2. Introduction
Definition – Humectant is a hygroscopic substance used to keep
things moist. They are used in many products , including food
cosmetic and medicines . A humectant attracts and retains moisture
in the air nearby via absorption,drawing the water vapour into or
beneath the object’s surface. In pharmaceuticals and cosmetics ,
humectants can be used in topical dosage form to increase the
solubility of a chemical compounds , active ingredients, increasing
the active ingredients ability to penetrate skin or its activity time.
3. Humectants are added to cosmetic creams partiularly of the oil-in-
water type, with a view to reducing the drying out of such creams on
exposure to the air. In addition , the hygroscopic properties of the
film of humectant which remains on the skin may be an important
factor in influencing the feel and condition of the skin after
application.
A humectant also provides control in use, by helping to reduce the
rate at which water disappers and fluidity decreases.
4. Drying out :
Drying out may occur either between the time of packing
and the time of use , or else during the usage period. During the first of
these periods the most important means of preventing drying out is an
efficient closure, and the humectant present plays a subsidiary role since
there is only the small space above the product to be sturated with water
vapour. In the case of emulsion products, the type of emulsion will have
a big effect in respect of water loss.
Water-in-oil emulsion such as all purpose creams were
successfully marketed for many years in slip lid tins, with the surface of
the cream covered by a disk of metal foil , since they dry out at much
lower rates than oil-in-water creams which, even if of similar
composition , require at least a screw cap with a compressible wad. In
tropical climates, even thus will often not avoid drying with shrinkage,
cavitation or crust formation.
In case of full exposure to the atmosphere during use . Ex. By
incomplete closure, the quantities of humectant that can be
incorporated seem. Theoretically to be insufficient to prevent drying out.
They appear much more likely to affect in retaining moisture in the
residual film on the skin.
5. The case of toothpaste is somewhat different. As a system it
contains from 30-60% solid and he problems of drying out during
storage is mnimal if the tube is properly closed, but drying out
during use by failing to replace the tube cap is much more likely and
leads to serious difficulties in extrusion and use.
In pressurized packs there is the
additional problem of the exposed paste blocking the delivery orifice
even with normal use.
6. Properties required :
1. Hygroscopicity : The product must attract and retain an
appreciable amount of water under nomal conditions of
atmospheric humidity.
2. Viscosity : A low viscosity humectant has advantages in easy mixing
but conversely a high viscosity will tend to hinder unmixing , or
separation of emulsion phases.
3. Viscosity index : Change in viscosity with temperature should be
small.
4. Compatibility : Good compatibility, solubility, or solvent power for a
wide range of materials , are advantages.
5. Colour, Odour, Taste : The material should have a good colour ,
odour, and taste.
6. Toxicity : The material should be non-toxic and non-irritant.
7. Corrosion : The material should be non-corrosive.
8. Stability : The material must be non-volatile and should not solidify
nor deposite crystals at low temperatures.
9. Reaction : The material should preferably be neutral in reaction.
10. Availability : The material should be freely available and as
inexpensive as possible.
7. TYPE OF HUMECTANTS :
In general , humectants are divided into three
classes:
1. Inorganic humectant
2. Metal-organic humectant
3. Organic humectant
8. 1. Inorganic humectant :
Considering inorganic humectants,
exemplified by calcium chloride, viscosity characteristics are
favorable. They are non-volatile but not always non-crystalizable.
They are frequently low in cost, but fail badly on the points of
corrosion and compatibility. They find only limited use.
Ex. Calcium chloride.
2. Metal organic humectant :
The principal metal organic humectant is
sodium lactate. This material has greater hygroscopic power than
glycerine but it is incompatible with a few materials , can be
corrosive has a pronounced taste and may discolor . It has not
found a wide application in cosmetics but, has been recommended
for use in skin creams to prevent crust formation, particularly
becomes lactates are naturally in the body and sodium lactate
carries no risk of toxicity or dermatities.
The problem of pH can be overcome by
admixture with lactic acid which is also fairly hygroscopic.
9. 3. Organic humectants :
Organic humectants , which are the most
widely used type , are usually polyhydric alcohols, sometimes their
esters and ethers. Ethylene glycol is the simplest polyhydric alcohols
and we may progress from it in several ways.
Considering the main family of polyhydric
alcohols the progression is via glycerol, sorbitol . Another
pregression is along the sries of alkanes with two terminal hydroxy
group.
HYGROSCOPICITY :
Hygroscopicity is the tendency of a solid substance to
absorb moisture from the surrounding atmosphere. The process can
take on a number of forms. Thus, with a porous solid such as activated
carbon, water vapor will be physically adsorbed, both on the external
surface and within the pores, to form a condensed layer. The process
may initially take place at "active sites" from which spreading then
occurs. With other solids, such as silica gel, the interaction at the surface
may not be entirely of a physical nature and some loose chemical bonds
may be established. Many cellulosic materials, including hair, cotton and
wool, are hygroscopic and change their physical dimensions as a result
of the take-up of water. Such materials may be used as the active
elements in hygrometers (see Humidity Measurement).
10. The method most frequently used to determine hygroscopic
qualities is to construst a curve of relative humidity of atmosphere
against humectant concentration in equilibrium.
This is done by exposing small weighed amount of
solutions of known composition in atmosphere of controlled
humidity and weighing periodically. The controlled humidity can be
achieved in small desiccators charged with crystals wetted with their
own saturated solutions.
STABILITY :
Polyols, glycerine, sorbitol and propylene glycol were not
interchangable in that in a W/O cream the sample containing
propylene glycol had the greater stability whilst that containing
glycerene showed the most oil separation. Conversely in an o/w
emulsion , the sample containing glycerine remained fluid whilst
those contaning propylene glycol and sorbitol would not after
storage. On the basis of such observation it would appear that
glycerine promoted the formation of an o/w emulsion and whilst
propylene glycol favoured a w/o emulsion , their use in wrong types
leading to instability. Sorbitol was apparently midway , in such
properties, between these other two humectants.
11. Safety :
The three humectants widely employed in the cosmetic and
toilet industry at present time and which have been discussed in this
chapter –namely glycerine, sorbitol, propylene glycol are non-toxic
and dermatologically innocuous.
Ethylene glycol is not considered safe , since it is oxidised
in the body to oxalic acid and any absorption through the skin might
lead to renal calculus; for the same reason diethylene glycol is
considered toxic. The mono-ethyl ether diethylene glycol has been
widely used in cosmetic and toilet preparation.
Skin moisturizing :
It is not possible to define satisfactorily the role of
humectants in skin care. It would appear that the presence of a
humectant may be expected to stabilze the water content of the
residual film from a cream on the skin and prevent excessive drying
out. However , the manner in which adjustment is made with
changes in the ambient humidity, and whether the skin is dried or
moistened , will depend on the relative transfer rate of water
between the atmoshere and the film ,and the film and the skin. The
use of humectant in skin products both for the probable benefit to
the product while it remains in its container , and the possible
benefit to the skin during use appears to be justified.
