Estive para aqui a pensar e uma das razões para que eu não publique muito actualmente é o trabalho que tenho na Universidade mais as coisas que tenho para fazer em casa. Eu sei que são desculpas mas é a realidade. Daí, lembrei-me que poderia partilhar alguns dos meus trabalhos porque podem interessar e ajudar alguém e porque se relacionam com os temas que aqui abordo. Para quem anda no Instituto Superior Técnico ou noutra universidade, isto é para vocês. Fiz este trabalho para a cadeira de Superfícies, Interfaces e Colóides e tive um 16 (dá para ter a noção que não está mau). 

Se acham que isto é conteúdo que vos atrai pff digam que trabalhos não me faltam. Em seguida segue o trabalho em inglês. 

It is defined that a colloid is any particle that has some linear dimension between 10^-9 m =10 Å and 10^-6 m= 1 μm. Colloids dimension lies between atomic dimensions and bulk dimensions. Two of the important consequences of the size range of colloids are that colloidal materials have enormous surface areas and surface energies, and the properties of colloidal particles are not always those of the corresponding atoms or molecules. Therefore, colloid and surface science meet in systems of more than one phase. 

Colloids are also defined by their affinity to the carrier fluid in which they are dispersed, being characterized by lyophilic and lyophobic. The terms hydrophilic and hydrophobic are used when water is the medium or solvent. 

Lyophilic colloids are soluble macromolecular materials in which the individual particles are of colloidal dimensions. However, there are macromolecules of colloidal dimensions containing both lyophilic and lyophobic components, which is the case of micelles. In the colloidal size range, continuous phase and dispersed phase refer to the medium and to the particles, respectively. Thus, these are solvent and solute in lyophilic systems. The system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. Lyophilic dispersions are true solutions, although this term ignores the colloidal size of the solute molecules. Lyophobic colloids depend on the the nature of the phases.

Colloids are used in the cosmetic industry to formulate cleansing, solubilization, adsorption, conditioning, and foaming agents. In the following chapters it will be described examples of different types of colloids such as emulsions, gels and foams found in cosmetic products.

Oil-in-water emulsions are composed of an oil phase that is dispersed as droplets in a continuous aqueous phase. Its composition is very important because it affects its physical properties which, in turn, impact the product behavior during manufacturing, packaging and application. Therefore, the texture is linked to the emulsion’s composition. 

Xanthan gum is a natural hydrophilic polymer used in O/W emulsions at 1% and it gives high viscosity at low shear rates, a strong non-thixotropic shear-thinning character and a viscoelastic behavior, which brings firmness and high degrees of stringiness and stickiness. Xanthan gum does not bring significant gloss, difficulty of spreading or absorbency. 

Texture properties of cosmetic emulsions are also analyzed to understand the effect of the oil phase level in the aqueous phase. 

Integrity of shape, penetration force, wetness (perceived amount of water while applying the product), ease of spreading (required force to move product over skin), grease (perceived amount of grease while applying the product which can be rich, thick and butter-like feeling), stickiness and gloss (amount of reflected light from product once absorbed on skin) are examined to describe and distinguish the texture of the emulsions and solutions. 

The amount of both the xanthan gum (Rhodicare T) and the oil phase (Dimethicone, Isohexadecane, Paraffin and Stearic acid) impact the textural properties of emulsions as well as solutions. Moreover, the effect of the oil phase content is greater than the effect of the amount of xanthan. Increasing the oil phase in emulsions enhances the consistency, grease degree and residual stickiness of the creams, while xanthan in solution influences integrity of shape, penetration force, wetness, ease of spreading and gloss.

Pectin is a plant polysaccharide (natural polymer) that consists of an α -(1-4)-D-galacturonic acid chain interrupted by 1,2-linked rhamnose units. Therefore, pectin has hydrophilic gelling agent that are affected by its structure and that are used as the dispersing phase in cosmetic emulsion gels.

Emulsion gels possess the advantage of both emulsions (ability to penetrate the skin, control of rheological properties, appearance and degree of greasiness) and hydrogels (greaseless, easily spreadable, easily removable, compatible with several excipients, and water-soluble or miscible) and the structured dispersing phase (the gel) improves the emulsion stability. By modifying the properties of gel and emulsion we are changing the macroscopic properties like texture and spreadability. 

The gels are also used, together with a common surfactant to prepare olive oil emulsion gels suitable to design new cosmetic products. The emulsion complex modulus is related to the oil fraction and properties of the dispersing phase. The emulsion gels are based on commercial citrus pectins, the non-ionic surfactant is Tween 60 (polyoxyethylene sorbitan monostearate) and the oil phase is virgin olive oil. By studying oil in water emulsions we can assess the macroscopic emulsion properties to the gel characteristics (pectin concentration) and oil fraction. 

We can conclude that the complex modulus of gels varies with pectin content and the LM (low-methoxyl) pectin with the most consistent gel is the one that produces structured emulsions.

Biphasic systems are used in the cosmetic industry due to its properties arising from the presence of two different phases. Emulgels or filled gels are characterized by the structure of the continuous phase and these materials have an emulsion-like behavior and the dispersing phase is a solid-like one. Suspensions are characterized by gelled internal phase. Bigels or biphasic gels are characterized by the structure of both phases and they can merge the advantages of emulsions and gels. 

For cosmetic formulations, specifically face and body/skin care creams, bigels have to be humectant, emolient, skin softeners and a carrier for active ingredients like antioxidants, vitamins and colouring agents or pigments which should permeate through the skin, which means that they need specific properties like smoothness, thickness and spreadability, often influenced by the texture of the product. Bigels are based on sulphurous hyperthermal water and olive oil. Hyperthermal water has anti-inflammatory, anti-pruriginous, keratoplastic and keratolytic effects, while olive oil has anti-inflammatory, anti-neoplastic and anti-aging effects. The aqueous phase is structured by using a hydrocolloid which acts as a gelling agent and a thickener, and olive oil is organogelled with a mixture of monoglycerides of fatty acids. 

Lotions or creams are formed by two immiscible phases, one polar which is the aqueous phase and one nonpolar which is the oil phase. Creaming, coalescence, flocculation and sedimentation represent instabilities in the phases. So, to improve spreadability and smoothness, and enhance storage stability it is necessary to analyse the mechanical properties of both phases by thickening and/or gelation. 

A sample consistency growth is expected when increasing the addition of organogel (Myverol, Ascorbyl palmitate, Algae, α -tocopherol and Extra virgin olive oil) fractions to the base emulsion and a transition from an O/W system to a more complex spatial arrangement of the phases similar to that observed for bicontinuous emulsions occurs. Hence, a complex matrix-in-matrix system is formed, which could be considered a bicontinuous bigel at the highest fractions of organogel. The addition of a structured oil phase to an existing O/W system can modifythe microstructure and the macroscopic properties. It is very hard to predict the bigel structure because its process is out-of-equilibrium.

Foams have 2 or 3 phases namely a hydrophilic liquid continuous phase with a foaming agent, a gaseous dispersion phase and a hydrophobic dispersed phase. Volume fraction of gas (phase volume) and diameter of the bubbles are two fundamental parameters in determining the structure and behaviour of foams as dispersed systems. Foam drainage kinetics is an important property for the efficiency of the application. Therefore, different parameters such as nature and concentration of foaming agent, viscosity of liquid phase, temperature and pH of the system can affect the foam structure. 

It is necessary 3 steps to formulate a foam. Firstly, is the solution of foaming agent, without incorporated air. Secondly, is the emulsion of gas where solution starts to incorporate air, at the lower volume fractions air bubbles do not have contact to each other and there is no influence on bubble geometry. Thirdly and finally, is the foam where air bubbles have contact to each other through lamellae and their spherical geometry is disturbed. 

A foaming agent, an amphiphilic agent, is needed to generate and stabilize a foam. Foaming agents form micelles in the bulk of liquid phase, so the surface tension of water decreases and the surface pressure increases. There are 4 types of foams, which are foaming agents such as surfactants and proteins, foam stabilisers such as hydrocolloids, foam destroyers such as oils, alcohols and solvents, and foam inhibitors such as silicon oils, glycerides and polyamide. 

There are many cosmetic foams with washing and cleaning processes such as hair mousse, shampoos, conditioners and colourant, shaving foam and foam bath because foams absorb and penetrate quickly without leaving any greasy residue. Cosmetic foaming compositions can contain keratolytics, lubricating agents, germicide agents or sunscreens. Cosmetic foams can also be found in skin products acting like a barrier to skin irritants, one example is Bepanthen®, a foam used to improve the healing process of the skin.

In conclusion, to formulate new and innovative cosmetic products a fundamental understanding on physical and chemical science is necessary. Colloids are present in the development of cosmetics in many forms including gels, emulsions and foams. 

Further analysis and studies can be performed to improve the application of colloids in this industry.

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