Rational design of microparticles for enhanced fragrance delivery

Abstract

The effective delivery of fragrances in consumer products is a challenging problem. For example, fragrances in commercial laundry detergents or fabric softeners may not adsorb efficiently onto clothes during the vigorous washing process. In addition, fragrances consist of volatile ingredients that may evaporate prematurely while clothes are being dried, thus leaving little fragrance remaining on the cleaned fabrics. In order to enhance fragrance delivery, controlled-delivery systems are beginning to be utilized in industry. However, the design of these systems in industry is often conducted by costly trial-and-error experimentation. The objective of this thesis was therefore to develop a fundamental understanding of how the physical characteristics of a controlled-delivery system could be rationally designed to enhance fragrance delivery. This thesis focused primarily on the design of a microparticle controlled-delivery system to enhance the delivery of fragrances in commercial laundry detergents and fabric softeners. The microparticle controlled-delivery system is conceptualized to possess the following three desirable features that enable it to enhance fragrance delivery in commercial laundry products: 1. Fragrance loading in microparticles can prevent premature release of the fragrance and adverse interactions between the fragrance and other ingredients in a product formulation both during storage and during the application process. 2. Fragrance adsorption onto a targeted surface can be improved through an optimization of the microparticle physical properties, such as size, hydrophobicity, and surface charge density. 3. Fragrance release can be sustained over prolonged periods of time because of the diffusional resistance provided by the microparticles. A feasibility analysis was initially conducted to demonstrate that the microparticles indeed possessed these three features. Experimental and theoretical studies were then conducted to understand how these three microparticle features could be rationally designed and optimized. In order to facilitate a rational design of fragrance loadings in microparticles, a molecular thermodynamic theory was developed to predict fragrance loadings in both polymeric microparticles that were prepared by a solvent evaporation process and wax microparticles that were prepared by a hot melt-freezing process. The developed theory provided reasonable predictions of experimentally measured fragrance loadings in both the polymeric and wax microparticle systems. Accordingly, the theory can be utilized to choose a material to load a given fragrance and to provide guidelines for optimizing the manufacturing process in order to produce microparticles with desired fragrance loadings. Since the theory does not require input of any experimentally measured fragrance loadings, use of the theory to rationally design and optimize a microparticle system for the loading of a given fragrance can significantly reduce the amount of time and resources required to conduct an experimental design and optimization. Fundamental studies were also conducted with unprotected fragrances to identify the mechanisms that inhibit fragrance adsorption onto fabrics in order to better design microparticles that could enhance fragrance adsorption. These studies were performed under simulated laundry conditions using well-characterized surfactants that were representative of those found in commercial laundry products. The adsorption of relatively hydrophilic fragrances onto fabrics was found to be limited by the relatively high solubility of these fragrances in the aqueous solutions utilized to wash the fabrics. On the other hand, the adsorption of relatively hydrophobic fragrances onto fabrics was found to be limited by fragrance solubilization in surfactant micelles that may be present in the commercial laundry solutions. These studies led to the development of a theoretical model to predict the composition of an adsorbed fragrance mixture from the composition of the mixture in the bulk washing solution. Since the aroma associated with a fragrance depends on the composition of the fragrance, the model enables a manufacturer to a priori manipulate the fragrance mixture composition in a laundry product in order to produce the desired aroma on the fabric surface. The developed molecular-level understanding of the mechanisms affecting fragrance adsorption onto fabrics was also extended to rationalize trends observed in fragrance adsorption onto both fabrics and hair that were washed using commercial laundry and hair care products, respectively. The mechanisms identified to inhibit the adsorption of unprotected fragrances onto fabrics can be circumvented by loading the fragrances into microparticles, and therefore, a preliminary study was conducted to determine the factors th.J.t influence microparticle adhesion onto a targeted surface. In particular, the effect of rnicroparticle size on microparticle adhesion onto cotton and polyester fabrics was investigated. The release of unprotected fragrances during the drying of fabrics was also investigated to determine a reference for comparing the release of fragrances from microparticles. Fragrances were most quickly released at the start of the drying process when the fabrics were relatively wet. This release was governed by the diffusional flux of the fragrance from the aqueous liquid film present on the wet fabrics. On the other hand, when the fabrics became relatively dry, direct interactions between the fragrance molecules and the fabric surface controlled the fragrance release behavior. In addition, preliminary studies were conducted to quantitatively characterize the ability of the microparticles to sustain fragrance release from fabrics over the course of three to seven days.