standard conditions.

A qualitative consistent results concerning the

change of HLB value under influence of addi-

tives were obtained using different methods35).

Some unpublished example, which compares the effect of propylene glycol and dipropylene glycol

on the HLB of nonylphenyl poly (oxyethylene)

(8) ether (NPE8) using three different methods: cloud point, phenol index, and cmc is shown inFig.-1.

The quantitative differences between pre-

Fig.-1 Effect of propylene glycol (○) and

dipropylene glycol(●) on the phenol

index, cloud point and cmc of

nonylphenyl poly (oxyethylene) (8)

ether.

3

138 油 化 学

sented methods result from the facts that de-

pending on the factor which determine the measured property, the change of HLB is equivalent to a hypothetical change in the effective poly (oxyethylene) chain length (e.g. cloud point and phenol index measuerements)33),36),37) or alternatively equivalent to an hypo-

thetical changing of the effective hydrocarbon chain length (e.g., cmc measurement)32),33).

The overall effect of additives is resultant of the effects, often contrary directed, on the hydrophilic and lipophilic part of surfactant molecule. For example, the solvent effect of

glycols, diglycols or glycol ethers may be divided into two groups: primary on the hydrocarbon

part of surfactant molecule, weaking of hydro-phobic bond and increasing cmc, and secondary on the poly (oxyethylene) part, where the solvent may cause either an increase or a decrease in the cmc32),33). The resultant increase in cmc in the presence of such additives basically in-dicates that the nonionic surfactant molecule becomes more hydrophilic and vice versa.

Conducted in the author's laboratory inves-tigations indicate also that the change of HLB of nonionics depends on the kind and amount

of additive, concentration of surfactant, poly (oxyethylene) chain length and structural changes of hydrocarbon part of surfactant. In some instances, the changes in the effective HLB values of nonionic surfactants in the presence of additives may be considerable, and already in concentration of 1.0M may exceeds 25% of

primary value40). For example, in Table-1 are presented the HLB changes induced by various additives, using phenol index method.

4 Structural modifications

The true HLB values depend also on various structural modifications of surfactant molecules, which are not reflected in the classical equations derived by Griffin2) and Davies3). These struc-tural variations in nonionic surfactants include: presence of double and triple bonds, aromatic groups, stereoisomerism, positionisomerism, branching, polyoxypropylation, etc. In the case of nonionics there are some difficulties to clarify the effects of various changes in the hydrocarbon structure on the HLB, because the hydrophilic group must be exactly defined. The well known uncertainties in oxyethylene dis-tribution (and, hence properties) of poly (oxy-

Table-1 The change of the HLB values of nonylphenyl poly (oxyethylene) ethers (NPEX)

(0.025M) in the presence of various additives (1.0M)40)43).

4

第32巻 第3号 (1983) 139

ethylene) ethers obtained by direct ethoxylation

are always connected with risk in interpreting

of experimental data with any sense of security.

However, this problem may be partially solved

for well defined nonionic surfactants with the

aid of such methods as effective chain length

concept50),51), polarity index, cloud point52), etc.

These methods indicate, e.g. that surfactants

with unsaturated bonds, branching, aromatic

group, cis configuration have the effective HLB

higher than the comparable compounds with

similar molecular weight but saturated, with a

straight chain, of an aliphatic nature, with trans

configuration, respectively52).

The effective chain length of the surfactants

(neff) enables the calculation effective HLB value

of several modified surfactants, neff is that

value of n (actual number of carbon atoms in

the alkyl chain) which analogous straight-chain

compounds must have to yield the same degree

of surface activity as the structurally modified

one50),51). For example, for polyoxypropylated

nonionic (RPE) surfactants, the Griffin's equa-

tions2) are not applicable and Davies' equation3)

does not reflect the anomalous behaviour of

the ethylene oxide groups interposed between

the hydrophobic part and the ionic hydrophilic

group, and the neff concept gives correct effec-

tive HLB values for such surfactants53),54).

The ethylene oxide groups in ionic-nonionic

ethoxylated surfactants (CnExSO4Na) may man-

ifest a dual character, reinforcing the role of

either the nonpolar or the polar part of the

molecule, depending on the distance from the

ionic group. The terminal ionic group affects

the hydration of the ethylene oxide groups in

close proximity making it hydrophobic. When

ethylene oxide groups are added at some distance

from the ionic group, hydration can occur, and

then the ethylene oxide groups behave as fully

hydrophilic. From the relation between cmc and

n for straight chain surfactants (so-called calibra-

tion curve), is calculated neff value of structurally

modified surfactant (in this case by introduction

of ethylene oxide or propylene oxide groupings

into molecule). The calculated neff in this

manner may be subsequently used in the mod-

ified Davies' equation:50),51)

HLBD=ƒ°(hydrophilic group number)

-(0 .475•Eneff)+7 (3)

to calculate effective HLB of polyoxyethylated and polyoxypropylated ionic and nonionic sur-

factants53),54). Some examples of calculated in this manner effective HLB values for polyoxy-ethylated ionic and polyoxypropylated ionic and nonionic surfactants are presented in Table-2.

Table-2 neff and HLB values of polyoxyethylated and polyoxypropylated ionic and nonionic surfactants53),54).

5

140 油 化 学

In the case of polyoxyethylated and poly-

oxypropylated fluorocarbon surfactants Eq. (3)

must be suitably modified to:55)

HLBD=ƒ° (hydrophilic group number)

-0.870•En-0.475• (neff-n)+7 (4)

Another example. The hydrophobic equivalent

of a benzene ring expressed in the number of

alkyl carbon atoms, determined on the basis of

such properties as surface or interfacial tension,

indicate that the part. of lipophilicity in the

hydrophile-lipophile balance of molecule con-

taining a benzene ring is smaller than it used

to calculate the HLB value in the Griffin and

Davies equations. In the other words, the ef-

fective HLB values for polyoxyethylene alkyl

phenols owing to be higher than predict it in

the Griffin and Davies equations52). For ex-

ample, effective HLB value of NPE8 changes

from 12.3 to 13.1 depending on the hydrophobic

equivalent of benzene ring (6 to 3.5).

The application of only one method to esti-

mate the direction of the HLB change under

influence of structural modifications may be

inadequately. For example, from the cloud

point measurements, it results that branching

of the hydrophobic moiety lowers the cloud

point56) and such effect may correspond to a

decrease of HLB. In the contrary, higher cmc

values of the branching isomers in comparison

to cmc values of straight chain parent com-

pounds57) indicate that the hydrophilicity of

such surfactant molecule rise. Additionally,

there are several other indications, that branch-

ing of the molecules appears to contribute to

the greater solubility, which is a simple measure

of their hydrophilicity.

Several other examples reflecting the change

of calculated HLB under influence of structural

modifications of surfactant molecules were pre-

sented in the other paper52).

5 Temperature

The HLB is also highly temperature depend-

ent, because the interaction between water

and hydrophilic group or oil and lipophilic group

changes with temperature and this cause changes

in the solubility of a surfactant in water or oil

phase, and/or in the state of orientation of a

surfactant at the oil-water interface 58)•`60). The

hydration forces between the hydrophilic moiety

of nonionic surfactants and water are gradually

weakend with the rise in temperature. The

same surfactant is more hydrophilic at the lower

temperature than at higher and therefore emul-

sions stabilized with it inverts from O/W to

W/O type with the rise in temperature6). In

the other words, the temperature increase in

nonionic surfactant solution and the decrease of

oxyethylene chain length of surfactant at a

constant temperature exhibit practically the same

effect61), or effective HLB of the same surfac-

tant at higher temperature is lower than that

calculated.

Valuable data on the effect of temperature

on the HLB may be obtained from the cloud

point34), PIT (or HLB-temperature)6),10) or vis-

cosity measurements62). In the PIT method,

HLB changes also with the types of oils (addi-

tives), and in practice, reflects the real balance

of surfactant at an oil-water interface. The

HLB-temperature corresponds to the temperature

at which the hydrophilic-lipophilic properties of

the surfactant just balances for a given hydro-

carbon-water system and was extensively inves-

tigated by Shinoda et al.6),10),58),59),63)•`66) and

other authors67)•`76). The PIT method is prob-

ably the best, reflecting practically all factors

affecting HLB value (although not all till now

were examined) and the most widely used

method for the effective HLB determination

and its advantages in relation to the classical

HLB method were described by Shinoda6).

Other factors affecting the HLB have a sec-

ondary significance, but their existence can not

be overlooked.

6 Decomposition of nonionic

surfactants

Decomposition of nonionic surfactants follows

by autoxidation and hydrolysis and may be

also reason for change of HLB value with time.

At room temperature, aqueous solutions of

nonionics develop peroxides within a few weeks

after preparation and this process is accompanied

probably by degradation of the poly (oxyethylene)

chain as indicated by the cloud point, cmc and

surface tension measurements77),78). A conse-

quence of the reduction in hydrophilicity after

autoxidation is the change of the HLB of

nonionic surfactant. The poly (oxyethylene)

6

第32巻 第3号 (1983) 141

nonionics of the ether type are more resistant

to hydrolysis and they behave a relatively con-

stant HLB in comparison to these possesing

ester linkages (e.g. Polysorbates). This factor

has some significance in the shelf life prediction

of stable formulation with nonionic surfactants.

7 Pressure

As indicated by the investigations of the

effect of pressure on the cloud point of nonionic

surfactants, the alteration in the HLB of the

surfactant by the compression up to 150MPa

also occurs but it is less than that by the addi-

tion of one oxyethylene group in the poly

(oxyethylene) chain79). This factor has however

some significance in the process of autoclaving

of some pharmaceutical formulations.

8 Preparative methods

This effect may be demonstrated e,g. in the

case of initial location of surfactants prior to

emulsification. In the system where the HLB

and the volume of the internal phase are such

that it can form either an O/W or W/O type

of emulsion, the initial location of surfactant

appears to play an important role in determining

the type of emulsion, assuming that the rate of

surfactant migration proceeds relatively slow80),

81). The placing of the surfactants in their

respective phases prior to emulsification (i.e.,

hydrophilic surfactant in water and the lipophilic

surfactant in oil) appeared to promote the for-

mation of an O/W emulsion. A W/O type of

emulsion is formed when the entire surfactant

is placed in the oil phase. This phenomenon

may be interpreted as due to an increase in the

effective HLB value of the surfactant or in the

other words, the surfactant dissolved in water

behaves as if it was more hydrophilic than the

same surfactant placed initially in the oil phase

80)•`83).

In Fig.-2 is shown the change of emulsion inversion point (EIP) as a measure of the ef-fective HLB under influence of surfactant lo-cation. The increase effective HLB value results in the shifts of EIP towards smaller values .

9 Effective HLB of nonionic surfactant mixtures

It is well known, that the more stable emul-

Fig.-2 Effect of NPE14 location of the EIP

value for paraffin oil-water emul-

sions stabilized with NPE3-NPE14 mixtueres (2%wt/vol). C-concen-

tration of NPE14 initially placed in

the water phase.

sions are usually obtained by blending two or

more surfactants rather than a single surfactant

whose HLB agrees with the required HLB. In

the HLB system it has been generally assumed

that the HLB of a mixture of surfactants is the

algebraic sum of the HLB values of the individ-

ual surfactants1). In the light of the CER

concept26), it appears that it would have been

more meaningful to have used the volume per-

centage. On the other hand, the use of blends

of emulsifiers may be connected with molecular

association at the interface and this phenomenon

may lead to properties which may be far re-

moved from the mean of the properties of the

individual emulsifiers involved30),31),69). In lit-

erature appears several works in which authors

found deviations from linearity of HLB values

of surfactant mixtures, thus indicating that HLB

values may not be strictly additive in nature34).

84)•`93).

As indicated by cloud point89), PIT89),90), and

phenol index92) measurements, the magnitude of deviation depends strongly on the difference in poly (oxyethylene) chain length of two mixed

surfactants (the positive deviation). Thus, the effective HLB of nonionic surfactant mixtures differing remarkably in poly (oxyethylene) chain length is other than the calculated ones as is shown schematic in Fig.-3. Deviations from additivity in HLB values may be explained on the basis of hydration change of the poly

(oxyethylene) shell with the change in com-position of mixed micelle94). Also the kind of

7

142 油 化 学

Fig.-3 The schematic plot for the deter-

mination of the effective HLB of

surfactant mixtures.

hydrophobic group89) and concentration of sur-

factants have an influence on the deviation of

HLB values from additivity93).

10 Effective HLB of ionic surfactants

In this paper we discussed chiefly the be-

haviour of nonionic surfactants from the point

of view of effective HLB value. It is interesting

to note that also in the case of ionic surfactants

the effective HLB can be changed fairly widely

as indicate studies by Shinoda et al95). This

effect may be obtained, e.g.:

-by changing the types, valencies and concen-

trations of counterions96), 97) as well as under

influence of other additives97),98)

-by change of temperature97),

-by use of surfactants which possess two (or

more) hydrocarbon chain99)•`101) or use of sur-

factants with other structural modifications, of

molecules50),102)•`107),

-by partially replacing the ionic surfactant with

lipophilic or hydrophilic cosurfactants108)•`111)

Thus, Shinoda et al. demonstrated that in-

stead of hydrophilic and lipophilic nonionic sur-

factant mixtures, hydrophilic ionic and lipophilic

nonionic surfactant, or hydrophilic nonionic and

lipophilic ionic surfactant mixtures may simi-

larly behave or exhibit the same effective HLB.

For example, in mixtures ionic-nonionic surfac-

tants, the small amounts of ionic surfactant

serve to adjust optimum HLB (PIT) of mixed

surfactants or temperature stability, and nonionic

surfactant, which is the main solubilizer, favors

effective solubilization112),113)

11 Conclusions

Because the much formulations with nonionic surfactants are stable and have the optimum of activity at the exactly defined HLB (so-called required HLB), the overlooking above discussed

factors in the modeling of final product may be the cause for their decreased stability and activ

-ity. Knowledge of the effects of various fac-tors affecting the HLB and use so-called effec-tive HLB facillitates the choice of the best surfactant for a specific purpose such as emul-sification, solubilization, detergency, etc., and reduce of empirical approache to solving this

problem.(Received Oct. 7, 1982)

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10