BIOSURFACTANTS AND BIOPOLYMERS 471 Biosurfactants are made of a head which is a hydrophile and a tail which is a hydrophobe. The degree of each of these determines the hydrophilic–lipophilic balance which contrib- utes to the biosurfactant’s properties. The physicochemical attributes of biosurfactants can be modifi ed based on application. These properties include surface tension, surface rheology, and interfacial tension. Another important aspect would be to identify the critical micellar concentration values of biosur- factants which helps to study the biosurfactant effi ciency in terms of foaming and cleansing. Biosurfactants arrange themselves into thermodynamically favorable formations at the sur- faces or interfaces of liquids. At CMC, they begin to make up structures known as micelles, bilayers, and vesicles. Thermodynamic arrangements of biosurfactant molecules decrease sur- face and interfacial tension of liquids that are not miscible, and allow for enhanced solubility. Lowering of surface tension also improves foaming and cleaning capabilities (142,143). It is necessary to recognize that low molecular weight biosurfactants are still classifi ed as biosurfactants because of their surface activity however, higher molecular weight struc- tures will fall into the category of bioemulsifi ers and not biosurfactants. These are surface inactive agents which aid in emulsifi cation (36,37). The biosurfactant effects on surface activity have been showcased in various studies. For instance, in the glycolipids groups, RLs can reduce the surface tension of water from 70 mN/m to a lower limit value of 25 mN/m and the interfacial tension of the water/n- hexadecane system to less than 1 mN/m. The CMC of RLs ranges from 10 to 30 mg/L, which ensures easier foaming than synthetic surfactants with higher CMC values. Zhu et al. (144) found that in the binary system of RL/cocamidopropyl betaine and ternary system of RL/CAPB/SL, where CAPB is a common zwitterionic synthetic surfactant, RLs domi- nated at both interfaces. In case of SLs, both acidic and lactonic variants of the biosurfactant can decrease the surface tension of water in a similar fashion—from 72 mN/m to 30 mN/m and the interfacial tension of water/n-hexadecane and water/vegetable oil systems to 1–5 mN/m (145). The low CMC and molecular weight of SL increase the solubility of the oil by micelle formation (146). The surface activity properties were found to be functions of the hydrophobic chain length. If the alkyl ester chain length of the SL increased by the addition of one carbon unit, the CMC decreased by half (147). Similarly, trehalose lipids lower the surface tension to a range between 25 and 40 mN/m, and interfacial tension of the same oil/water system to 1–17 mN/m. The CMC of TL is as low as 2 mg/L. Lipopeptide surfactins have a CMC range of 25–50 mg/L. They reduce the surface tension of water to 27 mN/m and the interfacial tension of oil/water to 1 mN/m (145,147). In lipopeptide groups, surfactin is the most surface active agent (148). The surface properties are proportional to the hydrophobicity and amino acids present (149). Surfactins compared with sodium do- decyl sulfate and BAS have better foaming capabilities because of their low CMC values (150). Bioemulsifi ers bind tightly to hydrocarbons or oils in an emulsion and prevent them from de-stabilizing an emulsion by coalescing/fl occulation/etc. This is performed by increasing their kinetic stability, and an important contributing factor is their chemistry (151,152). The combination of fatty acids, proteins, and occasionally sugar polymers contributes to their stabilizing nature. Emulsan, for example, can emulsify oils even when present in minute weight percentages because of the lipophilic sites of the fatty acids. In case a pro- tein is present, that acts as the hydrophobic site. Uzoigwe et al. (73) have extensively studied and described the different stabilizing mechanisms of emulsan, alasan, manno- proteins, etc.

JOURNAL OF COSMETIC SCIENCE 472 RHEOLOGY Rheology is an important parameter when it comes to cosmetic formulations. The viscos- ity of a formulation is often used as a control parameter. Beauty and personal care prod- ucts are often exposed to a varied range of shear rates during processing, packaging, and fi nally when used by the consumer. This makes it vital to control the rheological proper- ties of a cosmetic product across the shear rate range. In addition to their functional properties like fi lm formation, biopolymers are often added as viscosity modifi ers in cos- metic formulations (153–155). The viscoelastic properties of chitosan have been widely studied (156–158). Hwang and Shin (156) explored the effect of polymer concentration on the rheological response of chitosan solutions. They recorded an increase in the bulk viscosity as the concentration of chitosan was increased. Furthermore, they also reported that the chitosan solution exhibited greater shear thinning fl ow at greater chitosan concentrations. They attributed this to the fact that at increased polymer concentrations, the movement of the individual polymer chains becomes more restricted. Thus, the time taken to create new entanglements to replace the ones initially deformed increases, making it more pseudoplastic (156). In another study, Chen and Heh (153) observed the impact of the molecular weight of chitosan on the rheological properties of moisture creams. The bulk viscosity of the creams increased when chitosan of greater molecular weight was added to the formula- tion. Also, the viscosity of the moisture cream with 0.5 wt% chitosan was found to be greater than that of the same moisture cream formulation with 2 wt% of a traditional viscosity modifi er like glycerol monostearate. The bulk rheological properties of xanthan gum depend on the average molecular weight as well as the acetate and pyruvate contents which in turn vary depending on the operational conditions during processing. Casas et al. (154) determined the effect of temperature and other parameters like fermentation time on the molecular structure of xanthan gum pro- duced as well as their rheological properties. At low temperatures, the synthesized xan- than gum had high molecular weight, and thus exhibited high viscosity. On the other hand, high processing temperatures (34°C) resulted in low viscosity polymers. At a con- centration greater than 0.3 wt%, the viscosity of xanthan solution is independent of salt, making it excellent for viscosity building in ion-containing solutions (159). Xu et al. (160) studied the shear thinning properties of xanthan gum in aqueous solutions which was due to the disruption of the polysaccharide aggregates at high shear rates. An increase in the apparent viscosity of the polymer solution was observed with increasing polymer concentrations. Xanthan gum solutions of high concentrations have a yield stress which makes it ideal as a suspending agent in various cosmetic formulations. This yield stress arises from the numerous hydrogen bonds present in the helix structure of the bio- polymer. Xanthan solutions have a dominant elastic fl ow behavior which makes them ideal rheology modifi ers (85). Carboxymethyl cellulose is another polysaccharide that is extensively used to modify rheo- logical properties in cosmetic applications in various creams, lotions, or toothpastes (155). Carboxymethyl cellulose exhibits shear thinning behavior through a mechanism similar to those mentioned previously in this section. Edali et al. (161) generated hyster- esis loops of the shearing cycles by fi rst increasing the shear stress to a fi xed value, and then holding the stress constant at the maximum value, followed by reducing the shear