Characterization of Zinc Oxide Nanoparticle Suspensions for Sunscreen

Table of Content

ZnO as a Sunscreen Agent
The Acorn Area
Determination of Surface Area from NMR Relaxation Time
Results of Surface Area Measurements
About XiGo Nanotools


Zinc oxide was first produced in the Bronze Age as a by-product of copper-ore smelting, zinc oxide (ZnO) and used as a skin therapeutic for healing of wounds and for treatment of eyesores. ZnO is also an effective fungistat.

As zinc oxide has unique thermal, optical electronic and chemical properties, it is used in a broad and diverse range of industrial, medical, pharmaceutical, agricultural and cosmetic applications such as pharmaceutical grade (USP). ZnO is widely used today in burn ointments, as a skin protectant and in baby diaper rash creams.

Ultraviolet radiation is a well-documented health hazard and it has been proved that UVB (290 -320nm) is a complete carcinogen and the consequence of exposure to it is skin cancer. However the use of sunscreen formulations containing actives that attenuate only UVB radiation appear to increase the risk for melanoma as sunburn, as a warning for excessive exposure, is blunted.

ZnO as a Sunscreen Agent

Microfine ZnO is a safe and effective sunscreen agent that provides broad-spectrum (290 - 400nm) UV protection.

The microfine grade of ZnO is differentiated from the regular USP grade material in that it has a particle size (PS) less than 200nm. Microfine ZnO is commercially sold as high solids suspensions in a range of non-aqueous vehicles including mineral oils, fatty esters (such as isopropyl myristate), capric/caprylic triglycerides and silicone fluids, and then incorporated into the "oil phase" of the specific W/O or O/W emulsion system used to deliver the active to the skin.

While formulating zinc oxide based sunscreen products, it is important to ensure the right processing to remove agglomerates, as dispersion quality has a strong effect on the performance of the final product.

In practice, a batch premix is prepared and this is transferred to a milling device. Milling is done through four basic processes, impact, shear, extension and cavitation. A variety of milling machinery is available, including ball-and bead-mills, attritors and high-speed dispersers.

As surface area changes with 1/d2,  particle surface area measurements are sensitive to the presence not only of fines but also of agglomerates. Surface area measurement is normally done using BET (N2) gas adsorption, but this requires the material under test to be a dry powder.

The Acorn Area™

The Acorn Area™ from Xigo Nanotools determines the wetted surface area of suspensions of particulate materials such as microfine zinc oxide. The Acorn Area takes advantage of the fact that liquid that is bound to a particle surface has a much lower relaxation time than the free or bulk liquid.

When compared to traditional methods, the Acorn Area measures suspensions directly and requires neither sample pretreatment nor temperature control. There are no assumptions made about the sample particle size (distribution) or shape used in the determination of surface area; it is measured directly. Hence it is a much simpler measurement technique and as little as 0.1ml of sample is needed.

Determination of Surface Area from NMR Relaxation Time

The formula for calculating the surface area from the measured NMR relaxation time is:

Rav = ψp S L ρp (Rs-Rb) + Rb      (1)

where Rav is the average spin relaxation rate constant, p is the particle volume to liquid volume ratio, S is the total surface area per unit weight, L is the surface layer thickness of liquid, ρb is the bulk particle density, Rs is the relaxation rate constant for the bound solvent and Rb is the relaxation rate constant for the free or bulk solvent.

Using a standard reference material it is possible to define a constant, Ka = L ρb (Rs – Rb) so that the equation (1) reduces to:

Rav = Ka S ψp + Rb      (2)

The surface area can then be calculated from:

S = Rsp Rb/ Ka ψp


Rsp = Rav/Rb - 1

A more accurate method is to use the slope of a plot Rsp as a function of different volume ratios.

Results of Surface Area Measurements

In order to show the applicability of the Acorn Area to the measurement of the surface area of ZnO, two series of high solids suspensions of a microfine grade of ZnO were used.

In the first, the ZnO was initially simply dispersed at approximately 52%wt/wt in C12 - C15 alkyl benzoate under high shear (rotor/stator) mixing.

In the second, a dispersing agent, polyhydroxystearic acid was added at a concentration of 0.1% and the ZnO concentration increase to 60%wt/wt. Both pre-mixes were then further processed using a cavitation device run at different (increasing) processing pressures.

From the final bulk batches, each suspension was sampled three times from the final bulk batches, the surface area measured directly and the data averaged. The results, summarized in Table 1 and Figures 1 and 2 demonstrate the effect of the milling process on surface area for both formulation compositions.

Figure 1. Surface Area and Viscosity as a Function of Mill Pressure No Dispersant

In the first series seen in Figure 1, milling at pressures over 2000 psi is counter-productive.

Given the impact on performance features, together with the economic implication of over-milling, a direct measurement of the wetted surface area without dilution or other sample preparation is critical in production quality control (QC) - The Acorn Area makes that possible.

Since measurement is quick, milling can be monitored virtually in real-time to obtain optimum conditions thus saving time and money.

In Figure 2, the second series also shows the dramatic effect of the addition of a dispersing agent. Using only 0.1% PHSA, the solids loading can be considerably increased without any detrimental change in pre-mix viscosity but the surface area increases from 50m2g-1 to 180m2g-1 and these factors all provide economic advantages or a manufacturer.

The Acorn Area can help to optimize the amount of dispersant used.

Figure 2. Surface Area and Viscosity as a Function of Mill Pressure With Dispersant

In Table 1, the surface area data and that from rheological measurements are compared on the same samples and rheological characteristics determine suspension functionality. Even though the flow behavior of real commercial suspensions is complex, the systems are visco-elastic (VE)and it is directly related to the particle size distribution (PSD) of the dispersed phase.

Also, the correlation of milling with surface area measurement is quite clear. Rheological measurements do not appear to be as sensitive. This kind of instrumentation is costly and requires a skilled operator and so, unlike the Acorn Area, does not conveniently lend itself to QC use.

Table 1. Summary of Surface Area and Rheology of ZnO Dispersions

(a) 52% ZnO – no dispersant
0 50 300 20
500 320 22 8
1000 345 18 8
2000 368 18 8
3000 317 20 10
(b) 60% ZnO – 0.1%PHSA
0 180 800 0.2
1000 340 33 0.6
1500 354 36 1.5
2000 324 45 3

About XiGo Nanotools

XiGo Nanotools was founded by Sean Race and Dr. David Fairhurst in 2005 with the mission to provide new innovate “tools” for the emerging nanomaterials industry.

The Acorn Area is designed to measure the wetted surface area of concentrated dispersions with little or no sample preparation, providing a viable complementary technique to BET surface area, analyzing nanoparticles as they are made or used, dispersed in liquids.

Xigo Nanotools' goal is to provide scientists, researchers, and corporations with tools that are easy to use and serve as wide and diverse a customer base as possible. They have incorporated the latest technology available into an integrated, high quality package that provides precise measurements in a very small footprint.

This information has been sourced, reviewed and adapted from materials provided by XiGo Nanotools.

For more information on this source, please visit XiGo Nanotools.

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