Chemistry Reference
In-Depth Information
TABLE 9.3. HLB Ranges and Their General Areas of
Application
HLB Range
General applications
2-6
W/O emulsions
7-9
Wetting and spreading
8-18
O/W emulsions
3-15
Detergency
15-18
Solubilization
Obviously, the ranges in which surfactants of various HLBs can be employed are
quite broad. Specific requirements for many systems have been tabulated in the
works cited in the Bibliography list for this chapter. While such tabulations can
be very useful to the formulations chemist, it must be kept in mind that there is
nothing particularly magic about a given HLB number. Many surfactants or surfac-
tant mixtures may possess the same HLB, yet subtle differences in their chemical
structures, solution chemistry, and specific interactions with other system compo-
nents may result in significant differences in performance. Particularly important
may be the formation of interfacial complexes, as noted above. Even though the
additive nature of surfactant mixture HLBs [(Eq. (9.11)] has not been found to
be linear over a wide range of compositions, over the short range of one or two
HLB units usually encountered in formulation work, linearity can usually be
assumed with little risk. It is therefore possible, in most cases, to fine-tune a surfac-
tant mixture with a minimum of experimental effort.
As alluded to above, one approach to the application of surfactant HLB to for-
mulation is to match that of the surfactant to the oil phase being employed. The
HLB of the oil can be determined empirically or calculated using the procedures
discussed previously. It is usually found that the additivity principle will hold for
mixtures of oils in a way similar to that for surfactants, possibly even to the extent
of nonlinearity in cases where oil structures differ significantly. Therefore, in for-
mulating an emulsion, it is possible to determine the HLB of the oil phase and to
vary the surfactant or mixture HLB to achieve the optimum performance. HLB
numbers of some commonly used oil phases are given in Table 9.4.
It should be noted that HLB numbers are most often used in connection with
nonionic surfactants. While ionic surfactants are included in the HLB system, the
more complex nature of the solution properties of the ionic materials makes them
less amenable for the normal approaches to HLB classification. In cases where an
electrical charge is desirable for stability or for some other functional reason, it has
been suggested that surfactants having limited water solubility and a bulky hydro-
phobic structure that inhibits efficient packing into micelles should be most effec-
tive as emulsifiers. Surfactants such as the sodium trialkylnaphthalene sulfonates
and dialkylsulfosuccinates, which do not readily form large micelles in aqueous
solution, have found use in that context, usually providing advantages in droplet
size and stability over simpler materials such as sodium dodecylsulfate.
