7.3.2 HLB systemIn spite of many ad

7.3.2 HLB system
In spite of many advances in the theory of
stability of lyophobic colloids, resort has still
to be made to an empirical approach to the
choice of emulsifier, devised in 1949 by Griffin.
In this system we calculate the hydrophile–
lipophile balance (HLB) of surfactants, which
is a measure of the relative contributions of
the hydrophilic and lipophilic regions of the
molecule. Values of the effective HLB of
surfactant mixtures can be calculated.
The HLB number of a surfactant is calculated according to an empirical formula. For
nonionic surfactants the values range from 0
to 20 on an arbitrary scale (see Fig. 7.12). At
the higher end of the scale the surfactants are
hydrophilic and act as solubilising agents,
detergents and ow emulsifiers. To maintain
stability, an excess of surfactant is required in
the continuous phase; hence, in general,
water-soluble surfactants stabilise ow emulsions and water-insoluble surfactants stabilise
wo emulsions. Oil-soluble surfactants with a
low HLB act as wo emulsifiers. In the stabilisation of oil globules it is essential that there
is a degree of surfactant hydrophilicity to
confer an enthalpic stabilising force and a
degree of hydrophobicity to secure adsorption
at the ow interface. The balance between the
two will depend on the nature of the oil and
the mixture of surfactants; hence the need to
apply the HLB system. The HLB of polyhydric
alcohol fatty acid esters such as glyceryl
monostearate may be obtained from equation
(7.14)
(7.14)
where S is the saponification number of the
ester and A is the acid number of the fatty
acid. The HLB of polysorbate 20 (Tween 20)
calculated using this formula is 16.7, with
S # 45.5 and A # 276.
Typically, the polysorbate (Tween) surfactants have HLB values in the range 9.6–16.7;
the sorbitan ester (Span) surfactants have
HLBs in the lower range of 1.8–8.6.
For those materials for which it is not possible to obtain saponification numbers, for
example beeswax and lanolin derivatives, the
HLB is calculated from
(7.15)
where E is the percentage by weight of oxyethylene chains, and P is the percentage by
weight of polyhydric alcohol groups (glycerol
or sorbitol) in the molecule.
Figure 7.12 The HLB scale and the approximate ranges into which solubilising agents, detergents, emulsifiers and
antifoaming agents fall.
Calculation of HLB of a polysorbate
Polysorbate 20 has a molecular weight of
approximately 1300 and contains 20 oxyethylene groups and two sorbitan rings. Thus,
Hence,
If the hydrophile consists only of oxyethylene
groups (CH 2CH 2O, mol. wt. # 44), a simpler
version of the equation is
(7.16)
giving the upper end of the scale (20) for
polyoxyethelene glycol itself.
Some HLB values of typical surfactants used
in pharmacy are given in Table 7.2. A more
detailed list is given in Tables 6.7 and 6.8.
Group contribution
The HLB system has been put on a more quantitative basis by Davies, who calculated group
contributions (group numbers) to the HLB
number such that the HLB was obtained from
(7.17)
Some group numbers are given in Table 7.3.
Choice of emulsifier or emulsifier mixture
The appropriate choice of emulsifier or emulsifier mixture can be made by preparing a
series of emulsions with a range of surfactants
of varying HLB. It is assumed that the HLB of a
mixture of two surfactants containing fraction
f of A and (l 0 f) of B is the algebraic mean of
the two HLB numbers:
(7.18)
For reasons not explained by the HLB
system, but from other approaches, mixtures
of high HLB and low HLB give more stable
emulsions than do single surfactants. Apart
from the possibility of complex formation
at the interface, the solubility of surfactant
components in both the disperse and the
continuous phase maintains the stability of
the surfactant film at the interface from the
reservoirs created in each phase. In the experimental determination of optimum HLB,
creaming of the emulsion is observed and is
taken as an index of stability. The system with
the minimum creaming or separation of
phases is deemed to have an optimal HLB. It is
therefore possible to determine optimum HLB
numbers required to produce stable emulsions
of a variety of oils. Table 7.4 shows the
required HLB of surfactants to achieve stability
of five oils. A more sensitive method would be
to determine the mean globule size in emulsions using modern techniques such as laser
diffraction methods to produce data such as
those in Fig. 7.13. For the mineral oil-in-water
emulsion stabilized by a mixture of two
Figure 7.13 Variation of mean globule size in a mineral
oil-in-water emulsion as a function of the HLB of the
surfactant mixtures present at a level of 2.5%. Surfactants:
Brij 92–Brij 96 mixtures.
Source: P. Depraetre, M. Seiller, A. T. Florence and F. Puisieux (unpublished)
nonionic surfactants, an optimal HLB of
between 7.5 and 8 is identified.
At the optimum HLB the mean particle size
of the emulsion is at a minimum (Fig. 7.13)
and this factor would explain to a large extent
the stability of the system (see equations 7.1
and 7.2, for example).
Although the optimum HLB values for
forming ow emulsions are obtained in this
way, it is possible to formulate stable systems
with mixtures of surfactants well below the
optimum. This is sometimes because of the
formation of a viscous network of surfactant
in the continuous phase. The high viscosity of
the medium surrounding the droplets prevents their collision and this overrides the
influence of the interfacial layer and barrier
forces due to the presence of the adsorbed
layer.
The HLB system has several drawbacks. The
calculated HLB, of course, cannot take account
of the effect of temperature or that of additives. The presence in emulsions of agents
which salt-in or salt-out surfactants will.
respectively increase and decrease the effective
(as opposed to the calculated) HLB values.
Salting-out the surfactant (for example, with
NaCl) will make the molecules less hydrophilic and one can thus expect a higher
optimal calculated HLB value for the stabilising surfactant for ow emulsions containing
sodium chloride. Examples are shown in
Fig. 7.14 in which the effects of NaCl and NaI
are compared.

Kết quả (Việt) 1:

7.3.2 hệ thống HLBMặc dù nhiều tiến bộ trong lý thuyếtsự ổn định của hệ keo lyophobic, khu nghỉ mát có vẫn cònđược thực hiện cho một cách tiếp cận thực nghiệm để cácsự lựa chọn của emulsifier, nghĩ ra năm 1949 bởi Griffin.Trong hệ thống này chúng tôi tính toán hydrophile-sự cân bằng lipophile (HLB) của bề mặt, màlà một thước đo của sự đóng góp tương đối củavùng Purifying và lipophilic của cácphân tử. Giá trị của HLB hiệu quả củahỗn hợp chất có thể được tính toán.HLB, số một chất được tính theo một công thức thực nghiệm. Chononionic bề mặt các giá trị nằm trong khoảng từ 0đến 20 trên một quy mô tùy ý (xem hình 7,12). Tạicuối cao quy mô các bề mặtPurifying và hành động như các tác nhân solubilising,chất tẩy rửa và ow natri NaNO2. Để duy trìsự ổn định, một vượt quá chất là cần thiết tronggiai đoạn liên tục; do đó, trong tổng hợp,hòa tan trong nước bề mặt ổn định ow nhũ tương và không hòa tan nước bề mặt ổn địnhwo nhũ tương. Các bề mặt hòa tan dầu với mộtthấp HLB hoạt động như natri NaNO2 wo. Ở ổn định dầu giọt nó là điều cần thiết mà cómột mức độ chất hydrophilicity đểCác hạt này một lực lượng enthalpic cánh và mộtmức độ hydrophobicity để an toàn hấp phụtại các ow giao diện. Sự cân bằng giữa cáchai sẽ phụ thuộc vào bản chất của dầu vàhỗn hợp của bề mặt; Vì thế sự cần thiết phảiáp dụng hệ thống HLB. HLB của polyhydricrượu acid béo Este chẳng hạn như glycerylmonostearate may be obtained from equation(7.14)(7.14)where S is the saponification number of theester and A is the acid number of the fattyacid. The HLB of polysorbate 20 (Tween 20)calculated using this formula is 16.7, withS # 45.5 and A # 276.Typically, the polysorbate (Tween) surfactants have HLB values in the range 9.6–16.7;the sorbitan ester (Span) surfactants haveHLBs in the lower range of 1.8–8.6.For those materials for which it is not possible to obtain saponification numbers, forexample beeswax and lanolin derivatives, theHLB is calculated from(7.15)where E is the percentage by weight of oxyethylene chains, and P is the percentage byweight of polyhydric alcohol groups (glycerolor sorbitol) in the molecule.Figure 7.12 The HLB scale and the approximate ranges into which solubilising agents, detergents, emulsifiers andantifoaming agents fall.Calculation of HLB of a polysorbatePolysorbate 20 has a molecular weight ofapproximately 1300 and contains 20 oxyethylene groups and two sorbitan rings. Thus,Hence,If the hydrophile consists only of oxyethylenegroups (CH 2CH 2O, mol. wt. # 44), a simplerversion of the equation is(7.16)giving the upper end of the scale (20) forpolyoxyethelene glycol itself.Some HLB values of typical surfactants usedin pharmacy are given in Table 7.2. A moredetailed list is given in Tables 6.7 and 6.8.Group contributionThe HLB system has been put on a more quantitative basis by Davies, who calculated groupcontributions (group numbers) to the HLBnumber such that the HLB was obtained from(7.17)Some group numbers are given in Table 7.3.Choice of emulsifier or emulsifier mixtureThe appropriate choice of emulsifier or emulsifier mixture can be made by preparing aseries of emulsions with a range of surfactantsof varying HLB. It is assumed that the HLB of amixture of two surfactants containing fractionf of A and (l 0 f) of B is the algebraic mean ofthe two HLB numbers:(7.18)For reasons not explained by the HLBsystem, but from other approaches, mixturesof high HLB and low HLB give more stableemulsions than do single surfactants. Apartfrom the possibility of complex formationat the interface, the solubility of surfactantcomponents in both the disperse and thecontinuous phase maintains the stability ofthe surfactant film at the interface from thereservoirs created in each phase. In the experimental determination of optimum HLB,creaming of the emulsion is observed and istaken as an index of stability. The system withthe minimum creaming or separation ofphases is deemed to have an optimal HLB. It istherefore possible to determine optimum HLBnumbers required to produce stable emulsionsof a variety of oils. Table 7.4 shows therequired HLB of surfactants to achieve stabilityof five oils. A more sensitive method would beto determine the mean globule size in emulsions using modern techniques such as laserdiffraction methods to produce data such asthose in Fig. 7.13. For the mineral oil-in-wateremulsion stabilized by a mixture of twoFigure 7.13 Variation of mean globule size in a mineraloil-in-water emulsion as a function of the HLB of thesurfactant mixtures present at a level of 2.5%. Surfactants:Brij 92–Brij 96 mixtures.Source: P. Depraetre, M. Seiller, A. T. Florence and F. Puisieux (unpublished)nonionic surfactants, an optimal HLB ofbetween 7.5 and 8 is identified.At the optimum HLB the mean particle sizeof the emulsion is at a minimum (Fig. 7.13)and this factor would explain to a large extentthe stability of the system (see equations 7.1and 7.2, for example).Although the optimum HLB values forforming ow emulsions are obtained in thisway, it is possible to formulate stable systemswith mixtures of surfactants well below theoptimum. This is sometimes because of theformation of a viscous network of surfactantin the continuous phase. The high viscosity ofthe medium surrounding the droplets prevents their collision and this overrides theinfluence of the interfacial layer and barrierforces due to the presence of the adsorbedlayer.The HLB system has several drawbacks. Thecalculated HLB, of course, cannot take accountof the effect of temperature or that of additives. The presence in emulsions of agentswhich salt-in or salt-out surfactants will.respectively increase and decrease the effective(as opposed to the calculated) HLB values.Salting-out the surfactant (for example, withNaCl) will make the molecules less hydrophilic and one can thus expect a higheroptimal calculated HLB value for the stabilising surfactant for ow emulsions containingsodium chloride. Examples are shown inFig. 7.14 in which the effects of NaCl and NaIare compared.